Optical Sensor Developments for Measuring the Surface Strains in Prestressed Concrete Members C.-H. Wu*, W. Zhao*, T. Beck † and R. Peterman ‡ *Department of Industrial and Manufacturing System Engineering, Kansas State University, Manhattan, KS, 66506, USA † Mechanical and Nuclear Engineering, Kansas State University, Manhattan, KS, 66506, USA ‡ Civil Engineering, Kansas State University, Manhattan, KS, 66506, USA ABSTRACT: Conventionally, transfer-length strain measurements are performed using mechanical gauges such as the Whittemore gauge, or demountable mechanical (DEMEC) strain gauges, and others devices using ‘contact’ measuring principles. These methods involve tedious surface prepa- ration, and are also prone to significant human errors and inaccuracies. Furthermore, these mechanical sensors can only detect lateral displacements. This paper presents a new optical sensor of measuring prestress concrete surface strains. It makes use of the laser-speckle displacement that is detected by cross correlating the associated optical signals from a Charged-Coupled Device (CCD) sensor. The sensor was designed to be able to measure the surface displacement compo- nents without being affected by other surface motions that are generally present during the concrete detensioning process. Experiments were conducted on a compressed concrete beam and a real prestressed concrete member during the manufacturing process. The results from the optical strain sensor showed good consistency with contact measurements made by using both a foil strain gauge and a Whittemore gauge. KEY WORDS: optical sensor, prestressed concrete, surface strains Introduction Surface strain profile measurement is used by the Concrete Manufacturing Industry to estimate certain parameters such as Transfer Length and Develop- ment length that are critical to the design and man- ufacturing of prestressed concrete beams [1]. The method that is most commonly used by industry is achieved manually using a Whittemore mechanical strain gauge, as shown in Figure 1. However, the Whittemore gauge has the disadvantages of having low resolution, bulky size, involve extensive pre-test surface preparations, and has poor repeatability due to human measurement errors. For a reasonable strain profile measurement, tens of ‘points’ must be bonded onto the concrete surface, which is very time- consuming and labour-intensive. Furthermore, the measurement results are heavily influenced by the users’ measuring habits and skills. Our experience has shown that even well-trained field engineers can produce significantly different readings. A consider- able amount of training and experience are required to achieve consistent and reliable readings. Another popular strain gauge used for the concrete surface strain measurement is the metallic wire on foil strain gauge. However, in real-life applications, the detected resistances are affected by many operating factors such as the ambient temperature, surface material properties, and the adhesives or bonding conditions between the metallic conductors and the tested surface. Consequently, when the foil strain sensors are used in a field environment, these operating factors can significantly impair the accu- racies of the final measurements [2]. The two types of gauges discussed above are both ‘contact’ strain gauges that require careful mounting of reference ‘points’ or otherwise a direct surface mount of sen- sors (as is the case foil strain gauges). The concrete surface must be clean and sufficiently smooth to obtain reliable readings, which severely limits the practical application of these contact strain gauges in concrete surface strain measurement. The optical speckle technique has evolved into a powerful tool for the measurement of surface strains since digital signal processing hardware and software have become widely available nowadays [3, 4, 5, 6]. It has the advantage that virtually no surface prepara- tion is required, it works well for various surface structures and roughness and has high resolution [7, 8, 9]. A typical measurement is fulfilled by capturing e376 Ó 2009 Blackwell Publishing Ltd j Strain (2011) 47, e376–e386 doi: 10.1111/j.1475-1305.2009.00621.x An International Journal for Experimental Mechanics