Influence of internal curing on the pore size distribution of high strength concrete

Construction and Building Materials 192 (2018) 50–57

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Influence of internal curing on the pore size distribution of high strength concrete Joo Hyung Kim a, Seul Woo Choi a, Kwang Myong Lee a, Young Cheol Choi b,⇑ a b

Department of Civil, Architectural and Environmental System Engineering, Sungkyunkwan University, Suwon-si 16419, Republic of Korea Department of Civil and Environmental Engineering, 1342 Seongnamdaero, Sujeong-gu, Seongnam-si, Gyeonggi-do 13120, Republic of Korea

h i g h l i g h t s
A series of high strength concrete specimens are tested with internal curing.
Varying proportions of expanded lightweight aggregate are used for internal curing.
The strength, early age autogenous shrinkage, and pore structure are determined.
Expanded lightweight aggregate affects both the strength and shrinkage of concrete.
An ideal proportion of expanded lightweight aggregate is determined.

a r t i c l e

i n f o

Article history: Received 10 May 2018 Received in revised form 3 October 2018 Accepted 15 October 2018

Keywords: High performance concrete Internal curing Shrinkage Pore distribution Microstructure-analysis

a b s t r a c t Internal curing methods, which supply water inside concrete using highly absorbent materials, have been investigated to reduce early-age shrinkage in high strength concrete. This study investigates changes in the concrete pore structure caused by internal curing. High strength concrete was mixed with 0–25% replacement of fine aggregate with expanded lightweight aggregate, the compressive strength and autogenous shrinkage of the specimens were measured, and a micropore analysis was performed. The increase in shrinkage reduction with increasing expanded lightweight aggregate replacement was confirmed, observing that the hydration and meso-pore concentration in the cement matrix increased, resulting in a denser structure. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction High-performance concrete (HPC) is increasingly used in various fields because it has excellent mechanical and durability performance owing to its exceptionally dense internal structure [1]. However, self-desiccation can easily occur at early age in an HPC that uses a large amount of binder with a low water-to-binder ratio (W/B) compared to that of normal concrete, and this causes a large degree of autogenous shrinkage [2]. Such shrinkage leads to internal and external restraint cracks caused by the internal aggregate of the concrete and by the surrounding structural members, respectively. Therefore, many studies have been conducted on the reduction of shrinkage in HPC. Recently, interest in internal curing (IC) methods that reduce self-desiccation by supplying water inside the concrete has been increasing. High-absorption

⇑ Corresponding author. E-mail address: [email protected] (Y.C. Choi). https://doi.org/10.1016/j.conbuildmat.2018.10.130 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

materials, such as lightweight aggregate (LWA), super absorbent polymer (SAP), pumice, zeolite, and wood-derived materials are commonly used for such IC purposes [3–7]. Lightweight aggregate can be used not only as a material for reducing the unit weight of concrete but also as IC material owing to its possesses internal pores that can store water; thus, it has been included in many IC studies. These studies were mainly conducted to investigate the effect of LWA replacement ratio, water saturation status, and initial water condition on the autogenous shrinkage of concrete [8–13]. Other studies have been conducted to determine the effect of LWA particle size distribution and raw material characteristics on IC concrete properties [9–13]. In general, the interfacial transition zone (ITZ) formed with LWA is smaller than that of normal aggregate. This is because the wall effect does not occur due to the porosity of the LWA surface [14], and instead cement hydration products are readily formed on the LWA surface [15]. The area of the ITZ also expands and becomes denser due to the water supply absorbed by the LWA [16]. In

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general, the effects of LWA on the physical properties of concrete vary depending on the absorption, porosity, and pore characteristics and condition of the LWA [14–18]. Through SEM image analysis, Bentz et al. [19] determined that in IC mortars, paste penetrated the LWA surface and formed hydration products. Regarding to the durability resulting from IC, Bentz [20] verified through mortar experiments that chloride ion penetration resistance increased because the internal structure of the concrete became denser due to a decrease in the volume fraction of the percolated ITZ zone and increase in hydration. Lura et al. [21] analyzed the relationship between the pore structure of IC agents and the moisture sorption isotherm using mercury intrusion porosimetry (MIP), and determined that the effects of IC increase when the IC agent has larger pores. Until now, studies on IC concrete have been mainly conducted to verify the shrinkage reduction effect of various IC applications, and to observe the microstructure of the concrete through image analyses of the ITZ. However, there is insufficient focus on pore changes in concrete due to the IC effect in the current body of work. Therefore, in this study, the IC effect of LWA was investigated through measurement of the compressive strength and autogenous shrinkage of a high-performance concrete (W/B 20%) using expanded shale lightweight fine aggregate (ELWA) as an IC agent. Furthermore, the effects of ELWA replacement ratios in the range of 0–25% on pore size distribution were analyzed using MIP.

Cement Silica fume

Fig. 1. Sizee distributions of binders.

Q

2. Materials and methods 2.1. Materials In this study, ordinary Portland cement (OPC) with a density of 3.15 g/cm3 and a Blaine fineness of 3250 cm2/g, and silica fume (SF) with a density of 2.20 g/cm3 and a Blaine fineness of 16,584 cm2/g were used. The chemical composition and mechanical properties of the raw materials are summarized in Table 1. The C3S, C2S, C3A, and C4AF compositions of the OPC were 55.3%, 17.9%, 7.1%, and 10.7% by mass, respectively, as calculated by the Bogue equation [22] based on the chemical composition. Fig. 1 depicts the pore size distribution of the binders (cement and silica fume) used in the mixture. The median particle sizes (d50) of the cement and silica fume were 13.5 and 0.14 lm, respectively. The normal fine aggregate and coarse aggregate used in this study satisfied the requirements of the ASTM C33. The fine aggregate was marine sand with a dry bulk density of 2.6 kg/m3, absorption of 1.13%, and fineness modulus of 2.8. Crushed granite with a dry bulk density of 2.7 kg/m3, absorption of 0.37%, fineness modulus of 7.3, and a maximum dimension of 13 mm was used as the coarse aggregate. In this study, ELWA was used as an IC agent. The ELWA was fabricated using clay as the main ingredient by molding it into spherical shapes through pelletizing and extruding works. Spherical shapes were then baked at a temperature of 1150–1200 °C. The chemical composition of the ELWA is also presented in Table 1. Fig. 2 depicts the XRD pattern of the ELWA, indicating that quartz and albite were the main compositional minerals. The dry bulk density and fineness modulus of the ELWA were 1.67 kg/m3 and

Q

A

A

A

Q Q A A Q A Q A

Q

Q

Fig. 2. XRD spectrum of ELWA (Q = Quartz low, A = Albite (heat-treated)).

4.7, respectively, and its porosity is as shown in the scanning electron microscope (SEM) image in Fig. 3. The internal porosity of the aggregate had a clear influence on the IC effect. According to existing research, when the pores of an IC agent are smaller than 100 nm, there is no influence on the IC effect owing to difficulty in releasing the stored water into the paste due to capillary suction phenomenon inside the pores [23]. Fig. 4 shows the pore size distribution of the ELWA as determined by MIP. The pores of the ELWA were mostly in the range of 100– 3000 nm in size, with 89.7% of pores being greater than or equal to 100 nm, indicating that this aggregate would act as a suitable IC agent. Because the hydration saturation state of the ELWA has a direct influence on the IC effect, it is necessary to closely examine the absorption capability of the aggregate. In this study, the

Table 1 Chemical compositions and mechanical properties of materials. Material

Cement Silica fume ELWA

Chemical composition (%) SiO2

Al2O3

MgO

CaO

SO3

Fe2O3

K2O

Na2O

LOI.

21.20 97.96 65.4

4.64 0.18 24.7

1.87 0.06 1.8

61.9 0.29 2.35

2.31 1.03 –

2.91 0.14 1.32

1.22 0.13 2.15

0.29 0.05 1.56

2.48 1.6 0.98

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2.2. Mixture proportions and methods

Fig. 3. SEM image of ELWA showing the porosity.

Fig. 4. Pore size distribution of ELWA.

The mixture proportions of the concrete used in this study are listed in Table 2. To fabricate a high strength concrete exhibiting large autogenous shrinkage, the water-to-binder ratio was set at 20% and silica fume replaced 10% of the cement by mass. In all mixtures, the ratios of water, cement, silica fume, coarse aggregate, and chemical admixture were fixed. The ELWA used for IC was replaced at 5 different levels: 5% (CL5), 10% (CL10), 15% (CL15), 20% (CL20), and 25% (CL25) of the normal fine aggregate volume in the control mixture (CL0). To satisfy the requirement of 10% absorption in the pre-soaked state, the ELWA was used after 3 days of soaking in water. Considering the standard particle size of fine aggregate for concrete purposes, the maximum volume replacement was limited to 25% at the time of designing the mixtures because the particles in the ELWA were mostly around 2.36 mm in size. To identify the curing characteristics of the concrete, a slumpflow test was conducted in accordance with the ASTM C1611, and a setting time measurement was conducted in accordance with ASTM C403. To perform the compressive strength test, U100  200 mm cylinders were fabricated and demolded after 24 h curing under a constant temperature of 20 ± 3 °C and a relative humidity of 60 ± 5% without being submerged in water (to confirm the internal curing effect). The compressive strength was measured at the ages of 1, 3, 7, and 28 days in accordance with ASTM C39. A PMF-series embedded strain gauge (Tokyo Sokki Kenkyujo Co., Ltd.) was installed at the center of a 100  100  400 mm rectangular mold to measure the autogenous shrinkage of the concrete, as shown in Fig. 6. To minimize sticking and friction between the mold and the concrete, the inside of the mold was lined with Teflon sheets. The strain was first measured immediately after casting the concrete and the mold was removed 24 h after casting, aluminum tape was used to seal the test specimens to prevent moisture loss from the concrete. The strain was then measured periodically during curing of the specimens and used for the autogenous shrinkage measurements [25]. All concrete specimens were then air-cured at a constant temperature of 20 ± 3 °C and a relative humidity 60 ± 5% RH. The pore structure analysis was performed at the age of 28 days using MIP (AutoPore IV 9500) to identify the pore structure inside the concrete. Portions of mortar excluding the coarse aggregate were collected in 4–6 mm sizes from the pertinent test specimens for the measurement samples [26,27]. After drying the collected sample for 24 h at 40 °C, the internal pore characteristics were analyzed by applying mercury intrusion up to a maximum pressure of 33,000 psi. The surface tension of the triple distilled mercury was assumed to be 0.484 N/m at 25 °C. The density and contact angle of the mercury were set at 13.546 g/ml and 140°, respectively.

3. Results and discussion 3.1. Slump flow, setting time, and compressive strength

Fig. 5. Time dependent water absorption of ELWA.

absorption was measured over time through volume changes using a pipette, as in [24]. In the absorption results shown in Fig. 5, it can be observed that the absorption of ELWA at 1, 3, and 14 days was 7%, 10%, and 25%, respectively; and as the soaking time increased, it was confirmed that the absorption continuously increased.

In this study, the slump-flow and setting time were measured to determine if pre-wetted ELWA has any influence on the initial properties of fresh concrete. Furthermore, the compressive strength was measured at different ages to investigate the dependency of compressive strength on curing time. The results of these tests are listed in Table 3. As the ELWA replacement ratio increased, the slump-flow increased slightly, but this increase was almost negligible. The initial and final setting time were 124–129 min and 427–438 min, respectively for all mixtures. Because the pre-wetted ELWA used in the mixtures was soaked

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J.H. Kim et al. / Construction and Building Materials 192 (2018) 50–57 Table 2 Mixture proportions of concrete specimens. Mix

CL0 CL5 CL10 CL15 CL20 CL25 *

W/B (%)

20

Unit mass (kg/m3)

S/a (%)

41

HRWA* [(C + SF)%]

Water

Cement

Silica Fume

Coarse agg.

Fine agg.

ELWA

144 144 144 144 144 144

662 662 662 662 662 662

74 74 74 74 74 74

977 977 977 977 977 977

651 619 586 554 521 488

0 21 42 63 84 105

0.63 0.63 0.61 0.62 0.63 0.61

HRWA = High range water reducing admixture.

Fig. 6. Schematic diagram of the measurement of shrinkage in concrete.

Table 3 Results of slump-flow, setting time, and compressive strength tests. Mix

Slump-flow (mm)

Initial setting time (min)

Final setting time (min)

CL0 CL5 CL10 CL15 CL20 CL25

610 605 615 610 625 620

125 128 124 127 126 129

427 432 434 438 435 431

for 3 days, making its absorption speed very slow, it had almost no influence on the water-to-binder ratio of the concrete, as the water movement (absorption or release) between the pre-wetted ELWA and the mixing water was insignificant during the concrete mixing and setting process [28,29]. As shown in Fig. 7, the compressive strengths of the IC mixtures (CL5–CL25) were 7–26% lower than that of control mixture (CL0)

Fig. 7. Results of compressive strength according to age.

after 1 day. The compressive strength changes were different depending on both the ELWA content and age. In the case of CL15–CL25, the compressive strength reduction was approximately 3–10% that of CL0 at all ages. However, the compressive strength of CL5-CL10 was almost similar after 3 days. In general, if excessive amount of the porous ELWA is used, the compressive strength of the concrete exhibits a decreasing trend with increasing ELWA owing to the low strength characteristics of the lightweight aggregate compared to that of the normal aggregate (NA). However, some researchers have reported that an appropriate amount of lightweight aggregate results in an increase in the compressive strength of concrete through densification of the ITZ between the aggregate and cement matrix, owing to the IC effect [3,9]. Fig. 8(a) shows an SEM image of specimen CL5 captured in BSE mode, to determine the NA, ELWA, paste, and hydration products in the IC specimen. The BSE mode represents the density differences of the matter in the material with different shades: when the density is low, the color is darker. It is clear from the image that the cement paste near the ELWA is brighter than the cement paste near the NA, indicating that the density of the cement paste near the ELWA is relatively higher than that near the NA. This is because as water from the ELWA is supplied to the ITZ, the hydration of the surrounding cement increases and the structure becomes denser. Furthermore, by examining the ITZ between the ELWA and the paste in the SE mode image in Fig. 8(b), it was confirmed that the cement paste penetrated inside the ELWA surface. This mechanical interlocking effect causes the binding force between the ELWA and cement paste to increase [17]. Because of the increased hydration of the cement paste and its increased binding force with the ELWA, the IC mixture using 10% of ELWA or less exhibited higher compressive strength compared to CL0. On the other hand, the IC mixtures using 15% ELWA or more exhibited lower compressive strength compared to CL0 because the increase in the number of pores inside the concrete and the low strength of the ELWA had a higher influence than the effect of the interaction between the ELWA and the cement matrix.

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(a) Cement paste and aggregate (BSE mode)

(b) Interfacial zone of ELWA(SE mode)

Fig. 8. SEM images of IC specimen CL5.

of the water stored inside the ELWA was most apparent at the early age.

3.2. Autogenous shrinkage Fig. 9 illustrates the autogenous shrinkage of specimens measured after the setting time. As can be observed in Fig. 9(a), as the quantity of ELWA replacement increased, the autogenous shrinkage of the concrete decreased. For the autogenous shrinkage at the age of 60 days, the shrinkage reductions of CL5, CL10, CL15, CL20, and CL25 were 10, 24, 33, 44, and 59%, respectively, higher than that of CL0, and the autogenous shrinkage decreased in proportion to the ELWA replacement ratio. This is because as the quantity of ELWA replacement increased, the water supplied to the internal paste matrix increased, increasing the hydration of the paste. As the water consumed by the hydration reaction of the cement particles was replaced by the release of water inside the ELWA, the shrinkage due to low water-binder ratio was reduced. At the age of 1 day, the autogenous shrinkage compensation ratios of CL5, CL10, CL15, CL20, and CL25 were 50, 58, 63, 69, and 78%, respectively. These were the highest shrinkage compensation ratios observed at any age. At the age of 20 days, the slope of the autogenous shrinkage compensation ratio decreased noticeably because the hydration of the cement proceeded at a fast rate due to the low 20% water-to-binder ratio of the specimens. Because of this low ratio, a large quantity of water was consumed at the early age, and the internal curing effect provided by the release

CL0 CL10 CL20

Fig. 9. Autogenous shrinkage of specimens.

CL5 CL15 CL25

3.3. Microstructure From the compressive strength and autogenous shrinkage results presented in Sections 3.1 and 3.2, the effects of IC on the compressive strength increased and shrinkage reduction was confirmed, which is consistent with results reported in other studies [7–13]. Due to the continuous release of water stored inside the ELWA, further hydration of the cement matrix around the aggregate was facilitated, and as a result, the microstructure can become denser. Fig. 10 depicts the pore distribution of specimens CL0–25 at 28 days. As can be shown in Fig. 10(a), the pore volume was largest when the pore diameter was 10–50 nm, and it was confirmed that the increase in the pore volume was highest for this pore size class as the content of ELWA increased. This result can be confirmed using the pore size distribution shown in Fig. 10(b); when the pore diameter is 10–40 nm, the peak pore volume increases the mixture quantity of ELWA increases. Table 4 summarizes the pore volume by classifying the distribution of pore volume in the mortar by the pore size into the gel micro-pore R1 class (<10 nm), meso-pore R2 class (10–50 nm), middle capillary pore R3 class (50–100 nm), and large capillary pore R4 class (>100 nm). The bulk density exhibited a decreasing trend as the ELWA replacement ratio increased because the ELWA has a lower density than the normal aggregate, resulting in a relative decrease in the bulk density of the concrete. The total porosity increased with increase in the ELWA replacement ratio as the ELWA itself is a highly porous material. Fig. 11 shows that compared to the CL0 mixture, specimens CL5–CL25 showed no significant change in pore distribution in the R1 and R3 classes, but the pore volume in the R2 and R4 classes changed significantly. Because the R4 class constitutes 86.7% or more of the pore distribution of the ELWA used as the IC material, the increase in the ELWA replacement ratio had a particularly direct influence on the pore increases in the R4 class inside the mortar. As mentioned earlier, pores of 100 nm or smaller do not contribute to any IC inside the concrete due to the influence of capillary pore action [22]; therefore, such an increase in the large-pore R4 class is very conducive to IC by facilitating the smooth movement of water between the cement paste and the ELWA inside the concrete. In the R2 class, as the ELWA replacement ratio increased, the IC effect increased, increasing the hydration inside the concrete [25]. As the hydration increased, the volume of meso-pores (<50 nm)

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R1

R2

R3

R4

fraction of the ICZ, Lu et al. [34] derived an equation through statistical analysis of the statistical geometry of the composite matrix. From the results reported by Garboczi et al. [35], the ICZ volume fraction V ICZ can be calculated with the following equation:

CL0 CL5 CL10 CL15 CL20 CL25


 V ICZ ¼ 1  V ELWA  ð1  V ELWA Þexp pq cr þ dr 2 þ gr 3

(a) Cumulative pore size distribution at 28 days

CL0 CL5 CL15 CL25

ð1Þ

where q is the number of particles per unit volume, and c, d, and g were determined by Garboczi et al. [33]. Using Eq. (1), the ICZ for ELWA can be calculated; in this study, this was determined from the mixture used in the test. The mean diameter of ELWA is 2.05 mm and the fraction of internal curing with respect to the distance from the ELWA surface was determined. It was assumed that the water effect thickness due to ELWA was 1.0–1.5 mm. The results of the calculation of the ICZ volume according to the ELWA mixing ratio are shown in Fig. 12. From Fig. 12, it was confirmed that the water effect thickness of the ELWA used in this experiment was approximately 1.3 mm. As the mixing ratio of ELWA increases in Fig. 12, the fraction of ICZ increases depending on the ratio of ELWA; this is manifested as an increase in the fraction of internal curing. A large quantity of cement paste is located on the ELWA surface where water is supplied, and this leads to increase in cement hydration. Furthermore, these calculated results show a similar trend with measured results, and agree with the protected paste volume concept [30] and the effect of internal curing using SAP [36]; when the pore volumes are identical, a system in which small pores are distributed finely is denser and more durable than a system consisting solely of large pores. 4. Conclusions In this study, an analysis of changes in the micro-pore structure occurring due to the IC effect when using ELWA in high performance concrete was performed, and the results are summarized as follows:

(b) Intrusion pore size distribution at 28 days Fig. 10. Pore structures of specimens characterized by MIP.

inside the concrete increased [28,29], indicating an increase in the share of R2 class pore volume. 3.4. Estimation of internal curing zone Bentz [30] explained the ‘‘protected paste volume concept” addressing the water supply of saturated lightweight aggregate inside concrete, and conducted studies to determine the internal curing zone(ICZ) on the surface of lightweight fine aggregate when additional water was provided [30–32]. To estimate the volume

(1) The use of up to 10% ELWA yielded compressive strength similar to that of Plain (CL0), but the compressive strength decreased when the ELWA exceeded 10%. The autogenous shrinkage tended to decrease as the amount of ELWA increased. Therefore, it was confirmed that the ELWA used in this study is suitable material for concrete internal curing. (2) It was confirmed through SEM image analysis that the structure of the cement matrix near the ELWA became denser, and this in turn influenced the strength improvement of the concrete. On the other hand, because the total pore ratio increases proportionally with increase in the ELWA replacement ratio, it was determined that using of a large amount of ELWA produces a negative effect on the compressive strength of concrete. (3) The ratio of 10–50 nm meso pores increased in the cement matrix due to IC effect, and the increase in hydration of the cement matrix was confirmed through the changes in

Table 4 Parameters of the pore structure of mortar specimens at 28 days. Mix

CL0 CL5 CL10 CL15 CL20 CL25

Bulk density (g/mL)

Porosity (%)

Pore volume intrusion (ml/g) R1 (<10 nm)

R2 (10–50 nm)

R3 (50–100 nm)

R4 (>100 nm)

2.23 2.20 2.10 2.01 1.96 1.94

12.53 13.59 14.27 15.36 17.58 18.91

0.00307 0.00315 0.00347 0.00408 0.00419 0.00456

0.0251 0.02828 0.03111 0.03488 0.0465 0.05054

0.00763 0.00512 0.00563 0.00559 0.00675 0.00734

0.02055 0.02536 0.02789 0.03207 0.0324 0.03522

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R1 R2 R3 R4

Acknowledgment

( < 10nm) (10~50nm) (50~100nm) ( > 100nm)

y = 0.1061x + 2.2806 R² = 0.9373

y = 0.0564x + 2.1866 R² = 0.9554

This research was supported by the Korea Ministry of Environment (MOE) as ‘‘The advancement of scientific research and technology development in environmental science program” (No. 2017000150001). This research was also financially supported by the Technology Advancement Research Program (TARP) (Grant No. 18CTAP-C129778-02) funded by the Ministry of Land, Infrastructure and Transport of the Korean government. References

Fig. 11. Size pore volume of ELWA replacement.

Fig. 12. Volume fraction of the internal curing zone taken up by the ELWA for 28 days.

the pore structure. Furthermore, it was confirmed that as the cement matrix produced hydration products by penetrating the ELWA, the 100 nm or larger pores, which were more numerous owing to the use of ELWA, were largely filled. Therefore, it was determined that instead of estimating the mechanical and durability characteristics of IC concrete solely through the pores of LWA, it is necessary to understand the various interactions formed between the LWA and the cement matrix. (4) Because IC is mainly applied to high strength concretes for the autogenous shrinkage reduction, there is a degree of neglect of the strength and durability according to the uses of the internal curing agent to provide porosity. However, it was been determined that the pore changes in the cement matrix and the pore-filling effect provided by IC due to LWA can lead to an increase in both the durability and lifespan of a concrete structure.

Conflict of interest None declare.

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