How do you calculate the compressive strength of concrete cubes?

Answer 1

Concrete specimens are tested using the ASTM39 Test Standard Test Method

for Compressive Strength

of Cylindrical Concrete Specimens by using two samples made from the same test at the same age, usually at strength of 28 days old.

Design engineers use the specified strength ƒ´c to design structural elements. This specified strength is incorporated in the job contract documents. The concrete mixture is designed to produce an average strength (ƒ´cr) higher than the specified strength, such that the risk of not complying with the strength specification is minimized.

Cylindrical specimens

for acceptance testing should be 6 x 12 inch (150 x 300 mm) size or 4 x 8 inch (100 x 200 mm) when specified. The Concrete strength is calculated by dividing the maximum load at failure by the average cross sectional area.

Concrete compressive strength requirements can vary from 2500 psi (17 MPa) for residential concrete to 4000 psi (28 MPa) and higher in commercial structures. Higher strengths up to and exceeding 10,000 psi (70 MPa) are specified for certain applications.

Cylinders are placed in a Compression testing machine and loaded to failure from 20 to 50psi. The type of break should be recorded by a Certified Technician.

Example 1:
For a concrete sample that is broken after 28 days
The cylinder is 6” diameter, so it has an area of 3.14 x Diameter squared /4

A = 3.14 x 6 x 6/4
A = 28.26 square inches

If the force required to break the cylinder was 97,500 pounds

Then the compressive strength is 97,500 pounds / 28.26 square inches = 3,450 psi.
3,450psi needs to be compared to the Design strength determined by the Design Strength determined by the Design Engineer.

Example 2:
Given: f 'c = 3000 psi
Concrete test cylinders averaged 2800 psi.
PSI
Core 1 =2900
Core 2 =2850
Core 3 =2450
Average compressive strength of 3 cores = 2730 psi.
Find: Is the concrete in the structure adequate under CDOT specifications?
Solution:
Test Evaluation:
f 'c = 3000 psi
Average compressive strength of 3 cores - 2730 psi
Do any compressive strengths differ from the average by more than 10%?
10% of Average compressive strength = 273 psi
Core 1: 2900 - 2730 = 170 psi, < 273 therefore OK
Core 2: 2850 - 2730 = 120 psi, < 273 therefore OK
Core 3: 2730 - 2450 = 280 psi, > 273 therefore -
discard core and re-compute average compressive strength using two remaining cores.
New average compressive strength = 2875 psi