To ensure that the design strength of concrete and the quality of concrete construction are achieved, multiple tests are conducted on hardened concrete. These tests are crucial in determining the reliability and durability of the concrete structure.
Various factors can affect the strength and quality of hardened concrete, such as the mixing proportions, curing conditions, and environmental exposure. Therefore, it is essential to test the concrete using standardized methods to evaluate its properties accurately.
Some of the common tests performed on hardened concrete include compressive strength tests, flexural strength tests, and splitting tensile strength tests. These tests measure the resistance of concrete to various types of stress and strain.
Additionally, tests are conducted to assess the durability of concrete, such as the water permeability test, chloride penetration test, and carbonation test. These tests evaluate the concrete’s ability to withstand environmental factors such as moisture, salt, and carbon dioxide.
Overall, these tests on hardened concrete are vital to ensure the structural integrity of concrete structures and to verify that they meet design specifications and industry standards.
Tests on Hardened Concrete
Suitable testing is necessary to determine the suitability of hardened concrete structures. There are two main categories of tests that can be conducted: destructive and non-destructive tests, as well as chemical tests.
Non-destructive tests are preferred as they do not damage the concrete structure during the testing process. These tests can provide accurate results without the need for repair or surface restoration. On the other hand, destructive tests require some degree of damage to the concrete surface in order to obtain results. Once the surface has been tested, repairs are necessary to restore the surface.
Destructive tests are conducted by slightly damaging the concrete surface, which can provide valuable information about the quality and durability of the concrete. Non-destructive tests, however, are able to provide results without any damage to the concrete structure.
Overall, the testing of hardened concrete is crucial in determining its suitability and durability. By conducting a range of tests, including both destructive and non-destructive tests, the quality of the concrete can be evaluated and necessary repairs can be made to ensure its continued performance. Chemical tests can also be conducted to provide further insights into the composition and properties of the concrete.
Destructive Tests on Hardened Concrete
Hardened concrete is commonly subjected to destructive testing to evaluate its strength and durability. The primary destructive tests performed on concrete include the cube test, tensile strength test, and concrete core test.
The cube test is a method of measuring the compressive strength of concrete. It involves creating cubes of concrete and subjecting them to compression using a hydraulic press. The amount of force required to crush the cube is then measured to determine the concrete’s compressive strength.
The tensile strength test, on the other hand, measures the tensile strength of concrete. This test involves applying a tensile force to a concrete specimen until it fractures. The amount of force required to cause the fracture is then measured to determine the tensile strength of the concrete.
Finally, the concrete core test is a method of assessing the strength of in-place concrete. It involves drilling a core sample from the hardened concrete structure and subjecting it to compression testing in a laboratory. This test provides information about the quality of the concrete and can help determine if any repairs or replacement are necessary.
1. Concrete Cube Test
The concrete characteristics are determined by conducting a compressive cube strength test on the concrete. This test involves using two types of specimens: cubes of 15cm x 15cm x 15cm or 10cm x 10cm x 10cm, depending on the size of the aggregate being used. However, for most projects, cubical moulds of size 15cm x 15cm x 15cm are commonly used.
The specimens are tested using a compression testing machine after being cured for either 7 days or 28 days. The load is gradually applied at a rate of 140 kg/cm2 per minute until the specimens fail. By conducting this test, the compressive strength of the concrete can be determined, which is a crucial characteristic of the material.
2. Tensile Strength Test
Concrete structures are prone to developing cracks when subjected to tensile stresses. Therefore, it is crucial to determine the tensile strength of concrete as it is a key factor in assessing the structural integrity of such constructions. The tensile strength of concrete structures can be evaluated by various methods.
2.1 Split Cylinder Test
The tensile strength of a material is determined indirectly using a test method specified in IS: 5816 – 1970. In this test, a specimen measuring 30cm x 15cm is used, which is placed on a compression testing machine. The load is applied uniformly and diametrically along the length of the cylinder until the cylinder fails. The failure of the cylinder occurs along the diameter in a vertical direction. This test method allows for the determination of tensile strength based on the failure characteristics of the cylinder specimen under compression.
Fig.1: Arrangement for Split tensile strength
To prevent direct stress on the specimen due to the point of application, plywood strips are placed between the specimen and the loading plates. As the load progresses, tensile stress is formed, causing the cylinder to split into two halves along the vertical plane. This splitting is caused by the indirect tensile stress, and the tensile stress that results in the splitting is denoted as “ft”. The compressive load at the point of failure is represented as “P”, while the cylinder length is denoted as “L” and the cylinder diameter as “D”.
2.2 Flexure Test
The flexure test guidelines outlined in BS 1881: Part 118: 1983 specify the procedure for testing a concrete beam specimen with dimensions of 15x15x75 cm. The span of the beam must be three times its depth, as shown in the accompanying figure. During the test, equal loads are applied at one-third of the distance from each end support, resulting in equal reactions at the supports. It is important to note that the stress experienced at the bottom beam fiber increases with the application of load, and this increase occurs at a rate of either 0.02 MPa or 0.10 MPa, depending on the specific conditions of the test.
Fig.2: Experimental Arrangement for Flexural Strength Test
The modulus of rupture is a measure of the tensile strength of concrete and is used to determine the theoretical maximum tensile stress at the bottom face at the point of failure. In the case of low strength concrete, a lower rate is used during testing, while for high strength concrete, a higher rate is employed. The modulus of rupture is estimated to be approximately 1.5 times the tensile stress determined through the splitting test. The splitting test is a common method used to assess the tensile strength of concrete, and the modulus of rupture provides an indication of the concrete’s ability to resist cracking or failure under tension. By calculating the modulus of rupture, engineers and construction professionals can better understand the performance characteristics of concrete in different strength ranges and ensure that appropriate design and construction practices are employed accordingly.
3. Core Strength Test
When evaluating the strength of concrete in a finished structure, cylindrical cores are commonly used. These cores are obtained by using a rotary cutting tool to extract a cylindrical piece from the structure. After the core is removed, it is soaked, capped, and subjected to compression testing to determine the strength of the concrete in the actual structure.
It is important to note that the strength of the concrete in the core can be influenced by both the ratio of core height to diameter and the location from which the core was taken. The strength at the top surface of the element is typically the lowest and increases as the depth within the element increases.
A standard cylinder test can be achieved by using a core height-to-diameter ratio of 2. This ratio is often used as a benchmark for testing the strength of concrete cores. Overall, core sampling and testing of concrete is an essential practice for assessing the strength and quality of concrete in a finished structure.
Non-Destructive Tests on Concrete
Hardened concrete is subject to various non-destructive strength tests to ensure its durability and reliability. The most common tests used for this purpose include the rebound hammer test, ultrasonic pulse velocity test, pull-out test, and penetration resistance test.
The rebound hammer test, also known as the hardness test, is a widely used technique to measure the surface hardness of concrete. It involves striking the concrete surface with a spring-loaded hammer and measuring the rebound distance of the hammer. This test is useful in assessing the surface strength of concrete.
The ultrasonic pulse velocity test is a non-destructive technique that uses ultrasonic waves to determine the velocity of sound in concrete. By measuring the time taken for the waves to travel through the concrete, this test can estimate the concrete’s density, uniformity, and compressive strength.
The pull-out test involves the insertion of a steel rod into the concrete and applying a tensile force to it. This test measures the strength of the bond between the concrete and the steel rod. It is commonly used to assess the strength of concrete in structures such as bridges, dams, and other large-scale infrastructure.
The penetration resistance test is a non-destructive method that involves applying a load to the concrete surface using a mechanical or hydraulic device. The depth of penetration is then measured, and the results are used to estimate the concrete’s strength and durability.
Other non-destructive tests, such as the maturity method, impact echo test, and spectral analysis of surface waves (SASW), are also used to evaluate the strength and durability of concrete. These tests help ensure that the concrete used in construction projects meets the required strength and durability standards, ensuring the safety and longevity of the structures.
1. Rebound hammer (hardness) test.
The rebound hardness test is a method used to determine the strength of concrete, and it involves using a tool known as the Schmidt hammer. This hammer is held against the surface of the concrete, and a spring-driven metal mass is used to strike it. The resulting rebound is then measured on a scale, which provides an indication of the concrete’s strength.
The basic principle behind the test is that the strength of the concrete affects the amount of rebound produced. Specifically, the higher the strength of the concrete, the greater the rebound will be. As a result, a higher rebound number on the scale indicates a stronger concrete.
The Schmidt hammer is a commonly used tool in the construction industry, and it provides a simple and quick way to assess the strength of concrete on site. By using this tool, construction professionals can determine whether the concrete they are working with is strong enough to meet the requirements of the project, or whether it needs to be reinforced or replaced.
2. Ultrasonic pulse velocity test
The ultrasonic pulse velocity test is a non-destructive technique used to determine the strength of a concrete section. It involves measuring the velocity of ultrasonic pulses as they pass through the concrete from a transmitter to a receiver. This velocity is then compared to a correlation chart to determine the concrete’s strength. Essentially, the higher the velocity of the ultrasonic pulses, the stronger the concrete is likely to be. This method is particularly useful as it allows for an estimation of concrete strength without causing any damage to the structure.
3.Pull Out Test
The pull-out test is a method used to measure the force needed to extract a steel rod that has been embedded in hardened concrete, with the steel being cast into a specially shaped form within the concrete. This test is performed to evaluate the bonding strength between the steel reinforcement and the surrounding concrete matrix. The force required to pull out the steel rod from the hardened concrete is measured, and this value provides valuable information about the performance and durability of the concrete structure. The pull-out test is a widely used technique in civil engineering and construction industry to assess the quality and integrity of concrete structures, such as reinforced concrete beams, columns, and slabs. By determining the force needed to pull out the steel rod from the hardened concrete, engineers and construction professionals can ensure that the concrete structure is designed and constructed to meet the required strength and safety standards.
Fig.3: Pull out Test Testing Machine & Concrete Core is Pulled
When pulling out steel, a cone-shaped piece of concrete with a slope of 45 degrees is used. The force needed to extract the concrete is dependent on the compressive strength of the concrete.
4. Penetration Resistance Test
Penetration resistance tests can be performed on concrete to assess the relative strength of different structures or compare the strength of concrete within the same structure. It is important to note that the equipment used for these tests cannot provide absolute values of strength and should not be relied upon to do so.
To conduct these tests, ASTM C-803 provides a standard test method called “Penetration Resistance of Hardened Concrete”. This method outlines the procedure for measuring the penetration resistance of concrete using a device known as a Schmidt hammer.
By striking the concrete surface with the hammer and measuring the rebound, an estimate of the concrete’s compressive strength can be made. However, it is important to recognize that the results of penetration resistance tests are only indicative of relative strength and should be used in conjunction with other test methods for a more comprehensive assessment of concrete strength.
5. Other non-destructive tests
Equipment has been specifically designed to accurately measure various properties of concrete. This includes crack widths and depths, which are important indicators of the health and integrity of the concrete structure. By measuring these properties, engineers and construction professionals can identify areas of concern and take appropriate action to address them before they worsen.
In addition, equipment has been developed to measure the water permeability and surface dampness of concrete. These factors can greatly impact the durability and longevity of the structure, as excess moisture can lead to corrosion and other forms of deterioration. With accurate measurements of water permeability and surface dampness, professionals can take proactive steps to prevent these issues from occurring or worsening.
Another important aspect of concrete health is the depth of cover and the location of reinforcing bars. Reinforcing bars provide additional strength to concrete structures, but they must be properly placed and secured to be effective. By measuring the depth of cover and the location of reinforcing bars, engineers and construction professionals can ensure that the concrete structure is properly reinforced and can withstand the necessary loads and stresses.
Finally, equipment has been developed to measure the electrochemical potential of reinforcing bars. By analyzing the electrochemical potential, professionals can determine the presence of corrosion and take appropriate steps to prevent or address it. This is crucial for maintaining the long-term health and durability of concrete structures, as corrosion can weaken the reinforcement and lead to structural failure.
Chemical Tests on Hardened Concrete
There are various chemical tests available that can measure different properties of concrete. One of these properties is the depth of carbonation, which can be determined through a range of chemical tests. Additionally, the cement content of the original mix can also be measured using specific chemical tests.
Another important property of concrete that can be measured through chemical testing is the content of salts such as chlorides and sulfates. It is crucial to determine the content of these salts, as they can react and cause the concrete to disintegrate or even cause corrosion of the reinforcement. Therefore, testing for the presence of these salts is essential for maintaining the durability and longevity of concrete structures.
In addition to the above properties, the alkali content of concrete can also be measured through various chemical tests. The alkali content is an important factor to consider, as high levels of alkalis can cause damage to concrete by reacting with certain types of aggregates. Therefore, testing for the alkali content of concrete is necessary to ensure the stability and durability of concrete structures.