Mechanical and structural characterisation of extrusion moulded SHCC

[摘要] ENGLISH ABSTRACT: SHCC (Strain-Hardening Cement-based Composite) is a type of HPFRCC (High Performance FibreReinforced Cement-based Composite) that was designed and engineered to overcome theweaknesses of ordinary concrete. It shows a high ductility as it can resist the full tensile load at astrain of more than 3%. This superior response is achieved with multiple cracking under tensileloading which has a pseudo strain-hardening phenomenon as a result.The purpose of the research project reported in this thesis document was to design and build a newpiston-driven extruder for the production of SHCC as well as R/SHCC (reinforced SHCC) elementsand to investigate and characterise the structural and mechanical behaviour of extrusion mouldedSHCC.A new piston-driven extruder, specifically for academic purposes, was designed based on theprinciples of fluid flow mechanics. Although fluid flow is not an ideal model to represent the flowof viscous material through an extruder, it was deemed sufficient for this specific study. A newextruder with the capacity to extrude SHCC and R/SHCC was built. Provision was made that thisextruder can be fitted with extruder dies and transition zones of varying shapes and sizes.A comparative study between unreinforced as well as reinforced cast SHCC and extruded SHCC aswell as a suitable R/C (Reinforced Concrete) was conducted. Three-point bending tests,representative of the envisioned structural application, were performed on specimens of each of thecomposites.The unreinforced cast SHCC and especially the unreinforced extruded SHCC have a comparativelevel of performance to the cast R/C. These specimens displayed a similar cracking pattern ofmultiple cracks, although less pronounced in the extruded SHCC. The extruded SHCC has superiorfirst cracking and ultimate strength in comparison to cast SHCC, but with accompanying lowerductility.The reinforced SHCC specimens failed in a combination of flexure and shear. The extrudedR/SHCC specimens formed multiple diagonal cracks before failure, while the cast R/SHCCspecimens formed only a few diagonal cracks, before delaminating along the reinforcement. Thehigher shear capacity and thus the ability to form multiple diagonal cracks of the extruded R/SHCC can be ascribed to the better fibre orientation of the specimens in the longitudinal direction, whilethe cast specimens have a random orientation of fibres. R/SHCC and especially extruded R/SHCCcould be a far superior structural material to R/C.Mechanical characterisation of extruded SHCC was done with the use of uni-axial tensile andcompressive tests. The results of these tests were compared with the results of uni-axial tensile testspreviously performed on cast SHCC as well as uni-axial compressive tests that were performed oncast SHCC in this research study.The extruded SHCC displayed superior tensile performance in terms of first cracking and ultimatestrength in comparison to cast SHCC, but with accompanying lower ductility. In terms ofcompressive performance the extruded SHCC has a higher ultimate strength, but with a lowerductility than cast SHCC. The extruded SHCC also has a much higher E-modulus than cast SHCC.This can partly be attributed to the difference between the water/binder ratios of the cast andextruded SHCC, but can mainly be ascribed to the lower porosity as a result of high extrusion forcesinvolved in the manufacturing of extruded SHCC.A simple bending model for SHCC has also been introduced. This model is based on themechanical characteristics of SHCC. The model somewhat underestimates the resistance moment ofthe extruded and cast SHCC, but this underestimation is more pronounced in the case of the castSHCC. Various reasons for the underestimation is discussed, but it is postulated that the mainreason for the difference in experimentally determined and the calculated resistance moment of thecast SHCC is the possible variation in ingredient properties and specimen preparation and testing,since the characterisation of the cast SHCC was done over a long period of time and by differentresearchers. The bending model is however deemed sufficient for the design purposes of SHCC.

[发布机构] Stellenbosch University