Test: RCC Design- 4 - Civil Engineering (CE) MCQ

Prestressed concrete import initial compressive stresses to counter tensile stresses due to loading.

Reinforcement bars are placed in a form and stressed or stretched with forces on each end of bars pulling them. When concrete is poured around the stressing bonds before pressure is released, it creates tension. 

When the tension is released the steel tendency to try to resume its original shape and adds a compressive force to the concrete laterally, giving it strength to span distances.

Elastic shortening:

• Pre - Tensioning:
  • When the prestress is transmitted to the concrete member, there is contraction due to prestress. This contraction causes a loss of stretch in the wire. 
• Post - Tensioning:
  • ​No loss due to elastic deformation if all the wires are simultaneously tensioned. If the wires are successively tensioned, there will be loss of prestress due to elastic deformation of concrete.

Elastic shortening occurs only in pre-tensioning and may not occur in post-tensioning if all wires are tightened at same point of time.

As per clause 11.1.7.1 of IS 1343,

In case of single wires used in pre-tensioning system, the minimum clear spacing shall not be lesser than greater of the following:

1) 3 times the diameter of wire, and

2)  times the maximum size of aggregate.

According to 1343: 1980, clause 5.2.4.1,

The approximate value of shrinkage strain for design shall be assumed as follows:

(i) For Pre-tensioning = 0.0003
For Post − tensioning =

For t = age of concrete at transfer (days)

At t = 3 days

Shrinkage strain will be:

In Freyssinet system, high tension steel wires 5 mm to 8 mm diameter about 12 in number are arranged to form a group into a cable with a spiral spring inside. In the Gifford Udall system the wires are stressed and anchored one by one in a separate cylinder using small wedging grips called Udall grips.

In the Lee-Mc Call system, high tensile alloy steel bars are used as the prestressing tends.

In the Magnel Blaton system cable of rectangular section is provided. The wires are arranged with four wires per layer. The geometric pattern of the wires is maintains in the same from throughout the length of the cable by providing grills or spacers at regular intervals.

Loss of prestress due to anchorage slip

 

= 2.1 N/mm²

% loss of prestress = 

⇒ 2.593 – 5.185

⇒ -2.592 N/mm²

Shrinkage strain in concrete in pretension

ϵ_cs = 3 × 10^-4  

∴ loss in prestress, Δσ = ϵ_cs x E_s 

= 3 × 10^-4 × 2 × 10⁵

= 60 N/mm²

% loss in prestress = 60/500 × 100 = 12%
Shrinkage strain in post – tension,

 

= 2 × 10^-4

∴ loss in prestress = ϵ_c × E_s

= 2 × 10^-4 × 2 × 10⁵

= 40 N/mm²

% loss in presstress in post tension = 

Prestressing force (P) =   

Loss due to elastic shortening = mf_c

f_c = compressive stress at the level of steel

m = modular ration = E_s/E_c

 

f_c = 4.948 + 0.9277

f_c = 5.876 MPa

loss due to elastic shortening = 6.32 × 5.876

= 37.13 MPa.

Loss due to creep of concrete = mϕf_c

ϕ = creep coefficient = 1.6

loss due to creep of concrete = 6.32 × 1.6 × 5.876

= 59.42 MPa.