CONCRETE TESTING METHODS EXPLAINED

CONCRETE

Concrete is a composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregates. Components of Concrete are Cement, Coarse aggregate, Fine aggregate, water and air.

Depending upon its state, Concrete is classified into 2 types:

1. Hardened concrete: It is strong in compression strength but lacks in tensile strength. Its properties are strength, durability, impermeability and dimensional variations.

1. Fresh concrete: It can be placed into forms to produce large variety of structural elements. Its properties are workability, segregation and bleeding.

QUALITY TESTS ON FRESH CONCRETE

1. Workability Test: The workability of concrete means the ability to do work with concrete. Workability tests are Slump Test, Compaction factor test, vee-bee consistometer test, Flow test, K-Slump test and kelly ball test.

2. Air Content: Air content measures the total air content in a sample of fresh concrete but does not indicate what the final in-place air content is, because a certain amount of air is lost in transportation Consolidating, placement, and finishing. Air content in concrete can be measured with Pressure air method which is based on the principles of Boyle's law.

3.Setting Time: The action of changing mixed cement from a fluid state to a solid state is called “Setting of Cement”. Initial Setting Time is defined as the period elapsing between the time when water is added to the cement and the time at which the needle of 1 mm square section fails to pierce the test block to a depth of about 5 mm from the bottom of the mold. Final Setting Time is defined as the period elapsing between the time when water is added to cement and the time at which the needle of 1 mm square section with 5 mm diameter attachment makes an impression on the test block. 

These are the most common tests on the Fresh Concrete. The other tests are  Segregation resistance, Unit weight, Wet analysis, Temperature, Heat generation and Bleeding.

QUALITY TESTS ON HARDENED CONCRETE

Most Common Quality Tests on hardened concrete are:

1. Compressive strength: Compressive strength is the resistance of the material to break under compression. The compressive strength of concrete cube test provides an idea about all the characteristics of concrete 

2. Tensile strength: The tensile strength of concrete is one of the basic and important properties which greatly affect the extent and size of cracking in structures. Moreover, the concrete is very weak in tension due to its brittle nature. Hence. it is not expected to resist the direct tension. So, concrete develops cracks when tensile forces exceed its tensile strength. Therefore, it is necessary to determine the tensile strength of concrete to determine the load at which the concrete members may crack.

3. Modulus of elasticity: Modulus of elasticity of concrete is the ratio of stress to the strain of the concrete under the application of loads. 

4. Permeability Tests on Concrete: When concrete is permeable it can cause corrosion in reinforcement in presence of oxygen, moisture, CO2, SO3- and Cl- etc. This formation of rust due to corrosion becomes nearly 6 times the volume of steel oxide layer, due to which cracking develops in reinforced concrete and spalling of concrete starts.

5. In situ test on concrete: There are various in-situ test conducted on hardened concrete, both destructive and non-destructive. Some of them are concrete pull out tests, Break off tests, Schmidt Hammer test. 

 Other quality tests are conducted to test the following:

• Modulus of rupture
• Density
• Shrinkage
• Creep
• Freeze/thaw resistance
• Resistance to aggressive chemicals
• Resistance to abrasion
• Bond to reinforcement
• Absorption

Slump Test and Compression Test 

Sump test and compression test are the two major quality tests on concrete. If necessary, it is desired to conduct fresh concrete temperature and hardened concrete density determination tests. 

The reasons for the selection of compressive strength test and slump test in practice for quality

control testing of concrete are:

• Most of the concrete properties are related to the compressive strength that is obtained by compressive strength test.
• Compressive strength test is the easiest, most economical or most accurately determinable test.
• The variability of concrete is best studied by means of compressive strength tests.
• The quality of the mix is judged by the slump test. This studies the variation of construction materials in the mix. These tests focus on the water-cement ratio of the concrete mix.
• The slump test is easy to conduct. It determines the quality of concrete very fastly before its placement. The placement standards are as recommended by the respective concrete practice codes.
• Slump test is conducted at the site which does not require any lab arrangement or expensive testing machines. Hence this test is economical.
• We conduct the slump test before pouring into the form work. Hence if there is an issue with the concrete quality, the tested batch can be rejected. This would help in bringing up a defective structural member and avoiding future dismantling and repair.