Materials 2015, 8, 5537-5553; doi:10.3390/ma8085261

OPEN ACCESS

materials ISSN 1996-1944 www.mdpi.com/journal/materials Article

Early-Age Strength of Ultra-High Performance Concrete in Various Curing Conditions Jong-Sup Park 1 , Young Jin Kim 1 , Jeong-Rae Cho 1 and Se-Jin Jeon 2, * 1

Structural Engineering Research Institute, Korea Institute of Civil Engineering and Building Technology, Goyang-si, Gyeonggi-do 411-712, Korea; E-Mails: [email protected] (J.-S.P.); [email protected] (Y.J.K.); [email protected] (J.-R.C.) 2 Department of Civil Systems Engineering, Ajou Univeristy, Suwon-si, Gyeonggi-do 443-749, Korea * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +82-31-219-2406; Fax: +82-31-219-1613. Academic Editor: Luciano Feo Received: 11 July 2015 / Accepted: 7 August 2015 / Published: 24 August 2015

Abstract: The strength of Ultra-High Performance Concrete (UHPC) can be sensitively affected by the curing method used. However, in contrast to the precast plant production of UHPC where a standard high-temperature steam curing is available, an optimum curing condition is rarely realized with cast-in-place UHPC. Therefore, the trend of the compressive strength development of UHPC was experimentally investigated in this study, with a focus on early-age strength by assuming the various curing conditions anticipated on site. Concrete specimens were cured under different conditions with variables including curing temperature, delay time before the initiation of curing, duration of curing, and moisture condition. Several conditions for curing are proposed that are required when the cast-in-place UHPC should gain a specified strength at an early age. It is expected that the practical use of UHPC on construction sites can be expedited through this study. Keywords: Ultra-High Performance Concrete; curing; strength; cast-in-place

1. Introduction Ultra-High Performance Concrete (UHPC) has been one of the most active research fields of concrete recently, because it can contribute to the longer life and economic efficiency of structures [1,2]. Recent extensive studies carried out on UHPC have further improved the quality of high strength or high

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performance concrete [2–4]. While UHPC has superior mechanical properties in terms of compressive and tensile strengths, ductility, and toughness, as well as high flowability and durability, strict quality control is required to ensure these target standards for its production. The term UHPC usually covers various types of cementitious composite materials developed for similar purposes. Similarly to ordinary concrete, UHPC can be either fabricated as precast members at a plant or cast in-place at a construction site. Apart from fully cast-in-place UHPC structures, although a precast-type UHPC structure is planned for stable quality control and acceleration of construction [5], some components, such as the joints of precast UHPC segments, still need to be cast in-place [6]. These cast-in-place components of a UHPC structure can significantly affect the overall quality of the structure and construction speed, since they are cast possibly under limited and unfavorable conditions on the site. Another important application of cast-in-place UHPC is the rehabilitation of existing structures [1,7]. The quality of cast-in-place UHPC can be considerably affected by the mixing, placing, and curing methods used. Some projects have been conducted to extend the use of UHPC to the field, focusing on cast-in-place technologies [1,7,8]. One of the difficulties of casting the UHPC on site is the need for a specially-designed mixer for UHPC that is usually used in a laboratory or a plant. As one of the strategies to cope with this problem, a portable mixer optimized for the UHPC mixture has been developed because the quality of the UHPC using a conventional mixer may be subject to large variation [8]. Another important factor that affects the quality of cast-in-place UHPC is the curing method. Possible curing methods on site may differ from those of precast UHPC segments fabricated in the ideal conditions of a plant. Generally, in precast UHPC, a standard steam curing method is adopted to obtain rapid strength development. However, in cast-in-place UHPC, which is the main focus of this study, curing methods are often limited in terms of curing temperature, curing period, and moisture condition. Several studies have focused on determining whether a specified compressive strength of UHPC can be attained at 28 days under normal moist curing without heat treatment [1,3,7]. In some cases, however, the specified strength needs to be obtained within an earlier age of UHPC to accelerate construction speed. Therefore, this study presents experimental results on the characteristics of the early-age strength development of UHPC in various curing conditions conceivable at the site. Regarding the terminology for the early age, there is not a clear definition of how short the early age of concrete is. However, 7-day compressive strength, which was determined as the focus of this study by referring to the days required for standard steam curing in plant production, can be regarded as the early-age strength in comparison to 28-day strength that is usually adopted for the design purposes. Factors considered in the experimental program include curing temperature, delay time before the initiation of curing, duration of curing, and moisture condition. The strengths were compared with those of the specimens cured by standard high-temperature steam. Through the analysis of the test results, several requirements for curing are proposed that are required when the specified strength of UHPC should be attained in an early age even though it is cast in-place. 2. Curing of UHPC 2.1. Development of K-UHPC For the mix design of UHPC, the superior mechanical properties need to be considered, such as strength, ductility, and toughness, in addition to the high flowability of fresh concrete and durability.

rength, ductility, and toughness, in addition to the high flowability of fresh concrete and durabilit herefore, various mix proportions of UHPC have been proposed, so far, depending on the targ operties. The K-UHPC developed by the Korea Institute of Civil Engineering and Buildin Materials 2015, 8 5539 echnology is the main focus in this study [4]. Figure 1 shows the schematic composition of K-UHP nd Table 1 presents the specific mix proportion of the K-UHPC which is expressed as a ratio of mas Therefore, various mix proportions of UHPC have been proposed, so far, depending on the target he mixture consists cement, silica fume, filling powder, fine aggregate, shrinkage reducing agen properties. TheofK-UHPC developed by the Korea Institute of Civil Engineering and Building Technology xpansive isagent, superplasticizer, steel fibers. Coarse aggregates areofnot included in the the main focus in this study and [4]. Figure 1 shows the schematic composition K-UHPC and Table 1 mixtur presents the specific proportionPortland of the K-UHPC whichSilica is expressed as aused ratio of The mixture he cement in Table 1 ismix Ordinary cement. fume formass. K-UHPC requires th consists of cement, silica fume, filling powder, fine shrinkage reducing agent, expansive agent, ecific surface area of more than 150,000 cm2/gaggregate, and SiO 2 content of more than 96%. The fillin superplasticizer, and steel fibers. Coarse aggregates are not included in the mixture. The cement in owder hasTable the 1average particle size of 10 μm and SiO2 content of more than 96%. Fine aggregate use is Ordinary Portland cement. Silica fume used for K-UHPC requires the specific surface area of r K-UHPC to 2 /g silica sand with the diameter of less than 0.5 mm. The glycol-base more is thanlimited 150,000 cm and SiO 2 content of more than 96%. The filling powder has the average particle of 10 µm and SiO of more than 96%. Fine aggregateexpansive used for K-UHPC limited to silica rinkage size reducing agent and calcium sulfa aluminate-based agentisare added to cope wi 2 content sandconcrete; with the diameter of less than mm. The glycol-based reducing agent and calcium rinkage of in particular, the 0.5 autogenous shrinkage shrinkage that is induced by self-desiccation whe sulfa aluminate-based expansive agent are added to cope with shrinkage of concrete; in particular, the e water-to-binder ratio is adjusted to be very low to attain high strength. Also, polycarboxylic acid-base autogenous shrinkage that is induced by self-desiccation when the water-to-binder ratio is adjusted to perplasticizer used to ensure high flowability even with a verysuperplasticizer low water-to-binder ratio. The ste be veryislow to attain high strength. Also, polycarboxylic acid-based is used to ensure withmm a very lowthe water-to-binder ratio. of Themore steel fibers theMPa. diameter of length 0.2 bers havehigh theflowability diametereven of 0.2 and tensile strength than have 2,000 The of th the tensile strength moreand than 20 2,000 MPa. The length of fibers can be chosen among 13, bers can mm be and chosen among 13,of 16 mm depending onthethe required tensile characteristic 16 and 20 mm depending on the required tensile characteristics. The binder in the water-to-binder ratio he binder in the water-to-binder ratio in Table 1 indicates cement plus silica fume. The desig in Table 1 indicates cement plus silica fume. The design recommendations for K-UHPC [4] provide commendations for K-UHPC [4]material. provide detailed information on each material. detailed information on each

1. Composition of K-Ultra-HighPerformance Performance Concrete (K-UHPC) (not to scale). Figure 1.Figure Composition of K-Ultra-High Concrete (K-UHPC) (not to scale). Table 1. Mix proportion of K-Ultra-High Performance Concrete (K-UHPC) (ratio of mass).

Table 1. Mix proportion of K-Ultra-High Performance Concrete (K-UHPC) (ratio of mass). Item

Value

Item

Value

Item

Value

Item Value Item Value Item Value Water-to-binder ratio 0.2 Filling powder 0.3 Expansive agent Water-to-binder ratio 0.2 Filling powder 0.3 Expansive agent0.075 0.075 Cement 1 Fine aggregate 1.1 Superplasticizer 0.018 Cement 1 Fine aggregate 1.1 Superplasticizer 0.018 Silica fume 0.25 Shrinkage reducing agent 0.01 Steel fiber (volume fraction) 1.5%–2% Silica fume 0.25 Shrinkage reducing agent 0.01 Steel fiber (volume fraction) 1.5%–2%

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The specified compressive strength, cracking strength, and tensile strength of K-UHPC are as high as 180, 9.5 and 13 MPa, respectively. In order to ensure these target strengths, initial curing and high-temperature steam curing are recommended in sequence [4]. The initial curing is maintained at 20 ˝ C for 12–48 h, with 24 h recommended, immediately after casting. The following high-temperature steam curing is performed at 90 ˝ C for 24–72 h, typically, with 48 h recommended. It was confirmed that strengths that exceed the specified strengths can be obtained even at an early age immediately after curing if the above curing criteria are fulfilled. These criteria were introduced by taking into consideration the curing of precast members. However, when the K-UHPC is cast in-place, the criteria are barely met in many cases, as a result of the difficulty in controlling the temperature and moisture due to the limited circumstances of the site. This study focuses on the minimum curing conditions of cast-in-place K-UHPC (in circumstances where standard steam curing is not available on site) that are required to ensure a similar target strength to that of precast K-UHPC at an early age. Other properties of K-UHPC, such as shrinkage behavior, including the dominant autogenous shrinkage, were investigated in the previous studies [9]. 2.2. Previous Studies Since the concept of concrete maturity was established by Carino et al. [10], the importance of curing temperature and age in strength development has been generally accepted. This is the reason why the high-temperature steam curing is preferred in a precast concrete plant. In UHPC, however, an additional closer relationship exists between curing temperature and strength than that in normal concrete, because most UHPC includes a large amount of silica fume due to the various advantageous characteristics [2,11,12], such as a considerable strength increase. Silica fume is transformed to calcium silicate hydrate by reacting with calcium hydroxide through the pozzolanic reaction. This type of reaction tends to be substantially activated under a high temperature [2,11], which is why it is recommended for most UHPC to be cured under a high temperature to ensure rapid strength development. On the other hand, the moisture condition of UHPC containing silica fume should be given special attention in order to cope with the dominant self-desiccation [1,11]. The Silica Fume Association recommends moist curing of the concrete containing the silica fume for at least 7 days [12]. Based on these previous studies, the control of curing temperature and moisture condition would have a crucial effect on the strength development of cast-in-place UHPC. However, it is usually difficult to apply an ideal curing scheme in terms of temperature and moisture when the UHPC is cast on site, because the construction site has an inferior condition to a laboratory or a precast concrete plant; a realistic curing scheme should, thus, be devised on site. Some researchers have focused on determining whether the UHPC can attain the specified compressive strength at 28 days when subjected to ambient or room temperature and sufficient moisture for a certain period [1,3]. However, sometimes the specified strength needs to be ensured within a shorter period in order to advance the completion date of the structure, even under inferior site conditions, as investigated in this study. Ishii et al. [5] demonstrated that the early-age strength of the UHPC that they applied to a pedestrian bridge was reduced from 215 to 147 MPa when the steam curing temperature was lowered from 90 to 70 ˝ C. Koh et al. [13] reported the results of 20 ˝ C curing of K-UHPC as shown in Figure 2. It was observed that 190 MPa was attained at 91 days under moist curing, which is similar to the

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observed that 190 MPa was attained at 91 days under moist curing, which is similar to the strength Materials 2015, 8 5541 expected immediately after 90 °C steam curing, while only 60% of 190 MPa was attained at 7 days. In comparison, only 80% of 190 MPa was achieved even at 91 days under a dry condition, demonstrating strength expected immediately after 90 ˝ C steam curing, while only 60% of 190 MPa was attained at the importance of moistureonly in curing. al. achieved [14] reported without 7 days. In comparison, 80% ofAhlborn 190 MPaetwas even atthat 91 the daysstrength under a of dryUHPC condition, the 90 °C steam curing was reduced to 69% and 83% of theetspecified strengththat at the 7 and 28 days, demonstrating the importance of moisture in curing. Ahlborn al. [14] reported strength ˝ respectively. studied the effect of delay theofsteam curing and concluded of UHPCAdditionally, without the 90they C steam curing was reduced to time 69% before and 83% the specified strength at that 7a delay even as longAdditionally, as 10 or 24they days did not affect strength after the and 28time days,ofrespectively. studied the significantly effect of delay time the before the steam and concluded that et a delay time analyzed of even as the longdegree as 10 orof24hydration days did not significantly steamcuring curing. Schachinger al. [15] using nuclear affect magnetic the strength after the of steam curing. Schachinger et al.temperatures [15] analyzedranging the degree of 20 hydration resonance and strength UHPC in terms of curing from to 90 using °C. They nuclear magnetic resonance and strength of UHPC in terms of curing temperatures ranging from 20 to at a showed that the gradual increase of the degree of hydration of silica fume in the specimen cured 90 ˝ C.low Theytemperature showed that the gradual of the degree of hydration of by silica theseveral specimenyears. relatively delayed theincrease specified strength development asfume muchin as cured at a relatively low temperature delayed the specified strength development by as much as several Nakayama et al. [16] also showed that the early-age strength of UHPC decreased as the curing years. Nakayama et al. [16] also showed that the early-age strength of UHPC decreased as the curing temperature lowered. Matsubara et al. [6] applied a heating system that can realize 40 or˝60 °C during temperature lowered. Matsubara et al. [6] applied a heating system that can realize 40 or 60 C during curing of the cast-in-place joint components of a precast UHPC. The precast UHPC used attained curing of the cast-in-place joint components of a precast UHPC. The precast UHPC used attained 180 180 MPaMPa with a curing temperature of 85 °C and duration of 30 h. The same strength level was with a curing temperature of 85 ˝ C and duration of 30 h. The same strength level was achieved achieved the cast-in-place components with temperature a curing temperature or 60 °C for maintained even even in the in cast-in-place components with a curing of 40 or 60of˝ C40maintained 7 days. for ˝ and 40 °C with˝ that at 7 days. Honma al. compared [17] compared the strength development UHPC cured Honma et al.et[17] the strength development of UHPCofcured at 20 and at 4020 C with that at 90 C 90 °C steam curing. difference of strengths eachtemperature curing temperature was atsignificant at 7 and steam curing. The The difference of strengths at each at curing was significant 7 and 28 days, but became less significant at 91 days. Thedays. chemical microstructure during curing and 28 days, but became less significant at 91 Themechanism chemical and mechanism and microstructure during hydration of concrete are presented in some references curing and hydration of concrete are presented in some[18,19]. references [18,19].

Figure 2. Compressive strength development of K-UHPC with different curing conditions [13].

Figure 2. Compressive strength development of K-UHPC with different curing conditions [13]. 3. Curing Tests of UHPC 3. Curing Tests of UHPC 3.1. Test Variables and Preparation of Specimens

3.1. Test Variables and Preparation of Specimens As discussed previously, the quality of UHPC is largely affected by curing conditions, such as curing Astemperature discussedand previously, the quality UHPCpreparing is largely by system curing on conditions, such as moisture condition, etc. of However, the affected steam curing a construction curing and moisture condition, etc. However, preparing theuneconomical steam curing sitetemperature (which is necessary for ensuring rapid strength development) would be andsystem involve on a construction site (which is its necessary foruse ensuring rapid strength development) would bethe uneconomical some difficulties due to temporary during curing and the required movability along casting place of some concrete. Therefore, would be very important to determine an the efficient curing method foralong and involve difficulties dueit to its temporary use during curing and required movability

the casting place of concrete. Therefore, it would be very important to determine an efficient curing

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cast-in-place UHPC by taking into account the site condition, construction period, economy, and required strength of UHPC. In this study, the test variables are determined by relaxing the conditions of the prototype curing method of K-UHPC [4,13], which has been deonstrated to be sufficient for ensuring the specified compressive strength of 180 MPa immediately after curing. The prototype follows the order of initial moist curing for 24 h after casting, form removal, and steam moist curing with 90 ˝ C for 48 h. As shown in Table 2, additional conditions considered in this study are: the lower curing temperatures of 20, 40 and 60 ˝ C; the duration of the initial curing (also called delay time in this study) that is shortened to 12 h or extended to 48 h; the duration of the main curing (also called curing time or continuing time in this study) that is shortened to 12 or 24 h; and the type of moist curing conditions. The moisture conditions during curing are categorized into four types. The enclosed or sealed condition is realized by tightly wrapping the specimen with polyethylene sheet to ensure that the internal moisture does not evaporate. A dry condition is provided by a dry heating chamber, while a constant temperature and humidity chamber as shown in Figure 3 is used to apply the water or steam condition. Table 2. Variables of curing test. Curing Temperature (˝ C)

Delay Time (h)

Continuing Time (h)

Moisture Condition

12 20

24

24 48

enclosed, water

72 12 12

24 48 12

40

24

24 48 12

48

24 48 12

12

24 48 12

60

24

24 48 12

48

24 48

90

24

48 72

dry, enclosed, water, steam

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Figure 3. Constant temperature and humidity chamber. Figure 3. Constant temperature and humidity chamber. The target curing temperature of the chamber is attained at a 15 ˝°C C increase per hour and the same rate of temperature variation is applied when descending. The curing time is evaluated based only on Although moisture is continuously supplied duringduring the initial the period periodofofconstant constanttemperature. temperature. Although moisture is continuously supplied the curing initial period with thewith standard curing method of K-UHPC, considering any adverse situation, it is assumed curing period the standard curing method of K-UHPC, considering any site adverse site situation, it is in this study that study the specimen subjectedistosubjected a dry condition during the initial curing, regardless of assumed in this that the isspecimen to a dry condition during the initial curing, the form removal conducted at 12 h after casting. the Therefore, initial curing also called regardless of the form removal conducted at 12 h Therefore, after casting. theperiod initialiscuring periodthe is delaycalled time in study because curing is notthe actually during this period. In the also thethis delay time in thisthe study because curingperformed is not actually performed during thisenclosed period. condition, however, the specimen is sealed immediately after immediately the form is removed, can is be removed, expected In the enclosed condition, however, the specimen is sealed after theasform on can site.beFurthermore, other moisture conditions last onlyconditions as long aslast the only curing thethe enclosed as expected on while site. Furthermore, while other moisture as time, long as curing condition is maintained until the strength is until measured at 7 daysissince this situation be easily applied time, the enclosed condition is maintained the strength measured at 7 dayscan since this situation can be easily applied on site. Because the purpose of this how studyclosely is to examine how attains closelythe thespecified strength on site. Because the purpose of this study is to examine the strength attains the specified compressive within an early strengths age, 7-day strengths are compressive strength within an earlystrength age, 7-day compressive are compressive measured according to the measured according the standard test method [4,20].strength The average compressive strength the is calculated standard test methodto[4,20]. The average compressive is calculated by averaging strengths by averaging the strengths the test threevariable. specimens each of testthe variable. Theisshape of the with specimen is a of the three specimens for of each Theforshape specimen a cylinder 100 mm cylinder 100 mm mmheight diameter and 200to mm height according to the relevant diameter with and 200 according the relevant specifications [4,21]. As specifications will be shown[4,21]. in the As will be shown in the themost laterwidely-used part, these are the most widely-used comparison provided in comparison the later part,provided these areinthe dimensions, as far as thedimensions, cylindrical as far as the cylindrical shape aisspecimen concerned. Although specimen withindifferent size was used in shape is concerned. Although with differenta size was used some previous studies, thesome size previous studies, size not exceed 150 mm diameter andIt300 in height atassumed most. Itthat canthe be did not exceed 150the mm in did diameter and 300 mm in in height at most. canmm be sufficiently sufficiently assumedofthat thesmall-sized internal temperature of these small-sized specimens used inorpractice, internal temperature these specimens used in practice, whether it is a cylinder a cube, whether it is adistributed cylinder or a cube, istouniformly distributed according toand, the ambient is uniformly according the ambient curing temperature; thus, thecuring effect temperature; of the shape and, thus,ofthe effect of on thethe shape and size distribution of a specimen the temperature distributionis and related and size a specimen temperature andon related strength development negligible. strength development is negligible. components of K-UHPC using a dedicated mixer The components of K-UHPC were The mixed using a dedicated mixerwere that mixed was developed for UHPC [8]. that was specimens developedwere for UHPC test specimens prepared [4,21] by following related The test prepared[8]. by The following the related were specifications in terms the of placing, specifications in terms of placing, consolidation, finishing, and ensuring plane ends. consolidation, [4,21] finishing, and ensuring plane ends. The test variables variables of ofthis thiscuring curingtest testbasically basicallyinclude includefour fourcases casesofof curing temperature, three cases The test curing temperature, three cases of of delay before the initiation of main curing, three of cases main time curing four cases of delay timetime before the initiation of main curing, three cases mainofcuring andtime fourand cases of moisture moisture are summarized in Table 4 with for explanations for several condition.condition. These are These summarized in Table 2 and Figure2 4and withFigure explanations several abbreviations. abbreviations. In Figure 4, if one of“M”, the letters “T”,“CT” “M”,is“DT” is given as ittest is, variables then the In Figure 4, if one of the letters “T”, “DT” and givenand as it“CT” is, then the entire entire to this letter arecorresponding included in thecase. corresponding case. T-M-24-48 For example, T-M-24-48 relatedtest to variables this letterrelated are included in the For example, indicates the indicates for 48from h starting from casting, 24 h after casting, all the curing and cases thatthe arecases curedthat forare 48cured h starting 24 h after with all thewith curing times andtimes moisture moisture conditionsconditions included. included.

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Figure 4. Abbreviations of variables of curing test. Figure 4. Abbreviations of variables of curing test. Effect of Curing Temperature 3.2. Effect

The average compressive strength of the specimens made with standard steam curing (24 h initial ˝ C) was was 201.0 201.0 MPa MPa as shown in Table 3, which is 112% curing and subsequent 48 h main curing at 90 °C) of the specified strength of 180 MPa. The The test test results, results, as affected by various curing temperatures, with other conditions remaining the same as those of the standard steam curing, are presented in Table 3. As decreased, the the average average compressive compressive strengths strengths also also reduced, reduced, which which results results in 97%, the curing temperature temperature decreased, ˝ C, respectively. 76% and 61% of the specified strength for the curing temperatures of 60, 60, 40 40 and and 20 20 °C, respectively. Table 3. Compressive Compressive strengths according to curing temperatures (T-S-24-48). (T-S-24-48). Table Variable Variable

Average CompressiveStandard CompressiveStrength Strength(MPa) (MPa) Average Standard Deviation Compressive Compressive Deviation Strength (MPa) Strength (MPa) (MPa)(MPa) #1#1 #2 #3 #2 #3

2-W-24-48 2-W-24-48 * * 4-S-24-48 4-S-24-48 6-S-24-48 6-S-24-48 9-S-24-48 9-S-24-48

116.7 116.7 142.3 142.3 179.9 179.9 200.6 200.6

110.2 110.2 133.6 133.6 170.9 170.9 201.5 201.5

101.0 101.0 133.0 133.0 171.4 171.4 200.8 200.8

109.3 109.3 136.3 136.3 174.1 174.1 201.0 201.0

6.44 6.44 4.25 4.25 4.13 4.13 0.39 0.39

** Water Water curing curing is is provided provided instead instead of of steam steam curing curing in in this this case. case.

Figure Figure 55 shows shows the the characteristics characteristics of of strength strength development development according according to to curing curing temperature temperature and and moisture condition with other conditions remaining the same. The compressive strength was moisture condition with other conditions remaining the same. The compressive strength was proportional proportional to the curingregardless temperature, of the moisture condition. Overall, the enclosed to the curing temperature, of theregardless moisture condition. Overall, the enclosed condition resulted condition resulted in fairly good strength development when compared with other moisture conditions, in fairly good strength development when compared with other moisture conditions, especially at lower especially at although lower temperatures, althoughwere onlytaken passive measures were taken to prevent the temperatures, only passive measures to prevent the evaporation of water in concrete. evaporation water in concrete. As condition mentioned previously, condition was maintained As mentionedofpreviously, the enclosed was maintainedthe untilenclosed the strength was measured at 7 days, until the strength was measured at 7 days, so that the remaining water in the concrete could be used for so that the remaining water in the concrete could be used for hydration and strength development. hydration development. other moisture conditions were maintained during However, and otherstrength moisture conditions However, were maintained only during the curing time, whichonly means the the curing time, which means the specimens were exposed to a dry environment during the remaining specimens were exposed to a dry environment during the remaining time before strength measurement. time before strength measurement. As a result, for a curing temperature of 40 °C, the highest As a result, for a curing temperature of 40 ˝ C, the highest compressive strength was 149.0 MPa in compressive strength was 149.0 MPa in the enclosed condition, while the lowest was 131.7 MPa in the dry the enclosed condition, while the lowest was 131.7 MPa in the dry condition. The strength obtained condition. The strength obtained from the enclosed condition was approximately 10% higher than that from the enclosed condition was approximately 10% higher than that of other moisture conditions of other moisture conditions for the curing temperature of 40 °C. Even for higher temperatures, the for the curing temperature of 40 ˝ C. Even for higher temperatures, the strength level in the enclosed strength level in the enclosed condition was still high as it was only 2.4% and 1.0% lower than the condition was still high as it was only 2.4% and 1.0% lower than the highest strengths obtained in steam

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highest strengths obtained in steam curing at 60 and 90 °C, respectively. Therefore, for strength curing at 60 of andK-UHPC, 90 ˝ C, respectively. development of K-UHPC, alsoconcrete effective by development it is also Therefore, effective tofortrystrength to maintain the water containeditinis the to try to the maintain thewith water containedsuch in theasconcrete by wrapping the surface with a material suchwater as wrapping surface a material polyethylene sheet instead of providing an active polyethylene sheet instead of providing an active water supply to the concrete. In feasible, other words, whenmost a supply to the concrete. In other words, when a continuous supply of water is not the next continuous supply is not the next most effective strategy is to protect surfaceeffect from on effective strategy is of to water protect the feasible, surface from evaporation. A dry condition had anthe adverse condition had an adverse effect the strength allwater rangesneeded of temperature due to the theevaporation. strength in A alldry ranges of temperature due to the on evaporation ofinthe for hydration. evaporation water needed Therefore, of it the is apparent that for thehydration. strength development of K-UHPC is accelerated as the curing Therefore, it is apparent that the strength development of presented K-UHPC in is accelerated the attain curingthe temperature increases. Although the 7-day strength of 60 °C Figure 5 didas not ˝ temperature increases.strength, Although 7-day strength of 60 C presented in Figure didconsiderable. not attain theAs specified compressive thethedifference between these two strengths was 5not specified compressive strength, the difference between these two strengths was not considerable. As will will be shown later, in some other cases cured at 60 °C, the specified strength was exceeded by be shown later, in some other cases cured at 60 ˝ C, the specified strength was exceeded by adjusting the adjusting the delay time. delay time. It is also important to estimate when the specified strength is attained for cases where the specified It is also important to estimate when the specified strength is attained for cases where the specified strength is not reached in 7 days. Several of the cases exceeded the specified strength at 28 days, as strength is not reached in 7 days. Several of the cases exceeded the specified strength at 28 days, as will will be discussed later. Many of the other cases may eventually attain the specified strength, as can be be discussed later. Many of the other cases may eventually attain the specified strength, as can be seen seen in Figure 2 and as confirmed in previous studies. Whether or not the delayed strength in Figure 2 and as confirmed in previous studies. Whether or not the delayed strength development is development is acceptable depends on the progress schedule of the site and construction period. acceptable depends on the progress schedule of the site and construction period. Figure 5 shows an almost linear strength development for all moisture conditions. Therefore, the Figure 5 shows an almost linear strength development for all moisture conditions. Therefore, the following result of of regression regressionanalysis analysisthat thatrelate relateearly-age early-age followingpredictive predictiveequations equations are are proposed proposed as as aa result compressive forenclosed enclosedand andwater water conditions: compressivestrength strengthtotocuring curingtemperature, temperature, representatively representatively for conditions: f /= f T =90 0.0062T + 0.4627 (20 °C ≤ T ≤ 90 °C, for enclosed condition) fc7c7{fc7,Tc7,=90 “ 0.0062T ` 0.4627 (20 ˝ C ď T ď 90 ˝ C, for enclosed condition) f /= f T =90 0.0065T + 0.4246 (20 °C ≤ T ≤ 90 °C, for water condition) fc7c7{fc7,Tc7,=90 “ 0.0065T ` 0.4246 (20 ˝ C ď T ď 90 ˝ C, for water condition)

(1)(1) (2)(2)

where fc7 fisc7 the 7-day compressive strength (MPa), fc7, Tf c7, compressive strength for afor90a°C = 90T =is90the where is the 7-day compressive strength (MPa), is 7-day the 7-day compressive strength ˝ ˝ curing (MPa), (MPa), and T is temperature (°C). (These equations are are normalized with 90 Ctemperature curing temperature andcuring T is curing temperature C). These equations normalized 2 2 respect to fc7, T to The coefficients of determination (R ) of(REquations (1) and high as 0.9675 with respect . The coefficients of determination ) of Equations (1)(2) andare (2)asare as high as = 90f.c7, T = 90 and respectively, 0.9955, respectively, indicates linear regressioncan canprovide provide aa sufficiently and0.9675 0.9955, which which indicates that that linear regression sufficientlyreliable reliable estimationofofthe the strength. strength. Similarly, equation can be for other delay of estimation Similarly,such sucha predictive a predictive equation canproposed be proposed forcases otherofcases time,time, curing time,time, and moisture conditions. delay curing and moisture conditions.

Figure 5. Compressive strengths according to curing temperatures (T-M-24-48).

Figure 5. Compressive strengths according to curing temperatures (T-M-24-48).

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3.3. 3.3. Effect Effect of of Curing Curing Time Time The increasing the length of time the main curing on curing strengthon development is investigatedis Theeffect effectof of increasing the length ofoftime of the main strength development in this section.in Figure 6 shows the average of specimens by curing time that were investigated this section. Figure 6 showsstrengths the average strengthsasofaffected specimens as affected by curing cured at each temperature with the delay timewith fixedthe as 24 h. For delay moisture time that were cured at each temperature delay timethis fixed astime, 24 h.regardless For this of delay time, ˝ condition, the specimens of while 7-day the strength curedofat 7-day 90 Cstrength exceeded the specified regardless while of moisture condition, specimens cured at 90 °C strength, exceeded the the specimens cured at the lower temperatures not reach the specified 180 MPa, even with of a specified strength, specimens cureddid at lower temperatures did strength not reachofthe specified strength ˝ curing timeeven of 48with h. aHowever, the of specimens that were at 60 that C almost attained the °C specified 180 MPa, curing time 48 h. However, thecured specimens were cured at 60 almost strength, as shown in Figure 6c. Therefore, it would be possible for these specimens to reach the specified attained the specified strength, as shown in Figure 6c. Therefore, it would be possible for these strength with such as a slightly increased curing time an adjustment the delay specimens to some reach measures, the specified strength with some measures, such as and a slightly increasedofcuring time time, asadjustment will be presented later. time, as will be presented later. and an of the delay

Figure 6.6. Compressive (T-M-24-CT). Figure Compressive strengths strengths according according toto curing curing times times (T-M-24-CT). (a)Curing Curingtemperature temperature °C (2-M-24-CT). (b)temperature Curing temperature = 40 °C (a) = 20=˝ C20 (2-M-24-CT). (b) Curing = 40 ˝ C (4-M-24-CT). (4-M-24-CT). (c) Curing temperature = 60 °C(d) (6-M-24-CT). (d) Curing temperature = 90 °C ˝ ˝ (c) Curing temperature = 60 C (6-M-24-CT). Curing temperature = 90 C (9-M-24-CT). (9-M-24-CT). ˝ In C, the In the the case case of of specimens specimens cured cured at at 20 20 °C, the strength strength increase increase according according to to curing curing time time was was so so marginal that the specified strength could not be attained within 7 days. Referring to the previous marginal that the specified strength could not be attained within 7 days. Referring to the previous study ˝ study on K-UHPC in Figure when applying a temperature ofthe 20 specified C, the specified on K-UHPC shownshown in Figure 2, when2,applying a temperature of 20 °C, strength strength can only can only be ensured in the long term with a continuous supply of moisture. The strengths obtained be ensured in the long term with a continuous supply of moisture. The strengths obtained from

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from enclosed and water conditions at each curing time do not significantly differ in the case of 20 ˝ C. As mentioned previously, the enclosed condition that can be relatively easily realized on site provides a better treatment of moisture and strength development in this study, regardless of curing temperature, as shown in Figure 6. Although the specified strength was not reached at 40 and 60 ˝ C with the curing times considered, a regression equation can be used to estimate the appropriate curing time if the specified strength is to be ensured in 7 days. For the enclosed condition that shows a better strength development, it appears that the linear regression can provide the best fit, resulting in the following equations: fc7 “ 0.5972t ` 121.82 (for curing temperature of 40 ˝ C)

(3)

fc7 “ 0.4158t ` 149.57 (for curing temperature of 60 ˝ C)

(4)

where f c7 is the 7-day compressive strength (MPa) and t is the curing time (h). R2 of Equations (3) and (4) are 0.8885 and 0.9907, respectively, with sufficient accuracy. According to Equations (3) and (4), the specified strength can be ensured in 7 days with the curing time of 97 and 73 h, i.e., approximately 4 and 3 days, for curing temperatures of 40 and 60 ˝ C, respectively. Matsubara et al. [6] stated that 180 MPa was attained after 7 days curing at 40 or 60 ˝ C. Consequently, the K-UHPC of this study shows a better strength development performance even with a shorter curing time when compared with the UHPC developed by Matsubara et al. [6]. At 90 ˝ C, increasing the curing time from the standard 48 to 72 h was also effective, except for the dry condition. In contrast, for the dry condition, the strength slightly decreased due to the excessive evaporation of internal water which induced drying and micro-cracks under a very high temperature [13]. Figure 7 shows the relationship between strength and curing time for delay times other than 24 h. At a shorter delay time of 12 h, which corresponds to the immediate initiation of curing after form removal, the strength was higher in the water condition than that in the enclosed condition, especially for that shown in Figure 7a at 40 ˝ C. It is because the advantage of the enclosed condition is less distinct when the delay time is short. However, the enclosed condition resulted in better strengths in the delay time of 48 h (Figure 7b,d) than other moisture conditions, as did in the delay time of 24 h (Figure 6b,c). As shown in Figure 7b, at a curing temperature of 40 ˝ C, the longer delay time of 48 h meant that the effect of increasing the curing time on strength development was less clear. Figures 6c and 7c,d show that the strength development at 60 ˝ C curing in the enclosed condition is quite satisfactory; the specified compressive strength can almost be reached or can even be exceeded in 7 days with the curing time of 48 h adopted in the standard steam curing of K-UHPC. On the other hand, it is estimated that at a curing temperature of 40 ˝ C, a few days curing time may be needed to reach the specified strength in 7 days, as shown in Figures 6b and 7a.

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Figure 7. 7. Compressive (a) Curing Curing Figure Compressive strengths strengths according according to to curing curing times times (T-M-DT-CT). (T-M-DT-CT). (a) temperature == 40 ˝ and delay time = 12 h (4-M-12-CT). (b) Curing temperature = 40 °C and temperature 40°C C and delay time = 12 h (4-M-12-CT). (b) Curing temperature = 40 ˝ C delaydelay timetime = 48= 48 h (4-M-48-CT). (c) (c) Curing temperature == 6060°C˝ Cand and h (4-M-48-CT). Curing temperature anddelay delaytime time == 12 12 hh (6-M-12-CT). (d) time = 48 h (6-M-48-CT). (6-M-12-CT). (d)Curing Curingtemperature temperature==6060°C˝ Cand anddelay delay time = 48 h (6-M-48-CT). 3.4. Effect of Delay Time 3.4. Effect of Delay Time According to a previous study on the standard steam curing of K-UHPC [13], if the delay time According to a previous study on the standard steam curing of K-UHPC [13], if the delay time before before high-temperature curing of 90 °C is too short, strength degradation may possibly occur because high-temperature of to 90a˝high C is too short, strength possibly because cement hydrate is curing exposed temperature beforedegradation it hardens tomay form a tight occur structure and iscement prone hydrate is exposed to a high temperature before it hardens to form a tight structure and is prone to to internal micro-cracks. The reason why the delay time or the initial curing period before the main internal micro-cracks. why delay time the Honma initial curing period before the that main steam curing is set at 24The h is reason based on thisthe previous studyor[13]. et al. [17] demonstrated if steam curing is set at 24 h is based on this previous study [13]. Honma et al. [17] demonstrated the high-temperature curing initiates after the initial setting that corresponds to the penetration that if the of high-temperature initiates after theregardless initial setting corresponds to UHPC the penetration resistance 5 MPa, similar curing strengths are obtained of thethat delay time in the with the resistance of 5 MPa, similar strengths are obtained regardless of the delay time in the UHPC with the specified compressive strength of 150–200 MPa. In comparison, the initial setting of concrete mixture specified 150–200 MPa. In comparison, initial settingtoofASTM concrete mixture is definedcompressive by the time strength when theofpenetration resistance equals 3.5 the MPa according C403 [22]. is defined by the time when the penetration resistance equals 3.5 MPa according to ASTM C403 [22]. Ahlborn et al. [14] reported that a delay time as long as 10 or 24 days before high-temperature curing Ahlborn et al. [14] reported that a of delay time as long as the 10 or 24 days curing at 90 °C induced a slight decrease strength. However, delay timesbefore of thishigh-temperature study are the shortest at ˝ at C induced a slight decrease of strength. However, times ofatthis study areare thenot shortest at 1290 h when the initial setting time has already passed, andthe aredelay the longest 48 h, which as long 12 h when initial setting has already passed, are the longest at 48 temperatures h, which are that not as as the delaythe time of the studytime by Ahlborn et al. [14]. and Furthermore, the curing arelong the as the delay time of the study by Ahlborn et al. [14]. Furthermore, the curing temperatures that are the main focus in this study are 40 and 60 °C, which are lower than the curing temperature of 90 °C main focus in this study are 40 and 60 ˝ C, which are lower than the curing temperature of 90 ˝ C adopted

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in the above studies carried on the the delay a result, the effect the adopted in theprevious above previous studiesout carried outeffect on theofeffect of thetime. delayAs time. As a result, the of effect delay strength development is not distinct in this in study. of the time delayontime on strength development is not distinct this study. As seen in Figure 8, with the curing time of 48 h adopted for standard curing, andthe with the As seen in Figure 8, with the curing time of 48 h adopted for standard steamsteam curing, and with curing ˝ curing temperature 40°C, or 60 the effect the delay time ranging on strength the strength temperature of 40 orof60 the C, effect of theof delay time ranging from from 12 to12 48toh 48 onhthe waswas not is, increasing the delay result a consistent increase or decrease of the strength. not clear. clear. ThatThat is, increasing the delay timetime did did not not result in ainconsistent increase or decrease of the strength.

Figure 8. Compressive Compressive strengths strengthsaccording accordingtotodelay delay times before curing (T-M-DT-48). Figure 8. times before curing (T-M-DT-48). (a) (a) Curing temperature = 40 ˝ °C (4-M-DT-48). (b) Curing temperature = 60˝°C (6-M-DT-48). Curing temperature = 40 C (4-M-DT-48). (b) Curing temperature = 60 C (6-M-DT-48). 3.5. Discussion 3.5. Discussion The results of this study and the relevant previous studies are compared in Table 4. The specified The results of this study and the relevant previous studies are compared in Table 4. The specified compressive strengths of the UHPC are considered to be in the range from 150 to 200 MPa. compressive strengths of the UHPC are considered to be in the range from 150 to 200 MPa. The comparison is limited to the lower curing temperatures, ranging from 20 to 70 °C, than usual The comparison is limited to the lower curing temperatures, ranging from 20 to 70 ˝ C, than usual steam steam curing temperature of 90 °C. Also, the development of early-age strength of less than or equal to curing temperature of 90 ˝ C. Also, the development of early-age strength of less than or equal to 7 days 7 days is compared, according to the focus of this study. It can be identified that this study took more is compared, according to thethe focus of thisstudies study. It be identified thataspects this study took more variables variables or parameters than previous tocan investigate various regarding the curing of or parameters than the previous studies to investigate various aspects regarding the curing of UHPC. UHPC. It is notable that in a few studies including this study, the specified compressive strength could It isattained notable that a few as studies including this study, the specified strength be attained be in asinearly 7 days, with careful consideration ofcompressive the temperature andcould duration of the in as early as 7 days, with careful consideration of the temperature and duration of the curing. In order to curing. In order to achieve this purpose, at least 40–60 °C was required as curing temperature, whereas ˝ achieve this purpose, least 40–60 was required as The curing temperature, room temperature at room temperature at at around 20 °C C was insufficient. shape and size whereas of the specimens of various ˝ around 20 C was insufficient. The shape and size of the specimens of various studies are also compared studies are also compared in Table 4. in Table 4. In addition to the early-age strengths measured at 7 days, 28-day strengths were also measured, In addition to the the early-age strengths at 7 of days, 28-day strengths were also although they are not main focus of thismeasured study. Some the specimens that were cured at measured, 60 °C and although they are not thethemain focus compressive of this study.strength Some of specimens curedstrength at 60 ˝ C that closely approached specified at 7the days exceededthat thewere specified at and that closely approached the specified compressive strength at 7 days exceeded the specified strength 28 days with the strength increase of 4.3% on average, which included 6-E-12-48, 6-W-12-48 and at 28 days with strength increase 4.3% on average, which included 6-E-12-48, 6-W-12-48 and 6-S-12-48. Otherthe specimens required of a longer term to attain the specified strength. When comparing 6-S-12-48. specimens required aamong longer the termdifferent to attain studies, the specified strength. comparing the the results Other of strength measurement the length of When time the curing was results of strength amongathe different studies, the length time the curing was maintained maintained shouldmeasurement be noted to ensure reasonable comparison. For aof long-term strength measurement should be noted to some ensurestudies a reasonable a long-term strength measurement at 28 days or at 28 days or later, appliedcomparison. moist curing For continuously until the strength was measured [3,13], later, some applied moistthis curing continuously until the strength was measured [3,13], while while otherstudies studies, including study, maintained moist curing only for a short period [1].other The studies,developed including this study, et maintained moist curing only for aatshort period [1]. continuously The UHPC developed UHPC by Wille al. [3] attained 190–200 MPa 28 days when stored in

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by Wille et al. [3] attained 190–200 MPa at 28 days when continuously stored in water at 20 ˝ C and the K-UHPC of this study reached 190 MPa at 91 days in a similar condition [13]. The UHPC developed in the Sustainable and Advanced Materials for Road InfraStructure (SAMARIS) project [1] achieved 182 MPa at 28 days in ambient temperature with 8 days of moist curing, although around 150 MPa was attained at 7 or 14 days. However, the previous studies mentioned above focused on a long-term strength of at least 28 days; therefore, because this study deals with the early-age strength of UHPC at 7 days, it can provide distinct and useful data for UHPC cast in-place for field application. Table 4. Comparison of the test results with previous studies on UHPC. Shape and Size of Specimen (mm)

Specified Water-to-Binder Compressive Ratio/Mineral Strength Admixture (MPa) (% of Binder)

Study

Variables

This study

Curing temperature, delay time, continuing time, moisture condition

Cylinder (Φ 100 ˆ 200)

180

0.2/Silica fume (20%)

SAMARIS [1]

-

Cylinder (Φ 110 ˆ 220)

180

0.123/Silica fume (21%)

Ishii et al. [5]

Curing temperature

Not specified

180

Not specified

Matsubara et al. [6]

Curing temperature

Cylinder (Φ 100 ˆ 200)

180

0.152/Silica fume (not specified)

Cylinder (Φ 76 ˆ 152)

200

Not specified

Cylinder (Φ 100 ˆ 200)

150

0.15/Silica fume (15%)

Cylinder (Φ 100 ˆ 200)

150–200

0.12–0.2/Silica fume (10%–20%)

Ahlborn et al. [14] Nakayama et al. [16]

Honma et al. [17]

Curing temperature, delay time Curing temperature, delay time, continuing time

Curing temperature

Early-Age Strength Development (Curing Condition) 180 MPa in 7 days (40 ˝ C for 4 days or 60 ˝ C for 2 days in moist condition) 150 MPa in 7 days (20 ˝ C for 7 days in moist condition) 147 MPa in 4 days (70 ˝ C for 2 days) 180 MPa in 7 days (40 ˝ C or 60 ˝ C for 7 days) 137 MPa in 7 days (20 ˝ C for 7 days) 135 MPa in 7 days (60 ˝ C for 3 h in moist condition) 90–100 MPa in 7 days (20 ˝ C for 7 days in moist condition) and 130–170 MPa in 7 days (40 ˝ C for 7 days in moist condition)

According to the experimental results of this study, it would not be possible to ensure the specified compressive strength of K-UHPC within 7 days with a curing temperature of 20 ˝ C, regardless of the moist curing method employed. The minimum condition to ensure the specified strength at 7 days derived in this study is a curing period of 48 h (2 days) with a temperature of 60 ˝ C under a moist condition, and with the delay time of 12 to 48 h before the curing begins. A maximum of 3 days curing period would be sufficient to ensure the specified strength in 7 days. A certain type of heat treatment would be required to ensure a curing temperature of 60 ˝ C, such as a heating system that is easily available on site as adopted by Matsubara et al. [6] in an actual pedestrian bridge. On the other hand,

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at a curing temperature of 40 ˝ C, at least a 4 days curing period is required to attain the same strength level. In the hot weather conditions in some countries, an environmental temperature of 40 ˝ C may be obtained simply by covering the structure with a thick and tight plastic sheet after casting, even though a heating facility is not available on site. 4. Conclusions This study presented experimental results on the effect of various curing conditions of K-UHPC on early-age strength development in order to increase the field applicability of K-UHPC. Based on the results of the foregoing investigation, the following conclusions can be drawn: ‚ As the curing temperature increased, the development of the compressive strength of K-UHPC was accelerated. The 7-day strength was almost linearly proportional to the curing temperature, which ranged from 20 to 90 ˝ C. Some specimens cured at 60 ˝ C for 48 h attained 180 MPa in 7 days, which is the specified compressive strength of K-UHPC, under the condition that proper delay time along with careful moisture treatment are provided. A dry condition should be avoided, especially in UHPC based on a low water-to-binder ratio. The enclosed condition where the concrete surface is covered with a thin plastic sheet was also effective when compared with moist curing. ‚ The strength development of K-UHPC was also proportional to the curing period, regardless of the curing temperature. In order to ensure the specified strength in 7 days, the curing time of 48–72 h was appropriate for a curing temperature of 60 ˝ C, while a longer period of at least 96 h may be necessary for 40 ˝ C based on the regression analysis. However, the specified strength is not attained in 7 days if a curing temperature of 20 ˝ C is maintained even until strength measurement. ‚ The effect of delay time ranging from 12 to 48 h prior to initiation of the main curing on the strength development of K-UHPC was not clear, regardless of the curing temperature. In field application, therefore, the delay time would not have an adverse effect on the strength unless it is either too long or too short. Acknowledgments This research was supported by a grant (13SCIPA02) from Smart Civil Infrastructure Research Program funded by Ministry of Land, Infrastructure and Transport (MOLIT) of Korea government and Korea Agency for Infrastructure Technology Advancement (KAIA). Author Contributions Jong-Sup Park conceived, designed and performed the experiments. Young Jin Kim supervised this project as a principal investigator. Jeong-Rae Cho contributed to the literature review for this study. Se-Jin Jeon analyzed the data and wrote the paper.

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Conflicts of Interest The authors declare no conflict of interest. References 1. Denarié, E.; Brühwiler, E.; Znidaric, A. Full scale application of UHPFRC for the rehabilitation of bridges—from the lab to the field. 2005. Available online: http://samaris.zag.si/ (accessed on 11 August 2015). 2. Richard, P.; Cheyrezy, M. Composition of reactive powder concretes. Cem. Concr. Res. 1995, 25, 1501–1511. [CrossRef] 3. Wille, K.; Naaman, A.E.; Parra-Montesinos, G.J. Ultra-high performance concrete with compressive strength exceeding 150 MPa (22 ksi): A simpler way. ACI Mater. J. 2011, 108, 46–54. 4. Korea Concrete Institute. Design Recommendations for Ultra-High Performance Concrete K-UHPC; KCI-M-12-003. Korea Concrete Institute: Seoul, Korea, 2012. 5. Ishii, T.; Nishio, H.; Matsuyama, T.; Miyajima, A.; Yokohata, K.; Gotou, M.; Xin, J.; Hirai, Y. Manufacture and construction of a PC through girder type pedestrian bridge using ultra high strength fiber reinforced concrete. In Proceedings of 8th International Symposium on Utilization of High-Strength and High-Performance Concrete, Tokyo, Japan, 27–29 October 2008; pp. 1270–1275. 6. Matsubara, N.; Ohno, T.; Sakai, G.; Watanabe, Y.; Ishii, S.; Ashida, M. Application of a new type of ultra high strength fiber reinforced concrete to a prestressed concrete bridge. In Proceedings of 2nd International Symposium on Ultra High Performance Concrete, Kassel, Germany, 5–7 March 2008; pp. 787–794. 7. Brühwiler, E.; Denarié, E. Rehabilitation of concrete structures using ultra-high performance fibre reinforced concrete. In Proceedings of 2nd International Symposium on Ultra High Performance Concrete, Kassel, Germany, 5–7 March 2008; pp. 895–902. 8. Korea Institute of Civil Engineering and Building Technology. Development of Design and Construction System Technology for Hybrid Cable Stayed Bridge; KICT 2012-075. Korea Institute of Civil Engineering and Building Technology: Goyang-si, Korea, 2012. 9. Koh, K.; Ryu, G.; Kang, S.; Park, J.; Kim, S. Shrinkage properties of ultra-high performance concrete (UHPC). Adv. Sci. Lett. 2011, 4, 948–952. [CrossRef] 10. Carino, N.J.; Lew, H.S.; Volz, C.K. Early age temperature effects on concrete strength prediction by the maturity method. Am. Concr. Inst. J. Proc. 1983, 80, 93–101. 11. American Concrete Institute. Guide for the Use of Silica Fume in Concrete; ACI 234R-96. American Concrete Institute: Farmington Hills, MI, USA, 1996. 12. Holland, T.C. Silica Fume User’s Manual; Federal Highway Administration: Washington, DC, USA, 2005. 13. Koh, K.T.; Park, J.J.; Ryu, G.S.; Kang, S.T. Effect of the compressive strength of ultra-high strength steel fiber reinforced cementitious composites on curing method. J. Korean Soc. Civ. Eng. 2007, 27, 427–432.

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14. Ahlborn, T.M.; Mission, D.L.; Peuse, E.J.; Gilbertson, C.G. Durability and strength characterization of ultra-high performance concrete under variable curing regimes. In Proceedings of 2nd International Symposium on Ultra High Performance Concrete, Kassel, Germany, 5–7 March 2008; pp. 197–204. 15. Schachinger, I.; Hilbig, H.; Stengel, T. Effect of curing temperature at an early age on the long-term strength development of UHPC. In Proceedings of the 2nd International Symposium on Ultra High Performance Concrete, Kassel, Germany, 5–7 March 2008; pp. 205–212. 16. Nakayama, H.; Ishinaka, M.; Naruse, H.; Fujii, K.; Nakase, H. Development of processing technology of super high-strength precast concrete column. In Proceedings of 8th International Symposium on Utilization of High-Strength and High-Performance Concrete, Tokyo, Japan, 27–29 October 2008; pp. 959–966. 17. Honma, D.; Kojima, M.; Mitsui, K. Curing methods and strength development of ultra-high strength concrete with 150–200 N/mm2 . Proc. Jpn. Concr. Inst. 2012, 34, 1234–1239. 18. Neville, A.M. Properties of Concrete, 4th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 1996. 19. Mehta, P.K.; Monteiro, P.J.M. Concrete: Microstructure, Properties, and Materials, 4th ed.; McGraw-Hill Education: New York, NY, USA, 2013. 20. American Society for Testing and Materials. Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens; ASTM C39/C39M-15a. ASTM International: West Conshohocken, PA, USA, 2015. 21. American Society for Testing and Materials. Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory; ASTM C192/C192M-14. ASTM International: West Conshohocken, PA, USA, 2014. 22. American Society for Testing and Materials. Standard Test Method for Time of Setting of Concrete Mixture by Penetration Resistance; ASTM C403/C403M-08. ASTM International: West Conshohocken, PA, USA, 2008. © 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).

Early-Age Strength of Ultra-High Performance Concrete in Various Curing Conditions.

The strength of Ultra-High Performance Concrete (UHPC) can be sensitively affected by the curing method used. However, in contrast to the precast plan...
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