The Auto-Shrink system measures the unrestrained autogenous shrinkage of a specimen of cement paste or mortar cured under sealed conditions (ASTM C1698, Test Method for Autogenous Strain of Cement Paste and Mortar). Auto-Shrink permits evaluation of the relative autogenous shrinkage potential of different cementitious systems. Excessive autogenous shrinkage may lead to microcracking that increases the permeability of concrete.
When cement hydrates through chemical reactions with water, the volume occupied by the products of hydration is less than the original volume of cement and water. This phenomenon is known as “chemical shrinkage.” If concrete is cured under sealed conditions (no external source of moisture), the reduction in paste volume due to hydration causes internal tensile stresses that can lead to microcracking. The microcracking, in turn, reduces concrete’s resistance to penetration of water and deleterious substances. If a specimen of paste or mortar is cured under sealed conditions and allowed to change in volume, the chemical shrinkage of the paste will cause autogenous shrinkage of the specimen. The Auto-Shrink digital dilatometer is designed for linear measurement of autogenous shrinkage in hardening cement-based materials. A special corrugated plastic mold is used to prevent moisture loss and allow the specimen to shrink freely. With Auto-Shrink, it is possible to measure the time dependent deformation of many different specimens simultaneously over periods of weeks or even years.
Auto-Shrink is intended primarily for measurements after setting of cement pastes or mortars with a maximum aggregate size of 2 mm. To minimize the influence of temperature variations, the dilatometer should be used in a thermostatically controlled room. Background information on the measurement technique used in Auto-Shrink can be found in the following reference:
Mejlhede Jensen, O. and Freiesleben Hansen, P. “A Dilatometer for Measuring Autogenous Deformation in Hardening Portland Cement Paste,” Materials and Structures, 1995, 28 (181) 406-409
The Auto-Shrink digital dilatometer is composed of the following basic elements: