HOW TO DETERMINE OF BOND STRENGTH BETWEEN STEEL & CONCRETE? [PULL OUT TEST]

Theory

The bond strength is the measure of the effectiveness of the grip between concrete and steel and has no standard quantitative definition. In pull out tests on plain bars, the maximum load generally represents the bond strength that can be developed between concrete and steel. With plain bars the maximum load is not very different from the load at the first visible slip, but in the case of deformed bar, the maximum load may corresponds to a large slip which may not be obtained in practice before other types of failure occur. It is preferable therefore when comparing plain and deformed bars to determine not only the maximum load but also the load at the arbitrary amount of slip and also plot the complete load slip curves for the plain and deformed bars under comparison. One such basis of comparison is the load at a relative movement (slip) between steel and concrete of 0.125 mm at the free end of the bar in a pull out test.

Objective

To determine the bond strength between ordinary mild steel bars and cement concrete.

• In a reinforced concrete beam with overlapping reinforcement
• By pullout test

Apparatus

• Beam mould (100 mm x 150 mm x 1830 mm)
• Cylinder mould (dia = 150 mm & height = 300 mm)
• Aligning stand
• 50 KN (5 tons) transverse testing machine or a loading reaction frame
• 100 KN (10 tons) universal testing machine with pullout test arrangement
• Dial gauges
• Tamping bar
• Trowels
• Graduated jar
• Platform weighing balance
• Concrete mixer
• Table vibrator

Procedure

1. Prepare 1:2:4 concrete mix and having water cement ratio equal to 0.6. for this take 17 kg cement, 34 kg sand, 68 kg coarse aggregate and 10.2 kg of water.
2. Cast the concrete beam of size 100 m x 150 mm x 1830 mm. using two 10 mm bars having an effective cover of 12.5 mm and overlapping of 200 mm in the middle. The four pieces of bars will have hooks at one end only. Cast three cubes along with the beam for finding crushing strength of concrete.
3. Cast three cylinders upto a height of 200 mm with a 10 mm dia rod placed at the centre of cylinder.
4. Remove the specimens from the moulds after 24 hours and put them in water. Test the beam after 27 days of wet curing applying the load gradually on 50 KN transverse testing machine or on a loading reactions frame. Test the three cubes in compression.
5. Perform the pull out test on 100 KN universal testing machine using pull out test attachment at 28 days age.
6. Attach a dial gauge for finding out the slip between steel and concrete and plot the curve between load and slip.
7. Note the load at 0.125 mm slip and at bond failure.

Observation & Calculation

Record observations in the table as given below.

R.C. Beam Test
Load at first crack (N)
Ultimate load at bond failure, P (N)
Maximum B.M. at first crack (Nm)
Maximum B.M. at failure (Nm)
Actual effective depth (mm)
Crushing Strength of concrete (N/mm2)
Ultimate bond stress

 

Calculate ultimate bond using the following formula

σ_bu = P_b / (π x d x l)

where,

σ = ultimate bond stress

P_b = Bond failure load

d = dia of bar

l = length of bar (in this case it is 200 mm)

Pull Out Test
Load at 0.125 mm slip (P1)
Load at bond failure (P2)
Average bond strength at 0.125 mm slip (N/mm2)
Average bond strength at failure (N/mm2)

 

Average bond strength at 0.125 mm slip = P₁ / (π*d*l)

Average bond strength at failure = P₂ / (π*d*l)

Precautions

• Use an aligning stand while casting the cylindrical specimen, to keep the bars in vertical positions.
• The beam and cylinder specimens would be tested just after removing from curing tank.