A precast destiny?

"Stainless reinforced precast concrete, if used properly, can deliver all-round positive environmental benefits for the concrete industry, the contractor and customer alike."

Corrosion of reinforcement within concrete is often first evidenced by the tell-tale bleeding marks of ferrous corrosion products. Corrosion can eventually lead to concrete spalling and loss of tensile capacity of reinforcement which can lead to collapse of the structure. Many methods are offered to prevent corrosion in reinforced concrete and there is also a large category of protective or preventative responses to corrosion which promise a cheap solution to the customer. They often 'reward' the customer with the promise of a cheaper initial project cost. Many readers will be familiar with the 'Law of Fives': 1 extra spent today can save 5 in putting right construction defects which can save 25 on the in-service maintenance which can save 125 in rehabilitation costs. Much of the data on problems relating to corrosion tend to support this proposition.

Precast production quality

In reinforced precast concrete we have a good example of where the balance of the concrete and stainless steel reinforcement comes as close to perfection in the use of concrete as possible. The use of precast offers factory production quality with far greater controls on placement of reinforcement resulting in uniform cover between the structure surface and the reinforcement within"1. The resulting product is consistent showing little of the variation often arising with in-situ applications which, being unique, vary from one hour to the next.

Stainless steel reinforcement

The use of stainless steel reinforcement allows reduced cover and a relaxation of crack-widths. From an architectural and aesthetic standpoint, the beauty of exploiting reduced cover will result in the creation of more slender structures. These can obviously be best produced in controlled factory environments. Stainless steel reinforcement does not corrode and is forgiving in precast as well as in-situ applications. Before the additional benefits are discussed it should be noted that it is an expensive material. It tends to track the cost of high tensile carbon steel at between four and ten times the latter's cost. Even during the recent sharp rise of high tensile steel prices, stainless has remained roughly within this band.

Aside from providing maintenance-free, long-term structures, it has the benefit of being a fully recyclable material at the end of the life of the structure.

In certain circumstances, stainless and carbon steel reinforcement can be coupled in concrete with little, if any, risk of initiating galvanic coupling between the two materials. No isolators are required. This means that stainless steel can be used highly selectively in a structure where limited cover may be required or where only part of the structure is in danger of being exposed to an aggressive environment. For example, decorative panelling on a sea front which is precast, may have the front of the panel reinforced with stainless reinforcement but the rear of the panel may only require carbon steel reinforcement. Because stainless steel is a weak cathode compared to carbon steel reinforcement, its presence in the continuously connected cage reinforcement, which is providing the tensile properties for the panels, will act like a resistor to the carbon steel reinforcement's initiating corrosion - so long as they are coupled without isolators. How the stainless and carbon steel are actually used within the structure will depend upon the lapping of the two types of reinforcement.

Stainless steel is similar to carbon steel reinforcement in that it follows the same rules for bending and lapping and requires no special treatment when fixed. It must be noted, however, that the processing (cutting and bending) of stainless must be undertaken on machinery clean of any carbon steel contamination. If carbon steel is impacted into the stainless steel during processing for a project, this impacted carbon steel may be a source of future pitting corrosion. Specifying engineers do not need to worry about treating stainless steel 'specially' in terms of design. It should be specified as if it were carbon steel reinforcement and used selectively in those areas most at risk of corrosion.

Environmental benefits

Some of the positive environmental benefits and practicalities of using stainless steel reinforcement in precast are:

* the reduction in cover of the concrete saves on the quantity of cement used

* given that the cost of cement is increasing, a reduction in cement quantities will help off-set the cost of stainless steel used in the structure

* the reduction in the quantity of concrete means a lighter precast structure and reduced transportation costs

* it may also assist in terms of ease of placement when arriving on site.

case studies

BaIlM College, Oxford

Stainless steel reinforcement was successfully used in the steps of post-graduate residences for Balliol College at the Master's Field in Oxford (see Figures 1 and 2). The project involved the use of an attractive coloured aggregate with very thin cover in a passive environment. The restricted area within the stairwell encouraged the architect and specifying engineer to opt for slender steps. Durability was an important consideration. Therefore the only practicable solution was to use stainless steel reinforcement. Cover in this case was limited to 15mm with 4OMPa concrete strength.

Westminster University, London

Westminster University in London is one of Britain's more recent universities, designed to be in use for centuries to come (see Figures 3-5). The architects were primarily interested in efficiency of delivery linked to a bright, white coloured concrete and the obvious need to avoid unsightly bleeding of corrosion products through the precast units at some point in the future. The benefit of using stainless reinforcement was again fully utilised in this case. The engineers only used 20mm cover with 4OMPa concrete strength and a white Portland cement.

The simplest of the stainless steel alloys, Steel Designation No. EN 1.4301, was used in both examples. The cost of using stainless reinforcement is often unnecessarily high due to the specification of alloys that are more corrosion resistant than is actually required. The use of EN 1.4301 in the Progreso Pier(2), which has adequately withstood the test of time for 60 years, amply illustrates the robust nature of this alloy.

Precast concrete balconies

Precast components for concrete balconies provide an important area where stainless is now being used. Historically, the reinforcement in the balcony and the cantilever reinforcement securing this to the building structure was carbon steel. These early reinforced concrete balconies have been given the unfortunate name of the 'silent killers'. This is due to the lack of any tell-tale bleeding through the concrete and paintwork prior to balcony failure and collapse. The corrosion product bleeds back into the wall cavities rather than through the exterior surface of the concrete. Replacement by stainless reinforcement eliminates this risk whilst not compromising the quality of concrete or appearance of the reinforced structure.

Further development of stainless reinforcement in precast concrete

There are many areas in which the interaction of stainless reinforced precast concrete offers exciting, elegant, reliable and cost-effective solutions. For example, reinforced precast tunnelling sections where the overall cover and, therefore, quantity of concrete are reduced and car parks where precast slabs, reinforced with stainless reinforcement, will withstand any corrosion induced by the wetting /drying action of car tyres depositing chlorides. Slab thickness could also be reduced at no expense to the performance of the car park.

Concluding remarks

It is to be hoped that stainless reinforcement, used in conjunction with concrete, will enable the plasticity of reinforced concrete to be tested to the limits of engineering convention. These limits are best executed in the controlled environment offered by precast production techniques. Stainless reinforced precast concrete, if used properly, can deliver all-round positive environmental benefits for the concrete industry, the contractor and customer alike.