Strain is the change in length as compared to original length that is before any variation. Strains in concrete are the reduction in volume of concrete after the application of loading then change in volume with respect to volume of concrete before applied loading is the measure of strains in concrete.
Strain = change in length / original length
Whenever a load (any load that may be live load, wind load, dead load) acts on a concrete specimen than because of these stresses, strains will be yield in the surface of concrete specimen, incase when applied stress value crosses the elastic limit of specimen than strains will begin propagating in the specimen.
Amount of these strains depends on the properties of specimen, like ductility, brittleness and strength. Specimen having more strength against applied stresses will show smaller strains and ductile material shows more strains before failure.
Amount Of Strains In Concrete
Concrete is a brittle material and it shows smaller value of strains before failure. Concrete fails suddenly and that is the big disadvantage of concrete in construction. To overcome this effect concrete is mostly use as a reinforced concrete (having steel bars inside the concrete).
Reinforced bars introduce ductility in concrete. Ductile material shows large value of strains before failure and due to this structure gives enough time for evacuation of building which saves life that is not possible in case of simple concrete.
Up to some limits strains are reversible (elastic in nature), which recovers while applied stresses are being removed. Plastic strains yield in the specimen before reaching peak point (maximum strains specimen able to bear before failure).
Elastic and plastic zone of specimen depends on the stiffness, brittleness and ductility of specimen. Specimen having higher strength will lead to smaller strains and elastic portion in this case will be more.