Durability of rapid-set (ettringite-based) concrete

Abstract

Rapid-set concretes are currently used to repair structures such as bridge decks, substructure elements on bridges (e.g. piers and columns), pavements, and components of buildings. As the name implies, rapid-set concrete results in a high strength in a short period of time (e.g. ≥ 20 MPa in 3 hours) in order to minimize construction times and disruption to the travelling public. Although they are capable of reaching a high-early strength, they are not only meant to be a short-term solution but to survive the life of the structure. Unlike ordinary Portland cement (PC) based systems, where ettringite (C3A·3C$·H32) is a minor constituent, the hydrates in rapid-set concrete systems are primarily composed of ettringite, which forms within the first few hours of hydration. The formation of ettringite is achieved through the use of cementitious systems that are rich in calcium and aluminum bearing oxides, which include binders that contain calcium aluminate cement (CAC) or calcium sulfoaluminate cement (C$A), together with calcium sulfate (C$). High-early strength Portland cement (HEPC) with high doses of set accelerator and calcium sulfoaluminate cements (C$A) are currently being used in rapid-repair products, however, there exists limited information concerning their long-term durability in aggressive conditions often encountered in service. A new rapid setting concrete composed of a ternary blend (PC-CAC-C$) of Portland cement (PC), calcium aluminate cement (CAC), and calcium sulfate (C$) has been developed and is expected to achieve both mechanical and durability characteristics similar to that of concrete produced with either HEPC or C$A cement. This dissertation presents mechanical and durability performance data, from both laboratory and field studies, of ettringite systems based on the new ternary cement (PCCAC-C$) and two commercially-available cements that utilize calcium-sulfo-aluminate (C$A) cement. As a comparison, the performance was compared to that of portland cement based systems including high-early strength portland cement. The effect of carbonation on both the mechanical and durability properties of the various systems was also studied to determine the stability of ettringite under various environments. The use of X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) was used to investigate the microstructural properties. In addition to studying the durability performance of these systems, modifications were made to an existing chloride-penetration model to allow for the impact of carbonation on chloride ingress to be assessed. This included making adjustments for the changes in the physical (pore-structure) and chemical (chloride binding) resistance to chlorides.