Beating the freeze
Concreting and cold conditions are not generally compatible. But new product research being carried out in the US could mean that concreting is not ruled out by a drop in temperature.
The basic issue is that of freezing water within the mix. As the ice crystals expand, so the concrete is liable to spall. At -3degreesC, 90 per cent of the water will freeze and the concrete will lose up to 50 per cent of its usual strength, and this will not be regained because fractures will have occurred within the concrete.
However, if the concrete is able to delay freezing for sufficient time to start to mature (as the water starts to chemically combine with the cement), the amount of water liable to freeze diminishes and the concrete gains strength normally - even if more slowly.
The National Building Specification (NBS) standard guidance states that when curing, 'maintain surface temperature above 5degreesC' for at least four days. The figure of 5degreesC reflects the point, gleaned from empirical study, at which 'normal' conditions, are deemed not to exist. Below 5degreesC, strength gain in the mix is significantly slowed, lengthening the time at which formwork can be stripped.
BS 8110: Part 1 says that 'at no point' should the temperature of the concrete 'fall below 5degreesC until the concrete in the structural element reaches a strength of 5N/mm 2'. Once this strength has been reached, the concrete is usually deemed to be out of danger.
To produce quality concrete, the air temperature must also be monitored and controlled to avoid temperature variations, causing the mix to dry out locally. Even if the concrete is warmed and insulated, strong winds can create significant temperature gradients across and within the concrete.
Over the years, a variety of admixtures have been introduced to accommodate the vagaries of the climate. Current site practices involve heating the materials to 10degreesC, insulating the finished works and surrounding the works area with wind breaks. This can be a very costly exercise indeed.
New research, by the US Army Corps of Engineers Construction Productivity Advancement Research (CPAR) program in consultation with companies in the private sector, have developed prototype admixtures which may change the nature of cold weather construction.
Low-temperature admixtures are chemicals that lower the freezing point of water and speed up the hydration of the cement. Even though many admixtures can perform these tasks separately, the researchers have created an admixture which can do both without adversely affecting the strength of the concrete or harming the steel reinforcement.
Field evaluation tests have been carried out by Charles Korhonen and Robert Ryan at a US Army site in Michigan. Panels of 150mm thick reinforced concrete were laid in exposed conditions in a snow covered site. The temperature average was -8.7degreesC (although minimum night time temperatures during the course of the tests fell to -16.5degreesC). A control panel of concrete, with no admixture, was laid in a heated enclosure (with air temperatures of approximately 29degreesC).
The concrete was mixed in external conditions with unheated aggregates and 'minimally heated water'. The batches were delivered to site within 30 minutes and laid in a further 30 minutes. The fresh concrete slumps were: 50mm for the control mix, and 130-140mm for the treated mixtures.
Results were striking. Not only did the admixture enable the concrete to gain strength at 4.7degreesC which actually exceeded the strength attained by the control samples at 5degreesC, but tests have revealed no detrimental side effects for concrete. The admixture does not contain chlorides or alkalis so does not induce corrosion or cause alkali-silica reactions, nor does it interfere with the air-entraining agents.
The conclusions suggest that safe working temperatures of concrete can be reduced to -5degreesC, greatly improving construction programmes. In Michigan, although notably colder than the UK, the construction season might be increased by 60 days per annum. Costs for the admixture are in the region of 38 per cent of the costs of protection works, excluding cost benefits to the work programme.
Korhonen is working towards 'getting low-temperature admixture technology into practice as soon as possible'. His latest success has been laying the concrete floors of the ice-storage facility at Sequoyah Nuclear Power Plant at -8degreesC. Unfortunately, as yet, none of these low temperature admixtures are commercially available.
Additional data supplied by Charles Korhonen, US Army Corps of Engineers, to contact him e-mail marie. c. darling @erdc. usace. army. mil A new guide, Handy Hints for Cold Weather Building by Castle Cement, is available from 0845 722 7853 or from www. castlecement. co. uk The people at box consultants are an interesting bunch. The practice is a multidisciplinary outfit, mixing engineers, designers and architects who have captured a niche in research and development of architectural 'product design'. It has worked on schemes as diverse as the Millennium Dome (research into the structural uses and fire resistance of cardboard in the Local Zone) to kinetic artwork at the Science Museum (with Darrel Viner). It is very keen on a collaborative approach to design. It 'lends people, borrows disciplines and creates in-between roles'.
One of its recent breakthroughs, in conjunction with dlm architectural designers, has been the development of a new 'freeze-dried' fabric which can create stable threedimensional stretched shapes for a range of architectural applications.
The first use, for which the research was principally undertaken, was for a display format at the British Council's 'The Fabric of Fashion' exhibition, where garments were suspended over a highly reflective internal surfaces of a series of stretched forms.
Lycra is fixed around a former and stretched to shape. Held in this position the material is liberally sprayed with epoxy resins and allowed to dry before coating with paint. In line with its environmental commitments, the firm is currently exploring using 80 per cent reconstituted resins, for 'hazard-free' production.
Curvatex, as the new material has been called, enables the creation of weatherproof, stable, complex curved surfaces without the need for formwork or extensive framing structures. Effectively, this material has all the benefits of GRP but it requires no moulds. It has many applications - from acoustic panelling and abstract shopfitting items to construction formers, canopies or sculptural forms.
As well as the display potentials, this unique method of creating rigid three-dimensional shapes opens up a range of design possibilities; box consultants is now exploring the potential for convex shapes.
For more information contact Tom Barker, tomb@bconsultants. co. uk Nils Norman is currently hosting an exhibition at the Institute of Visual Culture in Cambridge, enigmatically entitled, 'Phase 1: Geocruiser.The Mother Coach:
Earth'.After 11 March, it will be going on national tour, which is handy given that the exhibition is a bus.
With such a sci-fi-esque title, there can only be a sense of disappointment to find that is represents a cross between a traveller's battle bus and a mobile library - Mad Max meets Gardeners'Question Time.
The 'cruiser' is intended to show proposals for 'an alternative redevelopment of urban niches and public places'. It houses a library, offering 'free access to books on radical gardening, eco-activism, self-sustainability and collectivism' and contains a photocopy machine operated by solar power. The rear of the bus has a built-in greenhouse. Why am I reminded of 1970s TVdramas of what life will be like after the bomb goes off?
Antifreeze concrete being placed in northern Michigan at an ambient temperature of -10degreesC.The concrete was placed and finished in the usual manner and covered with a sheet of plastic to minimise moisture loss during curing. No thermal protection was applied. The concrete temperature dipped to -4.7degreesC and not only did not freeze but gained strength while in those conditions The CI/SfB system of library classification is, in equal measure, the bane and the salvation of many architectural offices. Manufacturers' infuriating habit of omitting dates on their product brochures, let alone a full index of reference codes, never made an adoption of the system a straightforward option for many.
Uniclass (Unified Classification for the Construction Industry) - launched in 1997 to tie together some of the discrepancies between CI/SfB and other classification standards - has had some success in complementing and, in some circumstances, supplementing CI/SfB.
The new guidance offers conversion tables from one to the other, and adds more numbers to make the location of documents more thorough. Instead of (31.5) 'Doors' read 'L411'; however, instead of 'acoustic doors' under the same CI/SfB reference, the Unicode classification has a subset 'L41124'. The possibilities for a joint reference code are possible, although some clashes are inevitable.
My advice would always be to get a librarian. The problem is that unless they are at your beck and call, even if they have put the literature in the right place, you will have to understand the system yourself to retrieve it. The Uniclass wallchart, which is designed to be a simple guide to accessing the system, has simplified things a little too much and could provoke the same heartache as did the CI/SfB system - although, to be fair, most product information in Unicode has been situated in one section, Section L. Finally, I note that there is no Uniclass reference or date on the cover.