The California Department of Transportation (Caltrans) needed a concrete pavement that would deliver a surface ready for traffic in 4 hours. Different admixture manufacturers experimented with various combinations to try to achieve this. Finally, Master Builders came up with what they call "4x4 Concrete," an innovative mixture that develops 400-psi flexural strength in just 4 hours after placement. This material costs about half as much as current fast-setting hydraulic cement concrete and can be produced with standard locally available materials. Chumo Construction recently placed over 500 cubic yards of this new mix on a remove-and-replace job for the California Department of Transportation with great success. We asked Tom Pyle, chief, Office of Rigid Pavements & Structural Concrete with Caltrans, and James Anderson, engineering service western manager for Master Builders, to discuss this new development in high-early-strength concrete for paving repair.
Although we don't actually want concrete to crack, we certainly expect it to, and all good designs plan for it. Indeed, the reinforcement in concrete doesn't even begin to work until it cracks. We even put in lines of weakness and beg the cracks to follow, which of course they sometimes don't.
Simply said, shotcrete is sprayed concrete or mortar. There are two different types of shotcrete: wet mix and dry mix. With dry mix, the dry ingredients are mixed and blown in a stream of compressed air through the hose to the nozzle where water is added immediately before the material exits the nozzle; only enough water for hydration is required. Elimination of surplus water results in high-quality concrete and virtually eliminates shrinkage and cracking.
The concrete repair contractor can choose from a wide array of specialty and conventional repair materials. This large selection gives the contractor greater opportunity to match material properties with job demands, but it can also increase his chance of selecting an inappropriate material. No matter how carefully a repair is made, using the wrong repair material will likely lead to early repair failure.
Unlike new construction, repair and restoration work involves varied and unusual operations. Also, in many instances, neither the final design nor construction budget can be established before proceeding with at least some of the work. This is particularly true in major restoration where surprises almost always greet the contractors and designers once removals are commenced and the actual existing construction can be viewed.
