CONCRETE WORK -2
Apparatus - Mould shall consist of a metal frustum of cone having the following internal dimensions -
Bottom diameter ………………………………………………..20 cm
Top diameter ……………………………………………………10 cm
Height ……………………………………………………………30 cm
The mould shall be of a metal other than brass and aluminum of at least 1.6 mm (or 16 BG) thickness. The top and bottom shall be open and at right angles to the axis of the cone. The mould shall have a smooth internal surface. It shall be provided with suitable foot pieces and handles to facilitate lifting it from the moulded concrete test specimen in a vertical direction as required by the test. A mould provided with a suitable guide attachment may be used.
Tamping rod shall be of steel or other suitable material 16 mm in diameter 60 mm long and rounded at one end.
Procedure - The internal surface of the mould shall be thoroughly cleaned and free from superfluous moisture and any set concrete before commencing the test. The mould shall be placed on a smooth horizontal, rigid and non-absorbent surface viz. leveled metal plate. The operator shall hold the mould firmly in place while it is being filled with test specimen of concrete. The mould shall be filled in four layers, each approximately one quarter of height of
mould. Each layer shall be tamped with twenty-five strokes of the rounded end of the tamping rod. The strokes shall be distributed in a uniform manner over the cross section of the mould and for the second and subsequent layers shall penetrate into the underlying layer. The bottom layer shall be tamped through out its depth, after the top layer has been rodded, the concrete shall be struck off level with trowel or the tamping rod, so that the mould be exactly filled. Any mortar, which shall leak out between the mould and the base plate, shall be cleaned away. The mould shall be removed from the concrete immediately after filling by raising it slowly and carefully in a vertical direction. The molded concrete shall then be allowed to subside and the slump shall be measured immediately by determining the difference between the height of the mould and that of the highest point of specimen.
The above operations shall be carried out at a place free from vibration or shock, and within a period of two minutes after sampling.
Result - The slump shall be recorded in terms of millimeters of subsidence of the specimen during the test. Any slump specimen which collapses or shears off laterally gives incorrect result. If this occurs, the test shall be repeated with another sample.
The slump test shall not be used for very dry mixes as the results obtained are not accurate.
WORK TEST FOR CONCRETE – MANDATORY LAB TEST
A-O One sample (consisting of six cubes 15x15x15 cm shall be taken for every 20 cum or part thereof at concrete work ignoring any part less than 5 cum or as often as considered necessary by the engineer. The test of concrete cubes shall be carried out in accordance with the procedure as described below. A register of cubes shall be maintained at the site of work in Annexure 4-A.8. The casting of cubes and all other incidental charges, such as curing, carriage to the testing laboratory shall be borne by the contractors. The testing fee for the cubes, if any, shall be borne by the department.
A-1 Test procedure
A-1.1 Mould - The mould shall be of size 15 cm x 15 cm x15 cm for the maximum nominal size of aggregate not exceeding 40 mm. For concrete with aggregate size more than 40 mm, size of mould shall be specified by the engineer keeping in view the fact that the length of size of mould should be about four times the size of aggregate. The moulds for test specimens shall be made of non-absorbent material and shall be substantially strong enough to hold their form during the moulding of test specimens. They shall not very from the standard dimensions by more than one per cent. The moulds shall be so constructed that there is no leakage of water from the test specimen during moulding. All the cube moulds for particular site should, prior to use, be checked for accuracy in dimensions and geometric from and such test should at least be made once a year.
Each mould shall be provided with a base plate having a plane surface and made of non-absorbent material. This plate shall be large enough in diameter to support the moulds properly without leakage. Glass plates not less than 6.5-mm thick or plain metal not less than 12 mm thick shall be used for this purpose. A similar plate shall be provided for covering the top surface of the test specimen when molded.
Note - Satisfactory moulds can be made from machine or steel castings, rolled metal plates or galvanized iron.
A-1.2 Sample of concrete - Samples of concrete for test specimen shall be taken at the mixer or in the case of ready mixed concrete from the transportation vehicle discharge or as directed by engineer. Such samples shall be obtained by repeatedly passing a scoop or pail through the discharge stream of concrete. The sampling operation should spread over evenly to the entire discharging operation. The samples thus obtained shall be transported to the place of molding of the specimen. To counteract segregation, the concrete shall be mixed with a shovel until it is uniform in appearance. The location in the work of the batch of concrete thus sampled shall be noted for further reference. In case of paving concrete, samples shall be taken from the batch immediately after deposition of the sub grade. At least five samples shall be taken from different portion of the pile and these samples shall be spread as evenly as possible through out the day. When wide changes occur during concreting, additional samples shall be taken if so desired by the engineer.
A-1.3 Preparation of test specimens - The interior surfaces of the mould and base plate shall be lightly oiled before the concrete is placed in the mould. The samples of concrete obtained as described under the test specimen shall be immediately molded by one of the following methods as indicated below.
When the job concrete is compacted by manual methods, the test specimen shall be molded by placing, the fresh concrete in the mould in three layers, each approximately one-third of the volume of the mould. In placing each scoopful of concrete the scoop shall be moved around the top edge of the mould as the concrete there slides from it, in order to ensure a uniform distribution of concrete within the mould. Each layer shall be rodded 35 times with 16 mm rod, 60 cm in length, bullet pointed at the lower end. The strokes shall be distributed in uniform manner over the cross section of the mould and shall penetrate into underlying layer. The bottom layer shall be rodded throughout its depth. After the top layer has been struck off with a trowel and covered with a glass plate at least 6.5 mm thick or a machined plate. The whole process of molding shall be carried out in such a manner as to be preclude the change of the water cement ratio of the concrete, by loss of water either by leakage from the bottom or over flow from the top of mould,
When the job concrete is placed by vibration and the consistency of the concrete is such that the test specimens cannot be properly molded by hand rodding as described above, the specimens shall be vibrated to give a compaction corresponding to that of the job concrete. The fresh concrete shall be placed in mould in two layers, each approximately half the volume of the mould. In placing each scoopful of concrete the scoop shall be moved around the top edge of the mould as the concrete there slides from it, in order to ensure a symmetrical distribution of concrete within the mould. Either internal or external vibrator may be used. The vibration of each layer shall not be continued longer than is necessary to secure the required density. Internal vibrators shall be of appropriate size and shall penetrate only the layer to be compacted. In compacting the first layer, the mould shall be filled to the extent that there will be no mortar loss during vibration. After vibrating the second layer enough concrete shall be added to bring level above the top of the mould. The surface of the concrete shall then be struck off with a trowel and covered with a glass or steel plate as specified above. The whole process of molding shall be carried out in such a manner as to preclude the alteration of water cement ratio of the concrete by loss of water, either by leakage from the bottom or over flow from the top of the mould.
A-1.4 Curing and storage of test specimen - In order to ensure reasonably uniform temperature and moisture conditions during the first 24 hours for curing the specimen and to protect them from damage, moulds shall be covered with wet straw or gunny sacking and placed in a storage box so constructed and kept on the work site that its air temperature when containing concrete specimens shall remain 22° C to 33° C. Other suitable means which provide such a temperate and moisture conditions may be used.
Note - It is suggested that the storage box be made of 25 mm dressed tongued and grooved timber, well braced with battens to avoid warping. The box should be well painted inside and should be provided with a hinged cover and padlock.
The test specimen shall be removed from the moulds at the end of 24 hours and stored in a moist condition at a temperature within 24° C to 30° C until the time of test. If storage in water is desired, saturated lime solution shall be used.
A-1.5. Testing - The specimens shall be tested in accordance with procedure as described below -
- The tests shall be made at an age of concrete corresponding to that for which the strengths are specified.
- Compression tests shall be made immediately upon removal of the concrete test specimen from the curing room i.e. the test specimen shall be loaded in damp condition. The dimensions of the test specimens shall be measured in mm accurate to 0.5 mm.
- The metal bearing plates of the testing machine shall be placed in contact with the ends of the test specimens. Cushioning materials shall not be used. in the case of cubes, the test specimen shall be placed in the machine in such a manner that the load is applied to sides of the specimens as cast. An adjustable bearing block shall be used to transmit the load to the test specimen. The size of the bearing block shall be the same or slightly larger than that of test specimen. The upper or lower section of the bearing block shall be kept in motion as the head of the testing machine is brought to a bearing on the test specimen.
- The load shall be applied axially without shock at the rate of approximately 140 kg. sq cm per minute. The total load indicated by the testing machine at failure of test specimen shall be recorded and the unit compressive strength is calculated in kg per sq cm using the area computed from the measured dimensions of the test specimen. The type of failure and appearance of the concrete shall be noted.
REBOUND HAMMER TEST. (MANDATORY FIELD TEST)
If a rebound hammer is regularly used by trained personnel in accordance with procedure described in IS: 13311 (part II) and a continuously maintained individual charts are kept showing a large number of readings and the relation between the readings and strength of concrete cubes made from the same batch of concrete, such charts may be used in conjunction with hammer readings to obtained an approximate indication of the strength of concrete in a structure or element. If calibration charts are available from manufacturers, it can be used. When making rebound hammer tests each result should be the average of attest 12 readings. Readings should not be taken within 20 mm of the edge of concrete members and it may be necessary tom distinguish between readings taken on a troweled face and those on a molded face, when making the test on a precast units, special care should be taken to bed them firmly against the impact of the hammer.
ADDITIONAL TESTS FOR CONCRETE
C-0 In case the concrete fails when tested as per the method prescribed in Annexure 4-A.5, one or more of the following check tests may be carried out at the discretion of engineer to satisfy the strength of the concrete laid. All testing expenditure shall be borne by the contractor. The number of additional tests to be carried out shall be determined by the engineer. He shall be the final authority for interpreting the results of additional tests and shall decide upon the acceptance or otherwise. His decision in this regard shall be final and binding. For the purpose of payment, the Hammering test results only shall be the criteria. Some of the tests are outlined below -
C-1 Cutting cores - This method involves drilling and testing cores from the concrete for determination of compressive strength. In suitable circumstances the compressive strength of the concrete in the structure may be assessed by drilling cores from the concrete and testing. The procedure used shall comply with the requirements of IS: 1199 and IS: 516.
The points from which cores shall be taken shall be representative of the whole concrete and at least three cores shall be obtained and tested. If the average of the strength of all cores cut from the structure is less than the specified strength, the concrete represented by the cores shall be liable to rejection and shall be rejected if a static load test (C-4) either cannot be carried out or is not permitted by the engineer.
C-2 Ultrasonic test - If an ultrasonic apparatus is regularly used by trained personnel in accordance with IS: 13311 (part I) and continuously maintained individual charts are kept showing a large number of readings and the relation between number f readings and the relation between the reading and strength of cubes made from the same batch of concrete, such charts may used to obtain approximate indications of the strength of concrete in the structures. In cases of suspected lack of compaction or low cubes strength the results obtained from the ultrasonic test results on adjacent acceptable sections of the structures may be used for the purpose of assessing the strength of concrete in the suspected portion.
C-3 Load Tests on Individual Precast Units - The load tests described in this clause are intended as check on the quality of the units and should not be used as substitute for normal design procedures. Where members require special testing, such special testing procedures shall be in accordance with the specification. Test loads shall be applied and removed incrementally.
C-3.1. Non-destructive tests - The unit shall be supported at its designed points of support and loaded for five minutes with a load equal to the sum of the characteristic dead load plus one and a quarter time the characteristic imposed load. The deflection is then recorded. The maximum deflection after application of the load shall be in accordance with the requirements defined by the engineer. The recovery is measured five minutes after the removal of the load and the load then reimposed. The percentage recovery after the second loading shall be not less than that after the first loading nor less than 90% of the deflection recorded during the second loading.
At no time during the tests, shall be unit show any sign of weakness of faulty construction as defined by the engineer in the light of reasonable interpretation of relevant data.
C-3.2. Destructive tests - The units is loaded while supported at its design point of support an must not fail at its design load for collapse, within 15 minutes of time when the test load becomes operative. A Defection exceeding 1/40 of the test span is regarded as failure of the unit.
C-3.3. Special tests - For very large units or units not readily amenable to the above test e.g. columns, the precast parts of composite beams and members designed foe continuity or fixity, and the testing arrangement shall be agreed upon before such units are cast.
C-4. Load tests of structures or parts of structures - The tests described in this clause are intended as check where there is a doubt regarding structural strength. Test loads are to be applied and removed incrementally.
C-4.1. Age at tests - The test is to be carried as soon as possible after the expiry of 28 days from the time of placing of the concrete. When the test is for a reason other than the quality of concrete in the structure being in doubt, the test may be carried out earlier, provided that the concrete has already reached is specified characteristic strength.
C-4.2. Test load - The tests loads to be applied for the limit states of deflection and local damage are the appropriate design loads i.e. the characteristic dead and superimposed loads. When the limit state of collapse is being considered the test load shall be equal to the sum of characteristic dead load plus one and a quarter times the characteristic imposed load and shall be maintained for a period of 24 hours. In any of the test temporary supports of sufficient strength to take the whole load shall be placed in position underneath but not in contact with the member being tested. Sufficient precautions must be taken to safeguard persons in the vicinity of the structure.
C-4.3. Measurements during tests - Measurements of deflection and crack width shall be taken immediately after application of the load and, in the case of 24h loaded period, after removal of the load and after 24h recovery period. Sufficient measurements shall be taken to enable side effects to be taken into account. Temperature and weather conditions shall be recorded during the tests.
C-4.0. Assessment of results - In assessing the strength of a structure or a part of the structure following a load test, the possible effects of variation in temperature and humidity during the period of the test shall be considered.
The following requirements shall be met -
- The maximum width of any crack measured immediately on application of the test load for local damage, is to be not more than 2/3 of the value of the appropriate limit state requirement.
- For members spanning between two supports the deflection measured immediately on application of the test load for deflection is to be not more than 1/500 of the effective span. Limits shall be agreed upon before testing cantilevered portions of structures.
- If maximum deflection in mm shown during 24 h under load is less than 40 L² / DD where L is effective span in m and D is overall depth of construction in mm, it is not necessary for the recovery to be measured and the requirements (d) does not apply, and
- If within 24 hours of the removal of test load for collapse as calculated in clause (a) a reinforced concrete structure does not show a recovery of at least 75 per cent of the maximum deflection shown during the 24 h under load, the loading should be repeated. The structure should be considered to have failed to pass the test if the recovery after second loading is not at least 75 per cent of the maximum deflection shown during the second loading.
FORMWORK AND SCAFFOLDING
1. Concrete is the most widely used construction material today because of its durability, mouldability and other characteristic. Concrete in its plastic stage has no form and therefore, needs to be molded to the required shape. Formwork includes the mould in contact with the wet concrete and all the necessary supports, hardware and bracing. The hardware supports and bracings are generally referred to as centering or false work. Scaffolding is the structure made to provide access to the point of working.
2. In the early days, formwork was generally rigged up by carpenter with available timber and nails as best as possible, using rule of thumb approach. Along with the growth in the development of concrete construction, formwork techniques have also developed side by side. With the technological advancement and introduction of new materials such as plywood, steel, aluminium, polypropylene, fibre reinforced plastics etc more rational approach is being made in the design of formwork.
3. Formwork - The basic objectives of the formwork designer should be to achieve the following:
4. Safety: to build substantially so that formwork is capable of supporting all dead and live loads, without collapse or danger to workmen and to the concrete structure.
5. Quality: To design and build forms accurately so that the desired size, shape and finish of the concrete is attained.
6. Economy: To build efficiently saving time and money for the contractor and owner.
7. Safety must find the first place in the design, construction, erection and stripping of formwork and centering systems.
8. Design consideration - To achieve the above basic objectives of formwork design the following should be considered.
a) Correct assessment of loads that come over forms with due consideration to pressures that arise from wet concrete.
b) Selection of proper forming material considering its strength, durability and cost.
c) Selection of proper supporting systems, either of wood, steel or aluminium. Proprietary supporting systems that are standardised and proved by tests should be adopted with advantage.
d) Provision for proper ties/anchors for the forms and bracing for support.
e) Provision of proper and safe working/access platforms for labour and equipment.
f) Proper scheduling, stripping and refixing of shores.
It is important to realize that centering design requires the same skill and attention to details as the design of permanent structure of like type.
9. Loads on forms - The loads on vertical forms are to be assessed from consideration of:
a. Density of concrete, b. Slump of concrete, c. Rate of pour, d. Method of discharge, e. Concrete temperature, f. Vibration, g. Height of discharge, h. Dimensions of section cast, i. Reinforcement details, j. Stiffness of forms
10. Form material and type - The choice of the form material mainly depends on the availability and cost of the material. Form materials include timber, plywood, hardboard, plastic fiber board, corrugated boxes, steel, aluminium, plaster of Paris etc., Thin metal sheets, neoprene craft paper, hardboard, fibre board and gypsum are generally used as forms liners attached to inside face to improve or alter the surface texture of concrete. Timber, plywood and steel are the main materials used in our country.
11. Timber - Traditional material for formwork has been timber due to its easy availability, relatively low cost and ease for shaping. The disadvantages of timber are warping, twisting, deterioration under stress of heat and contact with wet concrete. It is common practice to support formwork for slab in buildings with timber ballies cut to approximate sizes with wedges used underneath them for final adjustments. These make weak points and are seldom prevented from displacement. Timber ballies are generally not straight and do not transmit load axially.
12. Plywood - The advantages of plywood are large panels for economical construction and removal, choice of thickness, physical properties, good finish and economy from repeated uses.
13. Steel - Steel has been an important material for fabrication of standard as well as special purpose forms, accessories and hardware. Steel is also extensively used for making horizontal and vertical shores. Because of the known characteristics of steel, design calculations for the system can be precisely made. Steel formwork system also facilitates to maintain accurate alignment, level and dimension with excellent surface finish.
Readymade forms are modular panel systems and accessories that can be adopted to build formwork for various sizes and shapes. Tailor made or special purpose made forms is fabricated to order and include tunnel forms, bridge girder shutters, dam shutters etc.
14. Climbing formwork - Most commonly used formwork system is the Climbing Forms. This system basically consists of form panels assembled with or without whalers and supported by vertical strong back members (generally called soldiers) of various designs. The Climbing Form System for large and deep concrete pours may incorporate special features such as, working platforms, adjustable push-pull struts for aligning the formwork and also roller mechanism for shifting the form assembly to allow tying reinforcement and fixing other inserts, in case of thin walls. Various types of anchorage’s are used to fix or support the Climbing Form Assembly to the previous concrete lift complete floor height in case of shear walls in buildings, deep pours in piers, abutments of bridges and duct walls are typical examples where such systems are used and generally handled by cranes. For smaller structures and shallow pours, lighter soldiers are used and the Form assembly is usually dismantled in small sections and refixed from pour to pour manually, with external access scaffolding.
15. Slip forms - Slip form construction also known, as sliding forms of construction is similar to extrusion process. The rate of movement of forms is regulated so that when forms leave the concrete it is strong enough to retain its shape while supporting its own weight. Vertical slip form is used for bins, soils, bridge piers etc. where as horizontal slip form is used for canal lining, tunnel inverts etc. Recent developments in slip form techniques enable construction of tapered structures like chimneys, cooling towers etc where simultaneously with moving of forms, vertically, mechanical/hydraulic jacks also adjust the forms circumferentially to the required sizes as the slide progresses.
16. Suspended forms - This is a climbing system of formwork used for construction of chimneys, silos etc. the forms for outside of the structure is suspended from a concreting platform which in turn is suspended from a central scaffold tower by means of chain pulley block. The formwork system incorporates a radial shift mechanism for adjusting the outer form to the required diameter. The inside forms are usually the climbing types.
17. Travelling or moving forms - Travelling or moving forms are usually made of steel and are generally resorted for construction of long stretches of similar section such as tunnel linings, sewers, galleries, culverts etc. Substantial saving in time and labour is possible by using travelling forms. Travelling forms are tailors made form fabricated/assembled to shape and supported by framework or gantry structure which is fitted with wheels for movement either manually or by electric or hydraulic motors. Hinges or other stripping devices are provided in the shutter itself for collapsing the formwork by means of jacks or turnbuckles. In telescopic type the form is so designed that with one mobile gantry several units of formwork can be handled by telescoping one section of formwork through the other.
18. Aluminum forms - Certain aluminium alloys are used for making forms, which are similar to steel forms. They are lightweight and reduce handling costs.
19. Concrete hardware’s - Formwork systems generally incorporate a variety of hardware’s such as ties and anchors for resisting lateral pressure exerted by green concrete. Form ties are tensile units consisting of an internal tension member and an external holding device. The ties can be continuous single unit or internal disconnecting type. Form anchors are devices embedded in previously poured concrete and are used for securing formwork for the subsequent lifts.
20. Formwork supports or centering - Various types of formwork supports have been developed in steel. They have been specially designed to cut labour cost in erection and stripping and to make them versatile by incorporating an adjustability feature in most cases. Generally formwork supports are either single leg type or multilegged type such as a frame or a tripod or a trestle. The single leg type is called a prop or a shore and is generally tubular and telescopic type. It incorporates adjusting features through a collar or nut to provide infinite adjustment in height. The props are usually used for supporting formwork upto heights of about 5 M. Beyond this height, they may be used in tiers in which case they are properly tied and braced to form a rigid structure. Bracings can be provided by means of tubes and clamps.
Among the multilegged support systems the common ones are of prefabricated tubular frames in a variety of shapes and modular sizes which can be assembled one over other to get the required heights and also spaced at suitable intervals depending on the loads to be carried. The forms are usually braced together by means of ledgers and cross braces to form a rigid structure. For finer adjustments in height, there are special accessories like screw jack either at top or bottom or both.
Like vertical formwork supports or shores there are also many types of horizontal formwork supports available. These are usually latticed or boxed beams which also telescope one into the other and cater for a range of spans. These horizontal supports rest either on beam forms or other shores at ends. The need for intermediate supports is eliminated and free access and working space is obtained during construction.
An important development in the formwork system particularly for flat slab and multistoried construction is the drop head system. Drop head is fitted on top of the prop or supports which continue to support the slab while the remaining form for the decking could be struck for reuse, there by affecting a great economy in the formwork costs. With this system only an extra set of shores would be required to get faster cycle of slab construction.
Various scaffolding systems may also be adopted and used to act as centering especially when the heights of supports involved is large such as in the case of high industrial buildings, motorway decks, high shell or barrel roof hangers etc.
21. Scaffolding - Practically in all stages of construction, scaffolds are required to provide temporary platforms at various levels to carry out all these works which can not be conveniently and easily carried out either from ground level or any other floor of the building or with the use of a ladder.
22. Timber scaffold - Timber has been used for building scaffold from time immemorial and continues to be used even today. The most common type of scaffolding used in India even today is bally or bamboo scaffold. Barring a few cases where bally or bamboo scaffolding is neatly erected, properly braced and well tied to the building, invariably such scaffolds are in crooked and awkward shapes presenting a dreadful sight particularly on tall building where a stronger and safer scaffolding is called for. The draft revision of IS 3696 suggests limiting bamboo and timber scaffold up to maximum of 18 M. height.
23. Metal scaffold - By and large metal scaffolds are made of steel tubes. Many countries have formulated standard specifications and codes of practice for metal scaffolding. IS: 2750 for steel scaffolding and IS: 4014 parts 1 and 2 for steel tubular scaffolding are relevant Indian Standards.
Metal scaffolds are broadly two types viz. Tubes and fitting type and prefabricated unit frame type.
Tubes and fittings type consists of plain tubes, which are, used for making uprights, transoms, ledger and putlog. Various type of clamps viz. Right angle or double coupler, swivel coupler, putlog coupler, joint pins etc are available for connecting tubes.
Many designs of prefabricated unit type of scaffold have been developed by proprietary concerns and are now being extensively used in most of the construction sites through out the world. Units have been designed incorporating the following basic features.
i) Prefabrication of adjustable components with few or no loose parts.
ii) Simple and fool-proof devices as far as practical to ensure maximum safety.
iii) Speed and ease in erection and dismantling at site by unskilled workers.
iv) Known characteristics of each component enabling complete calculation of loading to ensure use of minimum materials.
v) High degree of versatility and durability enables hundreds of uses for a wide range of applications.
Some of the prefabricated types of scaffoldings available are as follows:
24. Unit frame or three pieces frame - This consists of two verticals and one horizontal member with specially designed end fittings and when three are assembled together it forms a H frame. The end fittings on the horizontal also incorporate a fixing device for the longitudinal ledger. The unit frames can be erected one above the other and are spaced at suitable intervals depending on the duty of the scaffolding. The manufacturers provide complete data on loading capacities. The advantage of this type of three piece frame is that the units can be spaced at any required intervals and also the platforms can be had at any required levels and hence scaffold of this type may be truly called as all purpose type.
25. Welded frame type - These scaffold frames are made as welded units consisting of two uprights and one or more cross members to form a rectangular or H frame. Such frames can be erected one over the other to the required height. Lengthwise such frames are connected either by scissors type cross braces or ledgers. In this system the length of the ledger of cross braces decides the longitudinal spacing of the frame. Accessories such as base plate, adjustable stirrup head etc are also supplied to complete the system. The frames are made of tubes in different grades viz. Light duty or heavy duty as required.
26. Wedge lock or collar grip type - Wedge lock type scaffold consists of verticals, ledgers, transoms and diagonals. The uprights have housing welded on them at regular intervals. The transoms, ledgers and diagonals have specially designed wedge lock assemblies fitted at ends, which engage in the housing on the uprights. This type of scaffolding can be erected very fast and does not require any special tool except a small hammer to drive the wedges in. Necessary accessories are also supplied to complete the system. This is extensively used for building scaffold towers inside chimneys, silos etc and also in ship building.
29. Scaffold boards - Scaffold boards for platform are generally in timber, particularly in pinewood because of its lightweight and strength. Apart from timber boards, Steel planks are also available. They are generally made with thin M.S.Sheet with pressed or cold-formed flanges and provided with anti-skid surface treatment. It may be noted that steel planks would not be suitable for platform in extreme tropical climate and also where oil/grease or such other slippery materials are likely to fall on platforms.
30. Safety requirements - Codes of practice specify the construction details of scaffolding and also give guidelines for bracketing and tying of scaffolds for stability. Single pole scaffolds shall be braced longitudinally and the double pole scaffold shall be braced both longitudinally and transversally, so that the scaffolds from a rigid and stable structure. The scaffold shall be effectively tied to a building or adjacent structure to prevent movement of the scaffold either towards or away from the building or structure. In extreme wind conditions, it may be necessary to provide additional ties, guys or other suitable supports as decided by the engineer.
31. Scaffolding systems.
1. Metallic scaffolding is mainly of steel although aluminum is also finding increasing use as a raw material. Steel scaffolding generally includes the following.
2. Tubes & fittings. This is the commonest type of metallic scaffold first used in 1908 by a British company. This system is versatile but cumbersome and time-consuming to use since it involves a lot of joints and several loose components, which necessitate safety precautions during erection. It is recommended only for limited applications such as access scaffold for not a very tall building and for old structures/connections.
3. Welded frame-Type-Fabricated - Steel frames and cross braces systems frames are placed at regular intervals one over the other and inter-connected by cross braces for rigidity and stability. This is sturdier and safer, easy to erect and dismantle, and is suited for most staging and scaffolding jobs. But the system has some limitations in use due to the fixed size of components. It is ideal for access scaffold, heavy staging of industrial buildings, bridges, flyovers, aqueducts, etc.
4. All-purpose units / Wedge-lock type scaffolds: These scaffolds are fairly versatile but require more time to erect and dismantle compared to the welded frame type of scaffold. They are suited for access scaffolding and slab staging of industrial structures. All-purpose units consist of two vertical and one horizontal unit which are interconnected by ordinary 40 mm. NB M.S. tubes called ledgers.
5.CUPLOK systems - This is among the most versatile modular scaffolding arrangements in the world. Its unique node point connection makes it a fast assembly scaffolding. The absence of loose parts and a unique cup action allows four horizontal units to be fixed or released in a single operation by means of only a hammer. Careful selection of raw materials for various components such as higher grade YST-240 tube, malleable cast iron top cups, deep drawn steel of bottom cups and drop-forged ledger blades makes it a sturdy and yet light scaffolding system. It is ideally suited for all access scaffolds and slab staging for any type of construction. The prime feature of CUPLOK is that since its vertical member has cup joints at every 500 mm. One has to just change the location of the horizontal units (thereby reducing or increasing l/r ratio) for different loading conditions without changing the size or thickness of the vertical tubes. Modular scaffolding systems have been effectively used for boiler maintenance, chimneystacks, access, flyovers, silos as well as offshore structures/ship building and repairs. These systems prove economical as they cut down erection time significantly.
6. Slab shuttering & support systems - From the days of timber shuttering & wooden props, slabs shuttering and centering have come a long way. The various slab shuttering systems are:
a) Conventional span–prop arrangements: Adjustability of the components makes the system versatile for normal slab shuttering.
b) Shuttering for heavier slab/deck slab - Specially designed shutters are made for jobs such as slab of industrial building, flyovers, bridges, etc.
c). Metriform unit – Decking arrangements: These consist of modular Metriform beams and panels while supporting the slab on drop-heads fitted over steel props or scaffolds. Slab shuttering can be removed in three days instead of the regular seven days thereby considerably increasing the rotation of shuttering materials. It is thus ideal for today’s time-bound projects.
d) Shuttering for waffles troughs - Made out of moulded plastic materials to give architectural finish.
e) Flying form - This includes the crane-handled formwork of a complete floor slab of a building for speedy completion. All the slabs of the building should be identical in this case.
f) Support staging - Slab/beam staging is normally effected through adjustable steel props or any type of system scaffold depending on the height and load of the structure to be taken on support staging.
7. Wall / column shuttering - The construction of RCC walls/columns requires sturdy shuttering to take care of concrete pour pressures. The systems generally available are:
a) Conventional channel/heavy duty soldier - This consists of steel panels connected side by side with soldiers. Heavy duty soldiers are used for one-sided shuttering such as for RCC piers, retaining walls, etc. these are ideal for lift walls, shear walls, RCC piers, columns etc.
b) Heavy duty/ strong back arrangement - This is meant for a pour height of up to 5 m. using J-4 or Slimlite back-up soldiers. Shuttering can be of steel/ply with soldiers provided as back-ups, behind the shutters. It is ideal for fast concreting, with the help of pumps and can be crane-handled.
8. Special shuttering - Construction of special structures also requires suitable formwork. Some of the applications are:
a) Slip form of chimneys/silos: Hydraulically lifted complete shuttering by means of heavy duty jacks enables concreting of a tall chimney in hardly any time as more often the slipping (or concreting) is continuous once it starts.
b) Dam shuttering: Special heavy duty hinged soldiers along with heavy shutters are used to match the profile of a dam.
c) Canal lining: Mobile shutters are specially designed to move along the canal, for the concrete lining.
d) Bridge shuttering: Shutters for girders are specially designed to take care of concreting loads.
9. Conclusion - It is obvious that modern shuttering and scaffolding systems, which are continuously evolving, are among the most important aspects of construction and maintenance. Unfortunately, so far neither the industry nor the engineering institutions have really gone into the relevance and details of this equipment are which should be utilized for effecting proper and economical designs for particular applications. With the advent of professional scaffolding organisations and realisation of the need on the part of the industry for safer, faster and economical construction, one hopes for the development of this long-neglected but important area in the near future is going to be a reality.
General - The use of admixtures in concrete, of late, has assumed greater importance in the field of concrete technology, There are quite a few new materials in the market. They are used in cement as an additive, in mortar and concrete as admixtures. Such additives and admixtures are sometimes collectively called construction chemicals. About ten years ago although such materials were in use in other countries, they were not freely available in our country. Sometimes, they were imported at great cost for specific use but such admixtures were not widely used. Therefore not only the state and central government department specifications, but also engineering practices by private bodies did not give much importance to the use of these construction chemicals. On the other hand, in other advanced or advancing countries, concrete is very rarely made without the use of one or other admixtures.
For the last nearly ten years, a number of international firms, manufacturing construction chemicals, have transferred their technologies to India. In collaboration with companies like MC-Bauchemie of West Germany, Fosroc Chemicals of U.K., Sika Qualcrete of Switzerland, and Feb. Ltd. of U.K., this list is illustrative but not exhaustive, Indian companies have started manufacturing a wide range of construction chemicals. In addition to the above, there are number of Indian companies manufacturing a wide range of admixtures and construction chemicals. All the same these companies have made valuable contribution to Indian construction industries.
In the past, admixtures have been used very rarely with the exception in the fifties of Surkhi as pozzolanic admixture and air entraining agents as workability agents in multipurpose dams. In cold weather regions, calcium chloride was also used as an accelerating admixture. In addition a number of integral waterproofing compounds were in the market. Either due to lack of knowledge or due to non-availability of other appropriate and efficient materials, cement has been used as an all-purpose, ubiquitous material. When better workability is required we have used more cement and more water. For grouting of dams was used cement. For cladding of walls with tiles, stones, marble or granite, was used cement. In short, in the field of construction, cement was used for all purposes.
While cement is a good bonding material, it has many disadvantages. Long term drying shrinkage is one of the worst facets of cement. The excessive heat of hydration of rich concrete, the dissolution of Ca (OH)2, susceptibility of concrete to sulphate attack and carbonation are some of the other disadvantages that make concrete an inefficient material. It was once thought that concrete structure could not be repaired as the bond between old concrete and new concrete could not be obtained satisfactorily. It was also thought that concrete structures once made did not require repair and maintenance, Some of these perceived disadvantages and misconceptions no longer hold good. We know now that modern construction chemicals available today all over the world and in India have helped to enhance the versatility of concrete as an efficient construction material in a wide variety of situations in the field of construction engineering.
Modern concrete admixtures and construction chemicals that are manufactured in India in collaboration with some of the international companies named earlier are dealt with. These companies have been working in a number of countries in different climatic conditions for more than two to three decades. For general information, the extended topic, construction chemicals, has also been included in this section since these construction chemicals are in a way associated in improving the performance of concrete. It is hoped that will be of immense practical use to all concerned with concrete technology as applied to construction engineering, since it attempts to lay out the state of the art as far as concrete admixtures are concerned.
The following materials will be discussed in this section:
(a) Plasticizers and Super Plasticizers
(b) Retarding Plasticizers and Retarders
(c) Accelerating Plasticizers and Accelerators
(d) Air entraining Agents
(e) Water Proofing Materials
(f) Polymer bonding agents
(g) Floor Hardeners and Dust Proofers
(h) Concrete Curing Compounds
(i) Polymer Mortar for Repair and Maintenance
(j) Adhesives for Tiles, Marble and Granite
(k) Mould Releasing Agents
(l) Grouting Agents
(m) Joint Sealant
(n) Decorative cum Protective Paints
(o) Concrete Repair System
(p) Installation Aids
(a) Plasticizers and super-plasticizers
Plasticizers - Right workability is the essence of good concrete. Concrete in different situations requires different degree of workability. A high degree of workability is required in situations like deep beams, thin walls of water retaining structures with high percentage of steel reinforcement, column and beam junctions, Tremie concreting, pumping of concrete, hot weather concreting, for concrete to be conveyed for considerable distance and in ready mixed concrete industries. The conventional methods followed for obtaining high workability is by improving the gradation, or by the use of relatively higher percentage of fine aggregate or by increasing the cement content. There are difficulties and limitations to obtain high workability in the field for a given set of conditions. The easy method generally followed at the site in most of the conditions is to use extra water unmindful of the harm it does to the strength and durability of concrete. It has been stressed time and again to the harmful effect of using extra water than necessary. It is an abuse, a criminal act, and un-engineering to use too much water than necessary in concrete. At the same-time, one must admit that getting required workability for the job in hand with set conditions and available materials is essential and often difficult. Therefore engineers at the site are generally placed in conflicting situations. Often he follows the easiest path and that is adding extra water to fluidize the mix. This addition of extra water to satisfy the need for workable concrete is amounting to sowing the seed of cancer in concrete.
Today plasticizers and super plasticizers help an engineer placed in intriguing situations. These plasticizers can help the difficult conditions for obtaining higher workability without using excess of water. One must remember that addition of excess water will only improve the fluidity or the consistency but not the workability of concrete. The excess water will not improve the inherent good qualities such as homogeneity and cohesiveness of the mix, which reduces the tendency for segregation and bleeding. Whereas the plasticized concrete will improve the desirable qualities demanded of plastic concrete. The practice all over the world now is to use plasticizer or super plasticizer for almost all the reinforced concrete and even for mass concrete to reduce the water requirement for making concrete of higher workability or flowing concrete. The use of super plasticizer has become almost a universal practice to reduce water/cement ratio for the given workability, which naturally increases the strength. Moreover, the reduction in water/cement ratio improves the durability of concrete. Sometimes the use of super-plasticizers is employed to reduce the cement content and heat of hydration in mass concrete.
The organic substances or combinations of organic and inorganic substances, which allow a reduction in water content for the given workability, or give a higher workability at the same water content, are classified as plasticizing admixtures. The advantages are considerable in both cases: in the former, concretes are stronger, and in the latter they are more workable.
A good plasticizer fluidizes the mortar or concrete in a different manner than that of the air-entraining agents. Some of the plasticizers, while improving the workability, entrains air also. As the entrainment of air reduces the mechanical strength, a good plasticizer is one, which does not cause air-entrainment in concrete by more than 1 or 2%.
Super-plasticizers constitute a relatively new category and improved version of plasticizers for concrete, the use of which was developed particularly in Japan and Germany from the sixties.
Super-plasticizers can produce:
(i) At the same w/c ratio, much more workable concrete than the plain ones.
(ii) At the same workability, a considerable decrease in the w/c ratio, and therefore concrete having higher strength.
In case of (i) above, it is possible to obtain the so called "flowing concrete' 'or "self leveling concrete" which is pumpable or needs very little compacting efforts for compaction. In the fluidized concretes, the phenomenon of aggregate segregation or water separation are practically absent, and anyhow much reduced than in the case of normal plasticizers.
Super-plasticizers are generally grouped in the following calories:
1. Sulphonated melamine - formaldehyde condensates (MSF).
2. Naphthalene Sulphonate - formaldehyde condensates (NSF).
3. Modified Lignosulphonates (LS).
4. Other types.
Each category includes products differing from one another because either the base component can have different molecular weights or may have undergone some chemical changes or other chemical substances can be present. As a consequence each commercial product will have different action on cements. Whilst the dosage of conventional plasticizers do not exceed 0.25% by weight of cement in case of lignosulphonates, or 0.1% in case of carboxylic acids, the products of groups (1) or (2) are used in considerably high dosages (0.5% to 3.00%), since they do not entrain air. Group (3) includes admixtures which have an effective fluidizing action, but at the relatively high dosages, they can produce undesirable effects, such as accelerations or delays in setting times. 13.2 Moreover, they increase the air-entrainment in concrete.
Plasticizers and super plasticizers are water based. The solid contents can vary to any extent in the product manufactured by different companies. Cost should be based on efficiency and solid content, but not on volume or weight basis. The composition of different samples can be qualitatively compared by "Infrared spectrometry".
Effects of super-plasticizers on fresh concrete
It is to be noted that dramatic change in workability is not achieved when super-plasticizers are added to very stiff, zero slump concrete. A mix with an initial slump of about 2 to 3 cm can only be fluidized with plasticizers or super-plasticizers. An improvement in slump value can be obtained to the extent of about 25 cm depending upon the initial slump and dosage. It has been seen that slump increases with increase in dosage. But there is no improvement in slump beyond certain limit of dosage. As a matter of fact the overdose may harm the concrete. A typical curve is shown in Figure1
Fig 1 Effects of additions of super plasticizers on the workability of a concrete. Cement content 300 kg/m3, w/c ratio = 0.6
The type of cement affects the plasticizer's influence in fluidizing the concrete. The compatibility of particular super-plasticizer with that of cement to be used must be taken into account. This can be done by simple field trial.
A negative property of super-plasticizers is the workability loss with time (slump loss). Figure 2 shows the typical slump loss with time.
Fig 2 Slump loss with time
Different methods are proposed to reduce the slump loss with time, such as, for example, adding super-plasticizer to concrete immediately before its placing or delayed addition, or at small successive doses or over by over dosing it.
The delayed addition of super-plasticizer reduces the loss of slump. Figure 3 shows the effect of delay on slump loss. It is suggested that concrete be first mixed with most of the water and the super-plasticizer is then added with the remaining water.
Fig 3 Effect of the delayed addition of sulphonated melamine formaldehyde
condensate on slump loss
The effect of repeated dosages or super-plasticizers to reduce the loss is shown in Figure 4.
One of the practical methods for reducing the slump loss is to formulate super-plasticizers, incorporating retarding admixtures. By this way, slump can be maintained for about 3 hrs.
Effect of super-plasticizers on hardened concrete - It is a shared opinion that, by working at constant w/c ratio, the strength of concrete is normally improved by the use of super-plasticizers on account of better compactability of concrete. Over dose of super-plasticizers bring about a small reduction of strength. If super-plasticizers are used to reduce the w/c ratio, by keeping the slump equal to that of reference concrete, much higher strengths are generally obtained. This effect is found also on mortars. Within certain limits, the effect is proportional to the amount of super-plasticizers added but beyond certain dosages, strength tends to decrease. It is therefore important to avoid excessive dosages. As regards the long-term strength of concrete containing super-plasticizers, there are no exhaustive case studies. However, unsatisfactory results have not been reported.
Fig 4 Effect of repeated dosages of sulphonated Naphthalene formaldehyde
condensates on slump
At the same w/c ratio, sulphonated melamine formaldehyde condensate (MSF) and naphthalene sulphonated formaldehyde condensate (NSF) do not considerably modify drying/shrinkage of concrete. At the same consistency, they sometimes reduce drying/shrinkage appreciably.
The total creep is higher when concrete contains Naphthalene Sulphonate at high w/c ratio (0.64). On the contrary, when the w/c ratio is low, the difference in creep between samples with or without super-plasticizers is insignificant.
Impermeability plays a primary role on the durability of concrete and since this depends on w/c ratio, super-plasticizers should exert a favourable effect Super-plasticizers, owing to the reduction in w/c ratio, reduce the penetration of chlorides and sulphates into the concrete and therefore improve their resistance to the de-icing effect of salt or sea water. For the same reason, the resistance to the sulphate attack is also improved.
Mechanism of action of super-plasticizer
Interpreting the mechanism of the action of plasticizers and super-plasticizers is rather complex owing to the variety of compounds forming the family of these admixtures and to the composite nature of cement and concrete. However, the following can be forwarded to explain the mechanism by which these admixtures bring about in improving the plasticity of the concrete.
The fluidizing effect is, at least partly, the consequence of retarding phenomena caused by the plasticizers and super-plasticizers on the cement hydration. Infact, the mixing water reacts more slowly with the cement and therefore it remains available for longer time to fluidize the mix.
The dispersing action of plasticizers is attributed to the adsorption of plasticizer molecules on the cement grains as well as to the resulting changes in the surface charge and zeta potential of the solid particles. The charges of the same sign cause repulsive forces, which results in the solid dispersion and therefore increase the mix plasticity.
From what has been said above, it can be seen that use of plasticizers and super-plasticizers can make significant contribution for improving the strength and durability of cement concrete. The innovation and use of these materials started during 1970 and has become one of the milestones in the short history of cement concrete. It is a boon in the hands of concrete technologists and construction engineers.
Table gives the list of a few commercial names of plasticizers and super-plasticizers available in India.
(b) Retarding plasticizers and retarders - it is usual to incorporate the retarding admixtures as an integral part of super-plasticizers. Therefore, retarding plasticizers are available in the market today. They are suitable for achieving long retardation for avoiding the construction joints. The retarding plasticizers are frequently used to reduce the loss of slump, in hot weather concreting or for conveying concrete over long distances.
Retarding plasticizers. Retarding Plasticizers are sometimes required for slip form method of construction. Since retarding super-plasticizer retard the setting action of cement particles over considerable time, they regulate the evolution of heat of hydration in mass concrete. Sometimes they are of special requirements for roughening the surface of concrete roads and pavements. In ready mixed concrete industries, the retarding super-plasticizers often become essential requirement.
Retarders. . Sometimes it will be advantageous to mix high performance set retarding admixtures separately into the concrete. These set retarding agents can be admixed separately along with normal plasticizers or super-plasticizers.. The dose of retarders independently depends on the situation.
(c) Accelerating and accelerators - Certain ingredients are added to accelerate the strength development of concrete to super-plasticizers to offer high early, strength to the concrete. Such accelerating super-plasticizers, when added to concrete result in faster development of strength. This enables early stripping of moulds ensuring speedy construction as well as overall economy on account of lesser formwork requirements. Accelerating plasticizers can be used in prestressed concrete works as well as in precast industry for manufacturing building elements, poles, pipes, piles, concrete sleepers etc. Accelerating plasticizers can give strength of about 1.6 times that of reference concrete in one day.
Accelerators - The use of calcium chloride as an accelerating material has been widely produced. However, some recent studies have indicated that calcium chloride is not the right material for acceleration of strength and the addition of chloride in any form should be avoided from the durability consideration, particularly for reinforced and prestressed concrete. As such it is particularly emphasized that all modern admixtures should be free from chlorides.
Even the BIS specifications for ordinary and high strength cement, is reconsidering the reduction of permissible chloride content in the cement, from what is permitted in the current code of practice.
The modern accelerating admixtures are completely chloride free. Some of the accelerators produced these days are so powerful that it is possible to make the cement set into stone-hard condition in a matter of 10 minutes or less. With the availability of such powerful accelerators the under water concreting has become comparatively easy. Similarly the repair work that could be carried out to the waterfront structures in the region of tidal variations has become easy. The use of such powerful accelerators has facilitated the basement waterproofing operations. In the field of prefabrication also it has become an invaluable material. As these materials could be used even up to -10 degree Celsius, they find an unquestionable use in cold weather concreting.
(e) Waterproofing materials - In spite of many fold advancement made in Concrete Technology and the ability to produce high qualify concrete, it has not been possible to really make waterproof structures. The problem of water proofing of roofs, walls, bathrooms, toilets, kitchens, basements, swimming pools, and water tanks etc. have not been much reduced. There are number of materials and methods available in the country for waterproofing purposes. But most of them fail due to one or the other reasons. Waterproofing has remained a complex problem. A successful waterproofing not only depends upon the quality and durability of material but also the workmanship, environment and type of structures. Leaving all other aspects, the material part is only discussed below.
It should be remembered that the use of plasticizers, super-plasticizers, air-entraining agents, pozzolanic materials and other workability, agents, help in reducing the permeability of concrete by reducing the requirement of mixing water and hence they can also be regarded as waterproof material. In addition, there are other materials and chemicals available for water proofing concrete structures.
These materials can be grouped as follows:
(i) Integral waterproofing compounds
(ii) Waterproofing liquid membrane
(iii) Waterproof slurry coatings
(iv). Water repellant materials
(v) Injection grout for cracks
(vi). Waterproof tile adhesive and
(vii) Sealing materials for rising dampness
(i) Integral Waterproofing Compounds - The performance of integral waterproofing compounds is covered in IS 2645. They have limited utility in making the concrete waterproof. They are not of much help for the possible cracks on account of drying shrinkage and cracks on account of thermal variations. They are of some help in a situation where concrete is continuously in wet or damp conditions.
(ii) Waterproofing liquid membrane - For roof waterproofing, a membrane forming material is an ideal material. The membrane should be tough, wear resistant, solar reflective, elastic, elastomeric and durable. They are to be applied on roof strictly as per manufacturers' instructions with special care to surface preparation. Attempts are being made to formulate polyurethane and polymerized bitumen as membrane forming waterproof materials for roof.
(iii) Waterproof slurry coatings - Waterproofing of concrete, brick masonry and cement bound surfaces can be obtained by a specially made slurry coatings. Slurry consists of specially processed hydraulically setting powder component and a liquid polymer component. These two materials when mixed in a specified manner form brushable slurry. Two coats of this slurry when applied on roof surface or on any other vertical surface in a basement, water tank or sunken portion of bathroom etc. forms a long lasting waterproofing coat. This coating requires curing for a week or so. The coating so formed is elastic and abrasion resistant to some extent. To make it long lasting mortar screeding or tiles may protect the coatings.
A modified version of the above has been made to give a better waterproofing and abrasion resistance to the treatment. This will make the coating tough, more elastic and waterproofing. This modified version of waterproofing system is especially applicable to terrace gardens, parking places, basements, swimming pools, sanitary areas etc. This coating also gives protection to chlorides, sulphates and carbonation attack on bridges and to underground structures.
(iv). Water repellant material - Sometimes shrinkage or thermal cracks appear on mortar joints of masonry walls or on plastered surfaces. A spray of transparent water repellant, silicone based material can make it effective for duration of two to three years.
(v) Injection grouting for cracks - Injection grouting is one of the powerful methods commonly adopted for stopping leakages in dams, basements, swimming pools construction joints and even in the leaking roofs. A few years back cement was used for grouting purposes. Cement is not an ideal material for grouting, as it shrinks while setting and hardening. Non- shrink or expansive cementing material is the appropriate material. We have quite a few materials available in the market for filling up cracks and crevices in concrete structures to make them waterproof or for repair and rehabilitation of structures. The grouts are produced with selected water repellant, silicifying chemical compounds and inert fillers to achieve varied characteristics like water impermeability, non-shrinkage, free flowability etc. They are suitable for gravity grouting as well as pressure grouting. Grouting of concrete structure is one of the powerful methods for strengthening and waterproofing of unhealthy structures.
(vi). Waterproof tile adhesive - Wet areas in buildings such as lavatory, bathroom and kitchen are the vulnerable places for leakage. Normally glazed tiles are used as one of the methods of waterproofing wet areas. Generally neat cement paste is used for fixing glazed tiles. The present practice often gives unsatisfactory results on account of poor joint filling by cement paste. The paste applied to the back of the tiles does not flow towards the edges of the tiles and as such joints remain unfilled. The white cement applied later for filling joints also becomes ineffective. It is a common sight that paints and plasters peel off behind such wet areas due to lack of waterproofing.
There are adhesives for fixing glazed tiles, ceramic tiles or marble in wet areas in buildings. They are first screeded directly on to the wall 2 to 3 mm thick and then glazed tiles or other tiles are pasted onto this screed. As this screed is a powerful adhesive as well as an effective waterproofing material, it provides a waterproofing coat to the wall. Adoption of such polymer based hydraulically setting ready to use waterproof tile adhesive in the wet areas will go a long way to make wet areas of buildings waterproof.
(vii) Sealing materials for rising dampness - Often old buildings are not provided with damp-proof course. The water from the ground rises by capillary action. This rising water brings with it the dissolved salts arid chemicals, which result in peeling of plaster affecting the durability of structure, and also make buildings un-hygenic. Attempts were made to cut the wall in stages and introduce new DPC, but this method has not only been cumbersome but also ineffective. Now materials that can be injected into the wall at appropriate levels to seal the capillaries and thereby to stop the upward movement of water are available. The Table 2 gives the brand name of some of the water proofing materials manufactured by different companies for different purposes.
(f) Polymer bonding agents - It is a well-known fact that there will not be perfect bond between old concrete and new concrete. Quite often new concrete or mortar is required to be laid on old concrete surface. For example, for providing an overlay on an existing pavement, in providing a screed over roof for water proofing or repair work etc. The bonding characteristics can be greatly improved by providing a bond coat between old and new concrete surface or mixing the bonding agent with the new concrete or mortar. The use of bonding agent distinctly improves the adhesion of new concrete or mortar to old surface. The mixing of bonding agents with concrete or mortar improves the workability also at lower water cement ratio and thereby reduces the shrinkage characteristic. It also helps in water retention in concrete to reduce the risk of early drying. It further improves the water proofing quality of the treated surface
(g) Floor hardeners and dust proofers - Floor is one of the parts of any building, particularly the industrial building, continuously subjected to wear and tear. The factory floor, on account of movement of materials, iron tyred trollies, vibrations caused by running machines is lively to suffer damages. Wear resistant and chemical resistant floor must be provided in the beginning itself. Replacing and repairing of old floor will interfere with the productivity and prove to be costly.
In the past, materials such as ironite, Hardonate, Metarock and other liquid floor hardeners were used to give better performance. But performances of these materials were not found to be satisfactory. Now we have modern floor toppings materials composed of carborundum or emery powders, systematically graded, mixed with processed and modified cement. This mixture when sprinkled over wet concrete floor of sufficient strength and depth is found to give an effective wear resistant, dust free, non-slip floor. The quantity to be sprinkled is depending upon the degree of wear resistance required.
One difficulty is experienced in the application of wear resistant hard top materials on the wet base concrete. If the sprinkling of this material is done when the base concrete is too wet, the finishing operation will make these hard wearing topping material sink, thus making the process ineffective. On the other hand if the sprinkling is delayed, the base concrete will have been set and hardened to such an extent that the hardtop material will not become integral part of the floor. The hard topping material should be sprinkled at the appropriate time for optimum result.
Recently vacuum de-watering method is frequently adopted for casting factory floor, road, air field pavements and concrete hard standing. In India TREMIX SYSTEM or JAMSHEDJI VACUUM de-watering system is popular. Employment of vacuum de-watering of concrete for factory floor by itself will give improved performance to factory floor. In addition, vacuum de-watering offers an ideal condition for wide casting the floor topping on the top of the concrete floor slab. The hard wearing, sized and graded aggregate forms the top surface of concrete floor to offer tremendous abrasion resistance.
There are also certain materials which when applied on the concrete floor, convert the lime rich cement compounds into silicified products which gives extreme chemical and mechanical resistance and also dust-proofing qualities.
(h) Concrete curing compounds - On account of the requirement of large quantity of water for curing, especially for vertical surfaces, sloping surfaces such as canal lining, water curing becomes costly and often inefficient Continuous uninterrupted curing of concrete does not really take place at the site.
(i) Polymer mortar for repair and maintenance - Sometime concrete surfaces require repair. The edge of concrete columns may get chipped off; or ceiling of concrete roof may get peeled off, or a concrete floor may get pitted in course of time. Hydraulic structures often require repairing. Prefabricated members such as pipes, poles, posts and roofing elements often gets chipped off while stripping from form work, handling and transportation, in the past cement mortar is used for any kind of repair and as an universal repair material. Cement mortar is not the right kind of material for repair. Now there are many kind of repair materials, mostly polymer modified, are available for effective repair. They adhere very firmly to the old concrete surface on account of greatly improved bond characteristics. These materials are often stronger than the parent materials.
(f) Adhesives for glazed tiles, marble and granite - The normal practice followed for fixing glazed tiles in bathroom, lavatory, kitchen, and other places is the use of stiff neat cement paste. The existing practice, though somewhat satisfactory in the indoor conditions from the point of fixity, such practice is unsatisfactory' when used in outdoor conditions and also from the point of view of waterproofing quality. The cement paste applied at the back of tiles does not often flow towards the edges of the tiles and as such water enters at the edges, particularly when white cement applied to the joints become ineffective. In large n amber of cases it is seen that paintings and plaster gets affected behind these glazed tiles supposedly applied to prevent moisture movement from wet areas.
Cement paste is not the right material for fixing the glazed tiles. There are polymers based, hydraulically setting, ready to use waterproof tile adhesive available in the market. They offer many advantages over the conventional method of tile fixing such as higher bond and adhesion strengths, faster work, and good waterproofing quality to the wall. They are suitable for exterior and overhead surfaces. No curing of tile surface becomes necessary. If the wall and plastered surface is done to good plumb, a screeding of only 1-2 mm thickness of this modem material will be sufficient to fix the tiles in which case, the adoption of this material will also become economical. The modem tile adhesive material will offer special advantages for fixing glazed tiles in swimming pools both on floor and at sides. It provides one more barrier for the purpose of waterproofing.
Many a time, the glazed tiles fixed on the kitchen platform or bathroom floor gets dirty or damaged. It requires to be replaced. Normal practice is to remove it by chipping the old tile, screed cement paste or mortar and then lay the new tiles. With modem tile adhesive, it is not necessary to remove the old tile. Tile adhesive can be screeded on the existing tile and new tiles are laid over the old tiles. The bonding quality is such that good adherence takes place tile over tile. This saves considerable cost and time and the operation becomes simple.
Marble and granite are increasingly used for cladding wall surfaces both internally and externally. Marble and granite have become the most common treatment for external treatment of prestigious buildings. They are used in the form of tiles or large panels. In the past for fixing thin marble and granite tiles cement paste was used and for fixing large slabs and panels, epoxy and dowel pins were used. Now there are specially made ready to use high strength polymer bonding materials available, which can be used with confidence, both for internal and external use. Requirement of dowels is eliminated in most of the cases except for cladding of large panels at very high level for extra safety. Marble and granite cm even be fixed on boards or inclined surfaces or even underside of beams and ceilings by the use of new powerful adhesive.
(k) Mould releasing agents - Wooden planks, ordinary plywood, shuttering plywood, steel plates etc is used as shuttering materials. Concrete when set and harden adhere to the surface of the formwork and make it difficult to demould. This affects the life and quality of shuttering materials. At times when extra force is used to demould from the formwork, concrete gets damaged. Sometime mould surface could be cement plastered surface, in which case the de-moulding or stripping of concrete member becomes all the more difficult. In the past to reduce the bond between form work and concrete, some kind of materials such as burnt engine oil, crude oil, cow dung wash, polyethylene sheet etc. were used. All the above are used on account of non-availability specially made suitable and effective mould releasing materials. Now we have specially formulated mould-releasing agents, separately for absorptive surfaces like timber and plywood and for non-absorbent surface like steel sheet is available.
(l) Grouting agents - Grouting aspects have been discussed earlier while dealing with waterproofing of basement slab and other concrete structure showing excess of permeability. Apart from the above, grouting has become one of important in civil engineering construction. Grouting below base plate or machine foundations, grouting of foundation bolt holes in industrial structures, grouting of prestressed concrete duct, grouting in anchoring and rock bolting systems, grouting of curtain walls, grouting of fissured rocks below dam foundation, grouting the body of the newly constructed dam itself, grouting of deteriorated concrete or fire affected structures to strengthen and rehabilitate, grouting of oil wells are some of the few situations where grouting is resorted to.
The grout material should have high early strength and ultimate strength, free flowing at low water content, should develop good bond with set concrete, essentially it should be non-shrink in nature.
The grouting materials can be broadly classified into two categories. One is free flow grout for use in machine foundations, foundation bolts and fixing crane rails etc. The second category of grout is meant for injection grouting to fill up the small cracks, which will be normally done under pressure.
(m) Joint sealant - Joints in buildings, bridges, roads and airfield pavements are inescapable. They may be expansion joints, construction joints or dummy joints. Such joints must be effectively sealed to facilitate movement of structure, to provide waterproofing quality or to improve the riding qualities. While providing large openings and windows in buildings there exists gap between wall and window frames, through which water flows inside. Such gaps in the windows should also be effectively sealed. The gaps resulting in installation of sanitary appliances are required to be sealed. There were no effective materials in Indian market hitherto.
(n) Decorative and protective paints - It was a popular belief that concrete structures do not require protection except when used as a pile in corrosive or sulphate infested environment or when concrete piles are subjected to abrasion and mechanical wear and tear. Lately it was found that concrete structures built in, and around coastal region, in and around fertilizers and chemical factories, cooling towers and chimneys need definite protection against disruptive action of aggressive environmental agencies. Sometimes it is also necessary that concrete structures such as flyovers and bridges should be made to look decorative and beautiful.
Many beautiful structures built near seashore get affected within a matter of few years. The delicate sections of sun breakers, louvers, facia, facades, sun shades and chajjas crack and spall off within a matter of ten years particularly when the cover provided to these thin members becomes inadequate. Hand mils of bridges become most vulnerable. Even bridge girders gel affected and show premature distress. Carbonation is supposed to be one of the main reasons for initial straining and eventual cracking of the concrete members. The above
degradation can be retarded by giving protective coatings particularly to the delicate concrete members.
Of late it has been realized that life of concrete structures in general and their elements in particular can be greatly enhanced if they are painted with crack bridging protective coatings. Such protective coatings are intentionally made decorative. They cover up staining, craziness and cracks in plaster and further provide resistance to carbonation and moisture penetration, while making the concrete structures decorative.
(o) Concrete repair system - It was once thought that concrete structures are durable and last almost forever. But now it is realized that concrete is not as durable as it was thought to be. It was also the earlier belief that concrete needs no protection. It is discussed above that concrete needs to be maintained and protected. Another wrong notion that prevailed was that concrete couldn’t be repaired. Now there are materials and methods for effective repair of damaged concrete structures, which is discussed below.
Concrete is constantly under attack of environmental pollution, moisture ingress, penetration of chlorides and sulphates and other deleterious chemicals. The durability of concrete is then affected. Of all forces of degradation, carbonation is believed to be one of the potent causes of deterioration of concrete.
Carbonation is the process where-in atmospheric carbon dioxide reacts with concrete's alkaline calcium hydroxide in the presence of moisture, or humidity, to convert it into calcium carbonate; the pH value of pore water of fresh concrete is from 12.5 to 13.5. On this strong alkaline environment the embedded steel reinforcement does not corrode. The carbonation process lowers the pH value of pore water to below 9 and disturbs the passivating protection to the reinforcement. The reinforcement then begins to rust. The volume of rust is about 2.5 times the volume of parent metal replaced. This extra volume of rust causes stress that pushes the protective cover resulting in cracks and spoiling. This process progressively increases with time, till such time the reinforcement is fully exposed. This kind of carbonation of concrete and subsequent progressive deterioration and failure is aptly described as concrete cancer.
The depth of carbonation depends upon the strength level of concrete, permeability of concrete, duration and whether the concrete, is protected or not.
Figure 5 shows the depth of carbonation for protected and unprotected concrete.
The Figure 6 shows the depth of carbonation with respect to strength (grade) of concrete.
In the past, there was no effective method of repairing cracked, spalled and deteriorated concrete. They were left as such for eventual failure. In the recent past, guniting was practiced for repair of concrete. Guniting has not proved to be an effective method of repair. But now very effective concrete cancer and increase the longevity of the structure. The repair materials used are stronger than the parent material. The efficient bond coat, effective carbonation resistant fine mortar, corrosion inhibiting primer, protective-coating makes the system very effective. Where reinforcement is corroded more than 50%, extra bars may be provided before repair mortar is applied. The whole repair process becomes a bit closely but often repair is inevitable and the higher cost does not matter much.
Fig 5 Depth of carbonation for protected and unprotected concrete
Fig 6 Depth of carbonation with respect to strength (grade) of concrete
(p) Installation aids – Many a time we leave holes or make holes in walls, staircases, gate pillars, etc. for fixing wash basin, lamp shades, hand rails or gates etc. Invariably, the holes made or kept, are larger than required. The extra space is required to be plugged subsequently. Material used in the past is cement mortar. Cement mortar takes a long time to set and harden, remain vulnerable for damage and it also shrinks. We have now specially manufactured materials, which will harden to take load in a matter of 10 – 15 minutes and work as an ideal material from all points of view for the purpose of fixing such installations. They can also be used for fitting of antennae, fixing of pipes and sanitary appliances etc
Water tanks, deep pump houses, basements, pipes carrying water or seepage, sometimes develop cracks and leaks. Such leakage’s can be plugged by using a special ready to use mortar for quick and reliable sealing and plugging of any kind of leaks. This stiff mortar is kept pressed against the crack for 5 to 7 minutes. Sometimes depending upon the shape of the crack etc., the first attempt may not bring 100% result. Probably a second attempt or third attempt may be required. Figure 9 illustrates the use. It is also used in the grouting operation for fixing the nozzle and also for stopping the grout escaping from elsewhere.
CONCRETE WORK --- LIST OF BUREAU OF INDIAN STANDARDS
Tin bronze ingots and castings (3rd revision) Reaffirmed 1993.
Coarse and fine aggregate from Natural source for concrete (2nd revision) Reaffirmed 1990.
Code of practice for plain and reinforced concrete (3rd revision) (Amendments 2) Reaffirmed 1991.
Method of sampling and analysis of concrete. Reaffirmed 1991.
1200 (Part II)1974
Method of measurement of building and civil engineering work Part 2 (concrete works). (3rd revision) (Amendments 2) Reaffirmed 1991.
Bitumen felt for water proofing and damp proofing (4th revision) (previously 13220-1982)
Batch type concrete mixers. (2nd revision) Reaffirmed 1990.
Method of test for aggregate for concrete work.
- Part 1 particle size and shape (Amendments 2) Reaffirmed 1990
- Part 2 Estimation of deleterious materials and organic impurities (Amendments 1) Reaffirmed 1990.
- Part 3 Specific gravity, density, voids, absorption and builking – Reaffirmed 1990.l
- Part 4 Mechanical properties (Amendments 3) Reaffirmed 1990.
General requirements for concrete vibrators immersion type. Reaffirmed 1993.
General requirements for screed board concrete vibrators. (1st revision) Reaffirmed 1990.
Integral cement water proofing components (1st revision) (Amendments 1) Reaffirmed 1992.
Cinder as fine aggregate for use in lime concrete (1st revision) (Amendments 1) Reaffirmed 1992.
Broken butnt (clay) coarse aggregate for use in lime concrete. (2nd revision) Reaffirmed 1991.
Flyash for use as pozzolana and admixtures (1st revision) Reaffirmed 1992.
Section wrenches for fire bridge use (1st revision) Reaffirmed 1992.
Form vibrators for concrete. Reaffirmed 1991.
7861 (Part 1) 1981
Code of practice for extreme weather concreting (Part 1) recommended practice for hot weather concreting (Amendments 1) Reaffirmed 1990.
7861 (Part 2)1975
Code of practice for cold weather concreting (Part 2) Recommended practice for cold weather concreting (Amendments 1) Reaffirmed 1992.
Admixture for concrete Reaffirmed 1990.