Improving efficiency and effectiveness in the design, manufacturing and construction of the beam and block slab systems

Abstract

Beam and block slab systems have become a preferred suspended flooring technology in South Africa. Their structural efficiency and relatively low cost makes them suitable for low to medium cost developments. Like all other structural components, they are required to demonstrate sound structural integrity. Concerns were raised by some manufacturers and users in Durban (South Africa) about (a) the lack of basic technical information which makes it difficult to identify methods of improving efficiency and effectiveness of these flooring systems in general, (b) the efficiency and effectiveness of concrete masonry rebated filler blocks - with respect to the load carrying capacity and protecting the structural topping from fire, (c) what constitutes acceptable quality of a deliberately roughened precast concrete surface, (d) interfacial tensile bond strength of special connections and (e) an alternative rib that can span 5 metres without temporary props. These issues were investigated by the student. Thus, this project aimed at improving the structural efficiency and effectiveness in designing, manufacturing and constructing beam and block slab systems was undertaken in Durban, South Africa, between 2012 and 2013. Pilot studies (involving questionnaires), interviews with manufacturers, site visits, and testing of non-structural and structural components were also undertaken. The first aim (in order to address concern (a)) was to provide users of beam and block slab systems with basic technical information about the possible ways to improve efficiency and effectiveness in the design, manufacturing and construction of beam and block slab systems by undertaking an exploratory (pilot) study to better understand users of these systems concerns. The second aim (to address concern (b)) was to investigate, by conducting a series of strength to weight ratio tests, how efficient or inefficient these filler blocks are, examine the structural integrity with respect to the integrity of the manufacturing methodologies and the product thereof, and formulate a method to quantify the fire-resistivity of concrete masonry rebated filler blocks to the structural topping with respect to confining fire. The third aim (to address concern (c)) was to determine what constituted acceptable quality of a deliberately roughened precast concrete surface through a literature review and by conducting a survey to learn about the construction methodologies used by manufacturers. Site visits were undertaken to validate information given by the contractors. The fourth aim (to address concern (d)) was to determine interfacial tensile bond strength through physical testing of deliberately roughened concrete ribs which are sometimes used in special connections. The fifth aim (to address the last concern (e)) was to make an assessment by undertaking a basic comparison study between one local beam and block slab system that uses a shallow rectangular precast pretensioned rib to beam and block slab systems used in the United Kingdom and propose an ideal section (precast pretensioned rib) that spans up to 5 metres without temporary props. With respect to the first aim, it was found that the lack of technical knowledge, including access to critical information about the design philosophy, manufacturing and construction standards of these flooring systems leads to reluctance in selecting them. The outcome of the second aim is that all concrete masonry rebated filler blocks tested were found to be effective because they supported more than the required construction load but some were shown to be inefficient as more materials, such as binders, are wasted in producing over-strength filler blocks and also, undertaking trial mix designs and the testing of samples prior to batch production will reduce costs. A method is formulated in the thesis that could also show that concrete masonry rebated filler blocks provide significant protection to the structural topping thereby preventing fire progression. With respect to the third aim, although a broom or brush is effective in providing a surface roughness (Rz) of 3 mm, it is not always efficient when considering factors like the variation in uniformity, appearance of laitance and roughening frequency, which are not addressed by the South African codes. The outcome of the fourth aim is that connections should be designed such that they do not rely purely on the tensile bond strength but through reinforcing bars (or ties) taking the full tension load causing delamination. With respect to the fifth aim, a basic comparison study indicates that T-section beams are more efficient than common rectangular ribs (±150 mm wide x ±60 mm deep) since they can eliminate completely the use of temporary props for spans of up to 4.51 m. Consequently, further research is underway to design an inverted T-section rib by using high strength precast pretensioned concrete that can span up to 5 m without using temporary props.