Comparison of Pre-Stressed Concrete, Partially Pre-Stressed Concrete and Reinforced Concrete in Terms of Different Aspects
1. INTRODUCTION 1. 1Background The report will mainly focus on the differences between reinforced, prestressed and partially stressed concrete. These concrete reinforcing methods differ in the period they have been used. Reinforced concrete was developed by a nursery owner in Paris in 1867, Joseph Monier, applying it to practical use for the first time (Salmon & Wang, 2007). Prestressed concrete’s discovery dates back to the late 1800’s.
Freyssinet was one of the first engineers who used this method and realised that, for this method, high quality concrete with very high tensile steel wires, stressed as high as possible was needed (Chris Burgoyne, 2005). The chief purpose of reinforcement methods is to strengthen concrete in its tensile capacity. Concrete can support loads when in compression, but it cannot handle any tensile stress, which leads to the cracking, shrinking and creeping of concrete. Therefore, reinforcement is placed at areas in the concrete where tensile stresses develop.
In this report the reinforcement methods will for the most part be discussed in terms reinforcement in beams. Stresses in a beam develop as illustrated in the following figure: Choosing the right methods of reinforcement for a specific concrete element will depend on the properties of the concrete mix and the materials of which the reinforcement is made. The properties of the materials include tensile strength, compressive strength, elasticity, creep and shrinkage, durability, expansion coefficient and other related properties.
Other aspects, such as the economical implementation and resources, must also be taken into account when choosing reinforcement. In practise the best method of reinforcement to apply in concrete, in a specific structure, has to be identified in order to be able to design an appropriate structure which can uphold the forces that act upon it. The properties of the reinforcement thus have to be known. The report discusses the different uses and properties of reinforced concrete, prestressed concrete and partially prestressed concrete to finally determine which method would be appropriate for a specific application.
The report further elaborates on the types of methods used for reinforcement. 2. REINFORCED CONCRETE Reinforced concrete was invented by Joseph Monier in 1849, who recognized and applied its potential uses (Salmon & Wang, 2007). Reinforced concrete is used to eliminate tensile stresses in concrete. By reinforcing the concrete, its ductility and toughness is enhanced. Reinforced concrete is installed by placing bars, grids, plates of fibres on strategic positions in the cement mix when casted. Ferroconrete is iron or steel reinforced concrete (http://www. encyclopedia. com/doc/1O1-ferroconcrete. tml). The different types of reinforced concrete includes in-situ concrete (concrete is poured into a cast and reinforcement placed in on site) and precast concrete (concrete parts are manufactured and reinforced at a factory and ready to use on site) (Curry, 2011). 2. 1Types of reinforcement used STEEL REBAR Textured steel rods are placed in the concrete when it is poured. During curing the concrete then adheres to the rebar allowing stress to be transferred between the different materials, thus transferring the tensile stresses which form in the concrete, to the steel rebar (Darwin et al. 2003). STEEL PLATE Steel plates are used in the place of rebar. It takes less time to fix the plate to the concrete and it often has higher strength, because it is placed on the outside where the tensile stress is the greatest. Occasionally, steel plates are used when maintaining a concrete element (Darwin et al. , 2003). FIBER REINFORCED CONCRETE This concrete compound consists of glass, steel or plastic fibers and is mostly used in ground floors and pavement. The cost of fiber reinforced concrete is relatively low (Darwin et al. 2003). NON-STEEL REINFORCEMENT This type of reinforcement is a modern technique. Fiber Reinforced Polymer (FRP) and Glass Reinforced Plastic (GRP) are two types. FRP is useful when the environment or structure does not allow the use of steel, because of magnetic instruments which function in the vicinity. This type of reinforcement has no corrosion, but its fire resistance is quite low (Darwin et al. , 2003). * Throughout the report, the main focus of reinforced concrete will be ferroconcrete, but more specifically, steel rebar. 2. 2Uses
Reinforced concrete is mainly used in the construction of structures. The components in which it can be found are beams, columns, slabs, buildings, bridges, walls and frames. Reinforced concrete is used in all concrete components, sometimes on its own and sometimes in combination with prestressed concrete, depending on the tensile stress that needs to be taken up by the rods (Trelfall, A. , et al. , 2008). Reinforced concrete on its own is usually used in smaller structures such as one to two storey houses or structures with a small horizontal span. . 3Properties CORROSION PROTECTION The steel rebar in the concrete has to be protected from corrosion (reacting with oxygen) especially in areas where it is wet and cold. When the rebar corrodes (rusts), the steel flakes and the bond between the concrete and rebar is broken. The rebar cannot fulfill its duty to take up tensile stresses. Protection from corrosion can be achieved by one of the following methods: – epoxy coated steel – hot dip galvanizing – stainless steel rebar – corrosion inhibitors mixed in the concrete
Adequate protection can sometimes be achieved just by design and good cement mix, although this is not always the case. Sealants also have to be applied to ensure that corrosion does not take place ( Darwin, D. 2000. ). COSTS This concrete method is probably the cheapest one available for removing tensile stresses in concrete. This is mostly a result of the fact that no special equipment is needed for the installation of this reinforcement. Reinforcement is only placed in concrete, without any specialized procedures. DEFLECTION
Acceptable deflection is controlled by the serviceability requirements for a structure, for example, the total allowable deformation of the interacting components which are supported by the component undergoing deflection. General reinforced concrete does not have a significant influence on deflection and cannot be precisely predicted (Salmon & Wang, 2007). Reinforced concrete can control the deflection in a beam to a small extent, causing the beam to bend less. ELASTICITY & STRENGTH Strength is defined as the maximum load that a material can carry.
The strength of concrete usually increases when other properties of the material improves. Combined with the simple tests for strength, it makes strength a common and reliable way of measuring the quality of concrete. The reversible deformation of material is known as elasticity. When a material displays elastic behaviour, when subjected to a stress, the strain developed is fully recovered when the stress is removed from the material. Materials have a critical level, the elastic limit or yield stress, at which deformation changes from elastic to plastic.
Plastic deformation is permanent and cannot be recovered. Elastic deformation is important to prevent the failure of materials and must be considered when analyzing different concretes. The type of concrete used in engineering projects must always be able to elastically absorb the stresses applied to it. When only considering different concretes, re-enforced concrete has a relatively small elasticity. The elastic limit of re-enforced concrete is generally less than 450 MPa. When only considering different concretes, the tensile strength of reinforced concrete is relatively high.
It performs much better than normal concrete when subjected to tensile stress. The compressive strength of reinforced concrete is high, but it does not perform much better than normal concrete. This results in a small ultimate stress for failure in re-enforced concrete, as seen in fig 2. 3. PRESTRESSED CONCRETE Prestressing is based on the idea of creating permanent stresses in a structure on purpose before it is subjected to any service load (Marshall et al. , 2000: 1-1). Prestressing is also described as a method for overcoming concrete’s weakness in tension (http://www. ement. org/basics/concreteproducts_prestressed. asp). As a beam is subjected to service loads, it experiences compressive and tensile stresses (and the corresponding strains) which is a maximum in the mid span section of the beam. The resulting stresses caused by prestressing counteract those stresses induced by the acting loads, reducing the compressive and tensile stresses as well as beam deflection (Hewson 2006: 4). Concrete is known to be weak in tension, but strong in compression (Marshall et al. , 2000:1-1).
For this reason, the designer of a structure aims to reduce the tensile stress in concrete to a minimum. When subjecting an unreinforced concrete beam to an increasing load so that the induced flexural tensile stress exceeds that of concrete, the beam will development cracks and immediately fail. To solve this problem, reinforcement steel is provided in the tension zone to carry the tensile force required for equilibrium of the cracked section. The concept of Prestressing is based on the prevention of those flexural cracks forming when subjected to service loads.
Concrete will not crack when no tensile stress is present in the beam. This can be met by neutralizing the tensile stress to any desired degree by providing suitable prestressing. The definition of prestressed concrete as given by the ACI Committee on Prestressed Concrete: “Concrete in which there have been introduced internal stresses of such magnitude and distribution that the stresses resulting from the given external loadings are counteracted to a desired degree. On reinforced-concrete members the prestress is commonly introduced by tensioning the steel reinforcement” (Marshall et al. 2000:1-1). General uses for prestressing: Floors in high-rise buildings and the entire containment vessels of nuclear reactors. Figure 3 explains the steps of prestressing clearly: 3. 1 Types of Prestressing Prestressing can be accomplished in two ways: PRE-TENSIONED CONCRETE: In this method prestressed reinforcement is tensioned before concrete is placed (Marshall et al. , 2000: 1-1). The tendon is stressed by means of a hydraulic jack against an anchor frame. As soon as the concrete has obtained sufficient strength, the force is released into the concrete (Hewson, 2006:6).
Cured concrete adheres and bonds to the bars. This good bond between concrete and tendons protect the tendon from corrosion and allow direct transfer of tension (http://www. cement. org/basics/concreteproducts_prestressed. asp). Uses for pre-tensioning: Balcony elements, lintels, floor slabs, beams or foundation piles. POST-TENSIONED CONCRETE: In this method prestressed reinforcement is tensioned after the concrete is placed (Marshall et al. , 2000: 1-1). Concrete is cast around plastic curved duct, tendons is fished through and tensioned as soon as the concrete has hardened.
The tension in the wires is maintained by wedging in position after the jack has been removed. The duct is then grouted to protect the tendons from corrosion (Pretorius, 2011). Uses of post-tensioning: To create monolithic slabs for house construction in locations where expansion soils create problems for typical perimeter foundation, bridges (http://www. cement. org/basics/concreteproducts_prestressed. asp), parking garages and barrier cables. 3. 2Properties USES Prestressing is widely used in any structure type.
As prestressing is more expensive than reinforcing, it is used with caution. Prestressing is more generally used in larger structures such as bridges, balconies, large span slabs, etc. COST Cost may be seen as one of the most important factors that determine the use of this method in any structure. There are many factors that influence the cost of prestressed concrete such as: Specialized equipment for the tensioning process, specialized personnel, grouting process (if applicable), elements, etc. These factors make this method a more expensive method to use. DEFLECTION
Service loads cause a downward deflection in concrete beams, while prestressing induces an upward deflection in the beam. The resultant deflection can be either an upward or a downward deflection depending on the magnitudes of the deflection of each. The aim of prestressing is to minimize the downward deflection and, by doing so, eliminate the tension present in the mid span section of the beam. Deflection can be controlled quite successfully by means of prestressing. Tensioning is measured to a desired magnitude until satisfying compression is accomplished. CORROSION
As prestressing eliminates cracking, no path is available for any moisture to pass through the concrete to the wires or strands, eliminating the problem of corrosion. However, different protection systems are used to protect tendons from corrosion as prestressing is used in the design of structures in aggressive environments as well as in water retaining structures. Figure 4 shows the different methods used for post-tensioning. As illustrated in (a) grout in used to fill the duct after tensioning. The grout fills the voids and creates a corrosion free environment.
As an alternative grease or wax filler is used as shown in (b). A third option is to cover the individual strands in plastic wires and filling it with grease or wax. This is then inserted into the duct and grouted in. ELASTICITY AND STRENGTH When only considering different concretes, prestressed concrete has a relatively high elasticity. The elastic limit of prestressed is between MPa and MPa. When only considering different concretes, the tensile strength of prestressed concrete is very high. Prestressed concrete performs better than any other concrete when subjected to tensile stress.
The compressive strength of prestressed concrete is much lower than normal concrete, but it is still very high when compared to other materials. The compressive strength of prestressed concrete is sacrificed for a larger tensile strength. This results in a high ultimate stress for failure in prestressed concrete, as seen in Figure 2. 4. PARTIALLY PRESTRESSED CONCRETE The principle of partially prestressed structures is to allow the concrete to crack under service loading and to limit the crack widths (Hewson 2006: 82).
This method occupies the range between reinforced concrete and fully prestressed concrete. Thus reinforced concrete and fully prestressed concrete represent the boundaries of the range of possibilities which exist for partially prestressed concrete (Marshall et al. , 2000: 1-1). The university of Hong Kong and Beijing states that this method has been accepted and is becoming normal practice in many regions. According to Pretoruis (2011), partial prestressing is not generally used in practice, but is useful in special cases. 5. RESULTS AND DISCUSSIONS 5. 1Generalisation
When designing a structure, specific needs need to be met. These needs are specified by the structure design, cost, risk, client needs, etc. The above mentioned must be considered when deciding whether to use either prestressed concrete, reinforced concrete or partially prestressed concrete. 5. 2Explanation When comparing prestressed concrete with reinforced concrete, the main difference lies in the fact that prestressed concrete is a higher strength method, which produces a couple of effects such as: better control over deflection and eliminated cracking and tension. This means that the full ection of the beam participates in the carrying of service loads (Marshall et al. , 2000). Advantages of prestressing to reinforcing: •Better corrosion protection (thus, it can be used in harsh environments and water retaining structures) •Lighter materials is used. This minimizes cost in terms of transport / handling. •Less material is used because the entire section participates in the carrying of loads. •Longer spans can be accomplished because of light weight material and lower quantities of materials. •Thinner slabs, which means lower cost and lighter supporting structures. Improved deflection control is accomplished in prestressing. Advantages of reinforcing to prestressing: •Cheaper and time efficient method. •More commonly used, seeing that smaller structures is more common. Because reinforced concrete and prestressed concrete can be implemented in many ways, depending on the type of structure and the design requirements, comparing them economically is complicated. Aspects affecting cost, such as design effort, specialized hardware and specialized personnel, should be taken in consideration, when either can be implemented.
Marshall (2000) explains: “A comparison can be seen as inappropriate because a specific prestressing level can always be found within a spectrum of possibilities to yield the best solution to a given problem. ” The following graph explains the volume concrete needed to support a structure of a specific span for prestressed and reinforced concrete. 5. 3Comparison Prestressed concrete: The entire concrete beam is in compression and participates in the carrying of service loads. Some tension is present in the concrete beam and does not participate in the carrying of service loads. This method offers improved deflection control
Lighter material is used in this method such as wires, cables and beams. Specialized equipment is used to tension tendons. A larger span can be accomplished without thickening the concrete beam. Reinforcement is protected better from corrosion and is therefore more suitable in aggressive environments and water-retaining structures. Reinforced concrete: No tension present in concrete beam, thus no cracks in concrete beam. Some cracks is apparent in the tension section of the beam. Deflection is not as controllable as in prestressed concrete. Heavier materials is used in this method such as steel rods.
No specialized equipment is needed for reinforcing. The larger the span needs to be, the thicker, as thus heavier, the beam should be. Reinforcement is protected from corrosion to a good extent, but is less than that of prestressed concrete because of cracking that occurs. 6. CONCLUSION Prestressing and reinforcing are two very effective methods for strengthening concrete. Each has its own advantages and effective applications. The aim is to know what properties to compare when choosing a method for a specific structure design, keeping in mind that using both is also an option.
After investigating the different methods and the properties of each, it is clear that the decision is not a straight forward one to make. Structures in aggressive environments, structures that retain water and bigger structures are more likely to make use of prestressed concrete, while smaller, more common structures make use of reinforced concrete. Partially prestressed concrete should be considered when it is not clear which method should be used. After deciding which method to use, it will be wise to make the calculations of total cost of each before finalizing the decision. After all, the decision rests on minimum cost and risk.