International Concrete Abstracts Portal

This paper deals with the properties of styrene butadiene latex (SBR)-modified lightweight aggregate concrete (LWAC) for thin precast components, made with two Brazilian lightweight aggregate (LWA). Water reduction capacity, air content, flow table after 2 h, compressive strength, splitting-tensile strength, flexural strength, modulus of elasticity, stress-strain behavior and water absorption were tested. The 7-day compressive strength and the dry concrete density varied from 39.7 to 51.9 MPa and from 1460 to 1605 kg/m3, respectively. The modulus of elasticity at 7 days, which varied from 17.9 to 22,6 GPa, was lower than that typically observed for normal-weight concrete at the same compressive strength level. The inclusion of SBR in the LWAC decreased W/(C+S) and water absorption level. The inclusion of SBR in the LWAC decreased W/(C+S) and water absorption and increased splitting-tensile and flexural strengths.

In this work, some experimental results for determination of fracture energy and brittleness number for high-performance concrete are presented. Three-point bend tests were conducted for different concrete mixture proportions, with compressive strengths of 70 MPa to 90 Mpa. The tests were performed using crack mouth opening displacement control in a closed-loop servo-hydraulic system. The experiments involved the testing of 75 single-notched beams of four different sizes in order to study the size effect. The compositions of the concrete were established according to, those specified by IBRACON (Brazilian Concrete Institute) in order to match the concrete commonly used by companies that operate in Brazil. The results found in this work by the method proposed by RILEM show that the size of the specimens influences the value of the obtained fracture energy, it being larger as the size of influences the value of the obtained fracture energy, it being larger as the size of the specimen increases, thus suggesting that the RILEM method is not valid in characterizing fracture energy as a material parameter. The results from this work found that the fracture energy obtained by the method proposed by Bazant and Pfeiffer can be adopted as a fracture parameter of the material, since its value is independent of the size of the specimen.

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.

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.