Concrete Products

JAN 2019

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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72 • January 2019 TECHNICAL TALK PRACTICE Typically, the maximum internal temperature is specified as 158°F, but sometimes as high as 160°F, because the risk for DEF increases above that temperature. Mass concrete temperature differential. In mass placements, the difference of temperature between the hotter and cooler part of the element—defined by many specifications as the core and sur- face—is the temperature differential. Large temperature differentials in mass concrete structures can cause severe cracking. Temperature differentials cause tensile stresses in concrete. If the stresses are greater than the concrete strength, cracking will occur. These cracks can be severe, so to reduce the risk for damage, many agencies state that the temperature difference between the hottest part of the concrete and the closest exterior surface should not be greater than 35°F. In some situations, this requirement may be too restrictive; in others, cracking may still occur below the recommended temperature differential. MONITORING MANDATE Every mass concrete project requires the contractor to monitor the temperature for several days after placement, sometimes even up to three weeks or longer. The temperature monitoring procedure that a contractor intends to follow usually must be described in a thermal control plan. The design calculations used in the plan must conform to the governing agency's specifications and be developed and approved in advance of construction. A thermal control plan includes calculations for both the maxi- mum internal temperature and maximum temperature differential the concrete element is expected to experience. These calculations are based on the size and shape of the element being cast, the mix design proposed, ambient conditions, and curing processes. The purpose of the calculations is to show the owner-agency that internal concrete temperatures will be within the specified requirements for maximum temperature and maximum differential allowed. Contractors can take advantage of advanced technology to monitor internal concrete temperatures and temperature differentials: Sen- sors record and store temperature data internally, while specialized equipment and software acquire and store recorded data. Specifica- tions for temperature sensor placement vary from project to project. The thermal control plan will identify the locations where concrete temperature sensors will be placed. However, temperature-monitor- ing sensors should generally be placed at the hottest location of the concrete and a few inches below the nearest surface from the hottest location. Installed sensors should continue to monitor concrete temperature for the amount of time specified in the thermal control plan. After placement, mass concrete temperature readings may be acquired as Walnut Wind Farm turbine foundation

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