Hybrid Timber Concrete (HTC) composite floor

The timber-concrete composite technique has been developed extensively in Europe, and has been widely used for flooring systems of new buildings as well as to repair timber floors in existing buildings. The current method of fabricating a timber concrete composite floor is to cast a concrete layer on top of the timber on site, so that the timber is on the tensile face and the concrete on the compression face. This is a reasonable and effective structural solution to reduce floor vibration. There have been a few projects that place concrete layer at the bottom, i.e. the tensile face, so as to satisfy acoustic and thermal requirements. This calls for a new Hybrid Timber Concrete (HTC) system in which both timber and concrete layers are prefabricated and can then be erected on site.

The new Hybrid Timber Concrete (HTC) flooring system

A new HTC floor system has been developed to meet the aforementioned requirements, it also has the advantage of being easy to maintain, and has the potential to be pre-stressed. In this system the concrete layer will be prefabricated to avoid shrinkage issues, increase overall quality of construction and decrease production time. The timber layer is formed from cross-laminated timber (CLT). A connection has been developed to join the CLT and concrete layers via a “dry-dry” situation. However, this takes away the advantages of allowing concrete to set over a shear connector to form a shear key. In order to increase the performance of the connection, the connector is to be pre-stressed to increase the composite action between the CLT and concrete layers.

Experiments and Results

To explore the feasibility of the HTC system, a series of tests have been planned. So far, we have completed static and dynamic tests on the connections, and in-service dynamic tests on a floor supported on two edges.

Connection tests and results

In the connection tests, two parameters have been considered, they are the dimensions of fasteners (16 and 30mm) and the pre-stress level (9.4, 6.5, and 2.15kN). Two different levels of force, 0.5 and 7kN, were imposed on the specimens during the tests, to reflect serviceability loads. The pre-stress forces in the fastener were measured by a load cell, and the specimens underwent 1000 cycles before the tests were terminated. The results show that pre-stressing the fasteners in HTC system has the direct effect of increasing the stiffness and damping of the system, which contributes to lower deflections and fewer vibration problems. It was observed that a decay in stiffness occurred in all the tests, which means that a regular maintenance plan for this system would be required should it be used in a real flooring system.

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Dynamic floor tests and results

A HTC flooring system dimensioned 3×0.6×0.22m was fabricated for in-service vibration tests. The specimen has two layers, a 5-ply CLT layer on the top and a concrete layer at the bottom. The timber and concrete layers were connected by fasteners designed for this system. The floor system was simply supported at both ends, with a servo-control shaker mounted at the middle of the floor.

The parameters for the dynamic tests include: (1) the number of fasteners (2, 4, and 8 fasteners); and (2) the pre-stressing level (9.4, 6.5, and 2.15kN) in fasteners. The frequency sweeps were carried out by the shaker, the data recorded was then analysed by Fast Fourier Transform (FFT) to convert from time-domain to frequency domain so as to identify the natural frequency of the system. The damping ratios of the system were then calculated by the half-power bandwidth method.

The results have shown that the natural frequency of the system increases with the increase in pre-stressing forces. The damping ratios of different conditions range from 5-9%, and a lower damping ratio was observed to be associated with a higher pre-stress. This results in a need for more experiments so as to find out the optimum combination of fastener number and pre-stressing level.

Future experimental plans

To explore the full feasibility and the potential of the HTC flooring system, more experimental work is needed to investigate, for example: (1) the spacing rules for the fasteners; (2) the optimum pre-stressing level; (3) the time interval between maintenance; (4) different boundary conditions; and (5) ultimate limit state behaviour of the flooring system.

Energy efficiency of the HTC flooring system

To investigate the energy efficiency of this new system against current practice, a preliminary dynamic energy simulation model has been developed. The HTC flooring system was assumed to be used in office buildings located in London and Athens. The heating and cooling energy consumptions in the office building throughout a year have been considered. The results have shown that the energy consumption of the office building with HTC flooring system is 9% lower than that of current practice (concrete on the top) in London; and 3% in Athens. Although more detailed analyses are needed, the preliminary analyses have shown that HTC flooring system can contribute to the reduction of energy consumption in an office building.

Summary

The new HTC flooring system developed at the University of Bath, with a concrete layer on the tensile face of the floor and timber on the compression face, is different from conventional timber-concrete composite floors. It might be less intuitive from a structural perspective, however, the appropriate fasteners will turn the whole floor into a pre-fabricated, pre-stressed system that is easy to maintain and energy efficient.

[Note]

This project was partly funded by IStructE, if you are interested to get more information please get in touch.