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.
Issue link: http://www.aspiremagazinebyengineers.com/i/532296
SAFETY AND SERVICEABILITY When Should Mass Concrete Requirements Apply? by John Gajda and Jon Feld, CTLGroup The question of when a placement should be considered to be mass concrete is often debated. From the American Concrete Institute (ACI) definition, mass concrete is “any volume of concrete in which a combination of dimensions of the member being cast, the boundary conditions, the characteristics of the concrete mixture, and the ambient conditions can lead to undesirable thermal stresses, cracking, deleterious chemical reactions, or reduction in the long-term strength as a result of elevated concrete temperature due to heat from hydration.” This is an excellent definition, however, it does not provide the simple and measurable guidance that most engineers and contractors are looking for; it does not provide a quantitative definition, such as a thickness based definition. ACI 301, Specifications for Structural Concrete,1 attempts to provide simple and measurable guidance in the “notes to the specifier section” at the back of the document, by stating that placements that are 4-ft thick and greater should be considered mass concrete. In placing the guidance in the notes section, rather than the specification section, the actual thickness that is specified is left to the discretion of the specifier. In this same section, ACI 301 also states that placements with a minimum cementitious materials content of 660 lb/yd^3 should also be considered mass concrete. This latter guidance, although well meaning, has resulted in treatment as mass concrete for placements which probably should not be considered mass concrete. For example, we have written letters demonstrating that a 1-ft-thick wall constructed with a concrete containing 675 lb/yd^3 of cementitious materials will not behave as mass concrete. So when should a placement be treated as mass concrete? When the maximum temperature in the placement exceeds the typical industry standard limit of 160°F and/or when the temperature difference in the placement exceeds the typical industry standard limit of 35°F between the interior of the placement and a point that is 2 to 3 in. below/inside the center of a nearby surface. Placements that will not exceed these limits under typical placement conditions do not need to be considered mass concrete. What exceeds these limits is a function of the placement thickness and the concrete mixture proportions, specifically the type and quantities of the cementitious materials that are used. Mass Concrete Mechanism Temperature builds within a concrete placement when the rate of heat generation by the hydration of cementitious materials exceeds the rate of heat loss through the surfaces of the placement. Concretes with a high cementitious materials content will generate heat quite quickly; quicker than the heat can escape. Thick placements also trap heat such that the cementitious materials at the center cannot readily dissipate the heat. Saying this differently, if one were to use a concrete mixture that contains, for example, 600 lb/yd^3 of cementitious materials (where 75% is Type I/II portland cement and 25% is class F fly ash) in an 8-in.-thick bridge deck, the rate of heat dissipation would be fast enough that the concrete does not get overly hot and therefore does not behave as mass concrete. However, if the same concrete mixture was used in a 6-ft-thick placement, the story would be different; the interior of the placement would get quite hot since the heat cannot dissipate as quickly as it is generated; the concrete placement would behave as mass concrete.