Drying Shrinkage
Aggregate's role in concrete is to limit the contraction of the cement paste, thus reducing the likelihood of cracking. Aggregates with higher modulus of elasticity, cubic shape, and rough particle surface textures offer more restraint to concrete shrinkage.
The degree of shrinkage can vary considerably depending on the type of aggregate used. Quartz, granite, and limestone aggregates are generally linked to low concrete shrinkage, whereas sandstone and some basic igneous rock aggregates may allow comparatively higher shrinkage.
To ensure the quality of structural concrete, BS EN 12620:2002+A1 specifies that the drying shrinkage of aggregates used should not exceed 0.075% when tested to BS EN 1367-4. This requirement is reflected in PD 6682-1 Annex B and BS 8500-2.
Concrete shrinkage values in the UK, using aggregates that are not known to have high shrinkage, typically range from 0.03% to 0.045%.
Test Method
To evaluate the drying shrinkage of concrete and prevent cracking in reinforced concrete members that can lead to a deterioration of durability in concrete structures, a quantitative test method is required. The following method can be used to estimate concrete shrinkage and control cracking during the design stage.
Materials:
Aggregate sample (fine and coarse)
Cement
Water
Equipment:
Mixing equipment
Moulds of specified dimensions (200 × 50 × 50 mm)
Oven set to 110°C
Method:
Mix the aggregate sample with cement and water to form a concrete mix.
Cast the mix into the moulds and compact them properly.
Wet the prisms for 24 hours at a temperature of 20±2°C.
Remove the prisms from the moulds and dry them in the oven at a temperature of 110°C for 72 hours.
Measure the change in length of the prisms from the wet to the dry state and calculate the average.
The excess drying shrinkage of the concrete is attributed to the aggregate and is expressed as a percentage of the final dry length of the prisms.
EN 1367-4 details this method for determining the drying shrinkage of aggregate in concrete. According to EN 12620, if disruptive cracking of concrete occurs due to the properties of the aggregate, the drying shrinkage associated with the aggregates used in structural concrete shall not exceed 0.075% when tested in accordance with EN 1367-4.