International Concrete Abstracts Portal

The precast industry has always been looking for ways to improve production. Be that ease of casting or finishing, faster turnaround or better economics due to less damage or reduced concrete costs. The advent of self consolidating concrete (SCC) has enabled some of these aspects to be realised. The development of SCC and new mixture design procedures has improved certain facets of the precast industry. However, the excessive use of fillers or very high cement contents has had equal drawbacks for the use of SCC in this environment. With the advances in software allowing very precise particle packing analyses to be made of the materials, new mix designs with lower total binder contents - and little or no fillers - are possible. Designs with supplementary cementitious materials (SCMs), including silica fume, can be very effective for SCC, not only giving excellent flow and non-segregation, but also enhancing the finished quality of the concrete. This paper reviews the use of silica fume in SCC, information from the Technically Optimised Piling Concrete (TOPIC) research in the UK, and gives examples of the use in some precast operations in Sweden.

Sixteen self-consolidating concrete (SCC) mixtures were developed for use in precast, prestressed bridge beams in Texas. The mixtures featured two different sets of aggregates-namely with river gravel or crushed limestone coarse aggregate-and varied in sand-aggregate ratio, paste volume, and paste composition. The 16-hour compressive strengths (release strengths) ranged from 4,500 to 10,500 psi (30 to 70 Mpa) depending on the mixture proportions and curing temperature history. The 28-day compressive strengths ranged from 11,000 to nearly 15,000 psi (75 to 100 Mpa). The SCC mixtures were developed to achieve the necessary release strengths while balancing the requirements for adequate workability and durability. This paper discusses the need for higher paste volumes and sand-aggregate ratios to achieve SCC workability requirements and the implications for hardened properties. Semi-adiabatic and isothermal calorimetry measurements performed on concrete and paste specimens, respectively, and compressive strength measurements indicated that although the SCC mixtures exhibited slightly delayed setting times in some cases, they generated heat at a faster rate, generated more total heat, and developed higher 28-day strength for a given release strength. Compared to conventional mixtures with the same release strength, the SCC mixtures exhibited unchanged or slightly reduced shrinkage except when one specific admixture was used.

Self-consolidating concrete (SCC) is being implemented throughout the US. Some advantages of SCC include its ease of placement, reduced labor requirements for placing the material, reduced noise when placing, and its improved finish quality. Clearly there are benefits of using this material. However, the AASHTO LRFD specifications were developed based on material characteristics of conventional, normal strength concretes. Because of this, engineers and designers are reluctant to specify and use SCC for bridge applications, possibly making the potential benefits of this material underutilized. This research investigated compressive strength development, modulus of elasticity (MOE), modulus of rupture (MOR), and splitting tensile strength (STS) of SCC mixtures specifically designed for precast, prestressed, concrete bridge girders. The experimental program included two target 16-hour compressive strength levels and two coarse aggregate types (river gravel and crushed limestone) with varying volume fractions. The measured mechanical properties for the SCC mixtures were compared with the results of conventional concrete (CC) mixtures of similar release strengths, as well as the estimated values from the 2006 AASHTO LRFD prediction equations. Results indicate that the AASHTO equations either predict the mechanical properties of SCC fairly well or underestimate the properties of SCC.

SP-247CD This CD-ROM is a collection of papers prepared for a session held at the ACI 2007 Fall convention in Puerto Rico on the hardened properties and performance of SCC developed for use in precast prestressed applications. The papers relate to SCC in prestressed applications and are organized as follows: 1) mixture proportioning; 2) mechanical properties; 3) time-dependent deformations; 4) flexural and shear behavior; 5) bond behavior; 6) prestress losses; and 7) the structural behavior of full-scale precast prestressed elements made with SCC.

As a part of an ongoing study at the University of Minnesota, self-consolidating concrete (SCC) has been developed successfully with locally available materials from two precast concrete plants for use in precast prestressed bridge girders in the State of Minnesota. Four SCC mixes (i.e., two mixes per plant) were designed, evaluated, and used to cast four SCC precast prestressed bridge girders. Variations in the SCC mixes included cementitious materials (ASTM Type I and Type III cement, and Class C fly ash), natural gravel and crushed stone as coarse aggregate, and several admixtures. In addition to SCC girders, a conventional concrete girder was cast simultaneously on the same precasting bed for each plant. The girders were instrumented to monitor both short-term and long-term performance, which included transfer length, camber, and prestress losses. In addition, companion cylinders were cast to monitor compressive strength and modulus of elasticity over time. The test results indicate that the overall performance of the SCC girders was comparable to that of conventional concrete girders and could be predicted using existing design equations.