The influence of curing techniques and chemical admixtures on the properties of concrete

Includes bibliographical references.The deterioration of concrete in South Africa is becoming of major concern to the construction industry. The maintenance of reinforced concrete structures is an extremely expensive exercise and is a continuing necessity. Concrete curing is a practice that is understood to be a necessity within industry, but is often overlooked as a result of time and/or economic constraints. The objective of the study is to ascertain whether or not the implementation of better quality and alternative curing techniques will improve the durability properties of the concrete. Curing is defined as the maintenance of appropriate moisture and temperature conditions to permit the continuation of the hydration or pozzolanic reaction. The objective of curing is to ensure the progress of hydration reactions causing the filling and discontinuity of capillary voids by hydrated compounds in newly placed concrete. Modern curing methods are generally classified as wet or sealing. Wet methods include fogging, sprinkling, ponding, immersion and wet coverings. Sealing methods include plastic coverings and membrane forming curing compounds. Crystallising permeability reducing admixtures may be included in the concrete mix design in order to decrease the penetrability of concrete by decreasing the interconnectivity of the pore structure. Curing methods need to be employed in order to assure specified durability limits are acquired, as durability constraints are implemented in industry. Various methods of curing were tested in order to establish the effect of the techniques on the durability properties of concrete. Samples were placed in water and in winter (Western Cape, South Africa) and simulated summer environments. Various curing techniques were then employed within each of the exposure environments. The curing methods were damp hessian, cling wrap, two curing compounds and two crystallising permeability reducers (PRA's). Samples were also left untreated in each environment as reference samples. Compressive strength, oxygen permeability, water sorptivity, chloride conductivity, bulk diffusion and accelerated carbonation tests were conducted. The results obtained in the study concur with those presented in literature. Prolonged periods of moist curing are significantly beneficial to the compressive strength and durability properties of concrete , however, full water immersion is not a feasible alternative for large or insitu-cast concrete elements. Results of the study show that d amp hessian was the best method to ensure superior durability properties. The sealing of samples with curing compounds in a cool and wet environment (winter) is not recommended, whereas it is marginally beneficial, as was clingwrap, in a hot and dry environment (summer). The crystallising PRA's provided mixed results and were favourable where excess moisture was available and fairly ineffective in dry conditions. The durability properties of concrete are markedly affected by the curing technique implemented