International Concrete Abstracts Portal

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.

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.

This Symposium Publication includes 30 papers selected from the Third Conference on High-Performance Concrete (HPC) and Performance and Quality of Concrete Structures, held in Brazil. Topics covered include chloride penetration models in reinforced concrete structures, high-strength concrete with crushed and natural sand, sewage sludge ash as an addition in concrete, and the mechanical properties of polymer-modified high-performance lighweight aggregate concrete. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP207

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.