Test on Hardened Concrete

By Aina Parasher|Updated : June 16th, 2022

Test on Hardened Concrete: The test on hardened concrete is performed to check and control the quality of the concrete works used in the structure. In addition to quality control, various test on hardened concrete also plays an important role in determining different physical properties such as strength and elastic properties of concrete. These tests are necessary for checking whether concrete is suitable for its intended use.

A test on hardened concrete is performed to determine properties of hardened concrete such as strength, creep properties, volume change, permeability, etc. Tests on hardened concrete are grouped into two, namely, destructive tests and non-destructive tests. These tests are further discussed in detail in this article.

Table of Content

Destructive Test on Hardened Concrete

Destructive tests are those tests in which the concrete being tested gets destroyed. This group of test on hardened concrete is usually performed on concrete samples to determine the strength properties of concrete. Destructive tests are used to assess service life and uncover design flaws that might not be visible under normal working conditions. The main destructive test on hardened concrete are mentioned below:

  • Compressive Strength Test
  • Flexural Strength Test
  • Splitting Cylinder Test                                    

Compressive Strength Test

The compressive strength on hardened concrete is determined by using cubical or cylindrical specimens. Prismoidal concrete specimens can also be used for testing but it is not generally used in our country. 15 cm cube is considered if the maximum nominal size of aggregate is greater than 20 mm, otherwise 10 cm cube is used. In the case of a cylindrical specimen, the length to diameter ratio is maintained at 2:1. Generally, cylindrical specimens of 30 cm length and 15 cm diameter are considered. The concrete specimen is cured for 7 days or 28 days and is tested in Universal Compression Testing Machine. The specimen is subjected to a gradual load of 14 N/mm2 per minute till failure. An average of results from 3 specimens having a maximum deviation of 15% is taken as the compressive strength of the concrete.

Flexural Strength Test

Flexural strength test on hardened concrete is determined by using a standard specimen of size 15 cm x 15 cm x 70 cm. If the maximum nominal size of the aggregate is less than 20 mm, a specimen of size 10 cm x 10 cm x 50 cm may be used. When using the standard specimen, it is placed over roller supports of a diameter of 38 mm having a spacing of 60 cm in between them. The specimen is loaded with the help of rollers of the same diameter having a spacing of 20 cm. The load on the specimen is increased at a uniform rate of 0.7N/mm2/min till failure. The flexural strength of concrete is expressed as the modulus of rupture (fb).

fb=PL/bd2 (if a > 20 cm)

fb=3Pa/bd2 (if 17 cm < a < 20 cm)

Where,

a = distance between the line of fracture and nearest support

b and d = width and depth of specimen

L = Distance between the roller supports

P = Load at failure

Splitting Cylinder Test

Determination of tensile strength using the Splitting Cylinder Test on hardened concrete is widely accepted. In this test on hardened concrete, tensile strength is calculated indirectly by loading cylindrical concrete specimens horizontally in a compression testing machine. Application of compressive load leads to induced tensile stress in the specimen leading to splitting failure of the specimen. Generally, the cylindrical specimen is 30 cm in length and 15 cm in diameter. 

The load on the specimen is gradually increased at a rate within the range of 1.2 N/mm2/min to 2.4 N/mm2/min till failure. Splitting tensile strength can be calculated as:

Splitting tensile strength, fc=2P/πLD

Where,

P = maximum compressive load at failure

L = Length of cylinder

D = diameter of cylinder

Non-Destructive Test on Hardened Concrete

Non-destructive test on hardened concrete cause little to no damage to the concrete specimen on which it is used. After performing these tests, the concrete specimen can still be used further. Unlike destructive test on hardened concrete mentioned earlier, these tests can be performed on concrete in situ, therefore providing the actual characteristics of the concrete. The main non-destructive tests are listed below:

  • Pull-out Test
  • Penetration Test
  • Ultrasonic Pulse Velocity Test
  • Rebound Hammer Test
  • Maturity Test
  • Radioactive Test

Pull-out Test

The pull-out test is commonly used to diagnose strength issues early on. It can, however, be used to assess the concrete strength of an existing structure. A specially shaped steel rod with one end enlarged is embedded in concrete. Once the concrete hardens, the rod is pulled out. The force required to pull out the steel rod is related to the compressive strength of concrete.

Penetration Test

In penetration test on hardened concrete, steel probes are driven into the concrete by means of a power-actuated gun. Depth of penetration is measured using a depth gauge which provides an indication of the compressive strength of the concrete. The penetration test can be used to determine the relative strength of concrete in the same or different structures.

Ultrasonic Pulse Velocity Test

In the ultrasonic test on hardened concrete, the velocity of ultrasonic waves in concrete is measured and correlated to the quality of the hardened concrete. The apparatus consists of a transmitter and receiver. The transmitter generates the ultrasonic pulse which is received by the receiver placed on the other side of the specimen. Travel time of the pulse through known distance in the specimen is noted and is used to calculate the velocity of the pulse in the specimen which can further be related to the quality of concrete. The higher the velocity, the better will be the quality. It can also be used to determine the internal condition of the test concrete such as cracks, discontinuities and deterioration inside the concrete.

Rebound Hammer Test

The rebound hammer test on hardened concrete is performed using an apparatus called Schmidt’s Rebound Hammer. This rebound hammer consists of a spring-controlled mass that slides over the plunger in a tubular casing. When the hammer is pressed against the surface of concrete, the spring-controlled mass will rebound. The amount of rebound is noted over a scale and is termed as rebound number. This rebound number can further be used to indicate the strength of the concrete. It is also used to check the uniformity and quality of concrete.

Maturity Test

Maturity is a value that indicates the degree of progression of curing in concrete which is directly related to its age and temperature. The maturity of concrete is a direct measure of its strength. It can be determined by the maturity meter.

Radioactive Test

Concrete absorbs X-rays and Ɣ-rays passing through it. This degree of absorption depends on the density of concrete. In the radioactive test on hardened concrete, the density of concrete is evaluated by measuring the degree of absorption of the rays by concrete. Radium and radio-cobalt are used as the source for these rays.

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FAQs on Test on Hardened Concrete

  • Destructive testing destroys the concrete specimen that is being tested. Non-destructive testing (NDT), on the other hand, causes no physical damage to the specimen being tested and allows it to be utilised further after the test. Destructive testing is generally used to determine the properties of concrete whereas non-destructive test on hardened concrete is generally used to determine defects in concrete.

  • The result obtained from the rebound hammer test on hardened concrete depends on several factors. These factors are listed below:

    • The smoothness of the surface that is under test.
    • Age of the concrete.
    • Size, shape and rigidity of the sample.
    • Internal and external moisture condition of concrete specimen.
    • Type of cement and aggregates used.
    • Angle on which the rebound hammer is placed on the concrete surface.
  • The pulse velocity measured during the ultrasonic velocity test on hardened concrete is related to the quality of the concrete under the test. Higher is the pulse velocity, the better will be the quality. The general quality of the concrete for corresponding pulse velocity is shown below.

    • Excellent- >5000
    • Good- 4000-5000
    • Questionable- 3000-4000
    • Poor- 2000-3000
    • Very Poor- <2000
  • When the compressive load is applied to the concrete specimen, it leads to the generation of complex compressive stresses in the concrete due to the restraining effect of the steel platens used over the specimen in the testing machine. This restraining effect is observed throughout the height of the cubical specimen but is restricted to part below the point of application of load in the cylindrical specimen. Therefore, the cylindrical specimen provides more accurate results than the cubical specimen.

  • Non-destructive test on hardened concrete is conducted for determining the compressive strength and other parameters of concrete from existing structures without destroying them. These tests can give immediate results and provide actual properties of concrete. It is used in new as well as the existing structures. For new structures, NDT is used to check and control the quality of the material used. In the case of existing structures, NDT is used to check for the internal conditions of the structure such as defects, cracks, deterioration of reinforcements etc. 

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