Concrete has a lot of excellent properties, making them the most preferred structural members, but has its weakness too.
CASE 2 (REINFORCEMENT CEMENT CONCRETE ON LOADS)
To overcome these cracks reinforcement steel is provided in the tensile zone which can take up the tensile loads coming on the structure and protect the member from cracks. (Ref: Fig)
Concrete with steel functions as a composite member and poor tensile strength and ductility of concrete are countered by reinforcement steel of high tensile strength and ductility.
WHAT ARE THE DEMERITS OF CONCRETE?
Even though concrete owes the property of good compressive strength, it has the following disadvantages.
- a) Tensile strength is weak
- b) Brittle
- c) Non ductile
A good designer can anticipate the areas of failure and the structure shall be designed to overcome it and then only it can be called an optimised design.
The Design is always based on a Design criteria ( Goal of the design). A limit state design requires the structure to satisfy two principal criteria.
a) Ultimate limit state
b) Serviceability limit state
1) Ultimate limit state
The structures are designed based on the ultimate limit state and will not collapse even in the worst condition. If the proposed structure for a bridge can handle a load of traffic without a collapse then we can say the structure is satisfying the Design criteria of the ultimate limit state.
The structures has to be checked for serviceability conditions like stability analysis, deflection checks etc. During the service stage if the structure deflect due to moments then it does not satisfy the criteria of serviceability.
Now let us see what happens when an RCC structure like a bridge when subjected to service loads.
DEFLECTION OF RCC STRUCTURES
The moments generated when an RCC structure is subjected to service loads deflect the structure as shown in the figure. Reinforcement steel in the tensile zone is ductile can elongate to compensate the loads, but concrete because of its poor tensile characteristics may develop cracks.
The cracks absorb moisture and gradually rust the reinforcement steel leading to the spalling of concrete and paves way for the ultimate collapse of the structure
Prestressed concrete is introduced to minimise the deflection cracks, increase the structure strength, and to equip designers with optimised design possibilities while dealing with large spans.
Prestressing is a method of inducing Compressive stress into a structural member by tensioning the steel, before subjecting to service loads..
The prestressing method is used for a lot of structures like bridges, large spanned auditoriums, silos, reservoirs, pile foundations, precast elements, etc.
SOME STRUCTURES WITH PRESTRESSED CONCRETE
HOW PRESTRESSED CONCRETE WORKS
The figure below explains how an RCC member subjected to loads deflects and gets cracked.
AN RCC BEAM ON LOADS CRACKS ON DEFLECTION
PRINCIPLE OF PRESTRESSING
Prestressing is a method of inducing compressive stresses in a structural member that counter the stresses developed in the structure when subjected to loads. In prestressed concrete, the steel/tendons are stretched along the axis, and then the concrete is poured.
When the concrete reaches the desired strength the tendons are released. On release, the tendons induce the compressive stress in the structural member.
The compressive stress induced in the structural member on releasing the tensioned steel tendons counterbalances the compression due to service loads applied in the service stage. (Dead load and live loads). The tensioning of steel produces negative deflections in the member which balances the compressive stress due to loads and prevents the concrete from cracking.
Prestressing gives the designers the much-needed flexibility in designing large spanned structures and deriving economical designs that are not possible in RCC.
METHODS OF PRESTRESSING
b) AND POST TENSIONING
In this method, the tendons are stretched before placing concrete and once the concrete attains the desired strength the tendons are released before application of service loads.
High-strength steel tendons are placed between abutments and stretched by around 70% of its ultimate strength or as per design requirements as shown in the figure. Concrete is poured with tendons in the stretched condition. Tendons are released when the concrete reaches its desired strength. On release, the tendons try to regain its original shape. During this process, compressive stress is induced in the member which counters the compressive stresses generated on exposed to service loads.
The post-tensioning method is used in precast girders of bridge spans, metro lines, flyovers, railway sleepers, piles and precast elements which are subjected to heavy loads. The structures are prestressed in the prestressing yards, conveyed and lifted for erection at site.
The structures to be pre-tensioned have size limitations as they have to be carried from fabrication yard to site.
B) POST TENSIONING METHOD
In the post tensioning method tendons are tensioned after concrete is poured. In this method ducts or profiles are placed in concrete before casting as shown in the figure.
Once the concrete hardens and reaches the sufficient strength, the tendons are then passed through the ducts or profiles before subjecting to service loads. The steel is tensioned using jacks as per the design requirements.
In bonded type , after tensioning the tendons are grouted with special grouts. In unbounded type, tendon grouting is not necessary.Post tensioning is done at site.
The post tensioning method is used in via ducts, segmental construction of large bridge spans, large slabs, reservoirs, big silos of cement plant , coal washeries etc
what are tendons
Tendons consist of single wires, multi-wire strands or threaded bars that are most commonly made from high-tensile steels, carbon fiber or aramid fiber.