Construction with concrete
Construction with concrete – our most frequently used, man made material.
Fundamental properties of concrete structures
What we normally call concrete consists of cement, water and ballast. The ballast may consist of sand, natural stone, crushed rock material,expanded clay clinker etc. If other binders than cement are used, e.g. bitumen, a clarifying addition is made and we then talk about bitumen concrete. In the following the word concrete is used for concrete materials, in which cement and water are used as a binder. Since concrete is more or less fluid in the fresh state and hard after the cement having hydrated, the most varying shapes can be created by casting or spraying.
By varying amounts and properties of the components a concrete material can be produced that has properties which vary considerably within wide ranges. Concrete itself has high strength in compression but very low strength in tension. Structures which could be realized under those conditions would be very uneconomical. In order to realize structural elements which can withstand other actions than compression only, it is necessary to improve the ability of the concrete to withstand tensile forces. This is accomplished by introducing cast in steel bars in the concrete. The concrete is said to be reinforced. The technique of designing the reinforcement of the concrete is equally important as the technique of producing a concrete with the required properties.
If a relatively large amount of water is used at the production of the concrete, an easily workable concrete mass is achieved, which is at risk to separate during casting. The shrinkage during the hardening process will be large and the strength will be low. If a smaller amount of water is used, the strength will increase and shrinkage will normally be smaller. The workability, however, will be very bad so it will be very difficult to compact the concrete and the strength and the durability will be low for that reason. By using different additives and filler materials the water amount can be decreased and still a good workability can be achieved. This is a prerequisite for high strength concrete. The shrinkage can normally not be reduced to zero, however. Cast in reinforcement does not shrink and the surroundings (e.g. a rock) do not. Restrained shrinkage is therefore a loading case that must be considered by the structural engineer. Concrete under permanent load is slowly deformed. This phenomenon is called creep. Creep causes increased deformation, which causes problems at large structures as bridges with post tensioned reinforcement.
The reinforcement of concrete consists normally of cast in steel bars or mesh made by steel bars which are welded together. Lately the technique of mixing fibres of steel or other material in the concrete has bee developed. The length of a steel fibre is normally around some tens of millimeters and its diameter a few tenths of a millimeter. The production of fibre reinforced concrete is comparatively expensive, however, and it is not possible to achieve the same load carrying capacity with fibres as with steel bars. The steel bars must interact with the concrete. That is normally achieved by deformations on their surface, which causes a grip in the concrete. Even if the stiffness of the reinforcement is larger than of the concrete, the steel bars do not affect the behaviour of the concrete at loading to begin with. The reason for that is that the amount of steel is much smaller than the amount of concrete. "The concrete does not experience the steel" before it cracks. After cracking the steel bars become active and start taking up forces. The width of the cracks increases hereby. The cracking is a problem both for the aesthetics and for the durability. The problem can be mastered by designing the reinforcement so that the cracks are kept small. Cracks with a width of 0,2 mm are not visible at some distance and they do not affect the alkaline environment of the concrete which protects the steel bars from corrosion.
When the concrete cracks its stiffness becomes reduced. That means that the deflections of a beam e.g. can be unacceptable if the beam is very slender. For such structures tensioned reinforcement is often used – normally post tensioned reinforcement. This technique implies that tubes are cast in the concrete. When the concrete has become hard steel wires are pulled through the tubes. The steel wires are then tensioned by hydraulic jacks and anchored in the concrete by means of special devices. By this the concrete is initially in compression and a flexural load causes the compressive stresses to decrease before they turn into tensile stresses, when cracking occurs. The concrete structure becomes stiffer and the risk of corrosion decreases since the cracks are small or nonexistent.
Tendencies and research areas
On the materials side there is an ongoing development towards higher strengths. In practical use today strengths that are two or three times the strengths that were used in the 1970s can be utilized. In laboratories a concrete with the same strength as of low quality steel can be produced. Higher concrete qualities put higher demands on the reinforcement that must be capable to interact with the stronger concrete. The analysis of new concrete qualities by laboratory testing and computer simulation of their fracture mechanics is an important research area.
Fibre reinforced concrete is a material that partly has other properties than bar reinforced concrete. The production also means specific problems, which are not at hand in ordinary concrete production. A common application of fibre reinforced concrete is sprayed concrete for rock reinforcement. The development of new fibre materials, their analysis and development of engineering methods for their design is also ongoing.
Failures in the production of sufficiently durable concrete for outdoor structures have led to a substantial research volume. Actual issues are the transport and freezing of water in the pore system of the concrete, the effect of deicing salt, the carbonization of the concrete under the impact of moisture and carbon dioxide in the air, corrosion protection of the reinforcement and repair techniques.
The demands on environmental protection leads to an increased use of crushed rock material as ballast. Such concrete demands a different recipe than ordinary concrete with natural ballast. It is not fully understood which mechanical properties such a concrete has, either. Reuse of old concrete for the production of new concrete is also of some interest.
Higher demands on labour conditions have made it interesting to develop a concrete that does not require vibration for the compaction during casting. Reinforcement made of lighter materials than steel like fibre reinforced plastics are also interesting in this respect.
The research at the division of concrete structures, KTH, will be directed towards the use of newly developed materials for reinforced concrete in close cooperation with materials researchers.