Tests on Hardened Concrete in Structures

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Various tests on Hardened Concrete is done to ensure the design strength of concrete and quality of concrete construction is achieved.

Tests on Hardened Concrete

The test conducted on hardened concrete can be classified into two main categories:

1. Destructive Tests on Concrete
2. Non-destructive Tests on Concrete
3. Chemical Test on Concrete

The suitability of the hardened concrete structure is determined by conducting suitable tests. It is preferred to have such testing without any destruction or damage to the concrete structure. Non-destructive tests are test that will undergo no damage to the structure and take the results. In Destructive tests, the results can be only taken by slightly damaging the concrete surface. Once the surface is tested, the surface has to be repaired. We will discuss the destructive and non-destructive tests in the concrete one by one in detail.

Destructive Tests on Hardened Concrete

The common destructive tests conducted on concrete are: The main destructive tests on hardened concrete are as follows.

1. Cube test
2. Tensile Strength Test
3. Concrete core test

1. Concrete Cube Test

Concrete Characteristics is determined by characteristics compressive cube strength test of concrete.For cube test two types of specimens either cubes of 15cm X 15cm X 15cm or 10cm X 10cm x 10cm depending upon the size of aggregate are used. For most of the works cubical moulds of size 15cm x 15cm x 15cm are commonly used. These specimens are tested by compression testing machine after 7 days curing or 28 days curing. Load should be applied gradually at the rate of 140 kg/cm2 per minute till the Specimens fails. Read More:Cube Test on Concrete

2. Tensile Strength Test

The concrete structures are highly vulnerable to tensile cracking and hence the determination of tensile strength of concrete is very important. The tensile strength of concrete structures is determined by:

2.1 Split Cylinder Test

Here, the tensile strength is determined indirectly. The test is performed based on IS: 5816 – 1970. The test specimen employed is 30cm x 15cm which is placed over a compression testing machine.The load is applied over the specimen diametrically and uniformly through the cylinder length till the cylinder undergoes failure. The failure of the cylinder will be along the diameter in vertical direction. Between the specimen and the loading plates, plywood strips are placed to avoid direct stress due to direct point of application. The tensile stress formed with the progress of load will split the cylinder into two halves. The splitting takes place along the vertical plane. This is caused due to the indirect tensile stress caused. Let the tensile stress created that results in splitting be f_t . If ‘P’ is the compressive load at the failure, “L” is the cylinder length and diameter of the cylinder being ‘D’,

2.2 Flexure Test

The guidelines for performing the flexure test is as per BS 1881: Part 118 : 1983. Here a concrete beam specimen of dimension 15x15x75 cm is loaded. The span of the beam specimen must be three times the depth. As shown in the figure below, equal load application is done at one third distance form the end supports. The reactions are equal at the support. The bottom beam fibre experiences increase in stress with the increase in load application. The increase of stress is at a rate of 0.02 MPa & 0.10 MPa.   For low strength concrete we make use of low rate and for high strength we use high rate. The theoretical maximum tensile stress at the bottom face at failure is calculated. This is termed the modulus of rupture. It is about 1.5 times the tensile stress determined by the splitting test. Modulus of Rupture is given by,

3. Core Strength Test

Cylindrical cores are cut from the finished structure with a rotary cutting tool. The core is soaked, capped and tested in compression to give a measure of the concrete strength in the actual structure. The ratio of core height to diameter and the location where the core is taken affect the strength. The strength is lowest at the top surface and increases with depth through the element. A ratio of core height-to-diameter of 2 gives a standard cylinder test. Read more: Core Sampling and testing of concrete.

Non-Destructive Tests on Concrete

The main non-destructive tests for strength on hardened concrete areas follows.

1. Rebound Hammer (Hardness Test)
2. Ultrasonic Pulse Velocity Test
3. Pull Out Test
4. Penetration Resistance
5. Other non-destructive tests

1. Rebound hammer (hardness) test

The Schmidt hammer is used in the rebound hardness test in which a metal hammer held against the concrete is struck by another spring-driven metal mass and rebounds. The amount of rebound is recorded on a scale and this gives an indication of the concrete strength. The larger the rebound number is, the higher is the concrete strength.

2. Ultrasonic pulse velocity test

In the ultrasonic pulse velocity test the velocity of ultrasonic pulses that pass through a concrete section from a transmitter to a receiver is measured. The pulse velocity is correlated against strength. The higher the velocity is, the stronger is the concrete.

3.Pull Out Test

The pull out test will determine the force that is required to pull out a steel rod specially shaped from hardened concrete to which the steel was cast. Pulling out of steel is done with a cone of concrete that have a slope of 45 degrees. The force required to pull the concrete out is related with the compressive strength of the concrete.

4. Penetration Resistance Test

Penetration resistance tests on concrete offers a means of determining relative strengths of concrete in the same structure or relative strength of different structures. Because of nature of equipments, it can not and should not be expected to yield absolute values of strength. ASTM C-803 gives this standard test method titled “Penetration Resistance of Hardened Concrete”

5. Other non-destructive tests

Equipment has been developed to measur

1.  Crack widths and depths
2.  Water permeability and the surface dampness of concrete
3. Depth of cover and the location of reinforcing bars
4. The electrochemical potential of reinforcing bars and hence the presence of corrosion

Chemical Tests on Hardened Concrete

A complete range of chemical tests is available to measure

1. Depth of carbonation
2. The cement content of the original mix
3. The content of salts such as chlorides and sulphates that may react and cause the concrete to disintegrate or cause corrosion of the reinforcement
4. Alkali Content

Read More on Chemical Tests on Concrete