Concrete decisions18 November 2004
The use of controlled permeability formwork on concrete surfaces can improve concrete quality and durability, says David Wilson
ONE of the first modern controlled permeability formwork (CPF) liners, Zemdrain by DuPont, was developed about 15 years ago. Since then great strides have been made not only with product development, but also in general acceptance of the CPF technology. Several hundred technical reports and papers have been produced on the topic and in the field over 3Mm2 of concrete face area has been cast worldwide.
For those unfamiliar with CPF, the CIRIA Report C511, Controlled Permeability Formwork  gives a detailed review of the technology and its benefits. So far for water and power structures CPF has only been used selectively, but structures in these sectors have perhaps more to gain than others.
Records show that ad-hoc CPF systems were used as long ago as the 1930's to improve the quality of concrete surfaces. During a period of dam building in the Scottish Highlands in the 1950’s attempts were made, not always very successfully, to improve sloping surfaces by using absorbent board. However it was only in the late 1980's that modern CPF systems started to emerge. One of the earliest applications of Zemdrain was for a dam in Turkey where the liner was used to avoid surface defects on the spillway surface. Initially the potential benefits of using a CPF liner were seen as: removing blowholes; reducing formwork pressures; and improving concrete durability
The removal of blowholes was initially the most obvious benefit of using a CPF liner particularly when using inclined top shutters and this still remains a primary benefit. The desire to reduce formwork costs was seen as the principal benefit to be gained from a reduction in pressures, however initial research showed that although pressures were dissipated faster; peak pressures remained unchanged. The expected durability benefits have now been confirmed and this has now become the primary driver for the specification and use of CPF.
Initially technical information on CPF performance was limited and early liner use was mainly limited to applications requiring blowhole removal. As detailed technical information and independent recommendations became available, liner use became more widespread. CPF use is now covered in several guides and standards:
• Formwork - A guide to good practice, Concrete Society 1995.
• Plain formed concrete finishes, Concrete Society 1999.
• Controlled Permeability Formwork, CIRIA 2000.
• BA57 Bridges - Design for durability, Highways Agency 2001.
• BRE, 2001, Special Digest 1. Concrete in aggressive ground.
• BS8500-1: 2002, Concrete - complementary British Standard to BS EN206-1.
• Guide to the construction of reinforced concrete in the Arabian Peninsula, CIRIA/Concrete Society 2002.
The initially perceived benefits of using CPF are still seen as important but they do not tell the whole story. Concrete specialists have always understood that the outer surface of the concrete, the covercrete, contains the poorest quality concrete in the structure. Since historically there has been no means of remedying this situation, the problem has to a large extent been ignored. Except of course for coating companies who would not put a specialist coating onto an unprepared traditionally cast surface. They would usually remove this weak, porous, contaminated surface by sand blasting and replace it with a faring coat prior to coating application.
Despite this situation, almost the entirety of research work on concrete durability has focused on improving the product or protecting it with coatings or penetrants. The effects of process and what happens at the concrete/formwork interface have been largely ignored. CPF, however, addresses many of these important issues of process, including:
• How to remove the excess air and water, which is trapped at the interface between the concrete and the oiled impermeable, faced formwork (plywood or steel), which causes blowholes and an increase in the w/c ratio (reduction in cement content) in the outer 20mm of the surface.
• How to avoid softening and contamination of the surface due to the use of release agents and curing agents.
• How to improve the early age curing of the concrete.
None of these aspects are properly addressed by increasing concrete strength, using different cement types or by the introduction of any number of wonder chemical admixtures. The above process problems can be overcome by the use of CPF with the consequent improvement in concrete quality and durability in the covercrete zone.
Indeed recent research work reported by McCarthy , shows that for both site and laboratory work the use of CPF in combination with any of these traditional durability enhancers provides the best durability solution. CPF gives a high quality well cured surface in the outer 20mm while the other durability enhancers can improve the mass durability.
In spite of industry approvals, the overwhelming technical evidence and over a decade of use with several million square metres of concrete face area cast with a CPF liner, overall usage remains limited. Zemdrain has been used on many water, dam and power projects throughout the world, but in terms of total concrete face area, the use is still minimal. Although also used for bridges, marine and water retaining structures, the overall take up of CPF remains disappointing. Why should this be and is it a technical or commercial problem?
The construction industry always seeks to pigeonhole all products and CPF liners are perceived as no different. On that basis, CPF has been conveniently categorised into two separate pigeonholes: With all the other durability enhancers (cements, admixtures and coatings), thus the unique benefits of CPF are largely unrecognised or ignored; and as a means of removing blowholes on a par with 'bagging up', thus the benefits of CPF use are largely ignored and producing a virtually blemish free surface first time is equated to a repaired surface with a limited life
By ignoring the benefits of CPF, usage is being limited, to the detriment of concrete. But what exactly are current industry attitudes?
Surface durability is not critical
Some engineers believe that surface concrete is of the same quality as the heartcrete and that by using different cements or admixtures that the problem of surface durability is solved. The fact is that when traditional oiled impermeable faced formwork is used, then no matter what amendments are made to the product, the quality of the outer 20mm of the concrete surface will always be the poorest in the structure due to water gain.
By pigeonholing CPF as just another durability enhancer, specifiers do not readily pick up on the critical uniqueness of this technology. Surface durability is of great importance for most water and power structures. The only available means of removing excess water from the concrete/formwork interface is the use CPF. By removing this excess water the concrete in the outer 20mm of the surface is transformed into the best in the structure. The properties of the heartcrete can always be improved by using any of the other durability enhancers.
Blowholes do not matter
For most structures it appears to be the perceived wisdom that blowholes do not really matter, as they have no structural significance and that they are just minor surface defects. This should certainly not be the case for water and power structures. However, the attitude that a repaired surface is as acceptable as producing a virtually blemish free ‘as struck’ surface has taken hold. As a consequence ‘bagging up’ is an accepted fact in the concrete industry. Increasingly even these cosmetic repairs are being avoided and the long-term effects are ignored.
Blowholes do matter as they are a symptom of process problems and can lead to premature surface deterioration. CPF use removes the majority of blowholes and pinholes, even on inclined upward facing surfaces, such as for spillways.
The release agent solution
A whole panoply of release agents are now available which are promoted as giving a variety of benefits to the finished concrete. The reason for using a release agent is to retard surface reaction to enable the easy removal of the formwork and prevent adherence to the concrete. By doing this, the surface is softened, more porous and also contaminated.
Contamination provides a potential food source for biological growth and inhibits the performance of most coatings and surface penetrants. The softer more porous surface also encourages quicker degradation from environmental attack. Release agents are not used with CPF. Visits to 8 to 10 year old marine and water retaining structures bear witness to this effect with CPF cast surfaces displaying insignificant degradation and biological growth when compared to traditionally cast surfaces.
Curing is not critical
The curing affected zone in concrete can go as deep as 50mm (i.e. the full depth of the cover). It is openly acknowledged that curing is not performed satisfactorily on many sites, whilst the importance of curing has been extensively highlighted, . Poor curing affects the longer-term quality of the concrete. CPF liners have been proven to act as curing membranes until form removal as they retain water in their structure and have also been used to provide longer term curing whilst the form is kept in place. According to Price, , ‘CPF appears to offer a means of reducing the sensitivity of the concrete to the curing process and providing a fail-safe enhancement of concrete surface properties’.
By pigeonholing CPF and by ignoring the fact that the surface of the concrete is of a lower quality it is easier to ignore process problems. As the deleterious effects on durability of using oiled impermeable faced formwork can be ignored, it is easier to keep offering new generations of chemical admixtures. It is also more convenient to do something in the batching plant than to introduce a new procedure on site. Different procedures often carry unnecessarily high price tags and potential operational savings are ignored. It can appear to some that savings in operation costs like cleaning and maintenance are not worth considering. Costs of using CPF have reduced due to the introduction of Type III liners, which are consistently reusable. At construction stage the additional costs of using CPF are minimal. If a true whole life costing is performed taking account of maintenance and refurbishment costs then the use of CPF offers significant savings.
CPF and water and power structures
The improvements to concrete produced by using CPF liners like Zemdrain can be of great benefit for water and power structures:
• Surface blemish removal – CPF virtually eliminates blowholes which can cause serious problems for concrete in contact with fast moving water
• Release agents not used – CPF gives a dense, low permeability surface which is uncontaminated, less degradable and less supportive of biological growth
• Curing is improved – CPF acts as a curing membrane until form removal and if kept in place additional water can be added for prolonged curing, resulting in a well cured low porosity surface
• Mechanical durability improved – Surface hardness, tensile strength and abrasion resistance are all significantly improved
• Resistance to environmental ingress - CPF has been proven to give durability benefits equivalent to an extra 15 to 20mm of cover, with significantly improved resistance to the ingress of all air and water borne elements
• Coating benefits – for extremely aggressive locations requiring a protective coating, CPF use produces a surface that does not require any preparatory sand blasting or filler layers prior to primer and coating application
Zemdrain CPF has been used on a number of water and power projects. A few examples are given below:
• Lingese dam, Singen, Germany – The upstream face of an existing arch dam was refaced with the new concrete being cast using Zemdrain‚. A number of other dams in Germany have also used this technique to improve the quality and durability of the new concrete surface.
• Pollan dam, Donegal, Ireland – This 130m long central concrete dam with an integral 50m long spillway section had an outer skin of facing concrete of a higher grade than the hearting concrete. In addition, due to the aggressive acidity of the reservoir water, all of the upstream and down stream exposed concrete faces for this new structure were cast using Zemdrain.
• Conecuh river dam spillway, Alabama, US – The designers required a dense, durable and stable blowhole free surface for increased erosion resistance. Zemdrain‚ was used during the casting of a 60cm overlay to the sloping spillway to extend the life of the dam and three unit hydroelectric plant.
• Lake St. John, Quebec, Canada – An existing unsafe dam was removed and replaced with a similar dam. Zemdrain‚ was used on all surfaces in contact with water, as the client wanted a dense, durable concrete surface without blowholes for increased erosion resistance, improved freeze/thaw resistance, longer life and reduced life cycle maintenance costs.
• Power station, Great Yarmouth, England – Zemdrain‚ was used to improve the mechanical resistance of the concrete in outlet structures
The evolution of CPF has been quite remarkable. In a relatively short time frame it has been shown conclusively that CPF offers unique benefits to the engineer in ensuring durable low maintenance concrete. Costs with the new generation of liners have reduced, such that there should now be few economic arguments against liner use for quality and durability enhancement.
For further information contact David Wilson, Zemdrain Consultant, Max Frank Gmbh. Email: [email protected]