Effect of Formwork Removal Time Reduction on Construction

applied sciences

Article Eﬀect of Formwork Removal Time Reduction on Construction Productivity Improvement by Mix Design of Early Strength Concrete

1, 2, 3 3, 3 Taegyu Lee † , Jaehyun Lee †, Jinsung Kim , Hyeonggil Choi * and Dong-Eun Lee 1 Department of Fire and Disaster Prevention, Semyung University, 65 Semyung-ro, Jecheon-si, Chungbuk 27136, Korea; [email protected] 2 Department of Safety Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul 01811, Korea; [email protected] 3 School of Architecture, Civil, Environment, and Energy Engineering, Kyungpook National University, 80 Daehakro, Bukgu, Daegu 41566, Korea; [email protected] (J.K.); [email protected] (D.-E.L.) * Correspondence: [email protected]; Tel.: +82-53-950-5596 These authors contributed equally to this work. † Received: 4 September 2020; Accepted: 6 October 2020; Published: 11 October 2020

Abstract: In this study, we examined the effects of cement fineness, SO3 content, an accelerating agent, and chemical admixtures mixed with unit weights of cement on concrete early strength using concrete mixtures. C24 (characteristic value of concrete, 24 MPa) was used in the experiment conducted. Ordinary Portland cement (OPC), high fineness and SO3 OPC (HFS_OPC), and Early Portland cement (EPC) were selected as the study materials. The unit weights of cement were set to OPC 330, 350, and 380. Further, a concrete mixture was prepared with a triethanolamine (TEA)-based chemical admixture to HFS. A raw material analysis was conducted, and the compressive strength, temperature history, and maturity (D h) were examined. Then, the vertical formwork removal time was evaluated · according to the criterion of each country. Finally, the time required to develop concrete strength of 5 MPa was estimated. Results showed that the early strength of concrete mixed with HFS and EPC was greater than that exhibited by concrete with an increased unit weight of cement with OPC. In addition, when HFS was used with EPC, its strength developed early, similar to the trend exhibited by EPC, even at low temperatures.

Keywords: early strength of concrete; cement ﬁneness; SO3 content; accelerating agent; maturity; Ordinary Portland cement

1. Introduction Early strength concrete is used as an elemental technology for reducing construction time by realizing early formwork removal at construction sites [1,2]. Early strength concrete can be investigated via two methods: (1) Appropriate management of the proportions of materials mixed in the concrete, and (2) acceleration of cement hydration by varying the curing temperature [3,4]. It is possible to decrease the setting time of concrete and increase the curing speed by reducing the water to cement (W/C) [5]. For mixtures used in the ﬁeld, a reduction in the W/C ratio increases the design strength by increasing the unit weight of cement. This method can improve the strength of concrete in a certain range when applied to the mixture design of a site. However, the method does not consider economic eﬃciency and cannot be used to improve concrete mixture design [6]. Ordinary Portland cement (OPC) is commonly used in concrete mixtures owing to its chemical stability. However, it exhibits limitations in terms of developing early strength in concrete [7]. Early strength in concrete can be developed using early strength cement, which increases the C3S content and

Appl. Sci. 2020, 10, 7046; doi:10.3390/app10207046 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7046 2 of 15

fineness and decreases the C2S content; thus, for early strength development, early strength cement is better than OPC [8]. Despite the excellent early strength development, the early strength cement has not been commonly used because it is difficult to develop a stable long-term strength, hence reducing the economic efficiency. Bentz et al. [9–13] reported that a higher W/C ratio may have a greater influence on the early strength development and cement with a higher fineness can reduce the formwork removal time under the curing conditions at actual construction sites. It is possible to develop the early strength of cement for which the fineness of OPC is increased 2 to 380 m /kg, and the SO3 content is increased to 3.1% in the scope of the 13–20 ◦C temperature range [14,15]. Rixon et al. [16–18] conducted research on the early strength development of OPC by reducing the setting time using triethanolamine (TEA). Further, according to the research results reported by Aggoun et al. [19], rapid curing occurred when the TEA accelerating agent was used in large quantities. A retarder was also used to control the rapid curing. However, the chemical decomposition of the accelerating agent and retarder was also reported in some cases. These studies have low reproducibility for the actual production of concrete, and they also have limitations in determining economical mixtures by fully considering the climate conditions of local areas. In particular, when the temperature of a local area is irregular and the curing temperature is less than 15 ◦C, accurate strength prediction for determining the formwork removal time [20–24] is difficult. In this study, we examined the influence of unit weight of cement in concrete that uses OPC on early strength development by considering the conditions of construction sites and referring to the concrete mixture of C24 (characteristic value of concrete, 24 MPa). In addition, high fineness and SO3 OPC (HFS_OPC) and early Portland cement (EPC) were selected, for which the fineness and SO3 content were varied considering the economic efficiency of mixture design. Further, the effect of the addition of a polycarboxylic (PC) superplasticizer mixed with an accelerating agent on the acceleration of early strength was analyzed. This study aims to examine the influencing factors of cement for early strength development by curing temperature and to develop the optimal mixture with economic efficiency. In addition, the formwork removal time was predicted, and its influence on the construction productivity was examined. In conclusion, this study aims to contribute toward enhancing the utilization of construction sites by investigating the effects of various concrete material factors, amount of cement, type of cement, chemical admixture, and SO3 on the early strength.

2. Materials and Methods

2.1. Materials Table1 summarizes the chemical composition of the binders by X-ray ﬂuorescence (XRF) analysis. OPC (ASTM type I), HFS_OPC, and EPC (ASTM type III) were used as the concrete binders. For OPC, the density and ﬁneness were 3150 kg/m3 and 330 m2/kg; for HFS, they were 3130 kg/m3 and 380 m2/kg; and for EPC, they were 3160 kg/m3 and 488 m2/kg, respectively.

Table 1. Chemical composition of the used binders by X-ray ﬂuorescence (XRF) analysis.

Chemical Composition (%) Materials L.O.I. (4) CaO Al2O3 SiO2 MgO Fe2O3 SO3 K2O Others OPC (1) 60.34 4.85 19.82 3.83 3.30 2.90 1.08 0.86 3.02 HFS (2) 61.00 4.51 19.22 4.14 3.35 3.13 1.04 0.79 2.82 EPC (3) 61.44 4.72 20.33 2.95 3.42 3.73 0.95 0.79 1.67 (1) (2) (3) OPC: Ordinary Portland cement; HFS: High Fineness and SO3 ordinary Portland cement; EPC: Early Portland cement; (4) L.O.I.: Loss on ignition. Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 15

Appl. Sci.For2020 washed, 10, 7046 sea sand, the fineness, density, and absorption were 2.01, 2600 kg/m3, and 0.79%,3 of 15 respectively. For crushed sand, the fineness, absorption, and density were 3.29, 0.87%, and 2570 kg/m3, respectively. Crushed coarse aggregate with a size of 25 mm, absorption of 0.76%, and density Figure1 shows the results of the sieve analysis curves on the aggregates. Washed sea sand and of 2600 kg/m3 was used as the coarse aggregate. For the chemical admixture, a PC superplasticizer crushed sand were used as the fine aggregates. Further, the fineness modulus of the fine aggregates and a TEA-based PC superplasticizer were used. was 2.84.

(a) (b)

FigureFigure 1. GradationGradation sieve sieve analysis analysis test test result result for forthe aggregates: the aggregates: (a) Fine (a) aggregates Fine aggregates and (b and) coarse (b) coarseaggregates. aggregates.

2.2. ExperimentalFor washed Plan sea sand,and Mix the Proportions ﬁneness, density, and absorption were 2.01, 2600 kg/m3, and 0.79%, respectively. For crushed sand, the ﬁneness, absorption, and density were 3.29, 0.87%, and 2570 kg/m3, Table 2 shows the experimental plan. For the concrete strength, the concrete of C24 was respectively. Crushed coarse aggregate with a size of 25 mm, absorption of 0.76%, and density of referenced, which is most commonly used at construction sites. For C24 (330 P) that used OPC, the 2600 kg/m3 was used as the coarse aggregate. For the chemical admixture, a PC superplasticizer and a W/C was set to 0.50 and the unit weight of cement was set to 330 kg/m2. In general, the unit weight TEA-based PC superplasticizer were used. of cement is increased to secure the early strength of concrete at construction sites. In this study, C27 2.2.(characteristic Experimental value Plan of and concrete Mix Proportions = 27 MPa) and C30 (characteristic value of concrete = 30 MPa) were selected, which increased the unit weight of cement. The unit weight of cement of C27 (350P) and Table2 shows the experimental plan. For the concrete strength, the concrete of C24 was referenced, C30 (380P) was 350 and 380 kg/m2, respectively. which is most commonly used at construction sites. For C24 (330 P) that used OPC, the W/C was set In addition, HFS_OPC and EPC, which increased the fineness and SO3 content of cement, were to 0.50 and the unit weight of cement was set to 330 kg/m2. In general, the unit weight of cement is additionally selected and examined to improve concrete strength considering the economic efficiency increased to secure the early strength of concrete at construction sites. In this study, C27 (characteristic of concrete mixtures. For such concrete mixtures, the unit weight of cement was set as 330 kg/m2, value of concrete = 27 MPa) and C30 (characteristic value of concrete = 30 MPa) were selected, which similar to that of C24. HFS (330P) increased the fineness and SO3 content of OPC, and it is effective in increased the unit weight of cement. The unit weight of cement of C27 (350P) and C30 (380P) was 350 developing the early strength [14,15]. However, as it has limitations in developing the early strength and 380 kg/m2, respectively. of concrete, ePC (early polycarboxylic superplasticizer with TEA) was added to HFS [23].

Table 2. Experimental plan. Table 2. Experimental plan. Unit Weight Curing Mix Cement Unit Chemical Series W/C of Cement TemperatureCuring Evaluation Item MixID. CementType Weight 3 ChemicalAdmixture Series W/C (kg/m ) Temperature(◦C) Evaluation Item ID.330P 0.50Type OPC of Cement 330 Admixture Slump (mm) Chamber(°C) I 350P 0.47(kg/m3) 350 PC Air contents (%) (13 ◦C) 330P380P 0.50 0.43 OPC 330 380 Compressive▪ Slump strength (mm) (MPa) 330P OPC 330 PC (1) - Cylinder Mold (∅100 200) Ⅰ 350P 0.47 350 PC Chamber (13 °C) ▪ Air contents× (%) 330HFS HFS 330 PCRoom Temp. - 18, 24, and 72 h II 380P 0.430.50 380 (2) ▪ Compressive strength (MPa) 330HFS_ePC HFS 330 ePC (20 ◦C) Temperature history (◦C) (1) 330P330EP OPC EPC 330 330PC PC- Cylinder and Maturity Mold (D h)(Ø 100 × 200) 330HFS HFS 330 PC Room Temp. (20 - 18, 24, and· 72 h Ⅱ (1) PC: Polycarboxylic0.50 superplasticizer-based type admixture. (2) ePC: early Polycarboxylic superplasticizer with 330HFS_ePC HFS 330 ePC (2) °C) ▪ Temperature history (°C) triethanolamine (TEA). 330EP EPC 330 PC and Maturity (D∙h) (1) PC: Polycarboxylic superplasticizer-based type admixture. (2) ePC: early Polycarboxylic In addition, HFS_OPC and EPC, which increased the ﬁneness and SO content of cement, were superplasticizer with triethanolamine (TEA). 3 additionally selected and examined to improve concrete strength considering the economic eﬃciency of concrete mixtures. For such concrete mixtures, the unit weight of cement was set as 330 kg/m2,

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similar to that of C24. HFS (330P) increased the fineness and SO3 content of OPC, and it is effective in developing the early strength [14,15]. However, as it has limitations in developing the early strength of concrete, ePC (early polycarboxylic superplasticizer with TEA) was added to HFS [23]. To identify the effect of the curing temperature on concrete strength, chamber (13 ◦C) and water curing (20 ◦C) were examined. The slump (mm) and air content (%) of concrete were evaluated to ensure that they could be used onsite. As for hardened properties, a cylindrical mold (∅100 mm 200 mm) was fabricated, and its × concrete strength was measured at 18 h, 24 h, and 72 h. In addition, the temperature history (◦C) was measured, and the maturity (D h) was calculated based on the temperature history. The time required · to remove the vertical formwork on concrete was estimated at the curing temperature by examining the correlation between the concrete strength and maturity. The formwork removal time on concrete is specified in each country’s regulations. However, its decision criteria differ. This study referred to the Asian (including South Korea and Japan) criteria that present a detailed demolding strength value [24,25]. The strength criterion for the formwork removal of concrete was set as 5 MPa. Table3 shows the mixing proportions on concrete. For the mixing proportions on concrete, their corresponding practical values were used as reference, and the unit weight of water was 165 kg/m2.

Table 3. Mixing proportions on concrete.

Unit Weight (kg/m3) Series Mix W/C S/a (2) PC ePC ID. (1) (3) (4) (5) (6) (7) (8) (B %) (B %) (%) W C HFS EPC S G × × 330P 0.50 48.5 165 330 885 891 0.8 I 350P 0.47 48.5 165 350 876 883 0.8 380P 0.43 48.5 165 380 864 870 0.8 330P 0.50 48.5 165 330 885 891 0.8 330HFS 0.50 48.5 165 330 884 890 0.8 II 330HFS_ePC 0.50 48.5 165 330 884 890 0.8 330EP 0.50 48.5 165 330 885 891 0.8 (1) (2) (3) (4) (5) W/C: Water/Cement; S/a: Sand/aggregates; W: Water; Cement; High ﬁneness and SO3 ordinary Portland cement (6) Early Portland cement; (7) Sea sand + Crushed sand; (8) G: Gravel.

For the fine aggregates, crushed sand and washed sea sand were mixed at a volume ratio of 6:4. For the fresh properties on concrete, the slump was set to 180 25 mm and air content was 4.5 1.5%, ± ± respectively, to apply workability in the construction site. The concretes were mixed using a commercial machine (mixing speed: 5–50 rpm, mixing capacity: 180 L double-axial spiral mixing type, Woojin, Korea). The fine aggregate was added and mixed for 30 s. Then, the binder, water, and coarse aggregate were added and mixed for 30 s. To ensure sufficient workability, a superplasticizer was added and mixed for 30 s. The concrete mix was prepared in 150 s.

2.3. Test Methods

2.3.1. Properties of Raw Materials and Concrete Table4 shows the test methods for properties of the raw materials and concrete. This study attempted to examine the eﬀects of the unit weight of cement and factors related to the cement types on concrete strength development of concrete. The raw material analysis of cement was conducted to investigate the physicochemical properties of cement. Experiments were performed in accordance with ASTM C204 [26] for the particle size distribution of cement, ASTM C114 [27] for XRF, and ASTM C1702 [28] for the heat of hydration. Appl. Sci. 2020, 10, 7046 5 of 15

Table 4. Test methods for engineering properties of raw materials and concrete.

Series Test Item Test Method Particle size distribution (%) ASTM C204 Raw materials X-ray ﬂuorescence ASTM C114 (Cement) Heat of Hydration ASTM C1702 ASTM C873 Mechanical properties analysis Compressive strength (MPa) ASTM C39 (Concrete) Maturity (D h) ASTM C1074 ·

The slump of fresh concrete was evaluated according to ASTM C143/C143M [29], and the air content test method was evaluated according to ASTM C231/C231M-17a [30]. To secure the workability of concrete that arrived at the site, the fresh properties of concrete were evaluated after 60 min. Specimens for compressive strength test were fabricated with dimensions of ∅100 mm 200 mm. The × specimens were cured at 13 ◦C and 20 ◦C using a constant temperature and humidity chamber. The compressive strength of the concrete was calculated after measuring the maximum load using a 300-ton class universal test machine in accordance with ASTM C873 [31] and ASTM C39/C39M [32] at the ages of 18 h, 24 h, and 72 h.

2.3.2. Temperature History and Maturity on Concrete To obtain the temperature history of concrete, the hydration history was measured by embedding K-type thermocouples at the center of concrete specimen. The maturity was calculated as follows in accordance with ASTM C1074 [33]: X M(t) = (Ta T )∆t, (1) − 0 where M(t) = the temperature-time factor at age t (degree-days or degree-h), ∆t = time interval (days or h), Ta = average concrete temperature during time interval (∆t, ◦C), and T0 = datum temperature (◦C).

3. Results and Discussion

3.1. Fresh and Hardened Properties on Concrete Table5 shows the fresh properties on concrete. When all the concrete mixtures were evaluated, the slump could meet the target range of 180 25 mm both at the beginning and after 60 min. The air ± content of concrete met range of 4.5 1.5% for all the mixtures, and its reduction was not signiﬁcant ± even after 60 min.

Table 5. Fresh properties on concrete.

Slump (mm) Air Content (%) Mix ID. Initial After 60 min. Initial After 60 min. 330P 190 175 5.4 5.0 350P 195 185 4.8 4.4 380P 195 180 4.3 3.9 330HFS 200 185 4.4 4.0 330HFS_ePC 205 190 5.8 5.4 330EP 195 175 5.5 5.0

Figure2 shows the concrete strength at early age. As for the concrete strength that used OPC according to the unit weight of cement, no diﬀerence between the strength developments of 330P and 350P was reported. For 380P, the strength continuously increased even at 13 ◦C. At 20 ◦C, the concrete strength rate was found to be similar for all the specimens until 24 h. However, 380P showed a higher Appl.Appl. Sci. Sci.2020 2020, 10, 10, 7046, x FOR PEER REVIEW 66 of of 15 15

strength rate was found to be similar for all the specimens until 24 h. However, 380P showed a higher strengthstrength development development after after 72 72 h. h. In In addition, addition, for for the the concrete concrete mixtures mixtures that that used used OPC, OPC, a a compressive compressive strengthstrength of of 5 5 MPa MPa was was developed developed after after 24 24 h h and and 72 72 h h in in the the case case of of 20 20◦ C°C and and 13 13◦ C,°C, especially. especially.

(a) (b)

FigureFigure 2. 2.Concrete Concrete strength strength development development at at early early age: age: (a(a)) Cement Cement amount amount and and ( b(b)) cement cement type type and and chemicalchemical admixture. admixture.

ForFor 330HFS, 330HFS, thethe concreteconcrete strengthstrength waswas slightly higher compared to to OPC OPC until until 24 24 h h.. H However,owever, it itsignificantly significantly increased increased and and exceeded exceeded 10 10 MPa MPa at at 72 72 h h undunderer the curing curing condition condition at at 13 13 ◦ °C.C. For For 330HFS_EPC,330HFS_EPC, the the compressive compressive strengthstrength waswas 2.72.7 MPa MPa at at 18 18 h h and and close close to to 5 5 MPa MPa at at 24 24 h. h. It It increased increased withwith a a slope slope similar similar to to that that of of 330HFS 330HFS at at 72 72 h, h, thereby thereby exhibiting exhibiting very very high high strength strength development development comparedcompared to to 330P. 330P. 330EP 330EP exhibited exhibited a a di differencefference in in strength strength development development of of less less than than 1 1 MPa MPa compared compared toto 330HFS_ePC, 330HFS_ePC, but but showed showed a a significantly significantly higher higher compressive compressive strength strength increase increase rate rate at at 72 72 h. h. AtAt the the 20 20◦C °C curing curing temperature, temperature, concrete concrete exhibited exhibited a higher a higher strength strength development development rate thanrate thatthan atthat 13 ◦atC. 13 For °C. 330EP For 330EP and 330HFS_ePC, and 330HFS_ePC, the compressive the compressive strength strength reached reached 5 MPa 5 withinMPa within 18 h, 18 and h,this and valuethis value was reached was reached within within 24 h for 24330HFS h for 330HFS and in and the 24–72in the h24 range–72 h forrange 330P. for 330P_ePC 330P. 330P_ePC reached reached 5 MPa within5 MPa 24 within h at 13 24◦ hC, at and 13 °C, the and effect the was effect significantly was signific increasedantly increased at a higher at a curing higher temperature. curing temperature.

3.2. Temperature History and Maturity on Concrete 3.2. Temperature History and Maturity on Concrete FigureFigure3 3shows shows the the maturity maturity of of concrete concrete according according to to the the elapsed elapsed time. time. When When a a chamber chamber with with a a constant temperature and humidity is used, the temperature changes within the 2 C range at the set constant temperature and humidity is used, the temperature changes within ±the◦ ±2 °C range at the temperaturesset temperatures of 13 of◦ 13C and°C and 20 ◦20C. °C. In thisIn this study, study, the the maturity maturity was was calculated calculated as as the the average average chamber chamber curing temperature because the error range of 2 C was met, although there were some differences curing temperature because the error range of± ±2◦ °C was met, although there were some differences dependingdepending on on the the concrete concrete mixture. mixture. The The maturity maturity of of concrete concretelinearly linearlyincreased, increased,which whichexhibited exhibited higherhigher values values at at a a 20 20◦ C°C compared compared to to 13 13◦ C,°C, and and this this di differencefference significantly significantly increased increased over over time. time. Figure4 shows the relation between the maturity and concrete strength at early ages. For the specimens that used OPC in which the unit weight of cement was increased, the compressive strength development tended to increase according to the maturity, but its effect was not significant. The maturity to develop 5 MPa was 1375 D h(330P), 1108 D h(350P), and 827 D h(380P). For 380P in which · · · the unit weight of cement was increased, the maturity value to develop 5 MPa was approximately 60% that of 330P.

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Appl. Sci. 2020, 10, 7046 7 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 15

Figure 3. Maturity of concrete according to the elapsed time.

Figure 4 shows the relation between the maturity and concrete strength at early ages. For the specimens that used OPC in which the unit weight of cement was increased, the compressive strength development tended to increase according to the maturity, but its effect was not significant. The maturity to develop 5 MPa was 1375 D∙h(330P), 1108 D∙h(350P), and 827 D∙h(380P). For 380P in which the unit weight of cement was increased, the maturity value to develop 5 MPa was approximately 60% that of 330P. FigureFigure 3.3. MaturityMaturity ofof concreteconcrete accordingaccording toto thethe elapsedelapsed time.time.

(a) (b)

FigureFigure 4.4. RelationRelation between maturity and concrete strength at earlyearly ages:ages: (a) CementCement amount and (b)) cementcement typetype andand chemicalchemical admixture.admixture.

TheThe compressivecompressive strengthstrength ofof eacheach cementcement typetype significantlysignificantly increasedincreased withwith thethe maturity.maturity. TheThe maturity to develop 5 MPa was 708 D h for 330HFS and 408 D h for 330EP. It was 475 D h for maturity to develop 5 MPa was 708 D∙h· for 330HFS and 408 D∙h· for 330EP. It was 475 D∙h· for 330HFS_ePC,330HFS_ePC, which was almost similar similar to to the the value value of of 330EP. 330EP. For For 330HFS, 330HFS, the the maturity maturity to to develop develop 5 5MPa MPa was was also also approximately approximately(a) 50% 50% that that of of 330P, 330P, which which was was lower lower than than the the value value(b) of of 380P. 380P. This i indicatesndicates that using HFS or EPC is more efficient than increasing the unit weight of cement to secure the that usingFigure HFS4. Relat orion EPC between is more maturity efficient and thanconcrete increasing strength at the early unit ages: weight (a) Cement of cement amount to and secure (b) the concrete strength. concretecement strength. type and chemical admixture. 3.3. Effect of Cement Fineness on Concrete Early Strength 3.3. EffectThe compressiveof Cement Fineness strength on Concrete of each Earlycement Strength type significantly increased with the maturity. The maturityFigureFigure to5 5 shows develop shows the the 5 particle MPaparticle was size size 708 distribution distribution D∙h for 33 of 0HFSof OPC, OPC, and HFS, HFS, 408 and and D∙h EPC. EPC. for The 330EP.The volume volume It was was found 475 found D∙h to be forto inbe330HFS_ePC, thein the following following which order order was when whenalmost the the similar particle particle to size the size distributionvalue distribution of 330EP. of of each Foreach 330HFS, cement cement type thetype maturity was was analyzed: analyzed: to develop OPCOPC 5 (meanMPa was size: also 16.49 approximatelyµm; fineness: 50% 330 that m2 of/kg) 330P,< HFS which (mean was size:lower 16.31 thanµ them; fineness:value of 380P. 380 m This2/kg) indicates< EPC (meanthat using size: HFS 14.01 orµm; EPC fineness: is more 488 efficient m2/kg). than increasing the unit weight of cement to secure the concrete strength.

3.3. Effect of Cement Fineness on Concrete Early Strength Figure 5 shows the particle size distribution of OPC, HFS, and EPC. The volume was found to be in the following order when the particle size distribution of each cement type was analyzed: OPC

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(mean size: 16.49 µm; fineness: 330 m2/kg) < HFS (mean size: 16.31 µm; fineness: 380 m2/kg) < EPC (mean size: 14.01 µm; fineness: 488 m2/kg). Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 15

2 2 Appl.(mean Sci. size:2020, 1016.49, 7046 µm; fineness: 330 m /kg) < HFS (mean size: 16.31 µm; fineness: 380 m /kg) <8 EPC of 15 (mean size: 14.01 µm; fineness: 488 m2/kg).

Figure 5. Particle size distribution of ordinary Portland cement (OPC), high fineness and SO3 ordinary Portland cement (HSPC), and early Portland cement (EPC).

Figure 6 shows the relation of fineness of cement and concrete strength. The relative strength of Figure 5. Particle size distribution of ordinary Portland cement (OPC), high ﬁneness and SO ordinary concreteFigure was calculated 5. Particle sizeas follows: distribution of ordinary Portland cement (OPC), high fineness and3 SO3 Portland cementordinary (HSPC), andPortland early cement Portland (HSPC cement), and (EPC). early Portland cement (EPC). 푓푐푢 푅푒푙푎푡𝑖푣푒 푠푡푟푒푛푔푡ℎ = , (2) 푓 FigureFigure6 6 shows shows the the relation relation of of ﬁneness fineness of of cement cement and and concrete concrete330푝 strength. strength. The The relative relative strength strength of of concrete was calculated as follows: concretewhere fcu was= compressive calculated strengthas follows: with mixture and f330p = compressive strength of 330p. The relation between the fineness of cement and concretefcu푓 strength showed that the compressive Relative strength = 푐푢, (2) strength tended to increase with the fineness푅푒푙푎푡𝑖푣푒 of푠푡푟푒푛푔푡 cement.ℎ = The optimal, fineness, however, was found(2) f330푓330p 푝 to be between 4300 m2/kg and 4500 m2/kg. In addition, the high fineness of cement had a higher effect wherewhereon the ffconcretecucu = compressivecompressive strength strengthat low tempe with ratures.mixture and f330330pp == compressivecompressive strength strength of of 330 330p.p. The relation between the fineness of cement and concrete strength showed that the compressive strength tended to increase with the fineness of cement. The optimal fineness, however, was found to be between 4300 m2/kg and 4500 m2/kg. In addition, the high fineness of cement had a higher effect on the concrete strength at low temperatures.

FigureFigure 6.6. Relation between ﬁnenessfineness ofof cementcement andand compressivecompressive strengthstrength development.development.

TheIn particular, relation between when 330HFS the fineness was used, of cement the strength and concrete was approximately strength showed twice that that the of compressive 330P at 13 strength°C, and tendedthis effect to increasewas higher with at the early fineness ages, ofi.e., cement. less than The 24 optimal h. Although fineness, 330HFS however, was was found found to tobe befavorable between forFigure 4300 applying m 6.2 /Relationkg and the 4500betweenconcrete m2/ kg.fineness strength In addition, of cementof 5 MPa, the and high compressiveits finenessstrength ofstrengthdevelopment cement development. had awas higher found effect to onbe theslightly concrete lower strength than that at lowof 330EP temperatures. after 24 h. At temperatures higher than 20 °C, the relative strength increasedInIn particular,particular, with the when whenfinene 330HFS 330HFSss of cement was was used, used,in a themanner the strength strength similar was was to approximately thatapproximately of the 13 twice°C twice curing. that that of 330Pof 330P at 13 at◦ 13C, and°C, and this ethisffect effect was higherwas higher at early at ages,early i.e., ages, less i.e., than less 24 than h. Although 24 h. Although 330HFS was330HFS found was to befound favorable to be forfavorable applying for the applying concrete the strength concrete of 5strength MPa, its of strength 5 MPa, development its strength development was found to bewas slightly found lowerto be thanslightly that lower of 330EP than after that 24of h.330EP At temperatures after 24 h. At higher temperatures than 20 higherC, the than relative 20 °C, strength the relative increased strength with ◦ theincreased fineness with of cement the finene in ass manner of cement similar in a manner to that of similar the 13 to◦C that curing. of the 13 °C curing. This difference in fineness is significantly related to the hydration reaction of cement. Figure7 shows the variation in heat of microhydration with elapsed time for each cement type. HFS and EPC had higher microhydration heat values than OPC at early ages less than 24 h. Figure8 shows the Appl.Appl. Sci.Sci. 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 99 ofof 1515

ThisThis differencedifference inin finenessfineness isis significantlysignificantly relatedrelated toto thethe hydrationhydration reactionreaction ofof cement.cement. FigureFigure 77 Appl. Sci. 2020, 10, 7046 9 of 15 showsshows thethe variationvariation inin heatheat ofof microhydrationmicrohydration withwith elapsedelapsed timetime forfor eacheach cementcement type.type. HFSHFS andand EEPCPC hadhad higherhigher microhydrationmicrohydration heatheat valuesvalues thanthan OPCOPC atat earlyearly agesages lessless thanthan 2424 h.h. FigureFigure 88 showsshows thethe cumulativecumulativecumulative heat heatheat of ofof microhydration microhydrationmicrohydration for forfor each eacheach cement cementcement type. type.type. The TheThe plot plotplot of ofof the thethe cumulative cumulativecumulative microhydration microhydrationmicrohydration heatheatheat of ofof cement cementcement also alsoalso shows showsshows that thatthat HFS HFSHFS and andand EPC EPCEPC exhibited exhibitedexhibited values valuesvalues more moremore than thanthan twice twicetwice higher higherhigher than thanthan those thosethose of ofof OPCOPCOPC from fromfrom the thethe point pointpoint of ofof contact contactcontact with withwith water. water.water. Based BasedBased on onon this, this,this, the thethe total totaltotal amount amountamount of ofof heat heatheat of ofof each eacheach mixture mixturemixture can cancan bebebe calculated calculatedcalculated according accordingaccording to toto the thethe unit unitunit weight weightweight of ofof cement. cement.cement.

FigureFigureFigure 7. 7.7. Heat HeatHeat of ofof microhydration microhydrationmicrohydration according accordingaccording to toto elapsed elapsedelapsed time timetime for fforor each eacheach cement cementcement type. type.type.

FigureFigureFigure 8. 8.8. Cumulative CumulativeCumulative microhydration microhydrationmicrohydration heat heatheat according accordingaccording to toto the thethe cement cementcement type. type.type.

ThereThereThere was waswas no nono significant significantsignificant di differencedifferencefference in inin the thethe total totaltotal amount amountamount of ofof heat heatheat according accordingaccording to toto the thethe unit unitunit weight weightweight of ofof cementcementcement within withinwithin 24 2424 h, h,h, which whichwhich was waswas also alsoalso related relatedrelated to toto the thethe maturity maturitymaturity and andand compressive comcompressivepressive strength strengthstrength results resultsresults (see (see(see FigureFigureFigure4 a).4a).4a). In InIn addition, addition,addition, after afterafter 24 2424 h, h,h, the thethe amount amountamount of ofof heat heatheat significantly significantlysignificantly increased increasedincreased as asas the thethe unit unitunit weight weightweight of ofof cementcementcement increased, increased,increased, indicating indicatingindicating thatthatthat the thethe unit unitunit weight weightweight of ofof cement cementcement a affectsaffectsffects the thethe concrete concreteconcretestrength. strength.strength. Figure FigureFigure9 99 showsshowsshows the thethe variation variationvariation in inin the thethe cumulative cumulativecumulative microhydration microhydrationmicrohydration heat heatheat according accordingaccording to toto the thethe cement cementcement type. type.type.

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Figure 9. Variation in cumulative heat of microhydration according to each cement. FigureFigure 9.9. VariationVariation inin cumulativecumulative heatheat ofof microhydrationmicrohydration accordingaccording toto eacheach cement. cement.

InInIn addition,addition,addition, the thethe results resultsresults of ofof analyzing analyzinganalyzing the thethe amount amountamount of of heatof heatheat according accordingaccording to the toto the cementthe cementcement type typetype showed showedshowed that HFSthatthat andHFSHFS EPC andand exhibited EPCEPC exhibitedexhibited similar similarsimilar values valuesvalues until 24 untiluntil h, but 2424 the h,h, but amountbut thethe amountamount of heat from ofof heatheat EPC fromfrom tended EPCEPC to tendedtended decrease toto afterdecreasedecrease 24 h. afterafter In general, 2424 h.h. InIn forgeneral,general, concrete forfor concrete thatconcrete uses thatthat EPC, usesuses fast EPC,EPC, early fastfast strength earlyearly strengthstrength development developmentdevelopment is secured isis securedsecured but the long-termbutbut thethe longlong strength--termterm strengthstrength tends to tendstends decrease toto decreasedecrease [9,10]. [9[9,,10].10].

3.4.3.4. EEffectﬀect of CementCement SO 3 Contents on Concrete Early Strength 3.4. Effect of Cement SO33 Contents on Concrete Early Strength

FigureFigure 1010 showsshows thethe XRFXRF analysisanalysis resultsresults ofof OPC,OPC, HFS,HFS, andand EPCEPC cements.cements. TheThe SOSO3 contentscontents ofof Figure 10 shows the XRF analysis results of OPC, HFS, and EPC cements. The SO33 contents of HFSHFSHFS andandand EPCEPCEPC werewerewere observed observedobserved tototo bebebe higherhigherhigher thanthanthan 3%. 3%.3%. TheyThey werewere approximatelyapproximatelyapproximately 107–129%107107––129%129% higherhigherhigher thanthanthan valuevalue ofof OPC.OPC. TheThe SOSO3//AlAl 2O3 value that represents the early strength performance of concreteconcrete waswas value of OPC. The SO33/Al22O33 value that represents the early strength performance of concrete was 116.1%116.1%116.1% higher higherhigher for forfor HFS HFSHFS and andand 132.2% 132.2%132.2% higher higherhigher for forfor EPC EPCEPC than thanthan that thatthat of ofof OPC. OPC.OPC.

Figure 10. X-ray ﬂuorescence (XRF) analysis results of OPC, HSPC, and EPC. FigureFigure 10.10. XX--rayray fluorescencefluorescence ((XRFXRF)) analysisanalysis resultsresults ofof OPC,OPC, HSPC,HSPC, andand EPC.EPC.

FigureFigure 11 11 shows shows the the relation relation between between the the SOSO33 contentscontents of of cement cement and and compressive compressive strength strength Figure 11 shows the relation between the SO3 contents of cement and compressive strength development,development, whichwhich waswas foundfound toto bebe linear.linear. ASTMASTM C150C150 [[8]8] limitslimits thethe SOSO33 content of Portland cement development, which was found to be linear. ASTM C150 [8] limits the SO3 content of Portland cement toto 3.5%3.5% andand suggestssuggests anan optimaloptimal SOSO33 contentcontent ofof 3.1%.3.1%. In addition, ENEN 197-1197-1 [[34]34] limitslimits thethe SOSO33 content to 3.5% and suggests an optimal SO3 content of 3.1%. In addition, EN 197-1 [34] limits the SO3 content to 4.0%. The relative strength obtained in this study, unlike that of 330P, linearly increased with the SO toto 4.4.0%.0%. TheThe relativerelative strengthstrength obtainedobtained inin thisthis study,study, unlikeunlike thatthat ofof 330P,330P, linearlylinearly increasedincreased withwith thethe3 contentSO3 content in the in range the range smaller smaller than ENthan 197-1. EN 197 In- addition,1. In addition, the eﬀ theect oneffect the on relative the relative strength strength was higher was SO3 content in the range smaller than EN 197-1. In addition, the effect on the relative strength was

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Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 15 athigher the lower at the temperature lower temperature of 13 ◦C. Theof 13 relative °C. The strength relative of s 380Ptrength appears of 380P to haveappears increased to have because increased the overallbecause SO thecontent overall increasedSO3 content owing increased to the owing increase to inthe the increas unit weighte in the of unit cement. weight of cement. higher at3 the lower temperature of 13 °C. The relative strength of 380P appears to have increased because the overall SO3 content increased owing to the increase in the unit weight of cement.

FigureFigure 11. 11.Relation Relation betweenbetween SOSO33 contentscontents ofof cementcement andand compressivecompressive strengthstrength development.development. Figure 11. Relation between SO3 contents of cement and compressive strength development. InIn thethe casecase of EPC with with high high fineness, fineness, the the compressive compressive strength strength increased increased after after 24 24h despite h despite the In the case of EPC with high fineness, the compressive strength increased after 24 h despite the thereduction reduction in in the the microhydration microhydration heat heat because because the the high high SO SO33 contentcontent contributed to the the strength strength reduction in the microhydration heat because the high SO3 content contributed to the strength improvement [2,8]. The relative strength of HFS tended to be higher than value of OPC with the higher improvementimprovement [2,8 [2,8].]. The The relative relative strengthstrength of HFS tendedtended toto be be higher higher than than value value of ofOPC OPC with with the the cementhigher fineness cement and fineness SO3 content. and SO3 When content. EPC When was used, EPC the was eff ect used, was the increased effect bywas approximately increased by higher cement fineness and SO3 content. When EPC was used, the effect was increased by 1.5–2 times. approximatelyapproximately 1.5 1.5–2– 2times. times. MohammedMohammedMohammed andand and SafiullahSafiullah Safiullah [ [35]35[35]] and andand LeeLee andand LeeLeeLee [[14,1514[14,15,15] ] reported reported that that that the the the compressive compressive compressive strengthstrength strength significantly increased after 48 h when the SO content was between 3.0% and 3.2%. They also significantlysignificantly increased increased after after 48 48 h h when when the SO333 cocontentntent was was between between 3.0% 3.0% and and 3.2%. 3.2%. They They also also mentionedmentionedmentioned thatthat that increasingincreasing increasing thethe the fineness finenessfineness of of cementcement (for(for(for whichwhichwhich the the SO SOSO33 3content contentcontent was waswas increased increasedincreased to toto3.1% 3.1%3.1% here)here)here) hadhad hada alarge alarge large impactimpact impact onon on thethe the earlyearly early strength. strength.strength. InIn thisthisthis study,study, when when EPC EPCEPC was waswas additionally additionallyadditionally used usedused in inthein thethe SOSOSO33 contentcontent3 content rangera rangenge ofof of lessless less thanthan than 4%,4%, 4%, a aa strength strengthstrength developmentdevelopment similar similar to toto the thethe level levellevel of ofof concrete concreteconcrete using usingusing EPC EPCEPC waswaswas achievable. achievable. achievable.

3.5.3.5.3.5. ComparisonComparison Comparison ofof of EscapeEscape Escape TimeTime Time forfor forVetical Vetical Vetical Form Form Removal Removal FigureFigureFigure 12 12 12shows shows shows the the the variation variation variation in inin concrete concreteconcrete strength strength according according according to to to the the the averageaverage average temperaturetemperature temperature after after 12 12 h. Basedh. h. Based Based on the on onrelationship the the relationship relationship between between between the maturity the andmaturity concrete and and strength, concrete concrete concrete strength, strength, strength concrete concrete development strength strength afterdevelopmentdevelopment 12 h of curing after after time 12 12 h wash of of curing calculatedcuring timetime and waswas reviewed calculated by andand referring reviewed reviewed to the by by codes referring referring of each to to the country the codes codes [of36 eachof]. Thiseach wascountrycountry conducted [36]. [36]. This This toderive was was conducted conducted the optimal toto derive mixturederive the by optimal establishing mixture mixture an by objectiveby establishing establishing criterion an an objective forobjective the criterion formwork criterion for the formwork removal time of concrete. removalfor the formwork time of concrete. removal time of concrete.

FigureFigure 12. 12.Variation Variation in in the the concrete concrete the the strength strength accordingaccording to the average average temperature temperature after after 12 12 h.h. Figure 12. Variation in the concrete the strength according to the average temperature after 12 h.

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TheThe results results of of this this study study showed showed that that it it would would be be di difficultfficult to to apply apply the the 10 10◦ C°C and and 12 12 h h demolding demolding conditionsconditions suggested suggested by by AmericanAmerican Concrete Concrete Institute Institute (ACI)(ACI) [20[20]] because because compressive compressive strength strength developmentdevelopment waswas notnot observed in in all all the the mixtures. mixtures. Further, Further, the the application application of British of British Standards Standards (BS) (BS)EN EN[22] [ 22is difficult] is difficult except except for for330EP 330EP and and 330HFS_ePC. 330HFS_ePC. TheThe Asian Asian (South (South Korea Korea and and Japan) Japan) criterion criterion of of 5 5 MPa MPa [ 23[23,24,24]] was was found found to to be be applicable applicable only only to to 330EP330EP and and 330HFS_ePC 330HFS_ePC at 12 at h. 12 As h. the As accurate the accurate value for value demolding for demolding strength strengthwas presented, was presented, however, ithowever, is expected it that is expected the formwork that removal the formwork time can removal be predicted time bycan examining be predicted the compressive by examining strength the andcompressive applying strength the maturity. and applying Therefore, the inmaturity. this study, Therefore, the demolding in this study, strength the demolding of each mixture strength was of analyzedeach mixture using was the analyzed Asian criterion, using the as aforementioned.Asian criterion, as aforementioned. FigureFigure 13 13 shows shows the the variation variation in thein the elapsed elapsed time time at the at strengththe strength of 5 MPaof 5 forMPa concrete. for concrete. The 5 The MPa 5 developmentMPa development time was time examined was examined according according to the 10to the◦C condition10 °C condition suggested suggested by ACI by and ACI the and 20 ◦theC condition,20° C condition, which iswhich the curing is the curing criterion criterion managed managed in the field.in the Anfield. order An oforder 330P of (68 330P h) (68> 350P h) > (55350P h) (55< 380Ph) < 380P (41 h) (41< 330HFSh) < 330HFS (35 h)(35< h)330HFS_ePC < 330HFS_ePC (24 (24 h) 350P350P (37 (37 h) h) << 380P380P (28 (28 h) h) < <330HFS330HFS (24 (24 h) h)< 330HFS_ePC< 330HFS_ePC (16 (16h) h) < 330EP< 330EP (14 (14 h). h). The The formwork formwork removal removal time time was was shortened shortened as the temperature temperature increased, increased, and and 330HFS_ePC330HFS_ePC and and 330EP 330EP exhibited exhibited almost almost similar similar formwork formwork removal removal times. times.

FigureFigure 13. 13.Variation Variation in in the the elapsed elapsed time time at at strength strength of of 5 5 MPa MPa for for concrete. concrete.

Further,Further, future future studies studies should should be be conducted conducted on on securing securing the the demolding demolding time time of of the the formwork formwork usingusing environmentally environmentally friendlyfriendly materialsmaterials [37[37,38,38]] and and the the corrosion corrosion resistance resistance according according to to the the SO SO3 3 contentcontent [ 39[39].].

4.4. Conclusions Conclusions ThisThis study study evaluated evaluated the the e effectsffects of of the the unit unit weight weight of of cement cement on on concrete concrete that that uses uses OPC OPC on on early early strengthstrength development. development. HFS_OPC HFS_OPC and and EPC EPC were were selected, selected, and and the the e effectsffects of of cement cement fineness, fineness, SO SO3 3 content,content, and and chemical chemical admixture admixture were were analyzed. analyzed. The The results results are are summarized summarized as as follows. follows.

(1)(1) TheThe slumpslump andand airair contentcontent necessarynecessary forfor securingsecuring thethe workabilityworkability onon concreteconcrete met met the the set set target target rangesranges both at at the the beginning beginning and and after after 60 60min. min. The Thestrength strength development development of concrete of concrete was further was furtheraccelerated accelerated by the cement by the cementtype and type the and addition the addition of EPC ofthan EPC when than the when unit the weight unit weightof cement of cementwas increased. was increased. In addition, In addition, the strength the strength development development rate of rate concrete of concrete was higher was higher at lower at lowertemperatures. temperatures.

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(2) For the maturity of concrete, the strength development tended to increase according to the maturity. This is because the unit weight of cement increased for the concrete mixtures that used OPC. The effect, however, was not significant when the HFS_OPC and EPC were used. (3) When the amount of heat of each mixture was calculated based on the microhydration heat results of cement, no significant differences were found in the total amount of heat within 24 h depending on the unit weight of cement. In addition, analyzing the results of the amount of heat for each cement type showed that HFS and EPC exhibited significantly high values compared to OPC until 24 h, indicating that they are favorable for the development of early strength. (4) The relation between concrete strength and the cement fineness showed that the compressive strength tended to increase with the fineness of cement, and an optimal fineness between 4300 m2/kg and 4500 m2/kg was obtained. The microhydration heat was found to be higher when the fineness of cement was high than when the unit weight of cement was increased. The results of analyzing the amount of heat for each cement type showed that HFS and EPC exhibited significantly higher values compared with OPC up to 24 h. Thus, they are expected to be favorable for early strength development.

(5) A linear relationship was observed between the SO3 content and relative strength of cement when SO3 content was less than 4%. In addition, when the unit weight of cement was increased, the early strength was slightly increased owing to the increase in overall SO3 content. (6) The criterion of each country for the formwork removal time on concrete were examined and applying the Asian criterion was judged to be eﬀective, which presents a clear compressive strength of 5 MPa. The use of EPC and HFS_ePC can shorten the formwork removal time by 20–24 h at 10 ◦C and by 14–16 h at 20 ◦C.

Author Contributions: Conceptualization, T.L., J.L., and H.C.; methodology, T.L., J.L., and H.C.; investigation, T.L., J.L., J.K., H.C. and D.-E.L.; resources, T.L. and J.L.; writing—original draft preparation, T.L. and J.L.; writing—review and editing, T.L., J.L., J.K., H.C and D.-E.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT), grant number NRF-2018R1A5A1025137. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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