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

High performance concrete is generally specified to meet special requirements such as higher compressive strength, lower permeability, higher resistance to aggressive environments and longer durability. The design of structures must be based on the knowledge of all concrete properties and the determination of creep values of paramount importance in several cases. This paper presents the influence of water-cement ratio and level of hydration for concretes with compressive strengths ranging from 20 MPa to 75 MPa. Creep of four mixtures with water-cement ratios of 0.29, 0.37, 0.52 and 0.75 with 6% of silica fume and a fixed slump was determined. Specimens were loaded at ages 3, 7, 28 and 90 days and maintained with a constant load for 90 days. Concrete testing included creep, compressive strength, modulus of elasticity, autogenous deformation and drying shrinkage. The paper presents creep coefficients, autogenous volume changes, drying shrinkage and their correlation with age and water-cement ratio. Test results showed that high performance concrete presents lesser creep if compared with concretes with lower compressive strength and that differences between specific creep values range from 12% to 43%. High performance concrete presented significantly higher values of autogenous volume changes. Tests confirmed that drying shrinkage is directly related to the water content of the mixture, whereas similar values were obtained from tests performed on several specimens representing different mixtures with various compressive strengths but containing approximately the same amount of water.

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.