Experimental and Numerical Study of Hybrid Concrete Structures

Abstract

The hybrid concrete structures investigated in this project are beams composed by layers of different types of concrete. Normal density concrete (NC) is used as top (and bottom) layer combined with a layer of fibre reinforced lightweight concrete (FRLWC). Hence, the beams have low weight and the NC layer fulfil requirements for ductility in compression. Steel fibres are added to improve the performance of LWC. The intention was to investigate the structural behaviour.

The work was divided into an experimental study and a numerical verification. Small-scale tests constitute the basis for obtaining design parameters used in the design of the larger hybrid beams. These beams were subjected to a 4-point bending test in order to study the performance in terms of both shear and bending actions. The numerical verification is based on results from the uniaxial tension test (UATT) in terms of numbers of fibres at the critical section and corresponding load-displacement response. These results are used directly in analyses of the hybrid beams that experienced shear failure.

This study shows that the concept of combining NC and FRLWC in one cross-section is working well. No problem with the bond between the layers of concrete was registered. Important aspects for ensuring satisfactory bond were curing conditions and a rough and clean surface for the substrate. Efficient interaction between the materials was ensured as the overlay of stronger normal concrete attracts external forces and governs the behaviour of the hybrid specimen.

Steel fibre reinforcement of the lightweight concrete increased the ductility in tension and the amount of conventional shear reinforcement might be reduced or even completely avoided for some types of structural members. However, the structural performance of fibre reinforced concrete strongly depends on the fibre distribution and orientation, which are governed by the behaviour of the fresh concrete and boundary conditions (like the mould and steel bars).

The material properties of the FRLWC are investigated through small-scale tests: the uniaxial tension test, the 3-point bending test and compressive test on concrete cylinders/cubes. These tests provided information about the tensile strength, Young’s modulus, the residual flexural tensile strength and the compressive strength. Through 4-point bending tests on hybrid beams, the shear resistance and moment capacity were investigated. Fibre counting was carried out in order to relate the performance to the number of fibres crossing the critical section, which turned out to have a considerable influence on the performance of the FRLWC. Based on results from the UATT and initiation of diagonal cracks on hybrid beams subjected to shear failure, the tensile strength of the FLRWC increase with increasing number of fibres.

Regarding the 4-point bending tests on hybrid beams, the types of failure were in general as expected. However, the beams without stirrups designed for moment failure experienced shear failure. The remaining moment beams experienced flexural failure and the ultimate load agreed with the calculated moment capacity. The shear beams also behaved as expected, but in calculations the shear resistance of the lightweight concrete was under-estimated according to Eurocode 2. Use of fibre reinforcement should increase the shear resistance, but the fibre contribution was questionable for the beams with 0.5% fibre reinforcement. There was large deviation in the results but in general the ultimate load/load level at initiation of diagonal crack increased with increasing number of fibres. Hence, calculations taking into account the number of fibres on the critical section provided the best agreement with the achieved results.

Numerical analyses are carried out in order to verify the method of using small-scale results as direct input to analyses of larger beams. Based on number of fibres and the load-displacement response, basic material properties were obtained from the uniaxial tension test. These parameters were in turn used directly as material properties for the FRLWC in the analyses of hybrid beams. In general, the results showed good agreement between analyses and test specimens/beams. One important aspect is that the input data were not calibrated in order to achieve better agreement between analyses and test results. A study of the effect of crack bandwidth was also carried out. The results indicated that the use of square element size as crack bandwidth is a proper choice for the FRLWC in the shear zone of the hybrid beams. Using the square element size as crack bandwidth is also suitable for notched small-scale specimens when the FE model is adapted to the size of the notch.

The Ph.d. project shows that the concept of hybrid concrete beams is a promising approach for a new type of structure. The concept provides low self-weight of the structure, practical solutions in the construction phase and good premises for more efficient building.