Concrete Products

APR 2014

Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.

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Page 42 of 51 April 2014 • 41 TECHNICAL TALK BY TOM KUENNEN ity—and is 30 to 40 percent lighter—than a comparable solid precast full-depth panel made of normal strength concrete, due to the improved structural properties of UHPC, note Aaleti and Sritharan. The decreased weight of the UHPC panel has signifcant benefts, including increase in span length for a given girder size, increase in girder-to-girder spacing, improvement in bridge ratings when used for deck replacement projects, and reduction in seismic, substructure, and foundation loads when com- pared to solid precast deck panel systems. The presence of the steel fbers in UHPC and very minimal shrinkage of UHPC after steam curing of the precast elements also decreases the reinforcement requirements when compared to traditional precast deck panels, the authors affrm. "Following a successful full-scale validation test on a unit consisting of two panels with three types of connections under laboratory condi- tions, the waffe deck was installed successfully on a replacement bridge in Wapello County, Iowa," they write. "The subsequent load testing con- frmed the desirable performance of the UHPC waffe deck bridge." Using the lessons from the completed project and outcomes from a se- ries of simple and detailed fnite element analyses of waffe decks, a design guide was developed to help broaden the design and installation of the UHPC waffe deck panel cost effectively in new and existing bridges. Avoiding Internally Cured, Prestressed Deck Panels Internally cured concrete is recommended for reducing the occurrence of early-age cracking in concrete bridge decks, but its use with pre- stressed concrete panels is not optimal, say W. Spencer Guthrie, Ph.D., Brigham Young University, Provo, Utah, and Joseph M. Yaede, E.I.T, Ap- plied Research Associates, Champaign, Ill., in their peer-reviewed paper, Evolution of Early-Age Cracking in Concrete Bridge Decks Incorporating Pre-Stressed Concrete Panels and Internally Cured Concrete. "Deck cracking can be minimized by using a variety of techniques affecting structural design, materials selection, and construction proce- dures," Guthrie and Yaede say. "Unfortunately, some approaches used for accelerated bridge construction, which are desirable to minimize disruptions to the traveling public, may compromise long-term deck performance by increasing the probability of deck cracking." Prefabricated bridge elements for speedy bridge construction are advantageous because they minimize traffc interruption, lower con- struction time, improve construction safety, and are less disruptive to the environment, the authors say. To accelerate bridge construc- tion and eliminate the need for conventional formwork between the bridge girders during the concrete bridge deck pour, precast, pre- stressed, half-deck panels are often used. The prestressed concrete panels together with the cast-in-place portions are designed to act compositely with the prestressed girders and behave similarly to a monolithic concrete deck. However, several reports have noted that the use of prestressed concrete panels has led to transverse cracking in the concrete bridge deck, where the cracks in the C-I-P deck surface correspond with the butt joints between adjacent underlying prestressed concrete pan- els. Although these cracks are not believed to signifcantly affect the structural performance of the deck, such cracking may accelerate deck deterioration by allowing moisture and chloride ions to penetrate the concrete and initiate corrosion of the embedded reinforcing steel. The objective of Guthrie and Yaede's research was to monitor, docu- ment, and quantify early-age deck cracking on four newly constructed bridge that incorporated features related to minimizing cracking and accelerating construction. Incorporation of pre-wetted lightweight fne aggregate (LWFA) to promote internal curing helps maximize cement hydration, minimize self-desiccation, and minimize deck cracking, the authors write. Therefore two bridge decks were constructed using a con- ventional concrete mixture, and two were constructed using a concrete mixture containing pre-wetted lightweight fne aggregate to facilitate internal curing and thereby reduce cracking. "Each of the four bridge structures incorporated prestressed con- crete panels placed between the girders to accelerate bridge con- struction," note Guthrie and Yaede. "Deck distress surveys were conducted to quantify and compare the degree of surface cracking among the bridge decks at fve months, eight months, and one year following deck construction." The authors conclude: • The use of pre-wetted lightweight fine aggregate to promote internal curing within concrete is recommended for reducing the occurrence of early-age cracking in concrete bridge decks. • However, internally cured concrete will not achieve its maximum potential in terms of crack reduction when prestressed concrete panels are used in deck construction; as documented, both conventional and internally cured concrete decks are susceptible to reflection cracking from the butt joints between underlying prestressed concrete panels. • Use of internally cured concrete in a monolithic concrete deck with conventional formwork may yield significantly better performance in this respect, although the benefits of precast concrete panels to accelerated bridge construction would be lost. • Comparing the loss in deck service life from premature cracking with the benefits of accelerated construction resulting from the use of PCPs is recommended for future research. Concrete Products April 2014.indd 41 4/1/2014 11:39:42 AM

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