DETERMINATION OF COMPRESSIVE STRENGTH OF CONCRETE
The ultimate compressive strength of a material is that value of uni-axial compressive stress reached when the material fails completely. This test method covers the procedure for determination of compressive strength of cement concrete. It is used to ensure the suitability of the available materials or to determine suitable mix proportions. The maximum nominal size of aggregate should not exceed 38 mm.
(i) Universal Testing Machine of any reliable type, of sufficient capacity for the tests and capable of applying the load at the rate of approximately 140 kg/cm2 per minute. The permissible error shall be not be greater than ±2% of the maximum load.
(ii) Cube Moulds of 150 mm x 150 mm x 150 mm size preferably made of steel or cast iron, stout enough to prevent distortion.
(iii) Cylinders shall be of metal not less than 3 mm thick, capable of opening longitudinally. When assembled for use, the mean diameter shall be 150±2 mm and height 300±1 mm.
NOTE: Some standards allow use of plastic moulds for casting specimens for compressive strength test. The shapes of plastic moulds are likely to change after use. In such cases, the dimensions shall be thoroughly checked before reusing the moulds.
(iv) The base plate shall be of suitable dimension to support during filling without leakage, preferably attached to the mould by springs or screws.
(v) Tamping rod 16 mm diameter, 60 cm long with bullet pointed edge at the lower end.
3. Preparation of specimens
a) During assembling of the mould, the joints and inside of the mould and base plate are coated with a thin film of mould oil or light grease to prevent adhesion of the concrete
b) The size of test specimens shall be 150 mm x 150 mm x 150 mm. If the maximum nominal size of aggregate in the concrete is not more than 2 mm, 100 mm size cubes may be prepared for testing.
c) Samples of concrete for testing shall be taken immediately after mixing.
d) Each batch of concrete shall be tested for consistency test as per IS: 1199-1959 immediately after mixing. The concrete used for consistency test shall be re-mixed with the concrete sampled before cubes are made.
e) Fill the concrete into the mould in layers of approximately 50 mm. Move the scoop around the top edge of the mould, to ensure symmetrical distribution of concrete in the mould.
f) Each layer of the concrete is given 35 strokes with the tamping rod. For 100 mm size cubes, 25 stokes are specified. For cylindrical specimens, 30 strokes are to be given. The strokes shall penetrate to the underlying and bottom layers.
g) Tap the sides of the mould to close any air voids resulting from tamping process.
h) The surface of the concrete shall be finished level with the top of the mould, using a trowel, and covered with a glass or metal plate to prevent evaporatio
i) When mechanical vibration is used for compaction of the concrete specimen, each layer shall be vibrated by means of an electric or pneumatic hammer or vibrator or by means of a suitable vibrating table.
j) Cylindrical specimens shall be capped using a thin layer of stiff, neat Portland cement paste or with a sulphur mixture consisting of 2 or 3 parts of sulphur to 1 part of inert filler such as fire-clay. Capping may also be done with hard plaster (Plaster of paris shall not be used) having a compressive strength not less than 140 kg/cm2 attained in one hour.
k) The cubes cast along with the mould and base plate is stored at site for 24±½ hours at a place free from vibration. The mould shall be kept under damp matting, sacks or similar materials. The temperature of storage shall be within 22°C to 32°C.
l) After 24 hours, the cubes are carefully removed from the mould and kept in clean water at 24°C till they are transported to the laboratory for testing.
m) The test cubes shall be transported to the laboratory, well packed in damp sand or sacks, to arrive there in damp condition not less than 24 hours before the time of test.
n) On arrival of the test cubes in the laboratory they are kept in water at 27±2°C till the time of test.
o) Records of daily maximum and minimum temperatures shall be kept during the period specimen remains and site and in the laboratory.
a) The tests are normally carried out at 7 and 28 days. However during initial periods of the project, 1 day, 3 days and 14 days testing are also done to obtain the performance pattern of the mix.
b) At least three specimens, preferably from different batches, shall be made for testing at each selected age. The age is calculated from the time of adding water to the dry ingredient.
c) Specimen stored in the water is taken out and the surface water, girt and any projecting fins are removed. The bearing plate and the machine are also cleaned.
d) Measure the actual dimension of the specimen and record to the nearest 0.2 mm. Weigh the specimen and record.
e) Place the cube on the bottom plate in such a manner that the load shall be applied to opposite sides of the cubes in contact with the mould during casting. The position of the cube is carefully checked to be in the middle of the plate. No packing shall be used between the faces of the test cube and the plates.
f) The top plate is slowly brought down to touch the top of the cube specimen. A uniform seating shall be ensured by rotating the movable portion of the top plate with hand.
g) The instrument is then adjusted for any zero error. The loading is then started and continued gradually. The maximum load in kg applied on the specimen is noted.
h) The appearance of concrete and the failure pattern is also noted.
Calculate the compressive strength of the cube tested by dividing the maximum test load by the cross sectional area.
||Maximum load applied
| Mean cross-sectional area of the specimen
The compressive strength values are reported to the nearest kg/cm2 or to the nearest 10 units of N/mm2.
Average of three values shall be taken as the representative of the batch provided the individual variation is not more than ±15% of the average. Otherwise repeat tests shall be made.
6. Report The following information shall be included in the report:
(i) Location of the sample.
(ii) Structure or pavement component.
(iii) Date of casting.
(iv) Identification mark of the specimen.
(v) Date of test. (vi) Age of specimen at testing date.
(vii) Curing conditions.
(viii) Weight of specimen.
(ix) Dimensions of specimen.
(x) Cross sectional area.
(xi) Maximum load.
(xii) Compressive strength.
(xiii) Appearance of fractured faces of concrete and type of fracture. A typical format for reporting compressive strength test report is attached for guidance
DETERMINATION OF FLEXURAL STRENGTH OF CONCRETE
Flexural strength is a parameter that indicates the capacity of concrete to withstand bending stress. The flexural strength is expressed as Modulus of Rupture (MR) in MPa or Kg/cm2 and is determined by third-point loading or centre point loading method. Flexural tests are extremely sensitive to specimen preparation, handling, and curing procedure. Beams are very heavy and can be damaged when handled and transported from the jobsite to the lab. Allowing a beam to dry will yield lower strengths. Beams must be cured in a standard manner, and tested while wet. Meeting all these requirements on a job site is extremely difficult, often resulting in unreliable and generally low values.
a) The mould for casting flexural strength test specimens shall made of metal, preferably steel or cast iron and of sufficient thickness to prevent spreading or warping.
b) The mould constructed with the longer dimension horizontal and in such a manner as to facilitate the removal of the moulded specimens without damage.
c) Each mould provided with a metal base plate and two loose top plates of 4.0 x 0.6 cm cross-section and 5.0 cm longer than the width of the mould.
d) The height of the mould is 15.0±0.005 cm or 10.0±0.005 cm, and the corresponding internal width of the mould is 15.0±0·02 cm or 10.0±0.02 cm respectively.
e) A steel tamping bar 40 cm long, weighing 2 kg, with a ramming face 25 mm2.
f) The base plate supports and rigidly attached to the mould, without leakage during the filling and subsequent handling of the filled mould.
g) The testing machine may be of any reliable type of sufficient capacity for the tests and capable of applying the load at the rate specified in procedure. The permissible errors shall be not greater than ± 0·5 percent of the applied load where a high degree of accuracy is required and not greater than ±1·5% of the applied load for commercial type of use. The bed of the testing machine shall be provided with two steel rollers, 38 mm in diameter, on which the specimen is to be supported, and these rollers shall be so mounted that the distance from centre to centre is 60 cm for 15·0 cm specimens or 40 cm for 10·0 cm specimens.
h) In two-point loading system, the load shall be applied through two similar rollers mounted at the third points of the supporting span, that is, spaced at 20 or 13·3 cm centre to centre.
i) In centre point loading, the load is applied at the centre of the beam as shown below.
j) The load shall be divided equally between the two loading rollers, and all rollers shall be mounted in such a manner that the load is applied axially and without subjecting the specimen to any torsional stresses or restraints.
3. Preparation of specimens
a) The standard size of specimens for flexural strength test shall be 15x15x70 cm. Alternatively, if the largest nominal size of the aggregate does not exceed 19 mm, 10x10x50 cm size specimens are used.
b) The proportions of the materials, including water in concrete mixes used for determining the suitability of the materials available, are similar in all respects to those to be employed in the work.
c) The preparation of concrete shall be done in the same way as in the case of making compression test specimens.
d) The quantities of cement, each size of aggregate and water for each batch shall be determined by weight, to an accuracy of 0.1% of the total weight of the batch.
e) Where the proportions of the ingredients of the concrete as used on the site are to be specified by volume, they shall be calculated from the proportions by weight used in the test beams/cubes and the unit weights of the materials.
f) The concrete mixed by hand or preferably in a laboratory batch mixer, in such a manner as to avoid loss of water or other materials.
g) Each batch of concrete shall be of such a size as to leave about 10 percent excess after moulding the desired number of test specimen.
h) Each batch of concrete tested for consistency immediately after mixing by one of the methods described in IS: 1199-1959.
i) Provided that care is taken to ensure that no water or other material is lost, the concrete used for the consistency tests may be remixed with the reminder of batch before making the test specimen. The period of remixing shall be as short as possible, yet sufficient to produce a homogeneous mass.
j) Apply the mould oil to the inner faces of the mould to ensure that no water escapes during the filling of the mould, and also ensure that to prevent the adhesion of concrete.
k) At least three specimens, preferably from different batches, shall, be made from testing at each selected age.
l) The test specimens are stored in a place, free from vibration, in moist air of at least 90% relative humidity and at a temperature of 27±2°C for 24±½ hours from the time of addition of water to the dry ingredients.
m) After this period the specimens marked and removed from the moulds and, unless required for test within 24 hours, immediately submerged in clean, fresh water or saturated lime solution and kept there until taken out just prior to test. The water or solution in which the specimens are submerged are renewed every seven days and shall be maintained at a temperature of 27±2°C. The specimens are not allowed to become dry at any time until they have been tested.
n) The tests are conducted at recognized ages of the test specimens, the most usual being 7 and 28 days.
o) Ages of 13 weeks and one year are recommended if tests at greater ages are required. Where it may be necessary to obtain the early strengths, tests may be made at the ages of 24±½ hours and 72±2 hours.
p) The age shall be calculated from the time of the addition of water to the dry ingredients.
a) The dimensions of each specimen shall be noted before testing. No preparation of the surfaces is required
b) The bearing surfaces of the supporting and loading rollers shall be wiped clean, and any loose sand or other material removed from the surfaces of the specimen where they are to make contact with the rollers.
c) The specimen shall then be placed in the machine in such a manner that the load applied to the uppermost surface as cast in the mould, along two line spaced 20·0 or 13·3 cm apart.
d) The axis of the specimen is carefully aligned with the axis of the loading device.
e) No packing shall be used between the bearing surfaces of the specimen and the rollers.
f) The load is applied without shock and increasing continuously at a rate of loading of 400 kg/min for the 15·0 cm specimens and at a rate of 180 kg/min for the 10·0 cm specimens.
g) The load shall be increased until the specimen fails, and the maximum load applied to the specimen during the test shall be recorded.
h) The appearance of the fractured faces of concrete and any unusual features in the type of failure shall be noted.
The flexural strength of the specimen shall be expressed as the modulus of rupture fb calculated to the nearest 0.5 kg/cm2 depending on „a?, which is the distance between the line of fracture and the nearer support, measured on the centre line of the tensile side of the specimen in cm.
When „a? is greater than 20 cm for a 15 cm specimen or greater than 13.3 cm for a 10 cm specimen, the modulus of rupture fb shall be calculated to the nearest 0.5 kg/cm2 as follows:
fb = (p x l)/ bd2, where
b = measured width in cm of the specimen.
d = measured depth in cm \If the specimen at the point of failure.
l = length in cm of the span on which the specimen was supported.
p = maximum load in kg applied to the specimen.
If „a? is less than 17 cm for a 15 cm specimen or less than 11 cm for a 10 cm specimen, the results of the test shall be discarded.
When 'a' is less than 20·0 cm but greater than 17 cm for 15 cm or less than 13·3 cm but greater than 11 cm for a 10 cm specimen, fb shall be calculated as follows:
fb = (3p x a)/ bd2
6. Test report
The test report shall include the following information on each specimen:
(i) Identification mark.
(ii) Date of sampling.
(iii) Date of test.
(iv) Age of specimen.
(v) Curing conditions.
(vi) Size of specimen
(vii) Span length.
(viii) Maximum loa
(ix) Position of fracture (Value „a?).
(x) Modulus of rupture (kg/cm2) and
(xi) Appearance of concrete and type of fracture, if these are unusual
NON-DESTRUCTIVE TESTING OF CONCRETE USING REBOUND HAMMER
IS: 13311 - Part 2
The concrete test hammer invented by Ernst Schmidt and introduced by Proceq at the beginning of the 1950's remains to this day the most widely used non-destructive test instrument for a rapid assessment of the condition of a concrete structure. When the plunger of the rebound hammer is pressed against the surface of the concrete, the spring-controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the concrete. The surface hardness and therefore the rebound are related to the compressive strength of the concrete. The rebound value is read from a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer.
(i) Rebound Hammer consisting of a spring controlled mass that slides on a plunger within a tubular housing.
(ii) The impact energy required for rebound hammers for different applications are given in table 1
Table 1: Impact Energy for rebound hammers for different applications.
||Approximate Impact Energy required for rebound hammers (Nm)
||For testing normal weight concrete
||For light weight or small and impact sensitive parts of concrete
||For mass concrete in roads, airfield pavements and hydraulic structures
(iii) Testing anvil of steel having Brinell hardness of about 5000 N/mm2. The readings on anvil for different types of rebound hammers should be indicated by the manufacturer.
3. Correlation between compressive strength and rebound number
The correlation between compressive strength and rebound number can be obtained by simultaneously testing concrete cubes in compression testing machine and using the rebound hammer.
a) The concrete cubes are held in the compressive testing machine and rebound number is determined first. The compressive strength is then determined as per IS: 516.
b) The seating load of 7 N/mm2 is considered equal to 2.2 Nm of impact energy of the rebound hammer. This load is added to the test load of rebound hammer
c) For calibrating rebound hammers of lower impact energy (2.2 Nm), 150 mm cubes are sufficient. However for testing large structures, rebound hammer shall be calibrated using at least 300 mm size cubes.
d) Cubes shall be tested only under dry condition.
e) If wet curing is adopted, the specimens are taken out from water and kept in the laboratory atmosphere for 24 hours before testing.
f) Rebound hammer test on wet cubes is not recommended.
g) If wet strength is to be evaluated, a correlation between compressive strengths of wet and dry cubes cast from the same mix is determined.
h) Only the vertical face of the cubes casted shall be used for testin
i) The point of impact of the hammer needle shall be 20 mm away from the edges and the same point shall not be tested again
4. Factors affecting rebound index The rebound numbers are influenced by several factors mentioned below:
(i) Type of cement
Concrete made with high alumina cement gives 100% higher strength while concrete with supersulphated cement gives 50% strength than with ordinary Portland cement.
(ii) Type of aggregate
Concrete with normal aggregates such as gravel or crushed stones give similar correlations. But concrete with light weight aggregates needs special correlation.
(iii) Surface condition
Rebound hammer test is suitable for concrete with smooth surface. Open textured surfaces of masonry blocks, honey-combed concrete or no-fines concrete is not suitable for this test. The correlation assumes full compaction of the concrete. Trowelled or floated surfaces give higher rebound index than moulded surfaces.
(iv) Moisture content
In structural concrete, a wet surface may give up to 20% lesser compressive strength than an equivalent concrete with dry surface.
The relationship between hardness and strength of concrete varies as a function of time. The variations in the initial rate of hardening, subsequent curing, conditions of exposure etc also influence the relationship.
(vi) Age of concrete
The effect of age can be generally ignored for concrete between 3 days and 3 months old.
(vii) Carbonation of concrete surface
Influence of carbonation of concrete surface on the rebound number is very significant. In extreme cases, increased strength up to 50% is indicated when tested on carbonated concrete surface. Satisfactory results can be obtained after removing the carbonated layer from the concrete and testing on the uncarbonated concrete
a) A smooth clean, dry surface is selected for testing. A grinding wheel or stone may be used to make the surface smooth.
b) Rough surfaces resulting from loss of grout, spalling, application of tools etc do not give reliable results and hence shall be avoided.
c) The rebound hammer is held at right angles to the surface.
d) At least six readings are taken around each observation point.
e) After deleting outliers as per IS: 8900-1978, the average value will give the rebound index of the point of observation.
6. Reporting of results
a) If suitable correlation curves are established, rebound hammer test gives a convenient and rapid indication of the compressive strength of concrete.
b) If the concrete is heterogeneous or having internal micro-cracks or flaws, the rebound hammer indices will not be the same.
c) The error in test results when tested on non-uniform concrete surface can be up to ±25%.
d) If the accuracy of the rebound index can be checked with compressive tests core samples or cubes made with the same mix, the results can be predicted with increased perfection.