Compressive Strength of Concrete | Cube Test, Procedure, Apparatus & Results
In this Article today we will talk about the Compressive Strength of Concrete | Cube Test of Concrete as per IS Code | 7 Days Compressive Strength of Concrete | How to Determine Compressive Strength of Concrete | 28 Days Compressive Strength of Concrete | Concrete Cube Test Report | Compressive Strength Concrete Formula
Concrete being the major consumable material after water makes it quite inquisitive in its nature. The strength of concrete is majorly derived from aggregates, where-as cement and sand contribute binding and workability along with flowability to concrete.
What is the Compressive Strength?
Compressive strength is the capacity of material or structure to resist or withstand under compression. The Compressive strength of a material is determined by the ability of the material to resist failure in the form cracks and fissure.
In this test, the push force applied on the both faces of concrete specimen and the maximum compression that concrete bears without failure is noted.
Concrete testing helps us to majorly focus on the Compressive strength of concrete because it helps us to quantify the ability of concrete to resists Compressive stresses among structures where-as other stresses such as axial stresses and tensile stresses are catered by reinforcement and other means.
In technical point of view, Cube Test of Concrete as per IS Code
Compressive Strength of concrete is defined as the Characteristic strength of 150mm size concrete cubes @28 days.
Compressive strength of Concrete and its importance:
As we all know that concrete is a mixture of sand, cement, and aggregate. The strength of the concrete depends upon many factors like individual compressive strength of its constituents (Cement, Sand, aggregate), quality of materials used, air entrainment mix proportions, water-cement ratio, curing methods and temperature effects.
Compressive strength gives an idea of the overall strength and above-mentioned factors. Through conducting this test, one can easily judge the concrete strength psi and quality of concrete produced. Cube Test of Concrete as per IS Code
Factors affecting compressive strength of concrete:
Concrete is made homogenous by combining 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 matrix, which leads to failure of concrete.
Concrete technologists 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, then it would lead increased compressive strength.
This theory was later discarded, as experiments proved that greater size of aggregates showed increased strength in initial phases but reduced exponentially.
The sole reason for this strength drop was due to the reduced surface area for bond strength between cement matrix and aggregates and weaker transition zone.
Air entrainment in concrete was one of the concepts developed by cold countries in order to prevent damages due to freezing and thawing. Later on, as experimentation’s proved multidimensional benefits of air entrainment along with improved the workability of concrete at lower water/cement ratio.
As the achievement of the desired workability at lower water content helped one to achieve concrete with the greater compressive strength which in turn, leads to light concrete with greater compressive strength. 7 Days Compressive Strength of Concrete
We are all very aware of how excess water can be harmful to the strength of concrete. Cement being the major binding material in concrete needs water for hydration process, but that is only limited to about (0.20 to 0.25) % of cement content. The excess water turns out to be beneficial in contributing to workability and finishing of concrete.
The very aspect where excess water is considered harmful because as the water in the concrete matrix dries, it leaves large interstitial spaces among aggregate and cement grains. This interstitial space becomes primary cracks during compressive strength testing of concrete. How to Determine Compressive Strength of Concrete
Why do we test concrete for 7 days, 14 days & 28 days ?
Concrete gains maximum strength at 28days. Since in construction sector great amount of capital is at stake, so instead of checking strength at 28 days we can check strength in terms of concrete strength psi at 7 and 14 days to predict the target strength of construction work.
From the below table it is clear that, Concrete gains 16 % of its strength within 24 hrs, whereas concrete gains 65% of the target strength by the time of 7 days of its casting.
Till 14 days concrete shows 90% of the target strength and thereafter the gain in strength slows down and it takes 28days to achieve 99% of strength.
We cant judge the strength of concrete until it becomes stable. And we also won’t wait for 28 days to judge the concrete whether it suitable for construction or not to keep it balanced, concrete is tested at various intervals. How to Determine Compressive Strength of Concrete
|Age in Days||Percentage of Strength|
The maximum strength gain spike is observed till 14 days, so we test concrete at an intervals of 7 days, 10 days & 14days and If concrete fails to show results of 90% of its overall strength @14 days then that batching is rejected. How to Determine Compressive Strength of Concrete
Compressive Strength of Different Grades of Concrete at 7, 14, 21 and 28 Days:
|Concrete Grade||Compressive strength in N/mm² at 3 days||Compressive strength in N/mm² at 7 days||Compressive strength in N/mm² at 14 days||Compressive strength in N/mm² at 28 days|