Understanding Compressive Strength of Concrete: Testing Methods and Quality Assurance

From small homes to large urban development projects, the quality of concrete determines the strength and longevity of structures. We measure this quality by checking the compressive strength of concrete.

Let us understand what this means and what you should know as a home builder.

What is Compressive Strength of Concrete and Why is it So Important?

In simple terms, the compressive strength of concrete is its ability to resist a pressing or squeezing force.

Imagine the weight of your walls, slabs, furniture, and even people—all this load presses down on the building's columns and foundation. The concrete must be strong enough to bear this load without being crushed.

This strength is a key indicator of the overall quality and durability of your concrete. A good compressive strength means your concrete is dense, less porous, and will better resist wear and tear over time.

For larger constructions and urban development projects, the minimum acceptance criteria for compressive strength of concrete has been laid down by the Bureau of Indian Standards (IS: 456-2000)

Key Factors That Determine Concrete Strength

The final strength of the concrete does not happen by accident. You should know about the important things that affect it during your home construction.

Water-Cement Ratio

The water-cement ratio is the most important thing to consider. This is the amount of water you use compared to the amount of cement in your concrete mix. Using too much water will make your concrete weak. If you use too little water, the concrete will be very hard to work with.

Quality of Materials

• Cement: The compressive strength of cement is the first step. You must use fresh cement from a good brand like Bangur Cement that has not been tampered with.

• Aggregates (Sand and Stone): The sand and stones you use must be clean and strong. They should also have a good mix of different sizes. Impurities like clay or dirt in the aggregates can make the concrete very weak.

• Water: The water for mixing should also be clean. It must not have any oil, acid, or salt in it.

Compaction of Concrete

You must compact the concrete properly after pouring it. This process removes any air that is trapped inside. These air pockets are weak points that can make the structure weak. An engineer will normally use a tool called a needle vibrator for this job.

Curing of Concrete

Curing is a very important step for strength that people often forget. It is the simple process of keeping the concrete wet after you have poured it. This helps the cement to become strong over time. We will talk more about this now.

The Importance of Curing: Letting Your Concrete Gain Strength

You can think of curing as giving the concrete the right conditions to become strong. The cement needs water to continue its chemical reaction and gain strength. Your concrete will not reach its full strength if it dries out too fast.

• How is it done? You can do this by making small ponds of water on slabs. You can also cover the concrete with wet gunny bags. Sprinkling water regularly is another common method.

• How long? You should do curing for at least 7 to 14 days for normal work. Your engineer might ask you to do it for a longer time for more important parts of the house.

How is Concrete Strength Tested?

Engineers perform a test of concrete in two main ways. They can use destructive tests or non-destructive tests.

1. Destructive Testing (Cube Test)

This is the most common and reliable method.

• Procedure: While the concrete is being mixed for your slab or column, small cubes (usually 150mm x 150mm x 150mm) are filled with the same concrete. These cubes are then cured in a water tank.

• Testing: The cubes are tested in a machine called a Compression Testing Machine (CTM) at specific intervals, usually after 7 days and 28 days. The machine applies a crushing force, and the pressure at which the cube breaks is recorded. This gives the compressive strength of concrete.

2. Non-Destructive Testing (NDT)

Sometimes, you need to check the strength of an existing structure without damaging it. This is where NDT methods are useful, especially before repairing cracks in concrete.

• Rebound Hammer Test: The Rebound Hammer is a simple tool that strikes the concrete surface with a spring-loaded mass. The extent to which it bounces back (the rebound number) gives an indication of the surface hardness, which can be correlated to strength. It's a quick and easy test but gives only an approximate idea of the strength.

• Ultrasonic Pulse Velocity (UPV) Test: This test involves sending a sound wave through the concrete and measuring the time it takes to travel from one point to another. A faster travel time generally indicates denser, stronger concrete.

• Pullout Test: This involves measuring the force required to pull out a specially shaped steel rod that has been cast into the concrete. The Pullout Test gives a more reliable measure of strength compared to the rebound hammer.

Concrete Grades and Their Strengths

Frequently Asked Questions (FAQs)

What is the compressive strength of concrete?

The compressive strength of concrete is its ability to withstand a squeezing force without breaking. It's the most common measure of concrete quality and is usually expressed in N/mm².

How to calculate compressive strength of concrete at 7 days, 14 days, and 28 days?

As a general rule, concrete gains strength over time. It typically reaches about 67% of its final strength in 7 days, around 90% in 14 days, and its full designed strength at 28 days.

What is the 7-day compressive strength of concrete as per IS 456?

According to Indian Standard IS 456, the 7-day compressive strength of concrete should be at least 65% of its specified 28-day strength. For example, for M20 concrete, the 7-day strength should be at least 0.65 * 20 = 13 N/mm².

What is the compressive strength of M20 concrete?

The '20' in M20 signifies its strength. The characteristic compressive strength of M20 concrete is 20 Newtons per square millimeter (N/mm²) after 28 days of proper curing.

What is the correct mix for concrete?

The correct mix for concrete depends on the required strength for different parts of your house. Your structural engineer will specify the grades (like M20 for slabs, M25 for columns). Following these specified mix ratios is essential for the structural stability of your home.