Concrete material properties
Freezing of partially saturated air-entrained concrete: A multiphase description of the hygro-thermo-mechanical behaviour
Even though air-entrained concrete is usually used for concrete structures located in cold climates that are exposed to wet environments, frost damage is frequently detected during inspections. However, it is often hard to assess the extent and severity of the damage and, thus, there is a need for better tools and aids that can complement already established assessment methods. Several studies have successfully shown that models based on poromechanics and a multiphase approach can be used to describe the freezing behaviour of air-entrained concrete. However, these models are often limited to the scale of the air pore system and, hence, hard to use in applications involving real structures. This study proposes a hygro-thermo-mechanical multiphase model which describes the freezing behaviour of partially saturated air-entrained concrete on the structural scale. The model is implemented in a general FE-code and two numerical examples are presented to validate and show the capabilities of the model. The first concerns a series of experimental tests of air-entrained cement pastes, whereas the second aims to show the capability of the model to account for an initial non-uniform distribution of moisture. While the model predictions underestimate the magnitude of the measured strains, the results still show that the model can capture the general freezing behaviour observed in the experimental tests on the structural scale. Furthermore, the results demonstrate that the model is capable of describing freezing induced deformations caused by non-uniform moisture distributions.
Shear behavior of high-performance basalt fiber concrete—Part I: Laboratory shear tests on beams with macro fibers and bars
An experimental study on shear properties of high-performance concrete beams reinforced with basalt fiber reinforced polymer bars and macrobasalt fibers. The test specimens comprised seven beams for shear testing and five beams for measuring the residual tensile strength of fiber concrete. No stirrups were used in the test beams. As part of the experiments, the compressive strength and bulk electrical resistivity were measured. To assess the influence of fiber dosage on shear capacity of concrete beams, five-volume fractions of 43 mm macrobasalt fibers were investigated. The experimental results verified the hypothesis that shear capacity of concrete beams is associated with the fiber dosage. Furthermore, the results show a good correspondence with the predictions of Swedish standard and the fib Model Code 2010 predictions of shear strength of fiber reinforced concrete beams without shear stirrups.
Contact: Johan Silfwerbrand
Shear behavior of high-performance basalt fiber concrete—Part II: Laboratory punching shear tests on small slabs with macrofibers without bars
A study on punching shear capacity of high-performance concrete slabs reinforced with macrobasalt fibers. The test specimens comprised 14 small-scale slabs with circular notches. To investigate the effect of fiber content, 5 different fiber volume fractions of 43mm macrobasalt fibers have been studied. The results show that the punching shear capacity and the ductility of the slabs were slightly improved with increasing the fiber content. Furthermore, the results also indicate that the outcome of the experimental method is valid when the rotation of the slabs is negligible.
Contact: Johan Silfwerbrand
Naturally occurring radioactivity in some Swedish concretes and their constituents: Assessment by using I-index and dose-model
The reference level for effective dose due to gamma radiation from building materials and construction products used for dwellings is set to 1 mSv per year. Given the specific conditions presented by the EC in 1999, considering building and construction materials, an I-index of 1 may generate an effective dose of 1 mSv per year. This paper presents a comparison of the activity concentrations of 4 0K, 226Ra and 232Th of aggregates and when these aggregates constitute a part of concrete. The activity concentration assessment tool for building and construction materials, the I-index, introduced by the EC in 1996, is used in the comparison. A comparison of the I-indices values are also made with a recently presented dose model, where density variations of the construction material and thickness of the construction walls within the building are considered. There was a 16-19% lower activity index in concretes than in the corresponding aggregates. The model further implies that the differences between the I-indices of aggregates and the concretes' final effective doses are even larger. The difference is due, mainly to a dilution effect of the added cement with low levels of natural radioisotopes, but also to a different and slightly higher subtracted background value (terrestrial value) used in the modeled calculation of a revised I-index. Only very minimal contributions to the annual dose could be related to the water and additives used, due to their very low content of radionuclides reported.
A coupled hygro-thermo-mechanical model for concrete subjected to variable environmental conditions
It is necessary to consider coupled analysis methods for a simulation to accurately predict the long-term deformations of concrete structures. Among other physical fields that can be considered, both temperature and moisture have a significant influence on the deformations. Variations of these fields must therefore be included implicitly in an analysis. This project presents a coupled hygro-thermo-mechanical model for hardened concrete based on the framework of the Microprestress-Solidification theory. The model accounts for important features of concrete such as ageing, creep, shrinkage, thermal dilation and cracking; all of these under variable temperatures and moisture conditions. It is discussed how to implement the proposed model in a flexible numerical framework that is especially suitable for multi-physics analyses. The capabilities of the model are shown through the analysis of three experimental data sets from the literature, with focus on creep and shrinkage. Overall, the agreement between the analysis and experimental results is good. Finally, a numerical example of a concrete gravity dam with dimensions and loads typical to northern Sweden is analysed to show the capabilities of the model on a structural scale.