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 paper analyzes the mechanical behavior and its relation with the development of the hydration reaction in concretes with low water-to-cementitious material ratio made with binary and ternary cements containing limestone filler and blast furnace slag. It explores the maximum level of replacement of portland cement by both additions to obtain high early strength concrete. At 3 days, the compressive strength was 43 to 45 MPa and it was greater than 60 MPa at 28 days. All studied concretes present a very similar strength development. Results show that the combination of limestone filler and blast furnace slag is complementary: the limestone filler improves the early strength of concrete while the slag improves the later strength achieving to an optimal strength development. The concrete performance analyzed in terms of water penetration test also classified these concretes as very low permeability.

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.