International Concrete Abstracts Portal

Techniques for modeling the mechanical response of thin section cement-based composites intended for structural based applications are presented using a micromechanical approach. A layer model is used and the property of each layer is specified based on the fiber and matrix constituents in addition to the orientation and the stacking sequence in each lamina. The overall axial and bending stiffness matrix is obtained using an incremental approach which updates the material parameters. The simulation is conducted by imposing an incremental strain distribution, and calculating the stresses. A stress based failure criterion is used for the three failure modes of initiation of cracking, ultimate strength of matrix, and ultimate strength of lamina. As the cracking saturates the specimen, it results in a gradual degradation of stiffness. A continuum damage model based on a scalar damage function is applied to account for the distributed cracking. The model predicts the response of unidirectional, cross ply and angle ply laminae under tensile loading in longitudinal and transverse directions. The load-deformation responses under tension and flexure are studied. It is shown that by proper selection of modeling approach, parameter measurement, and theoretical modeling, a wide range of analysis tools and design guidelines for structural applications of FRC materials are attainable.

Cement-based composites, reinforced with randomly distributed short fibers exhibit a nonlinear behavior, called damage, which could be described in terms of microcrack initiation, growth and coalescence leading to the creation of macrocracks. A micromechanics-based continuum damage mechanics, MBCDM, model is proposed for the prediction of the effect of initial microcrack configuration and propagation on the macroscopic Young’s modulus and thermodynamic force associated with the chosen damage variable. Parametric studies for a number of periodic crack distributions in a two-dimensional case have been carried out. Both unreinforced (brittle) and pitch-based carbon fiber reinforced thin sheet cementitious materials have been considered. It is shown that despite the relative simplicity of the damage measure used, the model was able to capture the main effects of cracking patterns on the overall behavior of the composite. Simulation results also reveal that, whereas the evolution of the normalized stiffness is practically the same for all configurations over the entire range of damage variation, the damage thermodynamic force is different for each case. The results predicted by the proposed approach, appear to be consistent with experimental observations regarding the tensile behavior of CFRC composites.

The use of microwave technology to speed up the production of precast ferrocement secondary roofing slabs is explored in this paper. In particular, the use of discrete on-off microwave curing regimes and the effects of such regimes on the strength and durability of the ferrocement slabs are investigated. By a regime of on-off microwave application to maintain the temperature of the slab within a specified range during microwave curing, overheating of the slabs can be avoided. High early age strengths were attained in slabs cured using such regimes, with no strength loss at 28 days. In addition, the durability of such slabs need not be compromised. The use of an appropriate reduced power level during the later stage of the curing process was found to result in a marginal improvement in the near surface quality without any reduction in early age strength.

Fiber-reinforced cement board (FRCB) is increasing in consumer popularity because it is more durable than conventional wood products. However, concerns exist about the freeze-thaw durability of the material due to its laminated structure and high porosity. To overcome these weaknesses, some manufacturers have begun to press the material after it is formed. The objective of this work is to evaluate the effects of this new processing on the durability of the FRCB. Three commercially-available FRCB products – two that had been pressed and one that had not – were subjected to accelerated freeze-thaw cycling according to a modified version of ASTM Standard C1185. The flexural strength, interlaminar bond (ILB) strength and porosity were measured. The results indicate that pressure might improve the ILB and flexural strength of the FRCB after freeze-thaw testing. However, porosity is not affected by pressure after freeze-thaw.

Hybrid fiber reinforcement of cement composites is rapidly emerging as an innovative and promising way of improving mechanical performance and durability of cement-based materials. In the present paper, fracture behavior of medium, high and very high strength mortars reinforced with hybrid fibers was experimentally studied by using contoured double cantilever beam specimens. Different combinations of small steel fibers and fibrillated polypropylene micro-fibers are investigated. These composites are very suitable for thin sheet products such as roofing sheets, tiles, curtain walls, cladding panels, permanent forms, etc. Aim of the paper was to study the influence of matrix strength, fiber type and fiber combinations on the fracture toughness of the resulting fiber reinforced mortars. Results indicate that some combinations of fibers and matrix strengths exhibit a higher resistance to crack growth and evidence the contribution of polypropylene fibers to mortar toughness.