Monitoring the early strength gain of concrete in the field is a crucial task for the success of certain construction projects, such as multi-story building construction. This is because it is necessary to start critical shoring and reshoring steps, as well as the removal of formwork, which are all dependent on the strength of the concrete.
To monitor or measure the early strength gain of concrete in the field, there are several methods that can be used. One of these methods is the compressive test for field-cured cylinders. Another method is concrete maturity testing. Additionally, a field-cured cylinder can be attached to forms, and methods such as penetration resistance and pullout testing can be used.
Each of these methods or a combination thereof is utilized to produce compressive strength data that is necessary in the field to control the timing of shoring/reshoring and form removal. This data is critical for ensuring the safety and success of construction projects that require concrete to be utilized.
1. Field-Cured Cylinders
A method for evaluating concrete compressive strength involves casting cylinders in the field according to the ASTM C31/C31 M standard. The cylinders are then cured under the same conditions of temperature and moisture as the concrete they represent. After a specified age, the cylinders are tested in the laboratory in accordance with the ASTM C39/C39M standard to determine the concrete’s compressive strength. This method ensures that the conditions for testing the concrete cylinders are as close to the actual conditions of the concrete as possible.
2. Field-Cured Cylinders Attached to Forms
A method for measuring the early strength of concrete involves the use of special cylinder molds that are attached to forms in the field. This test is conducted according to the Standard Test Method for Compressive Strength of Concrete Cylinders Cast in Place in Cylindrical Molds (ASTM C873/C873M), which is only applicable to slabs where the depth of the concrete ranges from 125 to 300 mm.
The height of the mold used is equivalent to the thickness of the slab, and the specimens are cast and cured under the same conditions as the concrete slab itself. At designated intervals, the samples are removed from their molds and then subjected to testing in order to estimate the concrete’s compressive strength.
By using this technique, the bearing capacity of the slab can be determined, which helps in determining the appropriate time for the removal of strips and shores. This method is particularly useful in ensuring that the concrete has reached its required strength before any further construction work is carried out.
3. Concrete Maturity
To evaluate the compressive strength of concrete, it is necessary to establish a correlation between the results of the concrete maturity test and its strength. This correlation should be established at the outset for the specific mixture of concrete that will be used on the project. To do so, compressive strength cylinders should be tested at different stages of maturity. Once the correlation curve has been established, the concrete strength can be determined with knowledge of its maturity.
To conduct the maturity test, it is necessary to follow the specifications set out in ASTM C 1074. By doing so, it is possible to ensure that the test is conducted accurately and consistently. With the results of the maturity test, and knowledge of the correlation curve for the specific concrete mixture, it is possible to determine the compressive strength of the concrete. This information is critical in ensuring that the concrete is of sufficient strength for the intended application and can help to prevent problems with the construction project down the line.
4. Pullout Tests.
In the field of concrete construction, one of the crucial parameters that need to be evaluated is the compressive strength of the concrete mixture. The strength of concrete is often tested using cylinders or cores obtained from the actual structure being constructed. However, conducting such tests can be time-consuming and impractical, especially when dealing with large structures or structures in hard-to-reach locations. This is where pullout tests come into the picture.
Pullout tests involve inserting a steel rod or a similar instrument into the concrete structure and measuring the force required to pull it out. The results of these tests can be used to evaluate the compressive strength of the concrete in the field. However, to establish a correlation between the compressive strength and the pullout values for a specific concrete mixture, it is necessary to conduct tests on cylinders and pullout specimens at various ages.
This correlation is essential for determining the compressive strength of the concrete mixture being used on the project accurately. ASTM C 900 is a standard test method that specifies the procedure for conducting pullout tests. By following the guidelines set forth in this standard, engineers and construction professionals can obtain reliable data on the compressive strength of the concrete mixture being used on the project. This, in turn, can help ensure that the structure being built is safe, durable, and meets all the necessary specifications and requirements.
5. Penetration Resistance
One way to estimate the compressive strength of hardened concrete in the field is by using the penetration resistance test. This technique involves measuring the resistance of concrete to penetration at different ages, which can then be correlated with the compressive strength of laboratory-tested specimens for all concrete mixtures planned for use on the project.
To use this technique, it is important to develop a correlation between the penetration values and compressive strength at various ages for each type of concrete mixture to be used. By doing so, the penetration resistance results obtained from the field can be easily converted to estimate the concrete strength.
The ASTM C803/C803M standard provides guidelines for conducting the penetration resistance test for concrete. This test can be used to monitor the early strength gain of concrete and assess its suitability for use in construction projects. By measuring the penetration resistance of concrete, engineers can estimate its compressive strength and make informed decisions about its use in various applications. Overall, the penetration resistance test is a useful tool for evaluating the strength and quality of concrete in the field.
Fig. 4: Penetration Resistance Test For Concrete to Monitor its Early Strength Gain