Investigation of Structural Behaviour of Geopolymer Prestressed Concrete Beam

Production of ordinary Portland cement (OPC) is a carbon-intensive process that generates significant amounts of carbon dioxide (CO2) gas from the combustion of fossil fuels and thermal decomposition of limestone. Overall, cement industries are responsible for around 7% of global CO2 emissions which poses a considerable threat to global climate change because of its greenhouse effects. Geopolymer is an inorganic polymer material having similar binding properties to OPC which can be produced from aluminosilicate compounds, such as fly ash when activated by alkaline solution. The recent advent of geopolymer technology shows great potential to reduce carbon footprints by utilising industrial by-products, such as fly ash and ground granulated blast furnace slag (GGBS), and convert into effective binding material. The setting and hardening process of geopolymer binder is different from hydration of OPC, called “geopolymerisation” which is the condensation process of aluminate and silicate monomers to form a polymer chain. Generally, fly ash-based geopolymer concrete attains relatively lower early-age strength at ambient temperature due to the slow rate of reaction. However, geopolymer concrete based on GGBS or a combination of fly ash and GGBS can set and harden in ambient temperature with comparable early age strength to OPC concrete of same grade. In the recent past, several studies were carried out to investigate mechanical, serviceability, durability and microstructural properties of geopolymer concrete using different aluminosilicate materials. However, limited research has been carried out on applications of geopolymer binder in structural concrete, such as reinforced concrete beam, column and prestressed concrete beam. Prestressed concrete is a construction technique in which flexural tensile stress generated in the concrete member due to imposed load is counteracted by applying an initial prestressing compressive force. The use of prestressed concrete structures has been increasing in modern construction practices because they can withstand significantly higher flexural load with minimal deflection and cracks than conventional reinforced concrete (RC) members of similar cross-section. Generally, tensile strength of concrete is ignored in the design of conventional RC structures. However, tensile or flexural strengths of concrete are significant in the design of prestressed concrete structures where tensile strength of concrete limits the maximum permissible prestressing load according to ACI 318. Application of higher prestressing load can increase the load-carrying capacity of prestressed concrete structures and minimize their deflection under service load. Previous results showed that geopolymer concrete possesses higher indirect-tensile and flexural strength than OPC concrete for the same compressive strength. In addition, time-dependent losses of prestressing stress are the major serviceability problems of prestressed concrete structure which reduce the load-carrying capacity of structures and increase the deflection under service loads. The time-dependent losses of prestressing stress are directly proportional to the amount of shrinkage and creep strains of concrete. Having smaller drying shrinkage and creep strains, geopolymer concrete can result in better serviceability than OPC concrete in prestressed concrete structures. Thus, this study investigates the application of geopolymer concrete in the prestressed concrete beam which may be a worthwhile utilization of geopolymer concrete in concrete structures. Despite having higher mechanical strengths and durability properties than conventional OPC concrete, geopolymer concrete has not been widely used in structural grade concrete, so far. The safety hazards in mixing and handling of concrete due to the use of liquid sodium hydroxide in geopolymer binder is one of the barriers to the adaptation of geopolymer in concrete industry. In this study, the mechanical and serviceability properties of grade 50 MPa geopolymer concrete from sodium hydroxide-free one-part geopolymer binder are investigated under ambient temperature curing and compared against same grade OPC concrete. Development of strengths at an early age under accelerated curing is investigated to study the suitability of geopolymer concrete in precast prestressed concrete structures. Finite element models of prestressed concrete beams of three different lengths and sizes are analysed to investigate their load-deflection behaviours under imposed load for short-term and long-term durations using the Abaqus program. The effects of tensile strength of concrete in load-deflection behaviours of prestressed concrete beams are studied by comparing the results between identical geopolymer and OPC prestressed concrete beams. This study finds that geopolymer concrete has around 27% higher indirect-tensile and flexural strengths than OPC concrete of same strength grade which contributes to geopolymer prestressed concrete beams to withstand around 20% higher first-crack load than OPC concrete beams of same span. In addition, geopolymer prestressed concrete beams show a relatively smaller loss in prestressing stress which results in a smaller loss in flexural capacity of beams over the service life of the structure