In addition to cube manufacture, the strength of concrete can also be monitored in situ using thermal maturity monitoring. This involves the placement of temperature monitoring loggers in the concrete at key points to measure the temperature as the concrete hydrates. This data can detect problems with concrete curing and also warn of thermal issues in both hot and cold conditions. It can also be used to measure temperature differential between ambient and surface to prevent thermal cracking when striking formwork. Concrete maturity is calculated using the Sadgrove equation. Sadgrove first came up with the idea of concrete ‘equivalent age’ in 1971, and this method has been tried and tested before. It is a standard practice according to ASTM C1074.
- Temperature sensors are used to determine the early strength gain of concrete in situ, without the use of destructive testing methods.
- This method utilises Sadgrove’s Maturity Law which predicts concrete strength based on the in situ temperature, compared to a lab cured baseline
- Estimate of strength given is more accurate than cube results over the early stages of slab curing, as samples made in the lab do not take the in situ temperature of the concrete into account.
- To create an equation to allow concrete strength to be assessed with temperature sensors, a set of at least 6 cubes for the mix you would like to assess need to be made. These will be crushed at fixed intervals of 1,2,3,4,5 and 7 Days to form a calibration curve for the equation.
Figure: Sample concrete calibration data.
Temperature Standards & Specifications
The following standards, specifications, and reports address concrete temperature issues.
ASTM C1074 – “Standard Practice for Estimating Concrete Strength by the Maturity Method”
ASTM C31 – "Standard Practice for Making and Curing Concrete Test Specimens in the Field"
ASTM C39 – "Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens"
ACI 306R - "Cold Weather Concreting"
ACI 306.1 - "Standard Specification for Cold Weather Concreting"
ACI 207.2R - "Effect of Restraint, Volume Change, and Reinforcement on Cracking of Mass Concrete"
ACI 207.4R - "Cooling and Insulating Systems for Mass Concrete"