International Concrete Abstracts Portal

The main topic focuses on a materials science approach to evaluating three major strengthening mechanisms of high-performance concretes: reduced porosity by low water-cement ratio, absence of macro-defects, and synthetic composition mechanism. The substantially improved cement matrix materials can be obtained by deliberately using one or more of the preceding mechanisms. The preliminary experiments were carried out by two computer coupled techniques, one utilizing an electromechanical linear variable differential transformer (LVDT), while the toughening experimental technique was based on determining the J-integral to obtain K 1 c and G 1 c in an indirect way suing small-size specimens. An acoustic emission system was also used. At different concrete maturity stages, the acoustic emission signal generated from the microstructure is transformed due to wave propagation and the transducer response. The data are analyzed numerically. The results obtained through this study are expected to contribute to the establishment of a new strengthening concept of high-performance concrete. The objective of this paper is to sketch a new approach to a group of strengthening phenomena that are as important from a theoretical viewpoint as they are useful for technology.

An experimental investigation was conducted on the shear behavior of deep beams made with steel fiber reinforced high performance concrete (HPC). Twenty-six beam specimens with various shear span-effective depth ratios, steel fiber contents, amounts of vertical and horizontal web reinforcements were tested under static loads. In addition to the strength test, extensive instrumentations were designed for the measurements of average strains of reinforced concrete in the shear span and strains of web reinforcements. The web-shear cracking initiated as the first inclined shear crack. About 30% increase in the inclined shear strength and 25% increase in the ultimate shear strength can be achieved with addition of 1 .O% steel fiber for specimens having a/d= 1 .5. The strain of vertical web reinforcements became negative and the horizontal web reinforcements were stretched to yield state for specimens having a/d ratios approach 0.5. The measured load-deformation relationships of reinforced concrete and strains of web reinforcements were compared with the prediction of the softened truss model of steel fiber reinforced concrete proposed by other investigators. Good correlation was found from the comparisons.

Cement particles generally consist of micropores measuring 5 to several hundred. The micropores are too small to permit permeation of water due to water surface tension, but large enough to accommodate diffusion of steam under elevated pressure. The size of a water molecule has been scientifically determined. When dry cement particles are in contact with steam, heat immediately transfers from steam to cement, and part of the steam is forced into inner regions of the cement particles via the micropores. As a result, cement particles gain activation energy, and at the same time steam partially condenses due to energy dissipation to form moisture coating on the surfaces of cement particles as well as the interior surfaces of the micropores. Both the activation energy and condensation of steam enhance a rapid and complete hydration. Test results show that concrete made with the dry-mix/steam-injection procedure developed high CSH/CH ratios in paste and a high strength of 700 kgf/cm 2 (10,000 psi), approximately 2.5 times that of companion concrete made with the wet-mix procedure, in less than 1 day. Another test series demonstrated a 50 percent reduction of cement requirement in comparison with the wet-mixed concrete with an equivalent strength of 560 to 630 kgf/cm 2 (8000 to 9000 psi).

Two Canadian aggregates, a reactive siliceous limestone and nonreactive crushed granite, were evaluated for their potential alkali reactivity (AAR) in high-performance concrete. The concretes were proportioned to have high strength and cement content greater than 400 kg/m 3. Concrete mixes were made using a silica fume blended cement and a cementitious system in which 25 percent of a CSA Type 20 low-alkali cement was replaced by ASTM Class F fly ash. Also, control mixes were made with a CSA Type 10 high-alkali cement. The susceptibility to AAR of these concrete mixes was evaluated by casting concrete prisms and subjecting them to various accelerated storage conditions in the laboratory. For comparison purposes, mortar bars were also made, and tested according to the ASTM P 214 (1990) accelerated mortar bar test procedure. The AAR concrete prism tests performed in this study have shown that none of the concrete prisms made with silica fume blended cement and low-alkali cement incorporating fly ash showed significant expansion after 18 to 24 months of testing either in 1N NaOH or in exposure conditions of 38 C and relative humidity greater than 95 percent. The accelerated mortar bar test results, however, suggest that long-term testing may be needed to evaluate the effectiveness of blended cements in reducing expansion due to AAR, especially for highly reactive aggregates.

Results of an experimental study on the structural behavior of exterior beam-column joints made of high-strength concrete and subjected to large reversal loads are presented. Variables examined were the joint shear stress and the ratio of transverse reinforcement. Based on the experimental results, it was shown that properly designed and detailed high-strength reinforced beam-column joints display ductile hysteretic behavior.