International Concrete Abstracts Portal

Differential shrinkage - shrinkage difference between an old concrete substrate and a new-cast concrete overlay - causes stresses of substantial magnitude in repaired concrete structures. Consequently, it is important to determine normal stresses in the overlay and in the substrate as well as shear stresses in the interface. The literature on differential shrinkage problems goes back to the 1950s. Most theories use Bernoulli’s hypothesis as an important assumption. Bernoulli’s hypothesis states that plane sections remain plane after bending. It facilitates the computations. In this paper, Swedish laboratory tests on overlaid concrete beams have been reconsidered to test the validity of Bernoulli’s hypothesis. Strain measurements across the beam depth support Bernoulli’s hypothesis. Still the ultimate and generally accepted theory for computing stresses and strains in composite concrete structures subjected to differential shrinkage is missing but this paper shows that Bernoulli’s hypothesis may constitute one of the foundation-stones in such a theory.

Concrete cracks when the tensile stress exceeds the tensile strength. Tensile stress arises from restrained contraction. The main sources of contraction are early age effects, temperature drop and drying shrinkage. Creep offers significant relief to early-age contractions, but only a little to shrinkage. Tensile strength reduces under sustained stress, so the duration of the contraction is important. A new graph is presented to show that although cracking can occur at any age, it is most likely either during the first 3-10 days or after some years. This is because the creep relief provides a margin for temperature variations and some shrinkage to occur without exceeding the critical tensile stress. However this margin diminishes with time. A new flow chart is then presented which shows what controls are necessary at each of these two key stages. This differentiates between avoiding cracking and controlling cracking, an important distinction that is often confused in the approach to design. The case study of a feature wall where this was not appreciated is described.

This document presents research carried out in Chile for the study of the shrinkage behavior of Chilean concretes. The effect of the volume surface ratio, concrete slump, aggregate and cement type, nominal aggregate maximum size and admixture type were analyzed. A total of 82 different concrete mixtures with a mean cylinder compressive strength between 25 and 40 MPa at 28 days were studied, involving 492 test specimens. The evolution of the drying shrinkage strains was measured up to 1350 days of drying. The applicability of six different prediction models is discussed in the light of the measured shrinkage strains. The prediction of drying shrinkage with ACI-209, B3, CEB-MC90, GL2000 and Sakata 1993 and 2001 models were compared with measured results of Chilean concretes. The results showed that current shrinkage models were not adequate to predict the drying shrinkage of the tested Chilean concretes. However, it was found that for Chilean conditions the best result was obtained with the Sakata models having a coefficient of variation less than 30% when the testing data of all concretes was considered in the analysis. An appropriate methodology to carry out the updating of models was developed, based on the comparison of measured and predicted shrinkage values and the calibration of current proposed models. As a result an updated model to local conditions for use in design phase is proposed.

Effect of autogenous shrinkage on the shear strength of reinforced high strength concrete beams is investigated, in which shear beams with the distance from compressive fiber to the centroid of reinforcing bars(effective depth) of 250mm, 500mm and 1000mm are prepared, made of high autogenous shrinkage- and expansive- or low autogenous shrinkage-high strength concretes with water to binder ratio of 0.23, respectively. The test results show that the shear strength at diagonal cracking of reinforced high autogenous shrinkage-high strength concrete beams is decreased by 5-18% compared with that of reinforced expansive- or low autogenous shrinkage-high strength concrete beams. Ultimate shear strength of the former is also lowered 20-45% than that of the latter, in which the differrences of the failure mode as well as the size effect are observed. Moreover, a new concept of the equivalent tension reinforcement ratio for the evaluation of the shrinkage effect on the shear strength at diagonal cracking is proposed, which is tension reinforcement ratio modified by considering the effect of tension reinforcement strain due to deformation of concrete at early ages. The concept shows succesfully the linear relationship between the shear strength at diagonal cracking and the effective depth to -2/5 power independent of the magnitude of the early age deformation of concrete, and a design equation for the shear strength at diagonal cracking applicable to concrete compressive strength from nearly 90-130N/mm2 is proposed.

In most concrete structures, a gradual stress transfer occurs between concrete and steel due to the time-dependent concrete deformations. This aspect is of particular interest in partially prestressed concrete members where significant tensile stresses may develop due to the restrained concrete deformations. To study this effect, a long-term research project was started in the 1970’s. In 1979, 16 concrete prisms with dimensions 140 x 150 x 4000 mm were manufactured. They differed by the amount of passive reinforcement (rs respectively 0; 1.5; 3 and 6 %) and by the level of the applied compressive stress (sc = 0; 5; 10 or 15 N/mm²). The compressive force was applied at 28 days by means of unbonded strands. The prisms were placed in an air-conditioned room at 20 ± 1 °C and 60 ± 3 % R.H. During more than 25 years, deformation measurements were carried out to determine the redistribution of stresses due to the restraining effect of the passive reinforcement on shrinkage and creep of concrete. In this article the results of the deformation measurements will be given and compared with an analysis based on the model for creep and shrinkage given in the CEB-FIP Model Code 1990.