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2016SpringShotcreteEMag

While MoDOT has not had a large iconic project that makes headlines in trade magazines, they have made great strides in their knowledge and use of shotcrete over the last 5 years. Fig. 2: Shotcrete repair under a bridge Fig. 3: Proper curing is a critical part of shotcrete placement Mark Croarkin graduated from the Missouri University of Science and Technology in 1997 and has been a licensed professional engineer since 2002. With the exception of 1 year of experience in private industry, Croarkin’s career has been dedicated to MoDOT. His foundation started with Construction Inspection, but his experience includes working in six of MoDOT’s seven geographically diverse districts in materials, construction, design build, bridge maintenance, and maintenance positions. Recently, Croarkin has been promoted to lead all of MoDOT’s maintenance efforts as the St. Louis District Maintenance Engineer. From 2009-2016, Croarkin was responsible for the safety of all structures in the St. Louis region, as well as directing repair crews and prioritize contract repairs. His passion for finding better ways to maintain bridges earned him a reputation as a leader in innovation for the State of Missouri. This article describes his accounts of the road MoDOT took to allow shotcrete into the specifications. American Concrete Institute (ACI) or EFNARC (the  European Federation of National Associations  Representing producers and applicators of specialist building products for  Concrete) nozzleman certification is a must. MoDOT recognizes that the experience in the art of placing shotcrete is critical to its success. While there is currently only a certified nozzleman requirement, the experience of the entire placement team contributes to the quality of the repair. If a large project was to allow the use of shotcrete, experience requirements for the contractor with similar projects would likely be required. An area not as strictly enforced has been moisture requirements of dry material, which typically require the use of a predampener or a hydro-mix nozzle. To keep costs down on small applications, MoDOT has focused more on cracking issues and has left the application up to the nozzleman. We have had good performance with a few pre-bagged products that are prequalified for use on small quantity projects without requiring additional testing, although we always reserve the right to test. While wet-mix placement methods have been used, dry-mix shotcrete or the old tradename “gunite” is typically the method of choice due to the small quantities being placed at a time. During the learning process, some MoDOT trials were in less than ideal environments, and shrinkage cracks appeared. While shrinkage cracks are not typically a long-term issue in a material that is so dense, they definitely catch the attention of bridge inspectors. Most trials that resulted in shrinkage cracks were sealed with 100% acrylic overcoats such as BASF’s Thorocoat to seal the cracks. Temperature, wind, time to finish, and curing methods are topics that are typically discussed now to avoid shrinkage issues. The majority of shotcrete use in Missouri has been fairly small areas of substructure, deck, and superstructure repairs in locations exposed to high chlorides. The repair restores the area to protect other elements from the harmful chlorides used to melt snow. Shotcrete has the ability to produce a high-quality repair with minimal access to the location being repaired, which typically results in cost reductions. The specific characteristics that are so appealing to MoDOT engineers are cost savings, the low chloride permeability, and the high bond strength. Prepackaged products such as BASF Shotpatch 21-F, Euclid Chemical Eucoshot F, King Shotcrete MS-D1, and CTS Cement Low-P have been successful enough such that they are typically preapproved for small quantity use in Missouri. Numerous very-low, 28-day chloride permeability (around 300 coulombs) and pulloff strengths consistently exceeding hand patch or “form and pour” comparisons have made this decision easy. Shotcrete • Spring 2016 31


2016SpringShotcreteEMag
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