Flexural Strength Test of Concrete – Procedure & Calculation

Objective

To determine the Flexural Strength of Concrete, which is crucial when road slabs experience inadequate sub-grade support, wheel loads, or volume changes due to temperature and shrinkage.

Reference Standard

• IS: 516-1959 – Methods of tests for strength of concrete

Equipment & Apparatus

Equipment	Specifications
Beam Mould	- 15 x 15 x 70 cm (for aggregates < 38 mm) - 10 x 10 x 50 cm (for aggregates < 19 mm)
Tamping Bar	- 40 cm long, weighing 2 kg - Tamping section: 25 mm x 25 mm
Flexural Test Machine	- Two steel rollers (38 mm diameter) as supports - Rollers spaced at 60 cm (for 15 cm specimens) or 40 cm (for 10 cm specimens) - Load applied through two additional rollers at third points (20 cm or 13.3 cm apart)

Test Procedure

1. Specimen Preparation
• Fill the concrete into the mould in three layers of equal thickness.
• Tamp each layer 35 times using the tamping bar.
• Distribute tamping uniformly across the beam mould's cross-section.
2. Test Setup
• Clean the bearing surfaces of the supporting and loading rollers.
• Remove loose sand or debris from the specimen surface.
• Use steel rollers (38 mm diameter) for support and loading points.
• Place the specimen in the testing machine, aligning the longitudinal axis perpendicular to the rollers.
• Ensure correct roller spacing:
• Outer rollers: Distance = 3d
• Inner rollers: Distance = d (equally spaced)
3. Testing Process
• Test specimens immediately after removal from water while still wet.
• Apply load at a rate of:
• 400 kg/min (for 15 cm specimens)
• 180 kg/min (for 10 cm specimens)

Calculation of Flexural Strength

The modulus of rupture (fb) is calculated using:

Case 1: When a > 20.0 cm (for 15 cm specimen) or a > 13.0 cm (for 10 cm specimen)

Fb = p⋅l/b⋅d²

Case 2: When a < 20.0 cm but > 17.0 cm (for 15 cm specimen) or a < 13.3 cm but > 11.0 cm (for 10 cm specimen)

Fb = 3⋅p⋅a/b⋅d²

Where:

• a = Distance between the fracture line and the nearest support (cm)
• b = Specimen width (cm)
• d = Depth at failure point (cm)
• l = Supported length (cm)
• p = Maximum applied load (kg)

Reporting Results

• Flexural strength should be reported to two significant figures.

Safety & Precautions

✅ Wear hand gloves and safety shoes during testing.
✅ Switch off the machine after testing.
✅ Keep all metal parts greased to prevent corrosion.
✅ Ensure guide rods are firmly fixed to the base and top plate.
✅ Thoroughly clean all equipment before and after testing.

📌 Read More: For an in-depth guide on concrete strength tests, check out our detailed article here.

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