Effect of Curing Conditions on the Shrinkage of Ultra High-Performance Fiber-Reinforced Concrete

(e effect of curing conditions on the early age and long-term shrinkage of ultra high-performance fiber-reinforced concrete (UHPFRC) was systematically studied. (e shrinkage of the early age (0–168 h) and long-term age (0–90 d) of UHPFRC material was measured based on three kinds of humidity conditions (dry, sealed, and soaked) and curing temperatures (25°C, 40°C, and 75°C), respectively. In this paper, the hydration degree of different shrinkage stages was studied in combination with chemicalbound water experiment. Meanwhile, the influencing mechanism of curing condition on the shrinkage of UHPFRC was analyzed. (e results show that the early shrinkage rate of UHPFRC is accelerated with the increase of temperature, and the rate of shrinkage development at the latter stage is suppressed with the increase of temperature. With the increase of humidity, the early age shrinkage of UHPFRC and its increasing rate gradually decrease, which means drying condition> sealing condition> soaking condition. According to the long-term shrinkage results, increasing temperature has very significant inhibiting effect on the UHPFRC shrinkage in the sealed condition. Due to the majority of the in-site components of UHPFRC cured in the sealed condition, high-temperature curing has evident inhibition of early age shrinkage of UHPFRC. (erefore, promoting curing temperature is fairly effective at inhibiting the early age shrinkage of UHPFRC for the in-site structures.


Introduction
As a kind of typical fiber-reinforced concrete material, ultra high-performance fiber-reinforced concrete (UHPFRC) is known for the high strength, high durability, high toughness, and low defection.UHPFRC has a broad application prospect, which can be widely used in several environments [1].In pursuit of excellent properties of UHPFRC, some methods including reducing water-cement ratio [1], adding reactive powders [2], and adding compound chemical admixtures were carried out.
ese design approaches will accelerate the hydration of cementitious materials to form dense structure, while the larger self-drying shrinkage is also generated simultaneously.However, the internal structure of ordinary concrete with less autogenous shrinkage is relatively loose.
e shrinkage mechanism of ordinary concrete is also quite different from that of UHPFRC.Studies have shown that the autogenous shrinkage of cement hydration was only 50-100 με after 5 years [3], while the early age shrinkage of UHPFRC material reached up to 400 με at the age of 14 d [4].Since the early strength and shrinkage of UHPFRC grow rapidly, the shrinkage stress will be generated inside concrete by constraint effect.
en the probability of cracking also increases, which causes negative effect on mechanical properties and durability of the structure.erefore, it is of great significance to further study the shrinkage mechanism of UHPFRC.
UHPFRC is usually cured by heating and steaming.e curing condition has a very important effect on the concrete shrinkage.ere is great difference for the shrinkage between the steam curing and the standard curing.Studies showed that [5] the shrinkage of UHPFRC among 14 d age reached up to 400-800 με in the standard curing condition, while that of 28 d was only about 100-300 με in the curing condition of high-temperature steam.Fehling et al. [6] believed that the concrete was denser after the heat curing, which would not produce a larger shrinkage.Bouziadi et al. [7] studied the effect of different curing temperatures (20 °C, 35 °C, and 50 °C) on the total shrinkage of high-strength concrete; the results showed that the total shrinkage and the growth rate increased with the increase of curing temperature.Besides, the effect of curing temperature on early age shrinkage was higher than that of long-term shrinkage.Mounanga et al. [8] studied the shrinkage development in different curing temperatures (10-50 °C).It was found that there was a certain limiting value for the growth of shrinkage with increasing curing temperature.e shrinkage was lower than 40 με at 50 °C.Han et al. [9] studied the effect of environmental humidity and temperature on the drying shrinkage before the end of curing.
e results showed that higher environment humidity during curing led to higher drying shrinkage after the curing time.While the curing temperature increases, the drying shrinkage decreased accordingly.Tu [10] studied the development of UHPFRC shrinkage in different curing conditions (75 °C steam curing, 75 °C dry curing, 40 °C purl curing, 40 °C dry curing, and 20 °C purl curing).It was indicated that temperature and humidity had large effect on the shrinkage during the curing period.Raising both the temperature and the humidity increases the shrinkage during curing, but high-temperature curing also decreases the drying shrinkage after the end of curing.Above all, the curing temperature and humidity presented significant influences on the shrinkage of UHPFRC.However, in existing researches, the effects of the curing condition on the shrinkage of UHPFRC in early age and long-term shrinkage have not been clearly studied.It is difficult to propose the effective methods to restrain the UHPFRC shrinkage.
In this study, based on the shrinkage development of UHPFRC cured in standard curing room, the shrinkage development of UHPFRC cured in various curing conditions was systematically measured.ree kinds of humidity conditions (dry, sealed, and soaked) and curing temperatures (25 °C, 40 °C, and 75 °C) were chosen.e embedded vibrating wire extensometer was used to measure the early age and long-term shrinkage.At the same time, combining with the hydration degree in different stages of shrinkage, the changing mechanisms of UHPFRC shrinkage were discussed.

Materials and Mix
Proportions of UHPFRC.42.5 ordinary Portland cement (fineness 3400 cm 3 /g; the initial set 160 min, final set 220 min) is used for cementitious materials.Its clinker mineral compositions are shown in Table 1.
e chemical compositions of silica fume with an average particle size of 0.31μm are shown in Table 2. e fine aggregate is made of quartz sand customized by a sand factory in Beijing.It is divided into three grades: 1.25-2.5 mm, 0.63-1.25 mm, and 0.315-0.6mm and a ratio of 2 : 4 : 1. e measured apparent density and bulk density of quartz sand in different grades are shown in Table 3. Shortcut steel fiber with the diameter of 0.22 mm, length of 13 mm, and tensile strength of 2800 MPa is used as reinforced material.PCE with 29% water reduction rate and 31% solid content is used as superplasticizer.
UHPFRC is composed of graded quartz sand, cement, silica fume, steel fiber, superplasticizer, and water.e standard mixture ratio of UHPFRC commonly used in this research group is designed based on the packing theory [11].As shown in Table 4, the UHPFRC standard mixture ratio was regarded as the control group.e shrinkage mechanisms of UHPFRC in different conditions were investigated by varying the curing conditions.e detailed curing process is described as follows: Before demolding and keeping in the standard curing room (constant temperature and humidity), the experimental specimens of the sealed and soaked conditions should be covered by the membrane.At 1 d after molding, the mold was removed.e specimen of dry curing was kept in the standard curing room.e specimens of sealed curing were sealed with a plastic membrane and tape.As for soaked curing, the specimen completely submerged in water was placed in a sealed box.Moreover, before putting it in the standard curing room until the stipulated age, the experimental group of high-temperature curing was cured 72 h in the corresponding temperature (40 °C and 75 °C, resp.).

Test Methods.
According to the Chinese current code GB/T 50082-2009 (ordinary concrete long-term performance and durability test method standard), the measured methods of concrete shrinkage are divided into the contact measuring method and the noncontact measuring method.Because the UHPFRC needs to be cured at high temperature, the shrinkage is unable to be measured by these two methods during curing according to the current code.So the method of using the embedded vibrating wire extensometer was carried out.After comparing the two kinds of methods, Han et al. [12] found that the method of using the embedded vibrating wire extensometer could accurately measures the shrinkage of UHPFRC.e shrinkage of specimens in di erent curing conditions was measured by using the JMZX-212-type high-precision vibrating wire extensometer and the JMBV-1164-type automatic acquisition device from molding.e reading time was set for each hour in 7 d.After this, it changed to read a number every day until the age of 90 d. e SHT75 temperature and humidity sensor embedded inside the cube specimens (100 mm × 100 mm × 100 mm) was used to measure the internal humidity with cooperating SCTH2001 intelligent data collector.
e internal humidity of hardened slurry was read every other day from pouring until the age of 28 d.
e early age shrinkage of UHPFRC is closely related to the hydration reaction of cementitious materials.e hydration heat method can only measure the hydration degree in rst 7 d, while the chemical-bounding water test can measure that of any age.For acquiring the data of hydration degree comprehensively, the chemical-bound water contents were measured at 2, 4, 7, 14, 28, and 90 d  Advances in Civil Engineering 3 in di erent curing conditions, respectively.According to [13], after stopping the hydration reaction, the samples were dried 3 h at 110 °C in vacuum from about 10 g to constant weight m 1 , and the constant weight m 2 at 950 °C through the same method.e chemical-bound water content was computed through the di erence of two weights.

e Early Age Shrinkage of UHPFRC in Di erent Curing
Temperatures.
e development of early age shrinkage is mainly determined by the hydration degree of UHPFRC.Di erent curing temperatures have di erent e ects on the degree of cement hydration.In the study process of UHPFRC early age shrinkage, the UHPFRC in the three conditions of dry, sealed, and soaked were placed in the environment of 25 °C, 40 °C, and 75 °C, respectively.After curing for 72 h, the specimens were placed into the standard curing room until the prescribed curing time.e early age shrinkage in 168 h is shown in Figure 1.
e early age shrinkage of UHPFRC occurred mainly in the rst 96 hours.After that, the shrinkage curve gradually tended to be at.Meanwhile, the shrinkage remained basically unchanged.In di erent humidity conditions, raising the temperature would have a great impact on the early age shrinkage of UHPFRC.At room temperature (25 °C), the shrinkage of UHPFRC 168 h cured in three conditions of soaked, sealed, and dry was 194.8 με, 651.7 με, and 770.7 με, respectively.With the increase of temperature, UHPFRC early age shrinkage had growth of varying degrees.When the temperature was raised to 75 °C, the corresponding 168 h shrinkage was 551.6 με, 885.5 με, and 1321.9 με, which was increased by 183.2%, 35.9%, and 71.5%, respectively.Advances in Civil Engineering e early age shrinkage of UHPFRC would increase with the increase of curing temperature in all conditions.Faster development of early age shrinkage led to a higher shrinkage value, which increased corresponding risk of material cracking.e initial cracking of UHPFRC could occur within a short time. is was because the increase of temperature promoted the hydration rate of the cementitious materials in the concrete, and the hydrationhardening process was also enhanced.Due to the increase of cement hydration, the chemical shrinkage also increased quickly.On the contrary, hydration consumed a large amount of water in the material, which leaded to an increase in self-shrinkage inside the concrete.It was noteworthy that UHPFRC undergone some expansion reaching 200 με within 24-36 h in soaked conditions.Due to the sudden increase of air humidity in the curing environment after demolding, the hygroscopic expansion of concrete resulted in the expansion of the UHPFRC specimen.

3.2.
e Long-Term Shrinkage of UHPFRC in Di erent Curing Temperatures.After continuous measurement of the shrinkage development, the shrinkage in 90 days was obtained in di erent curing humidity conditions, as shown in Figure 2. In soaked and dry curing environments, the early age shrinkage rate of UHPFRC increased rapidly until the fth day and then remained stable in the latter period.e long-term shrinkage results showed that higher curing temperature led to smaller long-term shrinkage.Even rebounded phenomenon would occur.However, higher curing temperature led to higher shrinkage in the same curing time.e development of chemical shrinkage and autogenous shrinkage resulting in larger early age shrinkage and rapid decrease of humidity inside the concrete would be promoted by the hightemperature during curing.However, the accelerated hydration of the cementitious material produced a large amount of hydration products, which makes the hardened paste more compact.As a result, the external water hardly enters the interior of concrete after curing, which inhibited the further development of hydration.In other words, curing temperature accelerated the developing rate of UHPFRC shrinkage.erefore, curing temperature has a signi cant increase in the early shrinkage.It also has an inhibitory e ect on the longterm shrinkage.
As shown in Figure 2(b), UHPFRC in the sealed condition exhibited di erent shrinkage developments within 90 d at three di erent curing temperatures.e shrinkage curve before 28 d at 25 °C increased with age continuously.e shrinkage at the rst 7 d developed rapidly but gradually slowed down in the latter period.e curve tended to be gentle after 28 d, and the shrinkage at 90 d was 909.9 με. e shrinkage at 40 °C increased rapidly in the rst 5 d and then continued to increase until up to 875.5 με at 90 d. e early age shrinkage at 75 °C was similar to that at 40 °C, while the shrinkage decreased continuously after 5 d and decreased to 612.6 με at 90 d.As shown in the long-term shrinkage curve, the increase of curing temperature inhibited the selfshrinkage in the latter age.e inhibition was also more obvious with the increase of curing temperature.e chemical-bound water of the cement paste in three curing temperatures was measured at 1, 4, 7, 14, 28, and 90 d, respectively.Analyzing the mechanism with the hydration degree, the hydration degree was calculated as shown in Figure 3.
e shrinkage developing curve of the sealed condition showed that the e ect of curing temperature on shrinkage was phased.During curing, high temperature promoted the development of shrinkage, while the inhibiting e ect of high temperature on shrinkage began to emerge at the end of curing.e higher the temperature was, the more obvious the inhibiting e ect was.As could be seen from the hydration degree, the highest degree of hydration occurred at 75 °C and the degree of hydration at 40 °C is slightly higher than 25 °C.High temperature promoted the hydration reaction, which made early relative humidity decrease rapidly.
e autogenous shrinkage also developed rapidly during this period.As there was no water loss in the sealed condition, the hydration of the cementitious material would continue.
e hydration products lled the pore to make the structure denser, which resulted that the increase of curing temperature would inhibit the shrinkage.

e Early Age Shrinkage of UHPFRC in Di erent Humidity Conditions.
e early age shrinkage of UHPFRC in di erent humidity conditions was compared.e UHPFRC cured in di erent humidity conditions (dry, sealed, and soaked) was placed for 72 h at the environment temperatures of 25 °C, 40 °C, and 75 °C, respectively.e early age shrinkage in 168 h is shown in Figure 4.With the increase of moisture in the curing environment, the shrinkage of UHPFRC decreased signi cantly.About three hours after molding, there was a di erence appearing in the curing conditions of di erent humidity conditions.e shrinkage of UHPFRC in the drying condition was highest, the Advances in Civil Engineering growth rate of which was faster.However, the shrinkage of UHPFRC in the soaked condition and the sealed condition was almost the same.
e water absorption expansion occurred at 24 h after curing in the soaked condition.e expansion increased obviously with the increase of curing temperature.
e early age shrinkage developed rapidly during the stage of 24 to 48 h.Especially in the normal temperature condition, the moister the environment was, the slower the shrinkage developed was.In addition, after curing in the high-humidity environment, the larger shrinkage occurred in the short term after the end of curing (96 hours).Due to the thermal shrinkage and humidity shrinkage caused by the reduction of temperature and moisture, respectively, higher temperature led to higher shrinkage.erefore, when the UHPFRC components of high-temperature steam curing were translated from the curing phase into the use stage, the risk of cracking should be focused.Higher curing temperature also led to more obvious cracking risk.

e Long-Term Shrinkage of UHPFRC in Di erent
Humidity Conditions.After continuous measurement of the shrinkage development, the shrinkage in 90 days was obtained in di erent curing humidity conditions, as shown in Figure 5.
ere was a signi cant di erence in the long-term shrinkage of UHPFRC material in di erent curing conditions.At 25 °C, the shrinkage in the soaked condition was always lower than that in the other two conditions.e shrinkage in the dry condition and the sealed condition was similar to each other.At 40 °C, the sort by UHPFRC shrinkage was as follows: dry condition > sealing condition > soaking condition.
e correlation between this rule and environmental humidity was very good.At a high-temperature curing of 75 °C, the shrinkage

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Advances in Civil Engineering of UHPFRC material in the dry condition signi cantly increased, while shrinkage of the other two curing conditions was relatively close.e change of curing temperature caused the di erence in the e ect of environmental humidity on the early age shrinkage of materials.On the whole, with the increase of curing temperature, the early age shrinkage of UHPFRC in the sealed condition was closer and closer to soaked curing, which indicated that it was signi cantly inhibited.e inhibiting e ect was only signi cant for the UHPFRC in the sealed condition, but not for the other two conditions.e UHPFRC material was too dense to permeate after forming, so most of the actual structural components of UHPFRC material could be regard as a sealed curing state.e high temperature had the most obvious inhibiting e ect on the early age shrinkage of UHPFRC in the sealed curing condition.erefore, as for the practical engineering structures, improving temperature is a very e ective method to inhibit the early age shrinkage of material.

Conclusions
e early age shrinkage rate of UHPFRC in di erent humidity conditions is as follows: dry condition > sealing condition > soaking condition, which is consistent with ordinary concrete.e increase of curing temperature will signi cantly accelerate the process of UHPFRC material shrinkage, which increases the early age shrinkage.However, the rate of long-term shrinkage will be accordingly reduced.
e shrinkage of UHPFRC material at the same age always increases with the promotion of curing temperature.According to the development of long-term shrinkage, increasing temperature has very signi cant inhibiting e ect on the UHPFRC shrinkage in the sealed condition, which is much higher than the other two conditions.Due to the majority of the engineering components of UHPFRC cured in the sealed condition, high-temperature curing has evident inhibition on early age shrinkage of them.erefore, as for Advances in Civil Engineering the practical engineering structures, improving temperature is a very effective method to inhibit the early age shrinkage of UHPFRC.

Figure 3 :
Figure 3: e hydration degree of UHPFRC in the sealed condition.

Table 3 :
e size distribution of quartz sand.