Msikaba Bridge
The Msikaba Bridge is a 580-meter single-span cable-stayed bridge on the new N2 ‘Wildcoast road’ in South Africa. The bridge is being constructed to provide a vital link between the provinces of KwaZulu-Natal and Eastern Cape.
One of the key technical challenges to be resolved by SMEC during the construction of the Msikaba Bridge was the use of a unique grouting methodology for the post-tensioning in the anchor blocks. The anchor blocks are large reinforced concrete structures that largely rely on gravity to anchor each group of back-span stay cables, so they are critical to the structural integrity. The anchor structures feature U-shaped post-tension tendons extending to the full depth of the anchor block. These tendons provide structural capacity, anchoring the stay cable anchors securely to the full depth of the anchor block structure. The anchor blocks are 49 meters long, 10 metres wide and 17 metres deep.
The team had to overcome several unconventional circumstances which impacted the effectiveness of the grouting and bonding process. Traditionally, grouted post tensioning systems are employed in horizontal beam structures with limited change in elevation. Here the bonded post-tension tendons were designed to be installed at 41° which is unusually steep. This created variable hydrostatic pressure, which can cause water to bleed from the grout mix.
Grout trials and controlled testing
The engineers conducted full-scale grout trials to investigate the effects of the differential hydrostatic pressure on uncured grout. This enabled the team to develop a tailored grouting procedure for the project. During the trials it became apparent that eradicating the water bleeding phenomena entirely would be very difficult, regardless of water cement ratio. Therefore a ‘bleed management’ solution was a more viable option.
- Grout mix: SMEC examined different grouting materials, both imported and locally manufactured, and discovered that a locally sourced product performed better. The viscosity-modified grout altered the grouts flow characteristics and reduced the amount of bleed water. This finding highlighted the value of local knowledge and expertise.
- Use of a grout cap and grout reservoir: It was vital to prevent bleedwater from accumulating in the post-tension anchor. The team devised a solution which gave the bleedwater a free path to migrate up to a point where it was visible and where it could be managed. The grout cap was an extension device which was attached to the top of the anchorage and provided a channel for the bleedwater to migrate past the anchorage and into the reservoir from where it could be removed.
- Reinjection vents and curing time:To manage the bleeding phenomena the engineers used strategically placed reinjection vents to control the curing process. The reinjection of fresh grout within two hours of grouting was effective in pushing out the bleed-affected grout before it set.
- Flow times: In this instance, slowing the flow time had no effect on the quality or filling ability of the grouting mixture.
Demonstrating successful removal of bleedwater
Lessons learned
The case study of the Msikaba Bridge project has a number of implications for the construction industry:
- Differential hydrostatic pressure: Engineers need to be aware of the effects of differential hydrostatic pressure when grouting steep inclined tendons.
- Bleeding: The bleeding phenomena is more pronounced in steep inclined tendons, and engineers need to take steps to manage it.
- Industry standards: The industry norm for grouting standards does not specifically cater for all applications, and engineers need to develop tailored grouting procedures for unconventional applications.
- Full-scale grout trials: Full-scale grout trials can be used to investigate the effects of different factors on the grouting process and to develop tailored grouting procedures for specific applications.
Conclusion
The engineers on the project overcame several challenges to develop a successful grouting procedure for steep inclined tendons. The project also highlights the importance of conducting full-scale trials when developing new grouting procedures for unconventional applications. The lessons learned from this project can be used to improve the grouting of steep inclined tendons on future projects.
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