Development of practical and economical ultra high performance fibre reinforced concrete for prestressed beams

Abstract

Ultra High Performance Concrete (UHPC) is an ultra-high strength concrete with superior mechanical properties. The high compressive strength (more than 150 MPa) and high ductility make UHPC a suitable material for manufacturing pre-cast structural members. Its high compressive strength can be beneficial for high-rise buildings as well as long-span bridge decks by making lighter structures. However, manufacturing UHPC in large-scale is not appealing for industry and the main reason can be attributed to the manufacturing cost including the material and preparation cost as well as the cost of equipment required. Typical UHPC contains high cementitious materials and fine aggregate (less than 600 µm). In addition, UHPC often requires highly specific types of curing making it unpractical as well as expensive.

Although literature exists on development of UHPC and its applications, little has been published considering wider applications and practicality, particularly in South Africa. This thesis is an attempt to develop a more practical and economical UHPC utilizing local materials in South Africa while maintaining significant UHPC properties. The study contributes toward findings a solution for increasing the use of UHPC, thus allowing industry to take advantage of its superior properties.

A comprehensive study was conducted not only on the development of an economical and practical UHPC, but also on the development of practical curing regimes. Manufacturing UHPC prestressed beams required investigation of bond performance between UHPC and high strength prestressing wires. UHPFRC prestressed I-beams were tested under flexure to determine whether shear fibres can act as shear reinforcing, thus making it possible to eliminate shear stirrups and limit the web thickness.

The results revealed that manufacturing practical and economical UHPC is achievable by utilizing local materials available in South Africa. This accomplishment is possible as a result of several modifications to the typical UHPC mix design. These modifications include: 1) increasing the aggregate content to an aggregate to cement ratio (by mass) of 2.5 resulted in the reduction of the cement content to 593 kg/m3 ,2)increasing the aggregate size with a maximum particle size of 4.75 mm and 6.7 mm as a fine aggregate and coarse aggregate, respectively. Heat treatment in 80°C water for 2 days can be considered as a practical curing regime. During the curing procedure, the rising and falling of the water temperature should be controlled to avoid thermal shocking.

Experimental results on the bond performance between UHPC and high strength prestressing wires revealed shorter development length is required by inclusion of fibres in UHPC. In the presence of fibre, embedment lengths of 100 mm and more at the age of 7 days and older provides a sufficient development length. However, in the absence of fibre sufficient development lengths are achieved at higher embedment lengths of 125 mm and 150 mm.

Testing UHPFRC prestressed I-beams under flexure confirms that fibres can act as shear reinforcing.