Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 8
8 correspond to min < 700 -cm, or have very high chloride copper sulfate reference electrode (CSE) is commonly used content (> 500 ppm). to monitor earth reinforcements. Results from the test can The estimated service life of unprotected rock reinforce- provide a comparison between metallic elements at different ment systems in moderately aggressive ground conditions is locations at the same site and identify the presence of differ- approximately 50 years (Kendorski, 2003), but may be much ent metals, for example, zinc or iron. Coupons or dummy lower for very aggressive ground conditions, and particularly reinforcements assist in interpretation of half-cell potential for high strength steel subject to low pH environments measurements. Plain steel, galvanized steel, and zinc coupons (Withiam et al., 2002). may provide baseline measurements for comparison. Test Protocol and Linear Polarization Resistance Measurement Techniques LPR measurements are used to observe the instanta- Recommended practices for corrosion monitoring and con- neous corrosion rate. Lawson et al. (1993), Elias (1990, 1997), dition assessment of Type I and Type II reinforcements were Berkovitz and Healy (1997), and Elias et al. (2009) describe developed from previous studies including FHWA Demonstra- that application of the LPR technique to MSE reinforcements tion Project 82, Durability/Corrosion of Soil Reinforced Struc- and application to Type II reinforcements is similar. Polariza- tures (Elias, 1990), and NCHRP Report 477: Recommended tion resistance is measured from the response to an impressed Practice for Evaluation of Metal-Tensioned Systems in Geotech- current and the corrosion rate is computed via the Stern- nical Applications, (Withiam et al., 2002). These studies evalu- Geary equation (Stern and Geary, 1957). The surface area of ated application of test techniques at a number of selected field the test element must be known since the Stern-Geary equa- sites and developed recommended practices for corrosion tion is based on the corrosion current density, which is mea- monitoring and condition assessment. sured in terms of current per unit surface area. The measured resistance, PR, is actually the sum of the interface and soil resistance (PR = PR + Rs) with a correction for soil resistance Measurement Techniques often necessary (Elias, 1990). Early corrosion monitoring practices involved exhuming and examining samples of reinforcements for evidence of cor- Sonic Echo Test Measurements rosion, including loss of cross section. The practice of exhum- ing in-service reinforcements is limited to reinforcements that The sonic echo method (impact test) is used for evaluating are accessible and usually near the surface of the structure. cracking of grouts, fracture of tendons, and loss of section or However, special inspection elements may be placed and later loss of prestress for Type II reinforcements (Rodger et al., extracted from various positions along the wall face including 1997). The end of the reinforcement is impacted using a ham- the top, middle, and bottom of the wall (Jackura et al., 1987). mer or air gun, which generates elastic compression waves Corrosion rate may be estimated from weight loss and thick- with relatively low frequency content. The waves are reflected ness measurements, provided the original thickness or weight by changes in geometry or conditions along the length of the and composition (e.g., zinc thickness) of the reinforcements reinforcement, including the ends of the elements, transitions are known. Corrosion rate decays with time (Romanoff, 1957), from free to bonded zones, and irregularities that may be and a catalog of measurements made at different times is encountered along the length. Gong et al. (2005) and Liao required to assess the rate of metal loss with respect to time. et al. (2008) describe application of the sonic echo test to eval- Less invasive techniques employing nondestructive electro- uate the length and integrity of soil nail installations. chemical tests such as measurement of half-cell potential and linear polarization resistance (LPR) were implemented for cor- Ultrasonic Test Measurements rosion monitoring of MSE walls beginning in the later 1980s (Lawson et al., 1993; Elias, 1990) and are also applied to Type The ultrasonic test method is a good technique for evalu- II reinforcements (Withiam et al., 2002). With these tech- ating grout condition, fracture of elements, and abrupt niques, a large number of samples are monitored and frequent changes in the element cross section for Type II reinforce- measurements may be collected. ments. The method has many of the features of the sonic echo technique, except that the transmitted signal contains rela- tively higher frequencies. Half-Cell Potential An ultrasonic transducer is acoustically coupled to the The half-cell potential, Ecorr, is the difference in potential exposed end of the test element. Grease is used as an acoustic between the metal element and a reference electrode. A copper/ couplant. The time taken for sound pulses, generated at regular