mechanical performance of shotcrete made with a high-strength cement-based mineral accelerator

Construction and Building Materials 49 (2013) 175–183

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Construction and Building Materials

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Mechanical performance of shotcrete made with a high-strengthcement-based mineral accelerator

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.08.014

⇑ Corresponding author. Tel.: +82 2 450 3750; fax: +82 2 2201 0907.E-mail address: [email protected] (J.-P. Won).

Jong-Pil Won ⇑, Un-Jong Hwang, Cheol-Keun Kim, Su-Jin LeeDepartment of Civil & Environmental System Engineering, Konkuk University, Seoul 143-701, Republic of Korea

h i g h l i g h t s

� The mechanical performance of shotcrete that used a HS-CM accelerator was evaluated.� The shotcrete that used a HS-CM accelerator to enhance the long-term strength performance.� The test data were statistically analysed at the 95% confidence level.

a r t i c l e i n f o

Article history:Received 17 July 2013Received in revised form 2 August 2013Accepted 9 August 2013

Keywords:AcceleratorHigh strength cement-based mineralacceleratorPermanent liningShotcrete

a b s t r a c t

This research investigated the mechanical performance of shotcrete that used a high-strength cement-based mineral accelerator (HS–CM) to enhance the long-term strength performance. HS–CM was addedat 5–8% with respect to the cement weight; the cement-based mineral accelerator (CM) was mixed at 5%for comparison. The setting time, compressive strength, and flexural strength were measured. As the testresult, in case of setting time, HS–CM which used more than 6% was slower than CM at initial set; butfinal set was faster. Compared with the mixture made with the CM, the HS–CM had approximately thesame compressive strength at early age but higher compressive strength after 7 days. The trend in flex-ural strength was similar to that of the compressive strength.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Shotcrete is an important element in the construction of tunnelsor underground spaces. It is used as permanent supports to stabi-lise cross-sections after excavation. An accelerator is used toachieve the initial strength, reduce rebound, and suppress earlyground relaxation. The accelerator is important because it affectsnot only the early strength of the shotcrete but also the develop-ment of long-term strength, durability, and thickness [1–4].

Shotcrete accelerators are classified as alkali-free or silicate-,aluminate-, or cement-based minerals, depending on the mainmaterial that is present. The silicate accelerator gives a rapid initialset and a slow final set, while the aluminate accelerator provides aslow initial set and a fast final set. Both types suffer the samedeficiency of loss in strength and durability over the long term.Furthermore, their strong alkalinity may endanger workers andlead to environmental contamination. Increasing the acceleratoramount adds to the cost and also increases the rebound ratio[1–6]. To alleviate these problems, alkali-free and cement-based

mineral accelerators (CMs) that are environmentally friendly andprovide good long-term strength are now frequently used in con-struction sites [7–9].

The alkali-free accelerator has an aluminium compound as itsmain component and is used mainly in Europe; it is a replacementfor silicate and aluminate accelerators. It is less hazardous to hu-mans [5,6].

The CM is a powder accelerator that has calcium aluminate asits main component and is currently used in cement. In this appli-cation, large amounts of ettringite are formed. Its early accelerationis so powerful that cement can be laid in wet areas. It is character-ised by a lower rebound and lower long-term strength reduction. Itis less dangerous to humans at comparable loadings, and its smallparticle size and powdered form give it good working characteris-tics and facilitate site quality control [7–9].

A high-strength cement-based mineral accelerator (HS–CM)was developed to provide better long-term strength than a conven-tional CM. It also has excellent accelerating performance and earlystrength development. This research studied the accelerating andmechanical performance of shotcrete made with the HS–CM. Theresults were statistically compared with the data for mixturesmade with CM.

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Table 1Physical properties of cement.

Blain fineness (cm2/g) Specific gravity Stability (%) Compressive strength (MPa)

3 days 7 days 28 days

3,330 3.15 0.08 30 42 58

Fig. 1. Grading curve of the fine aggregate.

Table 2Properties of the accelerators.

Accelerator Component Type Specific gravity pH

HS–CM C12A7 Powder 2.78 11.65CM C12A7 Powder 2.76 11.7

Table 3Chemical components of the accelerators.

Accelerator Chemical components (%)

CaO Al2O3 Na2O SO3

HS–CM 40.06 32.30 1.69 16.71CM 40.14 29.57 12.59 0.94

Fig. 2. Test results for the setting time of the mortar with accelerators.

176 J.-P. Won et al. / Construction and Building Materials 49 (2013) 175–183

2. Materials and mix proportions

2.1. Cement and aggregate

Type I ordinary Portland cement was used; its physical properties are listed inTable 1. Coarse aggregate, with a specific gravity of 2.69 and a maximum size of10 mm, was used. The fine aggregate was washed sand having a specific gravityof 2.59 and a fineness modulus of 2.92. The particle size distribution of the fineaggregate is shown in Fig. 1.

2.2. High-strength cement mineral accelerator

A CM is based on the phenomenon that calcium aluminate mineral, which is atype of cement mineral, accelerates setting as it reacts with Portland cement. Itsprimary component is C12A7, which is amorphous and among the calcium alumi-nate minerals with the highest accelerating performance [9]. It is now widely used

Table 4Mix proportions.

Mixture Accelerator (C*%) Gmax (mm) Slump (mm) Air (%) W/B

Plain – 10 120 ± 2 4 ± 1 38CM 5

HS–CM 55.5678

in tunnel construction sites because of its excellent accelerating performance andstable strength development [7–9]. However, over the long term, its strengthenhancement is low compared with plain concrete; a high-strength admixtureshould be used to enhance the long-term strength. The HS–CM was developed toprovide early strength and long-term strength without needing a high-strengthadmixture. The HS–CM is a powder that has calcium aluminate as its main compo-nent and is added to a concrete mixture to improve adhesion and acceleratestrength development through rapid hardening. The HS–CM follows the samechemical reaction and manufacturing method and also uses 12CaO�7Al2O3 as theCM, but additionally contains fine (Blaine 4000–8000 cm2/g) powdered calcium sul-phur aluminate. This material forms the stable ettringite and enhances the long-term strength and durability as it reacts with cement, hardening accelerator, andanhydrous gypsum (added to enhance the speed of later-period hardening andcompressive strength development).

To produce the 12CaO�7Al2O3 powder, which is the main component of the HS–CM, first, quicklime and alumina by-products are mixed together and the mixture ismelted in a furnace. Then, the melt is rapidly cooled by spraying with water andcompressed air to form the amorphous 12CaO�7Al2O3, which is then ground to afine powder (Blaine fineness 5000–7000 cm2/g). To reduce costs, blast furnace slagthat has been ground to a powder is used as a finely powdered admixture. The hard-ening accelerator is made by using one or more of lithium carbonate, sodium car-bonate, magnesium sulphate, sodium sulphate, or aluminium sulphate. Table 2shows the basic properties of the HS–CM used in this research; its chemical compo-nents are listed in Table 3.

2.3. Mix proportions

The mechanical performances of shotcretes were measured for mixtures con-taining HS–CM at 5%, 5.5%, 6%, 7%, and 8% with respect to the cement weight. A mix-ture containing 5% of the CM was used as a control because that level is accepted asthe optimal mixing ratio. A high-strength mixture formulation with a designstrength of 45 MPa was used; Table 4 shows the mixing design values. The targetslump was 120 ± 20 mm with an air content of 4 ± 1%. Polycarbonate superplasticis-er was added to achieve the target slump. Samples are identified as CM or HS–CM,as appropriate, with a suffix corresponding to the mixing ratio.

(%) S/a (%) Unit weight (kg/m3) Admixture (C*%)

Water Cement Sand Gravel

60 190 500 942 652 0.6

Table 5Compressive strength test results.

Type Age Compressive strength (MPa) Mean (MPa)

Batch #1 Batch #2

1 2 3 1 2 3

Plain 1 day 20.77 19.36 21.06 21.66 20.55 20.97 20.737 days 38.36 36.09 39.85 37.13 38.67 38.08 38.0328 days 58.18 57.64 59.97 57.24 57.97 59.30 58.38

CM5 3 h 3.34 3.58 3.50 3.48 3.81 3.68 3.561 day 23.94 24.31 24.48 24.97 24.29 24.56 24.427 days 29.10 26.74 27.72 28.48 29.40 28.78 28.3728 days 38.39 37.06 37.74 37.76 37.46 38.04 37.74

HS–CM5 3 h 2.36 2.29 2.02 2.33 2.19 2.17 2.231 day 22.20 21.87 22.66 22.53 21.76 22.23 22.217 days 32.84 32.92 33.43 33.30 32.94 32.62 33.0128 days 49.33 48.73 48.78 49.82 50.11 49.68 49.41

HS–CM5.5 3 h 2.50 2.42 2.61 2.60 2.55 2.39 2.511 day 22.73 23.08 22.39 23.02 22.17 22.81 22.707 days 33.81 35.66 32.24 34.28 31.94 33.72 33.6028 days 50.14 48.91 47.71 49.82 49.96 50.17 49.45




Com

pres

sive

stre

ngth

(MPa

)

Mixture

Fig. 3. Compressive strength test results.

Table 6Analysis of variance of the compressive strength test results.

3hours Sum ofsquares

DFa Mean ofsquares

F-ratio Significance

Mixture 7.258 5 1.452 121.428b 0.000Error 0.359 30 0.012Total 7.616 35

1 dayMixture 52.292 6 8.715 36.293b 0.000Error 8.405 35 0.240Total 60.697 41

7 daysMixture 329.444 6 54.907 67.474b 0.000Error 28.482 35 0.814Total 357.926 41

28 daysMixture 1509.744 6 251.624 423.799b 0.000Error 20.781 35 0.594Total 1530.525 41

a Degree of freedom.b Statistically significant at the 95% confidence level.

J.-P. Won et al. / Construction and Building Materials 49 (2013) 175–183 177

3. Experimental

3.1. Setting time

The setting of shotcrete with the addition of accelerator is sofast that there is insufficient time for sifting unhardened shotcrete.Therefore, a mortar having a water/cement ratio (W/C) = 0.5 and asand/cement ratio (S/C) = 3 was prepared in accordance with theASTM C403/C403M-08 standard [10]. Mixtures were made induplicate.

3.2. Compressive strength

The compressive strength of the concrete made using the HS–CM was tested in accordance with the ASTM C39/C39M-12A stan-dard [11]. Sufficient cylindrical specimens 100 mm in diameterand 200 mm long were prepared such that 3 specimens could beused for each test at the ages of 3 h, 1, 7, and 28 days; the test

was run in duplicate. The prepared specimens were cured for1 day at a constant temperature of 23 ± 2 �C and a constant relativehumidity of 50% and then water-cured at 23 ± 2 �C. The specimenthat was tested 3 h after mixing was stored at room temperature.

3.3. Flexural strength

The flexural strength of concrete made using the HS–CM wasmeasured in accordance with the ASTM C78/C78M-10e1 standard

Table 7Multiple comparisons of the compressive strength test results.

(I) (J) Mean difference (I–J) Standard error Significance 95% confidence interval for the mean

Lower bound Upper bound

(a) 3 hCM5 HS–CM5 1.33833* 0.06312 0.000 1.2094 1.4672

HS–CM5.5 1.05333* 0.06312 0.000 0.9244 1.1822HS–CM6 0.99667* 0.06312 0.000 0.8678 1.1256HS–CM7 1.16333* 0.06312 0.000 1.0344 1.2922HS–CM8 1.26667* 0.06312 0.000 1.1378 1.3956

HS–CM5 CM5 �1.33833* 0.06312 0.000 �1.4672 �1.2094HS–CM5.5 �0.28500* 0.06312 0.000 �0.4139 �0.1561HS–CM6 �0.34167* 0.06312 0.000 �0.4706 �0.2128HS–CM7 �0.17500* 0.06312 0.009 �0.3039 �0.0461HS–CM8 �0.07167 0.06312 0.265 �0.2006 0.0572

HS–CM5.5 CM5 �1.05333* 0.06312 0.000 �1.1822 �0.9244HS–CM5 0.28500* 0.06312 0.000 0.1561 0.4139HS–CM6 �0.05667 0.06312 0.376 �0.1856 0.0722HS–CM7 0.11000 0.06312 0.092 �0.0189 0.2389HS–CM8 0.21333* 0.06312 0.002 0.0844 0.3422

HS–CM6 CM5 �0.99667* 0.06312 0.000 �1.1256 �0.8678HS–CM5 0.34167* 0.06312 0.000 0.2128 0.4706HS–CM5.5 0.05667 0.06312 0.376 �0.0722 0.1856HS–CM7 0.16667* 0.06312 0.013 0.0378 0.2956HS–CM8 0.27000* 0.06312 0.000 0.1411 0.3989

HS–CM7 CM5 �1.16333* 0.06312 0.000 �1.2922 �1.0344HS–CM5 0.17500* 0.06312 0.009 0.0461 0.3039HS–CM5.5 �0.11000 0.06312 0.092 �0.2389 0.0189HS–CM6 �0.16667* 0.06312 0.013 �0.2956 �0.0378HS–CM8 0.10333 0.06312 0.112 �0.0256 0.2322

HS–CM8 CM5 �1.26667* 0.06312 0.000 �1.3956 �1.1378HS–CM5 0.07167 0.06312 0.265 �0.0572 0.2006HS–CM5.5 �0.21333* 0.06312 0.002 �0.3422 �0.0844HS–CM6 �0.27000* 0.06312 0.000 �0.3989 �0.1411HS–CM7 �0.10333 0.06312 0.112 �0.2322 0.0256

(b) 1 dayPlain CM5 �3.69667* 0.28292 0.000 �4.2710 �3.1223

HS–CM5 �1.48000* 0.28292 0.000 �2.0544 �0.9056HS–CM5.5 �1.97167* 0.28292 0.000 �2.5460 �1.3973HS–CM6 �2.16500* 0.28292 0.000 �2.7394 �1.5906HS–CM7 �1.27667* 0.28292 0.000 �1.8510 �0.7023HS–CM8 �0.51167 0.28292 0.079 �1.0860 0.0627

CM5 Plain 3.69667* 0.28292 0.000 3.1223 4.2710HS–CM5 2.21667* 0.28292 0.000 1.6423 2.7910HS–CM5.5 1.72500* 0.28292 0.000 1.1506 2.2994HS–CM6 1.53167* 0.28292 0.000 0.9573 2.1060HS–CM7 2.42000* 0.28292 0.000 1.8456 2.9944HS–CM8 3.18500* 0.28292 0.000 2.6106 3.7594

HS–CM5 Plain 1.48000* 0.28292 0.000 0.9056 2.0544CM5 �2.21667* 0.28292 0.000 �2.7910 �1.6423HS–CM5.5 �0.49167 0.28292 0.091 �1.0660 0.0827HS–CM6 �0.68500* 0.28292 0.021 �1.2594 �0.1106HS–CM7 0.20333 0.28292 0.477 �0.3710 0.7777HS–CM8 0.96833* 0.28292 0.002 0.3940 1.5427

HS–CM5.5 Plain 1.97167* 0.28292 0.000 1.3973 2.5460CM5 �1.72500* 0.28292 0.000 �2.2994 �1.1506HS–CM5 0.49167 0.28292 0.091 �0.0827 1.0660HS–CM6 �0.19333 0.28292 0.499 �0.7677 0.3810HS–CM7 0.69500* 0.28292 0.019 0.1206 1.2694HS–CM8 1.46000* 0.28292 0.000 0.8856 2.0344

HS–CM6 Plain 2.16500* 0.28292 0.000 1.5906 2.7394CM5 �1.53167* 0.28292 0.000 �2.1060 �0.9573HS–CM5 0.68500* 0.28292 0.021 0.1106 1.2594HS–CM5.5 0.19333 0.28292 0.499 �0.3810 0.7677HS–CM7 0.88833* 0.28292 0.003 0.3140 1.4627HS–CM8 1.65333* 0.28292 0.000 1.0790 2.2277

HS–CM7 Plain 1.27667* 0.28292 0.000 0.7023 1.8510CM5 �2.42000* 0.28292 0.000 �2.9944 �1.8456HS–CM5 �0.20333 0.28292 0.477 �0.7777 0.3710HS–CM5.5 �0.69500* 0.28292 0.019 �1.2694 �0.1206HS–CM6 �0.88833* 0.28292 0.003 �1.4627 �0.3140


Table 7 (continued)



HS–CM8 0.76500* 0.28292 0.011 0.1906 1.3394

HS–CM8 Plain 0.51167 0.28292 0.079 �0.0627 1.0860CM5 �3.18500* 0.28292 0.000 �3.7594 �2.6106HS–CM5 �0.96833* 0.28292 0.002 �1.5427 �0.3940HS–CM5.5 �1.46000* 0.28292 0.000 �2.0344 �0.8856HS–CM6 �1.65333* 0.28292 0.000 �2.2277 �1.0790HS–CM7 �0.76500* 0.28292 0.011 �1.3394 �0.1906

(c) 7 daysPlain CM5 9.66000* 0.52082 0.000 8.6027 10.7173

HS–CM5 5.02167* 0.52082 0.000 3.9643 6.0790HS–CM5.5 4.42167* 0.52082 0.000 3.3643 5.4790HS–CM6 3.76500* 0.52082 0.000 2.7077 4.8223HS–CM7 5.33500* 0.52082 0.000 4.2777 6.3923HS–CM8 7.50833* 0.52082 0.000 6.4510 8.5657

CM5 Plain �9.66000* 0.52082 0.000 �10.7173 �8.6027HS–CM5 �4.63833* 0.52082 0.000 �5.6957 �3.5810HS–CM5.5 �5.23833* 0.52082 0.000 �6.2957 �4.1810HS–CM6 �5.89500* 0.52082 0.000 �6.9523 �4.8377HS–CM7 �4.32500* 0.52082 0.000 �5.3823 �3.2677HS–CM8 �2.15167* 0.52082 0.000 �3.2090 �1.0943

HS–CM5 Plain �5.02167* 0.52082 0.000 �6.0790 �3.9643CM5 4.63833* 0.52082 0.000 3.5810 5.6957HS–CM5.5 �0.60000 0.52082 0.257 �1.6573 0.4573HS–CM6 �1.25667* 0.52082 0.021 �2.3140 �0.1993HS–CM7 0.31333 0.52082 0.551 �0.7440 1.3707HS–CM8 2.48667* 0.52082 0.000 1.4293 3.5440

HS–CM5.5 Plain 4.42167* 0.52082 0.000 �5.4790 �3.3643CM5 5.23833* 0.52082 0.000 4.1810 6.2957HS–CM5 0.60000 0.52082 0.257 �0.4573 1.6573HS–CM6 �0.65667 0.52082 0.216 �1.7140 0.4007HS–CM7 0.91333 0.52082 0.088 �0.1440 1.9707HS–CM8 3.08667* 0.52082 0.000 2.0293 4.1440

HS–CM6 Plain �3.76500* 0.52082 0.000 �4.8223 �2.7077CM5 5.89500* 0.52082 0.000 4.8377 6.9523HS–CM5 1.25667* 0.52082 0.021 0.1993 2.3140HS–CM5.5 0.65667 0.52082 0.216 �0.4007 1.7140HS–CM7 1.57000* 0.52082 0.005 0.5127 2.6273HS–CM8 3.74333* 0.52082 0.000 2.6860 4.8007

HS–CM7 Plain �5.33500* 0.52082 0.000 �6.3923 �4.2777CM5 4.32500* 0.52082 0.000 3.2677 5.3823HS–CM5 �0.31333 0.52082 0.551 �1.3707 0.7440HS–CM5.5 �0.91333 0.52082 0.088 �1.9707 0.1440HS–CM6 �1.57000* 0.52082 0.005 �2.6273 �0.5127HS–CM8 2.17333* 0.52082 0.000 1.1160 3.2307

HS–CM8 Plain �7.50833* 0.52082 0.000 �8.5657 �6.4510CM5 2.15167* 0.52082 0.000 1.0943 3.2090HS–CM5 �2.48667* 0.52082 0.000 �3.5440 �1.4293HS–CM5.5 �3.08667* 0.52082 0.000 �4.1440 �2.0293HS–CM6 �3.74333* 0.52082 0.000 �4.8007 �2.6860HS–CM7 �2.17333* 0.52082 0.000 �3.2307 �1.1160

(d) 28 daysPlain CM5 20.64167* 0.44487 0.000 19.7385 21.5448

HS–CM5 8.97500* 0.44487 0.000 8.0719 9.8781HS–CM5.5 8.93167* 0.44487 0.000 8.0285 9.8348HS–CM6 7.61500* 0.44487 0.000 6.7119 8.5181HS–CM7 9.53000* 0.44487 0.000 8.6269 10.4331HS–CM8 15.52167* 0.44487 0.000 14.6185 16.4248

CM5 Plain �20.64167* 0.44487 0.000 �21.5448 �19.7385HS–CM5 �11.66667* 0.44487 0.000 �12.5698 �10.7635HS–CM5.5 �11.71000* 0.44487 0.000 �12.6131 �10.8069HS–CM6 �13.02667* 0.44487 0.000 �13.9298 �12.1235HS–CM7 �11.11167* 0.44487 0.000 �12.0148 �10.2085HS–CM8 �5.12000* 0.44487 0.000 �6.0231 �4.2169

HS–CM5 Plain �8.97500* 0.44487 0.000 �9.8781 �8.0719CM5 11.66667* 0.44487 0.000 10.7635 12.5698HS–CM5.5 �0.04333 0.44487 0.923 �0.9465 0.8598HS–CM6 �1.36000* 0.44487 0.004 �2.2631 �0.4569

(continued on next page)


Table 7 (continued)



HS–CM7 0.55500 0.44487 0.220 �0.3481 1.4581HS–CM8 6.54667* 0.44487 0.000 5.6435 7.4498

HS–CM5.5 Plain �8.93167* 0.44487 0.000 �9.8348 �8.0285CM5 11.71000* 0.44487 0.000 10.8069 12.6131HS–CM5 0.04333 0.44487 0.923 �0.8598 0.9465HS–CM6 �1.31667* 0.44487 0.005 �2.2198 �0.4135HS–CM7 0.59833 0.44487 0.187 �0.3048 1.5015HS–CM8 6.59000* 0.44487 0.000 5.6869 7.4931

HS–CM6 Plain �7.61500* 0.44487 0.000 �8.5181 �6.7119CM5 13.02667* 0.44487 0.000 12.1235 13.9298HS–CM5 1.36000* 0.44487 0.004 0.4569 2.2631HS–CM5.5 1.31667* 0.44487 0.005 0.4135 2.2198HS–CM7 1.91500* 0.44487 0.000 1.0119 2.8181HS–CM8 7.90667* 0.44487 0.000 7.0035 8.8098

HS–CM7 Plain �9.53000* 0.44487 0.000 �10.4331 �8.6269CM5 11.11167* 0.44487 0.000 10.2085 12.0148HS–CM5 �0.55500 0.44487 0.220 �1.4581 0.3481HS–CM5.5 �0.59833 0.44487 0.187 �1.5015 0.3048HS–CM6 �1.91500* 0.44487 0.000 �2.8181 �1.0119HS–CM8 5.99167* 0.44487 0.000 5.0885 6.8948

HS–CM8 Plain �15.52167* 0.44487 0.000 �16.4248 �14.6185CM5 5.12000* 0.44487 0.000 4.2169 6.0231HS–CM5 �6.54667* 0.44487 0.000 �7.4498 �5.6435HS–CM5.5 �6.59000* 0.44487 0.000 �7.4931 �5.6869HS–CM6 �7.90667* 0.44487 0.000 �8.8098 �7.0035HS–CM7 �5.99167* 0.44487 0.000 �6.8948 �5.0885

* Statistically significant at the 95% confidence level.

Table 8Flexural strength test results.

Type Age Flexural strength (MPa) Mean (MPa)

Batch #1 Batch #2

1 2 1 2

Plain 1 day 3.71 3.24 3.21 3.52 3.4228 days 6.39 6.06 6.79 6.46 6.43

CM5 1 day 3.57 3.65 3.51 3.49 3.5628 days 4.65 4.46 4.85 4.79 4.69

HS–CM5 1 day 3.45 3.36 3.41 3.20 3.3628 days 4.98 4.92 4.86 4.94 4.93

HS–CM5.5 1 day 3.36 3.41 3.35 3.29 3.3528 days 5.31 5.04 4.94 5.23 5.13

HS–CM6 1 day 3.48 3.42 3.39 3.30 3.4028 days 5.36 5.03 5.24 5.41 5.26

HS–CM7 1 day 3.27 3.21 3.32 3.35 3.2928 days 5.09 5.18 5.21 5.15 5.16

HS–CM8 1 day 2.83 2.91 2.99 3.04 2.9428 days 4.90 4.82 4.94 5.01 4.92

Flex

ural

stre

ngth

(MPa

)

Mixture

Fig. 4. Flexural strength test results.

Table 9Analysis of variance of the flexural strength test results.

Sum of squares DFa Mean of squares F-ratio Significance

1 dayMixture 0.866 6 0.144 10.700 0.000Error 0.283 21 0.013Total 1.149 27

28 daysMixture 7.710 6 1.285 47.709 0.000Error 0.566 21 0.027Total 8.276 27

a Degree of freedom.


Table 10Multiple comparisons of the flexural strength test results.



(a) 1 dayPlain CM5 �0.13500 0.08210 0.115 �0.3057 0.0357

HS–CM5 0.06500 0.08210 0.437 �0.1057 0.2357HS–CM5.5 0.06750 0.08210 0.420 �0.1032 0.2382HS–CM6 0.02250 0.08210 0.787 �0.1482 0.1932HS–CM7 0.13250 0.08210 0.121 �0.0382 0.3032HS–CM8 0.47750* 0.08210 0.000 0.3068 0.6482

CM5 Plain 0.13500 0.08210 0.115 �0.0357 0.3057HS–CM5 0.20000* 0.08210 0.024 0.0293 0.3707HS–CM5.5 0.20250* 0.08210 0.022 0.0318 0.3732HS–CM6 0.15750 0.08210 0.069 �0.0132 0.3282HS–CM7 0.26750* 0.08210 0.004 0.0968 0.4382HS–CM8 0.61250* 0.08210 0.000 0.4418 0.7832

HS–CM5 Plain �0.06500 0.08210 0.437 �0.2357 0.1057CM5 �0.20000* 0.08210 0.024 �0.3707 �0.0293HS–CM5.5 0.00250 0.08210 0.976 �0.1682 0.1732HS–CM6 �0.04250 0.08210 0.610 �0.2132 0.1282HS–CM7 0.06750 0.08210 0.420 �0.1032 0.2382HS–CM8 0.41250* 0.08210 0.000 0.2418 0.5832

HS–CM5.5 Plain �0.06750 0.08210 0.420 �0.2382 0.1032CM5 �0.20250* 0.08210 0.022 �0.3732 �0.0318HS–CM5 �0.00250 0.08210 0.976 �0.1732 0.1682HS–CM6 �0.04500 0.08210 0.589 �0.2157 0.1257HS–CM7 0.06500 0.08210 0.437 �0.1057 0.2357HS–CM8 0.41000* 0.08210 0.000 0.2393 0.5807

HS–CM6 Plain �0.02250 0.08210 0.787 �0.1932 0.1482CM5 �0.15750 0.08210 0.069 �0.3282 0.0132HS–CM5 0.04250 0.08210 0.610 �0.1282 0.2132HS–CM5.5 0.04500 0.08210 0.589 �0.1257 0.2157HS–CM7 0.11000 0.08210 0.195 �0.0607 0.2807HS–CM8 0.45500* 0.08210 0.000 0.2843 0.6257

HS–CM7 Plain �0.13250 0.08210 0.121 �0.3032 0.0382CM5 �0.26750* 0.08210 0.004 �0.4382 �0.0968HS–CM5 �0.06750 0.08210 0.420 �0.2382 0.1032HS–CM5.5 �0.06500 0.08210 0.437 �0.2357 0.1057HS–CM6 �0.11000 0.08210 0.195 �0.2807 0.0607HS–CM8 0.34500* 0.08210 0.000 0.1743 0.5157

HS–CM8 Plain �0.47750* 0.08210 0.000 �0.6482 �0.3068CM5 �0.61250* 0.08210 0.000 �0.7832 �0.4418HS–CM5 �0.41250* 0.08210 0.000 �0.5832 �0.2418HS–CM5.5 �0.41000* 0.08210 0.000 �0.5807 �0.2393HS–CM6 �0.45500* 0.08210 0.000 �0.6257 �0.2843HS–CM7 �0.34500* 0.08210 0.000 �0.5157 �0.1743

(b) 28 daysPlain CM5 1.73750* .11605 .000 1.4962 1.9788

HS–CM5 1.50000* .11605 .000 1.2587 1.7413HS–CM5.5 1.29500* .11605 .000 1.0537 1.5363HS–CM6 1.16500* .11605 .000 .9237 1.4063HS–CM7 1.26750* .11605 .000 1.0262 1.5088HS–CM8 1.50750* .11605 .000 1.2662 1.7488

CM5 Plain �1.73750* .11605 .000 �1.9788 �1.4962HS–CM5 �.23750 .11605 .053 �.4788 .0038HS–CM5.5 �.44250* .11605 .001 �.6838 �.2012HS–CM6 �.57250* .11605 .000 �.8138 �.3312HS–CM7 �.47000* .11605 .001 �.7113 �.2287HS–CM8 �.23000 .11605 .061 �.4713 .0113

HS–CM5 Plain �1.50000* .11605 .000 �1.7413 �1.2587CM5 .23750 .11605 .053 �.0038 .4788HS–CM5.5 �.20500 .11605 .092 �.4463 .0363HS–CM6 �.33500* .11605 .009 �.5763 �.0937HS–CM7 �.23250 .11605 .058 �.4738 .0088HS–CM8 .00750 .11605 .949 �.2338 .2488

HS–CM5.5 Plain �1.29500* .11605 .000 �1.5363 �1.0537CM5 .44250* .11605 .001 .2012 .6838HS–CM5 .20500 .11605 .092 �.0363 .4463HS–CM6 �.13000 .11605 .275 �.3713 .1113HS–CM7 �.02750 .11605 .815 �.2688 .2138

(continued on next page)


Table 10 (continued)



HS–CM8 .21250 .11605 .081 �.0288 .4538

HS–CM6 Plain �1.16500* .11605 .000 �1.4063 �.9237CM5 .57250* .11605 .000 .3312 .8138HS–CM5 .33500* .11605 .009 .0937 .5763HS–CM5.5 .13000 .11605 .275 �.1113 .3713HS–CM7 .10250 .11605 .387 �.1388 .3438HS–CM8 .34250* .11605 .008 .1012 .5838

HS–CM7 Plain �1.26750* .11605 .000 �1.5088 �1.0262CM5 .47000* .11605 .001 .2287 .7113HS–CM5 .23250 .11605 .058 �.0088 .4738HS–CM5.5 .02750 .11605 .815 �.2138 .2688HS–CM6 �.10250 .11605 .387 �.3438 .1388HS–CM8 .24000 .11605 .051 �.0013 .4813

HS–CM8 Plain �1.50750* .11605 .000 �1.7488 �1.2662CM5 .23000 .11605 .061 �.0113 .4713HS–CM5 �.00750 .11605 .949 �.2488 .2338HS–CM5.5 �.21250 .11605 .081 �.4538 .0288HS–CM6 �.34250* .11605 .008 �.5838 �.1012HS–CM7 �.24000 .11605 .051 �.4813 .0013

* Statistically significant at the 95% confidence level.


[12]. Sufficient rectangular bars 100 � 100 � 400 mm were pre-pared such that 2 specimens could be used for each test at the agesof 1 and 28 days. The prepared specimens were cured in the sameway as for the compressive strength tests.

4. Results and discussion

4.1. Setting time

The test results for each accelerator mixing ratio are shown inFig. 2. Both the initial and final sets progressed faster in the mortarcontaining the HS–CM and increased with increasing acceleratormixing ratio. In every case, the initial set was essentially completewithin 5 min. The final set was faster than the CM mixture at addi-tion levels of 6%, 7%, and 8% due to hardening accelerator compo-nent in HS–CM which accelerated the hardening.

4.2. Compressive strength

The compressive strength test results for each mixture of shot-crete made using the HS–CM are shown in Table 5 and Fig. 3. Theshotcrete made with the HS–CM was higher than the plain shot-crete but lower than the shotcrete made with the conventionalCM at short ageing times. However, after 7 days, the compressivestrength of an HS–CM mixture was higher than that of a CM mix-ture. After 28 days, it was 13% higher than the plain shotcrete butwas 35% higher than the shotcrete made with the CM. The mixturecontaining 6% HS–CM had the highest strength. The strength islower than the CM mixture at early ageing times because calciumsulfo aluminate contained in the HS–CM is slow in setting. As thesample aged, the constantly-created ettringite concurrently mini-mised the formation of calcium hydroxide and voids, leading tohigher strength [13].

The compressive strength data were statistically analysed toestablish the influence of each accelerator at the 95% confidence le-vel. Analysis of variance and a posteriori multiple comparison testswere carried out on the data for each accelerator mix; the resultsare shown in Tables 6 and 7. As results of variance analysis, the sig-nificances were lower than 0.05 (95% confidence level) regardlessof age. It was consider that the accelerator significantly affectedthe compressive strength. To confirm the effect of accelerator on

the compressive strength, the multiple comparison test was per-formed. Multiple comparison tests showed the compressivestrength of shotcrete made using the CM5 was significantly higherthan the strength of plain and HS–CM at 3 h and 1 day because theall of the significance between CM5 and the other mixture waslower than 0.05. At 7 and 28 days, the compressive strength ofthe plain shotcrete was significantly higher than that of other mix-tures, and the strength of the HS–CM6 was significantly higherthan that of CM5 instead. Therefore, at the early ages of 3 h and1 day, the CM5 mixture was higher than both plain shotcrete andthe shotcrete made using the HS–CM. Also, at ages of 7 and28 days, the shotcrete made using the HS–CM had clearly highercompressive strength than the shotcrete made using the CM5. Itmeans that the shotcrete made using HS–CM was more affectiveto compressive strength than CM5.

4.3. Flexural strength

Flexural strength test results for each mixture of shotcretemade using the HS–CM are shown in Table 8. The 95% confidenceinterval of the average flexural strength for each mixture is shownin Fig. 4. The flexural strengths were lower for the HS–CM mixturethan for the CM mixture at the age of 1 day, but were higher in theHS–CM mixture after 28 days. However, when the accelerator mix-ing ratio was above 6%, the strength decreased with increasing age-ing. It was consider that hardening accelerator was hider the actionof calcium sulfo-aluminate, so the final set was done before the en-ough compressive strength secure.

At 28 days, the flexural strength using the HS–CM6 which hadthe lowest strength reduction was 18% lower than plain shotcrete,and 12% higher than shotcrete made using the CM5.

Statistical analysis was performed to confirm the influence ofeach accelerator mix on the flexural strength at the 95% confidencelevel. Analysis of variance and a posteriori multiple comparisontests were carried out on the flexural strength data for each accel-erator mix; the results are shown in Tables 9 and 10. Analysis ofvariance showed that the accelerator significantly affected the flex-ural strength. As results of variance analysis, the significances werelower than 0.05 (95% confidence level) regardless of age. It wasconsider that the accelerator significantly affected the flexuralstrength. To confirm the effect of accelerator on the flexuralstrength, the multiple comparison test was performed. Multiple


comparison tests showed that after ageing for 1 day, the flexuralstrength of shotcrete made using the CM5 was significantly higherthan that of plain shotcrete and shotcrete made using the HS–CM6.As the analysis result based on CM5, the mean differences weresmall compared with plain and HS–CM6. It was shown that theflexural performance was not difference among the types of mix-ture at the early age. At 28 days, the flexural strength of plain shot-crete was significantly higher than that of the other mixtures, andthe flexural strength of the HS–CM6 was significantly higher thanthat of the CM5.

Hence, at the early age, the CM5 mixture had higher flexuralstrength than plain shotcrete and the shotcrete made using theHS–CM. At 28 days, the shotcrete made using the HS–CM exhibitedhigher flexural strength than the shotcrete made using the CM5.

5. Conclusions

This research evaluated the mechanical performance of shot-crete made using the HS–CM. All of the mixtures had excellentlong-term strength. The setting times, compressive strengths, andflexural strengths were measured for each addition level of theHS–CM and were compared statistically with data for the mixturesmade with the CM. Comparisons were made at the 95% confidencelevel.

(1) For the HS–CM mixtures, as the mixing ratio increased, theinitial and final sets appeared more quickly. When the mix-ing ratio of the HS–CM exceeded 6%, the initial set appearedmore slowly but the final set appeared more quickly thanthe CM mixture.

(2) At ages of 3 h and 1 day, the compressive strengths of theHS–CM mixtures were slightly lower than those of the CMmixtures. After 7 days, the strengths were good but wereslightly lower than that of plain shotcrete.

(3) Statistical analysis at the 95% confidence level showed thatan accelerator significantly influenced the compressivestrength at each mixing ratio. Multiple comparison testsshowed that at early ages, the compressive and flexuralstrength of shotcrete made using the CM5 was significantly

higher than the strength of plain shotcrete and the shotcretemade using the HS–CM. After ageing for a long time, thecompressive and flexural strength of plain shotcrete was sig-nificantly higher than that of other mixtures, and thestrength of the 6% HS–CM mixture was significantly higherthan that of shotcrete made with the CM5.

Acknowledgements

The support of Samsung C&T corporation and Union corpora-tion is acknowledged.

References

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[10] ASTM C403/C403M-08. Standard test method for time of setting of concretemixtures by penetration resistance. ASTM International. PA, USA. 2008.

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mechanical performance of shotcrete made with a high-strength cement-based mineral accelerator

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