Retention of flowability of SCC at the point of discharge at the jobsite is an important issue. Hot weather, long haul distances and delays on the jobsite can result in the reduction of flowability whereby the benefits of using SCC are reduced. Job site water addition to SCC may not always yield the expected increase in flowability and could cause stability problems.
Full capacity mixer truck loads may not be feasible with SCCs of very high flowability due to potential spillage. In such situations, it is prudent to transport SCC at a lower flowability and adjust the mixture with HRWR admixtures at the job site. Care should be taken to maintain the stability of the mixture and minimize blocking during pumping and placement of SCC through restricted spaces. Formwork may have to be designed to withstand fluid concrete pressure, and conservatively should be designed for full head pressure. SCC may have to be placed in lifts in taller elements. Once the concrete is in place it should not display segregation, bleeding or settlement.
How to Order SCC
When ordering and/or specifying SCC, consideration must be given to the end use of the concrete. Ready mixed concrete producers will generally have developed mixture proportions based on performance and applications. The required spread (slump flow) is based on the type of construction, selected placement method, complexity of the formwork shape, and the configuration of the reinforcement. ACI Committee 237 is completing a guidance document that will provide guidelines to select the appropriate slump flow for various conditions. The lowest slump flow required for the job conditions must be specified. This will ensure that SCC can be attained easily with required stability and at the lowest possible cost. The hardened concrete properties should be specified by the design professional, and based on the structural and service requirements of the structure. For the most part, hardened concrete properties of SCC are similar to conventional concrete mixtures. Based on the requirements of each project, SCC concrete designs can be submitted by the producer only after specification provisions regarding the performance of the concrete in its plastic and hardened state are clearly defined.