An Investigation into the critical factors affecting the performance of composite controlled permeable formwork liners: Part I – Drainage medium
Introduction
It is well-known that long term reinforced concrete durability is a function of the near surface resistance to detrimental environmental attack. Once initiated, reinforcement corrosion can quickly propagate, impairing the structural integrity. Durability has traditionally been addressed through increased cement contents to the bulk composition [1], [2], silane or microsilica additives [3], coatings, more durable concretes and better curing practices just to name a few. However, the requirement is critical only in the near surface region and increasing bulk composition and/or including additives leads to increased costs. In addition, the UK concrete repair industry is in excess of £100m per year, indicating that these traditional approaches are not always effective [4].
Controlled permeable formwork (CPF) is one of a few techniques developed specifically for improving the near surface of vertical and inclined concrete without employing the use of additives or surface coatings. This process is made possible by lining conventional formwork with a permeable material which is engineered to remain dimensionally stable whilst under the static pressure of the concrete to provide a path for removal of air and excess mix water. The three basic elements of a CPF system (Fig. 1) are: a filter membrane which allows the passage of air and water from the fresh concrete but retains cement and other fines, a drainage layer which allows transfer of air and water to outside the formwork, and finally a structural support to provide a profile to fix the filter and drainage layer whilst resisting concrete pressure.
The operating principle of CPF is to provide a path for air and excess mix water to escape from the concrete which is directly behind the formwork. The movement of air and water is generally encouraged through the use of vibrating pokers which help fluidise the mix, thereby reducing the water/cement (w/c) ratio near the surface. The general result of the fluid movement is a denser and stronger surface which is typically free from blowholes and aggregate bridging [3], [5], [6]. The reduction in w/c ratio is also reported to increase surface hardness [1], [2], [3], abrasion resistance [2], sorptivity [7], chloride and oxygen diffusivity [8], [9], [10], carbonation penetration and freeze-thaw resistance [3], [11], [12]. With chloride diffusivity being indicated as one of the main causes of deterioration in reinforced concrete structures throughout the world [13]. An additional benefit of surface densification from the use of a CPF liner is the reduction in depth to which non-sociable graffiti media can penetrate. As a result, removal has been found to be considerably easier compared with concrete cast against conventional impermeable wood or steel formwork [14]. Similarly, algae growth has been shown to be retarded and more easily removed when using CPF cast concrete [15].
The reported role of the drainage medium is to provide an immediate path for the water and air out of the formwork. A variety of methods are employed to achieve this including drilling closely spaced holes in the formwork. However, this inevitably weakens the formwork structure. Another method utilises an absorbent media, which inherently swells creating dimensional instability and requires replacing after every cast. A more practical approach of fixing a secondary structure to the back of the filter medium is generally utilised. A variety of approaches are used depending on the filter medium textile. Structures include a coarse lattice, embossed patterns, flannel and a coarser version of the filter medium structure. Backing layers typically add rigidity to the CPF liner and reduce the likelihood of wrinkling and sagging, especially in woven textiles.
The techniques required for using CPF have developed to such an extent that the chance of misuse is slight. However, first time contractors are still weary of the technology and potential cost implications. Attaching a liner to formwork requires additional materials and increased labour in assembling the formwork. The use of CPF liners are reported to double construction costs over that of conventional formwork [16], [17]. However, this initial outlay can be easily offset when the benefits of the system are considered. Benefits such as elimination of release agents, low grade formwork, reduced cosmetic repairs and reciprocal use [4]. But by far the most advantageous benefit is the enhance durability and increase service life of the structure which leads to reduced service costs.
Section snippets
Research program
The study comprises of critical elements associated with a functional drainage medium. A range of absorption levels, textured features and texture depth are the basis of this study. Here it is intended to investigate the potential influence that a range of the above parameters have on the final surface quality and specific properties of CPF cast concrete. Statistical software has been utilised to conduct an analysis of variance (ANOVA) to identify dominant combinations in the making of CPF
Production
The concrete used in this study was designed similar to that found in other related studies for comparative purposes. Concrete (0.1 m3) was produced in a ‘Cumflow’ type forced action pan mixer. A concrete temperature of 22 °C and an initial slump of 35 to 50 mm were recorded. The concrete mix proportions are seen in Table 1.
Formwork of dimensions 750 × 750 × 150 mm was produced from plywood and coated with a polyurethane varnish to represent an impermeable formwork (Fig. 2). The cavity was centrally
Test methods
Moisture absorption of the various drainage mediums was tested to EN 382-2 standards. However, the water used for the test was substituted with a cement filtrate to demonstrate moisture absorption under more realistic pH conditions for a formwork environment. The cement filtrate was produced by mixing water and Portland cement in a ratio of 1:0.5 by mass. The mixture was then filtered through a sheet of muslin and then through a sheet of filter paper. The results are expressed in terms of
Results and discussion
The aim of the drainage medium study was to assess the effects of textured features and board absorbency on assisting the removal of air and reduction of the near surface water cement ratio (w/c) from cast concrete when using a permeable filter medium. The three independent variables shown in Table 3 were chosen as they were thought to be the dominate features to assist in the removal of water and air from the filter layer. Texture orientation directions are defined in Fig. 4. A control system,
Conclusions
The current study investigated the influence of varying levels of board absorbency, texture depth and texture orientation on the near surface properties of cast concrete when used with a permeable filter medium. The results were analysed using statistical tools to identify dominate trends. A range of tests including surface quality, surface hardness, surface strength and sorptivity were used to determine the effectiveness of each system. All tests were conducted under laboratory conditions for
Acknowledgements
The authors are grateful to the DTI, TRADA and especially Michael Jones for his unwavering commitment in laboratory support.
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