Compressive Strength of Concrete - Cylinder Test | ASTM Standards

The compressive strength of concrete at the site is determined by conducting a compression test on a concrete specimen. The concrete specimen can take the shape of a cube or a cylinder. This is therefore called a cube test or cylinder test of concrete.

How to test the Compressive Strength of Concrete at the Site?

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

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

What is Compressive Strength of Concrete? 

Compressive strength of concrete is one of the topmost preferred design attributes for the design of concrete structures. Compressive strength as mentioned, is obtained by conducting a 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.

Compressive Strength (psi) = Maximum Load (lbf) / Cross-sectional Area (in²)

To convert to SI Units (MPa or N/mm²):

• 1 lbf = 4.44822 N
• 1 in² = 645.16 mm²

It takes the value in Mega Pascals (MPa) or in Pound-force per square inch (psi).

Why Compressive Strength of Concrete?

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

The concrete mix is designed for actual construction for an average strength of f’cr, which is higher than fc, so failure of strength goes under an acceptance criterion. This is the 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 a compressive test on concrete is to ensure that the strength value of concrete, 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 Cylinder 

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

Preparation

Dimension of Specimen

The cylindrical specimen used can be of standard sizes 6 x 12 inches (100 x 200mm) when the aggregate size is greater than 2 inches, or (150 x 300mm) concrete cylinders.

When the test is conducted at the site, it is good to employ smaller samples. It is recommended to use a minimum of two specimens from the concrete sample and test at the same age. The strength test is conducted at the age of 28 days. This strength is called as characteristic compressive strength of concrete.

Preparation of Specimen

The complete making, curing, protection, and transportation of specimens is followed as per ASTM C31. 

a. Making

The concrete is placed in the cylinder mold in three layers. 

• For concrete with a slump > 3 inches, compact with a rod. 
• For slumps between 1 to 3 inches, either rod or vibrate. 
• If the slump < 1 inch, vibrate it.

After filling, the top must be struck off and sealed to prevent moisture evaporation. Label it and store it. 

b. Curing and Transportation
Lift the specimen using a trowel and keep it vertical during transportation.
Leave the specimen undisturbed for 24 ± 8 hours at a temperature maintained at 60 to 80°F.

c. Protection

The samples after 48 hours can be demoulded and stored in 70 to 76 ° saturated lime water until it is taken to the lab for testing. If less than 48 hours, do not demould while taking to the lab.

The concrete cylinder cured under field conditions gives an indication of in-place concrete at a given time. The lab-controlled curing of the specimen gives an idea about the compliance of the specimen with the strength specifications. The second test can be conducted to determine the time of removal of formwork and shoring.

Procedure

Basic Requirements and Considerations

The basic requirements and specifications of the compression testing machine are specified in ASTM C39. 

1. 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 avoid any sort of disputes. 
3. The cylinder specimen is capped either by means of a sulphur mortar or a neoprene cap, whose specifications and details are given in ASTM C617 and ASTM C1231, respectively. When sulphur caps are used, install them either one day or at least two hours before testing the sample. To measure the concrete strength between 10 to 80MPa, a neoprene pad cap can be used. 
4. The pads used must be replaced after every 100 tests. 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 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

1. The load value at the failure is recorded. 
2. The ratio of failure load to the area of cross-section of the cylinder gives the compressive strength of the concrete. 
3. The test is conducted on a minimum of two samples at site at same age.

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

Report 

Sample Report Table – Compressive Strength of Concrete Cylinder (ASTM C39)

As per ASTM C31

• Location of the concrete in the structures represented by the test specimen
• Date/Time of Cylinder Specimen
• Individual Molding Cylinders
• Slump, temperature and density of specimen
• Details of field curing

As per ASTM C39

• Specimen No: or Identification Number
• Average Measured Diameter
• Cross-sectional Area
• Testing Age
• Maximum Load Applied
• Compressive Strength = Maximum Applied load/Area of cross-section
• Type of Fracture
• Other defects in the cylinder and caps.

Points to Remember 

• Any deviations from the standard procedure of testing result in a lower value of measured strength.
• The cylinder must be moulded on 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 makes large aggregates collect at the top.
• Store the sample on a level base with no vibration and maintained temperature.
• Use of a curing box helps to cushion and secure the concrete cylinder samples during transportation to the testing lab.
• The coefficient of variation of strength between companion cylinder specimens tested at the 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 re-evaluated.
• Always prepare reports of compressive strength and results. It is information for the project team to work on current and future projects. The strength test results must be shared with the concrete contractors, producers, and client.

Key Takeaways

Following ASTM standards, the compressive strength of concrete is determined using cylindrical specimens, whereas the strength of cement is evaluated through compressive strength testing of standardized mortar cube samples.

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

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