Plain cement concrete, ground granulated blast furnace slag (GGBS) concrete, and fly ash concrete were designed. Three wet curing periods were employed, which were 2, 5, and 8 days. The drying shrinkage values of the concretes were measured within 1 year after wet curing. The results show that the increasing rate of the drying shrinkage of concrete containing a mineral admixture at late age is higher than that of plain cement concrete regardless of the wet curing time. With the reduction of wet curing time, the increment of total drying shrinkage of concrete decreases with the decrease of the W/B ratio. The negative effects on the drying shrinkage of fly ash concrete due to the reduction of the wet curing time are much more obvious than those of GGBS concrete and plain cement concrete. Superfine ground granulated blast furnace slag (SGGBS) can reduce the drying shrinkage of GGBS concrete and fly ash concrete when the wet curing time is insufficient.
Drying shrinkage of concrete is a phenomenon that is caused by the moisture drying from the pore system of the hardened paste and thus leads to volume shrinkage of the concrete [
There are four prominent physical models proposed to account for the mechanism of drying shrinkage: capillary tension [
Fly ash is a by-product of the combustion of pulverized coal in power plants that acts as a pozzolanic material in concrete. Fly ash blended cement can make an improvement in the pore structure of concrete due its pozzolanic and microaggregate effects [
Blast furnace slag is also widely used as a mineral admixture in concrete [
The water/binder (W/B) ratio is another important factor affecting the drying shrinkage of concrete. In general, a high W/B ratio increases the drying shrinkage with the tradition explanation that a higher W/B ratio leads to a greater volumetric cement paste content and less restraint of deformation [
The wet curing time of the concrete before being exposed to the drying environment has an impact on the development of drying shrinkage. In this work, the influence of wet curing time on the drying shrinkage of concrete was experimentally evaluated with different mineral admixtures and W/B ratios.
Table
Chemical compositions of cement, GGBS, and fly ash (%).
CaO | SiO2 | Al2O3 | Fe2O3 | MgO | Na2 |
Loss on ignition | |
---|---|---|---|---|---|---|---|
Cement | 63.59 | 21.86 | 4.25 | 2.66 | 2.19 | 0.55 | 1.75 |
GGBS | 36.44 | 31.76 | 14.84 | 0.60 | 9.08 | 0.56 | 0 |
Fly ash | 2.44 | 48.67 | 30.95 | 5.62 | 1.15 | 0.78 | 7.65 |
The mix proportions of the 8 concretes were listed in Table
Mix proportions of the concretes (kg/m3).
Cement | GGBS | Fly ash | SGGBS | Coarse aggregates | Fine aggregates | Water | |
---|---|---|---|---|---|---|---|
C-0.50 | 360 | 0 | 0 | 0 | 1060 | 800 | 180 |
B-0.50 | 216 | 144 | 0 | 0 | 1097 | 763 | 180 |
F-0.50 | 216 | 0 | 144 | 0 | 1116 | 744 | 180 |
SB-0.50 | 216 | 108 | 0 | 36 | 1097 | 763 | 180 |
SF-0.50 | 216 | 0 | 108 | 36 | 1116 | 744 | 180 |
C-0.42 | 360 | 0 | 0 | 0 | 1077 | 812 | 151.2 |
B-0.42 | 216 | 144 | 0 | 0 | 1114 | 774 | 151.2 |
F-0.42 | 216 | 0 | 144 | 0 | 1133 | 756 | 151.2 |
Prism specimens measuring
Three curing methods were employed: (a) wet curing for 2 days and then dry curing (
The drying shrinkage of the concretes was measured right after the wet curing period. The drying shrinkage values were measured within 360 days’ drying. Chloride ion permeability test was carried out according to ASTM C1202, “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.” The nonevaporable water content values of the paste were obtained as the mass differences between the samples heated at 105°C and 1000°C. These results were normalized by the mass after heating at 105°C and corrected for the loss on ignition of unhydrated samples.
Autogenous shrinkage is defined as the change of the concrete volume without the transfer of moisture to the environment after the initial setting [
Figures
Drying shrinkage of plain cement concrete.
Drying shrinkage of GGBS concrete.
Drying shrinkage of fly ash concrete.
However, the three figures also show some different laws: (1) the drying shrinkage values of the three concretes differ from each other due to their different curing conditions. After wet curing for 8 days, the drying shrinkage of GGBS concrete is close to that of plain cement concrete, whereas the drying shrinkage of fly ash concrete is much less than that of the other two concretes. After wet curing for 2 days, drying shrinkage of GGBS concrete is still close to that of the plain cement concrete, whereas the drying shrinkage of the fly ash concrete is obviously larger than that of the other two concretes, which is contrary to that found under the condition of wet curing for 8 days. This indicates that prolonging the wet curing time decreases the drying shrinkage of fly ash concrete much more obviously than that of the GGBS concrete and plain cement concrete. This is mainly due to the low activity of fly ash at an early age. Therefore, more wet curing time is needed to ensure the full hydration of the cement and the activation of the fly ash by the Ca(OH)2 produced. (2) The increasing rates of the drying shrinkage of the three concretes show great differences from each other at the late age. For plain cement concrete, the increasing rate of drying shrinkage is very slow at the late age regardless of the length of wet curing time. However, the drying shrinkage values of GGBS concrete and fly ash concrete increase greatly at the late age after wet curing for 2 days. Comparatively, the increasing rate of drying shrinkage of fly ash concrete was larger at the late age. After wet curing for 5 or 8 days, the increasing rates of drying shrinkage of GGBS concrete and fly ash concrete at the late age are a little slower than those after wet curing for 2 days, and they are still much larger than that of plain cement concrete. Similarly, the increasing rate of drying shrinkage of fly ash concrete is comparatively larger at the late age.
This indicates that the wet curing time has a greater influence on concrete containing mineral admixtures than plain cement concrete. Two reasons may account for it: (1) when wet curing is insufficient (only 2 days), the reaction degrees of GGBS and fly ash are low, which makes their pore structures coarse and the connected porosity increases at late age. Thus, the moisture in concrete transfers quickly and the loss of moisture from the gels increases, leading to an increase of the drying shrinkage at the late age. Comparatively, the reaction degree of fly ash is lower and thus the pore structure pore is coarser at the late age, which results in a greater increase of drying shrinkage. (2) When wet curing for 5 days or 8 days, the reaction degrees of GGBS and fly ash are relatively high at the late age, which decreases the porosity and is a benefit for improving the pore structure at late age. Specifically, the proportion of pores smaller than 10 nm increases, leading to a high capillary pressure and an increase of the drying shrinkage at the late age [
The nonevaporable water (
Nonevaporable water contents of the hydration products of binders.
Figure
Elastic modulus of the concretes with W/B ratio of 0.42.
The entire elastic modulus of concrete is dependent on both the hardened paste and the aggregates. The length of wet curing time does not affect the elastic modulus of the aggregates, and thus it has less influence on the entire elastic modulus of the concrete than it has on that of the hardened paste. It is believed that the elastic modulus of plain cement paste and paste containing GGBS is reduced to some extent due to the decrease of the wet curing time. However, the entire elastic modulus values of plain cement concrete and GGBS concrete are little affected by wet curing time. Furthermore, the decrease of wet curing time has an obvious influence on the entire elastic modulus of fly ash concrete. This indicates that the elastic modulus of paste containing fly ash decreases much more significantly than that of paste containing GGBS and plain cement due to the reduction of the wet curing time. The trend of the entire elastic modulus of concrete is closely related to that of the stiffness of the concrete. Therefore, it can be concluded that as for the fly ash concrete, the decline of elastic modulus is one of the reasons for the increase of the drying shrinkage by decreasing the wet curing time. However, for the cement concrete and GGBS concrete, the influence on elastic modulus can be neglected.
Figures
Chloride permeability of plain cement concrete.
Chloride permeability of GGBS concrete.
Chloride permeability of fly ash concrete.
For plain cement concrete, the chloride permeability values of concretes with different wet curing periods are in the same grade (moderate level) at the age of 28 days. This indicates that the decrease of wet curing time does not affect the chloride permeability grade of plain cement concrete at 28 days. In the case of wet curing for 8 days, the permeability of plain cement concrete falls into the “low” grade at the age of 360 days, whereas the permeability of plain cement concrete after wet curing for 5 or 2 days is still in the “moderate” grade at 360 days. This indicates that shortening the wet curing time from 8 to 5 days can improve the chloride permeability grade of plain cement concrete at 360 days, while shortening the wet curing time from 5 to 2 days cannot.
For GGBS concrete, the chloride permeability values of concretes with different wet curing periods are in the same grade (low level) at the age of 28 days. This indicates that the decrease of wet curing time does not affect the chloride permeability grade of GGBS concrete at 28 days, which is similar to the case of plain cement concrete. In the case of wet curing for 8 or 5 days, the permeability of GGBS concrete falls in the “very low” grade at the age of 360 days, whereas the permeability of GGBS concrete after wet curing for 2 days is still in the “low” grade at 360 days. This indicates that shortening the wet curing time from 5 to 2 days can improve chloride permeability grade of GGBS concrete at 360 days, while shortening the wet curing time from 8 to 5 days cannot.
For fly ash concrete, the chloride permeability values of concretes after wet curing for 8 and 5 days are in the same grade (low level) at the age of 28 days, whereas those after wet curing for 2 days are in “moderate” grade. This indicates that shortening the wet curing time from 5 to 2 days can improve the chloride permeability grade of fly ash concrete at 28 days, while shortening the wet curing time from 8 to 5 days cannot. In the case of wet curing for 8 days, the permeability of fly ash concrete falls in the “very low” grade at the age of 360 days, whereas the permeability of fly ash concrete after wet curing for 5 or 2 days is still in the “low” grade at 360 days. This indicates that shortening the wet curing time from 8 to 5 days can improve chloride permeability grade of fly ash concrete at 360 days, while shortening the wet curing time from 5 days to 2 days cannot.
To study the influence of the reduction of the wet curing time on the drying shrinkage of concretes at different ages, the drying shrinkage of concrete at 30 days is defined as early drying shrinkage and the drying shrinkage of concrete at 360 days is defined as total drying shrinkage. Figures
The increments of drying shrinkage of concretes when the wet curing time decreases from 8 days to 5 days.
The increments of drying shrinkage of concretes when the wet curing time decreases from 5 days to 2 days.
Figures
However, there are also some differences between the two figures: (1) the increments of early drying shrinkage of GGBS concrete and plain cement concrete are very small (only approximately 10
It can be concluded from this study that the drying shrinkage of concrete with mineral admixtures increases significantly due to their low activity at an early age when the wet curing time is insufficient. This is especially the case for fly ash concrete, whose increments of early and total drying shrinkage are much larger than those of plain cement concrete due to the reduction of the wet curing time. The existing research results show that SGGBS improves reaction activity at the early and middle ages and has better resistance to capillary suction, which can decrease the drying shrinkage and improve the compactness and durability of concrete [
Figures
Drying shrinkage of GGBS concrete and SGGBS-GGBS concrete.
Drying shrinkage of fly ash concrete and SGGBS-fly ash concrete.
It can also be seen from these two figures that with the addition of SGGBS, the decrease of the amplitude of the drying shrinkage of GGBS concrete at early and middle ages is obviously larger than that at the late age. Furthermore, SGGBS decreases the drying shrinkage of GGBS concrete much more obviously than that of fly ash concrete at early and middle ages. However, with the addition of SGGBS, the decrease of the amplitude of the drying shrinkage of fly ash concrete increases with increasing age. This indicates that SGGBS can decrease the drying shrinkage of GGBS concrete at early and middle ages significantly but has limited effects on the drying shrinkage at the late age. However, SGGBS can decrease the drying shrinkage of fly ash concrete only at the late age. Two reasons may account for it. One reason is that SGGBS can accelerate the early and middle hydration process, which is beneficial to the formation of a fine pore structure and slows down the moisture loss. However, it leads to a high proportion of pores smaller than 10 nm in concrete [
The drying shrinkage of concrete within 1 year significantly increases with the decrease of wet curing time regardless of cementing components. As wet curing time is reduced, the increments of total drying shrinkage of concretes decrease with the decrease of the W/B ratio. Decreasing the wet curing time from 8 to 5 days has little effect on the increments of the early drying shrinkage of GGBS concrete and plain cement concrete. However, the increments of early drying shrinkage of GGBS concrete and plain cement concrete increased significantly when the wet curing time is reduced from 5 to 2 days. For any cementing component, decreasing the wet curing time from 5 to 2 days has greater effects on the early drying shrinkage of concrete but limited effects on the total drying shrinkage when compared to the effects of the wet curing time decreasing from 8 to 5 days.
The increasing rate of the drying shrinkage of concrete containing a mineral admixture at a late age is higher than that of plain cement concrete regardless of the wet curing time. The increasing amplitude of the drying shrinkage of fly ash concrete due to the reduction of the wet curing time is larger than that of GGBS concrete and plain cement concrete. The sensitivities to the change of wet curing time on the drying shrinkage of these two concretes are close to each other when the wet curing time is over 5 days, whereas the sensitivity to the change of the wet curing time of the drying shrinkage of GGBS concrete is higher than that of plain cement concrete when the wet curing time is less than 5 days. SGGBS can reduce the drying shrinkage of GGBS and fly ash concrete. Specifically, SGGBS can significantly decrease the drying shrinkage of GGBS concrete at the early and middle ages but has limited effects at the late age. However, SGGBS can decrease the drying shrinkage of fly ash concrete only at the late age.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Authors would like to acknowledge the Tsinghua University Initiative Scientific Research Program (20161080079).