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Browse from hundreds of recorded presentations from ACI Conventions and other concrete industry events.

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Technologies to Reduce Shrinkage and Cracking (ACI Fall 2020 Convention, Virtual Sessions) A potential shortcoming of ASTM C157, Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete, is the inability to observe behavior between the time of final set of concrete and the first 24 hours after casting. A modified version of ASTM C157, where length-change measurements begin approximately 5 to 5½ hours after casting was developed by University of Kansas Researchers to evaluate the effects of expansive admixtures on free shrinkage, where much of the early-age expansion would otherwise be missed. Results show that combinations of supplementary cementitious materials, internal curing via pre-wetted lightweight aggregates, shrinkage-reducing admixtures, and/or shrinkage-compensating admixtures in concrete produce significantly more swelling within the first 24 hours after casting than ordinary portland cement mixtures, a majority of which is not observed when using the standard test method. Furthermore, the test results for mixtures tested in accordance with both procedures show that the amount of drying shrinkage (length change from the end of curing through one year of drying) is virtually unaffected by the difference in test procedures. Apart from the reduction in shrinkage provided by the technologies listed above, the additional swelling further mitigates the potential for cracking in concrete.

Upcoming Presentation

June 28 - July 4

How the Choice of Various SSI Models Influences the Seismic Response of Medium-Span Bridges
by Nathalie Roy, University of Sherbrooke

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Exploiting SSI Effects in Structural Design of Bridges (ACI Spring 2021 Convention, Virtual Sessions) In the design stage, bridges are commonly modeled considering rigid foundations, thus neglecting the soil-structure interaction, which can have a significant impact on the evaluation of the base shear forces and superstructure displacements. This parametric study is about the influence of soil-structure interaction, using 3D models calibrated with ambient and forced-vibration tests, with different approaches for the energy dissipation and stiffness of the foundation soil. Superstructure displacements and base shears obtained with the rigid foundation model are compared with those obtained using linear soil-structure interaction models: (i) the simplified method proposed in the latest edition of the Canadian Highway Bridge Design Code (CSA S6-14); and (ii) the method proposed in the US National Earthquake Hazards Reduction Program (NEHRP-2012). The latter represents the foundation-structure interaction using a parallel damper and spring system, while the CSA S6-14 uses only a spring, without damping. The earthquake responses are calculated by time-history analyses for a case study bridge, using artificial accelerograms for different soil types. The influence of soil-structure interaction on displacements and shear forces is quantified. The decision to include soil-structure interaction into structural models, with various levels of complexity, can then be evaluated in the context of seismic design of highway bridges.

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