Concrete structures generally incorporate reinforced and/or prestressed members. Many concrete structures do require some form of reinforcement, most commonly steel bars or wires and/or other forms of reinforcement such as steel or synthetic fibres, in order to carry their design loads. However, this is not always the case, and, in some circumstances, structures may contain elements that are formed from plain concrete with no reinforcement required to perform their function.
Plain concrete (containing no reinforcement) can be used in situations where the tensile strength of the concrete alone is sufficient. Plain concrete members are generally limited to subbases and slabs on ground with low loading or in rare cases to industrial slabs with carefully constructed sub-base and designed jointing. Reinforced concrete is a material that combines concrete and reinforcement into a composite whole. Whilst steel bars, wires and mesh are by far the most widely used forms of reinforcement, other materials are used in special applications, e.g. carbon-filament reinforcement and steel/synthetic fibres. AS 3600 defines reinforcement as ‘steel bar, wire, or mesh but not tendons’, whereas it defines ‘tendons’ as a coverall for prestressing or post-tensioning tendons, bars or wires.
Prestressed concrete structures use a particular type of reinforcing system that increases the efficiency of the reinforcement. In prestressed concrete members, the concrete is placed in compression before the member is subjected to the applied loads. The compression force is provided by tensioned tendons (high tensile steel wires, strands or bars) before they are bonded to the concrete and then transfer this force to the concrete. Placing the concrete in compression increases its ability to withstand loads.
Reinforcement or prestressing of concrete combines the material properties of steel and concrete to provide a versatile construction material. Plain concrete (unreinforced) has a high compressive strength but a low tensile strength. Steel, on the other hand, has a very high tensile strength and compressive strength, but it is much more expensive to use steel for its compressive strength compared to concrete. By combining steel and concrete into a composite material, it is possible to make use of both the high tensile strength of steel and the compressive strength of concrete cost effectively.
Aside from strength properties, concrete has other beneficial attributes such as plasticity, which enables it to be moulded readily into different shapes, and relatively high fire resistance, which can be used to protect steel reinforcement embedded in the concrete. The aim of the reinforced concrete designer is to combine the reinforcement with the concrete in an efficient manner. The design requires sufficient of the (relatively expensive) reinforcement to be incorporated to resist the tensile and shear forces which may occur in the structure. The design will also utilise the (comparatively inexpensive) concrete to resist the compressive forces in the structure. To achieve this aim, the designer needs to determine not only the amount of reinforcement to be used, but also how it is to be distributed and where it is to be positioned. These latter decisions are critical to the successful performance of reinforced concrete and it is imperative that, during construction, reinforcement be positioned exactly as specified by the designer.
In reinforced concrete, the steel reinforcement carries all of the tensile stresses and in some cases, some of the compressive stresses. In prestressed concrete, the tendons are used primarily to keep the concrete in compression. The tendons are stretched (placing them in tension) and then bonded to the hardened concrete before releasing them. The force in the tendons is transferred to the concrete and so compressing it. In prestressed concrete, the steel does carry very high levels of tensile stress. Whilst it is well able to do this, there are some penalties attached. Firstly, because of the forces involved, considerable care must be exercised in stretching the tendons and securing them. Stressing operations should always be carried out or at least supervised by skilled personnel.
Secondly, the structure must be able to compress, otherwise the full, beneficial prestressing forces cannot act on the concrete. The designer must detail the structure so that the necessary movements can occur during stressing operations. Reinforcement of concrete using steel or synthetic fibres is carried out for a range of beneficial reasons. The quantity and type of fibre used is selected for the benefit that is to be achieved. Fibres are commonly used in conjunction with steel reinforcement but may be used with plain concrete as the only form of reinforcement in some cases.
Plain concrete is commonly used in ‘on-ground’ applications such as pavements, including bases, sub-bases or ‘blinding layers’ as well as wearing surfaces. The design of a pavement must include appropriate design of jointing detail and design of the thickness of the concrete base. This should be designed to maintain its integrity while transferring loads on the pavement to the sub-base and subgrade below it. Concrete used in plain concrete pavement is commonly specified by performance properties including compressive strength grade, flexural tensile strength and maximum drying shrinkage.
The types of stresses that occur in plain concrete are dependent on the structure being designed. In pavements, the key stresses relate to loading. A load on the surface of a plain concrete rigid pavement induces compressive stresses in the top surface as well as tensile stresses in the lower surface if the concrete is placed on a flexible sub-base and sub-grade. The magnitude of these stresses is dependent, in part, on the thickness of the plain concrete pavement and also on the properties of the sub-base and sub-grade. Much of the strength of a plain concrete pavement is dependent on the properties of sub-base and subgrade supporting the pavement. The principle property of the subgrade and sub-base are their combined elastic modulus (often represented as ‘modulus of sub-grade reaction’ and used in pavement design). The elastic modulus of subgrade materials is directly related to the modulus of subgrade reaction and is also related to a commonly measured and specified value of the sub-grade known as the California Bearing Ratio (or CBR). The other key location where stresses in plain concrete pavements must be considered are at the joints in the pavement base. If the joint is working correctly, it limits differential movement in the vertical direction between jointed slab segments while allowing some limited movement in the horizontal direction. To do this the impact of a vertical load on a slab needs to be restrained by shear force transfer across the joint in the vertical direction. Joints should allow for concrete dimensional change caused by shrinkage and thermal expansion/contraction without allowing differential vertical movement between two pavement segments under applied loads. such as footpaths.
Concrete has low shear strength in comparison to steel but has a moderately high abrasion resistance depending on its compressive strength grade. This supports a view that plain concrete is best suited for purposes where it is not required to rely heavily on its tensile or shear strength and uses such as concrete pavements are a good fit for these properties provided that the subgrade and sub-base are sufficiently high modulus of elasticity to provide support to the concrete pavement.
Some common applications are • Concrete road pavements; • Aircraft pavements; • Industrial pavements (interior); • ‘Blinding Layers’ over subgrade as a working platform for further concrete pavement construction; • Sub-base for unreinforced concrete highway construction; • High or low strength fill where washout resistance greater than that of granular road-base is required; • Gravity dam construction and roller compacted concrete construction more generally. The principal types of stresses that develop in structural elements or members are: • Compressive stresses – those which tend to cause the member to compact and crush; • Tensile stresses – those which tend to cause the member to stretch and crack; and • Shear stresses – those which tend to cause adjacent portions of the member to slide across each other. It is very rare that there is only one of these types of stresses found in a structural member. Generally, some combination of compressive, tensile and shear stresses will be encountered, and it is the job of the designer to determine these and locate the appropriate amount of reinforcement necessary to resist this combination of stresses. Whilst shear stresses can be quite complex in the way in which they act and react, two principal types can be distinguished – vertical and horizontal. Vertical shear stresses occur, for example, near the end supports of beams but are less near the centre of the beam where the vertical shear forces are more in balance. Horizontal shear stresses occur as the beam bends and the (imaginary) horizontal layers within it will try to slide over one another. When vertical and horizontal shear stresses react with one another, they produce what is known as diagonal tension which, in turn, tends to produce diagonal cracking. This commonly occurs near the ends of heavily loaded beams. To resist such cracking, reinforcement must be provided. This is done commonly by providing stirrups or, on occasions, cranking the horizontal reinforcement. The spacing between stirrups is closer near the supports and increases as the distance from the end of the beam increases.
When such a beam is loaded, either by a central point load or a uniformly distributed load along its length, it tends to sag or deflect downwards. This causes the top of the beam to compress and the bottom of the beam to stretch and go into tension. Reinforcement is placed in the bottom of the beam to resist the tensile stresses. Compressive reinforcement will not normally be required in the top of the beam due to high compressive strength of the concrete.
Simple Cantilevers – When a simple cantilever beam or slab is loaded, it tends to droop or deflect. Tensile stresses occur in the top of the beam or slab and compressive stresses in the bottom. In this case, therefore, the reinforcement is placed in the top of the beam. Fixed-Ended Beams – When a beam which is fixed at both ends is loaded it tends to bend. Tension will again occur in the bottom of the beam and in this case also in the top of the beam close to the supports. Reinforcement must be placed in the top near the supports and in the bottom across the centre. Multi-Span Beams and Slabs BOND AND ANCHORAGE – as has been noted already, steel and concrete act compositely when they are firmly bonded together. The strength of this bond is an important consideration in the design of reinforced concrete. It is dependent on the concrete being thoroughly compacted around the reinforcement and on the latter being clean and free of loose scale, rust or other material. Formwork oil, for example, will destroy the bond between steel and concrete. To ensure that adequate anchorage is achieved in the reinforcement, it is normally extended beyond the region of tensile stress for a sufficient length so that the bond between the reinforcement and the concrete can develop the tensile stress required at that point in the bar. Where this is not possible for some reason, or as an additional safety factor, bends or hooks in reinforcement are often used to provide the anchorage required.
POST-TENSIONING: When a member is to be post-tensioned, the concrete is first allowed to harden before the steel tendons are stretched or tensioned. They cannot therefore be allowed to bond with the concrete, at least not initially. Usually they are placed in ducts or holes which have been cast in the concrete, although sometimes they are greased and encased in a plastic tube to prevent bond. In other cases, the tendons are fixed to the outside faces of the member. After the concrete has gained sufficient strength, the wires or cables are tensioned and then fixed or anchored in special fittings cast into the ends of the concrete member. A wide variety of patented fittings and systems are available for this purpose.
APPLICATIONS Although both pre-tensioning and posttensioning systems are designed to apply prestress to concrete members, there are some practical differences in their fields of application. Pre-tensioning is normally confined to the factory production of repetitive units where the cost of the relatively large abutments or restraints, against which the prestressing jacks operate, can be justified. Alternatively, very strong and robust formwork may be constructed and wires are anchored against its ends. Post-tensioning is more flexible in its application and may be carried out on-site. It permits the use of curved tendon profiles and is also suited to a wide variety of construction techniques, such as ‘segmental construction’ and ‘stage stressing’. Since stressing is not carried out until the concrete has hardened, the concrete member itself provides the restraint against which the stressing jacks operate. 5 TYPES OF FIBRES: Another form of reinforcement of concrete is available with the use of fibres. The more common forms of fibres currently available are discussed in this section but it must be noted that new forms of fibres, including variations in material used to manufacture the fibre, shape and size are being developed all the time. The common broad material types used in fibres are summarised below: • Steel; • Synthetic/Polymer; • Glass; • Carbon; • Natural. Each of these materials will produce varying properties to the concrete. In addition, the characteristics of fibre reinforced concrete change with varying concretes, fibre materials, shape, size and dose rate. Steel fibres are the most commonly used fibres for increasing the strength of concrete elements and therefore are described by shape, aspect ratio, length and tensile strength. The steel fibre tensile strength, aspect ratio, length and shape all impact on the final concrete hardened properties containing these fibres. Synthetic fibres have been largely composed of polypropylene or nylon but newer fibres are being developed from other materials such as recycled waste. Fibres may contain polyolefin varieties such as polypropylene or polyethylene terephthalate and other suitable plastics. Synthetic fibres are commonly used to resist spalling of concrete during a fire. Synthetic fibres come in two main groups: • Macro Synthetic (also known as Structural Synthetic); • Micro Synthetic. Macro synthetic fibres are made from a number of polymers and were originally developed to provide an alternative to steel fibres in some applications. They generally have a high tensile strength and a moderate modulus of elasticity. Unlike polypropylene micro synthetic fibres, they can significantly increase the postcracking capacity of concrete. The properties of Macro and Micro Synthetic fibres are provided by EN 148892. Macro synthetic fibres come in varying lengths, aspect ratios and degree of surface texture to aid shear connection to the concrete. Micro synthetic fibres are commonly produced from polypropylene. Fibrillated fibres are defined by length (typically 6 mm to 15 mm) and generally have diameters of 0.030 mm to 0.040 mm. These are beneficial to concrete when correctly used. Some of these properties include reduction of plastic cracking in concrete and significant reduction of spalling of concrete subjected to fire when used at the correct dose rate. Glass fibres for concrete are generally composed of alkali resistant glass. Glass fibres have been more commonly used in the production of thin lightweight precast panels. In this application higher additions of glass fibre improve the tensile strength and toughness of the panels. Natural fibres and carbon fibres are less commonly used. Natural fibres may include basalt fibres and various types of plant sourced fibres. Asbestos fibres are also a natural fibre but are no longer used due to safety concerns. Other types of fibres noted have their own specific properties and impacts on concrete. In all cases the supplier’s information on their correct use must be considered before they are specified.
ACTION OF FIBRES IN CONCRETE: All fibre types can be used in conjunction with standard steel reinforcement to improve the structural and durability performance of a concrete member. It is possible that some varieties of fibre when used in the correct applications can be the only reinforcement in concrete. Plain concrete generally shows brittle behaviour leading to failure after the first tensile stress crack is formed under load. This property of plain concrete can be enhanced in some applications by using suitable fibres to provide a more ductile behaviour under load. Steel fibres and some structural synthetic fibres can provide useful tensile restraint and aid more ductile behaviour of loaded structures post cracking. Most types of fibres will reduce the likelihood of plastic cracking in concrete. Their performance at doing this is dependent on the individual product dose rate used. The dose rate for a specific fibre must be discussed with the fibre supplier. The drying shrinkage of concrete can be reduced through use of steel fibres. In this case optimising the combination of steel fibre dose rate in concrete requires coordination with the concrete mix to achieve suitable workability with this dose rate of fibre. Steel fibres, macro synthetic fibres and some specific micro synthetic fibres have proven to be useful for improving the impact resistance and abrasion resistance of concrete. Specific types of micro synthetic fibres have proven very useful in reducing or eliminating spalling of concrete under aggressive fire testing. The concrete mixture should be assessed for performance in a fire to ensure that the correct dose of fibres is used where spalling mitigation is required. Fire testing is carried out in accordance with AS 1530.4. Significant local and international research into the performance of steel fibres in providing enhancement to the shear resistance of concrete is being carried out. The result of some of this research is reflected in AS 3600. In AS 3600 a design value of the steel fibre impact on shear resistance of a reinforced concrete member is estimated based on the steel fibre component of the concrete. In all cases the degree of benefit provided by an individual fibre needs to be assessed and will depend on using an adequate dose of fibres to achieve the targeted enhancement.
APPLICATIONS FOR FIBRES IN CONCRETE: Fibre reinforced concrete is used in structures with or without the addition of conventional forms of post-tensioning, bar reinforcement or reinforcing mesh. The type of fibres used will depend on the benefit being sought and the economics of the solution. Common structures that use fibres as part of the concrete reinforcement system in Australia include: • Industrial pavements; • Concrete road pavements and roundabouts; • Precast concrete elements used in tunnel lining; • Underground shotcrete in tunnel and underground mining applications; • Sprayed concrete swimming pools in stable foundations; • Sprayed embankment stabilisation; • Footpaths and driveways; • Concrete road barriers; • Concrete elements where resistance to spalling in a fire is critical.
Source: Cement Concrete & Aggregates Australia – www.ccaa.com.au – excerpts from “Principles of Plain, Reinforced, Pre-stressed and Fibre Reinforce Concrete”