Bond between concrete and steel is an important parameter. As concrete grows old, the bonding generally deteriorates. In this article, the method to conduct pull out test to estimate the bond strength is discussed.
In the case of plain rebars, the maximum load almost same as the instant for first slip, while in the case of deformed bar, the load can increase even when there is a large slip. Therefore, it is a good practice to compare the plain and deformed bars in the same experiment.
The specimen size to test should be a cube of :
100 mm side in case of rebar with dia 12 mm or smaller
150 mm side in case of rebar over dia 12 mm
225 mm side in case of rebar over dia 25 mm
The testing rebar should pass though the centre of the cube. On one side, the bar should be long enough to put in the pull out apparatus (for grip) while on the other side it should extend about 1 cm beyond the concrete face. The concrete cube should be reinforced with a helix of 6 mm diameter plain mild steel bar at 25 mm pitch with diameter equal to the size of the cube. If this is not provided, the concrete may fail by splitting.
To control the length of bonding, PVC pipes can be used to separate rebar with concrete. (This is however not mentioned in IS2770)
While testing, load should be applied slowly with displacement not exceeding 1.25mm/min. The loading should be continued until (a) the steel bar yields , (b) concrete has failed or (c) slippage reaches 2.5 mm a the loaded end. Note that the slip in the other end may be different. The bond resistance at 0.025 mm displacement at the free end (slip) should be used for the bond stress calculation.
-  IS 2770 ( Part I ) • 1967 Methods of testing bond in reinforced concrete , Part 1 – Pull out test, Bureau of Indian Standards