Table of Contents for Concrete Testing-

1. What are the methods of concrete testing?
2. Test 1: Workability Tests
3. Test 2: Temperature Tests
4. Test 3: Concrete Setting Tests
5. Test 4: Compressive Strength Test
6. Test 5: Permeability Tests
7. Conclusion

What are the methods of concrete testing?

The concrete is tested for quality in the fresh state as well as the hardened state. The tests conducted on fresh concrete are as follows-

• Workability Test
• Temperature Test
• Setting Test
• Air Content
• Unit Weight
• Heat Generation
• Bleeding
• Segregation Resistance etc.

The hardened concrete is tested for-

• Compressive Strength
• Modulus of Elasticity
• Tensile Strength
• Permeability
• Absorption
• Density
• Shrinkage
• Bond
• Abrasion etc.

The quality control of concrete strictly requires tests for workability, temperature, compressive strength etc. The necessity and frequency of other tests are determined by the Engineer-in-charge or the organisational QA program.

The tests on fresh concrete are done at the point of production and point of placing concrete. The fresh concrete samples are taken for testing and the results are immediately published to evaluate the concrete quality against the standards.

The tests on hardened concrete are done after the concrete achieve sufficient strength. These tests are generally carried out in the laboratory.

This article covers three fresh concrete tests and two hardened concrete tests that are must-know for all Civil Engineers.

Before moving ahead, the sampling of concrete for all the tests shall be done following IS 1199.

Test 1: Workability Tests

The workability of concrete is the measure of ease of working with concrete. The term ‘working with concrete’ non-exhaustively implies preparation, mixing, transporting, placing and compaction. The workability increases with the increase in the water-cement ratio. Highly workable concrete is liable for shrinkage, develop lesser strength etc.

Importance of Workability Tests- The workability of concrete ensures easy transportation, superior pumping ability, better filling and compaction. However, it also deteriorates the strength of the concrete. Hence, it becomes necessary to keep a balance between the workability and strength parameters.

There are five methods of testing the workability of concrete-

• Slump Test
• Compaction Factor Test
• Flow Test
• Vee Bee Time Test
• Kelly Ball Test

In this article, I shall briefly explain only the first three methods and save the minute details and the other two methods for a dedicated article on the workability of concrete.

Relevant Codes: IS 1199, ACI 318-19, ASTM C-143

1.1 Slump Test

The slump test is also known as the slump cone test. A conical frustum of base diameter 200 mm, top diameter 100 mm and height of 300 mm known as slump cone is used as apparatus for it. It is the most common method of testing the workability of concrete at the site. Although, it is widely accepted yet the results are not fully reliable.

The method of performing the slump test as per IS code is mentioned below-

• The cone is cleaned and placed on a levelled surface generally over a metal plate.
• The concrete is filled into the slump cone into three layers. Each layer is uniformly compacted 25 times by a 16mm diameter steel rod. After filling concrete till the top of the cone, the top surface of concrete is levelled with a trowel.
• Remove the mould vertically without disturbing the concrete.
• After the concrete settles down, measure the settlement of concrete. This measurement is reported in millimeters and called ‘slump’.

Suitability of Slump Test- The slump test is suitable for medium workability of concrete i.e., the slump of 20-150 mm.

Result Interpretation- The ideal slump value should be as low as possible. This represents the lower water-cement ratio hence better strength gain and lower shrinkage. However, due to placing and compaction limitations at the site, the slump value is increased to increase the workability.

1.2 Compaction Factor Test

Compaction Factor Test measures the workability of concrete by self-compaction achieved due to falling from height. The apparatus contains two hoppers over each other and a cylinder at the bottom. Each hopper is fitted with a trap door at its’ bottom.

The method of performing the compaction factor test by IS code is mentioned below-

• Fill the first hopper completely with concrete and level the top with a trowel.
• Open the first hopper trap door to allow concrete to fall into the second hopper.
• Now open the trap door of second hopper and let the concrete fall into the cylinder.
• Weigh the concrete in the cylinder. Record it as W1.
• Now, again fill the concrete directly into the cylinder in three layers with uniform compaction of each layer by 16 mm diameter rod.
• Record the weight of the compacted concrete as W2.
• Calculate the compaction factor by dividing W1 by W2.

Suitability of Compaction Factor Test- The compaction factor test is suitable for concrete having very little workability. The concrete that is to be vibrated is tested by this method.

Result Interpretation- The value of the compaction factor for normal workable concrete lies between 0.80 – 0.92. The higher the compaction factor, the higher is the workability.

1.3 Flow Test

Flow test of Flow Table test is done for highly workable concrete. The test checks the consistency of concrete. The apparatus mainly contains a 70 cm x 70 cm table and a cone of 25 cm bottom diameter, 17 cm top diameter and 30 cm height.

The method of performing the flow table test by IS code is mentioned below-

• The cone is placed over the wet table and filled with concrete into two layers. Each layer is tamped 25 times by a standard tamping rod.
• The cone is lifted and the concrete is allowed to flow.
• The table is lifted for some height and dropped to further allow the flow of concrete.
• The flow diameter of concrete is measured in six directions and their average is recorded as ‘D’.
• The flow % is calculated as {(D-25)/25} x 100.

Suitability of Flow Table Test- The test is unsuitable for the low slump concrete.

Result Interpretation- The flow % percentage represents the consistency of the concrete. The higher the flow %, the higher is the consistency.

Test 2: Temperature Test

The temperature test is relatively simpler as compared to other tests. The only equipment required for testing is the thermometer.

Importance of Temperature Test- Thecement-water reaction is exothermic. Thus, to aid the chemical reaction to progress positively, the concrete temperature must be kept low. High concrete temperature also quickly evaporates the free water. Hence, it becomes necessary to check the concrete temperature at the batching plant and the poring placing.

Relevant Code- IS 7861-1

Result Interpretation- The ideal temperature for concrete to be placed is 10-32 degrees centigrade. A lower temperature is better for strength development and lesser pore water loss. The suitable methods shall be applied in hot regions to keep the concrete temperature low.

Test 3: Concrete Setting Tests

The concrete sets on mixing with water. The setting of concrete is defined in terms of initial setting time and final setting time. It should be clearly noted that the setting of concrete is related to the plasticity of concrete and not the strength gain.

Importance of Concrete Setting Tests- The concrete setting times are useful in determining the maximum allowed transportation time, placing time and finishing time etc. For different conditions, the concrete can be suitably mixed with additives that alter its’ setting time.

Relevant Codes: IS 8142, ASTM C191, ASTM C403, ASTM C266, ASTM C953

3.1 Initial & Final Setting Time

The initial setting time of concrete is the time after which the concrete starts losing its’ plasticity. The final setting time of concrete is the time by which the concrete loses all of its’ plasticity. The Penetration Resistance Apparatus is used to determine the setting time of concrete.

The step-by-step procedure to conduct the initial setting time test by IS code is as follows-

• A mortar test specimen sieved by IS 4.75 mm sieve from fresh concrete.
• The mortar is filled into a container in 50 mm layers and compacted. The minimum final height of the mortar specimen is 140 mm.
• The specimen is cleared of bleeding water from the surface and placed under the apparatus.
• The needle of the apparatus is brought to the level of specimen surface and a constant force is applied.
• The force applied and the time elapsed since cement and water mixing is recorded.
• For normal concrete at normal temperature, repeat the test after 3-4 hours of first test and keep repeating after every 1 hour of it. Continue the tests until the penetration resistance is 275 kgf/cm^2.

Result Interpretation- The setting time is defined as the elapsed time since mixing cement and water at which the penetration resistance is 35 kgf/cm^2. The final setting time is the elapsed time at which the penetration resistance is 275 kgf/cm^2. The result is reported in hours and minutes.

Test 4: Compressive Strength Tests

The compressive strength is the key characteristic of concrete. However, it develops cracks even if exposed to mild tension. The test can be conducted on a 150 x150 x 150 mm sized concrete cube or concrete cylinder of 150 mm diameter and 300mm height.

Relevant Codes: IS 516 for the testing procedure and IS 456 for sampling frequency. ACI-318.

The general steps for testing the compressive strength of concrete as per IS code is as follows-

• The oiled moulds are filled with concrete and well compacted.
• The top surface of concrete is levelled with the mould and excess concrete is strike-off by a trowel.
• After 24 hours, the cube/cylinder is carefully removed from the mould and put underwater for curing. The samples are marked with the grade of concrete and casting date.
• After completing the curing period of 7 days, half of the samples (at least 3) are placed in compression testing machine one-by-one and the value at which sample develops crack is noted. The average of samples is recorded as 7-day mean compressive strength of the sample. Same procedure is repeated for 28-day mean compressive strength.

Result Interpretation- The concrete is generally tested for 7 days & 28 days compressive strength. In some cases, it can also be tested for 3 days as well. The 3 days, 7 days and 28 days strength should be equal to or more than the 50%, 66% and 99.99% of the target mean strength respectively.

Test 5: Permeability Tests

The permeability is the property of concrete that allows the flow of water through it. The permeability test of concrete should not be confused with the water absorption test of concrete.

Importance of Concrete Setting Tests- The water causes corrosion to the reinforcement. It is a well-known fact that as the steel corrodes its’ volume increases and cracks the concrete. Hence, the permeable concrete will be susceptible to cracks and early deterioration due to corrosion of reinforcement. Therefore, it is necessary to check for permeability especially for underwater concrete or concrete frequently exposed to liquids.

Relevant Codes: IS 3085, ACI 228.2R

The test procedure for testing the permeability of concrete as per IS code is mentioned below-

• Three number cylindrical samples of 200 mm diameter and 120mm height re prepared.
• After 24 hours, the middle 100 mm diameter is roughened and remaining portion is sealed with cement paste. The samples are then cured for 28 days.
•  After completing the curing period, the roughened portion is applied with water pressure of 1 bar for 48 hours, 3 bars for next 24 hours and 7 bars for another 24 hours.
• The samples are then split by compression machine to study the water penetration depth and recorded for each sample.
• The average of water penetration depth is recorded.

Result Interpretation- The depth of water penetration should not exceed 25 mm else; the sample is considered to fail.

Conclusion

The five tests mentioned in the article are non-exhaustive. As already mentioned above, these tests are essential and conducted to ascertain the quality of concrete. However, quality assurance and control should start from purchasing material to curing and maintaining the concrete. For complete concrete quality control, follow these five steps that will enable you to meet the concrete quality standards.

The correction methods for the shortcomings in concrete shall be well discussed with the Engineer- In-Charge and laid down in QAP. The standardisation of correction methods shall ensure the overall quality control and prevent wastage of material.

Read more about the futuristic concrete types here.

The list of IS codes for all the tests can be read here.