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To evaluate the tensile strength of concrete, a flexural test is performed on concrete beam. This test checks the ability of unreinforced concrete beam to withstand failure in bending.

The flexural test on concrete can be conducted using 3-point load test.

What are the Applications of Flexural Test on Concrete?

Flexure test is an essential requirement for concrete mix design, it is employed in testing concrete for slab and pavement construction. In some countries, it is an essential requirement for concrete mix design.

Procedure of Flexural Test on Concrete

1. Take out the specimen from curing tank and place on the loading points of flexure testing machine.
2. Center the loading system in relation to the applied force and bring the block applying force in contact with the specimen surface at the loading points.
3. Start applying loads between 2 to 5 percent of the computed ultimate load.
4. Apply load on to the specimen continuously without shock till the point of failure at a constant rate (Indian standard specified loading rate of 400 Kg/min for 150mm specimen and 180kg/min for 100mm specimen, stress increase rate 0.06+/-0.04N/mm2.s).

The loading rate can be computed based on the following equation:

r = Sbd² / L

Where:

r = loading rate

S = rate of increase of extreme fiber

b = average specimen width

d = average specimen depth

L = span length 

Measure the cross section of the tested specimen at both ends and at the center to calculate average depth & height. 

Computation of Modulus of Rupture 

The following expression is used for estimation of modulus of rupture:

MR = 3PL / 2bd² 

Where: 

MR: modulus of rupture

P: ultimate applied load indicated by testing machine

L: span length

b: average width of the specimen at the fracture

d: average depth of the specimen at the fracture

Watch Moulding and Demoulding demonstration of Plastic and Polyurethane concrete moulds on You Tube Channel of Zeal International.

Zeal International is manufacturing Polyurethane beam moulds in India under make in India initiative. Polyurethane beam moulds are durable and lightweight. The are also easy to clean and maintain. These  moulds are rust proof and have longer work life in comparison to traditional moulds. Please visit http://www.zealinternational.com/materialtesting/concrete/moulding-equipment/plastic-beam-mould for more information on Polyurethane Beam Moulds.

To determine the compressive strength of concrete, 6 concrete samples are prepared using cube mould. Freshly prepared concrete is poured into 6 cube moulds and are kept for curing for 7 & 28 days. The preferred size of concrete cube used in concrete testing is 150mm x 150mm x 150mm. On 7th day, 3 concrete samples are extracted from their respective cube moulds and compression test is performed using compression machine. Similar process is repeated on 28th day with remaing 3 concrete cubes. 

When the force is applied on the concrete cube specimens, concrete starts to chip off and finally the specimen cracks. The force at which the specimen fails is noted and compressive strength of concrete cube is calculated. I.e, if the specimen cracks at 500kN force and size of specimen is 150 X 150 x 150 mm, the strength of cube will be (500 x 1000) / (150 x 150) N/sq. mm

Finally, the mean of all 6 concrete cube specimens, is used as the compressive strength of concrete.

Note: Only IS, BS or ASTM cube moulds should be used to prepare concrete samples, so as to maintain accuracy of test results.

To shorten the curing time, cube moulds can be kept in curing tank. Zeal International also supplies curing tanks to it’s customers. Click on the following links to know more about Curing Tank 
or Cube Mould

Concrete Cube Mould preparation for strength testing

Clean the cube mould thoroughly and oil it’s inner walls lightly. Freshly prepared concrete should be added to the mould and compacted with a steel tamping bar, with at least 30 to 35 tamps per layer for a 100 mm cube mould. Slightly overfill the mould and gently trowel the extra concrete to level it with top of mould.

Keep the freshly filled cube mould at a place away from extreme heat or cold. Also, cover the top with a plastic sheet to prevent surface evaporation. After 72 hours, extract the concrete cube from cube mould and place it in the curing tank. After the completion of curing process, concrete cubes are ready for testing.

You can watch a descriptive video for concrete cube mould preparation at: https://youtu.be/Y8q_3q6_o8k 

To know more about cube moulds, visit: http://www.zealinternational.com/materialtesting/concrete/moulding-equipment/cube-mould