Structural health monitoring of concrete systems.

摘要

Concrete material is widely used around the world for the construction of many infrastructure including highway bridges, residential and commercial building, dams, electric power generation plants, and nuclear power plants. It is therefore practical to continue development of both improved structural systems as well as innovative condition assessment techniques for new and existing concrete structures. This thesis includes two studies which focus on structural health monitoring of concrete systems. The first project deals with performance assessment of an in-situ precast approach slab system and the second is an investigation of acoustic emission (AE) as a condition assessment technique for alkali-silica reaction (ASR) in concrete. The first study is aimed at investigating the performance of precast concrete slabs placed on the approaches of a replacement bridge in Union County, S.C. The use of full depth precast panels in bridge construction has been limited by problems associated with the durability of connection joints between adjacent panels. The precast slabs being monitored are designed and constructed using an improved longitudinal joint detail consisting of interlocking looped reinforcement bars. The purpose of this research program is to assess the practicality of further implementation of approach slab system as an alternative to current construction methods. To evaluate the effects of service loads the approach slabs were instrumented with strain and displacement gages and measurements were collected periodically for a period of 18 months. A series of load tests were also performed on the approach slab system. Results of long-term monitoring and load tests show no adverse indications and the approach slabs appear to be functioning adequately. In the second study an accelerated alkali-silica reaction (ASR) test was designed to examine the ability of acoustic emission (AE) to detect this damage mechanism. ASR is a chemical reaction occurring between alkaline hydroxides within cement past and certain types of amorphous silica found in mineral aggregates. ASR causes an accumulation of internal pressure due to the formation of a hygroscopic gel which leads to expansion and cracking of the concrete. AE is highly sensitive to stress waves emitted from a sudden release of energy such as formation of cracks in concrete. This allows it to capture and identify propagating damage. AE has the potential to detect micro-cracks forming prior to expansion, which can be related to the degree of ASR damage. The experimental setup consisted of an adapted ASTM C1293 test, twelve specimens of dimensions 3x3x11.25 in. created using a highly reactive aggregate as well as an elevated alkaline content, and 3 control specimens of similar dimensions incorporating innocuous aggregates and low-alkaline cement. The specimens were placed in controlled environment with high temperature and relative humidity to accelerate the ASR reaction. Length change measurements and petrographic examination were performed periodically to detect ASR damage while AE activity was recorded continuously. The results of this study show that AE has the ability to detect ASR damage with a good agreement with length change measurements. Furthermore, AE cumulative signal strength can be related to the length expansion associated with ASR distress and the intensity analysis chart has the potential to classify ASR damage in concrete structures.