GGBFS Replacement for High Early Strength Concrete for Precast Industry

Shivaprakash G, M-Tech Structure, Sr. Manager, Admixture and Surface Treatment, Fosroc Chemicals

The Precast Concrete industry is one of the thriving industries in the wake of faster development with high quality standards. The initial capital cost for precast construction is higher than for conventional concrete. The chief requirement in precast concrete is high early strength. It becomes a challenge to reduce costs, while maintaining the high early strength, durability, workability, and environmental soundness.

In the investigation work presented in this paper, mix design was developed with 25 per cent replacement of OPC with fine reactive powder (GGBFS) at water-powder ratio of 0.27 with an objective to reduce costs and make concrete green. The concrete thus designed for wet properties and compressive strength at 10 hours, 12 hours and 14 hours was to be tested and expected to meet the requirements of high early strength for precast industry.

Precast concrete is a construction product produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site and lifted into place. Previously, precast construction technology was used mainly for bridges, tunnels, flyovers, underpasses and such structures, but of late, the precast industry has gained prominence in housing, given the acute shortage in housing, rising labour and input costs, and increased emphasis on quality and timely delivery.

Precast concrete has different requirements from normal concrete in terms of workability, water-cement ratio and early strength. Within the precast industry, the rheology of concrete varies with different precast elements and structures. These variations can be brought about in the concrete with the addition of the right type and dosage of chemical admixtures. Chemical admixtures play a pivotal role in developing the concrete to required rheology.

Governing factors in precast concrete are:

• Low water-cement ratio
• Medium workability
• Need for high early strength for early de-moulding to increase productivity
• Compatibility with cement replacements
• Compatibility with 100% crushed sand / manufactured sand.

Gaining high early strength is fundamental in the Precast industry since the casted elements of concrete need to be transported to storage or the construction site and lifted into place. This improves the efficiency of the precast plant by increasing the production cycles.