PRESTRESSED CONCRETE OVER REINFORCED CONCRETE AND ARCH

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COMPARISON BETWEEN PRESTRESSED CONCRETE, RCC AND ARCH

Concrete is a building material strong in compression but relatively weak in tension. There are two ways of overcoming this problem:

• Embed another material in the concrete which is strong in tension – reinforced concrete
• Remove the tension altogether – by arching or prestressing.

Reinforced concrete:

In reinforced concrete, steel reinforcement bars are provided to carry tension in those regions of the member where tension may occur, compensating for the low tensile strength of the concrete.

In practice, reinforced concrete beams are designed on the assumption that cracks will occur under their service loads. This is not generally true of a prestressed concrete beam.

Arch:

The arch form is one of the earliest solutions to dealing with a material which has no tensile strength. The construction of an arch is fundamentally different to that of any prestressed concrete member.

The shape of the arch is designed specially such that when loaded, tensile stresses do not occur in the structure. All compressive forces evolve directly from the load.

When a load is applied to the arch, the reactions at the abutments increase. This is equivalent to an increase in magnitude of the passive prestressing force. Therefore, the magnitude of the prestressing force within an arch is a function of the applied loading. This accounts for the great strength of arch structures.

Prestressed concrete:

Prestressing applies an initial compressive axial force to the concrete which greatly reduces or eliminates the internal tensile stresses.

It does this by a tensile stress to steel cable running through the concrete. The steel cable is then anchored and a compressive force is transferred to the concrete by bond forces as shown in figure.

The benefits of prestressing are:

• Cracking is greatly reduced or eliminated,
• Applying the prestressing forces below the neutral axis induces moments which oppose those caused by externally applied loads, thus significantly reducing deflection.
• Member sections are smaller than reinforced concrete sections for the same imposed loads.