materials

Article Evaluation of Mechanical and Shrinkage Behavior of Lowered Temperatures Cementitious Mortars Mixed with – Based Accelerator

Yusuke Tomita 1, Akira Yoneyama 1, Heesup Choi 1,* , Masumi Inoue 1, Jihoon Kim 2, Hyeonggil Choi 3 and Yuhji Sudoh 4

1 Department of Civil and Environmental Engineering, Kitami Institute of Technology, Hokkaido 090-8507, Japan; [email protected] (Y.T.); [email protected] (A.Y.); [email protected] (M.I.) 2 Faculty of Environmental Technology, Muroran Institute of Technology, Hokkaido 090-8585, Japan; [email protected] 3 School of Architecture, Civil, Environment, and Energy Engineering, Kyungpook National University, Daegu 41566, Korea; [email protected] 4 Basic Chemicals Department Chemicals Division, Nissan Chemical Corporation, Tokyo 103-6119, Japan; [email protected] * Correspondence: [email protected]

 Received: 23 July 2020; Accepted: 18 August 2020; Published: 20 August 2020 

Abstract: Recently, nitrite (Ca(NO2)2) and (Ca(NO3)2) have been increasingly used as the main components of salt- and alkali-free anti-freezing agents, for promoting concrete hydration in cold-weather concreting. With an increase in the amount of nitrite-based accelerator, the hydration of C3A, C3S, and βC2S in the cement is accelerated, thereby improving its early strength and effectively preventing the initial frost damage. Meanwhile, with an increase in the amount of nitrite-based accelerator, the expansion and shrinkage of the concrete—and, therefore, the crack occurrence—are expected to increase. In this study, various experiments were conducted on shrinkage, crack initiation, and the development of mortar containing a considerable amount of a nitrite-based accelerator. The result confirmed that, as the amount of nitrite-based accelerator was increased, the shrinkage was increased, and cracking in early age was more likely to occur, compared to the cases without the addition of this accelerator.

Keywords: frost-resistant accelerator; calcium nitrite; calcium nitrate; cracking; strength; pore volume; shrinkage; crack potential; degree of restraint

1. Introduction During cold-weather concreting, to prevent the initial frost damage, it is necessary to control the temperature until the concrete strength reaches 5 N/mm2 of the heat curing, using a temporary enclosure and heater [1–6]. On the other hand, under a very low temperature or worse conditions—such as a steep incline, narrow space, or windstorm—anti-freezing agents are used to prevent the initial frost damage and secure the initial strength by sheet curing. Generally, the allowable range of anti-freezing agents is from 4 to 8 C[7–10]. However, when the daily average temperature is below 10 C, it is − − ◦ − ◦ necessary to increase the amount of anti-freezing agent. At present, calcium nitrite and calcium nitrate are used as the main components of a salt- and alkali-free-type nitrite-based accelerator (CN) [9,10]. It has been proposed that increasing the amount of this nitrite-based accelerator contributes to good initial strength development in low-temperature environments, due to the hydration promotion of C3A in the cement—and that is due to the increased

Materials 2020, 13, 3686; doi:10.3390/ma13173686 www.mdpi.com/journal/materials Materials 2020, 13, 3686 2 of 13 Materials 2020, 13, x; doi: FOR PEER REVIEW 2 of 12 of C S and βC S[11–13]. According to the Japanese Architectural Standard Specification for solubility of C33S and βC2S [11–13]. According to the Japanese Architectural Standard Specification for ReinforcedReinforced ConcreteConcrete WorkWork (JASS5),(JASS5), shrinkageshrinkage crackscracks needneed toto bebe reducedreduced toto ensureensure thethe durabilitydurability ofof concrete. Therefore, the drying shrinkage rate of concrete has been stipulated to be under 8 10 4 [14]. concrete. Therefore, the drying shrinkage rate of concrete has been stipulated to be under 8× × 10−−4 [14]. CN accelerates the hydration reaction of the C A contained in cement and increases the amount CN accelerates the hydration reaction of the C33A contained in cement and increases the amount ofof ettringiteettringite andand monosulfatemonosulfate [[15–17].15–17]. This is becausebecause hydrates,hydrates, suchsuch asas nitrite–nitratenitrite–nitrate hydratehydrate (3CaO Al O Ca (NO /NO ) xH O), are formed through reactions between the C A (Al O ), NO , (3CaO·Al· 22O33·Ca· (NO22/NO33)2·xH2· 2O),2 are formed through reactions between the C3A3 (Al2O32), NO3 2−, and2− and NO in CN [15–17]. Therefore, the chemical shrinkage potential of the cement matrix increases, NO3− in 3CN− [15–17]. Therefore, the chemical shrinkage potential of the cement matrix increases, which whichleads leadsto a higher to a higher concern concern regarding regarding shrinkage shrinkage cracking. cracking. However, However, few few studies studies have have focused onon shrinkageshrinkage cracking.cracking. InIn thisthis study, study, the the authors’ authors’ aims aims are toare clarify to clar theify shrinkage the shrinkage behavior behavior of concrete, of concrete, and the initiation and the andinitiation development and development of cracks, whenof cracks, a considerable when a consid amounterable of amount CN is present of CN inis thepresent mixture. in the Therefore, mixture. variousTherefore, experiments various experiments have been conductedhave been toconducte quantitativelyd to quantitatively evaluate the evaluate physical the and physical shrinkage and crackingshrinkage properties cracking ofproperties mortar containing of mortar containing a considerable a considerable amount of amount CN. Figure of CN.1 shows Figure the 1 flowchartshows the forflowchart this study. for this study.

Physical Properties of Mortar with Nitrite·Nitrate-based Accelerator

– • Age : Casting – 14 days • NO2 = Nitrite ions – • Temperature : 10℃ – – • NO3 = Nitrate ions NO2 , NO3 Compressive Table Flow Strength

Strength Development Temperature History Initial Hardening (C3A, C3S, βC2S)

(C3A) Vo id S truc ture

Evaluation of Cracking Properties of Mortar with Nitrite·Nitrate-based Accelerator

Restrained Shrinkage Strain Tensile Strength Unrestrained Ring Test Shrinkage Strain (0.291·Fc0.658) Restrained Tensile Stress

Degree of Restraint Evaluation of Crack Potential

Evaluation of Cracking Characteristics of Mortar with Nitrite·Nitrate-based Accelerator

FigureFigure 1.1. StudyStudy flowflow chart.chart.

2.2. Experimental OverviewOverview

2.1. Materials and Procedures 2.1. Materials and Procedures Table1 presents the materials used in the experiments conducted in this study, and Table2 Table 1 presents the materials used in the experiments conducted in this study, and Table 2 presents their CN components. CN is a 45% mixed aqueous solution of calcium nitrite and calcium presents their CN components. CN is a 45% mixed aqueous solution of calcium nitrite and calcium nitrate. Table3 presents the mortar composition used in the experiments, where the water–cement nitrate. Table 3 presents the mortar composition used in the experiments, where the water–cement ratio is 50% and the sand–cement ratio (S/C) is 2.5 [10,11]. The standard amount of accelerator added ratio is 50% and the sand–cement ratio (S/C) is 2.5 [10,11]. The standard amount of accelerator added is 4%~7% of the cement mass, depending on the ambient temperature [10]. Assuming a case where a is 4%~7% of the cement mass, depending on the ambient temperature [10]. Assuming a case where a considerable amount of CN was added in extremely low temperatures, the amount of CN added was considerable amount of CN was added in extremely low temperatures, the amount of CN added was set to the four levels of 0, 7, 9, and 11%, as compared to the cement weight. The concrete temperature during the unloading has been specified as 10~20 °C by the Architectural Institute of Japan in the Materials 2020, 13, 3686 3 of 13 set to the four levels of 0, 7, 9, and 11%, as compared to the cement weight. The concrete temperature during the unloading has been specified as 10~20 ◦C by the Architectural Institute of Japan in the Recommendation for Practice of Cold Weather Concreting (Practical Guideline for Investigation (2010)). Therefore, in the experiment, to elucidate the behavioral assessment of the concrete expansion and shrinkage when a CN is added, both mixing and sealed curing were performed at 10 1 C and ± ◦ 85% 5% (Relative Humidity; RH), from Day 1 to Day 14, after placing. ± Table 1. Properties of the materials used (Data from [18]).

Materials (Code) Properties Normal Portland cement, density: 3.16 g/cm3 (Taiheiyo Cement, Cement (C) Tokyo, Japan) No. 5 silica sand, absolute dry density: 2.61 g/cm3, Water absorption: 0.26%, Fine aggregate (S) fineness modulus: 2.16 (Tochu, Tokyo, Japan) Nitrite nitrate-based accelerator = calcium nitrite (Ca(NO ) ); calcium Anti-freezing agent (CN) 2 2 nitrate (Ca(NO3)2) (Nissan Chemical, Tokyo, Japan)

Note: CN: Anti-freezing agent = nitrite + nitrate-based accelerator (Ca(NO2)2 + Ca(NO3)2).

Table 2. Properties of the anti-freezing agent (Data from [18]).

Code Component Component Ratio pH Specific Gravity Ca(NO ) 23.02% CN 2 2 9.3 1.43 g/cm3 Ca(NO3)2 22.81%

Table 3. Proportions of the mortar mix (Data from [18]).

Unit Content (kg/m3) Anti-Freezing Agent (C %) Type W/C (%) S/C × W C S CN CN0 0 CN7 7 50 2.5 281 562 1407 CN9 9 CN11 11 Note: W/C: Water–cement ratio; S/C: Sand–cement ratio; CN0: Mixing amount of anti-freezing agent = 0%; CN7: Mixing amount of anti-freezing agent = 7%; CN9: Mixing amount of anti-freezing agent = 9%; CN11: Mixing amount of anti-freezing agent = 11%.

2.2. Experimental Method As the amount of nitrite-based accelerator was increased, various physical and shrinkage behavior assessments were conducted at various ages, using the experimental method outlined below. Table4 shows the conditions and assessment method applied in this experiment.

Table 4. Experimental conditions and evaluation method (Data from [18]).

Subject and Method of Evaluation Temperature Condition Experimental Period Physical Properties Shrinkage Properties Flow test Un/Restrained shrinkage Compressive strength Tensile strength 10 ◦C Casting—14 days Temperature history Crack potential MIP Degree of restraint Note: Physical and shrinkage properties: All cases; flow test: Immediately after placement; temperature history: Casting—14 days.

The mortar’s fresh property was according to the “Flow test” in JIS R 5201 (Physical Testing Methods for Cement; Tokyo, Japan), and a flow test was performed immediately after mixing [19]. Materials 2020, 13, x; doi: FOR PEER REVIEW 4 of 12

To assess the compressive strength, sealed curing was performed at 10 ± 1 °C and 85% ± 5% RH, from Day 1 to Day 14, after placing the mortar into a φ 5 × 10 cm2 mold. Next, the compressive strength was measured on Days 1, 3, 7, and 14. For the internal temperature, a thermocouple (Tokyo, Japan) was installed at the center of a mold Materialsof 10 × 202020 cm, 132,, x; and doi: theFOR time-dependentPEER REVIEW change in the mortar temperature immediately after mixing,4 of 12 until Day 14, was measured. To assess the compressive strength, sealed curing was performed at 10 ± 1 °C and 85% ± 5% RH, For the MIP (Mercury Intrusion Porosimetry) test, as shown in Figure 2, in order to obtain a from Day 1 to Day 14, after placing the mortar into a φ 5 × 10 cm2 mold. Next, the compressive sample that is representative of the measurement of porosity, samples were collected at each age and strength was measured on Days 1, 3, 7, and 14. cut into 5 mm cubes. Then, in order to stop the hydration of the sample, the sample was immersed in For the internal temperature, a thermocouple (Tokyo, Japan) was installed at the center of a mold acetone for 4 h or more, and dried by the D-dry method for 1 week, and then the pore distribution of of 10 × 20 cm2, and the time-dependent change in the mortar temperature immediately after mixing, each sample was measured at a minimum diameter of 6 nm, with a maximum pressure of 220 MPa. until Day 14, was measured. As shown in Figure 3, the unrestrained drying shrinkage strain was measured by embedding a For the MIP (Mercury Intrusion Porosimetry) test, as shown in Figure 2, in order to obtain a reinforcing bar with a strain gauge that was attached to the center of a 10 × 10 × 40 cm3 mold. To make sample that is representative of the measurement of porosity, samples were collected at each age and the drying conditions identical to those of the ring specimen, and after demolding the specimen at cut into 5 mm cubes. Then, in order to stop the hydration of the sample, the sample was immersed in the age of 1 day, the exposed surface area ratio was adjusted to be the same as that of the ring acetoneMaterials 2020for 4, 13 h, or 3686 more, and dried by the D-dry method for 1 week, and then the pore distribution4 of of 13 specimen (~44%) by covering the circumferential direction with aluminum tape. each sample was measured at a minimum diameter of 6 nm, with a maximum pressure of 220 MPa. The restraint shrinkage strain is a strain generated by the restraint of the inner steel ring. The As shown in Figure 3, the unrestrained drying shrinkage strain was measured by embedding a heightTo of assess the ring the compressivetest piece was strength, changed sealed from curing152 to was75 mm, performed proposed at 10by AASHTO1 ◦C and (American 85% 5% reinforcing bar with a strain gauge that was attached to the center of a 10 × 10 × 40± cm3 mold. To make± AssociationRH, from Day of State 1 to Day Highway 14, after Transportation placing the mortar Officials; into 1998), a ϕ 5 to 10induce cm2 mold.uniform Next, dry theshrinkage compressive at the the drying conditions identical to those of the ring specimen, ×and after demolding the specimen at cross-sectionstrength was of measured the concrete on Days ring 1, (Figure 3, 7, and 4) 14.[20,21]. In this experiment, a Teflon sheet was installed the age of 1 day, the exposed surface area ratio was adjusted to be the same as that of the ring betweenFor thethe internalouter ring temperature, and the mortar a thermocouple to minimize (Tokyo, the restraint Japan) was from installed the outer at the ring center and of prevent a mold specimen (~44%) by covering the circumferential direction with aluminum tape. waterof 10 evaporation20 cm2, and on the the time-dependent upper part of the change test bo indy. the At mortar the age temperature of 1 day, the immediately lower wood after plate mixing, and The× restraint shrinkage strain is a strain generated by the restraint of the inner steel ring. The upperuntil Day Teflon 14, wassheet measured. of the ring specimen were removed, and thus, only the upper and lower surfaces height of the ring test piece was changed from 152 to 75 mm, proposed by AASHTO (American were Fordried. the For MIP determining (Mercury Intrusion the restrained Porosimetry) shrinkag test,e strain, as shown strain in gauges Figure 2were, in orderattached to obtainat three a Association of State Highway Transportation Officials; 1998), to induce uniform dry shrinkage at the locationssample that (h/2 is = representative 37.5 mm) in the of thecenter measurement of the inne ofr ring, porosity, and samplesthe change were in collectedthe strain atwith each time age was and cross-section of the concrete ring (Figure 4) [20,21]. In this experiment, a Teflon sheet was installed measuredcut into 5 mmimmediately cubes. Then, after in that. order Figure to stop 5 shows the hydration the apparatus of the of sample, the unrestrained the sample drying was immersed shrinkage in between the outer ring and the mortar to minimize the restraint from the outer ring and prevent test,acetone with for the 4 h specimen’s or more, and exposed dried by surface the D-dry area method ratio adjusted for 1 week, to be and the then same the poreas that distribution of the ring of water evaporation on the upper part of the test body. At the age of 1 day, the lower wood plate and specimen.each sample Figure was measured6 shows the at apparatus a minimum of diameterthe restrained of 6 nm, shrinkage with a maximumtest (ring test). pressure of 220 MPa. upper Teflon sheet of the ring specimen were removed, and thus, only the upper and lower surfaces were dried. For determining the restrained shrinkage strain, strain gauges were attached at three SamplingSampling locationlocation for for MIP MIP locations (h/2 = 37.5 mm) in the center of the inner ring, and the change in the strain with time was 10cm measured immediately after that. Figure 5 shows the apparatus of the unrestrained drying shrinkage test, with the specimen’s exposed surface area ratio adjusted to be the same as that of the ring specimen. Figure 6 shows the apparatus of th5mme restrained shrinkage test (ring test). 5mm MIP SamplingSampling locationlocation for for MIP MIP sample 20cm 10cm sample

5mm

CoreCore specimenspecimen of of 5mm each each age age MIP sample

20cm FigureFigure 2. SamplingSampling of of specimen specimen for for evaluation evaluation of of pore pore structure. structure. sample

As shown in Figure3, the unrestrained drying shrinkage strain was measured by embedding a Teflon sheet Strain guage reinforcing bar with a strain gauge that was attached to the center of a 10 10 40 cm3 mold. To make × × the drying conditions identical toCoreCore those specimenspecimen of the ring of of specimen, each each age age and after demolding the specimen at the age of 1 day, the exposed surface area ratio was adjusted to be the same as that of the ring specimen (~44%) by covering theFigure circumferential 2. Sampling of direction specimen with for evaluation aluminum of tape.pore structure.

Teflon sheet Strain guage Steel model

Figure 3. Overview of unrestrained drying shrinkage.

Steel model

Figure 3. Overview of unrestrained drying shrinkage. Figure 3. Overview of unrestrained drying shrinkage. The restraint shrinkage strain is a strain generated by the restraint of the inner steel ring. The height of the ring test piece was changed from 152 to 75 mm, proposed by AASHTO (American Association of State Highway Transportation Officials; 1998), to induce uniform dry shrinkage at the cross-section of the concrete ring (Figure4)[ 20,21]. In this experiment, a Teflon sheet was installed between the outer ring and the mortar to minimize the restraint from the outer ring and prevent water evaporation on the upper part of the test body. At the age of 1 day, the lower wood plate and upper Teflon sheet of the ring specimen were removed, and thus, only the upper and lower surfaces were dried. For determining the restrained shrinkage strain, strain gauges were attached at three locations (h/2 = 37.5 mm) in the center of the inner ring, and the change in the strain with time was measured immediately after that. Figure5 shows the apparatus of the unrestrained drying shrinkage test, with the specimen’s exposed surface Materials 2020, 13, 3686 5 of 13

area ratio adjusted to be the same as that of the ring specimen. Figure6 shows the apparatus of the

MaterialsMaterialsrestrainedMaterials 20202020 2020 shrinkage,, 1313, ,13, x;x;, doi:x;doi: doi: FORFOR testFOR PEERPEER (ringPEER REVIEWREVIEW REVIEW test). 55 ofof5 of1212 12

FigureFigureFigure 4.4. 4.OverviewOverviewOverview Overview ofof of of ringring ring ring testtest test test (restrained(restrained (restrained (restrained shrinkage).shrinkage). shrinkage).

FigureFigureFigure 5.5. 5.UnrestrainedUnrestrainedUnrestrained Unrestrained dryingdrying drying drying shrinkage.shrinkage. shrinkage. shrinkage.

FigureFigureFigure 6.6. 6.RestrainedRestrainedRestrained Restrained shrinkageshrinkage shrinkage shrinkage (ring(ring (ring (ring test).test). test). test).

3.3. 3. PhysicalPhysical Physical PropertiesProperties Properties ofof of MortarMortar Mortar withwith with CNCN CN

3.1.3.1.3.1. FreshFresh Fresh PropertiesProperties Properties FigureFigureFigure 77 shows7shows shows thethe the resultsresults results ofof of thethe the tabletable table flowflow flow testtest test inin in eacheach each case.case. case. TheThe The flowflow flow valuevalue value ofof of CN0CN0 CN0 waswas was setset set toto to 186186186 mm,mm, mm, basedbased based onon on thosethose those ofof of CN7,CN7, CN7, CN9,CN9, CN9, andand and CN100,CN100, CN100, whichwhich which werewere were decreaseddecreased decreased byby by 6.5,6.5, 6.5, 9.2,9.2, 9.2, andand and 25.9%,25.9%, 25.9%, Materials 2020, 13, 3686 6 of 13

3. Physical Properties of Mortar with CN

3.1. Fresh Properties Materials 2020, 13, x; doi: FOR PEER REVIEW 6 of 12 Materials 2020, 13, x; doi: FOR PEER REVIEW 6 of 12 Figure7 shows the results of the table flow test in each case. The flow value of CN0 was set to 186 respectively.mm,respectively. based on As thoseAs the the ofamount amount CN7, CN9, of of CN CN and was was CN100, increased, increased, which the the were flow flow decreased value value tended tended by 6.5, to to 9.2, decrease. decrease. and 25.9%, Figure Figure respectively. 8 8 shows shows Asthethe thehistory history amount of of the the of CNinternal internal was temperature increased, temperature the of of flow the the valuemort mortarar tended (~2 (~2 h). h). to The The decrease. temperature temperature Figure peaks8 peaks shows were were the 15.2 history15.2 °C °C for offor CN0,theCN0, internal 17.1 17.1 for for temperature CN7, CN7, 18.6 18.6 offor for the CN9, CN9, mortar and and (~2 22.0 22.0 h). for Thefor CN11.CN11. temperature As As the the peaksamount amount were of of 15.2CN CN ◦was Cwas for increased, increased, CN0, 17.1 the forthe temperature tended to increase. When CN was added, the NO2− and NO3− reacted rapidly with the CN7,temperature 18.6 for CN9,tended and to 22.0 increase. for CN11. When As CN the was amount added, of CN the was NO increased,2− and NO the3− reacted temperature rapidly tended with the to C3A in the cement to produce nitrite/nitrate hydrate [13,15,17,18,22]. These results indicate that when increase.C3A in the When cement CN to was produce added, nitrite/nitrate the NO2− and hydrate NO3− [13,15,17,18,22].reacted rapidly These with theresults C3A indicate in the cement that when to aproducea considerable considerable nitrite /amountnitrate amount hydrate of of CN CN is [is13 added, added,,15,17 ,hydration18 hydration,22]. These is is promoted, promoted, results indicate the the mortar mortar that when temperature temperature a considerable increases, increases, amount and and theofthe CN flow flow is added,value value decreases. hydrationdecreases. is promoted, the mortar temperature increases, and the flow value decreases.

20 20

15 15

10 10

5 Flow value (cm) value Flow 5 Flow value (cm) value Flow

0 0 CN0 CN7 CN9 CN11 CN0 CN7 CN9 CN11

Figure 7. Flow value. FigureFigure 7. 7.Flow Flow value. value. 30 30 CN0 CN7 CN0 CN7 25 CN9 CN11 25 CN9 CN11 20 20 15 15

Temperature (℃) 10

Temperature (℃) 10 5 5 Age (hour) Age (hour) Figure 8. TemperatureTemperature history (up to 2 h). Figure 8. Temperature history (up to 2 h). 3.2. Compressive Strength Properties 3.2.3.2. Compressive Compressive Strength Strength Properties Properties Figure 99 showsshows thethe compressivecompressive strengthstrength fromfrom 11 toto 14 14 days days for for each each case. case. TheThe compressivecompressive Figure 9 shows the compressive2 strength from 1 to 14 days for each case. The compressive strength on Day 1 was 4.38 N/mm2 for CN0, 5.15 for CN7, 6.51 for CN9, and 7.03 for CN11, and strengthstrength on on Day Day 1 1 was was 4.38 4.38 N/mm N/mm 2for for CN0, CN0, 5.15 5.15 for for CN7, CN7, 6.51 6.51 for for CN9, CN9, and and 7.03 7.03 for for CN11, CN11, and and tended tended totended increase to increasewith the withamount the of amount CN. Figure of CN. 10 show Figures the10 historyshows theof the history mortar’s of theinternal mortar’s temperature internal temperatureto increase with (~24 the h). amount In particular, of CN. the Figure temperature 10 show peakss the history at 0–4 hof and the 6–18mortar’s h increased internal [ 16temperature], and the (~24(~24 h). h). In In particular, particular, the the temperature temperature peaks peaks at at 0–4 0–4 h h and and 6–18 6–18 h h increased increased [16], [16], and and the the time time required required totime reach required the peak to reach decreased. the peak With decreased. an increase With in an the increase amount in theof CN, amount the oftemperature CN, the temperature tended to tendedto reach to increase;the peak moreover,decreased. the With amounts an increase of NO inand the NO amountincreased of CN, and the reacted temperature rapidly tended with the to increase; moreover, the amounts of NO2− and NO23− increased3− and reacted rapidly with the C3A in the increase; moreover, the amounts of NO2− and NO3− increased and reacted rapidly with the C3A in the C3A in the cement, which promoted hydration, increased the mortar temperature, and increased the cement,cement, which which promoted promoted hydration, hydration, increased increased the the mortar mortar temperatur temperature,e, and and increased increased the the amount amount of of nitrite/nitrateamount of nitrite hydrate./nitrate Therefore, hydrate. the Therefore, strength the on strength Day 1 was on considered Day 1 was consideredto have increased. to have However, increased. However,nitrite/nitrate the compressive hydrate. Therefore, strength, the by strength Day 3, tended on Day to 1 decreasewas considered as the amount to have of increased. CN was increased.However, thethe compressivecompressive strength,strength, byby DayDay 3,3, tendedtended toto decreasedecrease asas thethe amountamount ofof CNCN waswas increased.increased. Furthermore, this this tendency became became remarkable remarkable af afterter 7 7 days, days, and and the the strength strength in in the case of CN additionFurthermore, was below this tendency that of CN0. became With remarkable an increase af inter the 7 days, amount and of the CN, strength the amounts in the of case ettringite of CN additionaddition was was below below that that of of CN0. CN0. With With an an increase increase in in the the amount amount of of CN, CN, the the amounts amounts of of ettringite ettringite and and nitrite/nitricand nitrite/nitric acidhydrate hydrate produced produced also increased. also increased. Therefore, Therefore, on Day on 1, Day the structure 1, the structure became became dense densenitrite/nitric and the acid strength hydrate increased. produced After also 3 days,increased. when Therefore, CN was added, on Day a 1, considerable the structure amount became of dense H O and the strength increased. After 3 days, when CN was added, a considerable amount of H2O was2 wasand consumed, the strength with increased. an increase After in 3 the days, amount when of CN nitrite was/nitrate added, hydrate a considerable [15,17,18 amount,22]. This of relatively H2O was consumed,consumed, with with an an increase increase in in the the amount amount of of nitr nitrite/nitrateite/nitrate hydrate hydrate [15,17,1 [15,17,18,22].8,22]. This This relatively relatively reduced the amounts of C-S-H and Ca(OH)2 produced by the reaction in ordinary cement, and made reduced the amounts of C-S-H and Ca(OH)2 produced by the reaction in ordinary cement, and made thethe structurestructure withoutwithout CNCN denserdenser thanthan thatthat withwith CN.CN. Therefore,Therefore, thethe strengthstrength waswas consideredconsidered toto increase.increase. Materials 2020, 13, 3686 7 of 13

reduced the amounts of C-S-H and Ca(OH)2 produced by the reaction in ordinary cement, and made Materials 2020, 13, x; doi: FOR PEER REVIEW 7 of 12 Materialsthe structure 20202020,, 1313 without,, x;x; doi:doi: FORFOR CN PEERPEER denser REVIEWREVIEW than that with CN. Therefore, the strength was considered to increase.77 ofof 1212

40 ) 40 ) 2 )

2 1day 3day 7day 14day 2 1day1day 3day3day 7day7day 14day14day 30 3030

20 2020

10 1010 CompressiveStrength (N/mm CompressiveStrength (N/mm CompressiveStrength (N/mm 0 00 CN0 CN7 CN9 CN11 CN0 CN7 CN9 CN11 Figure 9. CompressiveCompressive strength. strength.

25 (Ⅰ) 25 (Ⅰ)(Ⅰ) CN0 CN7 CN0 CN7 CN9 CN11 20 CN9 CN11

(Ⅱ) (Ⅱ)(Ⅱ) 15

10 Temperature (℃) 10 Temperature (℃) Temperature (℃)

5

Age (hour) Age (hour) FigureFigure 10. 10. TemperatureTemperature history (up to 24 h). 3.3. Void Structure Properties 3.3. Void Structure Properties Figure 11 shows the void distribution on Day 1, where CN1 had a void distribution in the range of Figure 11 shows the void distribution on Day 1, where CN1 had a void distribution in the range 0.5 to5 µm, CN7 from 0.1 to 3 µm, CN9 from 0.05 to 0.5 µm, and CN11 from 0.03 to 0.1 µm. According of 0.5 to5 μm, CN7 from 0.1 to 3 μm, CN9 from 0.05 to 0.5 μm, and CN11 from 0.03 to 0.1 μm. to these results, the void diameter and volume tended to decrease as the amount of CN added was According to these results, the void diameter and volume tended to decrease as the amount of CN increased. In particular, in the cases of a considerable amount of CN being added (CN9 and CN11), added was increased. In particular, in the cases of a considerable amount of CN being added (CN9 many voids were formed within the range of 0.05 µm or less, which was considered to substantially and CN11), many voids were formed within the range of 0.05 μm or less, which was considered to affect dry shrinkage [16]. Figure 12 shows the void distribution by Day 14. In all cases, the void volume substantiallysubstantially affectaffect drydry shrinkageshrinkage [1[16]. Figure 12 shows the void distribution by Day 14. In all cases, tended to be smaller than that on Day 1. These results indicate that CN addition increased the amount thethe voidvoid volumevolume tendedtended toto bebe smallersmaller thanthan thatthat onon DayDay 1.1. TheseThese resultsresults indicateindicate thatthat CNCN additionaddition of nitrite/nitrate hydrate produced immediately after mixing, and promoted hydration; moreover, increasedincreased thethe amountamount ofof nitrite/nitratenitrite/nitrate hydratehydrate producedproduced immediatelyimmediately afterafter mixing,mixing, andand promotedpromoted the voids were filled, resulting in a good strength on Day 1. On the other hand, after 14 days, there hydration; moreover, the voids were filled, resulting inin aa goodgood strengthstrength onon DayDay 1.1. OnOn thethe otherother hand,hand, was no clear relationship between the void structure and the fact that the case of CN addition had a after 14 days, there was no clear relationship between thethe voidvoid structurestructure andand thethe factfact thatthat thethe casecase ofof lower compressive strength than that with no addition of CN. CN addition had a lower compressive strength than that with no addition of CN.

2.5 2.52.5 CN0 CN7 2 CN0 CN7 22 CN9 CN11 CN9 CN11 1.5 1.51.5

1 11

0.5 Pore volume (Vol.%) Pore 0.5 Pore volume (Vol.%) Pore Pore volume (Vol.%) Pore 0 00 0.001 0.01 0.1 1 10 100 1000 0.0010.001 0.01 0.01 0.1 0.1 1 1 10 10 100 100 1000 1000 Pore size (μm) Pore size (μm) FigureFigure 11. VoidVoid distribution (1 day). Materials 2020, 13, 3686 8 of 13 MaterialsMaterialsMaterialsMaterials 2020 2020 2020, 13 ,2020 13, ,x; ,13 x;doi:, ,13 doi:x; , FOR doi:x; FOR doi: FORPEER PEERFOR PEER REVIEW PEER REVIEW REVIEW REVIEW 8 of8 12of 12 MaterialsMaterials 2020 2020, ,13 13, ,x; x; doi: doi: FOR FOR PEER PEER REVIEW REVIEW 88 of 8of of128 12 of12 12

2.52.52.5 2.5 2.52.5 CN0CN0CN0CN0CN0CN7CN7CN7CN7CN7 2 2 2 2 CN0 CN7 22 CN9CN9CN9CN9CN11CN11CN11CN11 CN9CN9 CN11CN11 1.51.51.5 1.5 1.51.5

1 1 1 1 11

0.50.50.5 0.5 Pore volume (Vol.%) Pore Pore volume (Vol.%) Pore

0.5volume (Vol.%) Pore 0.5volume (Vol.%) Pore Pore volume (Vol.%) Pore Pore volume (Vol.%) Pore 0 0 0 0 00 0.0010.0010.0010.001 0.01 0.01 0.01 0.01 0.1 0.1 0.1 0.1 1 1 1 101 10 10 10010 100 100 1000100 1000 1000 1000 0.0010.001 0.01 0.01 0.1 0.1 1 1 10 10 100 100 1000 1000 PorePore Poresize sizePore (μm)size (μm) size (μm) (μm) PorePore size size (μm) (μm) FigureFigureFigureFigureFigureFigure 12. 12.12.Figure 12. Void 12.Void Void 12. Void 12. distribution Voiddistributiondistribution distribution Void distribution distribution distribution (14 (14(14 (14 days).(14 days).days). days).(14 days). (14 days). days).

4.4.4. 4.Shrinkage Shrinkage4. Shrinkage 4.Shrinkage 4.Shrinkage Shrinkage Properties Properties Properties Properties Properties Properties of of of Mortar Mortarof Mortar of Mortar of Mortar Mortarwith with with with CNwith CN CNwith CN CN CN 4.1. Restrained Drying Shrinkage 4.1.4.1.4.1. 4.1.Restrained Restrained 4.1.Restrained Restrained4.1. Restrained Restrained Drying Drying Drying Drying Drying ShrinkageDrying Shrinkage Shrinkage Shrinkage Shrinkage Shrinkage Figure 13 shows the restrained shrinkage strain results obtained for the ring test. Table5 shows FigureFigureFigureFigureFigure 13 Figure13 13 shows 13shows shows 13 shows 13 shows theshowsthe the therestrained restrained therestrained restrainedthe restrained restrained shrinkage shrinkage shrinkage shrinkage shrinkage shrinkage strain strain strain strain strain re restrain resultssults results results obtainedsults reobtained obtainedsults obtained obtained obtained for for for thefor the thefor thering forring thering ringthe test. ringtest. test. ring test. Table test.Table Table test. Table Table 5 5 Tableshows5 shows 5shows shows5 shows5 shows the occurrence of cracks in each test piece and the number of days until the crack occurred. First, thethethe theoccurrence occurrence theoccurrence occurrencethe occurrence occurrence of of of cracks cracksof cracks of cracks ofcracks in cracksin in each eachin each in each intest eachtest testeach test piece piece testpiece testpiece pieceand and pieceand and the theand the and thenumber number thenumber numberthe number number of of of days daysof days of days ofuntildays until untildays until theuntil the theuntil thecrack crack thecrack crackthe crackoccurred. occurred. crackoccurred. occurred. occurred. occurred. First, First, First, First, First,the the theFirst, the the the the shrinkage started after casting, after ~10 h for CN11, 11 h for CN9, 12 h for CN11, and 36 h for CN0. shrinkageshrinkageshrinkageshrinkageshrinkageshrinkage started started started started started startedafter after after after casting,after casting, casting,after casting, casting, casting, after after after after ~10after ~10 ~10after ~10h h ~10hfor for h~10for hforCN11, CN11, CN11,forh CN11,for CN11, 11 CN11,11 11 h 11h hfor 11for hfor 11 hforCN9, CN9, CN9,forh CN9,for CN9,12 12 CN9,12 h 12h h for12for hfor 12 hforCN11, CN11, CN11,forh CN11,for CN11, and CN11,and and and 36 36and 36 h and36h hfor 36for hfor 36 hforCN0. CN0. CN0.forh CN0.for CN0. CN0. In particular, in the case of CN addition, shrinkage gradually occurred ~6 h after casting, after which InInIn particular, particular,In particular,In particular, Inparticular, particular, in in in the thein the in thecase case inthecase casethe of caseof of CNcase CNof CN of CN addition, addition, of CNaddition, CNaddition, addition, addition, shrinkag shrinkag shrinkag shrinkag shrinkag shrinkagee graduallye gradually graduallye graduallye graduallye gradually occurred occurred occurred occurred occurred occurred ~6 ~6 ~6 h ~6h hafter ~6after hafter ~6 afterh casting,after hcasting, casting,after casting, casting, casting, after after after after whichafter which whichafter which which which cracks occurred in the process of increasing shrinkage. The cracking dates were 2.8 days for CN11, crackscrackscrackscrackscracks occurred cracksoccurred occurred occurred occurred occurred in in in the thein the in theprocess process in theprocess processthe process process of of of increasing increasingof increasing of increasing ofincreasing increasing shrinkage. shrinkage. shrinkage. shrinkage. shrinkage. shrinkage. The The The The cracking crackingThe cracking Thecracking cracking cracking dates dates dates dates dateswere were dateswere were were2.8 2.8 2.8were 2.8days days days2.8 days2.8 daysfor for fordays forCN11, CN11, CN11,for CN11,for CN11, CN11, 3.6 days for CN9, and 4.4 days for CN7. The cracks tended to occur faster as the amount of CN was 3.63.63.6 days3.6 days days3.6 days3.6 fordays for fordays forCN9, CN9, CN9,for CN9,for CN9,and and CN9,and and 4.4 4.4and 4.4 anddays4.4 days days4.4 days4.4 fordays for fordays forCN7. CN7. CN7.for CN7.for CN7.The The CN7.The The cracks cracksThe cracks Thecracks cracks tended crackstended tended tended tended tendedto to to occur occurto occur to occur tooccur faster faster occurfaster faster faster as as fasteras the asthe the as theamount amountas theamount amountthe amount amount of of of CN CNof CN of CN was ofwas CNwas CNwas was was increased. The restrained shrinkage strains at the time of cracking were 25 µ for CN11, 27 µ for CN9, increased.increased.increased.increased.increased.increased. The The The The restrained restrainedThe restrained Therestrained restrained restrained shrinkage shrinkage shrinkage shrinkage shrinkage shrinkage strains strains strains strains strains at strainsat at the theat the at thetime time atthetime timethe of timeof of timecracking crackingof cracking of cracking ofcracking cracking were were were were were25 25 25were μ 25μ μfor 25for μfor 25 μCN11,for CN11, CN11,forμ CN11, for CN11, 27 CN11,27 27 μ 27μ μfor 27for μfor 27 μCN9,for CN9, CN9,forμ CN9, for CN9, CN9, and 30 µ for CN7. On the other hand, in the case of CN0, no cracks occurred during the measurement andandandand 30 30and 30 μ and30μ μfor 30for μfor 30 CN7.μfor CN7. CN7.forμ CN7. for CN7.On On CN7.On Onthe the Onthe theotherOn other theother otherthe otherhand, hand, otherhand, hand, hand, in inhand, in the thein the in thecase case inthecase casethe of caseof of CN0, case CN0,of CN0, of CN0, of CN0,no no CN0,no cracks nocracks cracks no cracks no cracks occurred cracksoccurred occurred occurred occurred occurred during during during during during theduring the the themeasurement measurement themeasurement measurementthe measurement measurement period of this experiment. periodperiodperiodperiodperiod of periodof of this thisof this of this experiment. ofexperiment.this experiment. thisexperiment. experiment. experiment.

0 0 0 0 00 CN0CN0CN0CN0CN7CN7CN7CN7 CN0CN0 CN7CN7 CN9CN9CN9CN9CN11CN11CN11CN11 -10-10-10-10 CN9CN9 CN11CN11 -10-10

-20-20-20-20 -20-20 Steel Strain (με) Steel Strain (με) Steel Strain (με) Steel Strain-30 (με) -30-30-30 Steel Strain-30 (με) Steel Strain-30 (με)

-40-40-40-40 -40-40 AgeAge Age(days) (days)Age (days) (days) AgeAge (days) (days) FigureFigureFigureFigureFigureFigure 13. 13.13.Figure 13. Restraint 13.Restraint Restraint 13. Restraint 13. Restraint Restraint shrinkage. shrinkage.shrinkage. shrinkage. shrinkage. shrinkage. shrinkage.

Table 5. Crack configuration and cracking days. TableTableTableTable Table5. 5. Table5.Crack Crack 5.Crack 5.Crack Crack5.configuration configuration Crackconfiguration configuration configuration configuration and and and andcracking cracking andcracking crackingand cracking cracking days. days. days. days. days. days.

CrackingCrackingCrackingCrackingCrackingCrackingCracking

CaseCaseCaseCaseCase Case Case CN0CN0CN0CN0 CN0 CN0CN0 CN7CN7CN7CN7 CN7 CN7 CN9CN9CN9 CN9 CN9 CN9 CN11CN11 CN11CN11CN11 CN11 DaysDaysDaysDays Daysof of Daysof of of of Days of Cracking------4.4 4.4 4.4 days 4.4days days 4.4 days 4.4 daysdays days 3.6 3.6 3.6 days 3.6days days 3.6 days 3.6 days days 2.8 2.8 2.8 days 2.8days daysdays 2.8 days 2.8 days days CrackingCrackingCrackingCrackingCrackingCracking

4.2.4.2.4.2. 4.2.Restrained Restrained 4.2.Restrained Restrained4.2. Restrained Restrained Tensile Tensile Tensile Tensile Tensile Stress TensileStress Stress Stress Stress and and Stressand andCrack Crack andCrack Crackand CrackPotential Potential CrackPotential Potential Potential Potential TheTheTheThe restrained restrainedThe restrained Therestrained restrained restrained tensile tensile tensile tensile tensile stress tensilestress stress stress stress can can stresscan canbe be canbe calculated becancalculated calculated be calculated be calculated calculated by by by Equation byEquation Equation by Equation by Equation Equation (1) (1) (1) using (1)using using(1) using (1) using the the usingthe theradii radii theradii radiithe ofradii of of radiithe theof the of theconcrete concrete oftheconcrete concretethe concrete concrete and and and and and and steelsteelsteelsteel steelring, ring,ring,steel ring, ring, restrained restrainedring,restrained restrained restrained restrained shrinkage shrinkageshrinkage shrinkage shrinkage shrinkage strain, strain,strain, strain, strain, andstrain, andand and elastiand elasti elasti andelasti celastic cmoduluselasti moduluscmodulus cmodulus modulusc modulus of ofof theof the theof the steel of steelthesteel thesteel steelring, ring,ring,steel ring, ring, assuming assumingring,assuming assuming assuming assuming that thatthat that thethat the the that the the the Materials 2020, 13, x; doi: FOR PEER REVIEW 9 of 12 Materials 2020, 13, x; doi: FOR PEER REVIEW 9 of 12 concrete poured into the ring specimen had a uniform shrinkage in the shear plane with a linear concrete poured into the ring specimen had a uniform shrinkage in the shear plane with a linear behavior [21,23–25]. behavior [21,23–25]. Materials 2020, 13, 3686 (𝛾2−𝛾2) (𝛾2+𝛾2) 9 of 13 𝜎 = (𝛾 2−𝛾 2) ∙(𝛾 2+𝛾 2) ∙𝐸 ∙𝜀 2𝛾 2 (𝛾 2−𝛾 2) (1) 𝜎 = ∙ ∙𝐸 ∙𝜀 (1) 2𝛾2 (𝛾2−𝛾2) 4.2.Here, Restrained σθimax indicates Tensile Stress the and restrained Crack Potential tensile stress, γis and γos indicate the internal and external Here, σθimax indicates the restrained tensile stress, γis and γos indicate the internal and external radii of the steel ring, γic and γoc indicate the internal and external radii of the concrete, Est indicates radii of theThe steel restrained ring, γ tensileic and γ stressoc indicate can be the calculated internal by and Equation external (1) radii using of the the radii concrete, of the concreteEst indicates the elastic modulus, and εst indicates the restrained shrinkage strain. the elasticand steel modulus, ring, restrained and εst indicates shrinkage the strain, restrained and elastic shrinkage modulus strain. of the steel ring, assuming that the Figure 14 shows the restrained tensile stress calculated by Equation (1), which tended to increase concreteFigure 14 poured shows into the therestrained ring specimen tensile hadstress a uniformcalculated shrinkage by Equation in the (1), shear which plane tended with a to linear increase with the restraint shrinkage strain. The maximum restrained tensile stress was 1.8 N/mm2 for CN11, behavior [21,23–25]. 2 with the restraint2 shrinkage strain. The2 maximum restrained tensile stress was 1.8 N/mm for CN11, 1.9 N/mm for CN9, and 2.1 N/mm for(γ osCN7,2 γ isand2) ( γcrackingim2 + γom occurred2) after reaching the maximum 1.9 N/mm2 for CN9, and 2.1 N/mmσθimax 2= for CN7,− and cracking occurredEst εst after reaching the maximum(1) tensile stress. The restrained tensile stress2 γincreaos2 sed· (γ omwith2 γtheim2 )amount· · of CN added because of the tensile stress. The restrained tensile stress increased with− the amount of CN added because of the increase Here,in theσ stressindicates generated the restrainedin the inner tensile steel stress, ring.γ Thisand γwasindicate confirmed the internal to accelerate and external the crack increase in the θstressimax generated in the inner steel ring. isThis wasos confirmed to accelerate the crack occurrenceradii of thein the steel mortar. ring, γ Itand is consideredγ indicate that the internal the stress and relaxation external radii was of redu the concrete,ced by theE indicatestensile creep. occurrence in the mortar. icIt is consideredoc that the stress relaxation was reduced by thest tensile creep. On thethe elasticother modulus,hand, the and crackingεst indicates potential the restrained is calculat shrinkageed from strain. the ratio of restrained tensile stress to On the other hand, the cracking potential is calculated from the ratio of restrained tensile stress to tensile strength.Figure 14 Figure shows the15 shows restrained the tensile change stress in tensile calculated strength by Equation over (1),time, which and tended Figure to 16 increase shows the tensile strength. Figure 15 shows the change in tensile strength over time, and Figure2 16 shows the crackwith potential the restraint in each shrinkage case. The strain. tensile The strength maximum was restrained calculated tensile by stressEquation was 1.8(2) Nusing/mm thefor result CN11, from crack potential2 in each case. The tensile2 strength was calculated by Equation (2) using the result from the compressive1.9 N/mm for strength CN9, and [26]: 2.1 N/mm for CN7, and cracking occurred after reaching the maximum the compressivetensile stress. strength The restrained [26]: tensile stress increased with the amount of CN added because of the . increase in the stress generated in the𝜎 inner= 0.291 steel ∙ ring. 𝐹𝑐 This was confirmed to accelerate the crack 𝜎 = 0.291 ∙ 𝐹𝑐. (2) occurrence in the mortar. It is considered that the stress relaxation was reduced by the tensile creep. (2) Here, σB refers to the tensile strength and Fc refers to the compressive strength. OnHere, the σ otherB refers hand, to the the tensile cracking strength potential and iscalculated Fc refers to from the thecompressive ratio of restrained strength. tensile stress to In the case of CN addition, the cracking potential increased between the ages of 1 and 2 days. tensileIn the strength.case of CN Figure addition, 15 shows the the cracking change potent in tensileial strengthincreased over between time, and the Figure ages 16of shows1 and the2 days. The possibility of crack occurrence became very high at an early age, compared to the case of no CN The possibilitycrack potential of crack in each occurrence case. The tensile became strength very high was calculated at an early by age, Equation compared (2) using to thethe resultcase of from no CN addition. According to this result, under the restraint conditions in this test, with an increase in the addition.the compressive According strength to this [result,26]: under the restraint conditions in this test, with an increase in the amount of CN added, both the shrinkage and crack occurrence0.658 possibility increased. σB = 0.291 Fc (2) amount of CN added, both the shrinkage and crack ·occurrence possibility increased. 2.5 CN0 CN7

) 2.5 2 CN0 CN7 ) CN9 CN11 2 2 2 CN9 CN11 1.5 1.5 1 1 0.5

Residual Stress (N/mm Stress Residual 0.5

Residual Stress (N/mm Stress Residual 0 0 Age (days) Age (days) Figure 14. Restraint tensile stress. FigureFigure 14. 14. RestraintRestraint tensile stress. stress.

4

) 4 2 CN0 CN7 )

2 CN0 CN7 3 CN9 CN11 3 CN9 CN11

2 2

1 1 Tensile Tensile Strength (N/mm

Tensile Tensile Strength (N/mm 0 0 13 7 14 13 7 14 Age (days) Age (days) FigureFigure 15. Tensile strength. strength. Figure 15. Tensile strength. Materials 2020, 13, x; doi: FOR PEER REVIEW 10 of 12

1.2 CN0 CN7 1 CN9 CN11 0.8

0.6

0.4

Cracking Potencial 0.2

0

Age (days) Materials 2020, 13, 3686 10 of 13 Materials 2020, 13, x; doi: FOR PEER REVIEW Figure 16. Cracking potential. 10 of 12

1.2 4.3. Degree of Restraint CN0 CN7 1 Figure 17 shows the changes in the drying shrinkageCN9 strainCN11 over time. On Day 1, after the start 0.8 of drying shrinkage, the drying shrinkage strain was 1.5 μ for CN0, 10.5 μ for CN7, 29 μ for CN9, and 36 μ for CN11, indicating that the0.6 drying shrinkage tended to increase with the amount of CN added. On Day 14, after the start of drying0.4 shrinkage, the drying shrinkage strain was 170 μ for CN0, 315 μ

for CN7, 372 μ for CN9, and 430Cracking Potencial 0.2 μ for CN11, indicating that the drying shrinkage on Day 1 tended to

increase with the amount of CN0 added, and became more pronounced. These results confirm that drying shrinkage increases with the amount of CN added, and agree with the restrained shrinkage results; moreover, when a considerable amountAge (days)of CN was added, restrained shrinkage was considered to increase with drying shrinkage.FigureFigure 16. 16. CrackingCracking potential. Figure 18 shows the degree of restraint in the specimen by the restraining body which, in this Here, σ refers to the tensile strength and F refers to the compressive strength. 4.3.test, Degree was calculated of RestraintB using Equation (3), in order cto evaluate the cracking [23–25]. In the case of CN addition, the cracking potential increased between the ages of 1 and 2 days. 𝜀(𝑡) TheFigure possibility 17 shows of crackthe changes occurrence in becamethe drying𝜑=1− very shrinkage high at an earlystrain age, over compared time. On to Day the case 1, after of no the CN start 𝜀 (𝑡) (3) of dryingaddition. shrinkage, According the todryi thisng result, shrinkage under strain the restraint was 1.5 conditions μ for CN0, in this10.5 test, μ for with CN7, an 29 increase μ for inCN9, the and 36 μ foramountHere, CN11, φ of represents CNindicating added, the boththat degree the shrinkagedrying of restraint, shrinkage and crackεst represents te occurrencended to the increase possibility amount with increased.of therestrained amount shrinkage, of CN added. and εsh represents the amount of drying shrinkage. εst is calculated by taking the concrete age of 0 days as On Day4.3. 14, Degree after of Restraintthe start of drying shrinkage, the drying shrinkage strain was 170 μ for CN0, 315 μ forthe CN7, start of372 the μ forrestrained CN9, and shrinkage. 430 μ for CN11, indicating that the drying shrinkage on Day 1 tended to increaseThe with Figuredegree the 17 of showsamount restraint the of changes tended CN added, into the increase dryingand beca slight shrinkagemely more with strain pronounced.the over amount time. Onof These CN Day added, 1,results after theand confirm start was of ~0.84 that drying shrinkage, the drying shrinkage strain was 1.5 µ for CN0, 10.5 µ for CN7, 29 µ for CN9, and dryingfor CN7, shrinkage ~0.87 for increases CN9, and with ~0.88 the foramount CN11. of This CN added,confirmed and thatagree the with time the the restrained cracking shrinkageoccurred, 36 µ for CN11, indicating that the drying shrinkage tended to increase with the amount of CN added. results;caused bymoreover, the degree when of arestraint, considerable became amount earlier of as CN the wasamount added, of CNrestrained added shrinkagewas increased. was On Day 14, after the start of drying shrinkage, the drying shrinkage strain was 170 µ for CN0, 315 µ Meanwhile, in the specimen with no CN added, after decreasing to 0.80, the degree of restraint tended consideredfor CN7, to 372 increaseµ for CN9, with and drying 430 µ shrinkage.for CN11, indicating that the drying shrinkage on Day 1 tended to to gradually increase, reaching ~0.86 on Day 14. The results showed that when a considerable amount increaseFigure 18 with shows the amount the degree of CN of added, restraint and in became the specimen more pronounced. by the restraining These results body confirm which, that in this test,of CN wasdrying was calculated added, shrinkage the using increases cracking Equation with potential the (3), amount in reached order of to CNa maximum,evaluate added, andthe and agreecracking cracking with [23–25]. the occurred restrained when shrinkage the degree of restraint was between ~0.84 and 0.88. Therefore, it is considered necessary to examine the crack results; moreover, when a considerable amount of𝜀 CN(𝑡) was added, restrained shrinkage was considered occurrenceto increase under with the drying restraint shrinkage. conditions𝜑=1− when using CN. (3) 𝜀(𝑡)

Here, φ represents the degree100 of restraint, εst represents the amount of restrained shrinkage, and εsh represents the amount of drying shrinkage. εst is calculatedCN0 by takingCN7 the concrete age of 0 days as 0 the start of the restrained shrinkage. CN9 CN11 The degree of restraint tended-100 to increase slightly with the amount of CN added, and was ~0.84 for CN7, ~0.87 for CN9, and ~0.88-200 for CN11. This confirmed that the time the cracking occurred, caused by the degree of restraint,-300 became earlier as the amount of CN added was increased. Strain (με) Meanwhile, in the specimen with-400 no CN added, after decreasing to 0.80, the degree of restraint tended to gradually increase, reaching ~0.86 on Day 14. The results showed that when a considerable amount -5000.7 of CN was added, the cracking potential036912 reached a maximum, and cracking occurred when the degree of restraint was between ~0.84 and 0.88. Therefore,Age it (days) is considered necessary to examine the crack occurrence under the restraint conditionsFigureFigure 17. 17. Unrestrained whenUnrestrained using dryingdrying CN. shrinkage. shrinkage.

Figure 18 shows the degree100 of restraint in the specimen by the restraining body which, in this test, was calculated using Equation (3), in order to evaluate theCN0 crackingCN7 [23–25]. 0 CN9 CN11 -100 εst(t) ϕ = 1 (3) -200 − εsh(t) -300 Strain (με) -400 -5000.7 036912 Age (days) Figure 17. Unrestrained drying shrinkage. Materials 2020, 13, 3686 11 of 13 Materials 2020, 13, x; doi: FOR PEER REVIEW 11 of 12

1 CN0 CN7 CN9 CN11 0.9

0.8 Degree of restraint 0.7

Age (days) FigureFigure 18. 18. DegreeDegree of restraint.restraint.

Here, ϕ represents the degree of restraint, ε represents the amount of restrained shrinkage, 5. Conclusions st and εsh represents the amount of drying shrinkage. εst is calculated by taking the concrete age of 0 days asIn thethis start study, of the each restrained experiment shrinkage. was conducted in order to clarify the shrinkage behavior and cracking Theof mortar degree when of restraint the amount tended to of increase nitrite-based slightly withaccelerator, the amount used of CNas an added, anti-freezing and was ~0.84 agent, for was changed.CN7, The ~0.87 results for CN9, are and summarized ~0.88 for CN11. as follows: This confirmed that the time the cracking occurred, caused by the degree of restraint, became earlier as the amount of CN added was increased. Meanwhile, in the (1) specimenWhen the with amount no CN of CN added, was after increased, decreasing hydration to 0.80, accelerated, the degree of the restraint mortar tended temperature to gradually increased increase,immediately reaching after ~0.86 casting, on Day an 14.d the The fluidity results showeddecreased. that when a considerable amount of CN was (2) added,On Day the 1, cracking the addition potential of a reached considerable a maximum, amount and of cracking CN promoted occurred whenhydration the degree and formed of restraint a large wasamount between of nitrite/nitrite ~0.84 and 0.88. hydrate, Therefore, resulting it is considered in dense necessaryvoids and to increased examine thestrength. crack occurrence (3) underAs the the amount restraint of conditionsCN was increased, when using shrinkage CN. increased and its start time became earlier. (4) Under the restraint conditions in this study—from the results of the cracking potential and 5. Conclusions degree of restraint—as the amount of CN was increased, the shrinkage and crack occurrence possibilityIn this study,increased. each experiment was conducted in order to clarify the shrinkage behavior and cracking of mortar when the amount of nitrite-based accelerator, used as an anti-freezing agent, Authorwas Contributions: changed. The resultsConceptualization, are summarized H.C. as follows:(Heesup Choi), J.K., and Y.S.; Data curation, J.K., A.Y.; Investigation,(1) When M.I., the H.C. amount (Hyeonggil of CN was Choi), increased, and Y.S.; hydration Project accelerated, administration, the mortar Y.T.; Supervision, temperature increasedH.C. (Heesup Choi); Supervision,immediately H.C. after (Heesup casting, Choi); and Writing the fluidity – original decreased. draft, Y.T.; Writing – review & editing, H.C. (Heesup Choi).(2) On Day 1, the addition of a considerable amount of CN promoted hydration and formed a large Funding: Thisamount research of nitrite received/nitrite no hydrate, externalresulting funding. in dense voids and increased strength. (3) As the amount of CN was increased, shrinkage increased and its start time became earlier. Acknowledgments: This research was supported by Nissan Chemical Co. of JAPAN. (4) Under the restraint conditions in this study—from the results of the cracking potential and Conflicts ofdegree Interest: of restraint—as The authors declare the amount no conflict of CN of was interest. increased, the shrinkage and crack occurrence possibility increased. References Author Contributions: Conceptualization, H.C. (Heesup Choi), J.K., and Y.S.; Data curation, J.K., A.Y.; 1. Akama, T.; Inoue, M.; Sudoh, U.; Mikami, S. Fresh properties and early strength development of concrete Investigation, M.I., H.C. (Hyeonggil Choi), and Y.S.; Project administration, Y.T.; Supervision, H.C. (Heesup Choi); Supervision,using calcium H.C. nitrite (Heesup and Choi); water-reducing Writing—original agents. draft, Proc. Y.T.; Jpn. Writing—review Concr. Inst. 2012 &, editing,34, 155–159. H.C.(Heesup Choi). 2. AllTaniguchi, authors haveM.; Nakamura, read and agreed T.; Koike, to the published S.; Nishi, version H. Study of the on manuscript. effect and mechanism of accelerator for freeze Funding:protection.This Hokkaido research Res. received Organ. no North. external Reg. funding. Build. Res. Inst. (Res. Rep.) 2015, 358, 1–11. 3. Acknowledgments:Hama, Y.; Kamada,This E. The research properties was supported of concre byte Nissan containing Chemical a frost-resistant Co. of JAPAN. accelerator. Concr. J. 1999, 37, Conflicts3–8. of Interest: The authors declare no conflict of interest. 4. Hama, Y.; Kamada, E. 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