The curing of concrete is driven by the hydration of cement, an exothermic chemical reaction. As with any chemical reaction, its rate will increase or decrease depending on the temperature of its surroundings. Because this chemical reaction is exothermic, the concrete generates its own heat which speeds up the reaction. As a result, concrete at different points in the structure will gain strength at different rates.
The use of a maturity sensor allows you to estimate the concrete strength at any point in the structure. Choice of placement is therefore important.
Typically, sensors should be placed in the weakest parts of the element. This will ensure that construction activities do not commence too early, improving safety on-site. Sometimes, sections with weaker areas will not be subjected to the same load. Therefore, another good choice of placement is any area of the element determined to be critical in terms of early load-bearing capacity.
Attach the sensor to the rebar, the metal sensor tip must not touch the rebar
Attach the sensor within 100mm of the surface, but not touching the surface
1. Guidance in British Standards EN 13670
“Detailed estimates of the development of concrete properties may be based on one of the following methods: maturity calculation from temperature measurements taken at a maximum depth of 10mm below the surface.” Extract from BS EN 13670, 2009, F.8.5
2. Guidance in ASTM C1074
“Place temperature sensing elements so that they will be surrounded by concrete and not be in direct contact with metallic embedments or other features that will be partially exposed to the environment. If this practice is used to decide whether critical construction operations may begin, install sensors at locations in the structure that are critical in terms of exposure conditions and structural requirements.” Extract from ASTM C1074-11
3. Placement near reinforcement
As mentioned in ASTM C1074, placement of a sensor directly in contact with the reinforcement will not give a representative measure of the concrete’s temperature, and therefore will not give an accurate estimate of the concrete’s strength. For this reason, industry best practice is to place the sensor roughly 25mm from the reinforcement and the last point of tying (Advanced Concrete Technology, Volume 2).
4. Placement in a slab
There will be thermal variation at different depths in a slab, and water migration can also affect which parts of the slab are weakest. In most sections (with the exception being those heated from the outside) the points of lowest maturity and strength will be those nearer the upper surface (“pullout readings on the upper surface can be about 10% less than those taken at the soffit.” Advanced Concrete Technology, Volume 2). General guidance, therefore, is to place the sensor close to the top of the surface. In simply supported bending this is also the zone of maximum compressive stress.