International Concrete Abstracts Portal

Three well known mixture proportioning methods were used in this research ACI 211 Method, Argentina Portland Cement Institute ICPA Method and University of Sao Paulo USP Method. The mixture proportioning concepts, procedures and steps of each method are different but the target is the same - to achieve the best and the cheapest high strength concrete. This research was carried out using materials available in high seismic risk regions, near the Andes Mountain in the West of Argentina, where there is predominant rounded gravel from basaltic and granite rock, and rounded natural quartz sand. The advantages of the high strength concretes compel a solution between structural design, laboratory tests and field jobs, where mixture proportioning method have an important rule. All mixture proportioning method can help to achieve the best concrete but also must be economic, rapid, easy and allow secure changes in field without new laboratory tests. The evaluation criteria considered workability, cement content, compressive strength, tensile splitting test, modulus of elasticity and specific cost evaluation to distinguish between the different mixture proportioning methods so as to achieve the same final concrete properties. The USP Method was showed to be a useful method in laboratory procedures and more flexible when some field changes are necessary.

The Brazilian experience with precast concrete in building schools all over the country has shown the flexibility allowed by that technology. Indeed, it is a success story in many aspects, particularly in terms of efficiency answering acute social needs and repetitive programs. Now, after a number of years it is possible to evaluate its performance in terms of durability. Implicit in the design of precast elements is a strong concern for weight and in the case of light precast elements this concern is even bigger. The result is the use of very thin components with only a few millimeters of concrete over the reinforcement bars, resulting in accelerated concrete carbonation and steel oxidation. This paper reports the use of high performance concrete to build light precast concrete building elements as an answer to the mentioned problem.

This paper shows a study on the connection between precast beams and precast slabs using push-out tests. The connection is formed by steel bars associated with shear-key. The steel bars are bent in hoop shape and inserted in the pockets in slab, which are filled with cast-in-place concrete. The strength of cast-in-place concrete ranges from 50 MPa to 100 MPa and a maximum volume of 1.50 % of steel fibers is added. Results show the strength of the connection increases when the concrete strength increases, mainly when steel fibers are added. It is also observed that addition of steel fibers to high strength concrete in connections transfers failure from the shear-key to the precast concrete. This result suggests the definition of an upper limit to the relationship between the strength of cast-in-place concrete and the strength of precast concrete.

Lightly reinforced concrete beams fail in a brittle manner, due to steel fracture soon after cracking. In order to avoid such a brittle failure and provide certain ductility at failure, codes of practice give formulae for minimum longitudinal and transverse reinforcement. This reinforcement is meant to resist loads in excess of the first crack load and ensure that several cracks form before failure. These design provisions are based mainly on empirical studies and different code formulae lead to quite different amounts of minimum reinforcement, particularly for high strength concrete beams. In this article, the minimum shear reinforcement ratio of beams with different concrete strength is discussed on the basis of theoretical considerations and experimental results from this work and others.

Steam curing of concrete is widely used in the concrete industry nowadays. Under steam curing, the concrete is subjected to high temperatures for a short period of time. Compressive and tensile strength are accelerated, and Compressive and tensile strength are accelerated, and thus much higher strengths at early ages were achieved. The influence of curing temperature on the development of mechanical properties of concrete can be estimated by the maturity approach. Maturity functions of time and temperature are used to transform actual times in equivalent ages at a reference temperature. This investigation uses the maturity approach to assess the influence of steam curing in high-performance concrete with various dosages of silica fume addition (0%, IO%, and 20% of cement mass). A 6-hour steam curing period at 80°C was applied, with compressive and tensile strength of the concrete mixtures evaluated every hour. The characterization of the thermal sensitiveness of every mixture was performed according to ASTM C 1074. The apparent activation energies of mortar mixtures with the same sand-cementitious ratio as the concrete mixtures were obtained from the development of compressive strength at three isothermal conditions at 30°C, 55°C and 80°C. The results obtained permit an evaluation of the applicability of the maturity approach to steam-cured high strength concretes. The influence of silica fume addition levels on the development of compressive and tensile strength of steam-cured HSC was also investigated.