| Summary: |
Corrosion of reinforced concrete structures is an important problem throughout the world, leading to significant cost s of repair and remediation. Changes to the bulk composition of concrete can be used to improve durability but as the concrete surface itself is the first line of resistance to penetration of chlorides and carbonation, modification to the surface zone itself may be more effective. Controlled Permeability Formwork (or CPF) is one of the few techniques developed recently for directly improving the concrete surface zone. This technique reduces the near surface water/binder ratio and reduces the sensitivity of the concrete to poor site curing. Recent developments in CPF manufacturing intended to simplify the practical use of the technology and the role of CPF as part of a multibarrier strategy, this, along with other measures such as coatings and penetrants, can improve the durability of concrete structures.
CPF consists of using a textile liner on the formwork allowing air bubbles and surplus water to drain out but retaining cement particles and so enabling the water-cement ratio of the outer layer to become very low and the concrete to hydrate to a very dense surface skin as the filter makes enough water available at the right time to activate optimum hydration. The reduction of water-cement ratio of the outer layer is reported to increase surface strength, abrasion and freeze-thaw resistance and to reduce blowholes, water and air permeability, water absorption, depth of carbonation and also chloride and oxygen diffusion.
Various research programmes on CPF have already been carried out at FEUP. Very good results of a 1st phase led to the conclusion of its beneficial use in works where an optimum quality of the cover to the reinforcement may be crucial in terms of fulfilment of the design life of the structure. A 2nd phase refered to white concrete and the conclusion was that the CPF system used enhances durability, except for a slight reduction of  |
Summary
Corrosion of reinforced concrete structures is an important problem throughout the world, leading to significant cost s of repair and remediation. Changes to the bulk composition of concrete can be used to improve durability but as the concrete surface itself is the first line of resistance to penetration of chlorides and carbonation, modification to the surface zone itself may be more effective. Controlled Permeability Formwork (or CPF) is one of the few techniques developed recently for directly improving the concrete surface zone. This technique reduces the near surface water/binder ratio and reduces the sensitivity of the concrete to poor site curing. Recent developments in CPF manufacturing intended to simplify the practical use of the technology and the role of CPF as part of a multibarrier strategy, this, along with other measures such as coatings and penetrants, can improve the durability of concrete structures.
CPF consists of using a textile liner on the formwork allowing air bubbles and surplus water to drain out but retaining cement particles and so enabling the water-cement ratio of the outer layer to become very low and the concrete to hydrate to a very dense surface skin as the filter makes enough water available at the right time to activate optimum hydration. The reduction of water-cement ratio of the outer layer is reported to increase surface strength, abrasion and freeze-thaw resistance and to reduce blowholes, water and air permeability, water absorption, depth of carbonation and also chloride and oxygen diffusion.
Various research programmes on CPF have already been carried out at FEUP. Very good results of a 1st phase led to the conclusion of its beneficial use in works where an optimum quality of the cover to the reinforcement may be crucial in terms of fulfilment of the design life of the structure. A 2nd phase refered to white concrete and the conclusion was that the CPF system used enhances durability, except for a slight reduction of the degree of whiteness. Another programme concerned CPF application to concrete with partial cement replacement materials such as - rice husk ash (RHA), metakaolin (MTK) and silica fume (SF). RHA and MTK were both obtained in the laboratory at FEUP. The conclusions were that CPF further enhances concrete durability when using these partial cement replacements materials. Two CPF applications have been carried out, one at a Pre-cast Industrial site which was highly successful concerning surface finish and improvement of surface hardness. CPF was also applied at a sewage plant in Aveiro in a 40 m perimeter element, 2,5 m height, on the inside and outside face.
It is essential to carry on with the work in the laboratory and on site, specially concerning different CPF systems and their reuse and fully assess levels of concrete perfomance for one only use and successive reuses. |