Why record the temperature of concrete?
Grant Data Acquisition explores the importance of curing concrete with temperature recording to eliminate premature cracking.
Concrete, one of the most durable building materials, is a composite material composed of aggregates bonded together with cement. The concrete slurry formed when aggregate is mixed with cement and water is easy to pour or mould into shape. Within a couple of hours of pouring or placing concrete, the temperature of the setting concrete will start to increase due to the chemical reaction between the cement and water. Cement hydration is an exothermic process and the heat generated may cause problems relating to the strength and quality of finish. In large pours or pours with a large volume to surface area the temperature of concrete can increase up to 85°C, then cool over the following 7 to 10 days. Because concrete undergoes a cure process rather than simply drying like paint, how concrete is treated after it is poured is just as important as before. The temperature of a large concrete structure needs to be accurately and reliably measured and recorded whilst being fabricated. This is especially true for pours which are greater than 0.5m thick, which need to have their temperature monitored and controlled to prevent undesirable effects occurring.
Properly curing concrete leads to increased strength and lower permeability and avoids cracking where the surface dries out prematurely, care must also be taken to avoid freezing or overheating due to the exothermic setting of cement. Improper curing can cause scaling, reduced strength, poor abrasion resistance and cracking.
The common issues are:
Thermal cracking (Fig.1) caused by the core (centre) and edge (face) contracting at different rates as the concrete cools. To prevent this, the differential temperature between the core and edge needs to be maintained below 25°C.
Cracking (Fig.2) which can occur up to several years after the concrete has been poured due to the delayed formation of a mineral called ettringite (this condition is called DEF). When the ettringite later forms its growth leads to a 2-3 volume increase in the concrete causing cracking. DEF can be prevented by ensuring the core temperature does not exceed 65°C.
To reduce the risk of these issues, the use of thermal blankets, embedded cooling pipes and low heat concretes are often used.
How is the temperature recorded?
A very effective way of monitoring and recording the temperature of concrete through the curing process is by using a data logger (Fig. 3) and a network of thermocouple wire temperature probes (Fig. 4).
Fig. 3 – the CMK610-S data logger
The measuring equipment will be installed prior to pouring. Thermocouple wires are routed within the pour to the desired location normally where the perceived highest temperature will occur and the top and bottom edges.
The thermocouple wire is usually secured to reinforcement (steel reinforcing bars) using plastic cable ties. Where possible the thermocouple wires are secured to the underside of the reinforcement to help prevent damage, for example from poker vibrators.
Up to 6 thermocouple temperature probes are connected to the data logger, which is housed in heavy duty bright yellow waterproof case and labelled plastic bag to help prevent damage due to poor weather. Once in place the datalogger and thermocouple wires are left unattended to automatically record the temperature and are left in situ until the pour has set successfully.
The sacrificial thermocouple cables are cast into the concrete and are cut off at the surface when the concrete is fully cured and the temperature data downloaded from the logger. Data can also be retrieved from the logging system during the cure process.
Grant supplies the temperature measurement cable in 100m rolls with separate plugs so that these individual sensors can be custom made on site to suit any job.
The Concrete Maturity Meter kit (CMK610-S) comprises:
- OQ610 logger for 6 x K or T type probes
- Waterproof case and type K extension leads PC610-S
- SquirrelView software and USB connecting cable
Like to discover more about our range of process loggers? Why not get in touch with our expert team via live chat, email us @ , call us on +44 (0) 1763 260 811 or visit our product page https://www.grantinstruments.com/data-acquisition/process-loggers/oq610-s-concrete-maturity-meter .