Concrete’s tensile strength is a critical property that has a significant impact on the size and extent of structural cracking. Despite its brittle nature, concrete’s weak tensile strength means that it cannot withstand direct tension, resulting in cracking when subjected to tensile forces that exceed its tensile strength. Therefore, determining the concrete’s tensile strength is essential to identify the maximum load that concrete members can handle before cracking. One method of determining the concrete’s tensile strength is through the splitting tensile strength test on concrete cylinders, which follows the ASTM C496 (Standard Test Method of Cylindrical Concrete Specimen) protocol, similar to other codes such as IS 5816 1999. In the following sections, we will discuss various aspects of the split cylinder test on concrete specimens.
Apparatus for Splitting Tensile Test of Concrete
Testing Machine
To meet the requirements for testing machines, there are three specific criteria that must be met. The first requirement is that the machine must adhere to the guidelines set forth in Test Method C 39/C 39M. This ensures that the machine meets the necessary standards and can produce reliable results.
The second requirement is that the testing machine must be capable of applying loads continuously and without shock. This is important because it ensures that the load is applied uniformly and consistently to the specimen being tested, which is essential for obtaining accurate results.
The third requirement is that the machine must be able to apply loads at a constant rate within the specified range. Specifically, the load should be applied at a rate between 0.7 to 1.4 MPa/min (or 1.2 to 2.4 MPa/min according to IS 5816 1999) for splitting tensile stress until the specimen fails. This range ensures that the load is applied at an appropriate and consistent rate, which is necessary for accurate and reliable results.
Fig.1:Split cylinder testing machine
Plate or Supplementary Bearing Bar
When testing the cylinder, there are certain conditions in which a specific type of plate needs to be employed. This is necessary when the diameter or the largest dimension of either the upper bearing face or the lower bearing block is less than the length of the cylinder that is being tested. In such cases, a plate with a width of 50mm is used.
It is important to ensure that when using this plate, the load is applied over the entire length of the specimen. This means that the plate must be used in a manner that allows for the load to be distributed evenly across the entire length of the cylinder during testing. Failure to do so may result in inaccurate test results, which can be detrimental to the overall testing process.
Fig.2: Supplementary steel bar
Bearing Strips
In materials testing, it is common to use two bearing strips as part of the setup. These strips serve as a buffer between the specimen being tested and the upper and lower bearing blocks of the testing machine, or any supplemental bars or plates used in the setup.
The bearing strips are typically 3.2 mm thick and 25 mm wide, with a length that matches or slightly exceeds that of the specimen. This ensures that the entire surface of the specimen is supported by the strips, preventing any undue stress or damage during the testing process.
By using bearing strips in materials testing, researchers can ensure that their results are accurate and reliable. These strips help to distribute the load evenly across the specimen, reducing the risk of deformation or failure during the test. This is particularly important when working with delicate or brittle materials, where even slight variations in pressure can have a significant impact on the outcome of the test.
Fig.3:Bearing strip, Plywood
Sampling of Concrete Cylinders
Concrete specimen moulds
A steel mould, with a thickness of 3 mm, is required to be constructed. The design should allow for longitudinal opening of the mould, which will make it easier to remove the specimen. Additionally, a means of keeping the mould closed during use should be included.
The internal diameter of the mould should have a mean value of 15 cm, with a tolerance of ± 0.2 mm. Meanwhile, the height of the mould must be 30 cm, with a tolerance of +/- 0.1 cm.
A metal base plate mould will be provided to accompany the mould. Prior to usage, a thin layer of mould oil should be applied to the surface of the mould. This will prevent adhesion of concrete, which can be problematic during the removal process.
Fig.4:Cylindrical mould
Tamping Rod
The tool being referred to is a steel rod that is utilized for manual compaction of concrete specimens. This rod is required to be round and straight, with a diameter of 16 mm and a length of 600 mm. The tamping end of the rod must be rounded to a hemispherical tip of the same diameter as the rod. Additionally, the other end of the rod may also be rounded if preferred. These specifications ensure that the tamping rod is effective for its intended purpose of compacting concrete specimens.
Fig.5:Tamping rod
Concrete pouring and compaction
The process of preparing the mixture involves pouring it into an oiled mould in layers that are approximately 5 cm thick. Each layer is then compacted using either manual or vibration methods. If manual compaction is used, a tamping bar is required. It is important to distribute the bar stroke uniformly to ensure proper compaction. The minimum tamping bar stroke for each layer should be 30, and the strikes should penetrate the underlying layer. The rode should be applied for the entire depth of the bottom layer, and once the top layer is complete, compaction should be finished. To achieve a level surface, a trowel should be used, and the top of the mould should be covered with a glass or metal plate to prevent evaporation.
Fig.6:concrete specimen
Curing of Specimen
The casted specimen needs to be stored at a specific temperature for a certain period of time. It should be kept in a place where the temperature is maintained at 27° +/- 2°C for precisely 24 +/- 0.5 hours starting from the time when water is added to the dry ingredients. Once this period has passed, the specimen should be marked and taken out of the mould. It is recommended to immediately submerge the specimen in either clean fresh water or a saturated lime solution. The specimen should remain submerged until it is taken out just before the test.
It is important to renew the water or solution in which the specimens are kept every seven days. The temperature of the water or solution should also be maintained at 27° +/- 2°C. For design purposes, the specimen needs to be cured for a total of 28 days before the test can be conducted.
During the test, it is required to cast and test three specimens for each reading. The average tensile strength will be taken based on the results obtained from the three specimens. Following these guidelines will help to ensure accurate and reliable test results.
Fig.7:curing concrete specimen
Procedure of Splitting Tensile Test
To estimate the tensile strength of a wet concrete specimen, it is first necessary to remove it from the water after it has cured for a desired period of 7, 28 days or any other preferred age. Once removed, the surface of the specimen should be wiped dry.
To ensure that the specimen is in the correct position, diametrical lines should be drawn on both ends of the specimen to confirm that they are on the same axial plane. The weight and dimensions of the specimen should also be recorded.
The compression testing machine should be set within the required range. A plywood strip should be placed on the lower plate, followed by the specimen, which should be aligned so that the marked lines on the ends are vertical and centered over the bottom plate. Another plywood strip should be placed above the specimen.
The upper plate should be slowly brought down until it just touches the plywood strip. The load should then be applied continuously, without shock, at a rate of 0.7 to 1.4 MPa/min (or 1.2 to 2.4 MPa/min according to IS 5816 1999).
Finally, the breaking load of the specimen (P) should be noted down.
Fig.8:testing cylindrical concrete specimen
Calculations
The task requires the calculation of the splitting tensile strength of a specimen using a formula that involves several variables. The splitting tensile strength, denoted by T and measured in megapascals (MPa), can be calculated using the formula T=2P/piLD. In this formula, P represents the maximum load applied to the specimen as indicated by the testing machine, measured in Newtons (N). The variable D denotes the diameter of the specimen, measured in millimeters (mm), while L represents the length of the specimen, also measured in millimeters.
To calculate the splitting tensile strength using this formula, one needs to know the values of P, D, and L. Once these values are known, the formula can be applied to determine the value of T, which represents the splitting tensile strength of the specimen.
Fig.9:dimension of split cylinder specimen and imposed loads
Report
Report the following information:
- Identification number
- Diameter and length, mm
- Maximum load, N
- Splitting tensile strength calculated to the nearest 0.05 MPa
- Estimated proportion of coarse aggregate fractured during test
- Age of specimen
- Curing history
- Defects in specimen
- Type of fracture
- Lastly, type of specimen
Result
Splitting tensile strength of given concrete =……………….N/mm²