Development and structural applications of ultra high performance and lightweight high strength concrete

Muhamad Ibrahim, Sifan Mohamed (2023) Development and structural applications of ultra high performance and lightweight high strength concrete. Doctoral thesis, Northumbria University.

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Abstract

The concrete industry is prioritising economically viable and environmentally sustainable solutions, particularly in the advancement of high-strength concrete (HSC). This emphasis aims to expand structural applications for more effective construction solutions. The development should be customised to meet specific construction requirements and align with available resources. This thesis aims to advance the development of lightweight high-strength concrete (LWHSC) by in depth analysis of LWHSC mixes, utilising advanced machine learning (ML) techniques, and by forging the path with ultra-high performance concrete (UHPC) produced from locally sourced materials. Beyond this development, it delves into a comprehensive evaluation of LWHSC's structural competence, examining its infusion into hollow flange cold formed steel (HFCFS) beams. Simultaneously, the research undertakes an analysis of the flexural behaviour of thin UHPC panels. The outcome of this thesis would transform the realm of HSC solutions.

This research was initiated by an exhaustive assessment of 197 prior LWHSC mixes, meeting strict criteria of oven dry (OD) densities below 2000 kg/m³ and 28-day compressive strengths exceeding 60 MPa. Subsequently, utilising this comprehensive database, advanced machine learning (ML) techniques were employed for precise prediction of LWHSC compressive and splitting strengths, aiming for an efficient mix design guideline development for LWHSC. As a result, exceptional predictive accuracy was achieved by fine-tuned gradient boosting machine (GBR) model with R2 values of 0.94 and 0.92 for compressive and splitting strengths.

UHPC mixes were developed incorporating coarse fine limestone, basalt, and dolomite using particle packing theory and evaluated their mechanical properties and microstructural characteristics. Results showcased the viability of these UHPC mixes, showing satisfactory workability and impressive compressive strengths at 360 days range from 143.2 to 190.7 MPa, with splitting tensile strength between 5.3 and 9.3 MPa. After the development of UHPC, flowable LWHSC mixes were developed using fine sintered fly ash (LYTAG®) as LWA, replacing the normal weight aggregate of UHPC, yielding an impressive 84.3 MPa compressive strength with an OD density of 1951 kg/m³. Limestone powder (LP) partially replaced cement paste to reduce cement consumption, with the optimal replacement rate at 10%. Additionally, the mixability of polypropylene (PP) powder for eco-friendly LWHSC was explored, yielding promising results (70.2 MPa compressive strength; OD density of 1654 kg/m³) with 20% volume replacement indicating the potential for environmentally friendly LWHSC.

In terms of structural applications, the study examined the flexural and shear behaviour of hollow flange cold formed steel (HFCFS) beams filled with lightweight normal and LWHSC using finite element (FE) numerical simulation. Results showed that concrete infill significantly enhanced CF-HFCFS beam flexural and shear capacities by up to 54.5% and 8%, respectively. From a detailed parametric study, novel simplified design equations were developed to predict the ultimate moment capacities (mean: 0.99, COV: 5.2%) and ultimate shear capacities (mean: 1.00, COV: 2.0%) of CF-HFCFS beams. Finally, the study examined the flexural performance of thin UHPC panels reinforced with textile FRP. It investigated the impact of the developed UHPC mixes and pigments on the panel behaviour. Adding pigments reduced workability by 7%, however, had almost no effect on compressive strength. FRP significantly improved flexural capacity by approximately 23.3%, preventing sudden collapse. The unconfined stress-strain relationship indicated brittle behaviour, with modulus of elasticity of above 60 GPa due to UHPC's inherent brittleness. With overall results, this thesis offers a tangible testament to the transformative potential of this research in real-world construction applications.

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