Investigation of effects of Portland cement fineness and alkali content on concrete plastic shrinkage cracking

Abstract

From a material viewpoint, modern concrete's frequent propensity to plastic shrinkage cracking can be attributable to a combination of low water-binder ratio use and ever-changing properties of binding materials. To obtain a better understanding of this phenomenon, this paper explores the effects of cement fineness and alkali content on the plastic shrinkage cracking of concrete manufactured with two water-binder ratios. Results indicate that within the range 275-385m 2 /kg, cement specific surface area is approximately directly and inversely proportional to hydration rate and evaporation rate respectively; a trend generally leading to higher plastic shrinkage and resulting areas of plastic cracking. Similar effects were observed for alkali contents which resulted in increased levels of plastic shrinkage. Furthermore, while decreasing crack tendency was noted as alkali content increased from 0.4 to 0.8% by mass of cement, further increases in alkali content caused significant decreases of compressive strength and slump; thereby lowering overall concrete performance. It is also found that plastic shrinkage cracking is closely related to the kinetics of plastic shrinkage. In summary, the experimental programme confirmed that cement with relatively low surface area (less than 340 m 2 /kg) and low alkali content (less than 0.8%) is preferred for modern concretes with minimal plastic cracking problems.

Plastic Shrinkage Cracking of Self-compacting Concrete: Influence of Capillary Pressure and Dormant Period

“…Furthermore, it has been observed that a normal hardening cement boosts the cracking tendency of SCC in comparison to a rapid hardening cement [4]. Finer cement particles, on the other hand, increase the cracking severity [12,13], most probably due to autogenous shrinkage. However, coarser cements cause wider cracks, despite of lower cracking intensity [13].…”

“…Finer cement particles, on the other hand, increase the cracking severity [12,13], most probably due to autogenous shrinkage. However, coarser cements cause wider cracks, despite of lower cracking intensity [13]. Another study shows that superplasticizers (SP) increase the evaporation, delay the hydration, and lead to higher cracking risk [14].…”

Early age behaviour of recycled concrete aggregates under normal and severe drying conditions

“…Thus Building concretes showed lower shrinkage rate than civil engineering concretes in spite of higher drying flux for building concretes. Kai et al [30] found that an increase in cement surface area resulted in lower evaporation rate but higher plastic shrinkage. Slowik et al [29] also showed that early age cracking risk increases with decreasing particle sizes.…”

“…Plastic shrinkage depends on water-to-binder (W/B) ratio and cement fineness [29]. An increase in cement fineness and a decrease in W/B ratio led to a decrease in the bleeding rate and an increase the capillary tension level [30]. Plastic shrinkage strain is reduced when the evaporated water is replaced by bleeding water, which could be provided by saturated recycled concrete aggregates [31].…”

The durability of RPC with different content of silicon powder

“…After 300 times of freezing-thawing cycles, the mass loss of RPC with 40% SP is 0.32% and the relative dynamic modulus is 99.3%. Based on the reports about the shrinkage [6][7], freezing-thawing cycles [8][9], and permeability of chloride ions [10][11] for ordinary concrete and RPC, when the SF is replaced by 40% SP, the compactness of the mortar or concrete will be improved with "Filling effect" of SP, which is due to a small particle size that is smaller than cement particle size and bigger than SF particle size. Generally speaking, more compact or less water means less porosity and penetration channel for ions or solution, which finally leads to a better volume stability, a lower mass loss and a higher relative dynamic modulus under freezing-thawing cycles, and a lower electric flux under the penetration of chlorid ions (Table 4).…”