Scope of Application

This instrument is suitable for testing and comparing the bond strength between different types of concrete and the same reinforcing bars. When used in conjunction with 30-ton, 60-ton, or 100-ton universal testing machines, it measures the bond strength of concrete and assesses the concrete’s ability to resist rebar slip, conforming to GB/T50081-2002 “Standard Test Methods for Mechanical Properties of Ordinary Concrete” and SD105-82 “Code for Testing of Hydraulic Concrete.” The instrument adopts high‑precision smart meters in accordance with the new standards and utilizes a computer‑based data acquisition and processing system throughout the entire production and experimental process, offering advantages such as simple operation and highly accurate experimental data.

Technical Parameters

1. Specimen size: 150×150×150 mm

2. Dial indicator: 0–1 ± 0.001 mm

3. External Dimensions: 589 × 250 × 150 mm

4. Weight: 24 kg

Basic Principles

This test was conducted using the pull-out method. During the pull-out process, the distribution of bond stress is illustrated in Figure 1. Let L denote the perimeter of the rebar’s cross‑section, a the embedment depth of the rebar, and p the applied load. The bond stress per unit area between concrete and rebar is assumed to be uniformly distributed and is calculated according to the following formula: rn = p/L·a. When threaded reinforcing bars are used, the perimeter of the cross‑section is calculated based on the outer diameter. In pull-out tests involving threaded reinforcing bars and concrete, the concrete typically cracks longitudinally first, or the rebar reaches yield before failure—resulting in the inability to determine the ultimate bond strength. In this test, the bond strength was determined as the average of several bond stresses measured when the rebar and concrete experienced relative slip of 0.01 mm, 0.05 mm, and 0.10 mm at their free ends. As for smooth round reinforcing bars and concrete, the rebar often pulls out of the concrete; under such conditions, the ultimate bond strength can be accurately measured.

Instructions for Use

1. Cast the reinforcing steel and concrete together, then cure the composite. 2. Install the dial indicator on the end of the reinforcing steel that protrudes slightly less above the concrete, ensuring it is securely fastened. 3. Place the test specimen on the device, with the longer end of the reinforcing steel—extending above the concrete—passing through the central hole at the bottom of the device. 4. Mount the device onto the universal testing machine.

Test Procedure

⑴ The threaded reinforcing bars used in the test shall meet the requirements for concrete bond strength specified in “Hot‑Rolled Steel Bars for Reinforced Concrete Structures” (GB1499–91), with a calculated diameter of 20 mm (inner diameter 18 mm, outer diameter 22 mm). To ensure sufficient length for clamping by the universal testing machine and for mounting the measuring instruments, a standard length of 500 mm is generally recommended. The dimensions, shape, and thread pattern of the reinforcing bars must be identical. Before forming, the reinforcing bars must be thoroughly cleaned with a wire brush and wiped with acetone; no steel chips or oil residues are permitted. The top surface of the free end of the reinforcing bar shall be smooth and flat, with a concrete bond strength that matches the pre‑reserved recesses in the test mold. When absolutely necessary, smooth, round reinforcing bars with a diameter of 20 mm may also be used instead. Note: The calculated diameter of threaded reinforcing bars is equivalent to the diameter of smooth, round reinforcing bars having the same cross‑sectional area.

⑵ Concrete mixing shall be carried out in accordance with the provisions for cube confining strength tests. Six specimens shall be prepared for each test age. When installing the reinforcement, the free ends shall be embedded into the mold walls; the holes in the mold walls through which the reinforcement is passed shall be sealed with rubber rings and retaining rings to secure the reinforcement and ensure a watertight seal, as shown in the figure.

⑶ In addition to complying with the provisions of this book, the molding and curing of concrete shall also conform to the following requirements:

a. The maximum size of concrete aggregates should not exceed 40 mm.

b. For dry, dense concrete, a vibrating table should be used for compaction; however, the concrete bond strength specimens should still be placed in two layers.

c. After the specimen is formed and until the test age is reached, the reinforcing bars must not be disturbed. The formwork should be removed only after the concrete has developed sufficient bond strength—ideally after two days and two nights. When removing the formwork, first remove the rubber ring and the retaining ring, then carefully take off the formwork wall that has been slipped over the reinforcing bars.

d. After the test specimen is removed from the curing location, it should be tested promptly to prevent significant changes in its temperature and humidity.

e. During testing, first wipe the specimen clean and inspect its appearance. The specimen must not have any obvious defects or loose, skewed reinforcing bars.

f. Place the specimen onto a pad with a central hole, then insert it into the test chuck already mounted on the universal testing machine, ensuring that the lower chuck of the universal testing machine’s concrete bond strength test grips the rebar in the specimen securely.

g. Install the dial indicator mounting bracket on the test specimen and attach a dial indicator, ensuring that the tip of the dial indicator’s measuring rod points vertically downward and is in contact with the top surface of the rebar that protrudes slightly beyond the surface of the concrete test specimen.

h. Before applying the load, check that the dial indicator’s measuring rod is in good contact with the top surface of the rebar and that the dial indicator operates smoothly; make appropriate adjustments if necessary.

i. After recording the initial reading of the dial indicator, start the universal testing machine and pull the rebar at a loading rate not exceeding 400 N/a. For each increment of load (1000–5000 N), record the corresponding dial indicator reading.

j. Loading shall be stopped when any of the following conditions is met: ① The concrete bond strength reaches its yield point; ② The concrete cracks; ③ The reinforcement is pulled out of the concrete.

Concrete Reinforcement Bond Strength Tester Calculation result

⑴ Subtract the initial dial gauge reading from the dial gauge readings under each load level to obtain the slip deformation at that load level. ⑵ When threaded reinforcing bars are used, plot the relationship between load and slip deformation using the arithmetic mean of the slip deformations measured on six concrete bond‑strength test specimens, with load as the vertical axis and slip deformation as the horizontal axis. Select slip deformations of 0.01 mm, 0.05 mm, and 0.10 mm, then read the corresponding loads from the curve. The arithmetic mean of these three load levels, divided by the surface area of the reinforcing bar embedded in the concrete (π × outer diameter × length), yields the bond strength (in MPa). ⑶ When threaded reinforcing bars are used, the bond strength (in MPa) can be calculated by dividing the arithmetic mean of the maximum loads recorded during the pull‑out tests on six specimens by the surface area of the reinforcing bar embedded in the concrete. Note: The load–concrete bond strength–slip deformation relationship curve may also be plotted for analysis.