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Within concrete there are various oxides including calcium, potassium oxides and sodium. These oxides are in high concentrations and are soluble which allows the oxides to react with water. When the oxides have reacted with the water present in the concrete a high alkaline environment is developed within the concrete. The alkaline environment present cause the steel reinforcement bars to develop a passive layer on the surface of the bars. The newly formed passive layer works as an impermeable barrier to hold back chloride ions, which will reduce the rate of corrosion caused from oxidation. The passive layer can repair itself provided the alkaline levels within the concrete environment are maintained and do not change. This seems like a good system however in practice the steel will corrode. The breakdown of steel can be accelerated because of two main reasons, breakdown of passive layer from chloride ions and carbonation.
Carbonation happens because of carbon dioxide being present in the atmosphere combined with the alkaline hydroxides within the concrete. This causes a reaction between the carbonic acids and the alkaline hydroxides within the concrete. The steel reinforcement isn't directly affected by the carbonic acids or the alkaline acids, the reaction however neutralises the alkalis within the concrete. This is because the carbon dioxide reacts with the pore water, and as a result calcium carbonate is formed. With regards to carbonation there are various factors which can accelerate the rate of carbonation within concrete. The quality of the concrete mix is an important factor and the workmanship is just as important. Evidence of carbonation can been seen on a range of older buildings as pictured below in figure .
The reason for carbonation in older structures is that the structures often have a porous cover and a low content of cement. The result of this formation is that the pores form a network in which carbon dioxide can easily distribute itself throughout the concrete. The rapid spread of carbon dioxide will accelerate the reduction of alkalis within the concrete resulting in a faster carbonation of the concrete as there is low cement content.
Chloride ion migration is to blame for a great deal of corrosion problems within concrete. The chlorides may be present in the form of seawater or contamination of aggregates. De-icing salts used on highways may also contaminate the concrete, and contamination has also been caused by sea water spray for structures situated in coastal areas. When the steel surface of the reinforcement comes into contact with the chloride ions, the passive film is disrupted allowing the corrosion process to accelerate. In the United Kingdom corrosion from de-icing salts is not as bad as the United States of America as a waterproof membrane is used within the UK highways.
When corrosion takes place within reinforced concrete the characteristics of that structural member will change. Corrosion within reinforced concrete will result in a great strength loss to the structural member. With one weak structural member it could result in the structure as a whole being unsafe due to the mechanism of the corroded member. Corrosion can affect bonding mechanism between the concrete and steel, the torsion and shear strength and the load bearing capacity. The loss of bond strength is subject to the layer of corrosion forming between the concrete and the bars which will greatly reduce the strength of the member in every aspect. These are important considerations when designing such a structure.
The corrosion of steel within concrete is one of the biggest problems for industrial countries with large infrastructure networks (Broomfield 1997). In 1991 it was estimated that the annual cost of bridge deck repairs where anywhere between 25 - 100 million (£) in the UK (transportation research board, 1991). The highways agency (UK) made an estimation that corrosion caused on highways from salt to be around the figure of £616.5 million, an estimated £39 million has also been spent on bridge repairs (Wallbank, 1989). Costs can be categorised as short term costs, including design, construction and installation. Other costs can be categorised as long term, these may include modifications, maintenance, disposal and deconstruction. These categories may be grouped once more as direct and indirect costs. Direct costs are those associated with repair cost such as materials and labour. Indirect costs are those occurring from the process of repair such as the loss of toll for bridges or the cost of not being able to travel through that route for the user of the highway/bridge (Chang S.E et al, 1996). There are difficulties that can arise when reviewing the lowest cost of other solutions, when other parties give different levels of importance on the different types of cost (Humphreys,).
Short term costs of FRP
Fibre composite materials are currently expensive in comparison to traditional construction methods and materials when viewed on an initial basis. There are factors that govern the price of composite materials these can include; high cost of raw materials, processing and manufacturing costs and availability. The aerospace and maritime industries have adopted the fact that high price tags are often attached to such materials. As a result of the industry accepting these costs it has become common practice for materials to be priced so high. Like many materials the more quantity being used will lower the price. If bulk production is initiated it is believed the costs will be dropped, this can be achieved by other industries such as sport, aerospace and civil using fibre composites (Humphrey (16). There is debate as to whether this is an optimistic approach as a great deal of fibre composites used are imported between countries such as; Europe, Australia, the United States of America and Japan. The import and export costs of procuring fibre composites can inflate the price greatly to the point where it may be unfeasible to use fibre composites. It should be taken into consideration that manufacturers price according to quantity ordered, so it could be a while before anticipated price reductions are enough to influence prospective designers/clients (Humphreys).
FRP pricing in terms of short term is dependent on the fabrication of the fibre composites, with the addition of high material costs. The techniques for fabricating fibre composites are derived from the aerospace, marine and car industries. In comparison the construction industry requires different methods and techniques for fabrication in order to produce fibre composites suitable for larger scale structures. Design specifications are generally different to other industries, and this means that duplicate copies of a member are rarely used. The mass manufacture for industries such as aerospace where similar parts are used for a range of projects can benefit from price reductions due to the same form of fabrication. Te construction industry however requires bespoke parts for each structure, and this can make costs higher as the same method of manufacture cannot be used. Other short term costs to consider are transportation and erection of the fibre composites. It may be of benefit to produce large lightweight modular components, as lower weight can result in savings of transportation and craneage costs. By creating fewer but larger modular components erection time can also be reduced which can be of benefit to the contract programme. According to a study carried out by Meier (Humphrey's reference 17) details that it is difficult to put quantities to indirect savings but they are important to the overall cost. Meier believes that there are savings that can be made at the systems level, and this is because faster construction can be established meaning the community will be disturbed less. Meier also believes that by having a lower dead weight for the structure, savings can be made in the form of smaller lighter substructures and lighter construction equipment. Other industry professionals such as Shapira et al (Humphrey's 18) and Ehlen (humphrey's 19) believe that by closing roads in busy areas congestion, detours, administrative cost, environmental costs, reduced maintainance and downtime in industrial applications can produce benefits.
Costing of FRP Materials
For civil infrastructure projects the techniques for costing can vary for each project, this is often because of individual choice. It is common for parties with financial interests in construction projects will base their cost decisions on the initial cost of the building. This is generally because of the project budget and the requirement of value for money, and this means obtaining the best structure with little concern of the future performance. There are two techniques that are believed to consider the fine details surrounding the application of fibre composites within the civil engineering industry. The first technique was developed by EL-Mikawi and Mosallam (Humphrey's reference 20), this technique uses the Analytical Hierarchy Process (AHP). The technique is used to evaluate the project needs, management, manufacture and maintenance. This method does however neglect the comparison between tangible and non-tangible factors, these can include; impact on amenity, or cost/benefit analysis of the provided solutions. The second technique is called the Whole of Life (WOL) method, and this method is a derivative of life cycle costing. The WOL method is believed to be more detailed with the inclusion of; initial cost, operating cost, maintenance cost, retirement and disposal cost, replacement and refurbishment cost, taxes, depreciation and management costs. Costs that are ongoing are given a value determined by the predicted lifespan of the structure. This method is useful for economic comparison of parameters such as new and existing materials for structures that are designed for equal performance criteria including; strength, stiffness and service life.
Strength and stiffness
High specific strength and high specific stiffness offered by FRP materials are said to be potential benefits to the construction industry. This claim is based on the concept that FRP materials with these properties can contribute to lightweight structures in certain scenarios. By constructing with light weight materials the option to construct larger structures is feasible. The option of lighter materials also allows larger components to be fabricated meaning reduced transport and erection costs, and cuts can also be made to the substructure and foundations. The potential of using FRP materials as a replacement for traditional materials could as a whole produce a lighter building with a range of savings, this could however be difficult to physically produce. FRP costs are still high, for example an FRP bridge deck can cost up to ten times that of a traditional precast concrete plank that will cover a single span two way bridge (humprheys ). It should be noted that the use of a lighter deck such as those constructed using FRP, will result in much smaller transport costs. It is estimated that travelling costs can be reduced by up to 75%. The transport saving could only amount to a couple of thousands pound for projects close to the manufacturer, however the further the distance the greater the saving. Smaller savings can be obtained through lower lifting costs and substructure savings, these however are not considered significant in most cases.
Stiffness is often used to evaluate the performance of civil structures, and materials with a low gross elastic modulus such as FRP can result in the structure being over designed with regards to strength. Over designing of a structure is not a good prospect in terms of money, as this will result in the structure being financially uncompetitive. Stiff laminates can be forged using FRP however they are generally not cost effective and are known to have a lower failure strain which can reduce their application. The future of FRP may hold higher modulus laminates, produced at lower costs. With further research it may be possible to achieve increased fibre volume fraction and a higher gross modulus without the use of expensive techniques.
Tailorable properties of FRP
There are claims that that FRP allows designers to achieve versatility when compared to traditional materials for civil applications. This may be utilised by the careful selection of fibres and resins, the layout of the constituents is vital to produce structures with combination of performance features. The achievable characteristics include strength, durability stiffness, fatigue and impact which can all be produced at varying levels. In order to tailor these traits a fabrication method that utilises localised variations of laminate composition. Methods such as the pultrusion method, will allow the designer some allowable variation in fibre type on a ply by ply situation. The pultrusion method will allow for some variation but will not allow the designer to vary the resin composition or changes in laminate layup (Humphreys).
A main advantage of FRP is that it can be forged to withstand environmental conditions. The possibility of engineering different characteristics from a large choice of fibres and resins could be extremely useful to the civil engineering industry. FRP materials can exhibit such features as radiation resistance, dynamic loading, chemical resistance, deterioration through age and freeze thaw cycles. These abilities could be extremely useful for specialist civil applications. It should be noted that FRP has only been used within civil applications for a short period of time, this means that further testing over a longer period of time is required to determine FRP's durability. There is often debate over the standard of FRP durability over an extended period of time, liao (humprheys reference 22) believes that there has been careful consideration shown towards FRP's use in infrastructure applications. A study carried out by Karbhari (Humphreys reference 23) details that long term studies regarding FRP are sparse and do not cover structures within the service life bracket of seventy five to one hundred years. The civil Engineering Research Fund (DERF) carried out research regarding long term durability data. This study revealed main areas where FRP lacked in civil engineering applications, the two areas are, moisture effects, fatigue, creep, physical degradation and alkaline resistance (Humprheys reference 24).
Alkali resistance (carbon fibres)
Carbon fibres exhibit strong resistance to environmental conditions, and are resistant to atmospheric conditions, moisture, weak acids and solvents (ASM 1987 docirc.pdf). A study carried out by Judd (1971 docirc.pdf) revealed that carbon is resistant to all solutions of alkali and temperatures. During this study carbon tows had been soaked in a 50% w/v sodium hydroxide solution for 257 days. The results showed variations in elastic modulus and strength at around 15%. A further study carried out by Arockiasamy (1995 docirc.pdf) tested beams that have been prestressed with carbon fibre strands against 9 months of wet/dry cycles for 9 months in an alkaline solution. The results revealed no degradation had taken place with regards to flexural strength. These studies reveal that carbon fibres can endure direct placement within the concrete environment for prolonged periods of time.
Alkali resistance (Aramid fibres)
It is known that fibres such as Kevlar are resistant to a range of chemicals and solvents but are vulnerable to the affects of acids (ASM 1987 docirc.pdf). a study carried out by Dupont (1992 docirc.pdf) tested two Kevlar specimens Kevlar 29 in a 10% solution of sodium hydroxide for 1000 hours and Kevlar 49 in a 40% solution for 100 hours. The results showed Kevlar 29 lost 74% of its strength and Kevlar 40 lost only 3%. These results could prove indecisive as the solution and time the materials are prolonged for vary greatly, however it is expected that Kevlar 40 should perform better. Aramid bars exhibit less damage, and this is because of the resins forming protection for the aramid bars. Aramid bars produce a 2% - 10% loss when exposed to solutions formed of sodium hydroxide. It is estimated that aramid rods would decrease by 60% in air and 50% of the short term strength in a concrete environment over 100 years (Horn et al 1977; gerritsde et al 1992, 1995 docirc.pdf). from the tests carried out it can be seen that aramid bars will decrease in strength when being subject to concrete. It should be noted that higher modulus aramid bars will increase resistance to akalis and should be selected for concrete applications if required (docirc.pdf)
Alkali resistance (glass fibres)
It is already known that glass fibres are vulnerable in terms of chemical attack. The extent of glass fibres vulnerability means that acids/bases, alkalis/solvents and the concrete environment will deteriorate the fibres. The application of glass fibres within a composite could allow for chemical protection of the glass fibres. This seems feasible however this may not always be the case. This situation means that glass fibres may not be favoured for selection within a composite as it exhibits a great vulnerability.
The concrete environment
Creep rupture and static fracture are types of degradation affecting reinforced concrete. These forms of degradation gradually reduce the strength of a concrete member, the presence of water, acids and alkalis can accelerate the degradation until the fibres eventually crack. A study carried out by Diamond (1985 docirc.pdf) showed that the tensile and flexural strengths of glass fibre composites could be greatly reduced in the presence of water and alkalis over a short unexpected period. In order to avoid the rapid loss of strength the use of a twofold protection system must be applied to the concrete environment by using the correct resins. A matrix must be formed that is strong enough to stop the formation of matrix micro cracks and has as little as possible diffusion within the matrix.
Carbon fibres are not directly affected by water, but the matrix however is subject to the affects of water. The affects of water will indirectly affect the composites properties, for composites that are formed to be unidirectional strength can be reduced for compression, shear and small reductions to tensile strength (Ciriscioli, 1988; Sen et al., 1996c docirc.pdf). One experiment tested the affect of water by using concrete with externally reinforced graphite composites, the concrete/composites are then subjected to 100 freeze thaw cycles. This experiment showed little change to the composite, however the adhesive bond from the concrete to the composite had degraded (Chajes et al., 1994; Karbhari and Engineer, 1996) docirc.pdf). Kevlar (para amid fibres) are also affected by water at high temperatures. According to Allred (1984 docirc.pdf) aramid composites that have been dowsed in water for extended periods of time have lost flexural strength up to 35%. Water can also affect glass fibre after a few months of exposure, losses in strength of 10% can be expected with regards to tensile and flexural strength (Springer et al., 1981; Novinson et al., 1998; Faza et al. 1994; Pantuso et al., 1998 docirc.pdf). According to Rahman et al.(, 1996 docirc.pdf) the loss of strength may be down to negligence and poor testing of specimens.
It is known that aramids are the most vulnerable in terms of ultraviolet resistance. An experiment was carried out in Florida where a Kevlar 29 fabric was left in direct sunlight for 5 weeks. The test revealed that the Kevlar lost 49% of its strength. In comparison a thick rope only lost 31% of its strength after 24 months, this is because the outer fibres have protected the inner fibres from UV damage (Dupont 1992 docirc .pdf). Resins are heavily affected by UV; this means they require protection in order to form a reliable composite. The protection can be achieved by additives and coatings, and this will help combat reduction in shear, compression and tension.
It can be seen that some forms of FRP has some durability issues that can be a constraint for some applications. The short term strengths are acceptable however there are some issues regarding long term strength when in the presence of poor environmental conditions. It can be seen that the presence of water, ultraviolet light, acids, and alkalis can substantially reduce long term strength. There are recommendations put in place until official design measures have been formed.
Guidelines (docirc. Pdf)
GFRP usage should be restricted when in direct contact with concrete.
GFRP working stresses when using bars should be limited to 25% of the ultimate tensile strength.
GFRP tendons and bars should not be relied upon as a primary reinforcement within concrete, unless the reinforcement is not subject to continuous loading for prolonged periods of time.
GFRP can be used for secondary reinforcement, including column traverse reinforcement and column wraps.
ARFP should not be used in marine conditions.
ARFP with high modulus should be used for situations that expose the AFRP to concrete.
ARFP working stresses should be limited to 35% of the maximum tensile strength.
All FRP reinforcements should be protected from UV and moisture.
CRFP working stresses should be limited to 60% of the ultimate tensile strength.
These guidelines form important measures for designing FRP reinforced concrete members. These are however only guidelines and official literature is still required to give specific details for design.