One of the important properties of concrete is “tensile strength” as structural loads make concrete vulnerable to tensile cracking. Tensile strength of concrete is much lower than its compressive strength (that’s why steel is used to carry the tension forces). It has been estimated that tensile strength of concrete equals roughly about 10% of compressive strength. To determine the tensile strength, indirect methods are applied due to the difficulty of the direct method. Noting that the values obtained of these methods are higher than those got from the uniaxial tensile test. These indirect techniques are: 1- split cylinder test and 2- flexural test. In this article, the Splitting Tensile Strength test is discussed.
Splitting Tensile Strength Test
Compression testing machine, two packing strips of plywood 30 cm long and 12 mm wide, moulds, tamping bar (steel bar of 16 mm diameter, 60 cm long), trowel, glass or metal plate
Preparation of Samples
- The sample size is cylinder of diameter 15 cm and height of 30 cm. The mould used is metal with mean internal diameter of the mould is 15 cm ± 0.2 mm and the height is 30 +/- 0.1 cm. The mould should be coated with a thin film of mould oil before use to prevent adhesion of concrete.
- Concrete is placed into the mould in layers of approximately 5 cm thickness. Each layer is compacted either by hand or by vibration. When compacting by hand, the tamping bar is utilized and the stroke of the bar shall be distributed in a uniform way. The number of strokes for each layer should not at least 30. The stroke should penetrate in to the underlying layer and the bottom layer should be rodded throughout its depth.
- After compacting the top layer, the surface of the concrete should be finished level with the top of the mould, using a trowel and covered with a glass or metal plate to prevent evaporation of water.
- Curing: The test specimen should be stored in a place at a temperature of 27° +/- 2°C for 24 hrs. After this period, specimens are removed from the moulds to be submerged in clean fresh water or saturated lime solution for the specified curing period (such as 7 or 28 days). The water or solution should be renewed every 7 days.
Procedure of Splitting Tensile Test:
- After curing, wipe out water from the surface of specimen
- Using a marker, draw diametrical lines on the two ends of the specimen to verify that they are on the same axial place.
- Measure the dimensions of the specimen.
- Keep the plywood strip on the lower plate and place the specimen.
- Align the specimen so that the lines marked on the ends are vertical and centered over the bottom plate.
- Place the other plywood strip above the specimen and bring down the upper plate to touch the plywood strip.
- Apply the load continuously without shock at a rate of approximately 14-21 kg/cm2/minute (Which corresponds to a total load of 9.9 ton/minute to 14.85 ton/minute)
- Write the breaking load (P)
Calculation of Tensile Strength
Range Calculation for testing machine
According to IS456, split tensile strength of concrete = 0.7 * Fck
The splitting tensile strength, Tsp = 2P / π DL
Where P is the applied load, D is the diameter of the specimen and L is length of the specimen
Accordingly P = 0.7 Fck x π DL / 2
Expected load = P x factor of safety
Range to be selected for loading = (— to —)
Splitting Tensile Strength
Tsp = 2P/ pi DL where P here is the actual failure load
Thus, Splitting tensile strength of given concrete =……………….N/mm²
The strength should be the average of three tested specimens.