Susceptibility of Portland cement and blended cement concretes to plastic shrinkage cracking

A practical approach for reducing the risk of plastic shrinkage cracking of concrete

“…It is noticed that the CRP continues for some time after the disappearance of the bleed water from the concrete surface, as long as the concrete's intrinsic permeability is constant [14,17]. On the other hand, some studies showed that the evaporation rate of concrete samples during the CRP agreed well [14,15], or was even higher [18] than estimations based on the nomograph. The latter can be accidental, due to the fact that the wind speed utilized for obtaining the nomograph is based on measurements at 0.5 m above the evaporating surface, and air temperature and relative humidity at a level approximately 1.2 to 1.6 m higher than the evaporation surface [7].…”

“…As long as the short time windows between the decrease of bleeding rate and the time of final set, together with the relatively high bleeding rates observed in many concretes, in the past usually assured that there was enough accumulated bleed water to avoid the rise of capillary pressures, the rate approach cannot be considered conservative, in particular for the concretes used nowadays. In addition to the lower bleeding rates (and thus lower bleeding capacity) mentioned in point i, this is particularly because of the prolonged time window between the times of initial and final set caused by the application of plasticizers, or replacement of cement with inert fillers or with slow-reacting supplementary cementitious materials [14].…”

“…Therefore, the ACI nomograph is likely not able to provide accurate estimations of the rate of evaporation of concrete, at least not after the bleed water is removed completely from the surface of the concrete [7,13,16]. It is noticed that the CRP continues for some time after the disappearance of the bleed water from the concrete surface, as long as the concrete's intrinsic permeability is constant [14,17]. On the other hand, some studies showed that the evaporation rate of concrete samples during the CRP agreed well [14,15], or was even higher [18] than estimations based on the nomograph.…”

“…To address the complex problem of plastic shrinkage cracking, the strategy of modeling fresh concrete following a poromechanics framework has been used in the PhD project of the first author, see also [14,17,21,30,31]. In fact, the proposed model is able to couple the material properties and the geometry of a concrete element with the environmental factors, to finally estimate the cracking risk.…”

“…This nomograph was based on methods used in hydrology for predicting the rate of evaporation from lakes and reservoirs. It is worthwhile to mention that the rate of evaporation of free water and bleed water from mortars during the constant evaporation rate period (CRP) are almost identical [11], while for concretes, due to their larger aggregate volume (corresponding to a lower volumetric water content) [12], the CRP evaporation rate is likely lower [13][14][15]. Therefore, the ACI nomograph is likely not able to provide accurate estimations of the rate of evaporation of concrete, at least not after the bleed water is removed completely from the surface of the concrete [7,13,16].…”