Prestressing Systems Instructor: Dr. Sawsan Alkhawaldeh4) Prestr… · Dr. Sawsan Alkhawaldeh Department of Civil Engineering . Prestressing systems • Pre-tensioning; prestressing

Prestressed Concrete

Prestressing Systems

Instructor:

Dr. Sawsan Alkhawaldeh

Department of Civil Engineering

Prestressing systems • Pre-tensioning; prestressing steel is tensioned prior to

casting of member. • For harped prestressing profile, the bed is provided with

hold down devices.

• Post-tensioning; prestressing steel is tensioned after hardening of concrete.

Jacking systems • Prestressing force is applied to the tendons

using hydraulic jacks of capacity of 10 to 20 tons and a stroke from 6 to 48 in depending on (pre-tensioning or post-tensioning) and whether one individual or all the tendons are being stressed simultaneously.

• If all tendons are stressed simultaneously, large capacity jacks are needed with a stroke of at least (30) in where the cost will be higher in this case.

Jacking systems

Grouting for Post-tensioned tendons • The prestressing ducts have to be filled

under pressure with appropriate cement grout in an injection process to:

- Provide permanent protection and bond between the steel and the surrounding concrete for post-tensioned steel.

- Prevent corrosion of the steel.

• Grouting material consists of:

- Portland cement

- Water

- Admixtures

Grouting and anchorage system

Ducts

Ducts • Ducts should be of a type that does not permit

the entrance of cement paste. • They should transfer bond stresses as required. • They should retain their shape under the weight

of concrete. • All ducts should have grout opening at both ends. • The duct size (area) should be twice the net area

of presstressing steel for tendons that made up of multiple stands. If one strand is used, the area of the duct should be at least 0.25 in larger than the diameter of the strand.

• Ducts should be securely fastened after placement at close enough intervals to avoid displacement during concreting.