Ultrasonic Monitoring of Setting and Strength Development of ... - MDPI

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Ultrasonic Monitoring of Setting and Strength Development of Ultra-High-Performance Concrete Doo-Yeol Yoo 1 , Hyun-Oh Shin 2 and Young-Soo Yoon 3, * 1 2 3

*

Department of Architectural Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Korea; [email protected] Department of Civil Engineering and Applied Mechanics, McGill University, 817 Sherbrooke Street West, Montreal, QC H3A 0C3, Canada; [email protected] School of Civil, Environmental and Architectural Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Korea Correspondence: [email protected]; Tel.: +82-2-3290-3320

Academic Editor: Jorge de Brito Received: 22 March 2016; Accepted: 13 April 2016; Published: 19 April 2016

Abstract: In this study, the setting and tensile strength development of ultra-high-performance concrete (UHPC) at a very early age was investigated by performing the penetration resistance test (ASTM C403), as well as the direct tensile test using the newly developed test apparatus, and taking ultrasonic pulse velocity (UPV) measurements. In order to determine the optimum surface treatment method for preventing rapid surface drying of UHPC, four different methods were examined: plastic sheet, curing cover, membrane-forming compound, and paraffin oil. Based on the test results, the use of paraffin oil was found to be the best choice for measuring the penetration resistance and the UPV, and attaching the plastic sheet to the exposed surface was considered to be a simple method for preventing the rapid surface drying of UHPC elements. An S-shaped tensile strength development at a very early age (before 24 h) was experimentally obtained, and it was predicted by a power function of UPV. Lastly, the addition of shrinkage-reducing and expansive admixtures resulted in more rapid development of penetration resistance and UPV of UHPC. Keywords: ultra-high-performance concrete; setting property; early age tensile strength; ultrasonic pulse velocity; surface treatment

1. Introduction For several decades, concrete has been widely used in the field of civil and architectural engineering all over the world, because of its excellent mechanical strength, durability, and economic efficiency. However, because of some drawbacks of concrete associated with low tensile strength, low ductility, and low strength-to-weight ratio, its application to thin-plate structures, e.g., thin walls and long-span bridge decks, has been limited thus far. In order to overcome these drawbacks, reactive powder concrete (RPC), which is the forerunner of the currently used ultra-high-performance concrete (UHPC), was developed by Richard and Cheyrezy [1] in the 1990s. Since UHPC is made from a granular mixture optimized by the packing density theory with a low water-to-binder ratio (W/B) and since it includes a high volume content of steel fibers, it exhibits both superior strength and ductility with a unique strain-hardening response. However, because of the low W/B and the inclusion of high-fineness admixtures in UHPC, the evaporation rate of water from the surface is normally too large to be replenished by bleeding. Because of this, the surface exposed to the atmosphere dries and condenses very quickly, even while most of the interior mortar is still fresh. This causes plastic shrinkage cracks at the surface, as well as durability and aesthetic problems. In addition, much quicker initial and final setting times of UHPC Materials 2016, 9, 294; doi:10.3390/ma9040294

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Materials 2016, 9, 294

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are obtained when these are measured as per ASTM C403 [2], because the needle penetrates through the condensed surface. Therefore, there is a pressing need for new ideas of appropriate methods to prevent the rapid drying and setting of the UHPC surface. According to a previous study [3], both free and restrained shrinkage of UHPC increase steeply at a very early age, leading to a high possibility of shrinkage cracking. In order to predict the cracking potential precisely, the tensile strength development needs to be investigated along with residual stress resulting from the restraint of shrinkage. For this reason, Yoo et al. [4] developed a direct tensile test apparatus to measure the tensile strength before the final set, based on a previous study [5]. However, it is time-consuming to measure the tensile strengths at certain times (especially at a very early age); thus, the field application is limited. An alternative nondestructive method for determining the strength development of concrete has been adopted by several researchers [6–10]. Because of its many advantages (e.g., continuous measurement of microstructural change in concrete, strong relationship between ultrasonic pulse velocity (UPV) and cement hydration, etc.), the nondestructive method has attracted much attention from engineers for field applications. However, to the best of the authors’ knowledge, only very limited study [4] is currently available for predicting the tensile strength development of UHPC at a very early age using a nondestructive method. In order to improve the shrinkage cracking resistance of UHPC, a number of restrained shrinkage tests have been conducted by many researchers [3,11–15]. In particular, Yoo et al. [3] reported that the cracking potential can be mitigated by selecting a lower reinforcement ratio and by using a reinforcing bar with a lower stiffness such as a glass fiber-reinforced polymer (GFRP) bar. Park et al. [14] employed a ring test to investigate the effect of using shrinkage-reducing admixture (SRA) and expansive admixture (EA) on the restrained shrinkage behavior of UHPC. Based on the test results, they concluded that the combined use of 1% SRA and 7.5% EA exhibits the best performance compared to other mixtures such as a UHPC mixture without SRA and EA or a UHPC mixture with SRA or EA alone. Therefore, the UHPC mixture with 1% SRA and 7.5% EA has been adopted for several structures built in Korea [16]. The effectiveness of the combined use of 1% SRA and 7.5% EA in reducing the shrinkage crack width of UHPC slabs has also been reported by Yoo et al. [15]. Accordingly, the present study experimentally investigated the setting, tensile strength, and UPV evolutions of two types of UHPC (with and without SRA and EA) at a very early age (before 24 h). In order to determine the optimum surface treatment method for preventing rapid surface drying, four different methods using plastic sheet, curing cover, membrane-forming compound, and paraffin oil were adopted. In addition, the very early age tensile strength was evaluated by a newly developed tensile test apparatus, and a simple power function relationship was developed to predict tensile strength on the basis of UPV. 2. Test Program 2.1. Materials, Mixture Proportions, and Mixing Sequence The mixture proportions used in this study are summarized in Table 1. Type 1 Portland cement produced in Korea and silica fume produced in Norway were used as cementitious materials; their chemical compositions and physical properties are given in Table 2. Silica sand with a grain size smaller than 0.5 mm was used as a fine aggregate, and silica flour including 98% SiO2 with a diameter of 2 µm was used as a filler. Coarse aggregate was not included in the mixture to improve its homogeneity. To provide adequate workability and viscosity, 1.2% of a high performance water-reducing agent, polycarboxylate superplasticizer (SP) with a density of 1.06 g/cm3 , was included. A W/B of 0.2 was adopted, and in order to improve the tensile performance, 2% by volume of short straight steel fibers with a length of 13 mm and a diameter of 0.2 mm were included. The detailed geometrical and mechanical properties of the steel fibers are given in Table 3.

proper fluidity and viscosity, the steel fibers were carefully dispersed and then it was mixed for an additional 5 min. Once an adequate fluidity and viscosity for preventing the fiber segregation from the mortar was achieved, the UHPC was cast in a mold. All test specimens were cured in a room with a temperature of 23 ± 1 °C and a humidity of 60% ± 5% during testing. Materials 2016, 9, 294

Steel Fiber Sand (vf, %) Steel Fiber SP Silica Sand EA SRA 867.4 1.2 2.0 (v , (%) f %) 865.3 59.0 867.4 - 7.9 1.2 1.2 2.0 2.0

Table 1. Mixture proportions of UHPC (unit weight, kg/m3 ). Silica Silica Silica

Specimen Specimen UH-N

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Table 1. Mixture proportions of UHPC (unit weight, kg/m3).

Water

Cement

Fume

EA

Flour

Water Silica Fume Flour 160.3Cement 788.5 197.1 Silica 236.6

SP (%)

SRA

UH-A 160.3 165.5 788.5 786.6 197.1 196.7 236.6 236.0 UH-N 59.0and7.9 1.2 2.0 165.5 786.6 admixture, 196.7SRA = shrinkage-reducing 236.0 865.3 UH-A Note: EA = expansive admixture, SP = superplasticizer. Note: EA = expansive admixture, SRA = shrinkage-reducing admixture, and SP = superplasticizer.

Table 2. Chemical compositions and physical properties of cementitious materials and admixture. Table 2. Chemical compositions and physical properties of cementitious materials and admixture.

Silica Expansive Fume Admixture Composition % (Mass) Cement Silica Fume Expansive Admixture CaO 61.33 0.38 13.55 CaO 61.33 0.38 13.55 Al2O3 6.40 0.25 18.66 Al2 O3 6.40 0.25 18.66 SiO 2 21.01 96.00 SiO2 21.01 96.00 3.80 3.80 3.12 0.12 Fe2 O3Fe2O3 3.12 0.12 MgO MgO 3.02 0.10 3.02 0.10 SO3 SO3 2.30 51.3551.35 2.30K2 O 0.56 - F-CaO K2O 16.020.56 F-CaO 3413 200,000 311716.02 Specific surface (cm2 /g) 3 ) (cm2/g) 3.15 2.10 2.98 3117 Specific surface 3413 200,000 Density (g/cm 3 Density (g/cm ) 3.15 2.10 2.98 Composition % (mass)

Cement

Table 3. Properties of straight steel fibers. Table 3. Properties of straight steel fibers.

Aspect Diameter Length Aspect Ratio Diameter Length df d (mm) Lf (mm) (LfRatio /df ) f (mm) Lf (mm) (Lf/df)

0.20.2

13.013.0

65.0 65.0

Density Density 3 (g/cm (g/cm)3)

Tensile Tensile Strength Strength (MPa) (MPa)

Elastic Elastic Modulus Modulus (GPa) (GPa)

7.9 7.9

2500 2500

200 200

Image Image

2.2. Test Equipment and Procedure Because of its high autogenous shrinkage and cracking potential at an early age [15], the combined use of 1% SRA and 7.5% EA has been studied by several previous researchers [14,15], and has been 2.2.1. Penetration Resistance Test (ASTM C403) adopted for practical applications in Korea [16], as mentioned previously. Therefore, two different In proportions order to investigate the early setting properties of UHPC, a penetration mixture for UHPC (i.e., aage UHPC mixture without SRA and EA, calledresistance “UH-N”,test andwas a performed as per ASTM C403 [2]. Since the high volume of steel fibers resist the penetration of the UHPC mixture with 1% SRA and 7.5% EA, called “UH-A”) were used in this study, as given in Table 1. needle, a UHPC without steel fiber wasbyused in the Chemie present test by simply assuming that the Glycol-based SRAmixture (METOLAT P 860) produced Münzing GmbH in Heilbronn, Germany steel fibers no effect on theJapan setting properties. addition, very fine silica sand, well dispersed and CSA EA have produced in Tokyo, (Table 2) wereIn adopted. For fabricating specimens, a Hobart-type laboratory mixer with 20 L capacity was used. Since UHPC has very low W/B and no coarse aggregate, the mixing sequence was different from that for ordinary concrete. Cement, silica fume, silica flour, and silica sand were premixed for about 10 min. Then, water with SP was added and mixed for another 10 min. When the state of the mortar exhibited proper fluidity and viscosity, the steel fibers were carefully dispersed and then it was mixed for an additional 5 min. Once an adequate fluidity and viscosity for preventing the fiber segregation from the mortar was achieved, the UHPC was cast in a mold. All test specimens were cured in a room with a temperature of 23 ˘ 1 ˝ C and a humidity of 60% ˘ 5% during testing.


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2.2. Test Equipment and Procedure 2.2.1. Penetration Resistance Test (ASTM C403) In order to investigate the early age setting properties of UHPC, a penetration resistance test was performed as per ASTM C403 [2]. Since the high volume of steel fibers resist the penetration of the needle, a UHPC mixture without steel fiber was used in the present test by simply assuming that the steel fibers have no effect on the setting properties. In addition, very fine silica sand, well dispersed in the mortar, is assumed to have insignificant implications on the setting properties. In the case of UHPC—owing to its low W/B and high fineness admixtures—the water evaporates from the surface Materials 2016, 9, 294 4 of 13 faster than it can be replenished by bleeding. Thus, the exposed surface dries rapidly and condenses the mortar, is the assumed to have insignificant implications the setting properties. the penetration case of evenin while most of interior cementitious matrix is still on fresh. For this reason, ifInthe UHPC—owing to its low W/B and high fineness admixtures—the water evaporates from the surface resistance test is performed without taking into account the rapid surface drying effect, the initial fastersetting than it times can bewill replenished by bleeding.Therefore, Thus, the exposed surfacefour driesdifferent rapidly and condenses and final be overestimated. in this study, surface treatment even while most of the interior cementitious matrix is still fresh. For this reason, if the penetration methods using a plastic sheet, a curing cover, a liquid-type membrane-forming compound, and liquid resistance test is performed without taking into account the rapid surface drying effect, the initial and paraffin oil were adopted, as given in Table 4. Three cylindrical plastic molds with a diameter of final setting times will be overestimated. Therefore, in this study, four different surface treatment 150 mm and a height of 160 mm were used for each variable. The mortar surface was determined methods using a plastic sheet, a curing cover, a liquid-type membrane-forming compound, and by 10liquid mm paraffin below the top edge of the moldinto provide thecylindrical surface treatment methods. The needle oil were adopted, as given Table 4. Three plastic molds with a diameter penetrated the mortar to a depth of 25 ˘ 2 mm in 10 s, and the clear distance rule of needle impressions of 150 mm and a height of 160 mm were used for each variable. The mortar surface was determined was complied by 10 mm with. below the top edge of the mold to provide the surface treatment methods. The needle penetrated the mortar to a depth of 25 ± 2 mm in 10 s, and the clear distance rule of needle impressions was complied with. Table 4. Surface treatment methods on casting surface. Specimen UH-N Specimen UH-A UH-N UH-A

Table 4. SurfaceCover treatment Membrane methods on casting surface. Plastic Sheet Curing Forming Compound

O Sheet Plastic XO X

O Cover Curing O O O

Paraffin Oil

O Membrane Forming Compound O O

O Oil Paraffin OO

O

O

2.2.2. UPV Measurement

2.2.2. UPV Measurement

Since the microstructural changes of concrete can be consistently estimated by using UPV, Since the microstructural changes of concrete can be consistently estimated by using UPV, several studies [6–10,17,18] have been conducted to examine the early age setting and strength several studies [6–10,17,18] have been conducted to examine the early age setting and strength developments in concrete byby UPV Inorder ordertotomeasure measure UPV immediately developments in concrete UPVmeasurement. measurement. In thethe UPV immediately after after concrete casting, a monitoring system asshown showninin Figures 1 and concrete casting, a monitoring systemwas wasimplemented implemented as Figures 1 and 2. 2. Transducer

300 100

300

350 100

Foam rubber UHPC

220

Bolt

Acrylic plate

Acrylic plate

Foam rubber

Transducer

350

350

(a)

(b)

Figure 1. Schematic diagram of UPV monitoring system (unit: mm): (a) top view; (b) side view.


3 1

UHPC Acrylic plate

Foam rubber

Transducer

350

350

(a)


(b)

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Figure 2. Picture of UPV monitoring system.

Figure 2. Picture of UPV monitoring system.

The mold was composed of four acrylic plates with a thickness of 20 mm, and an acrylic deck. Foam rubber was inserted between the acrylic plates and applied at the bottom surface above the acrylic deck to prevent the propagation of signals through the molds. This is necessary because of the fact that, since UHPC has a very low elastic modulus at an early age, the ultrasonic signals can travel more easily through the acrylic plates than the UHPC matrix. In addition, four bolts were fastened to assure a good contact between the used matrix and the acrylic plates. A commercial device for UPV measurement including a function generator (Agilent 33220A, Santa Clara, CA, USA) for generating the waveform, a filter-amplifier (Krohn-hite 3364, Brockton, MA, USA) for removing random noise and amplifying the received signal, a digital oscilloscope (Agilent 54624A) to measure the signal, and an ultrasonic transducer pair with a nominal frequency of 54 kHz was employed for the UPV measurements [19]. The frequency is applicable for fresh UHPC because its UPV is slower than that of hardened UHPC. Each ultrasonic transducer was mounted inside a circular hole at the mid-height of the mold, and the UPV data was measured by sending a wave from the transmitting transducer into the UHPC matrix and then receiving it with the receiving transducer. The travel time of a wave to pass through the UHPC matrix was recorded by the digital oscilloscope, and the pulse velocity was calculated by Vp “ L{∆t p (1) where Vp is the pulse velocity, L is the length of the straight wave path through the specimen, and ∆tp is the travel time of the wave. The digital oscilloscope measured only the longitudinal wave velocity. In addition, the measurements started just after concrete casting, and continued for 24 h at 30 min intervals. The steel fibers inside UHPC mortar disturb the accurate measurement of UPV; thus, a UHPC mixture without steel fiber (identical to that used for the penetration resistance test) was adopted. 2.2.3. Direct Tensile Test In accordance with a previous research [5], a direct tensile test apparatus was produced to measure the very early age tensile strength of UHPC. The test apparatus and the geometry of the test mold are shown in Figure 3. A uniaxial load was applied by a universal testing machine (UTM, Shimadzu, Japan) with a maximum load capacity of 250 kN through displacement control at a rate of 0.8 mm/min during testing. Since the tensile strength at a very early age (before 24 h) is extremely low, a load cell with a maximum capacity of 5 kN and a sensitivity of 1.25 N was used up to the tensile strength of 5 kN, and after that, another load cell with a maximum capacity of 20 kN and a sensitivity of 5 N was used. Therefore, reliable data were obtained. When the tensile strength reached almost 2 MPa (shortly after the final set), the tensile strength was measured using a dog-bone test method, as shown in Figure 4 [20]. The cross-section of the dog-bone specimens was 50 mm ˆ 100 mm. The alignment of the test specimen was carefully checked using a plumb before testing. In order to improve the accuracy of the test results, the test setup was designed with so-called pin-fixed ends to avoid secondary flexural

0.8 mm/min during testing. Since the tensile strength at a very early age (before 24 h) is extremely low, a load cell with a maximum capacity of 5 kN and a sensitivity of 1.25 N was used up to the tensile strength of 5 kN, and after that, another load cell with a maximum capacity of 20 kN and a sensitivity of 5 N was used. Therefore, reliable data were obtained. When the tensile strength reached almost 2 MPa (shortly after the final set), the tensile strength was measured using a dog-bone test Materials 2016, 9, 294 6 of 13 method, as shown in Figure 4 [20]. The cross-section of the dog-bone specimens was 50 mm × 100 mm. The alignment of the test specimen was carefully checked using a plumb before testing. In order to improve the accuracy of the test results, the test setup was designed with so-called pin-fixed ends to stress and to assure the centric loadings. The UTM, identical to that used for very early age tensile avoid secondary flexural stress and to assure the centric loadings. The UTM, identical to that used strength, was under displacement at adisplacement rate of 0.8 mm/min. for very used early age tensile strength, was control used under control at a rate of 0.8 mm/min.

100

30 70 130 30

110

70

70 430

70

110

(a)

(b)

Figure 3. Schematic view of tensile test machine: (a) geometry (unit: mm); (b) test setup and failure mode.

Figure 3. Schematic view of tensile test machine: (a) geometry (unit: mm); (b) test setup and failure mode. Materials 2016, 9, 294

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(a)

(b)

Figure 4. Dog-bone test: (a) geometry of specimen (unit: mm); (b) test setup.

Figure 4. Dog-bone test: (a) geometry of specimen (unit: mm); (b) test setup. After concrete casting, all the specimens were immediately covered and adhered to a plastic sheet in order to prevent water evaporation until the testing time.

After concrete casting, all the specimens were immediately covered and adhered to a plastic sheet in order to 3.prevent water Test Results andevaporation Discussion until the testing time. 3.1. Setting 3. Test Results andCharacteristics Discussionof UHPC by Penetration Resistance Test Figure 5 shows the penetration resistance versus elapsed time curves for all test specimens. The

3.1. Settingdevelopment Characteristics of UHPCresistance by Penetration Resistance Test by the surface treatment method. of penetration was noticeably influenced

In the case of the plastic sheet, the penetration resistance was developed most rapidly; thus, the initial

Figureand 5 shows the penetration resistance versus elapsed time curves for all test specimens. final setting times were earlier than those of the other cases in Figure 5a. This is because the The development of penetration resistance noticeably by the surface treatment plastic sheet was shortly removed fromwas the surface wheninfluenced the needle penetrated, and it caused the method. rapid surface drying since it was exposed to the atmosphere. This led to an overestimation of the In the case of the plastic sheet, the penetration resistance was developed most rapidly; thus, the initial penetration with age. the other hand, thecases use ofinaFigure membrane-forming compoundthe plastic and final setting timesresistance were earlier thanOnthose of the other 5a. This is because resulted in the slowest development of penetration resistance and the longest initial and final setting sheet was times. shortly removed from the surface when the needle penetrated, and it caused the rapid This is because the membrane-forming compound easily permeated into the UHPC mortar surface drying since it was and exposed atmosphere. This led to an overestimation the penetration without bleeding, causedtoathe delay of the setting development. The delay of theofstrength development andthe pulse velocity in the concrete at an early age has been reported previously by resistance with age. On other hand, use of a membrane-forming compound resulted in the Kim et al. [21]. Using a curing cover or paraffin oil resulted in intermediate penetration resistance slowest development of penetration resistance and the longest initial and final setting times. This is development and setting times. However, because of the addition of water at the surface, the use of because thethe membrane-forming compound easily permeated mortar curing cover delayed the setting development slightly moreinto thanthe the UHPC use of paraffin oil.without Based on bleeding, and causedthea chemical delay ofshrinkage the setting development. The delay of the strength development test results performed by Yoo et al. [4], the paraffin oil had an insignificantand pulse 3) than effect on the cement hydration. In addition, since paraffin oil has a lower density (0.88 g/cmUsing velocity in concrete at an early age has been reported previously by Kim et al. [21]. a curing that of water and does not mix with water, it is able to prevent water evaporation without any cover or paraffin oil resulted in intermediate penetration resistance development and setting times. obstacle of hydration. Therefore, even though the use of a curing cover produced similar behavior to However, that because of the of water atthe the surface, the use of most the curing cover the of paraffin oil, addition the use of paraffin oil on surface was chosen as the appropriate way delayed for setting development more than the use of paraffin oil. Based on the chemical shrinkage test measuring theslightly penetration resistance of UHPC. The specimen UH-A exhibited more rapid development of penetration resistance with age, causing earlier initial and final setting times, compared to the specimen UH-N without EA and SRA. This is consistent with the findings from a previous study [15], and is caused by fact that the development of setting in UHPC mortar was accelerated by the formation of large quantities of ettringite at an early age, resulting from the use of EA. On the other hand, a more gradual increase of penetration resistance after the initial set was obtained for UH-A than that for UH-N. The measured


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results performed by Yoo et al. [4], the paraffin oil had an insignificant effect on the cement hydration. In addition, since paraffin oil has a lower density (0.88 g/cm3 ) than that of water and does not mix with water, it is able to prevent water evaporation without any obstacle of hydration. Therefore, even though the use of a curing cover produced similar behavior to that of paraffin oil, the use of paraffin oil on the surface was chosen as the most appropriate way for measuring the penetration resistance Materials of UHPC. 2016, 9, 294 7 of 13

Penetration resistance (MPa)

40 Curing cover

Paraffin oil

Final set (28 N/mm2)

30

Plastic sheet 20 Membrane forming compound

10

Initial set (3.5 N/mm2) 0 0

5

10

15

20

Elapsed time (hour)

(a) Penetration resistance (MPa)

40 Curing cover

Final set (28 N/mm2)

30 Paraffin oil

20

Membrane forming compound

10

Initial set (3.5 N/mm2) 0 0

5

10

15

20

Elapsed time (hour)

(b) Figure 5. Penetration resistance versus elapsed time curve: (a) UH-N; (b) UH-A.

Figure 5. Penetration resistance versus elapsed time curve: (a) UH-N; (b) UH-A. Table 5. Initial and final setting times.

The specimen UH-A exhibited more rapid development penetration resistance Specimen Surface Treatment Initialof Set (hour) Final Set (hour)with age, causing Plastic sheet 8.8 UH-N without 11.5 EA and SRA. This is earlier initial and final setting times, compared to the specimen Curing cover 10.2 13.2 consistent with theUH-N findings from a previous study [15], and is caused by fact that the development of Membrane forming compound 12.8 16.0 setting in UHPC mortar was accelerated large quantities12.8 of ettringite at an early Paraffinby oil the formation of10.5 age, resulting from the use of EA. On the cover other hand, a more gradual increase 10.7 of penetration resistance Curing 5.6 Membrane forming than compound 8.0 The measured 11.9 after the initial setUH-A was obtained for UH-A that for UH-N. initial and final setting Paraffin oil 5.8 10.2 times according to the surface treatment method are summarized in Table 5. 3.2. Evolution of UPV

Table 5. Initial and final setting times.

The typical ultrasonic waveforms of UH-N with paraffin oil at three different ages are shown in Figure 6. The waveforms were measured by recording the voltage signal at the receiving transducer Specimen Surface Treatment Set arrow (Hour) Final Set (Hour) with an oscilloscope (Figure 2). Point (A) with theInitial downward indicates the time when the input pulse was first transmitted. Point sheet (B) with the upward arrow Plastic 8.8 indicates the time when 11.5 the ultrasonic wave was received by the receiving transducer. Because of the increase of stiffness Curing cover 10.2 13.2 in the UHPC mortar with age, the travel time offorming the ultrasonic wave was noticeably reduced as the mortar aged. Membrane UH-N 12.8 In addition, a much higher compound dominant frequency in the waveforms was obtained at16.0 the ages of 12 and 24 h, compared to that of 8 h. This is because the stiffness (penetration resistance) of12.8 the mortar steeply Paraffin oil 10.5 increased after about 8 h, as shown in Figure 5a; thus, the pulse velocity through the mortar Curing cover 5.6 10.7 substantially increased from that point.

UH-A

Membrane forming compound Paraffin oil

8.0

11.9

5.8

10.2


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3.2. Evolution of UPV The typical ultrasonic waveforms of UH-N with paraffin oil at three different ages are shown in Figure 6. The waveforms were measured by recording the voltage signal at the receiving transducer with an oscilloscope (Figure 2). Point (A) with the downward arrow indicates the time when the input pulse was first transmitted. Point (B) with the upward arrow indicates the time when the ultrasonic wave was received by the receiving transducer. Because of the increase of stiffness in the UHPC mortar with age, the travel time of the ultrasonic wave was noticeably reduced as the mortar aged. In addition, a much higher dominant frequency in the waveforms was obtained at the ages of 12 and 24 h, compared to that of 8 h. This is because the stiffness (penetration resistance) of the mortar steeply increased after about 8 h, as shown in Figure 5a; thus, the pulse velocity through the mortar substantially increased from that point.

Voltage [10.0 mV/div]


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(A) (B)

Age = 8 hours Δt = 240 μs

Time [0.5 ms/div]


(a)

(A) (B)


Time [0.5 ms/div]


(b)

(A) (B)


Time [0.5 ms/div] (c) Figure 6. Ultrasonic waveforms from oscilloscope for UH-N with paraffin oil: (a) 8 h; (b) 12 h; (c) 24 h.

Figure 6. Ultrasonic waveforms from oscilloscope for UH-N with paraffin oil: (a) 8 h; (b) 12 h; (c) 24 h. The travel time of the ultrasonic wave in Equation (1) was obtained by subtracting the transit time of the ultrasonic wave through the acrylic plates from the measured total time. Figure 7 shows evolution of UPV calculated bywave Equation during the first h. Theobtained shape of UPV development The travelthe time of the ultrasonic in(1)Equation (1)24was by subtracting the transit was not significantly affected by the surface treatment method, whereas the UPV values at certain time of the ultrasonic wave through the acrylic plates from the measured total time. Figure 7 shows ages (after an increase point) were slightly lower when a curing cover or membrane-forming the evolution ofcompound UPV calculated by Equation (1)when during the 24 h. The shape with of UPV was used compared to the values paraffin oilfirst was used. This is consistent the development results of the penetration resistance test in Figure 5. However, the use of a plastic sheet, which was not significantly affected by the surface treatment method, whereas the UPV values at certain ages adhered to the exposed surface immediately after casting, provided similar UPV values as those of (after an increase point) wereisslightly lower when a curing or membrane-forming compound paraffin oil, which inconsistent with the findings from the cover penetration resistance test. This is

was used compared to the values when paraffin oil was used. This is consistent with the results of the penetration resistance test in Figure 5. However, the use of a plastic sheet, which adhered to

the air-filled space in the cement paste increased [22]. This observation was more obvious in the specimen UH-A than UH-N. Note in Figure 7 that specimen UH-A clearly exhibits an earlier starting time for the UPV increase than its counterpart. This is caused by the accelerated setting development at an early age, which, in turn, is caused by the large quantities of ettringite resulting from the addition of EA. On the other hand, a more gradual increase of the UPV was observed for specimen Materials 2016, 9, 294 9 of 13 UH-A than for UH-N, which is consistent with the penetration resistance test results. Similar UPV values at 24 h were obtained for both UH-N and UH-A. The values of UPV at 24 h obtained in this provided study (approximately m/s) similaroil, to the exposed surface immediately after casting, similar UPV3000 values as were thosequite of paraffin the UPVs of ordinarywith cement mortar atfrom 24 h,the reported by Reinhardt al. [17]. theplastic UPVs which is inconsistent the findings penetration resistanceettest. ThisSpecifically, is because the corresponding to the initial and setting timesUPV (in Table 5) were found to m/s 1217 m/s sheet was not removed from thefinal surface during testing, in contrast to be the685 case ofand penetration for UH-N,testing. and 1276Therefore, m/s and 1804 forthat UH-A, the case for when paraffin oil waswater used. resistance it is m/s noted the respectively, plastic sheet for is effective preventing rapid Thus, these UPV be used forsamples determining the initial test andsamples, final setting pointselements, of UHPC.etc.). evaporation at thevalues surfacecan of the UHPC (i.e., mechanical structural

Ultrasonic pulse velocity (m/s)

4000

Plastic sheet Curing cover Paraffin oil Membrane forming compound

3000

2000

1000

0 0

4

8

12

16

20

24

20

24

Elapsed time (hour)

(a) Ultrasonic pulse velocity (m/s)

4000

Curing cover Paraffin oil Membrane forming compound

3000

2000

1000

0 0

4

8

12

16

Elapsed time (hour)

(b) Figure 7. Evolution of of UPV in in UHPC mortar: (a)(a) UH-N; (b)(b) UH-A. Figure 7. Evolution UPV UHPC mortar: UH-N; UH-A.

Figure 8 shows a schematic view of typical UPV evolution. In agreement with previous research UPVs slightly decreased at a very early age, because when the cement was hydrated with water, [17,18,23], an S-shaped UPV development was obtained. In stage 1, the ultrasonic wave propagated the air-filled space in the cement paste increased [22]. This observation was more obvious in the through the fresh UHPC mortar (a water-like viscous phase), with a relatively low UPV of specimen UH-A than UH-N. Note in Figure 7 that specimen UH-A clearly exhibits an earlier starting approximately 400 m/s. At the onset of the hydration process, a slight decrease of UPV was obtained, time for the UPV increase than its counterpart. This is caused by the accelerated setting development resulting from the increase of air-filled pores [22]. As the hydration continued, a minimum quantity at an early age, which, in turn, is caused by the large quantities of ettringite resulting from the addition of EA. On the other hand, a more gradual increase of the UPV was observed for specimen UH-A than for UH-N, which is consistent with the penetration resistance test results. Similar UPV values at 24 h were obtained for both UH-N and UH-A. The values of UPV at 24 h obtained in this study (approximately 3000 m/s) were quite similar to the UPVs of ordinary cement mortar at 24 h, reported by Reinhardt et al. [17]. Specifically, the UPVs corresponding to the initial and final setting times (in Table 5) were found to be 685 m/s and 1217 m/s for UH-N, and 1276 m/s and 1804 m/s for UH-A, respectively, for the case when paraffin oil was used. Thus, these UPV values can be used for determining the initial and final setting points of UHPC. Figure 8 shows a schematic view of typical UPV evolution. In agreement with previous research [17,18,23], an S-shaped UPV development was obtained. In stage 1, the ultrasonic wave

Materials 2016, 2016, 9, 294 Materials 9, 294

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of hydration products was formed and then the UPV increased sharply (stage 2). The water-saturated

Ultrasonic pulse velocity (m/s)

propagated the fresh mortar (a water-like phase),products; with a relatively UPV of porous through solid structure wasUHPC continuously connected by viscous the hydration thus, the low waves approximately 400 m/s. At the onset of the hydration process, a slight decrease of UPV was obtained, propagated through the solid matrix instead of through the water-like viscous phase. Accordingly, a noticeable of increase the UPVpores was observed stage 2. Once most of the pores werequantity filled of resulting from rate the increase ofofair-filled [22]. Asduring the hydration continued, a minimum by hydration products, the slope of the UPV gradually sharply decreased and the to hydration products was formed and then theincrease UPV increased (stage 2).UPV Theconverged water-saturated a constant value in the solid structure (stage 3). porous solid structure was continuously connected by the hydration products; thus, the waves In the case when paraffin oil was used, the inflection points tA and tB in the S-shaped UPV propagated through the solid matrix instead of through the water-like viscous phase. Accordingly, a evolution curves are summarized in Table 6. The specimen UH-A exhibited a much shorter time to noticeable rate of increase of the UPV was observed during stage 2. Once most of the pores were filled complete stage 1, owing to the acceleration of stiffness development in the mortar that included EA. by hydration products, the slope of the UPV increase gradually decreased and the UPV converged to a In the same manner, a faster initiation of the steep increase in the shrinkage was obtained for UH-A constant value in the (stage (with 1% SRA andsolid 7.5% structure EA), compared to3). that for UH-N (without SRA and EA) [13]. Stage 1

Stage 2

tA

Stage 3

tB Elapsed time (hour)

Figure 8. Schematic view of UPV development (UH-N with paraffin oil).

Figure 8. Schematic view of UPV development (UH-N with paraffin oil). Table 6. Inflection points in UPV evolution curves.

In the case when paraffin oil was used, the inflection points tA and tB in the S-shaped UPV tB tA evolution curves are summarized inSpecimen Table 6. The specimen UH-A exhibited a much shorter time to (hour) (hour) complete stage 1, owing to the acceleration of stiffness development in the mortar that included EA. UH-N 9.8 17.2 In the same manner, a faster initiation of the steep increase in the shrinkage was obtained for UH-A UH-A 2.0 5.1 (with 1% SRA and 7.5% EA), compared to that for UH-N (without SRA and EA) [13]. 3.3. Early Age Tensile Strength Development Table 6. Inflection points in UPV evolution curves. The tensile strength development at a very early age is illustrated in Figure 9. Just as with the UPV evolution, S-shaped curves were obtained both UH-N UH-A. Before the tensile strength Specimen tA for (Hour) tBand (Hour) reached approximately 2 MPa (at an applied load of approximately 14 kN), the developed tensile test UH-N 9.8 17.2 apparatus was used (Figure 3), and after that, the dog-bone test method was adopted for measuring UH-A 2.0 5.1 the tensile strength (Figure 4). Specimen UH-A exhibited higher tensile strength at a very early age than UH-N did; this is consistent with the findings from the results of the penetration resistance test 3.3. Early AgeUPV Tensile Strength Development and the measurements. On the other hand, the tensile strengths became similar to each other at about 24 h, which is the same trend that was observed in the case of UPV evolution. For example, the The tensile strength development at a very early age is illustrated in Figure 9. Just as with the average tensile strengths were found to be 6.56 MPa for UH-N and 6.45 MPa for UH-A at about 22 h. UPV evolution, S-shaped curves were obtained for both UH-N and UH-A. Before the tensile strength In order to predict the very early age tensile strength of UHPC with respect to UPV, the simple reached approximately 2 MPa (at an applied load of approximately 14 kN), the developed tensile test power function previously suggested by Pessiki and Carino [6] was adopted in this study, as follows:

apparatus was used (Figure 3), and after that, the dog-bone test method was adopted for measuring b  aVp  higher tensile strength at a very early (2) age the tensile strength (Figure 4). Specimen UH-Aftexhibited than where UH-Nftdid; this is consistent with the findings from the results of the penetration resistance is the tensile strength (MPa), Vp is the pulse velocity (km/s), and a and b are the coefficients. test and the UPV measurements. On (2) thewas other hand, the tensile became similar to each In the current study, Equation chosen because it has astrengths simple formulation and exhibits theother at about 24 h, which is the same trend that was observed in the case of UPV evolution. For example, best fit with the experimental results compared to the other models suggested by Pessiki and Carino [7] and Pessiki Johnsonwere [9], which widely used the present time.MPa for UH-A at about the average tensileand strengths foundare tomost be 6.56 MPa foratUH-N and 6.45 22 h.


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In order to predict the very early age tensile strength of UHPC with respect to UPV, the simple power function previously suggested by Pessiki and Carino [6] was adopted in this study, as follows: ` ˘b f t “ a Vp

(2)

where ft is the tensile strength (MPa), Vp is the pulse velocity (km/s), and a and b are the coefficients. In the current study, Equation (2) was chosen because it has a simple formulation and exhibits the best fit with 2016, the experimental results compared to the other models suggested by Pessiki and Carino Materials 9, 294 11 of[7] 13 and Pessiki and Johnson [9], which are most widely used at the present time. Materials 2016, 9, 294 11 of 13 The The comparison comparison of of tensile tensile strength strength and and UPV UPV is is shown shown in in Figure Figure 10. 10. The The tensile tensile strength strength was was increased with exponential power function form. This consistent with the The comparison of in tensile strengthor UPV is shown in Figure The tensile was increased with the the UPV UPV in exponential orand power function form. This is is10. consistent withstrength the findings findings from and Carino Keating et [8][8] forfunction thethe relationship between compressive and increased with the UPV[7] in exponential oral. form. This is consistent with strength the findings fromPessiki Pessiki and Carino [7]and and Keating etpower al. for relationship between compressive strength UPV. In addition, the relationship betweenbetween and UPVand at abetween very early age was accurately from Pessiki Carino [7]relationship and Keating ettensile al. [8]strength for the relationship strength and UPV. Inand addition, the tensile strength UPV at compressive a very early age was 2 ) was found2 to be predicted a simple function, i.e.,function, thetensile coefficient determination and UPV. using In addition, thepower relationship between strength and UPV a(Rvery early (R age) was accurately predicted using a simple power i.e., theofcoefficient of at determination 0.99691 UH-N and 0.96742 for accurately using a simple power found tofor bepredicted 0.99691 for UH-N andUH-A. 0.96742 forfunction, UH-A. i.e., the coefficient of determination (R2) was found to be 0.99691 for UH-N and 0.96742 for UH-A. 8

Tensile strength (MPa) Tensile strength (MPa)

8 6

UH-N UH-A UH-N UH-A

by dog-bone test by dog-bone test

6 4 4 2

by developed test apparatus for very early agetest tensile strength by developed apparatus for

2

very early age tensile strength

0 0 0 0

4

8

12

16

20

24

Age 12 (hour) 16 20 24 Age (hour) Figure 9. Early ageage tensile strength development (before 2424 h).h). Figure 9. Early tensile strength development (before Figure 9. Early age tensile strength development (before 24 h). 4

8

8

Tensile strength (MPa) Tensile strength (MPa)

8 6

UH-N UH-A UH-N UH-A

6 4

ft = 0.249(Vp)3.057 ft = 0.249(Vp)3.057

4 2 2 0 0 0 0

ft = 0.146(Vp)3.478 ft = 0.146(Vp)3.478 500

1000

2500

3000

3500

500

Ultrasonic pulse 2000 velocity (m/s) 1000 1500 2500

1500

2000

3000

3500

Ultrasonic pulse velocity (m/s) Figure 10. Relationship between early age tensile strength and UPV. Figure 10.10. Relationship between early ageage tensile strength and UPV. Figure Relationship between early tensile strength and UPV.

4. Conclusions 4. Conclusions This study investigated the effects of the surface treatment method on the setting properties and 4. Conclusions This study investigated of In theaddition, surface treatment method on the strength setting properties and UPV evolution of two typesthe ofeffects UHPC. very early age tensile development This study investigated the effects of the surface treatment method on the setting properties and UPV evolution two types UHPC. In addition, very age tensile strength with development (before 24 h) wasofmeasured byof a newly developed tensile testearly apparatus and correlated the UPV. UPV ofdiscussions, two types by ofthe UHPC. Indeveloped addition, very early age tensile strength development (before (before 24above h) was measured a newly tensile testbe apparatus and correlated with the UPV. Fromevolution the following conclusions can drawn: From(1) theThe above the following can be drawn: use discussions, of a plastic sheet caused anconclusions overestimation of the penetration resistance, whereas the (1) The of a plastic sheet caused an overestimation of the penetration resistance, addition of ause membrane-forming compound led to an underestimation. The use of eitherwhereas paraffinthe oil addition of acover membrane-forming compoundvalues led to an underestimation. use of either oil or a curing resulted in intermediate with behaviors that The were similar to paraffin each other. or a curingthe cover resulted in chosen intermediate values with behaviors that were to each other. However, paraffin oil was to be most appropriate for measuring the similar penetration resistance 3), its immiscibility However, the paraffin oillower was chosen to(0.88 be most appropriate for measuring the penetration resistance of UHPC because of its density g/cm with water, and the fact that it


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24 h) was measured by a newly developed tensile test apparatus and correlated with the UPV. From the above discussions, the following conclusions can be drawn: (1) The use of a plastic sheet caused an overestimation of the penetration resistance, whereas the addition of a membrane-forming compound led to an underestimation. The use of either paraffin oil or a curing cover resulted in intermediate values with behaviors that were similar to each other. However, the paraffin oil was chosen to be most appropriate for measuring the penetration resistance of UHPC because of its lower density (0.88 g/cm3 ), its immiscibility with water, and the fact that it did not interfere with hydration. (2) The shape of UPV development with age was not affected by the surface treatment method. In contrast, the use of a curing cover or membrane-forming compound resulted in slightly lower UPV values at certain ages, compared to the results when paraffin oil was used. Attaching a plastic sheet to the exposed surface was effective at preventing water evaporation, and thus, it is considered to be a simple method for preventing the rapid surface drying of UHPC elements. (3) The specimen UH-A (with 1% SRA and 7.5% EA) exhibited more rapid development of penetration resistance, leading to quicker initial and final setting times, and an earlier starting time for the steep increase in the UPV evolution, compared to those of UH-N (without SRA and EA). This is mainly caused by the accelerated stiffness development in the UHPC mortar at an early age due to the large quantities of ettringite that result from the addition of EA. (4) The S-shaped tensile strength development of UHPC at a very early age (before 24 h) was successfully measured in this study. The tensile strength increased monotonically with the UPV values and could be accurately predicted by using a simple power function. Acknowledgments: This research was supported by a grant (13SCIPA01) from Smart Civil Infrastructure Research Program funded by Ministry of Land, Infrastructure and Transport (MOLIT) of Korea government and Korea Agency for Infrastructure Technology Advancement (KAIA). Author Contributions: Doo-Yeol Yoo and Young-Soo Yoon conceived and designed the experiments; Doo-Yeol Yoo and Hyun-Oh Shin performed the experiments; Doo-Yeol Yoo analyzed the experimental data and wrote the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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