Most of the carbonation studies are carried out by means of accelerated tests in conditioned chambers in order to obtain data in a shorter time, although the results are not always well correlated with field or laboratory data in natural tests. This work presents a comparative study between a reference portland cement concrete and pozzolanic mixtures with 10% to 50% fly ash, rice husk ash and silica fume in binary and ternary mixtures. Accelerated tests in a conditioned chamber with carbonation depth readings after 4, 8, 12 and 16 weeks, and natural tests with similar specimens exposed to the lab air environment with readings after 0.5, 1 and 2 years, were performed. The calculated rate between the carbonation coefficient of accelerated and natural tests was approximately 1 mm.week0.5 = 1 mm.year0.5 for concrete with w/cm < 0.45 and pozzolan contents up to 25%. For higher w/cm rates and w/cm < 0.45 and pozzolan contents up to 25%. For higher w/cm rates and pozzolan contents, the accelerated tests showed coefftcients 2 to 6 times higher than those observed in the natural test. This work presents preliminary data and it will be continued for 5 years more, in order to obtain readings after exposure to lab air for 4 and 8 years, after casting.

An experimental study was carried out to examine the effects of curing condition on compressive and tensile strengths, water absorption and drying shrinkage of styrene-butadiene latex (SBR)-modified lightweight aggregate concrete (LWAC). The 28-day compressive strength and the dry concrete density were about 40 Mpa and 1500 kg/m3, respectively. The results of the investigation suggest that the compressive and tensile streniths, water absorption and drying shrinkage of the SBR-modified LWAC is less sensitive to the lack of initial moist curing than the unmodified LWAC.

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.

The results of an experimental investigation of the effects of short steel fibers on the shear behavior of reinforced concrete beams are presented. Two types of steel fiber with different shapes, lengths and cross sections were used. Two series of reinforced concrete beams were cast and tested in the laboratory. Each series had a different ratio of transverse reinforcement as well as the addition or not of steel fibers in the concrete mix. The test results indicate the better performance of the beams made with fibers. These beams exhibited smaller crack width and spacing and consequently more stiffness and load carrying capacity. The results also show that the steel fibers improve the shear resisting mechanisms of the concrete represented by the aggregate interlocking and dowel action and behave as an additional transverse ireinforcement, consequently reducing the stresses in the stirrups. This finding suggests a code revision of the concrete contribuition for the shear design of beams when steel fibers are used. Their use can reduce the labor costs in the fabrication of these beams.

The characteristics of cement paste are very important for the workability, the mechanical properties and the durability of concrete, particularly in High Performance Concrete (HPC). Several problems of concrete such as slump loss, retardation in setting time or excessive heat of hydration, are directly related with some characteristics of the cement paste. This paper presents the results of a study made on cement pastes to select cementitious materials and chemical admixtures, and adjust mixture proportions before the concrete mixture design. Changes in the fluidity and the behavior of different super-plasticizers combined with cementitious materials including cements of different fineness, and different types or contents of mineral additions (calcareous fillers, natural pozzolans or silica fume) were comparatively studied by using the Marsh cone. It was found that the saturation point (defined as the dosage of superplasticizer over which the flow is not substantially modified) may change significantly with the type of superplasticizer, the fineness of the cementitious material and the incorporation of mineral additions (especially silica fume). Moreover, a quick and easy procedure was applied to analyze the behavior of different cement pastes regarding hydration process, setting time and heat development during the first ages.