DESIGN OF ENGINEERED CEMENTITIOUS COMPOSITES (ECC) FOR PROCESSING AND WORKABILITY REQUIREMENTS

Abstract

The design of fiber reinforced cementitious composites (FRCC) is typically governed by their mechanical properties in the hardened state, such as the tensile and compressive strength and strain capacity. The intricacies of processing and workability of FRCC, with fiber volume fractions ranging from 1.5 to 20% depending on the particular composition, are often of secondary importance in small-scale laboratory production. However, the fresh composite properties significantly influence the performance of the composite in the hardened state, often leading to substandard mechanical properties due to non-uniform fiber dispersion or inconsistent compaction. More importantly, the large-scale application of FRCC in practice is often not feasible since special mixing equipment or processing techniques are required to overcome the difficulties associated with processing and workability. In particular, the uniform dispersion of short, randomly oriented fibers in the cementitious matrix at fiber volume fractions of more than 1.5% typically requires force-based mixing equipment, such as high speed pan mixers, planetary mixers, or so-called omni mixers, which are commercially available at the laboratory scale (5dm 3 to 200dm 3 ). These specialized mixers are relatively expensive for large capacities and are rarely on hand in most concrete mixing plants and at construction sites. This paper focuses on the fresh mix design of Engineered Cementitious Composites (ECC), which represent one type of high performance FRCC with strain hardening and multiple cracking behavior. The particular version of ECC described in this study utilizes PVA fibers (d t =39μm, l t =12mm) at a volume fraction of 2%. The presented approach is guided by consideration of the practical requirements of producing cementitious composites at large scale under field conditions. The goal of this study is to design the composite such that mixing can be conducted in a conventional, gravity-based drum mixer while retaining the required workability and mechanical properties observed when mixing in a specialized, small-scale laboratory mixer. A case study is presented for the development of a flowable ECC with self-consolidating consistency. This paper will present possible approaches to meeting these requirements, including control of the particle size distribution and chemical composition of the cementitious matrix, adjusting the mixing sequence and intervals, and proper utilization of cement types and common chemical admixtures.