Fig. 1. Cracked massive element and close-up view of cracking.
Mass Concrete Specifications – Two States' Perspectives
Christopher C. Ferraro Ph.D., P.E., University of Florida and Kevin R. Pruski, PE, Texas Department of Transportation
In the mid-to-late 1990s, both the Texas Department of Transportation (TxDOT) and the Florida Department of Transportation (FDOT) began noticing cracking in many of the larger cast-in-place concrete transportation structures. Although the Standard Specifications and Design Guidelines relating to concrete construction for these agencies provide general guidance requirements to address heat generation for structural members based on minimum dimensional requirements, implementation was not always addressed properly. In some cases, the steps to limit temperature rise and temperature differential within the concrete member soon after concrete placement did not get implemented correctly. Thus, much of the cracking that was observed then can be attributed to temperature rise associated with concrete hydration. In the early 2000s, TxDOT implemented the results of research discussed in a previous issue of this newsletter.(1) During the same time period, FDOT incorporated the results from several research projects into the standard specification for road and bridge construction.(2) Both specifications adopted the methods outlined in ACI 207.(3) This article further discusses what TxDOT and FDOT have learned from their implementation of mass concrete requirements.. TxDOT recently updated the standard specifications for highway construction.(4) The section addressing concrete structure construction--in particular, mass concrete construction--was modified slightly to address what to do if the temperature controls shown in Table 1 were exceeded. As the use of the ConcreteWorks software to predict thermal profile in the member progressed, along with actual observation of the in-situ monitoring, it became clear that the biggest challenge was to keep the differential temperature within the member below the allowed temperature. Additional guidance is provided to instruct the contractor and inspection staff as to placement of sensors within the member. In practice, it is not the worst-case scenario related to delta rise that may be critical, but rather, the delta producing the highest temperature gradient that results in concrete cracking. The simple approach recommended is to compare the core temperature to the temperature at the nearest side face, 3 inches from the surface, in the same plane.
TxDOT has greater concern with limiting the maximum core temperature. Because the majority of aggregates locally available have potential for Alkali-Silica Reactivity (ASR), and therefore may be more prone to Delayed Ettringite Formation (DEF), keeping the core temperature below 160°F is strictly enforced. If the core temperature is measured to exceed the 160°F, removal of the member could be necessary unless material testing and petrographic analysis shows little chance for future occurrence of DEF.
Like Texas, Florida has recently updated the Standard Specification for Road and Bridge Construction. The FDOT specifications generally align with the TxDOT Specifications. However, there are some differences between the two states. The maximum allowable temperature per the Florida specification is 180°F. This is based on research indicating that the potential for DEF is mitigated in concrete which contains class F fly ash and slag.(5) FDOT has not approved a concrete mixture to be used for Mass Concrete applications without slag or fly ash since prior to 1980.(6) Thus, the pairing of warm climates and conventional use of high-quality pozzolans provides an extra degree of flexibility given the relative difficulty involved with minimizing temperature rise in concrete placed in warm climates.
Information and field experience collected by FDOT have shown that temperature differentials are often the cause of early-age cracking. Figure 1 shows a massive concrete footing that experienced full-depth cracking due to the temperature differential being exceeded at early ages. Therefore, the FDOT specifications have strict requirements for early-age control measures to prevent cracking due to temperature differentials and thermal shock. Florida requires the maximum allowable temperature differential between the core and 2" from the surface to be 35°F. However, the specification has recently introduced a temperature control measure to prevent thermal shock, which requires the temperature control mechanisms to remain in place until the core temperature is within 50°F of the ambient temperature.
The use of the thermal control plans, per the specifications in Texas and Florida respectively, has produced mass concrete that remains relatively uncracked based on field experience. If the collected field data shows non-compliance, the contractor is requested to revise the thermal plan for future placements and the member is thoroughly inspected for cracks. Crack sealing or other repair techniques are required if cracking is found.
Table 1: Specification Requirements - Mass Concrete, Thermal Control Plan