What causes loss of prestress?

What causes loss of prestress?

In the case of post-tensioning, the tendons are provided inside the duct of a precast concrete member. So the loss in prestress occurs due to friction between the concrete surface and the tendon in the process of tensioning. The loss of friction is also accompanied by the wobble effect.

How do you reduce losses in prestress?

If the initial stress in steel is known, the percentage loss of stress due to the elastic deformation of concrete can be computed. and the methowd of curing used of high-strength concrete with low water cement ratios result in a reduction in shrinkage and consequent loss of prestress.

Which parameter is related to loss in prestress due to friction?

Friction losses µ = curvature friction coefficient. α = cumulative angle (in radians) of change in the tendon-profile tangent between x and the jacking end.

How can we reduce the prestress losses in a prestressed concrete member?

Loss due to Creep in steel (Relaxation of steel) Choice of proper steel helps to reduce this loss. Prestressed wires have lesser creep. Galvanised wires also have no creep. Hence choice of proper steel will help to reduce the los of prestress due to creep.

What is loss due to slip of Anchorage?

Explanation: Loss of stress due to anchorage slip = (EsΔ/l), For a 30m long beam, loss of stress = (210×103×5)/(30×1000) = 35n/mm2, Loss of stress = 35/1000×100 = 3.5%.

What is friction loss in prestressed concrete?

Loss due to Friction. ➢ In post-tensioning members , loss of prestressing. occurs due to friction between the tendons and the surrounding concrete. ➢ The magnitude of this loss is a function of two factors: – The tendon form or alignment, called the curvature effect.

What are three factors that affect creep?

The amount of creep that the concrete undergoes is dependent upon 1) the magnitude of the sustained loading, 2) the age and strength of the concrete when the stress is applied, and 3) the total amount of time that the concrete is stressed.

What is friction loss in prestress?

P0= Prestressing force after immediate losses = Reduced value of prestressing force after elastic shortening, anchorage slip and loss due to friction. Strain compatibility • Loss due to elastic shortening is quantified by the drop in prestress (∆fp) in a tendon due to change in strain in tendon (∆εp).

What are the sources of prestress?

There are four sources of prestressing force: Mechanical, hydraulic, electrical and chemical.

What is Anchorage slip loss in prestress?

Explanation: Anchorage slip is the distance moved by the friction wedges (in post tensioned members) after releasing the jacks at the ends of the member and before the wires get fixed perfectly in wedges, the loss during anchoring which occurs with wedge type grips is normally allowed for on the site by over-extending …

What do you mean by loss of prestress?

The prestress losses are defined as the loss of tensile stress in the prestress steel which acts on the concrete component ofthe prestressed concrete section. In pretensioned concrete, the four major sources of prestress losses are elastic shortening (ES), creep (CR), shrinkage (SH) and relaxation (RE).

What is curvature effect and wobble effect?

➢ The magnitude of this loss is a function of two factors: – The tendon form or alignment, called the curvature effect. It is predetermined. – The local deviations in the alignment, called the wobble effect. It is a result of accidental or unavoidable misalignment since ducts or sheaths cannot be perfectly placed.

What causes the loss of prestressing force?

Loss due to friction between the post-tensioning tendon and duct during stressing  prestressing force varies along length of tendon. Wedge-type anchorages require some movement to engage the wedges and anchor the tendon  movement of the wedges causes tendon to shorten, reducing prestress force. Also known as seating loss.

How to calculate the loss of prestress due to Anchorage?

A general formula for computing the loss of prestress due to anchorage deformation Δa is Δfs = ΔaEsL where, Δa = Amount of slip L = Tendon length of the tendon Es = Elastic modulus of the prestressing steel The above equation is based on the assumption that the slip is uniformly distributed over the length of the tendon.

How to calculate the loss of steel stress?

Loss of steel stress is given as FR = f1 – f2, where f1 is the steel stress at the jacking end & L is length to the point under consideration. Thus, FR = f1−f2=f1−f1e− μα−KL=f1 (1−e− μα−KL) For tendons with a succession of curves of varying radii, it is necessary to apply this formula from section to section.

How does reverse friction prevent distribution along the tendon?

Distribution along the tendon is prevented by reverse friction as the tendon slips inward, & the steel stress throughout much of the tendon length may be unaffected by anchorage slip.

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