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Concrete In Australia : June 2008
TECHNICAL A new era of precast facades Sustainable design When the Romans ground lime or volcanic ash or burnt clay tiles, embodied energy was solely a function of time and muscle power. When Joseph Aspin produced Portland cement more than two centuries later, by heating a mixture of finely ground clay and limestone in the furnace, he also would have had little consideration for the energy required to produce this breakthrough cement. However currently designers are required to meet briefs Figure 2. Outward bowing can occur due to differential and thermal shrinkage. Transportation Micro-cracks may occur due to transportation and erection. These are typically not detected at construction stage. However over time they will grow in width and may cause structural weakening and corrosion of the reinforcement. Thermal and differential shrinkage Where the height or width of a panel is large with respect to its thickness and or the reinforcement is not symmetrical, differential shrinkage warping may occur. This can be quite severe and cause failures of connections and failure of sealants or water proofi ng at joints and interfaces with other façade systems. Furthermore thermal bowing can occur due to signifi cant temperature gradients across the section of the panel. Bowing is usually a combination of the above (figure 2). Intrinsic and environmental chlorides Buildings subjected to airborne chlorides, particularly where they face bodies of salt, are at a signifi cant risk of developing steel corrosion related concrete spalling. Many studies have documented the reach of these air-borne chlorides (aerosols) in terms of distance from the body of salt water. However, it is of interest to note that at some high-rise buildings the highest levels of chloride are encountered at the higher building levels, with corresponding defects and that the lower levels were in a relatively better condition. This is likely due to surrounding city building shielding effects and negative and positive wind pressure zones. While the practice of including chloride containing admixtures to accelerate the curing has now been discontinued, the effects are still being encountered. Testing for chlorides will in this case reveal that the chloride profi le will be such that the outer 20mm or so will have a signifi cantly lower chloride content than the inner concrete. This is believed to be due to chloride leaching from the outer concrete as a result of exposure to rain. Conversely, if an outer layer has higher chloride levels than the inner concrete, it is generally due to air-borne chlorides. 46 Concrete in Australia Vol 34 No 2 provided by owners or authorities, which typically require that the performance of the building achieves a given energy rating. This rating is an all encompassing assessment of the energy performance of the building as a whole. As the facades are the thermal fi lters of the exteriors of buildings, all components including precast concrete panels play a vital role in achieving the performance requirements. Designers and manufacturers will therefore be challenged to minimise the embodied energy, through substitution or by addition of other constituents to the concrete, to maximise the benefi ts of precast concrete panels. Furthermore, precast concrete elements are now being integrated into new facades to achieve the required rating. Aesthetic precast Precast concrete facade elements offer aesthetic opportunities which are diffi cult to achieve with insitu concrete. External surfaces can be sculptured and textured and colour differences can be striking using new materials which offer much longer colour retention. European Code EN1504 After more than 10 years of development the new European draft standard EN1504 for the protection and repair of concrete structures has been introduced. While this standard is aimed at the preservation and repair of existing panels, it contains an important section on the classifi cation and causes of defects. This is of direct relevance to designers of new panels and facades. Conclusion Precast concrete panels have been an important facade cladding material for many Australian buildings. While most of these have performed well and stood the test of time, many others have developed signifi cant defects. These problems need to be recognised and their causes understood and avoided in the future. A new era is dawning in the design and use of precast panels in facades. The worldwide recognition of climate change and need to minimise energy through more energy effi cient buildings will affect the design of facades. Designers will need to maximise the energy performance of the skin of buildings and are more likely to incorporate precast concrete panels into the exteriors. Signifi cant new developments and opportunities will lead to a new generation of facades.