Concrete Construction Planning
In a construction project, concrete is often on the critical path. Most of the jobsite activities can not be initiated unless the concrete that is placed for building structural elements such as beams and slabs has achieved sufficient strength (typically 75% of the specified compressive strength of concrete). If concrete is not cured properly in favorable temperature ranges (at least more than 5°C or 40°F based on ACI 306) after pouring, it is quite possible to have a slippage in the project plan and timeline due to the delayed strength development in concrete. Curing temperature is also important when it comes to mass concrete placements such as foundations. If the temperature differential between the surface and core of the mass concrete element exceeds a certain level, there is good chance of concrete cracking due to thermal stresses. Project managers, therefore, have a thermal control plan to ensure that concrete achieves the target strength values as scheduled during the first few days after pouring concrete so they can safely move to the next steps of construction such as formwork removal or post-tensioning with confidence.
Monitoring Concrete Curing and Hardening
A comprehensive thermal control plan describes the temperature monitoring procedure that the general contractor or concrete sub-contractor must implement when placing concrete in cold or hot weather conditions, as well as that for mass concrete elements. This plan generally includes details on the minimum and maximum allowable temperature values for different structural locations, maximum allowable temperature differential for mass concrete placements, type of equipment, sensors and tools for temperature measurement, frequency and duration of temperature readings, as well as relevant heating or cooling producers.
The in-situ concrete temperature measurements can also be used to estimate the strength of concrete using the maturity method (as per ASTM C1074) in real-time. The possibility of obtaining field strength of concrete (without relying of delayed lab test results), makes concrete temperature measurement even more important particularly in structural concrete elements. In addition to temperature measurement that is an essential part of every thermal control plan, some contractors have been leveraging the maturity method to optimize their project schedules and prevent delays in critical construction operations such as stripping forms, shoring and re-shoring removal, pulling post-tension cables, and opening traffic on concrete pavement. In addition, the simultaneous monitoring of concrete temperature and strength provides opportunities for the optimization of heating and cooling cost, as well as allowing for higher temperature differentials.
Different Types of Concrete Temperature Sensors and Maturity Meters
When it comes to the choice of concrete temperature or maturity measurement sensors and equipment, contractor have many options to select based on the cost, accuracy, and ease of use, and of course how all these considerations would fit their project needs and budget. The measurement systems available in the market are categorized as follows:
b. Wired Temperature and Maturity Loggers
c. Wired Concrete Sensors with External Wireless Transmitter
d. Fully Embedded Wireless Concrete Sensors
Each of these temperature/maturity monitoring systems have their own advantages and disadvantages and it is important for contractors to review these before developing their thermal control planning in order to maximize the ROI. It should also be noted that infrared laser-based temperature measurement tools are not recommended for monitoring concrete curing and hardening as they can not capture internal concrete temperature.