Sample butting

Two 900 mm cylinders were joined together to form a 1.8 m long unit in the butting frame, as shown in Figure.4.4. An integrity clamp consists of two components, upper half clamp and base half clamp. The base half clamp, without eyebolt hole, was set in the middle of butting frame for the purpose of fixing the concrete samples together at the butting section. A plastic film was paved on the inner surface of the bottom clamp component to prevent the epoxy binder from leaking out.

Figure 4.4 Butting frame and clamps

Subsequently, one end of 900 mm cylinder was placed in the centre of the clamp for epoxy painting. The concrete cylinder was lift and positioned in the butting frames by remote-control travelling crane. An air tact with 40 mm diameter was attached in the mid of butting surface to prevent epoxy invading into cylinders and to lead grout through. Epoxy binder and hardener mix, was used for butt-glue end surfaces of two concrete blocks. The epoxy mixture was evenly smeared on the outer rim of the concrete joint surface, as shown in Figure 4.5.

A squeezing tool was used to tighten the concrete cylinders together in the butting frame and to close the joint. A long 22 mm diameter solid rock bolt, threaded at both ends,

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was inserted in the concrete cylinder centre hole and tightened to attach the joint surfaces together after smearing with epoxy, where steel plates and spacers were attached to its two ends. The epoxy overflow from the joint when butting and the plastic film was wrapped up around circumference of middle joint. The whole assembly was left undisturbed for a period of 30 minutes, the bolt was then removed and the other top half of the outer confining steel clamp was mounted and tightened.

Figure 4.5 Butting and squeezing concrete samples

4.2.3 Pre-tensioning and grouting

For each test, a set of four concrete blocks was required to provide a total length of 3.6 m. Two sets of 1.8 m long concrete blocks were mounted together in the Pre-Tension and Grouting Frames (PTGF).The travelling head crane and nylon slings were used to lift the concrete block from the butting frame and position them on the PTGF as shown in Figure 4.6. Each set of concrete block was positioned against one side of the frame and the other end of the concrete cylinder was seated and supported by the central primary clamp. To prevent leaking during grouting process and reduce shear friction during the loading process, a rubber gasket (air tact) and two Teflon sheets were glued at the middle joint, as shown in Figure 4.7. The inner diameter of the Teflon sheet was equal to the outer diameter of rubber gasket.

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Figure 4.6 Concrete blocks positioned in the PTGF

Figure 4.7 Gasket and Teflon sheets attached on concrete end surface

The PTGF had two ends, as the literal meaning, one for grouting and the other for pre-tensioning, separately. A long steel tube with injection hole was welded to the trumpet annular steel plate (200 mm diameter) to be used for grouting, as shown in Figure 4.8.

An annular rubber plate was glued to the end of the circular steel plate to maintain uniform contact with the backbone. The inner diameter of the rubber plate was 80 mm and the outer diameter was 187 mm. This rubber plate worked as the double faced adhesive tape to make sure that the ends were sealed completely and the steel plate was well-attached on concrete cylinder. Similarly, a pre-tensioning trumpet was installed at the pre-tensioning end, whose steel tube was shorter and smooth. Likewise, this trumpet was glued and bonded with concrete block end by the same size rubber plate.

When the concrete blocks were positioned properly on the PTGF and all rubber and

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Teflon components were fixed on the predetermined position, two steel bolts in pre-tension end were screwed to push the steel plate towards the concrete to tighten it, just as the same way to butt two 900 mm concrete blocks. And then, the top half of the clamp was placed on the concrete cylinder at the middle joint and bolted together with the bottom half. A cable bolt was inserted into concrete sample. The tested cable was long enough to allow both ends to be stretching out of the concrete ends by some 40 mm in length. Silicon sealant and a small rubber gasket were used to seal the end of the grouting head as shown in Figure 4.8.

Figure 4.8 Trumpet seat on frame and silicon sealing grouting head

As shown in Figure 4.9, barrel and wedges were installed on the ends of cable outside of PTGF. A spacer and nut was covered by the 62.5 mm jack for pre-tensioning. The long screw bar was twisted on the screw pipe and it was confined by hydraulic cylinder by the end nut. A hand pressuriser provided pressure for tension jack and the pre-tension load was measured by master gauge. When the pressure came to the required level, the nut was tightened up to ensure the pressure kept on cable.

Figure 4.9 Cable pre-tension apparatus

Spacer Nut

B&W

Jack

End nut

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Figure 4.10 Angled samples for grouting

The whole PTGF was lifted and set on a steel-base frame to make space for grouting.

The side support bar was moved and hydraulic support was retracted. The upper frame was rotated and the slope angle was 65°, as shown in Figure 4.10. Normally a total of six cables were assembled in six pretension frames and lifted up to allow bottom up grouting simulating the field condition. Grout was ejected into the cable/concrete hole to encapsulate the cable in the concrete block using a dedicated grout mixer for pumping.

4.2.4 Assembly of shear apparatus and testing procedure

Once the mandatory time for curing was reached, each sample was disassembled from the frame and lifted to be mounted on to the shearing rig. The test sample was tied up by four nylon slings and hooked on the travelling lift (crane). Three steel blocks were utilized to control key positions of installation, as shown in Figure 4.11. The setup steel block was firstly put on the slide-way of the testing machine and it help to locate the middle joint face with clamp. And then, the setup block was replaced by the final stop block. The final stop block helps to position the joint face of concrete sample aligning with the guillotine face of test machine. Steel clamps were placed full length around the concrete blocks to provide confinement to the sample. The aim of using confinement on the concrete cylinders was to provide radial strength to the composite medium encapsulating the cable to replicate the in situ conditions of the forces applied on the cable bolt.

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(a) Setup block on slide-way (b) Alignment of the middle-joint

(c) Final stop block on the slide-way

Figure 4.11 Critical position of test sample during setup proceeding

Figure 4.12 The cross section view of the tested concrete sample

A hydraulic power pack was connected to the hydraulic rams to permit cable shear loading. The hydraulic pressure application was controlled manually, in order to keep the applied shearing load constant, although the application of shearing load rate was not always constant in different tests. During the loading process, the right section of the testing machine was compressed down and the bolted concrete blocks were sheared at the middle joint which was overlapping with the shear plane of the testing machine.

When the combined shear and tensile forces in the cable exceeded the cable strength, the cable bolt failed and the single shear test finished. Figure 4.12 shows the cross section view of the tested concrete sample.

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