How to Test Compressive Strength of Concrete ?

The compressive strength of concrete at the site is determined by conducting a compression test on a concrete specimen that takes either the shape of a cube or a cylinder. This is also called a cube test or cylinder test of concrete. 

How to test Compressive Strength of Concrete at Site?

The cube test is preferred mainly by British, European or Indian Codes. The cylinder test is preferred by American Standards. There is also empirical formula to convert the cube strength to corresponding cylinder strength or vice versa.

Also Read: What is the difference between Concrete Cube and Cylinder Strength test?

Let’s study in detail the concept, procedure and features of compression strength test of concrete at site.

What is Compressive Strength of Concrete?

Compressive strength of concrete is one of the top most preferred design attribute for the design of concrete structures. Compressive strength as mentioned is obtained by conducting compression test on a concrete sample (Cylinder or cube) of a given mix design. The failure load of the given sample divided by the area of cross-section of the sample gives the compressive strength of the specimen. In takes the value in Mega Pascals (MPa) or in Pound-force per square inch (psi).     

Why Compressive Strength of Concrete?

The concrete structures are deigned for a design strength of fc. If the mix design is M20, then design strength is 20MPa. This is checked for safe working and finalised in the project specifications.

Read More: What is M20?

The concrete mix is deigned for actual construction to produce an average strength of f’cr which is higher than fc so that failure of strength goes under a acceptance criteria. This is target strength of concrete (f’cr) which is mainly obtained from the historical records or design formulas.

Also Read: What is difference between Characteristic Compressive Strength and Target Mean Strength?

What is the Objective of Compressive Strength of Concrete?

The main objective of compressive test on concrete is to ensure that the strength value specified by the design requirements is met by the practically made concrete at the construction site. This way, the compression test on concrete is used for: 

1. Quality Control of Concrete
2. Acceptance or Rejection of Concrete 
3. Estimation of strength in a Structure 
4. Evaluation of adequacy of Curing 

Compression Test on Concrete As per ASTM Standards

The procedure from sample preparation to the testing is explained in ASTM C31.

Specimen for Compression Test on Concrete As per ASTM Standards

The specimen used can be of standard sizes 6 x 12 inches (100 x 200mm) when aggregate size is greater than 2 inches, or (150 x 300mm) concrete cylinders. When the test is conducted at site, it is good to employ smaller samples. It is recommended to use minimum two specimens from the sample concrete sample and test at same age. The strength test is conducted at the age of 28 days. This strength is called as characteristic compressive strength of concrete.

Read More: What is Characteristic Compressive Strength of Concrete?

Preparation  Concrete Cylinders for Compression Test

The complete making, curing, protection and transportation of specimen is followed as per ASTM C31. The concrete cylinder cured at field conditions gives an indication of in-place concrete at a given time. The lab controlled curing of specimen gives an idea about the compliance of the specimen with the strength specifications. The second test can be conducted to know the time of removal of formwork and shoring.

1.  The concrete is placed in the cylinder mold in three layers. For concrete with slump > 3inches, compact by a rod. For slump between 1 to 3 inches, either rod or vibrate. If slump < 1 inch, vibrate it.
2. After filling, the top must be strike off and sealed to prevent moisture evaporation. Label it and store it. Lift the specimen using a trowel and keep it vertical during transportation. 
3.  Leave the specimen undisturbed for 24 ± 8 hours at a temperature maintained at 60 to 80F.
4. The samples after 48 hours can be demolded and stored in 70 to 76 degree saturated lime water until it is taken to lab test. If less than 48 hours do not demold while taking to lab. 

Procedure to Test the Compressive Strength of Concrete as per ASTM

1. The basic requirements and specifications of compression testing machine is specified in ASTM C39. The diameter of the cylinder specimen used must be 3 times the nominal maximum size of aggregates used in the manufacture of concrete.
2. Always test the weight of the specimens before conducting the compression test. This helps to void any sort of disputes. 
3. The cylinder specimen is capped either by means of a sulphur mortar or a neoprene caps, whose specifications and details are given in ASTM C617 and ASTM C1231 respectively. When sulphur caps are used, install it either one day or at least two hours before testing the sample. To measure the concrete strength between 10 to 80MPa, a neoprene pad caps can be used. 
4. The pads used must be replaced after every 100 testing. When the strength tested is between 7000 and 12000 psi or 50 and 80MPa, it must be replaced after every 50 trials. 
5. Do not let the cylinder specimen to dry before testing. 
6. The diameter of the cylinder must be measured in two direction at the mid-height of the cylinder specimen. Then take the average. This value is taken to determine the cross-sectional area of the cylinders.                                                                                                                                   

   Note: Do not test the cylinder if the diameters measured in two directions differs more than 2%. 

7. The cylinder ends must be perpendicular to the cylinder axis. Any departure from this more than 0.5 degrees is not accepted. The specimen ends must be plane to within 0.002 inches i.e. .05mm.
8. Place the cylinder in center of the compression testing machine and it is loaded until failure. 
9. The Loading Rate is maintained to 28 to 42psi per second or 0.20 to 0.30MPa/s. 
10. Record the type of break and the corresponding load. Normally a Conical Fracture is observed. 

Calculation of Compressive Strength of Cylinder from Compression Test

The load value at the failure is recorded. The ratio of failure load to the area of cross section of the cylinder gives the compressive strength of the concrete.

Note: There are situation when the samples length to diameter ratio is between 1.75 and 1, which is rare condition in specimen. For this ASTM C39 have provided some correction factors.

The test is conducted on a minimum of two samples at site at same age.

Report Preparation of Compression Test at Site

As per ASTM C31

1. Location of the concrete in the structures represented by the test specimen 
2. Date/Time of Cylinder Specimen
3. Individual Molding Cylinders 
4. Slump, temperature and density of specimen 
5. Details of field curing 

As per ASTM C39

1.  Specimen No: or Identification Number
2. Average Measured Diameter 
3. Cross-sectional Area 
4. Testing Age 
5. Maximum Load Applied 
6. Compressive Strength = Maximum Applied load/Area of cross-section 
7. Type of Fracture 
8. Other defects in cylinder and caps 

Points to Remember in Compression Test on Concrete Cylinder

•  Any deviations from standard procedure of testing result in lower value of measured strength.
• · The cylinder must be molded in a level base.
• After filling use a mallet and tap the outside of the cylinder to remove voids and rod holes if any. This helps to avoid low cylinder breaks. 
• Do not overfill the last layer. This make large aggregates to collect at the top. 
• Store the sample om a level, base with no vibration and maintained temperature. 
• Use of curing box helps to cushion and secure the concrete cylinder samples during the transportation to the testing lab. 
• The coefficient of variation of strength between companion cylinder specimen tested at same age must be between 2 to 3%. If for more than one time in 20 trials, it exceeds 8%, then the procedure must be reevaluated. 
• Always prepare reports of compressive strength and results. It is an information for project team to work on current and future projects. The results of strength test must be shared with the concrete contractors, producers and client. 

Read More: What is Target Mean Strength in R.C.C Mix Design?

References: ACI 301-16; ACI 311.6-18; ACI 318-19; ACI 363R-10; ACI 363.2R-11; ASTM C31; ASTM C39; ASTM C215; ASTM C469