Every mass concrete placement project requires the contractor to monitor concrete temperature during curing. Many specifications will require the development of a thermal control plan, or will specify how and where to measure temperatures. Contractors use COMMAND Center to track the internal temperatures of in-place concrete during curing.
Mass placement is often defined as concrete that is continuously poured for an element whose minimum dimension is three feet or greater in any one direction. This definition, however, is not universal; owner agencies may vary their definition of mass placement anywhere from two to five feet. Job specifications will outline exactly what is considered mass placement for whatever job you are working on. The specifications will also describe what needs to be done when mass placement occurs—for most projects that means a thermal control plan is required.
As concrete hydrates, it releases heat. In mass placements, because there is so much concrete being placed at once and hydrating, a lot of heat is released, creating two primary concerns:
- Exceeding maximum internal temperatures.
- Exceeding temperature differentials between the hotter and cooler part of the element.
When internal concrete temperatures exceed maximum thresholds, there is an increased potential for drying shrinkage and delayed-ettringite formation (DEF). Both DEF and drying shrinkage can lead to cracking severe enough to result in costly repairs or reconstruction.
When temperature differentials between the hotter and cooler part of the element (usually defined by many specifications as the core and surface) exceed defined limits before enough strength has been achieved within the element, cracks—even severe cracks—may form, again resulting in costly repairs or reconstruction.
It is because of these concerns that most specifications require a thermal control plan. A thermal control plan explains the approach for determining the anticipated amount of heat that will be generated by the element during hydration, and it outlines the steps that will be taken to ensure that:
- The maximum internal concrete temperature does not exceed specified thresholds defined in the job’s specifications.
- The temperature difference within the concrete (most often between the core and surface) does not exceed the specified limit.
Typically, thermal control plans are either developed by the general contractor or are subcontracted to an engineering firm that specializes in thermal control plans for concrete. The development of a thermal control plan should include an analysis to determine the element’s temperature and strength gain. It should consider the size and shape of the element being cast, the mix design proposed, ambient conditions and curing processes. As part of the plan, key locations for monitoring temperature continuously throughout the curing process should be identified.
In general, construction teams must monitor temperatures at the center of an element and 2 to 3 inches below the surface. A traditional thermometer or infra-red laser is not sophisticated enough to adequately measure concrete temperatures in mass placements, so contractors use COMMAND Center temperature sensors to reach these areas and reliably track their temperature. Contractors place the sensors in designated locations, pour the concrete, and let the sensors automatically measure and log temperatures at predetermined intervals. The contractor can check the temperature history at any time.
COMMAND Center Sensors record and store temperature data internally. Modern sensor technology offers users the ability to define the desired time intervals for recording temperature data. Once the sensor records data, construction teams can download the data and use the free COMMAND Center software to generate reports of temperature history and temperature differential data for easy disbursement. For temperature monitoring only in mass placements, a 60-minute interval is most commonly specified for collecting temperature data.
Some mass concrete placement projects require installation of a redundant set of sensors at locations near the primary set. Data from these redundant sensors is usually only acquired and recorded in the event the primary temperature sensors fail. Should any of the primary temperature monitoring equipment fail, the contractor must take immediate steps to fix it. If the primary system cannot be fixed, the backup temperature monitoring system must be put into service. Because of the extremely low failure rates of COMMAND Center Sensors, redundancy is usually only necessary when required by specification.
Construction teams use COMMAND Center to properly monitor the temperature of mass concrete placements during curing. Failure to monitor mass placement temperatures can result in the element being rejected. Rejected concrete elements usually must be removed at the contractor’s expense.
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