Compressive Strength of Concrete

Hello friends, today in this post you will learn the Compressive strength test of concrete with calculations & detail stepwise procedure.
So, don't skip to read the post completely. After reading the post, you will completely understand the simple tests of the Compression test on concrete cubes.
So, let's start with these amazing topic...

What is the Compressive strength ?

In general, Compressive strength means the capacity of material or structure to resist or to withstand under compression.

The Compressive strength of a material means the ability of the material to resist failure in the form cracks as well as the fissure.

In this test, we apply pushing force on the both faces of concrete specimen and the maximum compression that concrete can withstand without failure is noted.

In Actual, it means the Compressive strength of concrete is also can be defined as the Characteristic strength of 150 x 150 x 150 mm size concrete cubes at 28 days.

Compressive strength of Concrete and its importance :-

As you know that, concrete is a mixture of cement, sand, aggregates & water. As the strength of the concrete depends upon many factors like individual compressive strength of materials used for manufacturing of concrete such as (Cement, Sand, aggregate), quality of materials used, air entrainment mix proportions, w/c ratio, curing methods used and temperature effects.

Factors affecting compressive strength of concrete:-

Coarse aggregate:-

Concrete is made from the homogenous mixture by combining the aggregates, cement, sand, water and various other admixtures. But even with proper mixing, there may arise some microcracks due to differences in thermal and mechanical properties of coarse aggregates and cement, which leads to failure of concrete.

Researchers came up with theoretical concepts regarding size of aggregates, which as the size of aggregate being the major contributor of compressive strength. So if the size of aggregate is increased, it will definetely increased compressive strength of concrete.

This theory was later discarded, after performing many experiments proved that greater size of aggregates showed increased strength in initial phases, but it get reduced exponentially.

The major reason for the decrease in the strength was due to the reduced surface area for bond strength between cement mix and aggregates and weaker transition zone.

Air-entrainment:-

Air entrainment in concrete concepts developed by cold countries in order to prevent damages due to the freezing and thawing of water particles in pores of concrete . Later after performing various tests, finally proved that the benefits of air entrainment improves the workability of concrete at lower water/cement ratio.

The desired workability at lower w/c ratio will helps to achieve concrete with the greater compressive strength, these will leads to light concrete with greater compressive strength.

Water/Cement ratio:-

We all know that excess water can cause harm to the strength of concrete. Cement being the major binding material in concrete needs water for hydration process, but in limit (0.20 to 0.25) % of cement content. The excess water will affect on the workability as well as the finishing of concrete.

The excess water is considered harmful because, if the water in the concrete mix dries, it leaves large interstitial spaces among aggregate and cement grains. This interstitial space may cause the cracks during compressive strength testing of concrete.

Why do we test concrete for 7 days, 14 days & 28 days:

As we know that, Concrete gains maximum strength at 28days. In the construction sector great amount of capital is at stake, due to these reason, instead of checking strength at 28 days, we check the strength in terms of concrete strength psi at 7 and 14 days to predict the target strength of construction work.

From the reference of below given table it is clear that, Concrete gains 16 % of its strength within 24 hrs, whereas concrete gains 65% of the target strength on the 7 days of its casting.

On 14 days, the concrete gains 90% of the target strength and thereafter the gain in strength slows down and it takes 28 days to achieve 99% of strength.

We cant judge the strength of concrete until it becomes stable. Also we can't wait for 28 days to determination of the suitability of concrete for construction works.

The maximum strength gain is observed till 14 days, so we test concrete at an intervals of 7 days, 10 days & 14 days and If concrete fails to show results of 90% of its overall strength at 14 days. then that concrete of that batch is rejected.

Compressive Strength of Different Grades of Concrete at 7, 14, 21 and 28 Days :

Compressive strength testing of concrete:-

For determination of Compression test, we require 150 x 150 x 150 mm size concrete cubes on a Universal testing machine or compressive testing machine.

Apparatus

As per IS: 516-1959 Compressive testing machine (2000Kn),15cm×15cm×15cm steel cube moulds or Cylinder having Dia. 15cm and length 30cm.

Below are the Compression test steps:-

Preparing of material for Cube test:
Mixing of concrete:

Required materials must be brought and stored at an approximate temperature of 27 ±3 degree Celsius. Cement must be uniformly mixed with a trowel, no lumps should be present in it.

The ingredients should not be mixed more than 2 minutes and the following pattern must be followed :

1>Calculated water, 2>50% coarse aggregates, 3>fine aggregates, 4>cement, 5>50% coarse aggregates.

The mixing process must be done on the rectangular non-porous pan until a homogenous mix is obtained.

Dry mixing of fine aggregates and cement>addition of Coarse aggregate with the even distribution>addition of calculated water in batch till consistency is achieved.

Casting of specimen
Compaction
Age of test

The moulds for concrete cubes should be made of cast iron and proper oiling should be done on inner side of each moulds for the easy removal of cubes. The specimen must be cast in 3 layers and properly compacted in order for prevention of honeycombing.

In compaction should be done with the tamping bar, 35 strokes must be done in all parts of a cube for proper compacting. This tamping bar has the dimension of diameter 16mm and length of 0.6m.

The Compressive strength on concrete cubes can be done on 1,3, 7, 14 and 28 days. In some cases, the strength of greater ages is required which is performed after 13 to 52 weeks of casting.

Number of specimens

It is mandatory to have at least 3 specimens for testing from different batches. The mean of compressive strength achieved by this specimen is used to determine actual strength of the batch.

Procedure for Compressive strength of concrete or Cube test :-

Place the prepared concrete mix in the steel cube mould for casting.

When the concrete get sets, After 24 hours remove the concrete cube from the mould.

Now we have to Keep the test specimens submerged underwater for stipulated time.

The specimen must be kept in water for 7, 14 or 28 days and for every 7 days the water is changed.

While performing the test, we must Ensure that concrete specimen must be well dried, before placing it on the UTM.

We have to also take, the Weight of samples in order to proceed with testing and it must not be less than 8.1 Kg.

Testing specimens are placed in the space between bearing surfaces.

Important thing to remeber that, we must take Care that to prevent the existence of any loose material or grit on the metal plates of machine or specimen block.

The concrete cubes are placed on bearing plate and aligned properly with the center of thrust in the testing machine plates.

Also note that, The loading must be applied axially on specimen without any shock and increased manner at the rate of 140kg/sq cm/min. till the specimen collapse.

Due to the constant application of load, the specimen starts cracking at a point & final breakdown of the specimen must be noted.

Calculations

Compressive strength of Concrete Formula:

The Compressive strength of specimen can be easily calculated by dividing maximum load carried by the specimen by cross-sectional area of the specimen cubes.

The surface area of specimen: = 150 x 150 = 22500mm² = 225cm²

Assume, The Max compression load noted as 450KN

1 KN = 1000N ; 450KN = 450×100 = 450000N

Compressive Strength of Concrete = 450000 / 22500 = 20 N/mm² = 203 Kg/cm²

The same calculation is done for the specimen at different ages as stated below:

Important Note: As per IS: 516-1959 Minimum three specimens should be tested at each selected age (that means three specimens at 7 days, three specimens @ 14 days & 28 days) If strength of any specimen varies by more than 15% of average strength, such specimen should be rejected.

Results of cube test
Here is the Video Explanation :

Average Compressive strength at 7 days = _____N/mm²

The average Compressive strength at 28 days = _____N/mm².

You can also take reference of below video for better understanding of Compressive strength of concrete :

Compressive Strength of Concrete