Thermal curing of concrete with conductive polymer technology.

Concrete is used on the majority of all construction projects. It is manufactured using the core constituents of a cement binder, typically Portland Cement, proportions of coarse and fine aggregates and water. The strength development of concrete is achieved by the addition of water that reacts with the binder in the form of a hydration reaction. This hydration reaction or strength development is dependant on two primary curing factors, time and temperature. Increasing the rate of strength development of concrete by elevating the curing temperature at early ages has advantages of maximising concrete production and reducing manufacturing time. Increasing the temperature of early age concrete up to 70 deg C is used in precast plants where it is critical for a minimum strength to be achieved within a certain time period to allow the removal of the concrete sections from moulds and forms. Various practices are adopted by precast concrete manufacturers to achieve the early strengths required to maximise production, both thermal and non-thermal. These include heating the various mix constituents of the concrete, using unnecessarily high cement contents within the mix, using large quantities of chemical admixtures or by increasing the ambient temperature of plants within which the elements are manufactured. In precast concrete plants the use of steam to elevate the curing temperature of the concrete is the most commonly adopted technique. However, it is inefficient and rarely provides controlled temperatures to the concrete as recommended in approved codes of practices and standards.This research programme has investigated the use of an alternative heating technology for concrete curing, optimising a unique Conductive Polymer Technology (CPT). The application and optimisation of the CPT material to provide heat curing to concrete within the laboratory, in-situ and within precast concrete plants has been investigated. The electrical properties of the CPT were investigated to determine their relationship with the size of the concrete elements. This was done for various CPT materials with different Characteristic Resistances. Having gained an in depth understanding of the electrical properties of the CPT, various heating Jackets were designed and manufactured to thermally cure concrete elements at early ages. The Jackets were designed with various outer protective materials. The effect of CPT curing on the strength and shrinkage, both at early ages and long-term, was determined. The thermal performance of the heating jackets was determined for each application including the uniformity of the heating provided into the concrete element. The interaction of the heat generated by hydration and CPT heating for larger elements was also investigated.The results showed that the CPT materials varied depending upon their manufacture and required target resistance. The thermal blankets had the capability to uniformly heat concrete elements at various ambient temperatures, to temperatures required by standards and approved codes of practice for accelerated curing of concrete. This uniform heating resulted in greater compressive strength of laboratory scale concrete elements with reduced shrinkage. The research identified the important parameters for CPT jacket design andmaterials selection e.g. the importance of the contact between CPT heating elements and the concrete element and the selection of appropriate insulation materialsThe test programme also investigated the durability of CPT under different exposure conditions. The results from testing the CPT material under conditions such as freeze thaw, heating & cooling and wetting & drying showed that wetting and drying had the most significant affect on the CPTs resistance, altering by 10%. Other tests of durability included punching holes of various sizes into the CPT samples to determine their effect on the CPT's resistance. This was found to be directly linked to the area of CPT material removed.The manufacture, performance and operation of the CPT materials has also been investigated to provide an understanding of its mode of heating and its effect on concrete curing. The concept of maturity has been used to determine relationships between strength development and thermal curing and energy requirement for thermal curing when using CPT.