International Concrete Abstracts Portal

Self-Consolidating Concrete (SCC) is recognized by those in the industry as a mixture that can flow into place and completely fill formwork with little or no vibration. This concept immediately brings to mind an image of a highly fluid concrete mixture that flows like water. It is the level of fluidity that provides the self-consolidation and ease of placement characteristics that both precast concrete producers and concrete con-tractors are anticipating when they use SCC. The characteristic of fresh concrete stability, however, is also important although it may sometimes be overlooked. Stability is critical both during the placement operations (dynamic stability) as well as once placement is complete (static stability). Because the stability of the SCC mixture has significant impact on the final hardened properties of the concrete, it should be considered during the mixture development and quality control process. This paper outlines some of the variables that influence SCC static stability and provides insight on how to control them.

During the last decades new cementitious materials were available. These represent a technical revolution with respect to the traditional concretes. The most important innovative "High Tech" materials are Self-Compacting Concretes (SCCs). In the present paper the compositions, the performances and some practical applications of high-performance SCCs are shown. In particular, some performance improvements carried out in our laboratories are shown for these specific uses: a) SCC for a Building Engineering application (S. Peter Apostle Church in Pescara, Italy) with white concrete characterized by a marble-like skin; b) SCC in the form of high-strength concrete with compressive strength over 90 MPa devoted to a work in the field of Civil Engineering (World Trade Cen ter in San Marino); c) SCC in the form of mass concrete structure with a reduced risk of cracking in duced by thermal difference between the nucleus and the skin of the elements; d) SCC in the form of lightweight precast concrete with a density of 1750 kg/m3, 28-day compressive strength of 35 MPa, and 28-day flexural strength of 5 MPa; e) SCC in the form of a shrinkage-compensating concrete for reinforced concrete walls 8 m high and 55 m long.

A comprehensive research program was undertaken to determine the influence of coarse aggregate concentration, binder type and content, and the use of set-modifying admixtures on lateral pressure exerted by self-consolidating concrete (SCC). Experimental columns measuring 200 mm in diameter and either 2100 or 2800 mm in height were used to determine the distribution of lateral pressure during the plastic stage of cement hydration. The effect of thixotropy of the concrete on pressure variations was investigated. Test results show that lateral pressure exerted by SCC is significantly affected by the development of shear strength properties of the plastic concrete, namely internal friction and cohesion. Mixtures incorporating greater coarse aggregate volumes and/or lower binder contents were found to exhibit higher degree of internal friction. This can reduce the mobility of the concrete and result in lower initial pressure. However, given that internal friction is an inherent property of the material which remains constant with time, the rate of drop in pressure was shown to depend mainly on the increase in cohesion. Therefore, mixtures containing higher binder contents and/or a set-accelerating admixture can exhibit sharper rate of pressure drop with time. Concrete with higher degree of thixotropy was found to develop lower initial lateral pressure and higher rate of pressure drop with time. This is attributed to the stiffening effect which enables the material to re-gain its shear strength when left at rest with-out any shearing action.

The use of precast panels made of new ultra-high performance reactive powder composite materials (RPCM) in civil construction as permanent formwork for concrete structures and buried forms for bridge slabs is a new application that has great applicability prospects due to the high strength and durability of these new materials. The bond at the interface between RPCM and fiber-reinforced ordinary cement concrete is studied in this paper using slant shear tests. The program aimed at testing the bond between a substrate of RPCM with different surface treatments and an overlay of ordinary concrete with different fiber contents. This study concluded that casting the bonding surface of the substrate on air-cell plastic sheets produces a rough surface with concavities and thus provides an easy and effective way to increase the bonding strength. The results of this method of surface preparation are comparable to conventional methods like sand-blasting or grinder treatment. Increasing the fiber content of the concrete overlay in-creases the bond strength with the RPCM substrate if the surface of RPCM is not treated. When the surface of RPCM is roughened to obtain mechanical interlock, the high fiber content of the concrete overlay negatively affects the bond strength. A volumetric fiber content of 0.1% in the concrete overlay is found to be adequate in all surface types.

Magnetic Resonance Imaging (MRI) is a nondestructive technique that can be used to spatially resolve distributions of certain nuclei. Lithium is a relatively sensitive nucleus for MRI. Therefore, it is possible to directly measure the distribution of lithium in cement based materials. Lithium salts are used in concrete to suppress alkali-silica reaction. The MRI relaxation parameters associated with lithium in cement-based materials are relatively short by traditional MRI standards. Due to the short relaxation parameters, special MRI measurement techniques and hardware considerations had to be developed in order to quantify lithium distributions in cement based materials. MRI has the potential to play an important role in concrete technology. While this method has been developed for laboratory studies, measurements could be made on cores extracted from existing concrete structures.