Split Tensile Strength Test of Concrete as per Indian Standard (IS 5816:1999)

 

Split Tensile Strength Test of Concrete as per Indian Standard (IS 5816:1999)

Aim
To determine the split tensile strength of cylindrical concrete specimens at a specified age using the indirect method described in IS 5816:1999.

Apparatus

1. Testing Machine: Compression testing machine (CTM) capable of applying a steady load.
2. Cylindrical Specimens: Standard dimensions of 150 mm diameter x 300 mm height.
3. Wooden Strips or Packing Materials: For uniformly distributing the load along the length of the cylinder.
4. Measuring Scale: To measure the dimensions of the cylinder.
5. Weighing Balance: To weigh the concrete before casting the cylinders.
6. Tamping Rod: For compacting the concrete in the molds.

Theory The split tensile strength test is an indirect method of determining the tensile strength of concrete. Concrete is known to be much weaker in tension than compression. This test involves loading a concrete cylinder along its length, causing tensile stress to develop perpendicular to the loading direction. The concrete eventually splits, and the tensile strength is determined from the load at failure.

The tensile strength is calculated using the formula:

$$\text{Split Tensile Strength} = \frac{2P}{\pi DL}$$

Where:

• $P$ = Load at failure (N)
• $D$ = Diameter of the cylinder (mm)
• $L$ = Length of the cylinder (mm)

Procedure

1. Preparation of Cylindrical Specimens:

   • Prepare the concrete mix as per the mix design (e.g., M25).
   • Fill the concrete into cylindrical molds in three layers, compacting each layer with the tamping rod by giving 25 strokes.
   • After filling, level the top surface of the cylinder and keep the molds undisturbed for 24 hours.
   • After 24 hours, demold the cylinders and immerse them in water for curing at a temperature of 27°C ± 2°C.

2. Testing Procedure:

   • After the specified curing period (usually 7 days or 28 days), remove the cylindrical specimens from the curing tank.
   • Measure the diameter and length of the cylinder.
   • Place the cylinder horizontally between the compression testing machine’s loading plates with the wooden strips on both sides to ensure uniform distribution of load along the length.
   • Apply the load continuously without shock at a constant rate until the specimen splits along its length. Record the load at failure.
   • Calculate the split tensile strength using the formula.

3. Observation Table

Sample No.	Curing Period (Days)	Diameter (mm)	Length (mm)	Failure Load (N)	Split Tensile Strength (N/mm²)
1	7	150	300	150000	2.12
2	28	150	300	250000	3.54
3	28	150	300	240000	3.40

Sample Readings

• Diameter of Cylinder: 150 mm
• Length of Cylinder: 300 mm
• Failure Load: 150 kN
• Split Tensile Strength: $$\text{Split Tensile Strength} = \frac{2 \times 150000}{\pi \times 150 \times 300} = 2.12 \, \text{N/mm}^2$$

Reporting the Results

• Report the split tensile strength of the concrete cylinder at both 7 days and 28 days.
• Take the average value from at least three specimens for each test period.
• Example:
  • Split tensile strength at 7 days: 2.12 N/mm²
  • Split tensile strength at 28 days: 3.54 N/mm²

Results should be compared with the target tensile strength for the mix design, such as M25 concrete, where a typical 28-day split tensile strength is around 3.0 - 3.5 N/mm².

Conclusion The split tensile strength test is essential for understanding the tensile properties of concrete, which plays a crucial role in concrete’s resistance to cracking under tensile stress. In this test, the concrete achieved a 28-day split tensile strength of 3.54 N/mm², which is in line with the expected values for M25 concrete. This indicates that the concrete possesses satisfactory tensile strength, which is vital for the overall durability and structural integrity of reinforced concrete members.