Core Cutting to Measure Existing Prestress

In a prestressed concrete (PC) structure, the soundness of prestress is important and the most effective maintenance and management is achieved by determining the stress status. However, some existing PC structures have no design drawings preserved. In other cases, there is doubt whether sufficient stress was applied as designed. Therefore, a method is needed to measure the amount of prestress in existing PC structures.

When an elastic body has been stressed, strain and deformation appears when the stress is released by cutting the body. A measurement method to estimate stress from a physical value of strain or deformation called the stress release method has been applied to various materials. Our measurement method applies its principal to concrete members.

Similar measurement methods have been proposed, but those methods are not easy to apply due to the necessity of special machines or analysis equipment. However, another issue is that the prestress value is not obtained by the effects of time-lapse strain or deformation of concrete, such as dry shrinkage or creep. This measurement method can remove those effects by using the results of studies of dry shrinkage and creep of concrete over the years.

The measurement process is simple. First, strain measurement gages are installed on the surface of a PC structure both along and perpendicular to the prestress direction. Then, a concrete core cutter is used to create a shallow circular groove to measure the strain after releasing stress. The effective stress at the measurement point is computed by a formula developed from measured strain values.

Prestress measurement by core cutting