International Concrete Abstracts Portal

In general, the structural member using high-strength concrete is accompanied by high brittleness, which may result in the unexpected dangerous failure. For economy and safety, high-strength concrete may be used for compressive members (vertical members) and low-strength concrete for flexural members (horizontal members). ACI 318-89 recommends that when the specified compressive strength of concrete in the column is greater than 1.4 times that specified for the floor system, the column concrete shall extend 600 mm into the slab from column face to avoid unexpected failure. The structural behavior of beam-column joints with two different compressive strengths of concrete for the beams and the columns has not been investigated adequately. ACI-ASCE Committee 352 recommends that for joints that are part of the primary system for resisting seismic lateral loads, the sum of nominal moment strengths of the column sections above and below the joint ( M c), calculated using the axial load, which gives the minimum column moment strength, should not be less than 1.4 times the sum of the nominal strengths of the beam sections at the joint ( M b). Thus, those recommended values should be examined before high-strength concrete can be used with confidence and convenience in structural members. The results showed that the ACI 318-89 extension distance of 600 mm is safe at least for members up to 300 mm in total depth, and the 2h (h is overall depth of the beam) extension distance was found to be safe also for members under flexural loading with a column-to-beam flexural strength ratio of 1.8.

Recently, there has been a great demand for high-quality concrete and concrete structures with high performance. In this context, silica fume is one of the most remarkable mineral admixtures that can give concrete high performance, such as high workability, strength, and durability. However, it is unclear as to the types of form silica fume takes in concrete, mortar, and cement paste. Some researchers point out that silica fume may be in high agglomeration. Therefore, it is very important to disperse silica fume in concrete effectively to get high-performance concrete. Consequently, this paper deals with the effect of physical treatment (ultrasonic homogenizer) and chemical treatment (superplasticizer) of silica fume on the properties of mortar. In this study, different silica fumes were used, one Japanese and five imported. The investigated properties of mortar were workability (flow values), compressive strength, and total pore volume. The study resulted in the following conclusions: 1) Silica fumes in the Japanese market were highly agglomerated in the natural state. This agglomeration of silica fume can be broken up by using some treatment methods, such as ultrasonic homogenizer and superplasticizer. 2) Physical treatment (ultrasonic homogenizer) before mixing mortar was useful to improve compressive strength and to decrease total pore volume of mortar containing silica fume. The use of superplasticizer could result in highly workable mortar. 3) The effectiveness of ultrasonic homogenizer treatment and that of superplasticizer treatment are different.

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.

Although high-strength concrete (HSC) has a brittle behavior in the case of specimens subjected to axial compression, a quite different behavior is obtained in the case of reinforced or prestressed concrete members subjected to bending. In this paper, five tests of HSC beams subjected to pure bending are described and analyzed to quantify their ductility and to deduce the real strain-softening behavior of their compressed zones. Three cases are studied: reinforced concrete, prestressed concrete, and partially prestressed concrete. The comparison of the experimental ultimate deformations (such as plastic rotations, curvatures, deflections) with the calculated values show that the strain-softening of compressed concrete may occur after the peak stress and can be defined by a k' coefficient varying from 0 to 1. For the tested beams, it was found that the use of HSC instead of normal strength concrete (NSC) results in the doubling of the plastic rotation capacity, for reinforced or prestressed beams subjected to pure bending.

Sixteen very high strength concrete beams (3600 x 350 x 200 mm) with and without steel fibers were tested under different combinations of shear force and bending moment. The beams were singly reinforced and without shear (web) reinforcement. The cylinder compressive strength of concrete was about 110 MPa. The main variables in this program were: shear span/depth ratio (a/d), the steel fiber content (V f), and the percentage of the longitudinal flexural reinforcement ({rho}). The test results showed that, adding steel fibers to high-strength concrete increased the ultimate shear strength, increased the stiffness, reduced the deflection, and transformed the failure mode into a more ductile one. Based on the test results, two empirical expressions have been proposed to predict the shear strength of steel fiber high-strength concrete.