Difference between Mass concrete and Reinforced concrete?
“any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change to minimize cracking.”
and reinforced concrete is defined by ACI 116R as:
“structural concrete reinforced with no less than the minimum amount of prestressing tendons or nonprestressed reinforcement as specified by ACI 318.”
Concrete is basically a good insulator. It doesn’t allow heat to pass through it easily. Hydration of cement is an exothermic reaction. When the dimensions of concrete are large enough, the heat generated during hydration of cement increases the temperature of the core of concrete, whereas the surface temperature of concrete being in contact with the atmosphere cools down due to loss of heat to the environment (depending upon the environmental conditions).
This difference in temperature between the core of concrete and its surface causes tensile stresses on the surface of concrete (as core expands due to rising in temperature) which when exceed tensile strain capacity of concrete causes it to crack. Such cracks are normally called thermal cracks. Thermal cracking mainly depends on cement content (per cubic meter of concrete), the volume of pour (dimensions of concrete to be cast monolithically), placement temperature of concrete and environmental conditions.
Mass concrete is normally placed in dams, bridge foundations, bridge piers, mat foundations, pile caps, thick walls, and tunnel linings. Mass concrete may or may not be reinforced depending upon the intended purpose of the structure. Most of the mass concrete is generally placed in gravity dams which doesn’t require reinforcements but even in these gravity dams, piers for spillway gates and spillway chutes are also heavily reinforced mass concrete.
Volumetric shrinkage is another phenomenon which can cause cracking in mass concrete due to thermal changes, autogenous or drying shrinkage.
The measures which are taken to reduce the cracks in mass concrete are by limiting the cement content (either by replacing cement with slag/fly ash or increasing the size of coarse aggregates), limiting placement temperature, cooling system, limiting the size of the pour and proper curing arrangements.