What is Compressive Strength of Concrete?
Compressive strength of concrete is the ability of concrete to withstand compressive loads. It is one of the most important properties of concrete, as it determines the load-bearing capacity of concrete structures. The compressive strength of concrete is measured in units of pounds per square inch (psi), Newton per mm² or megapascals (MPa).
Test for Compressive Strength of Concrete
The Compressive Strength Test evaluates the ability of concrete to withstand load pressure. It involves applying force to a concrete sample until it fails, providing crucial insights into its structural integrity and durability.
Why is Compressive Strength of Concrete Important?
Compressive strength of concrete is important for determines the load-bearing capacity of concrete structures mainly. Concrete structures are designed to withstand certain loads, and the compressive strength of the concrete must be high enough to support those loads. Also, compressive strength affects the durability of concrete structures. Concrete structures that are exposed to harsh environments, such as seawater or deicing salts, need to have a high compressive strength in order to resist degradation.
How is Compressive Strength Measured?
The compressive strength of concrete is measured by breaking concrete specimens in a compression-testing machine at a certain rate of loading. The specimens can be cubes or cylinders, and they are typically cast in molds and cured in a laboratory at specified temperature. The specimens are then placed in the compression-testing machine and loaded until they fail. The compressive strength is then calculated by dividing the failure load by the cross-sectional area of the specimen.
Test Procedure for Compressive Strength of Concrete
The following is a general test procedure for compressive strength of concrete:
Cast concrete specimens:
Concrete specimens can be cubes or cylinders. Cubes are typically 150 mm x 150 mm x 150 mm in size, and cylinders are typically 150 mm in diameter and 300 mm in height.
Cure concrete specimens:
Concrete specimens are typically cured in a laboratory for 7 days or 28 days as per requirement. During the curing of concrete cube or cylinder, the specimens are kept moist to prevent them from drying out.
Test concrete specimens in a compression-testing machine:
The concrete specimens are placed in the compression-testing machine and loaded until they fail. The failure load is recorded.
Calculate compressive strength:
The compressive strength is calculated by dividing the failure load by the cross-sectional area of the specimen.
Factors that Affect Compressive Strength of Concrete
The following factors can affect the compressive strength of concrete:
Water-to-cement ratio:
The water-to-cement ratio is the ratio of the weight of water to the weight of cement in a concrete mix. A lower water-to-cement ratio will result in a higher compressive strength.
Type of cement:
Different types of cement have different compressive strengths. Portland cement is the most common type of cement used in concrete, and it has a compressive strength of approximately 4000 psi (28 MPa).
Aggregate type:
The type of aggregate used in concrete can also affect its compressive strength. Aggregates that are strong and well-graded will result in a higher compressive strength.
Curing conditions:
Concrete needs to be cured properly in order to achieve its full compressive strength. Concrete that is cured in moist conditions will have a higher compressive strength than concrete that is cured in dry conditions.
Compressive Strength Requirements
The compressive strength requirements for concrete vary depending on the application. For example, concrete that is used in sidewalks and driveways will need a lower compressive strength than concrete that is used in bridges and high-rise buildings.
Compressive Strength of Hardened Concrete as Per IS:516 (Part-1)
Test Specimens
The test specimen shall be a cube or a cylinder, meeting the requirements of IS: 1199 (Part-5) and IS:516 (Part-4) for concrete cores. Cylindrical specimens cast in the lab or cores extracted from the structures, shall be capped as per 6.6 of IS 1199 (Part-5).
Damaged specimens shall not be tested. Specimens that are badly honeycombed shall not be regarded as being representative of the quality of concrete supplied.In general, standard cube and cylinder specimens shall not be tested if they are badly honeycombed as this is an indication of poor specimen making. When such specimens are tested, the test report shall include the fact that the specimen was honeycombed.
Apparatus for Test
The test shall be carried out using a compression testing machine conforming to IS 14858. The test machine shall be in calibration at the time of test. The calibration shall be carried out at least once per year.
Age at Test
Tests shall be made at recognized ages of the test specimens, the most usual being 7 and 28 days. Ages of 56 days, 90 days 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 h ± 30 min and 72 h ± 2 h. The ages shall be calculated from the time of the addition of water to the dry ingredients.
Number of Specimens
At least three specimens shall be tested at each selected age.
Test Procedure
- Preparation and Positioning of Specimens For specimens stored in water, excess moisture shall be wiped from the surface of the specimen before placing them in the testing machine. The dimensions of the specimens to the nearest 0.2 mm and their weight shall be noted before testing.The time between the extraction of the specimen from the curing tank, and the testing, shall be as short as possible or not more than 2 hours. During the time the specimen is outside the curing tank, it shall be protected from drying, may be by covering with wet cloth. All testing machine bearing surfaces shall be wiped clean and any loose grit or other extraneous material removed from the surfaces of the specimen that will be in contact with the platens.
- Do not use packing, other than auxiliary platens or spacing blocks, between the specimen and the platens of the testing machine. Cube specimens shall be compressed perpendicularly to the direction of casting.The specimen shall be centered on the lower platen to an accuracy of 1 percent of the designated size of cubic,or diameter of cylindrical specimens. Where physical means of ensuring centering are provided on the testing machine and they are in calibration, these shall be deemed to satisfy the requirements for accuracy of centering. If auxiliary platens are used, the top auxiliary platen shall be aligned with the top of the specimen. With two-column testing machines, cube specimens shall be placed with the troweled surface facing a column.
Loading
The load shall be applied without shock and shall be increased continuously at a constant rate of 14 N/mm2/min until no greater load can be sustained.The maximum load indicated shall be recorded.
Assessment of Type of Failure
For cube specimens, if the failure is satisfactory( see Fig. 1), this fact shall be recorded. If the failure pattern is unsatisfactory, this fact shall be recorded andthe type of failure recorded using the pattern number inFig. 2 closest to that observed.For cylindrical specimens, if the failure is satisfactory( see Fig. 3), this fact shall be recorded. If the failure pattern is unsatisfactory, this fact shall be recorded and the type of failure recorded using the pattern letter in Fig. 4 closest to that observed.3.6 Test Results The compressive strength is given by the equation
fc= F / Ac
Where,
fc = compressive strength, in MPa;
F= maximum load, in N; and
Ac = cross-sectional area, in mm2, of the specimen on which the compressive force acts.
Average of three values shall be taken as the representative of the batch provided the individual variation is not more than ± 15 percent of the average. Otherwise repeat test shall be made, however if there is no further sample, then the average of two closest values may be taken as the average result.The actual dimensions of test specimens shall conform to IS 10086. If the actual dimensions are within the tolerance limits as mentioned in IS 10086, the strength may be calculated on the basis of designated size. If the actual dimensions are outside this tolerance, the strength calculation shall be based on the actual dimensions of the test specimen, however, perpendicularity of the surface of specimens should be maintained as per IS 10086.The compressive strength shall be expressed to the nearest 0.5 MPa.
Test Report
The following information shall be included in the report:
a) Details of the concrete like grade, mix details, etc, in case of cast specimens; and details of the structure,like structure type, origin member/structure, in case of cored specimen;
b) Type of specimen: cast (cube/cylinder) or drilled core;
c) Size of the specimen, and capping details, if applicable;
d) Identification mark;
e) Age of specimen;
f) Date of test;
g) Curing conditions;
h) Weight of specimen;
j) Dimension of specimen;
k) Maximum load;
m) Details of the machine used for testing(manual/automated, loading range, date of calibration, etc);
n) Compressive strength of specimen (to the nearest 0.5 MPa); and
p) Type of failure (satisfactory or unsatisfactory and,if unsatisfactory, the nearest type)
