Research on drying shrinkage deformation and cracking risk of pavement concrete internally cured by SAPs

Construction and Building Materials 227 (2019) 116705

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Research on drying shrinkage deformation and cracking risk of pavement concrete internally cured by SAPs Jingyu Yang a, Yinchuan Guo a, Aiqin Shen a, Zhihui Chen a, Xiao Qin b,⇑, Ming Zhao c a

Key Laboratory for Special Region Highway Engineering, Ministry of Education, Chang’ an University, Xi’an 710064, Shaanxi, China Foshan University, Guangdong, China c Guangdong Road and Bridge Construction Development Co., Ltd., Guangdong, China b

h i g h l i g h t s
The scaled down slabs are used to simulate actual concrete pavement slabs.
The laws of shrinkage development under regulation of humidity by SAP are studied.
The internal humidity and warping stress of the slabs cured by SAP are analyzed.
The cracking risk and anti-cracking effect of concrete cured by SAP are studied.

a r t i c l e

i n f o

Article history: Received 21 April 2019 Received in revised form 8 August 2019 Accepted 10 August 2019

Keywords: Superabsorbent polymer Drying shrinkage Humidity distribution Humidity warping stress Inner wall strain Cracking risk

a b s t r a c t Drying shrinkage and cracking are important factors affecting the performance and service life of pavement concrete directly, and they can be effectively alleviated by internal curing with super absorbent polymer (SAP). The influence of SAP on drying shrinkage and humidity distribution was investigated via displacement and humidity sensors. Moreover, the inner wall strain of pavement concrete with or without SAP was analyzed, and the risk of cracking was evaluated under the condition of annular restraint. The results show that the internal curing with SAP could reduce the shrinkage strain at the center and corners of the pavement slab and be accompanied by a decrease in the humidity difference between the upper, middle and lower layer of the slab, which is extremely beneficial for suppressing the humidity warping stress of the pavement slab. The internal curing with SAP was the most effective for the shrinkage of the pavement slab within 7 days, and then, the anti-shrinkage effect began to decrease. After internal curing with SAP, the cracking risk of concrete was degraded, and cracking could be avoided. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The cement concrete pavement is a thin-plate structure, which is exposed to the external environment for a long time. Due to evaporation and hydration, the internal humidity of the concrete cannot be maintained in an appropriate range, which causes a negative pressure in the capillary pores and thus produces shrinkage and cracks inside the concrete [1,2]. The superabsorbent polymer (SAP) has been introduced into concrete as an internal curing material due to its ability to intelligently store and release water in a timely way that alleviates the shrinkage deformation caused by early hydration and surface drying evaporation [3–9]. Since SAP was first used as an internal maintenance material, its characteristics of liquid absorption and release have been exten⇑ Corresponding author. E-mail address: [email protected] (X. Qin). https://doi.org/10.1016/j.conbuildmat.2019.116705 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

sively studied. Yang J and Wang F et al. [10] found that the total ion concentration and Ca2+ ion concentration are important factors that can affect the absorption capacity of SAP and the hydration time during the hydration process. Jensen [11] and Esteves [12] studied the factors affecting the binding ability of Ca2+ and SAP. The results show that the multi-ion, high-concentration cement slurry has a great influence on the liquid absorption capacity of SAP. The particle size of SAP also significantly affects its water absorption kinetics. The smaller the particle size, the stronger the liquid absorption capacity [13]. In the hydration process of cement, with the gradual acceleration of the hydration process, the internal humidity of hardened cement mortar gradually enters the decline stage. Due to SAP’s unique characteristics of liquid absorption and liquid release, the water stored inside SAP begins to diffuse into the cement slurry under the influence of a hydraulic gradient. The humidity in different areas is raised, and the time from a relative humidity of 100% is obviously delayed [14,15]. In general, research

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J. Yang et al. / Construction and Building Materials 227 (2019) 116705

on the absorption and release of SAP and the effect on the internal humidity of cement concrete has made some progress, but there is a nonlinear humidity gradient in the vertical direction of concrete, which will produce certain warping stresses and deformations [16–18], and the law of change for the period when SAP plays a role is still not clear. The ultimate goal of SAP applications is to reduce shrinkage and cracking by increasing the relative humidity inside concrete. The internal curing water stored by SAP plays an important role in improving the relative humidity inside concrete, compensating for self-drying, and reducing autogenous shrinkage and the formation of potential cracks [19–25]. Geiker M R [26] found that the addition of SAP can reduce the overall shrinkage of cement concrete and improve shrinkage cracking significantly. Through longterm monitoring Wang F [27] found that the deformation of the internal curing concrete with a water-to-binder ratio of 0.23 was less than that of ordinary concrete with a water-to-binder ratio of 0.42. Assmann [28] and Mechtcherine [29] found that the autogenous shrinkage strain of concrete is 95 lstrain and 85 lstrain when the water-binder ratio is 0.36 and 0.42, respectively. However, the SAP test sample with a water-to-binder ratio of 0.36 + 0.06 (0.06 of which is additional water for SAP) is only 35 lstrain, indicating that SAP can reduce autogenous shrinkage effectively regardless of whether or not additional water is introduced. Mechtcherine V et al. [30] proposed that SAP can not only reduce autogenous shrinkage but also alleviate other types of shrinkage. Yao [31] found that SAP can reduce the crack width of ECC and reduce constrained shrinkage and drying shrinkage. Kong [23] and Assmann [28] analyzed the effect of SAP on the drying shrinkage of concrete and found that the sample with SAP added had a significant decrease in drying shrinkage deformation and total deformation. Mechtcherine [32] also found similar rules in fine aggregate ultra-high performance concrete, and if the time of external wet maintenance is prolonged, the post-shrinkage is also reduced significantly. Dudziak [33] and Craeye [34] found that SAP can reduce capillary pressure, and the water contained in it is easier to evaporate than capillary water, so that the evaporation rate of capillary water and crack reduction can be delayed. Kumar [35] showed that increasing the additional water intake can delay the cracking of the mortar specimen through experimental research, and no cracking occurs within 28 days. In summary, many scholars have researched the impact of SAP on the shrinkage and cracking of cement concrete, but most studies have considered small specimens. In practical application, cement concrete is basically used in the form of plates. Due to the effect of size, the suppression of the shrinkage and cracking of pavement and the compensation of vertical humidity warping stress cured by SAP need to be studied. In this paper, the shrinkage and humidity distribution of scaleddown pavement slabs at different locations were monitored by displacement and humidity sensors, and the variation law was analyzed. At the same time, the humidity gradient warping stress of the road surface panel was calculated to reveal the effect of SAP on the humidity warping stress of the road surface panel. Finally, the inhibitory effect of SAP on early cracking was analyzed by a ring constraint test. The research results provide a basis for the design and construction of the pavement concrete internally cured by SAP. 2. Materials and experimental methods

were 35.5 MPa and 52.6 Mpa respectively, and the flexural strengths were 6.6 MPa and 8.4 MPa, respectively. The fly ash with a specific surface area of 2700 cm2/g was I grade, and its activity index and fluidity were 75% and 91%, respectively. The chemical compositions of the cement and fly ash are shown in Table 1. The coarse aggregates used in this study were granite gravel with two particle sizes of 9.5–19.0 mm and 4.75–9.5 mm. When the bulk density was a maximum, the mass percentage of the two grades was 1:4, which met the grading requirements of pavement cement concrete. The fine aggregates were selected from river sand with a fineness modulus of 2.71. To improve the workability of the concrete, a polycarboxylate superplasticizer was used, and its main performance indicators are shown in Table 2. The above materials all met the requirements of Chinese standards. Sodium polyacrylate resin with particle sizes of 120–150 lm (C30-100), was used as the SAP. The optimal particle size was determined in preliminary tests by the research team. This kind of SAP has the advantages of high absorbency, strong gel strength, salt corrosion resistance, and antibacterial and heat resistance. The specific performance indicators are shown in Table 3. 2.2. Test design In this paper, concrete with a strength grade of C30 according to Chinese standards was designed, and the reference w/b was 0.37. Based on the Powers theory, the additional water-to-binder ratio of Wic/B in the cement concrete was calculated by Eq. (1). To prevent the introduction of additional water diversion to change the concrete reference mix proportion and its workability, the SAP content was calculated according to the maximum water absorption ratio of SAP in the test, which was determined by previous tests of the research group in the simulated cement slurry. The mixing proportion for the drying shrinkage tests of concrete is shown in Table 4. W B

 0:36; WBic ¼ 0:18 WB

ð1Þ

0:36  WB  0:42; WBic ¼ 0:42  WB

2.2.1. Shrinkage test To obtain the continuous transition process of early drying shrinkage of concrete, the MIC-YWC-5 displacement meter and the MIC-DCV-4 voltage data collector produced by Shenzhen Monitor Co., Ltd. were used. According to the size of pavement slabs in actual projects, a scaled pavement slab with a specific size of 400 mm  400 mm  100 mm was formed. Then, the displacement meter matching voltage data collector was used to monitor and collect the shrinkage displacement of the slab center and the long diameter direction of the corner. SAP plays a major role in reducing shrinkage and inhibiting microcracks of concrete during the 7 d period. The slabs were demolded after 24 h of standard curing and then was placed on the displacement sensors for testing. The distance between the center point and the edge of the slab was 200 mm, and the position of the corner was 50 mm from the edge of the slab. The data acquisition interval of the sensor was set to 1 h, the collection period was 28 d, the test ambient temperature was set to 20 ± 2 °C, and the relative humidity (RH) was controlled to be 60% ± 5% RH. The specific monitoring process of drying shrinkage is shown in Fig. 1. Because the transmission of the shrinkage probe test signal is outputted in the form of voltage (unit: mV), for the convenience of analysis, the voltage signal must be converted to the displacement value (micro strain le) by the voltage-displacement conversion formula provided by the manufacturer. The conversion formula is given in Eq. (2).




le ¼ ðU t  U0 Þ= 1000  A  400  106



ð2Þ

where Ut is the voltage value at age t; U0 is the initial voltage value; and A is the signal sensitivity. 2.2.2. Internal humidity test The pavement concrete has significant stratification characteristics along the depth direction of the slab, and there is a nonlinear humidity gradient from top to bottom. Therefore, for the reference group and the internal curing group, 400 mm  400 mm  100 mm thin pavement slab components were specially formed. The sensor heads were inserted at the center axis of the scale pavement slab. The sensor probes were 100 mm, 200 mm, and 300 mm from the edge or 1/4, 1/2, and 3/4 of the depth from the surface, respectively. The distribution of internal humidity was monitored for 28 days through the probes, the acquisition interval was 1 h, the test ambient temperature was set to 20 ± 2 °C, and the ambient relative humidity (RH) was controlled to be 60%±5% RH. An actual test photo and schematic diagram are shown in Fig. 2.

2.1. Test raw materials The two cementing materials used in this study were comprised of cement and fly ash. The cement was ordinary Portland cement (PO.42.5) with a specific surface area of 3360 cm2/g, and the times of the initial set and final set were 176 min and 235 min, respectively. At 3 d and 28 d, the compressive strengths of the cement

2.2.3. Ring restrained cracking test The ring restrained cracking test was carried out for the reference group and the internal curing group of the pavement concrete. The complete setup of the test system, with the attached strain gauges and the MIC-DCV-4 voltage collector, is shown in Fig. 3. The whole test equipment consisted of inner and outer steel rings of con-

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J. Yang et al. / Construction and Building Materials 227 (2019) 116705 Table 1 Chemical compositions of cementitious materials (wt%).

Cement Fly ash

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

Na2O

Loss of ignition

22.06 58.91

5.13 28.82

5.25 4.31

64.37 1.52

1.06 2.83

2.03 –

– 3.24

– 4.95

Table 2 Main performance index of superplasticizer. Water-reducing rate (%)

Air content (%)

Alkali content (%)

Chloride ion content (%)

14–28

2.8

5.7

0.2

crete, a cast steel base and a strain collection system on the inner wall. The height of the ring concrete was 100 mm, the outer diameter was 425 mm, the inner diameter was 305 mm, and the thickness of the ring was 12 mm. The attachment method of strain gauges in the test was studied by Han Yudong et al. [36]. The strain gauges were pasted horizontally to the center height of the inner wall of the steel ring. In comparison, they found that a large strain value can be obtained at this position, and thereby, the accuracy of the test is improved. The restraint test started after the concrete ring casting was completely finished for 1 d, during which all the wiring work of the strain gauges and strain collector was completed. Simultaneously, 1 d after complete pouring, the outer ring of concrete was removed to expose the outer surface to the dry environment. At the same time, the upper surface of the concrete ring was sealed with epoxy resin latex, and the drying test condition for the single-sided outer ring was constructed. The whole ring restraint test was carried out in a constant temperature and humidity environment. The ambient temperature was controlled to be 20 ± 2 °C, and the relative humidity (RH) was controlled to be 60% ± 5% RH, and the entire test was continuously monitored for 28 days or until a perforating crack appeared in the concrete ring. The change of strain on the internal wall is represented by the measured value of the strain gauges. The specific calculation formula is shown in Eq. (3).

Table 3 Index of superabsorbent polymer (SAP). Index

Unit

Specifications

Exterior Nominal particle size Absorptance Absorptance of 0.9% Normal saline Absorbed rate

– deionized water g/g mL/g

Granules or powder 120–830 450–550 70–100

s g/cm3 1% moisture dispersion

<28 0.7–0.75 5.5–6.8

q PH

lm

De ¼ K ð V t  V 0 Þ

ð3Þ

Table 4 The mixing proportion for the drying shrinkage test. Serial number

C30-J C30-100-0.145%

Mixture composition/(kg/m3) Water + Internal curing water

SAP

cement

Fly ash

sand

10–20 mm aggregate

5–10 mm aggregate

Water reducer

160 160 + 21.65

0 0.628

368

65

745

790

198

2.81

Note: C30-J represents reference group, C30-100-0.145%, 0.145% represents SAP content (The optimal content was determined in preliminary tests by the research team).

Fig. 1. Drying shrinkage test at the center and corner.

Fig. 2. Spatial distribution test of internal humidity.

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J. Yang et al. / Construction and Building Materials 227 (2019) 116705

Fig. 3. Ring constraint cracking test.

where De is the strain variation of the pavement concrete ring, and the unit is le; K is the measurement sensitivity of the strain gauge, and the unit is le/mV; V t is the measured value of the strain gauge at time t, and the unit is mV; and V 0 is the initial value of the strain gauge, and the unit is mV.

3. Results and analysis 3.1. Shrinkage gradient and humidity stress of pavement slab 3.1.1. Drying shrinkage at center and corner of pavement slab In the natural drying environment of 20 ± 2 °C and 60% ± 5% RH, the full curve of the drying shrinkage at the center and corner positions of two pavement slabs was measured. The curve is shown in Fig. 4. It can be seen from Fig. 4 that the shrinkage deformation value of the pavement concrete is basically in the order of C30-Jcorner > C30-J-center > C30-100-0.145%-corner > C30-100-0.145%center during the whole process of drying shrinkage development. In the first 50 h, a trend of accelerated growth appeared in the shrinkage development of the pavement concrete. The fastest growth was the shrinkage deformation at the corner of the reference group, followed by the center point of the reference group and the corner point of the internal-curing group. The strain value of the center point of the internal curing group basically tended to 0 within the first 50 h age, and even microexpansion occurred. After 150 h, the shrinkage deformation at the center and corner of the internal curing group began to accelerate significantly, and the increase of shrinkage deformation remained stable after 250 h. The main reasons for the above laws may be as follows (1) When the shrinkage stress was generated in the center of the pavement slab under the influence of humidity and temperature differ-

Fig. 4. Drying shrinkage development at center and corner.

ences, the direction of the bond between the hardened cement mortar and the aggregates was relatively scattered and could be mutually offset, while at the edge position, the stress on one side of the center of the slab was larger, so the macroscopic shrinkage deformation at the center was lower than that at the corner. (2) In the early age, the internal curing water would be released by SAP according to the internal humidity of the hardened cement mortar, which will accelerate the formation of AFt crystals (expansion). Combined with hydration kinetics of the cementitious materials, it can be seen that there was a peak period of hydration rate growth and hydration heat release within 72 h, and the rise of temperature generated by hydration heat caused the concrete to expand. (3) For SAP with particle sizes of 120–150 lm, the water was released rapidly before 75 h, and then the rate of water release began to decline at 75–150 h. Therefore, the shrinkage inhibition was the best during this period. Subsequently, the shrinkage deformation began to increase obviously which was due to the gradual ending of internal curing. The effect of internal curing basically ended after 250 h. As a result, the concrete began to maintain a stable shrinkage development. In the later stage, the difference of the shrinkage values at the corner between the reference group and the internal-curing group increased gradually during the development of shrinkage deformation. The shrinkage deformation of C30-J and C30-100-0.145% was 337.72 le and 268.12 le at 700 h (28 d) respectively, and the shrinkage rate can be reduced by 20.61% after the incorporated of SAPs. 3.1.2. The spatial distribution of humidity inside the pavement slab Fig. 5 exhibits the development of internal relative humidity IRH at different layers of pavement concrete slabs with age. It can be seen from Fig. 5 that the relative humidity of the upper, middle and lower layers of the reference group showed obvious gradient characteristics, and its order of magnitude was lower layer humidity > middle layer humidity > upper layer humidity. Before 100 h, the relative humidity of each layer in the pavement slab dropped rapidly, and the lowering speed of the upper layer was faster than that of the middle and lower layers. At this time, the concrete was in the hydration accelerated period. When the concrete entered the hydration stabilization period, the humidity of each layer dropped at an obviously slower rate, and the humidity decline speed was upper layer > middle layer > lower layer. The reasons are as follows (1) During the hydration acceleration period, the change of the humidity at each layer of the pavement slab was mainly dominated by the hydration of the cementitious material, which was affected by the external water evaporation simultaneously. At the same time, the upper layer of concrete was affected most seriously by evaporation due to direct contact with the external environment. Therefore, the relative humidity distribution of each layer dropped rapidly, and the upper layer dropped the fastest, followed by the middle layer. (2) In the stable hydration period, the evaporation effect of the external environment on the pavement slab was the main cause of humidity changes. Especially in the upper and middle layers of concrete, the rate of moisture decline would be faster, so the humidity difference between the layers was gradually expanded. (3) The process of vibrating adopted in the forming process of pavement concrete creates a mortar layer with higher water colloid is present on the surface layer, so that the humidity drop caused by the evaporation of water is more likely to occur in the upper concrete, followed by the middle layer and the lower layer concrete. After using SAP to maintain health, the descending speed of the relative humidity inside the pavement slab was obviously lower than that of the reference group. At the age of 150 h (7 d), all layers of the internal curing concrete could maintain a higher humidity state. The relative humidity of the upper, middle and lower layers in the internal curing concrete was 95.38% RH, 96.51% RH and

J. Yang et al. / Construction and Building Materials 227 (2019) 116705

5

Fig. 5. Full curve of humidity development in pavement slabs.

97.34% RH, respectively, which could be improved by 8.47%, 6.05% and 4.67%, respectively, compared with the reference group. After entering the hydration stabilization period, the relative humidity curves of each layer were still close to each other, indicating that there was no significant change in the humidity difference between the layers. It can be seen that SAP supplemented the moisture reduction caused by moisture evaporation in the middle and upper layers of the concrete effectively in the internal curing stage. The water loss of the concrete in the upper layer was the greatest, and SAP provided the most water replenishment for this layer. This behavior reduced the humidity gradient between the upper middle layer and the lower layer. This feature is extremely beneficial for suppressing the humidity warping stress of concrete pavement slabs. When the water release of SAP was finished, the humidity inside the slab gradually changed to upper layer < middle layer < lower layer under the effect of external drying, but the humidity between the three was still similar. 3.1.3. Humidity gradient warping stress of pavement slab The top of cement concrete pavement is exposed to external natural conditions. When the ambient humidity is lower than the internal humidity of the concrete, water can gradually rise to the surface through the capillary pores inside the concrete and then disappear into the air. As a result, a top-down humidity gradient is formed in the concrete along the depth direction, and there will thus be an upward warping deformation at the corner of the pavement slab. This will cause a void to appear at the corner of the slab and a gap between the edge of the plate and the foundation. Coupled with the vehicle load, the stress concentration will appear on the surface of the slab, which will eventually cause vertical fracture of the pavement slab from top to bottom, affecting the service life of cement pavement. Based on this, the humidity gradient and warping stress of concrete pavement slabs with SAPs will be studied, and the inhibition effect on warping stress caused by the humidity gradient after water compensation will be discussed. The calculation method of the warping stress of a concrete pavement slab based on the humidity gradient is given as follows: The relationship between material deformation and internal humidity was established, and the correlation between the shrinkage deformation of the cementitious material and the humidity inside the slab is shown in Eq. (4) [37,38].

ep ¼ a  RH þ b

ð4Þ

where ep is the shrinkage deformation of cementitious material. The regression coefficient a is the slope of the linear regression equation between shrinkage and relative humidity, and b is the intercept.

Meanwhile, to calculate the relationship between the shrinkage deformation of concrete materials and the humidity inside the slab more conveniently, the Pickett model is introduced in Eq. (4), as shown in Eq. (5).

ec ¼ ep  ð1  V A Þn ¼ ½a  RH þ b  ð1  V A Þn  106

ð5Þ

where ec is the shrinkage deformation of concrete; VA is the volume fraction of the aggregate in the concrete; and n is the shrinkage limit rate caused by the aggregate, and the value is 1.68. According to the conditions for continuous deformation of the elastomer, the warpage bending moment of the slab is



Mx



 1 ¼D

v

v

My

1

 ;

Cx

 ð6Þ

Cy

3

Eh where D ¼ 12ð1V 2 ; h is the thickness of the slab; E is the elastic Þ

modulus of the concrete; v is the Poisson’s ratio; Mx ; My are the warping moments of the board; and C x ; C y are the curvatures of the pavement slab in the x and y directions (the depth direction is z), and C x ¼ C y is assumed. Thus, the following formula can be obtained:

Z Mx ¼ M y ¼ M RH ¼ Z ¼

h=2

h=2

h=2

h=2

rðzÞzdz

½a  RH þ bð1  V A Þn  106 zdz

ð7Þ

It is assumed that the warpage deformation of the cement concrete pavement under the equivalent temperature gradient DT e is equal to that under the humidity gradient, and the humidity gradient is converted into the equivalent temperature gradient DT e [37– 39]. Then, the moment at the equivalent temperature gradient DT e can be obtained as shown in Eq. (8). 2

MT ¼

EDT e ah 12ð1  v Þ

ð8Þ

The following formula can be obtained from M T ¼ MRH .

DT e ¼

12 ah

 2

Z

h=2

h=2

½a  RHðzÞ þ bð1  VA Þn  106 zdz

ð9Þ

where DT e is the equivalent temperature gradient of depth z corresponding to RH(z); a is the coefficient of thermal expansion; and h is the thickness of the pavement slab. To simulate the distribution of actual humidity gradient stress for high-grade highway cement concrete pavement, the common pavement panel size parameters are selected, that is, the length,

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J. Yang et al. / Construction and Building Materials 227 (2019) 116705

width, and height are 4.5 m, 3.75 m, and 25 cm, respectively. Based on the parameters required in Eqs. (4)–(9), the law of relative humidity changes with depth inside the concrete slabs of the reference and internal-curing groups in Fig. 5 was calculated and converted. Then, the equivalent temperature gradients along the depth direction of the reference and internal-curing groups were obtained and are shown in Table 5. After calculating with Comsol finite element software, the internal stress nephograms in the pavement slabs were obtained and are shown in Fig. 6. Finally, the internal stress at the typical position of the pavement slab in the internal stress nephogram was extracted for visual analysis, and the internal stress table of humidity is shown in Table 6. It can be seen from the internal stress nephogram and the internal stress of humidity simulated by COMSOL software that the positions of the humidity stress peak in the reference group and the internal-curing group are both located at the edge of the surface of the slab. Therefore, the surface of the pavement slab shows a tendency of shrinkage and warping, and the stress exhibits a regular distribution for the slab. There is strip-type stress concentration approximately 25 cm away from the edge of the slab, and among them, the maximum stress is approximately 30 cm inward from the corner of the slab. The humidity stress for the rest of the slab is relatively small. This is also consistent with the phenomenon that fractures often occur near the edges of slabs in actual engineering practice. Simultaneously, the humidity stress at the center of the pavement slab is obviously smaller than that at the stress concentration, and it is only apptoximately 70% of the latter. By comparing the warping stress in the strip area on the surface of the two pavement slabs, the warping stress of the C30-J pavement slab can reach 217 KPa, and that of the C30-100-0.145% pavement slab is 98.2 kPa. The reduction can reach 54.75%, which shows that the warping stress at the slab edge is obviously reduced after curing with SAP. Because SAP with a particle size of 120– 150 lm has a better long-term curing and moisture control effect, it can greatly reduce the differences in humidity between the upper and lower layers of the slab, so that the humidity gradient stress can be effectively reduced. In general, the reduction of humidity warping stress of the pavement slab with SAP is closely related to the long-term curing effect of SAP. 3.2. Risk of cracking under early annular constraints 3.2.1. The strain of the inner wall After the concrete was poured, standard curing was carried out for 1 day, and then the outer mold was removed to create a singlesided drying condition for the concrete. The inner wall strains of the reference group and the internal-curing group are shown in Fig. 7. Taking the curve of the inner wall strain of the C30-J pavement concrete under the ring constraint with age in Fig. 7(a) as an example, the development characteristics were analyzed. During the 3-d period, the concrete was in the stage of rapid development with shrinkage, and at the same time, the concrete was restrained by the steel ring. In this way, the contractile tensile stress would be generated towards the inside of the ring, and the inner wall strain appeared as compression. Therefore, the compressive strain value would gradually increase with the development of shrinkage. At 2 d, the inner wall strain of the steel ring was approximately

Table 5 Equivalent temperature gradient in the depth direction. Serial number

C30-J

C30-100-0.145%

DT e /°C

18

10

16 le, and then it entered a stable development stage while the concrete entered the hydration attenuation period. As the elastic modulus of concrete continued to develop, the shrinkage strain of concrete in a dry environment would increase with time. At the same time, the compressive stress acting on the inner wall of the ring would increase with time, and the development rate of the inner wall strain was accelerated obviously at 4 d. When the shrinkage tensile stress inside the concrete ring reached the tensile strength of the concrete, the concrete ring would crack (corresponding to point A in Fig. 7(a)). Meanwhile, the shrinkage tensile stress in the ring was released, and the compressive stress acting on the inner wall was rapidly reduced. During this process, the peak of the inner wall strain was 78 le, and cracking occurred at 13.5 d. Thereafter, the constraining force on the concrete ring would gradually decrease, and the strain on the inner wall of the ring would gradually approach 0. It can be seen from Fig. 7(b) that within approximately 2 d, the inner wall strain of the internal curing group was positive and increased from 0 to approximately 10 le, indicating that the concrete ring was subjected to compressive stress, which might be due to the temperature rise caused by the hydration heat causing the steel ring to expand [40]. After a brief expansion deformation, the tensile stress on the inner wall gradually turned into compressive stress and increased continuously, but the overall growth rate was lower than that of the C30-J concrete. This suggests that internal curing with SAP can achieve a good effect of shrinkage inhibition and avoid the rapid growth of the inner wall strain. Unlike the C30-J concrete, the internal-curing group did not show a strain transition on the internal wall (i.e., the concrete ring was not cracked), while the inner wall strain tended to a stable value of approximately 55 le at 17 d. 3.2.2. Cracking risk assessment The cracking risk assessment of annular concrete based on inner wall strain can be quantitatively expressed by establishing a coordinate relationship between the net cracking time (t) and the cracking stress rate (Q), as specified in ASTM. The net cracking time (t) is the time from the start of the concrete in a single-side drying environment to the occurrence of cracks in the concrete ring. Because the concrete ring of the internal-curing group did not crack, the net cracking time of is calculated to be more than 28 days. In addition, the calculation method of the cracking stress rate (Q) is shown in Eq. (10), which is characterized by stress development per unit time.

Q¼

pffiffi drt ¼ Gjaav e j=2 t dt

ð10Þ

where G ¼ Estrisrichhc st and Est is the modulus of the steel ring, which is 180 GPa; hst is the thickness of the steel ring; ris is the inner diameter of the steel ring; hc is the thickness of the concrete ring; ric is the inner diameter of the concrete ring; and aave is the average   1 strain rate, and the unit is m  d 2. m Combined with the results of the ring cracking test, the calculated results of all parameters are shown in Table 7. Based on the classification range for the cracking risk of each parameter in Schedule 1.1 of ASTM C1581/C1581M-09a, a crack risk assessment diagram as shown in Fig. 8 can be plotted. The abscissa represents the cracking stress rate and the ordinate is the net cracking time. According to ASTM C1581/C1581M-09a, the risk level based on the horizontal and vertical coordinates is divided into four areas. The risk is gradually increased from left to right on the abscissa and is gradually increased from top to bottom on the ordinate. The risk of cracking decreases from the lower right corner to the upper left corner. The drop points of C30-J and

J. Yang et al. / Construction and Building Materials 227 (2019) 116705

7

Fig. 6. Internal stress nephogram.

Table 6 Internal stress of humidity at different positions. Serial number

Position of humidity stress peak

Humidity peak stress/KPa

Warping stress at slab edge/KPa

C30-J C30-1000.145%

Board surface edge Board surface edge

217 98.2

217 98.2

C30-100-0.145% were selected according to the calculation of the net cracking time and the cracking stress rate, as shown in Fig. 8. As seen from Fig. 8, the cracking risk of concrete can be significantly reduced by internal curing with SAP. After internal curing, the cracking risk of concrete was at the lowest level (Low), and the risk level was C30-J > C30-100-0.145%. With the incorporation of SAP, the cracking risk could be reduced by two grades. It can be seen that SAP significantly improves the concrete in terms of annular confined cracking. After internal curing, the cracking stress and net cracking time had different degrees of reduction, especially for the net cracking time, and the risk went from moderate-high to low after adding SAP. For both the cracking stress rate and the net cracking time, the risks were in the low risk range after internal curing.

4. Conclusions In this paper, the drying shrinkage, humidity gradient and humidity warping stress at different locations of concrete pave-

ment slabs with or without SAP were studied. Then, the influence of SAP on early concrete cracking was researched by annular restraint tests, and the cracking risk assessment was carried out. Based on the results, the following conclusions can be drawn: (1) The rules that the shrinkage deformation value of the reference group is larger than that of the internal curing group and that at the corner is larger than that at the center were followed throughout the shrinkage process for the concrete. The shrinkage inhibition effect of SAP was the best within 7 days, while the shrinkage deformation began to increase obviously after 7 d, and it basically maintained steady growth after 10 d. After incorporating SAP, the shrinkage rate at the corner of the slab was reduced by 20.61% for the reference group. (2) The gradient characteristic was presented in the humidity distribution in the vertical direction of the pavement slab, with the highest humidity in the lower layer and the lowest humidity in the upper layer. As a result of evaporation, the humidity of the upper layer decreased the fastest among the three layers, and the humidity difference between the layers increased gradually. After being cured by SAP, the humidity difference of each layer in the concrete was small, indicating that internal curing greatly reduces the depth of the surface drying and the humidity differences between the pavement slab horizons. Consequently, the increase in the humidity warping stress is suppressed, so that the risks of warpage cracking and shrinkage cracking for the pavement slab are significantly reduced.

Fig. 7. Inner wall strains of the annular pavement concrete.

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J. Yang et al. / Construction and Building Materials 227 (2019) 116705

Table 7 Parameters related to ASTM cracking risk calculation. Numbering

Net cracking time t/d

Modulus/GPa

Average strain rate  106/((m/m) d1/2)

Cracking stress rate Q/(MPa d1)

C30-J C30-100-0.145%

13.5 28

64.72

5.01 4.23

0.044 0.026

[6]

[7]

[8]

[9]

[10] Fig. 8. Risk assessment diagram of concrete cracking. [11]

(3) The incorporation of SAP resulted in a more pronounced reduction of the strain increase rate on the inner wall and the cracking of the concrete ring. The pavement concrete of the reference group began to crack at the age of 13.5 d, and the peak value of the cracking was 78 le. However, the concrete ring of the internal curing group did not crack, and the strain on the inner wall of the concrete ring tended to a stable value of approximately 55 le after a constant growth of 17 d. (4) Internal curing with SAP can decrease the risk of cracking for concrete significantly, which can reduce the risk of concrete cracking by two grades, and ensure that both cracking stress and net cracking time are in the low risk range.

[12] [13] [14]

[15] [16]

[17] [18]

[19]

Declaration of Competing Interest [20]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[21] [22]

Acknowledgements [23]

This study was financially supported by the National Natural Science Foundation of China (Grant No. 51778061) and the Guangdong Transportation Funding (Grant No. 2017-02-011). The authors thank the reviewers of this paper for their comments and suggestions. References

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