Concrete Products

FEB 2017

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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www.concreteproducts.com February 2017 • 71 TECHNICAL TALK SLABS ON GRADE "Minimizing Thermal Cracks in Concrete Pave- ments," the latest research brief from the Massachusetts Institute of Technology-hosted Concrete Sustainability Hub (CSHub), exam- ines transportation slabs' risk fracture due to thermal cycles. Pavements subjected to suf- ficient stress are susceptible to fracture that can significantly reduce their durability. Ther- mo-mechanical eigenstresses (self-generated stresses due to thermal cycles within concrete pavements) are known to result in axial forc- es (expansion and contraction) and moments (bending and flexing) that can lead to fracture. CSHub researchers model the risk of frac- ture due to temperature changes and propose a method for estimating the risk of thermal cracking in transient-state conditions—when pavement is still undergoing a change in temperature—and steady-state conditions. When a pavement is in a transient state, they note, both axial forces and moments are observed, however a constant temperature in the steady-state condition only induces axial forces. Researchers also report that the allowable temperature change under steady- state conditions scales with pavement thick- ness and joint spacing, and (as shown in the figure) that the energy release rate due to the change in thermal gradient can be high- er than its corresponding value in a steady- state condition. This means that fracture is most likely to occur before the pavement reaches the steady-state condition. Designs typically only consider the steady- state condition. This research suggests that incorporating information on the risk-of-frac- ture during the transient period can allow engineers to develop pavements that are less prone to cracking. Among key takeaways from the research: • Durable-pavement design requires a means to minimize the risk of slab fracture. • The proposed model identifies the important structural and material parameters affecting pavement fracture resistance due to thermal cycles for both transient and steady state thermal conditions. • The model can be used to investigate the risk of fracture due to eigenstresses generated through other types of distress mechanisms such as alkali-silica reactions and freeze- thaw cycles. CONCRETE SUSTAINABILITY HUB MODEL THERMAL CRACKS IN PAVEMENTS CSHub researchers calculated cracking risk over time following a temperature change. The process is time-dependent because heat takes time to diffuse. Each line represents a pavement with a differ- ent thermal diffusion coefficient (a mea- sure of how effectively the pavement is able to conduct heat). The temperature profile is expected to eventually be the same for all systems (represented by the dashed line), but intermediate values dif- fer due to the difference in heat transfer.

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