Compressive Strength of Concrete – Test, Results & Procedure

The compressive strength of concrete cube test is a measure of the performance and strength of a given concrete mixture. By this single test, one can judge whether concreting has been done properly or not and whether the structure has the required strength as per the specifications. The compressive strength of concrete varies from 2500 psi (17 MPa) to 4400 psi (30 MPa) for general residential and commercial constructions and several applications also use strengths greater than 10,000 psi (70 MPa) in certain commercial and industrial constructions.

The compressive strength of concrete depends on factors such as water-cement ratio, cement strength, quality of coarse and fine aggregate materials, quality control during the production of concrete, etc.

The compressive strength test is carried out either on a cube or cylinder by breaking the concrete specimen in a compression testing machine and it is calculated from the failure load divided by the cross-sectional area of the cube or cylinder resisting the load and reported in units of pound-force per square inch (psi) in US Customary units or megapascals (MPa) in SI units.

Various standard codes recommend a concrete cylinder or concrete cube as the standard specimen for the test. American Society for Testing Materials ASTM C39/C39M provides Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens.

Definition of Compressive Strength

Compressive strength is the ability of a concrete material or a concrete structure to carry the loads on its surface without any crack or deflection. A material under compression tends to reduce the size, while in tension, size elongates.

Compressive Strength Formula

The compressive strength formula for any material is the load applied at the point of failure to the cross-section area of the face on which the load was applied.

Compressive Strength = Load / Cross-sectional Area

Compressive strength test results are primarily used to determine that the concrete mixture as delivered on-site meets the requirements of the specified strength, fc’, in the job specification. Concrete cylinders tested for acceptance and quality control are made and cured in accordance with procedures described for standard-cured specimens in ASTM C-31 (Standard Practice for Making and Curing Concrete Test Specimens in the Field). For estimating the in-place concrete strength, ASTM C-31 provides procedures for field-cured specimens.

Cylindrical specimens are tested in accordance with ASTM C-39 (which is the Standard Test Method for the Compressive Strength of Cylindrical Concrete Specimens).

A test result is the average of at least two standard-cured strength specimens made from the same concrete batch and tested at the same age. In most cases, strength requirements for concrete are at 28 days.

Compressive Strength Test Acceptance

Design engineers use the specified concrete strength to design structural elements. This specified strength is incorporated in the job contract documents and is called the design strength of concrete.

The concrete mixture is designed to produce an average strength fc’ higher than the specified strength such that the risk of not complying with the strength specification is minimized.

To comply with the strength requirements of a job specification, the following acceptance criteria apply:

1. The average of three consecutive tests should equal or exceed the specified strength fc’.
2. No single strength test should fall below fc’ by more than 500 psi (3.45MPa) or by more than 0.10fc’ when fc’ is more than 5000 psi (35 MPa).

It is important to understand that an individual test falling below fc’ does not necessarily mean that the test has failed and specifications were not as per requirement. When the average of strength tests is as per the required average strength fc’, the probability that individual strength tests will be less than the specified strength is about 10% and this is accounted for in the acceptance criteria.

When strength test results indicate that the concrete fails to meet the requirements of the specification, it is important to recognize that the failure of concrete may also be due to the testing procedure. This is especially true if the fabrication, handling, curing and testing of the cylinders are not conducted in accordance with standard procedures.

Table: Compressive Strength of Concrete at Various Ages

The strength of concrete increases with age. The table shows the strength of concrete at different ages after 1 day, 3 days, 7 days, and 14 days in comparison with the strength at 28 days after casting.

Table: Compressive Strength of Different Grades of Concrete at 7 and 28 Days

Table: Minimum Concrete Strength Required in psi for Different Mixes

How to Test the Strength of Concrete

1. Cylindrical specimens for acceptance testing should be 6 x 12 inches (150 x 300 mm) size or 4 x 8 inches (100 x 200 mm) when specified.
   The smaller specimens tend to be easier to make and handle in the field and the laboratory.
   The diameter of the cylinder used should be at least 3 times the nominal maximum size of the coarse aggregate used in the concrete.
2. Recording the mass of the specimen before capping provides useful information in case of disputes.
3. To provide for a uniform load distribution when testing, cylinders are capped generally with sulfur mortar (ASTM C 617) or neoprene pad caps (ASTM C 1231).
   Sulfur caps should be applied at least two hours and preferably one day before testing.
   Neoprene pad caps can be used to measure concrete strengths between 1500 and 7000 psi (10 to 50 MPa).
   For higher strengths up to 12,000 psi, neoprene pad caps are permitted to be used if they are qualified by companion testing with sulfur caps.
   Durometer hardness requirements for neoprene pads vary from 50 to 70 depending on the strength level tested. Pads should be replaced if there is excessive wear.
4. Cylinders should not be allowed to dry out prior to testing.
5. The cylinder diameter should be measured in two locations at right angles to each other at mid-height of the specimen and averaged to calculate the cross-sectional area. If the two measured diameters differ by more than 2%, the cylinder should not be tested.
6. The ends of the specimens should not depart from perpendicularity with the cylinder axis by more than 0.5º and the ends should be plane to within 0.002 inches (0.05 mm).
7. Cylinders should be centered in the compression-testing machine and loaded to complete failure. The loading rate on a hydraulic machine should be maintained in a range of 20 to 50 psi/s (0.15 to 0.35 MPa/s) during the latter half of the loading phase. The type of break should be recorded. A common break pattern is a conical fracture (see figure).
8. The concrete strength is calculated by dividing the maximum load at failure by the average cross-sectional area.
   C 39 has correction factors if the length-to-diameter ratio of the cylinder is between 1.75 and 1.00, which is rare.
   At least two cylinders are tested at the same age and the average strength is reported as the test result to the nearest 10 psi (0.1 MPa)
9. The technician carrying out the test should record the date they were received at the lab, the test date, specimen identification, cylinder diameter, test age, the maximum load applied, compressive strength, type of fracture, and any defects in the cylinders or caps. If measured, the mass of the cylinders should also be noted.
10. Most deviations from standard procedures for making, curing, and testing concrete test specimens will result in a lower measured strength.
11. The range between companion cylinders from the same set and tested at the same age should be, on average, about 2 to 3% of the average strength. If the difference between two companion cylinders exceeds 8% too often, or 9.5% for three companion cylinders, the testing procedures at the laboratory should be evaluated and rectified.
12. Results of tests made by different labs on the same concrete sample should not differ by more than about 13% of the average of the two test results.
13. If one or both of a set of cylinders break at strength below ƒ´c, evaluate the cylinders for obvious problems and hold the tested cylinders for later examination. Frequently the cause of a failed test can be readily seen in the cylinder, either immediately or by petrographic examination. If it is thrown away an easy opportunity to correct the problem may be lost. In some cases, additional reserve cylinders are made and can be tested if one cylinder of a set broke at a lower strength.
14. A 3 or 7-day test may help detect potential problems with concrete quality or testing procedures at the lab but is not a basis for rejecting concrete, with a requirement for 28-day or other age strength.
15. ASTM C 1077 requires that laboratory technicians involved in testing concrete must be certified.
16. Reports of compressive strength tests provide valuable information to the project team for current and future projects. The reports should be forwarded to the concrete producer, contractor, and the owner’s representative as expeditiously as possible.

References

1. ASTM C 31, C 39, C 617, C 1077, C 1231, Annual Book of ASTM Standards, Volume 04.02, ASTM, West Conshohocken, PA, www.astm.org
2. Concrete in Practice Series, NRMCA, Silver Spring, MD, www.nrmca.org
3. In-Place Strength Evaluation – A Recommended Practice, NRMCA Publication 133, NRMCA RES Committee, NRMCA, Silver Spring, MD
4. How producers can correct improper test-cylinder curing, Ward R. Malisch, Concrete Producer Magazine, November 1997, www.worldofconcrete.com
5. NRMCA/ASCC Checklist for Concrete Pre-Construction Conference, NRMCA, Silver Spring, MD
6. Tips on Control Tests for Quality Concrete, PA015, Portland Cement Association, Skokie, IL, www.cement.org
7. ACI 214, Recommended Practice for Evaluation of Strength Tests Results of Concrete, American Concrete Institute, Farmington Hills, MI, www.concrete.org