Bond Strength of Concrete With Rebar

Bond Strength of Concrete Developed with Reinforcing Steel (ASTM Designation: C 234)

Purpose To determine the bond strength between a concrete and deformed steel reinforcing bars due to the adhesion of the paste to the steel, the friction between the steel and the concrete, and the bearing of the concrete against the lugs of the deformed steel bars.

Equipment and Materials • Two types of molds are required for the preparation of 15-cm (6-inch) molds for both vertical embedded bars and for top and bottom horizontally embedded bars. Molds for both types of specimens shall be constructed of watertight, easily assembled and disassembled steel forms at least 6 mm (¼ inch) thick, as shown in Figure 24. • 2 dial gages graduated to 0.25 mm (0.001 inch) with a range up to at least 13 mm (0.5 inch) with appropriate brackets • Suitable testing apparatus that can support the specimen on a machined steel bearing plate with dimensions no less than the cross section of the specimens and able to accommodate the reinforcing steel • 15 cm × 15 cm (6 in. × 6 in.) pieces of plywood with a center hole to permit the reinforcing steel to pass through at least 6 mm (¼ in.) thick to act as a cushion between the concrete specimen and the steel bearing plate of the testing apparatus • Three 15 cm (6 in.) diameter × 30 cm (12 in.) molds for determining the 28-day compressive strength of the concrete • No. 6 (19 cm) deformed steel reinforcing bars (RE-bars) ≥ 60 cm (2 ft) in length • All of the equipment and materials used in ASTM Designation: C 192, “Making and Curing Concrete Test Specimens”

Test Procedure The testing apparatus described herein differs from the one shown in the ASTM Method, although it is not presented as the mandatory piece of equipment. Furthermore, in the ASTM Method, the measurement of the embedded steel slippage from the concrete block is based on the movement of the loaded steel end minus the elongation due to the tension that the affected length of bar is undergoing. In the procedure described below, the measurements of the movement of the steel bar during the test are with relation to the nonloaded end of the bar and only one measuring gauge can be used for this purpose. The other measuring gauge is used to record the elongation of the RE-bar to assure that it has not exceeded the yield point of the steel. This is based upon the experience of the author. There is then no need to deduct any elongation of the RE-bar to obtain the net slippage of the RE-bar in the concrete. The sketches and pictures on pages 120 and 121 show how the procedure is carried out. 115

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Engineered Concrete: Mix Design and Test Methods, Second Edition

6 in. (150 mm)

A

A

6 in. (150 mm)

C

B

6 in. (150 mm)

6 in. (150 mm)

Side plates 1 in. (6 mm) 4 thick

Section A-A

B

Base plates 3 in. (10 mm) 8 thick

12 in. (300 mm) 6 in. (150 mm) 6 in. (150 mm)

6 in. (150 mm)

Caulk with small rope or rubber ring

C

Detail X

Detail X Triangular grooves 1 in. (13 mm) 2 deep

3 8

in. (10 mm) thick

Section B-B

View C-C

Figure 24 Molds for bond specimens. Copyright ASTM. Reprinted with permission.

1. Prior to casting the specimens, the insides of the steel molds should be coated with a thin film of mineral oil. Clean the RE-bars of rust and mill scale with a stiff wire brush. If there is any oil or grease on the RE-bars, carefully clean it off with a suitable solvent. A test set should consist of 3 vertical bar forms and 3 horizontal bar forms. Each of the latter will have a top horizontal and a bottom horizontal bar, for a total of nine test specimens for bond. Center and align the RE-bars in the forms. After the RE-bars are assembled in the forms, make the joints mortar tight by applying a suitable caulking compound on the outside of the forms, between the RE-bars and the forms. If the same concrete mixture has not previously been used for a compression test determination, a set of 3 specimens should also be prepared to find the 28-day compression strength, f ′c.



2. Using the same procedure as described in ASTM Procedure: C 192, mix the concrete and check and record the slump and the air content.



3. Fill the forms with two incremental layers of concrete for the vertically embedded RE-bars and in four layers for the horizontally embedded bars, tamping each lift with the tamping rod 25 times. If the concrete has a very low slump