J. Cent. South Univ. (2012) 19: 1155-1162

DOI: 10.1007/s11771-012-1122-2

Influence of subsequent curing on water sorptivity and pore structure of steam-cured concrete

HE Zhi-min(贺智敏)^{1, 2}, LONG Guang-cheng(龙广成)¹, XIE You-jun(谢友均)¹

1. School of Civil Engineering, Central South University, Changsha 410075, China;

2. School of Civil Engineering, Ningbo University, Ningbo 315211, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: Steam-cured condition is found to cause larger porosity and worse properties of concrete compared with normal curing condition. For the sake of seeking effective measurements to eliminate this bad effect of steam-cured condition on concrete, the water sorptivity and pore structure of steam-cured concretes exposed to different subsequent curing conditions were investigated after steam-curing treatment. The capillary absorption coefficient and porosity of the corresponding concretes were analyzed, and their mechanisms were also discussed. The results indicate that water sorptivity and pore structure of steam-cured concrete are greatly influenced by the curing condition used in subsequent ages. Exposure steam-cured concrete to air condition has an obviously bad effect on its properties and microstructures. Adopting subsequent curing of immersing steam-cured concrete into about 20 °C water after steam curing period can significantly decrease its capillary absorption coefficient and porosity. Steam-cured concrete with 7 d water curing has minimum capillary absorption coefficient and total porosity. Its water sorptivity is decreased by 23% compared with standard curing concrete and the porosity is 9.6% lower. Moreover, the corresponding gradient of water sorptivity and porosity of steam-cured concrete both decrease, thus microstructure of concrete becomes more homogeneous.

Key words: steam-cured concrete; water sorptivity; pore structure; curing condition

1 Introduction

Precast concrete elements in railway engineering infrastructures, such as sleepers, track slabs and pre-stressed concrete beams, are mainly produced by steam-cured concrete. Concrete exposed to elevated temperature shows an accelerated hydration and non- uniform distribution of hydration products [1-2], which results in a rapid increase of the compressive strength in early age. However, this can also cause a larger porosity and a decrease of strength in later age [3-4]. HO et al [5] reported that for heat-cured products, the quality of the concrete cover was poor and was equivalent to that of concrete with only 2-3 d of standard curing. In order to improve the properties and microstructure of steam- cured concrete and avoid above bad effects caused by steam curing process, many measurements, such as adoption of minerals admixture and optimization of steam-cured regime, were developed [6-7]. However, some defects such as crack and peeling can still be observed on the surface layer of steam-cured concrete, especially on the external surface layer exposed to steam room.

Although steam curing accelerates hydration of cement, the internal structure of steam-cured concrete does not completely form just within 10 h steam-cured period [8-10]. Curing becomes even more important if the concrete contains supplementary cementing materials such as fly ash or ground, granulated blast-furnace slag or silica fume, and the concrete is subjected to hot and dry environments immediately after casting [11-13]. If the potential of concrete with regards to strength and durability is to be fully realized, it is most essential to be cured adequately. Obviously, for steam-cured concrete, the subsequent curing after steam-curing treatment is essential to obtain its potential performance. Despite steam-cured concrete has been used widely in the precast industry to speed up production, investigations on steam-cured concrete were focused mainly on their strength development and other properties [7-10]. Research on how to eliminate the adverse effect of steam curing period on concrete by subsequent curing conditions is limited. This work is intended to find some simple and effective measurements to improve the performance and eliminate the flaws of steam-cured concrete by investigating the influence of subsequent curing conditions on water sorptivity and porosity of steam-cured concrete.

2 Experimental

2.1 Materials and mixing proportions

The cement used was reference Portland cement with grade P.O 42.5 provided by China Building Material Research Institute. Fly ash Grade I (FA) was supplied by Xiangtan Power Plant, and its surface area is 512 m²/kg. Ground granular blast furnace slag (GGBS) came from a commercial producer in Changsha, and its surface area is 470 m²/kg. Xiangjiang river sand was used as fine aggregate, and its fineness modulus is 2.71. Coarse aggregate was crushed lime stones. A superplasticizer of sulfated naphthalene formaldehyde base produced by Zhuzhou Bridge Plant was used in the mix, which is commercially named as TQN. Tap water was used as mixing water. Experimental mixing proportions and related parameters of samples are listed in Table 1.

2.2 Preparation of specimens and experimental apparatus

Concrete mixtures were prepared in the laboratory using a mixer. And after being mixed, concrete samples with a dimension of 150 mm cubes were cast immediately by vibrating for 3 min with 50 Hz vibrator. Some specimens were treated by standard curing at a temperature of (20±2) °C and a relative humidity more than 90% after demolding for 1 d. The other specimens were placed in steam room for curing according to following steam-curing regime after casting. The steam curing treatment had a total duration of 13 h, including preheating duration of 2 h, heating duration of 2 h, treatment duration of 8 h with constant temperature of  60 °C, and cooling duration of 1 h. After 13 h steam curing, the specimens were demoulded and then treated according to corresponding experimental conditions shown in Table 2.

To insure experimental test accuracy, the water sorptivity and porosity test were carried out by a uniform core sample with a size of d 100 mm × 48.5 mm. The core samples were drilled from cubic specimens with a size of 150 mm × 150 mm × 150 mm and three thin discs (48.5 mm in thickness) were cut from a core sample by means of a diamond saw. The shadow surfaces were used for sorptivity tests (see Fig. 1).

2.3 Water sorptivity of concrete test method

Water sorptivity measurement was carried out according to the method developed by HALL [14]. The experimental set-up is shown in Fig. 2. In this work, all the samples were dried at 105 °C for 3 h before water sorptivity test. The side of the specimens was coated with grease to achieve unidirectional flow.

The water level in the pan was maintained at a constant height (4 mm) throughout the experiment. At regular intervals (t＝0, 20, 40, 60, 80, 120, 160, 200, 240, 360, 720 and 1 440 min), the mass of the specimens was measured using a balance with a resolution of 0.1 g after removing the surface water using a dampened tissue. The amount of water absorbed was then calculated and normalized by the cross-section area of the specimen exposed to the water. Sorptivity tests in this work were carried out in room where ambient temperature is 25 °C and relative humidity is 85%.

Table 1 Mixing proportions of samples

Table 2 Subsequent curing regimes for specimens after steam-cured

Fig. 1 Three thin discs cut from core sample

Fig. 2 Experimental set-up for sorptivity tests

2.4 Concrete porosity test method

Concrete porosity was measured by mercury intrusion porometer method (MIP) and mass loss  method. The mass loss of water-saturated concrete under specified conditions is generally called as “The method of evaporable water content” [15]. The water-saturated discs were dried at 90.7% relative humidity by placing them above a saturated salt solution of barium chloride contained in desiccators and the  mass was recorded at intervals until the change in mass was negligible. The mass loss by drying was then converted to volume fraction of the bulk paste. This particular measure of coarse capillary porosity corresponds to pore size larger than 30 nm [16]. For the analysis convenience, the pores were subdivided into ranges according to the following rules: Pores with diameter larger than 30 nm are named as macro-pore, and pores with diameter smaller than 30 nm are named as capillary pore. The total porosity is obtained by testing the total mass loss of the sample from fully saturated state to fully dry state under 105 °C drying condition for 14 h. The porosity of capillary pores is the difference value between the total porosity and the porosity of macro-pores.

3 Results and discussion

3.1 Comparisons of water sorptivity between steam- cured concrete and standard curing concrete

To understand the effects of steam curing on the water sorptivity of concrete, the water sorptivity tests for 27 d aged concretes under steam-curing and standard curing conditions were carried out by experiments. Steam-cured concretes were continued to cure with standard curing condition until 27 d after steam curing was finished. The results of water sorptivity of different samples are shown in Fig. 3. It can be found from Fig. 3 that, there is an obvious difference of water absorption of samples located between top layer and internal layers of concrete both for steam-curing and standard curing conditions. This phenomenon is hereafter referred to as water absorption gradient. Under steam curing condition, water absorption difference of concrete samples located between top and middle positions is found to be significantly higher than that of samples located between middle and bottom positions. However, in the case of standard curing concrete samples, the corresponding water absorption gradient of samples is lower.

Fig. 3 Water absorption of sample C1 under standard curing and steam curing (C1-0 represents specimen located at top surface position; C1-1 represents sample located at 1 cm from top surface position; C1-3 represents sample located at 3 cm from top surface position): (a) Standard curing concrete;     (b) Steam cured concrete

Generally, water absorption of concrete sample is determined by its capillary pore structure. Water absorption difference of samples indicates different pore structures between them. Pore structures of concrete are affected by many factors. For the same mixing proportion, microstructure differences are mainly attributed to two aspects in this work. Firstly, the apparent density of each component of concrete is different, and the settlement of high apparent-density component (aggregates) is larger when vibrating, thus the relatively light cement pastes will raise toward top and cause the internal components non-uniform; Secondly, curing temperature has great influence on the concrete internal microstructure. In the case of high temperature, the fast hydration in the initial stage leads to a more heterogeneous distribution of hydration products and a higher coarsened porosity [7]. Based on the water absorption difference of samples discussed above, it can be included that for standard curing concretes, the water absorption gradient mainly results in the settlement of high apparent-density component. However, for steam cured concrete, the larger gradient in water absorption between top surface and internal of concrete is mainly caused by the steam curing period, especially the top surface exposed directly to steam room is damaged, which causes a higher water absorption coefficient.

Steam curing mainly exacerbates the inhomogeneity of top surface of steam-cured concrete and causes higher porosity. This demonstrates that the steam curing temperature greatly affects the microstructure of concrete near to the top surface directly exposed to steam. In this work, the effective depth of steam curing probably is located at 1 cm position from top surface.

3.2 Effect of subsequent curing conditions on water sorptivity of steam-cured concretes

The results of water sorptivity of steam-cured concrete (sample C1) subjected to different subsequent curing conditions are shown in Fig. 4. One can find from Fig. 4 that, there exist two different water sorptivity coefficients of samples in the whole test process regardless of standard curing concrete or steam-curing concretes. The initial water sorptivity coefficient at early age is significantly higher than the subsequent water sorptivity coefficient at later age for the same specimen. This indicates that the water absorption rate of concrete changes with time.

Subsequent water curing exerts significant effect on water sorptivity of steam cured concrete. Steam cured concrete under 27 d air curing (E2) condition achieves a highest water sorptivity coefficient, followed by standard curing (E1) condition, E3 condition and E4 condition, respectively. The samples subjected to 27 d water curing (E5) condition have the lowest water sorptivity coefficient, which is 34% lower than that of E2 condition, 23% for E1 condition, 21% for E3 condition and 8% for E4 condition, respectively. By comparing the water sorption of concrete under water curing, it is found that the water sorptivity coefficient continually decreases with the increase in water curing time. It is generally accepted that the hydration of cement can take place only when the vapor pressure in the capillaries is sufficiently high, about 80% of saturation pressure [17]. This is probably due to the lower degree of cement hydration caused by shorter periods of water curing and due to the micro-cracks forming on the surface of concrete resulted from the early dissipation of moisture from the concrete [18].

The adequate subsequent water curing is important to steam-cured concrete after steam curing treatment. According to this work, water sorptivity value of steam cured concrete under 7 d water curing (E4) condition is almost consistent with the value of sample under 27 d water curing (E5) condition, and is much lower than that of sample under standard curing. Thus, it can be suggested that subsequent 7 d water curing for steam- cured concretes can almost eliminate the adverse effect of elevated temperature on concrete cover.

3.3 Effect of subsequent curing conditions on water absorption gradient of steam-cured concretes

From Figs. 4(a) and (b), it is indicated that the change of water absorption gradient of standard curing concretes is little with the change of the specimen height. However, for steam curing concretes, it is observed that the noticeable gradient appears in absorption characteristics of samples located between top surface and the middle (9.7 cm below top surface) or the bottom surfaces of samples. Water absorption capacity of samples located on the top surface is the highest, and that of the middle surface is almost the same as that of samples located at the bottom surface. From Fig. 4, it is observed that the water absorption gradient of concrete decreases with the increase of water curing time. As far as the effect of subsequent curing condition on water absorption gradient of steam-cured sample located between top surface and the middle surface is concerned, after 3 d water curing, only slight decrease in the water absorption gradient of steam-cured concretes is observed. But for 7 d water curing condition, it can be seen that there is significant decrease in water absorption gradient. By comparing the variation of concrete water absorption gradient between 7 d water curing (Fig. 4(d)) and 27 d water curing (Fig. 4(e)), it is found that there is only slight difference. It could be included that subsequent 7 d water curing on steam cured concrete could effectively decrease water absorption gradient of samples.

Fig. 4 Water absorption of concrete C1 under different subsequent curing methods: (a) E1 condition; (b) E2 condition; (c) E3 condition; (d) E4 condition; (e) E5 condition

3.4 Effect of subsequent curing conditions on total porosity of steam-cured concretes

Figures 5-8 indicate the variation of total porosity of concretes C1, C2 and C3 with different subsequent curing methods. From the results shown in Figs. 5-8, it is observed that the porosity gradient of steam cured concretes (C1, C2 and C3) under 27 d air or 3 d water curing are both much higher than that of standard curing concretes.

From Fig. 6, total porosity of top slice of sample (C1) is the highest for steam-cured concrete under 27 d air condition, whereas the porosity of corresponding middle slice is in rather good accordance with that of the bottom slice and their total porosity values are as low as those of standard curing concrete, thus the porosity gradient of steam-cured concrete under 27 d air condition is the highest. However, this porosity gradient significantly decreases due to subsequent 7 d water curing or 27 d water curing. This phenomenon is identical to the variation of water sorptivity and water absorption gradient of steam cured concrete due to the effect of subsequent water curing discussed above.

Table 3 lists the data of total porosity of concretes C1, C2 and C3 under different subsequent curing methods. According to the data analysis, all slices of steam-cured concretes under subsequent 7 d or 27 d water curing have lower total porosities compared with that of samples located at the top slices of standard curing concrete. Subsequent water curing plays an important role in the reduction of total porosity of steam-cured concrete.

Fig. 5 Change of porosity gradient of standard curing concretes

Fig. 6 Change of porosity gradient of concrete C1

Fig. 7 Change of porosity gradient of concrete C2

Fig. 8 Change of porosity gradient of concrete C3

Table 3 Data analysis of concrete porosity

By comparing the different subsequent curing conditions, the concrete specimens cured by water continuously for 27 d exhibit the lowest porosity. It can be concluded that the longer the water curing age is, the lower the total porosity of concrete is. From Table 3, compared to the top slices of steam cured concretes under 3 d water curing, the total porosity of the corresponding sample under 7 d water curing is 9.6% lower. When water curing time is prolonged from 7 d to 27 d, the decrease of total porosity values is only 3.1%. Thus, for the steam-cured concretes, subsequent water curing for 7 d can significantly improve the cover concrete pore structure and decrease the total porosity. Especially, subsequent water curing can effectively reduce the total porosity of top surface of steam-cured concrete.

These results indicate that the correlation between water absorption of concrete and porosity is very close.

A more detailed study on the pore sizes distribution was carried out. The pore size distribution of samples selected from the top surface of steam-cured concrete was measured by mercury intrusion porometer (MIP method). Figure 9 shows the effect of subsequent water curing time on pore size distribution of the samples. The peak value in differential curve corresponds to the most probable pore diameter which can reflect the characteristic of the pore size distribution. The smaller the most probable pore diameter is, the smaller the threshold and median pore diameter are.

From the results shown in Fig. 9, it can be noted that subsequent water curing time has an significant influence on the pore size distribution of the samples. The peak values of pore size distribution curves are scattered for the samples with 1 d or without subsequent water curing, and there are some larger pore with diameter larger than 100 nm. However, the pore size distribution curves of samples with longer subsequent water curing are very regular and almost all pores are less than 100 nm. The median pore diameter is about 0.09 nm for the sample which has subsequent 7 d water curing. Moreover, when water curing age is prolonged from 7 d to 14 d, the obvious decrease of median pore diameter can be seen. Thus, subsequent water curing can effectively decrease the median pore diameter of steam- cured concrete. The results also show that the analysis on pore structure of concrete by MIP method is consistent with that by mass loss method.

Fig. 9 Pore size distribution curves of concrete C2 under different subsequent water curing conditions: (a) 0 d; (b) 7 d; (c) 1 d; (d) 14 d

4 Conclusions

1) The top surface of steam-cured concrete has a higher porosity and a higher water absorption coefficient due to direct exposure to steam, which is responsible for the larger gradient in water absorption of steam-cured concrete samples located between top surface and internal positions.

2) The subsequent water curing on concrete after steam curing treatment can decrease water absorption capacity and total porosity of steam-cured concrete. There is a noticeable decrease in water absorption and total porosity gradients of steam-cured concrete subjected to subsequent water curing for 7 d and 27 d.

3) The improved efficiency of subsequent water curing on water sorptivity and total porosity of steam-cured concrete is close to the water curing age. The reasonable age of subsequent water curing for the steam-cured concrete with mineral materials is 7 d after steam curing treatment.

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(Edited by HE Yun-bin)

Foundation item: Project(2008G031-18) supported by the Ministry of Railway Science and Technology Research Foundation of China; Project(2010R50034) supported by the Key Science and Technology Innovation Team Program of Zhejiang Province, China; Project(2010QZZD018) supported by Leading-edge Research Program of Central South University, China

Received date: 2011-04-25; Accepted date: 2011-09-01

Corresponding author: LONG Guang-cheng, Professor; Tel: +86-731-82656568; E-mail: longguangcheng@126.com