CHAPTER 1 INTRODUCTION

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1 1 CHAPTER 1 INTRODUCTION 1.1 GENERAL Civil engineering structures are essential for economic health and prosperity of any country. These civil engineering structures such as tall buildings, bridges, nuclear containments, pressure vessels, etc. are constructed using reinforced/prestressed concrete. Many of these were not built to last longer than 50 years. In a well-publicized 2005 report, the American Society of Civil Engineers (ASCE) (2005) stated that 27 per cent of the almost 600,000 bridges in the US are structurally deficient or functionally obsolete. This may be due to hammering action of vehicles, overloading, uneven settlement, use of inferior quality construction material and workmanship etc. Thousands of existing bridges world wide are presently in need of rehabilitation through major works of repairs. In the future, the rehabilitation of existing structures will constitute an exceptionally large field of operation that will extend for many years. Timely retrofitting measures are important and this will help to reduce damage and improve the service life of structures. To assess the safety and serviceability of the distressed structures, it is necessary to reliably estimate the existing level of strength/stress.

2 2 The existing stresses in concrete structures can be evaluated by two approaches. The first approach is to instrument the structure at the time of construction and then monitor periodically. This method is called structural health monitoring and will be useful for new structures. For old and distressed structures, stress/strength assessment has to be done through appropriate insitu testing and evaluation of test results. 1.2 NEED FOR STRESS ASSESSMENT IN CONCRETE STRUCTURES Reinforced and prestressed concrete is widely used for construction of major civil engineering structures. Prestressed concrete structures undergo distress with time due to environmental and other unfavorable operating conditions. To take decision on the possible repair measures, it is necessary to reliably estimate the existing level of stress. Assessing the existing level of stress in prestressed concrete structures in service is fairly a difficult task and the engineers are often faced with lack of actual design/construction information, environmental and operating (in-service) conditions. It is necessary to develop scientific and systematic methods relating the acquired data (through field investigations) to the existing level of stress, in order to estimate the residual strength and to decide upon the possible repair measures to rehabilitate the distressed prestressed concrete members. The aim here is to determine the in-situ stress on the surface of the prestressed concrete members which will enable the assessment of residual prestress in the prestressed members. It is always desirable to develop an in-situ stress evaluation method that would not adversely affect its performance and may cause minimal or nil damage. In evaluating various stress measurement techniques, three factors

3 3 are important: cost, time to take measurements and accuracy. Additional factors include ease of measurement, level of preliminary work, level of expertise required, acceptance of the concept and extent of damage occurred when using the technique. In-situ stress determination is based on the measurement of strain release due to local elastic stress relief, caused by core drilling and creation of stress-free boundaries. Determination of in-situ stress on the concrete surface is one way to assess the prestressing force available in the prestressed concrete members. 1.3 OBJECTIVE AND SCOPE OF RESEARCH There are few methods available to assess the existing stress in concrete structures that are predominantly under uniaxial stress state. These methods have some limitations which include the strain release is very less, difficult to apply for in-situ stress measurements in existing structures, ease of measurement, level of preliminary work, level of expertise required, etc. Also, the assessment of stresses in concrete structures under biaxial stress state is complex and there is hardly any reported work in the field of measurement of existing stresses in concrete structures under biaxial stress field. The main objective of this thesis is to develop methodologies and experimental techniques to assess the in-situ stress in concrete and prestressed concrete structural elements under uniaxial- and biaxial- stress conditions. Assessing the existing stresses in biaxial stress field using experimental investigation and interpretation of measurements are also considered in the present study.

4 4 Three experimental techniques namely, concrete core-drilling technique, concrete core trepanning technique and concrete core-drilling strain gage technique were developed to evaluate the in-situ stress under uniaxial and biaxial stress conditions. The detailed methodologies and procedures of experimental and numerical studies to evaluate in-situ stresses are presented in this thesis. 1.4 ORGANIZATION OF THE THESIS The present investigation deals with the development of methodologies and experimental techniques for measurement of in-situ stress in concrete structures subjected to uniaxial/biaxial stress condition. The efficacy of the methodologies developed is evaluated using finite element method. below. The thesis is organized into five chapters (Chapter 2 to 6) as described Chapter 2 presents a review of relevant investigations reported in the literature. It includes the details of different evaluation methods available. Chapter 3 discusses the development of concrete core-drilling technique. This technique was developed by using a special arrangement of electrical resistance strain gages suitably placed along radial- and tangentialdirections of the intended core. These gages are connected through a Wheatstone bridge circuit, in full bridge configuration, to magnify the response of measured strain for assessment of in-situ stress under uniaxial stress condition. Laboratory studies conducted to evaluate the reliability of this concrete core drilling technique are presented in detail in this chapter. Calibration constants evaluated based on experimental results are compared

5 5 with the values evaluated using numerical analyses and the comparisons are also presented. Chapter 4 focuses on evaluation of in-situ stresses under bi-axial stress state using the proposed concrete core trepanning technique. This technique employs a three element strain gage rosette to measure the strain release due to core drilling. The reliability of this technique was established by conducting experimental investigations, in the laboratory, on concrete specimens with known stress/strain field. The results of experimental investigation were compared with those obtained from finite element analysis. The results of these studies are also incorporated in this chapter. Chapter 5 presents a new method called concrete core-drilling strain gage (CDSG) technique to evaluate in-situ stresses under bi-axial stress state. This technique uses six strain gages suitably placed around the intended core for assessment of in-situ stresses. Three strain gages are placed radially and the remaining three placed tangentially and by combining the radial and tangential gages in a half-bridge Wheatstone bridge circuit. This configuration is adopted to improve the stress measurement accuracy. The suitability of the gage position and configuration has been justified using elastic analysis of a continuum with a through hole. Finite element analysis was carried out to evaluate the calibration constants. Calibration constants evaluated based on experimental results is compared with results of numerical analyses (i.e. obtained using Finite element method). Experimental studies were carried out to assess the existing stresses with a known stress field using the core drilling strain gage technique. Chapter 6 presents the conclusions drawn based on the investigations reported and suggests the scope for future work.