The rebound of dry shotcrete mixes can be a large 'add-on' cost in construction projects and it is in the interest of the contractor and the owner to keep these costs to a minimum. The type and nature of materials used are important factors affecting the rebound. Two types of aggregate (natural river sand and crushed aggregate) as well as partial replacement of Portland cement with silica fume and fly ash have been investigated. The same amount of steel fibres was added to all mixtures so that the ductility properties of shotcrete could be evaluated. Laboratory trails were undertaken to determine the influence of materials on the rebound and the mechanical properties of shotcrete. This study was expanded to assess the cost implication of using the above materials. Shotcrete manufactured using natural river sand and plain cement was used as reference. Results indicate that substantial reductions in the rebound and cost can be obtained when using crushed aggregate, with greater reductions being experienced when a combination of 8 % silica fume and 15 % fly ash is used as cement replacement. This seems to be a solution to the problem of obtaining sand for shotcrete in regions where natural sand are either not available in sufficient quantities or are of unsuitable quality.
A dearth of low-cost housing in South Africa is a current and widespread problem. Although spherical dome structures are not a common form of housing, the shape has many favourable characteristics, which enables a significant reduction in material and construction costs. However, dome structures have several challenging aspects - the analysis is complex and the construction difficult. This paper deals with both issues. The design adapts membrane equations from classical theory, but boundary conditions, which introduce complexities in the analysis, are dealt with in a simplified manner. A method of construction is also proposed to ensure quality control of materials and to ensure that the walls are constructed within acceptable tolerances. A new type of composite wall is also presented - cement-stabilised soil brick and a fibre-impregnated structural plaster. To verify the proposals, a prototype dome was constructed in Mozambique.
Recent timber trussed roof failures in South Africa, especially in Gauteng, of large-span trusses have necessitated a rethink about the analysis of timber trusses in general, but especially primary girder trusses, which support major loads from secondary tresses. The failures include roofs that collapsed (these having been reported in the press) to roofs where the bottom chord of multiple-ply girder trusses had rotated to such an extent that nail plates on the outer ply failed. These roofs were repaired before collapse could occur. Some of these failures are still under investigation and to divulge their location would be inappropriate. The author has identified eccentric loading as one of the possible reasons for some of the failures. In this paper he investigates all the possible factors that may influence the strength and stiffness of the trusses and he uses these to analyse three different span of girder truss. The author shows that by ignoring the eccentric loading the plate force may be underestimated by a factor in excess of 5. He shows a simple way of determining member and plate forces and suggests that these should be used when girder trusses are designed. This may then reduce the number of failures and should lead to timber roof structures that are more robust and more capable of accommodating erection errors.