ABSTRACT
Concrete, when it first came into use, was hailed as being long-lasting and largely maintenance free. The need for extensive repairs to concrete structures was hardly considered. And, with appropriate design of the concrete, with due recognition of the exposure conditions and a few simple rules in designing the mix and some simple pre-service testing, there is no reason why extended lifetimes for RC structures cannot be enjoyed. However, experience tells us that reinforced concrete is often not the maintenance-free material that some people expect and early failure sometimes ensues. It is worth exploring the reasons for this before proceeding.Far too often, the basic design of the concrete and attention to detail in its placement are lacking. In the author’s experience, many concrete mixes are not correctly designed and suffer a tendency to bleed and segregate, all of which renders the protection that should be afforded to the steel reinforcement compromised from the start.Add to that incorrect placement of the steel, with inadequate cover and you have a recipe for early failure, whatever the exposure conditions. In severe conditions, with exposure to marine or de-icing salt, for example, failure can be very rapid indeed. Concrete Society Advice Note 17 (Roberts, 2006) lists some simple rules for good-quality surface finishes for cast-insitu concrete (see next section). Not surprisingly, those same rules will help to provide durable structures when combined with due attention to selection of an appropriate concrete and cover for the exposure conditions it will experience. The author once worked on the old Severn crossing, now replaced by the new bridge. The concrete in this bridge was of such high quality that it had not carbonated at all in 35 years and attempts to demonstrate a Windsor Probe device for strength estimation, caused the pin to simply bounce off, as the concrete was so hard!There are other reasons why premature failure can occur: in the 1960s, for example, following pressure from the industry to improve formwork stripping times, the cement industry responded by grinding the cement finer and increasing the tricalcium silicate content (C3S) in the cement. Readymix producers were quick to spot that it was now possible to achieve the 28-day
design strength with less cement (and therefore a higher water to cement ratio). Whereas, previously, concrete tended to gain in strength significantly after 28 days, with the newer cements that strength gain was much lower. The end result of this was concrete that carbonated rapidly, resulting in the onset of reinforcement corrosion much sooner than expected. There has also been a tendency for contractors to leave the procurement of the concrete to the buying department. Left with a free hand, they will choose the cheapest concrete possible – typically with about a 50 mm slump (S1 consistence). This concrete may be totally unsuitable to place in the works, where a higher workability may well be required, so the temptation to add some water on site to improve placeability is huge. The result, again, is a higher than anticipated water to cement ratio and lower durability. The correct procedure, of course, is to decide on an appropriate set of concretes for the different parts of a contract, with appropriate workability in each case. These can then be called off as required, and the temptation to add water avoided!Designers can also improve the chances of a durable structure by avoiding placing drip details directly under a bar (or at least providing additional cover
or protection in these areas). Here, the drip groove is placed underneath a horizontal bar, with typically 5-10 mm cover at the top of the groove! The author cannot count the number of times this simple error has been observed on structures during his career!Another wonderful example of poor durability design is the use of so-called ‘reconstituted stone’ for window mullions and sills. These are made from a semi-dry mortar mix rammed into moulds and usually contain one or more steel rods for handling purposes. An alternative, sometimes found, comprises a rather poorly compacted concrete core, containing the steel rod, with a well-formed attractive stone-like, sandy coloured, mortar facing. Both types have a tendency to carbonate rapidly and corrosion of the steel then ensues with splitting and spalling of the units. Since these are often in use on high-rise structures and offices, the risk of falling material causing injury is high. Indeed, when tackled on this issue, one company, who shall remain nameless, advised the author that carbonation was no longer an issue, because they carbonate the units at the factory! He was of course referring to the other problem that reconstituted stone can suffer – flaking and crumbling of edges and corners, due to inadequate water and compaction in the mix. Carbonating the concrete hardens the surface and thus helps to avoid damaged edges, but compromises the durability of any units containing steel reinforcement, unless this is galvanised or otherwise protected.Billions of pounds are spent each year on the repair of structures. In the US, alone, for example, annual repair costs are estimated at 18-21 billion USD (source: American Concrete Institute). Getting it right is therefore of critical importance.In the remainder of this chapter we will explore some simple rules for avoiding problems in the first place and how non-destructive testing (NDT) and semi-destructive tests like coring and lab analysis can pay dividends in avoiding or diagnosing defects. 1.2 Get the concrete right
To ensure good-quality, well-finished and inherently durable concrete in the first place, a few simple steps need to be followed. 1 The correct quality of concrete, which has been designed to achieve an appropriate strength, durability for the exposure conditions it will experience and ‘finishability’.2 The use of the correct type and quality of form-face material and release agent suitable for the finish specified.3 Workmanship, both in producing the formwork and mixing, placing, compacting and finishing the concrete.
