THE CONCRETE BRIDGE MAGAZINE

WINTER 2018

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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42 | ASPIRE Winter 2018 T he addition of steel reinforcement to concrete in the 19th century enabled structural concrete elements to work not only in compression but also in tension, which caused cracking in tension areas. Other common causes of concrete cracking are plastic shrinkage, settlement, drying shrinkage, thermal stresses, chemical reaction (due to alkali-reactive silica or sulfates), freezing and thawing, corrosion of reinforcing steel, overload stresses, poor construction practices, and design or detailing errors. 1 Even if concrete cracking does not affect structural performance, it can adversely affect the durability of structures by allowing chloride or carbon dioxide deep into the structural element and initiating corrosion over time. To some extent, cracking can be minimized or controlled by employing appropriate design, detailing, and construction practices and by incorporating advanced c o n s t r u c t i o n m a t e r i a l s i n t o n e w concrete structures. 2 ACI 224R-01 2 Table 4.1 provides a general guideline for reasonable crack widths for reinforced concrete under service loads for new construction. Acceptable crack widths range from 0.004 in. (0.1 mm) to 0.016 in. (0.41 mm), with smaller widths for concrete in wet or aggressive environments and larger crack widths for drier exposures. It is up to the specifier to use sound engineering judgment along with applicable standards to determine crack widths that may lead to a loss in functionality of the concrete structure. For a concrete structure, once the allowable crack width is established and cracks are identified, their widths and depths and the cause of cracking should be determined. The width of cracks can be measured to a precision of 0.001 in. (0.025 mm) by using crack gauge cards or pocket microscopes. It is important that the actual width of the crack is measured at a clean and straight location along the crack and that the measurement does not include the width of chipped or worn edges of the crack. To determine the depth and cause of cracking, core samples can be taken and, if subjected to petrographic testing, analyzed for issues such as freeze- thaw resistance, alkali-silica reactivity, or delayed ettringite formation. The depth of cracking can also be evaluated by a nondestructive testing (NDT) method such as ultrasonic pulse velocity. Because the bond strength of epoxy to concrete is greater than the tensile strength of concrete, epoxy injection can restore the structural integrity of the concrete when applied to cracks 0.002 in. (0.05 mm) in width or greater. It is important to note that if the cause of cracking is corrosion of the reinforcing steel or if movement of the concrete is anticipated after repairs, epoxy injection may not be the best solution to the problem. 3 Shallow cracks on horizontal surfaces such as bridge decks or the tops of pier caps can be repaired by gravity feed S A F E T Y A N D S E R V I C E A B I L I T Y Evaluation and Epoxy-Injection Repair of Cracks in Concrete by Leo Mancs, Vector Construction Crack widths can be visually measured using a gauge card (middle) or pocket microscopes. All Photos: Vector Construction. After the surface has been cleaned, surface- mounted injection ports are installed. A temporary crack-sealer epoxy paste will be applied on the face of the crack between and around the ports before the injection process begins. Table 4.1 Guide to reasonable* crack widths, reinforced concrete under service loads Exposure condition Crack width in. mm Dry air or protective membrane 0.016 0.41 Humidity, moist air, soil 0.012 0.30 Deicing chemicals 0.007 0.18 Seawater and seawater spray, wetting and drying 0.006 0.15 Water-retaining structures † 0.004 0.10 *It should be expected that a portion of the cracks in the structure will exceed these values. With time, a significant portion can exceed these values. These are general guidelines for design to be used in conjunction with sound engineering judgement [sic] . † Exclusing [sic] nonpressure pipes. Note: Table 4.1, Guide to Reasonable Crack Widths, Reinforced Concrete Under Service Loads, was reproduced from ACI 224R-01 (Reapproved 2008) Control of Cracking of Concrete Structures with permission from the American Concrete Institute.

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