Structural behaviour of reinforced and prestressed high performance concrete (HPC) structural elements

Abstract

Concrete technology has been advanced in various ways to improve current mix designs based on cementitious materials, or by implementing the geopolymer approach, where waste materials are used as the main binder. The development of existing mix designs has led to the creation of ultra-high-performance concrete (UHPC), which exhibits superior mechanical and durability characteristics. However, UHPC is more susceptible to forming cracks due to autogenous shrinkage. Thus, it is necessary to control the formation of cracks to extend the life cycle of the concrete structural members. Moreover, the technology of geopolymer concrete (GPC) is still under development, and there are many variables in this process that increase the uncertainty of producing GPC. Therefore, this thesis consists of several manuscripts (some of them already published and some under consideration by journals) regarding the improvement of the self-healing performance of UHPC, the optimisation of GPC mix design and the structural applications of UHPC and GPC.

The first chapter investigates the mechanical, durability and self-healing performance of UHPC with a superabsorbent polymer (SAP) under various curing conditions. The SAP-to-binder ratios used in this study were 0.3% and 0.4%. Two approaches were adopted to examine self-healing performance under repetitive loads and sustained tensile loading for up to 28 days. The results show that incorporating an SAP into UHPC enhances the concrete’s elastic modulus, flexural strength and tensile strength. Also, the mixes exhibited compressive strength above 120 MPa after 90 days. Furthermore, the load recovery of the prisms under repetitive flexural loads and the prisms under sustained tensile loading demonstrated that the self-healing efficiency of the SAP incorporated into the UHPC mixes was higher than that of the control mixes.

The second chapter presents a study in which GPC was optimised using the Taguchi method, and it details the mix design to produce ultra-high-performance geopolymer concrete (UHPGC). Utilising an L9 orthogonal array, the investigation was structured into two phases, each examining a distinct set of variables across three levels to ascertain their impact on compressive strength. The first phase analysed the water-to-binder (W/B) ratio, silica-fume-to-binder ratio, potassium-silicate-to-potassium-hydroxide ratio and aggregate-to-binder ratio. The second phase explored the effects of the W/B ratio, fibre content, silicon-carbide-to-binder (SiC/B) ratio and superplasticiser-to-binder (SP/B) ratio. The Taguchi method facilitated a meticulous evaluation of 18 unique mix designs and the identification of an optimal mix that significantly elevated the 28-day compressive strength to 126 MPa. This optimal mix was characterised by a W/B ratio of 0.28, a fibre volume of 3%, a SiC/B ratio of 0 and an SP/B ratio of 2%. Furthermore, the study found a robust correlation between the empirical findings of the second phase and the predictions of the developed mathematical model, which substantiates the effectiveness of the Taguchi orthogonal array in enhancing the compressive strength of UHPGC.

The third chapter investigates the mechanical and durability characteristics of fibre-reinforced geopolymer composites (FRGCs). The Taguchi method was employed by using the L9 orthogonal array with four parameters across the three levels of each parameter; the parameters were the percentage of fibre and the W/B, SiC/B and SP/B ratios. The W/B ratio had the most significant impact on the composites’ mechanical and durability characteristics. Analysis of variance (ANOVA) with residual fitting was conducted to derive mathematical models for accurately predicting the compressive strength that exhibited a good correlation with the test results.

The fourth chapter studies UHPC-filled double-skin aluminium tube (UHPC-FDAT) columns under combined loading conditions. Four columns under concentric and eccentric loading conditions and a beam were tested. The findings show that increasing the loading eccentricity reduced the ultimate load with an increase in the corresponding mid-height lateral displacement. The specimens tested experimentally were also modelled numerically using the Abaqus software. The numerical results show a very high agreement with the results obtained experimentally. Therefore, a parametric study was conducted by changing one variable among compressive strength, inner tube thickness, outer tube thickness and hollow ratio under the same loading conditions. The outcome of this parametric study shows the dominant influence of the compressive strength of UHPC under axial loading where an increase in the outer tube thickness achieved a higher load while enhancing the loading eccentricity.

The fifth chapter examines FRGC-filled double-skin square steel tube (FRGC-FDST) columns. Four columns under concentric and eccentric loading conditions and a beam were tested. An interaction diagram was plotted experimentally and analytically to assess the capabilities of existing models to predict the results for this kind of column. The findings show good agreement between the experimental and theoretical analyses. Furthermore, finite elements modelling (FEM) was conducted with Abaqus to simulate the constructed columns in the experimental program. The numerical and experimental behaviours of the columns also agreed.