BOND-Test | GERMANN

Description

Principle

In the Bond Pull Off Test, a disc is bonded to a prepared testing surface and the disc is pulled off after a partial core has been cut around the disc (extreme left in following figure). The pull-off force, F, is divided by the cross-sectional area of the partial core to obtain the pull-off strength fp:
where d is the diameter of the partial core.
The types of failures that can occur in a BOND-TEST are illustrated above: (a) failure in the substrate indicates that the bond strength is greater than the tensile strength of the substrate; (b) failure at the interface provides a measure of the tensile bond strength between the overlay and the substrate; and (c) failure in the overlay indicates that the bond strength is greater than the tensile strength of the overlay. During a test, it is very important that negligible bending is introduced to the disc by the loading system. Otherwise, low and erratic test results will be obtained.

Failure type (a) is preferred because it shows that the bond strength of the overlay is greater than the tensile strength of the substrate. Note that failure occurs at the weakest link of the composite system, and one cannot predict which type of failure will occur. Only tests results with the same type of failure should be averaged in calculating the average pull-off strength.

The nature of the BOND-TEST has been investigated by finite element analyses (see Petersen, C.G., Dahlblom, O. and Worters, P., “Bond-Test of Concrete and Overlays,” Proceedings, International Conference on NDT in Civil Engineering, University of Liverpool, U.K., 1997; Bungey, J.H. and Madandoust, R., Factors influencing pull-off tests on concrete,” Mag. of Concr. Res., 1992, 44, No. 158) ). Failure in the BOND-TEST using a 75-mm disc was predicted to occur at a displacement of 0.02 mm to 0.03 mm and the nominal stress in the partial core before rupture is about 3 % lower than the uniaxial tensile strength of the substrate concrete.

Variability

For 75-mm discs, the coefficient of variation of replicate test results is about 8 to 10 % on concrete with a maximum aggregate size of 38 mm. For 50-mm discs, the coefficient of variation is 14 to 16 %.

The BOND-TEST procedure

1. Surface planing
The surface is ground with a diamond studded planing tool to expose the aggregates and to obtain a plane surface. The center knob that remains is removed with a separate wheel grinder. The dry surface is steel brushed and any dust or powder is blown away. The suction plate is used to stabilize the planing tool. Note that this operation is done without using cooling water.

2. Bonding the disc
A clean disc is bonded to the prepared surface using a rapid-curing adhesive (GRA). The GRA adhesive has a tensile strength of 10 MPa when fully cured, which takes 2 to 5 minutes at normal temperatures. The progress of hardening is observed in the small cup in which the two-component GRA was mixed. In cold weather conditions, the concrete surface and the disc are heated with a heat gun to accelerate curing of the adhesive.

3. Partial coring
A partial core is cut perpendicular to the surface using cooling water; the bonded disc serves as a drill guide (the inner diameter of the coring bit is slightly larger than the disc diameter) . The partial core is cut with the CORECASE. For tests to measure bond strength, the core is cut to a depth of 25 mm into the substrate or one-half of the core diameter, whichever is greater; for tensile strength of the substrate, cut to a depth of 25 mm.

4. Pull-off
The disc is loaded in direct tension at a controlled rate using a calibrated hydraulic pull machine. The machine, which is the same as used for pullout tests, bears against a circular counter pressure ring positioned centrally on the planed surface. The peak force in kN is recorded and used to obtain the pull-off strength by dividing by the cross-sectional area of the partial core. The type of failure, (a), (b) or (c), as shown on the previous page, is recorded.

The procedure and special equipment used for the BOND-TEST ensure that the disc is loaded in direct tension without bending. Bending may lower results by 20 to 50 %. The discs have sufficient stiffness to avoid distortion during testing. By bonding a clean disc on a planed, dry surface with the GRA adhesive, failure should not occur at the disc/overlay interface. Failure at the disc/overlay interface is an inconclusive test and must be repeated if the bond strength is to be evaluated.