6Spacers are ubiquitous in reinforced concrete, but their influence on durability is unclear. This paper presents 7 the first study on the effects of spacers on mass transport and microstructure of concrete. Samples with 8 different spacers, cover depths, aggregate sizes, curing ages and conditioning were subjected to diffusion, 9 permeation, absorption and chloride penetration, and to μXRF, BSE microscopy and image analysis. Results 10show that spacers increase transport in all cases, the magnitude depending on spacer type and transport 11 mechanism. Plastic spacers produced the largest increase, followed by cementitious spacers and then steel 12 chairs. The negative effect is due to a porous spacer-concrete interface that spans the cover where 13 preferential transport occurs. Spacers may seem low value, small and inconsequential, but because they are 14 placed every ≤ 1 m along rebars, their overall effect on ingress of external media is significant. This is not 15 currently recognised by standards or by most practitioners.
Introduction
20Spacers are essential components in reinforced concrete structures. Their function is to secure steel 21 reinforcements in the correct position within the formwork to prevent movement prior to and during 22 concreting so that the required cover is obtained in the finished structure. The size of spacer determines the 23 size of the cover depth to reinforcement, which in structural design, is defined according to the severity of 24 exposure environment, required durability and fire resistance. Achieving adequate depth and quality of 25 concrete cover is critical because it protects embedded steel reinforcement from the external environment. It 26 is well-known that inadequate cover is the major factor causing premature corrosion of reinforcement, the 27 principal form of degradation of concrete structures. In structural design, it is assumed that achieving the 28 specified cover ensures that the as-built structure achieves the expected design performance in terms of 29 durability, fire resistance and serviceability (crack width). 30Spacers are made of plastic, metal or cementitious materials, and are available in various sizes and 31shapes (see Fig.1). In this paper, we will use a generic term "spacers", but recognising that other terms may 32 be prevalent elsewhere, e.g. bar supports, wire chairs, bolsters, continuous runners, dowels etc. Although 33 many types of spacers are available commercially, they generally fall into one of six categories: a) plastic 34 spacers with integral clip-on action for horizontal rebars of 20 mm or less, b) plastic end spacers that fit ends 35 of rebar for end cover, c) plastic wheel/circular spacers for vertical rebars in columns and walls, d) 36 cementitious block spacers for bar size > 20 mm in heavily-reinforced sections; e) continuous line spacers 37 that are either cementitious or plastic, of constant cross-section in typically1 m lengths to support several 38 bars; and f) steel wire chairs that may be single, continuous or circ...
