International Concrete Abstracts Portal

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.

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.

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.

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.

This paper summarizes a testing program undertaken to evaluate the uniformity of bond strength between concrete and reinforcing bars positioned at various depths of experimental wall elements. In total, four self-consolidating concrete (SCC) mixtures and three conventional flowable mixtures were prepared with different combinations of viscosity-modifying admixtures and high-range water reducers. The concrete mixtures were used to cast experimental wall elements measuring 1.54 m in height, 1.1 m in length, and 0.2 m in width. Two of the walls were steam-cured, while the remaining four elements were air-cured. Each wall had 16 reinforcing bars, four per row positioned at four levels, that were subjected to pullout tests at 1 and 28 days of age. The concrete mixtures were prepared with Type III cement, 20% Class F fly ash substitution, and a low w/cm of 0.37, which is typical of structural precast concrete construction. The targeted 1-day compressive strength was 40 MPa. Uniform distribution of in-situ compressive strength and adequate bond to the reinforcing bars were obtained with relatively small variations along the experimental wall elements. The 1- and 28-day top-bar effect ratios varied between 1 and 1.4 for the majority of the test results. These values were lower for the air-cured mixtures compared to the steam-cured mixtures. The top-bar effect is shown to be sensitive to the type of VMA used in the SCC.