Limitations of Reinforced Concrete without Prestressing
Dead loads, imposed loads, deformations and load-independent effects such as temperature changes and shrinkage induces tensile forces in a RC beam. Since the tensile strength of concrete is low steel reinforcement bars are embedded in the concrete to carry all internal tensile forces. Prestressed concrete is an advanced form of reinforced concrete.
External loads -> tension in the bottom fibres -> cracking
For practical design applications, reinforced concrete beams are assumed to be cracked during service loads. The bending moments are resisted by concrete in compression above the cracking zone and reinforcing steel in tension. This flexural steel cannot prevent the cracking of concrete around the tension region though the required flexural strength is obtained.
The width of induced crack is proportional to the strain at the level of cracking which is dependent on strain in the steel. Hence the stress in the steel has to be limited to control and avoid excessive cracking under service conditions.
Correspondingly, high steel strain will result in large curvature, which will cause large deflection in the member. The benefit of using higher strength steel or concrete cannot be justified or completely utilized. The increased strain capacity offered by higher strength steel cannot satisfy serviceability criteria.
For prestressing concrete, steel which can be tensioned with large elastic strains is appropriate. This necessitates the use of high-strength steel for prestressing.
5 Advantages of Prestressed Concrete
Controlled or Reduced Cracking
Prestressing process involves application of an initial compressive pre-stress on a structure to minimize or eliminate the internal tensile forces this will help to control or eliminate cracking.
The prestressing force is transferred and sustained by highly tensioned steel reinforcement usually referred as tendons, reacting on the concrete. This will reduce or eliminate cracking thus a prestressed concrete section is significantly stiffer than the equivalent cracked reinforced section.
Prestressing will impose internal forces which are of reverse in sign to the external loads and therefore it will significantly reduce or even eliminate deflection in the member.
Service load behavior of the members are improved with reduced cracking and deflection, thus the use of high-strength steel reinforcement and concrete becomes economical and structurally efficient.
Lighter and Longer Members
Compared to RC counterparts, Prestressing results in lighter members, longer spans and increase in the economy of application of reinforced concrete. Pre stressing provides higher strength to depth ratios.