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.

Lightly reinforced concrete beams fail in a brittle manner, due to steel fracture soon after cracking. In order to avoid such a brittle failure and provide certain ductility at failure, codes of practice give formulae for minimum longitudinal and transverse reinforcement. This reinforcement is meant to resist loads in excess of the first crack load and ensure that several cracks form before failure. These design provisions are based mainly on empirical studies and different code formulae lead to quite different amounts of minimum reinforcement, particularly for high strength concrete beams. In this article, the minimum shear reinforcement ratio of beams with different concrete strength is discussed on the basis of theoretical considerations and experimental results from this work and others.

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.

Mortar joints are commonly used in precast concrete structures as connection between columns, walls and load bearing precast concrete facade elements. Usually, the mortar joint has a lower strength than precast elements and its deformability tends to be larger than the ones, which causes a non-uniform distribution of stress in the joint. The mortar joint represents the weakest link in the structural system and the mortar bearing capacity limits the bearing capacity of the precast concrete elements. This paper reports on the development of an experimental program to analyze the bearing capacity of precast high strength concrete columns connected by mortar joints produced with commercially available materials, with the purpose of making better use of the columns’ bearing capacity. It was found that the thicker the joint, the lower the system’s bearing capacity, and that different strains are produced, depending on the type of material used to fill the joint - grout or dry mortar, even when the same thickness and the same relation between strengths is maintained. The conclusion reached was that it is always advisable to adopt a relation equal to or higher than one, and that a 20mm mortar layer produces an optimal behavior, in terms of both strength and ductility.

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.