On the mechanism of plastic shrinkage cracking in fresh cementitious materials

Abstract

In this study, a continuum poromechanics approach is presented to model the plastic shrinkage cracking of fresh cementitious materials. The boundary conditions are according to the modified ASTM C1579-13 standard for mortars. The restrained deformations are linked to the restraint stresses according to the Cauchy-Navier equations of elasticity, assuming an incremental stressstrain relationship. The Bresler-Pister and Rankine failure criteria are utilized to model failure. The material parameters are adapted according to the Drucker-Prager and Griffith criteria. The crack initiation and propagation is verified experimentally by X-ray radiography. Eventually, the cracking mechanism is discussed and a safe capillary pressure limit is proposed. It is found that capillary pressure stiffening occurring before air entry, when deformations take place in the saturated state, is the predominant cause of plastic shrinkage cracking in the drying state.

In-situ crosslinking of superabsorbent polymers as external curing layer compared to internal curing to mitigate plastic shrinkage

“…When the evaporation rate exceeds the bleeding rate, the concrete surface starts to dry out and water menisci are formed in the pore system close to the surface. As a result, the negative capillary pressures occurring in the system reduce the inter-particle spacing and thus cause shrinkage of the material [6][7][8]. With ongoing evaporation the capillary pressures increase which leads to a breakthrough of the capillary pressure when air enters the pore system locally [1].…”

Reduction of shrinkage by Superabsorbent polymers (SAP) in fibre reinforced mortars

“…This additional reduction of shrinkage is the subject of the presented study. As it has been previously reported SAP ability to provide water during hydration process [26,27] can lead to notable reductions of autogenous shrinkage [28][29][30], plastic shrinkage [28,[31][32] and drying shrinkage [28,29] in plain cementitious materials (without SMCs and fibre reinforcement).…”

Assessing the dimensional stability of alkali-activated calcined clays in the fresh state: a time-lapse X-ray imaging approach

“…Likewise, the properties of alkaliactivated cement pastes in the fresh state control the amount of segregation and bleeding [16], which may lead to surface water evaporation and development of plastic shrinkage cracking. This process is initiated by capillary forces arising upon evaporation of the thin superficial water film, which lead to a stress accumulation that may eventually induce cracking once the tensile failure limit is exceeded [17,18]. Before setting occurs, the stress resulting from capillary pressure can be accommodated by movement of the dispersed particles, which is macroscopically manifested as a displacement of the surface [19,20] In general, it is not trivial, from the technical point of view, to quantitatively measure the strain of cement materials in their fresh state.…”

“…Here, the continuous collection of X-ray attenuation images with a small time interval provides a high-resolution time-lapse view of the sample interfacial displacement occurring during the early stage of reaction. A similar approach, based on X-ray imaging, was previously used to assess the initiation of plastic shrinkage crack propagation [18], although such an imaging approach has not been used to provide a direct visualization of the interface displacement in the fresh state.…”