International Concrete Abstracts Portal

The current design recommendations for concrete deep beams given in the ACI Code, Canadian Code, CEB-FIP Model Code, CIRIA Guide-2, etc., are based on research results on normal strength concrete. As such, these design recommendations may not be directly applicable to deep beams made of high-strength concrete. A summary of the authors' recent research on the shear behavior of deep beams made of high-strength concrete is presented. Experimental results on the ultimate shear strengths of single-span, continuous, and slender deep beams as affected by the strength of concrete, shear-span-to-depth ratio, and slenderness ratio, are compared to various design methods. It is found that the ACI method is overly conservative for all cases, the Canadian Code method is unconservative for higher strength concrete, the CEB-FIP method gives somewhat scattered predictions, and the CIRIA Guide-2 is slightly unconservative for all cases. A minor modification on the CIRIA Guide-2 method is shown to yield a reliable method for all the cases investigated.

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.

The influence of aggregate properties on the tension-softening behavior of high-strength concrete was studied. A new method to determine the tension-softening curve of concrete is proposed, based on the polylinear approximation analysis. The prediction method for the load-displacement relationships of concrete with cracks is developed by means of the fictitious crack model concept with the K-superposition method and constitutive law with the polylinear tension-softening curve. In this method, nonlinear crack equations were solved by the iteration program for evaluating softening inclinations. The polylinear approximation method for calculating the complete tension-softening curve from the actual load-displacement curve was established by using nonlinear crack equation analysis. Tension-softening curves of concrete with various aggregates and matrix strengths were measured, and their characteristics were discussed.

The interaction between aggregates and mortar matrix is an important factor for improving the strength and toughness of high-strength concrete. Experimental investigations were carried out on the influence of aggregate properties on fracture energy G F of high-strength concrete. The influence of mortar matrix strength and volume fraction of coarse aggregate on fracture energy has been tested by three-point bending in Mode I loading. The interaction between aggregates and mortar matrix was estimated by the energy balance concept on the fracture energy of aggregate inclusion G Fi and that of mortar matrix G Fm. According to the test results, G F is strongly influenced by strength properties and volume fraction of aggregates. It was clear that the improvement of toughness of high-strength concrete is obtained by using high-strength aggregate and by designing a large volume fraction of coarse aggregate.

Bond reinforcing bars and concrete under impact loading were studied for both plain and steel fiber reinforced concretes. Experiments consisted of both pullout tests and push-in tests. The design compressive strengths of the concrete were 40 MPa (normal strength) and 75 MPa (high strength) at 28 days. The impact loading induced bond stress rates ranging from 0.5 x 10 -4 to 0.5 x 10 -2 MPa/sec. The bond under stress rates ranging from 0.5 x 10 -8 to 0.5 x 10 -4 MPa/sec was also studied for comparison. Each reinforcing bar was instrumented with five pairs of strain gages to monitor the actual strains during the bond-slip process. All test data were collected by a high-speed data acquisition system at a sampling rate of 200 sec. Stress distributions in both the steel and concrete, bond stresses and slips, bond stress-slip relationships, fracture energy in bond failure, and internal crack development were investigated. It was found that compressive strengths increased the bond-resistance capacity and fracture energy in bond failure, and therefore had a great influence on bond stress-versus-slip relationship. This effect was increased by high loading rates and steel fiber additions, especially for the push-in loading mode.