MANUFACTURE OF FIBRE-REINFORCED COMPOSITE REINFORCING BARS FOR CONCRETE BY MICROWAVE ASSISTED PULTRUSION (MAP)
Jim Methven, Fayez Khalaf
Centre for Manufacure, UMIST
England

Keywords: microwave, pultrusion, re-bar

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A composite reinforcing bar (re-bar) for concrete must incorporate a sufficient number of surface undercuts that together form a mechanical key to the concrete matrix and prevent the bar slipping under an applied load.
A common method of achieving this by pultrusion is to imprint the weave of a coarse fabric (peel-ply) on the surface of the bar by pulling both though the pultrusion die and removing (peeling off) the fabric at the die exit. Such re-bars are expensive since their cost includes the cost of the peel-ply itself, the cost of the resin it "absorbs" and the cost of the reduced line speed that is imposed by the reduced rate of heat transfer to the bar itself within the die.
The continuous MAP process involves pulling a cylindrical core of catalysed vinyl ester resin and glass roving through a simple PTFE shaping bush and then though the centre of an overwinder. The overwinder comprises an arm that can rotate about the core axis and which holds a bobbin of glass roving, the end of which is attached to the core. As the core moves through the overwinder the arm winds roving around the core in a helix, the pitch of which is determined by the line speed and rotation speed. It is this helix that forms the undercut for mechanical keying to the concrete matrix. After passing through the overwinder the profile is heated and thereby crosslinked by passing though a travelling wave microwave applicator in the form of a 1m long WR340 (86mm x 43mm) aluminium waveguide.
Unlike conventional pultrusion, the MAP process does not use a rigid die but rather exploits the non-contacting nature of microwave energy to heat and to crosslink the profile so that the undercut formed during overwinding is preserved. In this work the re-bar cores were 9mm in diameter and produced at line speeds in the range 0.3m/minute to 1m/minute using microwave power between 500W and 1kW at 2450MHz.