This research explores the effect of various humidity conditions and the efficiency of shrinkage-reducing admixtures on the free shrinkage strain of ordinary Portland cement (OPC) concrete and its mechanical properties. An OPC concrete C30/37 mixture was replenished with 5% of quicklime and 2% of organic-compound-based liquid shrinkage-reducing agent (SRA). The investigation revealed that a combination of quicklime and SRA led to the highest reduction in concrete shrinkage strain. Polypropylene microfiber addition was not so effective in reducing the concrete shrinkage as the previous two additives did. The prediction of concrete shrinkage without quicklime additive was performed according to EC2 and B4 model methods, and the obtained results were compared with the experimental ones. The B4 model evaluates the parameters more than the EC2 model does and, therefore, was modified to calculate the concrete shrinkage for the case of variable humidity and to evaluate the effect of quicklime additive. The experimental shrinkage curve that best coincides with the theoretical one was that obtained by the modified B4 model.

This paper presents experimental, numerical and analytical analysis of newly cast and pre-cracked flexural reinforced concrete beams strengthened with CFRP. In total, 9 intermediate-scale composite beams were cast and tested using 4-point bending test setup. Midspan deflection, width of the cracks, concrete and CFRP strains were measured during the experimental program. Clear efficiency of composite pre-cracked beams was observed in comparison to newly cast beams: enhanced flexural capacity and increased stiffness after appearance of primary cracks in tension zone. Good agreement was found comparing experimental and theoretical (EC2) deflections of RC beams strengthened with CFRP. However, for more detailed verification, the analysis should be extended with more specimens. The shear stress at the end of CFRP sheets between the concrete and CFRP increased rapidly until reaching maximum slip value, when the reinforced concrete beam strengthened with CFRP reaches 60-90 % utilization of load bearing capacity. All experimental results were compared with numerical and analytical calculations. Experimental, numerical and analytical results were in sufficiently good agreement.

A time-dependent stress-strain state of reinforced elements induced by gradually developing shrinkage of concrete is analysed. Depending on environmental conditions the early age concrete shrinkage can produce a significant tensile stress in concrete that reduces the cracking resistance of the cross-section. To analyse this phenomenon a numerical model is proposed. For the exact analytical prediction of the time-dependent stress-strain state induced by restrained shrinkage, the key parameter is the coefficient of aging which is strongly dependent on the time-evolution of tensile stress of concrete and modulus of elasticity. The values of this coefficient obtained in the current analysis are suitable for the proper evaluation of the reduction of cracking resistance of the cross-section induced by restrained shrinkage evolving during concrete hardening process in a period prior to element loading. DOI: http://dx.doi.org/10.5755/j01.mech.24.1.17901

The application of FRP reinforcement in concrete structures is quite popular nowadays. In this article the cracking moment of flexural concrete structures reinforced by steel and FRP reinforcement bars is investigated. There is estimated that the behaviour of tensile concrete is nonlinear when crack opens. The nonlinearity was estimated by two different cases. In the first one the coefficient of elastic and plastic strain of tensile concrete were applied and in the second the stress-strain relationship were expressed by polynomial function. That function prepared according to compressed concrete stress-strain diagram described in design standard EN 1992-1-1. Those two different stress-strain relationships were used for cracking moment calculation. Also the cracking moment calculations were performed using mentioned standard EN and Technical building code STR 2.05.05:2005. In all cases the calculations were performed in assumption that the behaviour of compressed concrete is elastic when crack opens. The obtained results were analysed and presented base conclusions. DOI: http://dx.doi.org/10.5755/j01.sace.9.4.7428

An energetic Lagrangian approach is applied to formulate coupled dynamic equations for unknown time-dependent displacements with respect to girder mid-span and vehicle suspension interaction, along with their rates and accelerations. The paper provides for a non-dimensional set of two linear non-homogeneous differential equations of a second order with variable coefficients, and a new analytical solution is proposed by implementing Lindstedt’s singular perturbation method. This allowed us to obtain algebraic equations for girder mid-span displacement dynamic factors and suspension transmissibility coefficients vs. dimensionless time, three ratios of the loading frequency with respect to the girder and vehicle, and combined interaction frequencies related to free vibration. It is demonstrated that the derived system of Mathieu-type differential equations represents the free vibration problem of girder–suspension interaction subjected to the motion of a weightless vehicle, while four non-trivial fundamental solutions to the excitation of free-vibrating girder–suspension and suspension–girder systems are proposed. Simplified approximate analytical solutions are also given for dynamic factors of girder mid-span displacement. The analytically predicted results of dynamic variables are also compared with those determined numerically.