ABSTRACT
There are dozens of thousands aging post-tensioned concrete bridges built mostly in the 1960s–1970s. In part of the bridges the prestressing system was applied in a faulty manner due to various reasons. In some cases, due to stress from the environment added to faulty construction, bridges have collapsed.
As a substantial part of the existing post-tensioned bridges worldwide have already reached renovation time the condition of the bridges should be assessed. Usually, this is done by performing bridge special inspections. Unfortunately, regular inspection techniques do not provide indication on the condition of the prestressing system which constitutes an internal system inside the reinforced concrete structure. It is also important to keep in mind that the prestressing system has a major role in the bridge load carrying capacity. At least part of the old post tensioned bridges were planned as fully prestressed structures with only small amount of reinforcement. Therefore a fault in the prestressing system is serious.
By applying suitable NDT techniques it is possible to evaluate the tendon duct grout injection of post tensioned bridges as well as of other structures. The injection grout (cementitious based grout) has a crucial role in protecting the prestressing steel against corrosion which may be induced from different environment stresses. In addition the injection grout has the important role of intermediating the prestressing forces from the prestressing steel to the concrete structure.
Applying NDT techniques also helps in pinpointing logical locations for invasive measures, i.e. opening of tendon ducts. The main purpose of the invasive measures is to confirm the NDT evaluation of the grout injection and to evaluate the condition of the prestressing system components. Emphasis is towards corrosion evaluation of the prestressing steel.
According to the authors experience the suitable NDT techniques for injection evaluation of post tension tendon ducts are the ultrasound tomographer system (MIRA) and the Impact-Echo system. Both systems enable to perform fast, enough accurate and sufficiently deep scanning form one surface of the concrete structure. MIRA scanning tomogram (C scan) along a duct at the depth of the duct. Intensive reflections (red color) are due to absence of injection grout in the duct (confirmed). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig340_1.jpg"/>
NDT techniques may also be applied to grout injection evaluation during construction of new post tensioned bridges as quality assurance measures.
The author of this paper has had a leading role during the recent years as a chief bridge inspector in about 50 test cases of post tensioned bridge inspections in Finland, where NDT and invasive measures were performed for evaluating the condition of the prestressing system. This paper will review some of the author’s experience gained during these tasks.
In some cases acute problems were detected and required fast measures such as strengthening measures, setting of traffic load limitations and even removal of a bridge from service. Possibly some disasters were and will be avoided thanks to efficient evaluation of the prestressing system.
