Effect of Improper Curing on Concrete Properties that may Affect Concrete Durability

The use of concrete increasing annually due its favourable properties and readily availability. Demand for concrete will continue to increase and it will remain the world’s most important construction materials for many years to come. However, the use of Portland cement concrete has an environmental burned, and so in a drive to reduce the carbon footprint of construction, there is widespread attention directed towards the utilisation of wastes and industrial by-products to minimise Portland cement (PC) consumption. The cement industry increasingly uses additions, such as fly ash. The literature has established the use of fly ash as partial replacement of Portland cement to increase strength at later age and exhibit considerable enhancement in durability. However, such binders hydrate more slowly, so proper curing conditions become more important. Ideally, the durability of concrete should not be a concern. Some degree of weathering should be expected, but improper concreting procedures can cause the deterioration to be earlier than expected. Furthermore, since durability issues cannot be seen immediately, some assessment of the impact of improper concrete curing is needed. The study has involved casting of concretes prepared with either CEM I or a CEM I blend with 30% replacement with fly ash to investigate the impact of improper curing. Performance was evaluated in terms of compressive strength, drying shrinkage, transport properties and resistance to carbonation. Paste samples were characterised by TGA, XRD and SEM to follow hydration and microstructural development. Also since the degree of saturation is known to affect the compressive strength of concrete, and curing under ambient conditions will lead to changes in the degree of concrete saturation, the work checked the impact of the degree of saturation on compressive strength; to enable an accurate understanding of the impact of improper curing. Improper curing leads to reduced compressive strength development and increased drying shrinkage. Sorptivity and permeability values were increased. This is due to reduced levels of cement hydration, as water evaporates from the concrete surface. The effect of improper curing on resistance to carbonation revealed that samples improperly cured carbonated more than those ideally cured. This study has shown that the impact on sorptivity and permeability is far greater than the impact on compressive strength, with implications for the long-term durability of concrete. Composite cements, containing 30% fly ash, showed comparable strengths to CEM I concretes and improved transport properties when ideally cured. Additions of fly ash reduced the drying shrinkage. Improper curing however led to reduced performance. Strength was compromised by improper curing to a greater degree than for equivalent CEM I mixes. However, it was sorptivity and permeability which were most severely affected. This was due to the reduced degree of cement hydration leading preventing the pozzolanic reaction between the fly ash and portlandite. Also, higher carbonation depth was seen on fly ash samples that were not cured. Low strength concrete, which already has an inherently higher porosity, is more greatly affected by improper curing than high strength concrete. This is presumed to be due to the ease with which water can evaporate from the surface of the more porous matrix. Also, concrete workability has been found to be a factor which can help to reduce the embodied carbon of concrete, with stiffer mixes having lower carbon footprints. However, this study has shown that stiff concrete mixes may be less durable and more susceptible to improper curing. This may be explained by the lower overall water contents within the stiff mixes, and therefore the greater impact of surface water evaporation. The effect of changes in the degree of saturation showed the deleterious effects of improper curing, with the saturated, ambient cured samples all exhibiting lower strengths than the equivalent ideally cured samples. The large capillary pores developed due to improper curing was seen with lower calcium hydroxide contents. The reduced hydration products obtained support the result that lower degree of hydration was produced due to improper curing since the hydration of cement cannot continues in the dry environment. This study confirms the need for good site practice, and shows that embodied carbon should not be the only factor when considering the environmental performance of concrete. Rather, durability and whole life performance should also be considered.