Construction World December 2023

CIVIL ENGINEERING CONTRACTORS

simulating the strength gain of the in-situ floor slab by leaving the cube moulds next to the slab covered in plastic sheeting to mimic the curing method and weather conditions, the compressive strength results would represent realistic information off which to base post tensioning decisions. The first phase stressing required a minimum compressive strength of 9 MPa which was expected within 24 hours. However, due to the cold weather, 9 MPa was not achieved at 24 hours and the Phase 1 stressing only commenced at the 48-hour mark after confirmation from the laboratory that compressive strength had been attained. Four separate jacks worked in a coordinated sequence to ensure gradual pre-compression of the entire slab during both Phase 1 and 2 stressing. Quantibuild again relied on the industry leading expertise of Tim Dubber from Res-Spec to design the post-tensioned slab on grade floor slab design as an alternative. The 200 mm thick slab was designed with 112no. unbonded

monostrand cables (15,2 mm diameter), a bottom mat of reinforcing under columns only, and a top mat of reinforcing to deal with the practicalities of placing concrete without disturbing the profile of the cables. Two layers of 1mm thick HDPE liners were designed to form a slip surface between the slab and the subgrade. To achieve the pre-compression of the slab required to resist crack-inducing tensile forces, the floor would need to be stressed in two phases as the concrete’s compressive strength increased but before shrinkage commenced. On the project as a whole, Civil Concepts, the consultant, delivered well-engineered, practical designs that made construction an absolute pleasure. Additionally, Civil Concepts were open to tweaking several small details on various elements to allow for reduced construction times and/or savings without affecting the quality of the end product. One such example was the addition of four holes in the roof to speed up the stripping of the roof slab decking material and support work. 

shrinkage, and sufficient early strength gain before shrinkage occurs. Substantial time went into the testing on the first prestressed, round reservoir floor at the Khutsong 30 Mℓ Reservoir which meant that the lessons learned could now simply be applied considering the local aggregates and slight changes in admixtures. A water-cement ratio of 0,47 was used to increase permeability whilst a 52,5 N cement was utilized to achieve the early strength requirements and blended with fly ash at a 70/30 ratio. In terms of aggregates, granitic 19 mm stone was used with a combination of granitic crusher sand and fine plaster sand. In combination with a plasticising admixture type, Chryso 206, the mix performed well in terms of shrinkage with an initial drying shrinkage of 0,029%, and therefore no shrinkage compensating admixture was required. Placement of the 393 m 3 of concrete on the 50 m diameter floor slab took place on a particularly cold and wet day with a maximum temperature of 12 degrees which would influence the early strength gain. By sampling twenty-one cubes at various stages of the pour for early strength testing and further

18 22 nd Best Projects Awards 2023