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2lead to overly conservative service-life estimates and recommendations of resistance factors for use in LRFD that are potentially too low. Rock bolts, soil nails, and ground anchors have features that differ from those of MSES reinforce- ments, including components of single or double cor- rosion protection systems and the use of high-strength steel elements that are often pre-stressed during installation (soil nails are not pre-stressed). Thus, the performance and service lives of these types of earth reinforcements are influenced by the installation details, the existing conditions of elements incor- porated into the corrosion protection systems, and localized forms of corrosion such as stress crack cor- rosion or hydrogen embrittlement rather than uniform or general corrosion. Better information on the in- service conditions of rock bolts, soil nails, and ground anchors is needed to assess factors that may signifi- cantly affect their performance. Since construction records are often not available from rock bolt instal- lations, information from in-service reinforcements is needed relative to the geometry and quality of these installations. NCHRP Project 24-28 included proposals for future research to (1) address the high uncertainty regarding the performance of MSES reinforcements in marginal quality fills, (2) obtain additional perfor- mance data from older installations of MSES rein- forcements, and (3) implement more robust test tech- niques to obtain better information about the existing conditions of rock bolts, soil nails, and ground anchors. This digest summarizes results from NCHRP Project 24-28A, which addressed these proposals and further validated the results from NCHRP Project 24-28, including predictive models for corrosion potential, metal loss, and service life of metal rein- forced systems. The complete project final report for NCHRP Project 24-28A is available for download.2 RESEARCH APPROACH Marginal quality fills are classified according to minimum laboratory resistivity (AASHTO T 288, âDetermining Minimum Laboratory Soil Resistiv- ityâ), which values can be very different from val- ues measured in situ. Samples of fill for resistivity testing are often collected prior to construction from stockpiles representing potential sources of fill ma- terial. However, sources actually used for construc- tion are uncertain. Therefore, the variability inher- ent to the observed performance of marginal fills is likely due to uncertainty with respect to the fill prop- erties, which may also be inherently variable. These effects are not as prevalent for good or high quality fills wherein sources of materials are more certain, variability is less, and the rate of metal loss is not as sensitive to changes in fill resistance. Therefore, the approach to reducing the uncertainty with respect to performance of marginal quality fills is to obtain bet- ter measurements of fill resistivity, and correspond- ing correlations with measurements of corrosion rate. Thus, there is a need to identify and implement methods to measure fill properties at the time and location of the corrosion rate measurements. For rock bolts, soil nails, and ground anchors there is a need to extract more information on their existing condition. Testing and data analysis tech- niques must be refined to fill this need. Other research has applied the impulse response (IR) technique to study the condition of deep foundation elements, and these results have been useful in locating and identifying the size and shape of anomalies along concrete-drilled shafts. This is considered an improve- ment over the sonic echo (SE) technique, which is the existing practice for condition assessment of rock bolts. The SE technique is only useful in iden- tifying the locations of sources of reflections and providing qualitative information about pre-stress levels. Thus, the suitability of the IR test for rock bolt installations was explored as part of NCHRP Project 24-28A. NCHRP Project 24-28A included the following tasks: 1. Evaluate the effectiveness of the impulse response technique in determining the in-situ condition of rock bolt installations. An impor- tant component of this task was the installation and testing of âdummyâ rock bolts incorporat- ing planned anomalies to provide a basis for comparison. 2. Identify methods to determine resistivity for in-situ materials surrounding earth reinforce- ments at the same time and location as corro- sion rate measurements. An important part of this task was to verify these methods by con- structing an earth embankment that incorpo- rated earth reinforcements, conducting test- ing of the reinforcements, and comparing the 2http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp? ProjectID=727
