ABSTRACT
An increasing number of concrete structures have been damaged due to concrete internal expansive reactions.
In Portugal, several dams are affected by this phenomenon, motivating numerous studies in order to improve the ability to prevent, diagnose and mitigate this problem and their structural effects. More recently, some concrete bridges have also been known to be affected by the same type of degradation, requiring important works for their rehabilitation and, in some cases, making necessary its replacement.
This paper presents the procedures adopted in a bridge with high potential of concrete expansion due to alkali-silica reaction (ASR) and internal sulphate reaction (ISR), before their piers were submerged due to the construction of a dam downstream. After a brief description of the Bridge, as well as the procedure adopted to mitigate the development of the internal expansive reactions in concrete, this paper presents the monitoring system installed and the most relevant results achieved.
The bridge is located in the national road N315, in the Northeast of Portugal and crosses the river Sabor. It has a total length of 560 m, with a horizontal curved girder, a radius in plan of 270 m, and has five continuous spans, with a central span of 160 m, adjacent spans of 125 m and extreme spans of 75 m.
The deck is a prestressed reinforced concrete box-girder of variable depth from 9.0 m to 4.0 m. The central piers, 75 m and 68 m height, are based in a rectangular modified hollow cross section.
Laboratory tests to confirm and to prognosis the ASR and ISR effects were performed on concrete cores taken from the bridge on two piers and in the corresponding pile caps. These tests include microscopic methods, namely optical (MO) and scanning electron microscopy (SEM), to identify the products resulting from ASR and ISR, and residual expansion tests in order to prognosticate the future behaviour of the concrete affected by ISR and ASR.
Taking into account the information provided by the results of these tests, which indicate that there is a strong possibility of ASR and ISR occurrence, it was been proposed the application of a concrete waterproofing system in the bridge piers, before the reservoir filling. This waterproofing was applied in both surfaces of pier (external and internal), from its base till the maximum reservoir level (Figure 1). View of the bridge during the rehabilitation works. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig164_1.jpg"/>
The monitoring system installed inside the bridge was designed in order to assess the performance of the waterproofing system and for the early detection of structural effects resulting from the development of the ASR and the ISR in concrete. The system includes three main components: monitoring of the environmental conditions, monitoring of the effectiveness of the waterproofing system and structural monitoring.
The monitoring of the effectiveness of the waterproofing system is based on the measurement of temperature and humidity inside of concrete and in concrete resistivity measurements. Sensors for that purpose were placed in three sections of one of the central piers, at three levels, corresponding to different exposure conditions: permanently submerged, submerged part of the year and permanently emerged.
