Construction and Building Materials 208 (2019) 767–779

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Construction and Building Materials

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Properties of calcium sulfoaluminate cement made ultra-highperformance concrete: Tensile performance, acoustic emissionmonitoring of damage evolution and microstructure

https://doi.org/10.1016/j.conbuildmat.2019.03.0570950-0618/� 2019 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Key Laboratory of Advanced Civil EngineeringMaterials (Tongji University), Ministry of Education, 4800 Cao’an Road, Shanghai201804, China.

E-mail addresses: wukai@tongji.edu.cn, wangjunyan@tongji.edu.cn (K. Wu).

Jun-Yan Wang a,b, Zhen-Zhen Chen b, Kai Wu a,b,⇑aKey Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education, 4800 Cao’an Road, Shanghai 201804, Chinab School of Materials Science and Engineering, Tongji University, 4800 Cao’an Road, Shanghai 201804, China

h i g h l i g h t s

� ES-UHPC exhibits significant tensile strain-hardening behavior at early age.� ES-UHPC obtain high ductility and bonding between steel fiber and matrix at 1 d.� Damage evolution was monitored by acoustic emission method.� AE analysis proves that the ES-UHPC exhibits multiple cracking behavior at 28 d.� Ettringite is the main hydrates and the content increases rapidly from 4 h to 1 d.

a r t i c l e i n f o

Article history:Received 4 May 2018Received in revised form 11 December 2018Accepted 7 March 2019

Keywords:Calcium sulfoaluminate cementUltra-high performance concreteTensile performanceDamage evolutionAcoustic emissionMicrostructure

a b s t r a c t

The properties of early age strength ultra-high performance concrete (ES-UHPC) made with calcium sul-foaluminate (CSA) cement, including tensile performance, damage evolution monitored by acoustic emis-sion (AE) and microstructure were evaluated in this work. Results show that the ultimate tensile straineUtu of ES-UHPC can reach to 0.199% at 1 d, which is 4.23 times larger than that of normal UHPC (N-UHPC) at the same testing age. AE testing results indicate that both types of UHPCs exhibit multiplecracking behavior at 28 d. With the tensile strain developing, the two UHPCs show a similar damage evo-lution trend and multiple microcracks with crack-width smaller than 0.01 mm generated after 0.046%strain reached. Microstructure tests demonstrate that ettringite presented after 4 h and the contentincreased significantly from 4 h to 3 d, which contributed to the early mechanical properties for ES-UHPCs.

� 2019 Elsevier Ltd. All rights reserved.

1. Introduction

Concrete is one of the most commonly used construction mate-rials in the world [1,2]. With aging of these constructions, manyconcrete structures are in urgent need of effective and durablerepair [2–4]. For example, almost 27% of all highway bridges arein need of repair or replacement in the USA [5].

Various of advanced materials and techniques have been devel-oped and applied in restoration projects. Unfortunately, it has beenestimated that up to half of all concrete repairs fail and about 3/4 ofthe failures are attributed to the lack of durability [6,7]. In previous

studies, most researchers focused on increasing the strength ofrepair materials or the bond strength between a repair materialand the concrete substrate [8]. However, with the strength increas-ing, brittleness is even more pronounced, as it is more prone tocracking. To reach durable repairs of civil infrastructure, concreterepair materials should have high tensile ductility to suppress brit-tle fracture [7,9–11].

Ultra-high performance concrete (UHPC) is a type of cementi-tious composites characterized by remarkable mechanical anddurability properties, and it has gained extensive attention overthe past decades [12–15]. According to the Swiss standard SIA2052–2016 [16], UHPCs can be divided into three types based onthe tensile behaviors,: UO (strain softening), UA (ultimate tensilehardening strain is higher than 1500le) and UB (ultimate tensilehardening strain is higher than 2000le). Strain-hardening UHPC(UA and UB) can be classified as strain-hardening cementitious

Table 1Chemical compositions of binders (wt%).

Composition CaO SiO2 Al2O3 MgO Fe2O3 SO3 Na2O K2O LOI

ES-UHPC binder 37.0 29.0 14.3 3.1 2.9 8.2 0.19 0.44 0.3N-UHPC binder 50.1 34.4 5.2 3.2 2.3 2.1 0.14 0.44 2

Table 2Mix proportions of UHPC matrix (wt%).

Component Binder Sand Steel fiber Water

OPC CSA Ground quartz GGBS SP

ES-UHPC – 39.51 4.39 – 0.20 40.6 6.4 8.9N-UHPC 30.77 – 6.59 6.59 0.15 40.6 6.4 8.9

Fig. 1. Particle size distribution of raw materials.

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composite (SHCC). The findings in literature indicated that SHCChas a potential of being used as a damage-tolerant material, whichis characterized by tensile strain hardening behavior, and the for-mation of multiple and narrow cracks under increasing loadingup to failure localization [11,17,18]. The extremely high straincapacity is accompanied with a limited crack width [19]. Thisadvantage inspires engineers to use SHCC and other strain-hardening cementitious based material to replace normal concretematerial in repairing and retrofitting of existing structures [20].Besides great tensile ductility, the materials used in repairing pro-jects should also have the characteristic of high early strength,which is mainly determined by the property of cementitious mate-rial employed [7,21].

In recent years, calcium sulphoaluminate cement (CSA) hasattracted the attention from scientists as well as of industry dueto its unique properties in comparison with Portland cement, suchas rapid strength development and good durability in aggressiveenvironments [22–24].The main mineral component in CSA isye’elimite (C4A3—), which contributes to a higher early strengthand entails a lower grinding energy and less COS2 releasing [25–28]. The ye’elimite reacts rapidly with water and form an initialettringite (AFt) and aluminum hydroxide (Eq. (1)), which is fol-lowed by the precipitation of calcium monosulpho aluminatehydrate (AFm) once sulfates were depleted (Eq. (2)):

Table 3Properties of steel fibers.

Tensile strength/MPa Elastic modulus/GPa Density/(kg/m3)

2500 200 7850

C4A3 S�þ2C S

�H2 þ 34H ! C3A � 3C S

��32Hþ 2AH3 ð1Þ

C4A3 S�þ18H ! C3A � C S

��12Hþ 2AH3 ð2Þ

CSA cement has been manufactured and used on an industrialscale for decades, while it is rarely applied to prepare UHPC. Ifthe strain-hardening UHPC can possess high enough strength atan early age, it is expected to contribute to fast and durable con-crete repairing projects with a reduced environmental issue. Thisresearch therefore aims to propose a reformed kind of strain-hardening UHPC, which adopted CSA cement as binder to obtainhigh early strength, named as the early age strength UHPC (ES-UHPC).

To investigate the properties of ES-UHPC and evaluate its poten-tial application for repairing construction, a series of experimentshave been done including tensile strength, cracks evolution andmicrostructure in this study. Previous researches show that repairmaterials should have a large tensile strain capacity and this prop-erty must be kept at all ages during the service life [21]. Therefore,tensile tests were conducted on ES-UHPCs at different curing ages(4 h, 1 d, 3 d, 7 d, 28 d). The tests were also performed on normalUHPC (N-UHPC) manufactured with Portland cement at 1 d and28 d for a comparison. Acoustic emission (AE) technology wasapplied to monitor the damage evolution process of the two typesof UHPCs under direct tensile test at 28 d. In addition, themicrostructure evolution of ES-UHPC matrix was determined byusing X-ray diffraction (XRD), thermal gravity analysis (TG) andscanning electron microscopy (SEM) to further analyze the mech-anism of mechanical response of ES-UHPC at different curingperiod.

2. Experimental

2.1. Materials and mixtures

In this paper, the UHPC materials adopted, including ES-UHPCand N-UHPC are commercial products named TENACAL� contain-ing three components: water, UHPC premixed powder and steelfibers. The components of ES-UHPC and N-UHPC are identicalexcept the binder employed. The chemical compositions of the

Length/mm Diameter/lm Slenderness ratio

13 200 65

(a) Tensile specimen (b) Specimen dimension

Fig. 2. Photo and dimension of specimen for tensile tests.

Fig. 3. Setup of tensile test equipped with AE analysis system.

Fig. 4. Schematic diagram of acoustic emission analysis system.

Fig. 5. Setup of AE test.

Fig. 6. Setup of pensile lead test.

Fig. 7. Positions of AE transducers.

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applied binders were determined by X-ray fluorescence spectrom-etry (XRF) and are given in Table 1. Table 2 provides the mix pro-portion of the two types of UHPCs. The particle size distribution ofthe raw materials (binder and sand) was measured using a laserparticle-size analyzer, and is shown in Fig. 1. A type of straightsmooth steel fibers with brass coating were employed and thedosage is 2% by volume. Table 3 presents the physical propertiesand geometry of steel fibers used in this study. Considering the

low water-to-binder ratio, a kind of powdery polycarboxylatesuperplasticizer (SP) was used to achieve good workability andthe ratio of reducing-water is 23%. It should be noted that thesuperplasticizer is introduced as one part of UHPC premixedpowder.

2.2. Specimen preparation

Mixing and specimen preparation were both conducted at roomtemperature. A laboratory mixer with sixty-liter capacity was usedto prepare the UHPC mixture. The mixing speed was 56 r/min andthe mixing motor power was 3kw. Since UHPC has different com-positions and mixture proportions from those of normal concrete, aspecial mixing sequence was adopted. First, UHPC premixed pow-der was dry-mixed for about 1 min. Water was then added gradu-ally and mixed for an additional 3 min until the mixture showedenough fluidity and viscosity. After that, the steel fibers were dis-persed evenly into the mixture and mixed for another 3 min to

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maintain apparent uniform fiber distribution. Finally, the UHPCmixtures were cast into in the molds without vibration. After cast-ing, the specimens were subsequently covered with plastic sheetsto prevent the rapid losing of water.

To imitate the real curing condition in the field applications,where the repairing project will be exposed to air and needed toopen to traffic in 4 h [7], all ES-UHPC specimens were demouldedin 4 h and subsequently cured in air at a room temperature of20 �C until designed testing ages. The tensile specimen after curingis shown in Fig. 2(a). For each age, ES-UHPC specimens were pre-pared as follows: 1) three dog-bone shaped specimens for directtensile test and the dimension applied is given in Fig. 2(b). Sincethe width (50 mm) of the specimens is longer than three times ofthe length of the fibers (13 mm), the influence of the fiber orienta-tion distribution could be ignored [29]. 2) three 100 mm cubicspecimens for compressive strength test according to Chinese stan-dard GB/T 31387-2015; 3) and three 100x100x300 mm prisms forelastic modulus test according to Chinese standard GB/T 31387-2015.

To compare the tensile properties, N-UHPC specimens were alsoprepared using the same method as mentioned above, and thedemoulding time for N-UHPC was set at 1 d. Then N-UHPC speci-mens were cured in the same condition as ES-UHPC until 1 d and28 d for tensile test.

2.3. Testing procedure

2.3.1. Mechanical propertiesCompressive strength (100 mm cubic specimen) and elastic

modulus (100 mm � 100 mm � 300 mm prism specimen) of eachES-UHPC specimen were measured according to Chinese standardGB/T 31387-2015 [30] using a universal testing machine (UTM)with a maximum load capacity of 3000 kN.

Direct tensile tests were conducted using a universal testingmachine (WDW-300 servo-controlled testing system) running indisplacement control manner. The tensile test set up is given inFig. 3. In order to avoid secondary flexural stress and ensure acentric-loading condition, a set of customize fixture was designed.Four high-precision linear variable displacement transducers(LVDTs) attached to the metal frame were used to measure theelongation of the tensile specimen in all dimensions. The average

Fig. 8. Compressive strength and elastic m

elongation value obtained from four LVDTs could be used to calcu-late tensile strain e of the specimen according to Eq. (3).

e ¼ Dl1 þ Dl2 þ Dl3 þ Dl44L

� 100% ð3Þ

where,

� Dl1;Dl2;Dl3;Dl4: The test value of the four LVDTs;� L: The gauge length is 150 mm.

The tensile stress f can be calculated from the imposed tensileloading force divided by the original cross section area (Eq. (4)).The dimensions of the cross section of each specimen within thegauge length were 100 mm � 50 mm.

f ¼ N � 1000100� 50

ð4Þ

where,

� N: The tensile loading force (kN).

For direct tensile test, two modes of loading control wereadopted: preloading and formal loading. The preloading modewas conducted under displacement control applying a loading rateof 1 mm/s. The mode of control was switched to formal loadingmode immediately when the load reached 0.5 kN, and accordinglythe loading rate changed to 0.3 mm/s. The test ended when thestress was lower than 30% of the peak stress and the whole tensiletest procedure of each specimen lasted for 30 min.

2.3.2. AE analysisAcoustic emission (AE) is a non-destructive examination

method to detect damages in materials [31]. An AE source isdefined as the transient elastic wave formed due to the rapidrelease of strain energy within a stressed material by propagatingmicro-damages [32,33]. The schematic diagram of AE analysis sys-tem is given in Fig. 4. When load increases close to its limitation,damages will spread in materials. During this process, energy willbe released and elastic waves can be detected by AE transducers onthe specimen surface. The waves will be converted into electricalsignals, and amplified to be able to perform the analysis [34,35].Through the preamplifier, all electrical signals will be recorded

odulus of ES-UHPCs at different ages.

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and stored by a windows-based AE data operation program and theprogram can be used to extract the AE parameters for further anal-ysis. For example, the 3D coordinates (x, y and z) and original timeof the AE source can be calculated to describe the damage evolu-tion process of the determined specimen.

A great number of researchers have indicated that AE techniqueis a useful method to characterize and locate the damage in con-crete at different loading stages. Lim and Koo [36] detected flexuralfailure of reinforced concrete beams by using AE and correlated theAE events and crack pattern of the beam at different loading stages.Maji and Ouyang [37] used AE method to detect the fracture pro-cess zone of concrete. In this study, the dynamic damage failureof UHPC under uniaxial tensile test was investigated by using AEmethod to analyze the damage evolution inside UHPC at differentloading stages.

(a) 4h

(c) 3d

(e) 2

Fig. 9. Strain-stress curves of E

The AE measurement system shown in Fig. 5 was applied in thisresearch to collect AE data during the direct tensile test. 8 AE trans-ducers were mounted in matrix form on the double surfaces of thespecimen to gather AE signals originating from the internal dam-ages of the UHPC’s material. Positions of AE transducers adoptedin this study are provided in Fig. 7. Before the tensile test, pencillead tests following the ASTM E976-99 standard was conductedon UHPC specimens [38]. The HB or 2B pencil lead with 0.5 mmdiameter was used as a manual AE source. The pencil lead was bro-ken 3 times at each point, as shown in Fig. 6, and the distancebetween the two points was 40 mm. The propagation velocity ofAE wave through UHPC was determined by pencil lead test. Oncethe wave propagation velocity was known, AE source locationscould be calculated based on the arrival time difference of the sig-nals gathered by 8 AE transducers. Moreover, the noise level of the

(b) 1d

(d) 7d

8d

S-UHPCs at different ages.

Table 4The characteristic parameters for different ages of ES-UHPCs.

UHPC ages fUte Average eUte Average fUtu Average eUtu AverageMPa MPa % % MPa MPa % %

4 h 1 4.8 4.4 0.017 0.015 5.6 5.0 0.114 0.0832 4.1 0.011 4.4 0.0653 4.3 0.016 4.9 0.071

1 d 1 6.9 6.3 0.020 0.018 9.7 7.9 0.284 0.1992 6.3 0.018 7.3 0.1843 5.6 0.016 6.6 0.129

3 d 1 8.3 7.7 0.021 0.019 9.6 9.2 0.183 0.2312 7.7 0.019 9.6 0.2773 7.1 0.018 8.5 0.232

7 d 1 6.4 6.7 0.017 0.017 7.6 7.5 0.217 0.1952 7.6 0.019 8.3 0.2513 6.2 0.015 6.6 0.118

28 d 1 8.9 8.7 0.020 0.019 9.3 9.6 0.155 0.2202 8.1 0.017 9.1 0.2703 9.1 0.020 10.5 0.235

Fig. 10. Tensile strengths of ES-UHPCs at different ages.

Fig. 11. fUtu/fUte of ES-UHPCs at different ages.

Fig. 12. Tensile strains of ES-UHPCs at different ages.

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test environment would be learnt based on pencil lead test resultto set the corresponding threshold and amplification. In this study,the threshold of the AE system was set as 30 dB, the preamplifiergain was set as 60 dB, the sampling frequency of AE signals fromconcrete was set as 3 MHz and band characteristic was set as20 kHz to 200 kHz.

2.3.3. Microstructural determinationIn order to determine the reaction product, X-ray diffraction

and thermal gravity analysis was carried out on the correspond-

ing neat paste. The cement paste was prepared by mixing ES-UHPC premixed powder with a water to binder ratio of 0.2. After4 h, 1 d, 3 d, 7 d and 28 d of hydration, the obtained harden pastewere crushed into particles, and half of them were ground intopowders in isopropanol until pass the sieve of 0.063 mm. After-wards, the powder were dried in an oven at 40 �C for 30 minand transferred to a desiccator over silica gel immediately. Theobtained particles and powders were identified by SEM andXRD using Cu Ka radiation at 40 kV and 100 mA. Sample scan-ning was used with a range from 5� to 75� and sampling intervalof 0.02� 2h.

In this work, a Netzsch STA 449C simultaneous thermal ana-lyzer was also used to conduct TG analyses. The tests were carriedout on the20 mg of ground powder under N2 atmosphere at a heat-ing rate 10 �C/min under flowing nitrogen (100 cm3/min) from 30to 1100 �C.

In order to correlate the mechanical properties, a mercuryintrusion porosimetry (MIP) test was performed on the hardenedES-UHPC and N-UHPC binder after hydrated for 1 d. The sampleswere taken from the inner parts with 2.0 g. An AutoPore IV 9500automated mercury porosimeter was employed to determine thepore structure. The mercury was intruded and extruded with aminimum intruding pressure of 3.6 kPa and a maximum intrudingpressure of 227.5 MPa. The detectable pore radius of r was calcu-lated via Washburn-Laplace equation. The contact angle h of 140�and the surface tension of mercury c of 0.480 N/m were used inthe calculations.

(a) 1d

(b) 28d

Fig. 13. Strain-stress curves of ES-UHPC and N-UHPC at 1 d and 28 d.

Fig. 14. Pore size distribution of ES-UHPC and N-UHPC binder at 1 d.

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3. Results and discussion

3.1. Age-dependent compressive performance of ES-UHPC

Fig. 8 illustrates the compressive strength and elastic modulusof the ES-UHPC at different ages. The compressive strength of ES-UHPC reaches 62.0 MPa at 4 h, 75.4 MPa at 1 d, 83.1 MPa at 3 d,and 85.9 MPa at 7 d, which is equivalent to 56.16%, 68.30%,75.27% and 77.81% of the compressive strength obtained at 28 d,respectively. As shown in Fig. 8, the developing trend of the elasticmodulus versus curing period resembles that of the compressivestrength closely.

3.2. Age-dependent tensile performance of ES-UHPC

The tensile stress–strain curves of the prepared ES-UHPCs from4 h to 28 d are summarized in Fig. 9. Under uniaxial tensile loading,the ES-UHPC exhibited significant tensile strain-hardening behav-

Table 5The difference in tensile properties between ES-UHPC and N-UHPC.

UHPC ages UHPC types Average fUte Ratio Average eUteMPa % %

1 d ES-UHPC 6.3 573 0.018N-UHPC 1.1 0.012

28 d ES-UHPC 8.7 87 0.019N-UHPC 10.0 0.023

ior at different curing period. But some curves showed strainrebound at the end of the strain hardening domain, which couldbe due to the major crack presented outside the gauge length.The Swiss standard SIA 2052–2016[16] suggests that the behaviorof UHPC under tensile loading is characterized by the followingproperties: 1) elastic tensile strength fUte and elastic tensile straineUte stand for the first structural cracking point, mainly determinedby the tensile strength of the UHPC matrix; 2) ultimate tensilestrength fUtu and ultimate tensile strain eUtu are applied to charac-terize the peak point at the end of the strain-hardening branch,which leads to the maximum post-cracking stress and strain[39,40].

As shown in Fig. 9, the stress–strain curves of these ES-UHPCsstart with a steep linear ascending portion up to the elastic limitpoint (fUte, eUte), followed by strain-hardening branches. In the elas-tic tensile strain section, the discreteness of those curves is indis-cernible, which can be related to the ES-UHPC matrix’s propertyin the elastic tensile strain regime. When the curves entered thestrain hardening regime, the discreteness gets larger and con-nected with the bonding strength between the matrix and steelfibers. Moreover, the cost of testing time among the three speci-mens could also introduce some discreteness to the results, espe-cially at 4 h and 1 d.

Table 4 tabulates the detailed information about the tensileproperties including elastic tensile strength fUte, elastic tensilestrain eUte, ultimate tensile strength fUtu, ultimate tensile strain eUtuat different curing ages. Fig. 10 illustrates the development of ten-sile strength of ES-UHPCs with curing age. As shown in Fig. 10, it isobvious that the tensile strength of ES-UHPC shows a growth trendwith curing age in general. The strength develops rapidly at theearly age stage. The average elastic ultimate tensile strength fUteat 4 h, 1 d, 3 d and 7 d is 4.4 MPa, 6.3 MPa, 7.7 MPa and 6.7 MPa,equivalent to 50.57%, 72.41%, 88.51% and 77.01% of the averageelastic ultimate tensile strength fUte at 28 d (8.7 MPa), respectively.And the average ultimate tensile strength fUtu at 4 h, 1 d, 3 d and

Ratio Average fUtu Ratio Average eUtu Ratio% MPa % % %

150 7.9 527 0.199 4231.5 0.047

83 9.7 84 0.220 6211.6 0.356

(a) Tensile stress and number of AE events versus strain

(b) AE sources distribution

Fig. 15. Damage evolution process of the ES-UHPC at 28 d.

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7 d is 5.0 MPa, 7.9 MPa, 9.2 MPa and 7.5 MPa, accounting to 52.08%,82.29%, 95.83% and 78.13% of the average ultimate tensile strengthfUtu at 28 d (9.6 MPa), respectively. The strength increases quicklyfrom 4 h to 1 d, while it shows a slight variation later on. This couldbe related to the hydration process of ES-UHPC matrix.

The elastic tensile strength fUte is mainly influenced by the ten-sile strength of the ES-UHPC matrix, whereas the ultimate tensilestrength fUtu is determined by the bonding strength between the

matrix and steel fibers. Fig. 11 shows the ultimate tensile strengthto elastic tensile strength ratio (fUtu/fUte) of ES-UHPC, characterizingthe tensile strain hardening behavior at different ages. As shown inFig. 11, the fUtu/fUte grows to be the largest (1.254) at 1 d, satisfyingthe criterion of UB (>1.2) required in SIA 2052–2016[16]. The ratiofalls slightly from 1 d to 28 d since the growing rate of ES-UHPCmatrix’s strength is larger than that of the bonding strengthbetween the matrix and steel fibers after 1 d. The corresponding

J.-Y. Wang et al. / Construction and Building Materials 208 (2019) 767–779 775

mechanism will be determined in detail by the single fiber pullouttest.

As shown in Fig. 12, the change of the ultimate tensile strain eUtuversus curing period is relatively more apparent compared to theelastic tensile strain eUte, especially from 4 h to 1 d. At 1 d, the ulti-mate tensile strain eUtu (0.199%) approximates to the yield strain of

(a) Tensile stress and number of

(b) AE sources d

Fig. 16. Damage evolution proc

steel bar (0.200%) greatly, meaning that ES-UHPC has achievedhigh ductility under direct tensile loading at 1 d. Meanwhile, it isnoticeable that the difference between the elastic tensile strain eUteand ultimate tensile strain eUtu increases obviously from 4 h to 1 d,indicating that the ES-UHPC gets much more significant strainhardening response at 1 d and shows a better crack-controlling

AE events versus strain

istribution

ess of the N-UHPC at 28 d.

Fig. 17. Picture of microcracks of E-UHPC at different parts of the specimen after AE tests.

Fig. 18. X-ray diffraction patterns of hydrated samples at different ages (G–gypsum; B- belite; Y–ye’elimite; E–ettringite).

776 J.-Y. Wang et al. / Construction and Building Materials 208 (2019) 767–779

ability. There is little variation in strain hardening response from1 d to 28 d.

3.3. Comparison with N-UHPC in tensile property

In order to compare the tensile properties of ES-UHPC with N-UHPC, three N-UHPC specimens were prepared for direct tensiletest at the age of 1 d and 28 d, respectively. Fig. 13 summarizesthe strain–stress curves of the ES-UHPCs and N-UHPCs at 1 d and28 d. As shown in Fig. 13, the ES-UHPC exhibits significant tensilehigh strain-hardening behavior at 1 d while the N-UHPC exhibits

nearly strain-softening behavior at that time. The two types ofUHPC both exhibit a strain-hardening behavior at 28 d obviously.

The detailed tensile test results are tabulated in Table 5. Theexperimental results reveal that the elastic tensile strength fUte,elastic tensile strain eUte, ultimate tensile strength fUtu and ultimatetensile strain eUtu of ES-UHPC are 5.73, 1.50, 5.27 and 4.23 timeslarger than that of N-UHPC at 1 d, suggesting that the ES-UHPChas much higher strength and more significant strain hardeningbehavior at 1 d. This is because that the CSA cement used in ES-UHPC hydrated rapidly at the early stage, which benefiting thestrength development. Fig. 14 provides the pore structure of theES-UHPC and N-UHPC binder at 1 d based on MIP. As shown inFig. 14, the critical pore diameter of hydrated ES-UHPC binder ismuch lower than that of N-UHPC at 1 d. In addition, the totalporosity of hydrated ES-UHPC is also obviously lower than thatof N-UHPC. For ES-UHPC, the rapid hydration of CSA binder canresult in a denser concrete matrix and reduced porosity, makingan effective impact on the strength development.

At 28 d, the elastic tensile strength fUte and ultimate tensilestrength fUtu of ES-UHPC are lower than that of N-UHPC slightly,and the ultimate tensile strain eUtu is less than that of N-UHPC dis-tinctly. This result indicates that there is little difference instrength between ES-UHPC and N-UHPC at 28 d, but the strainhardening behavior of N-UHPC is more obvious than that of ES-UHPC.

3.4. Comparison with N-UHPC in damage evolution process at 28 d

According to Section 3.3, ES-UHPC exhibit better tensile proper-ties than N-UHPC at 1 d while the strain hardening behavior of N-UHPC is more obvious at 28 d. To further study the damage evolu-tion during the direct tensile test, AE non-destructive techniquewas applied and the difference between the two types of UHPC

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are evaluated. From the foregoing, the curing age has a greatimpact on the strength, especially at the early stage. The wholeprocess of AE sources determination combined with tensile testin this study takes about 4 h for each once. In order to decreasethe influence caused by time lag and curing age, AE test was onlyconducted at 28 d.

Fig. 15 and 16 show the tensile stress and number of AE eventsversus strain curves of ES-UHPC and N-UHPC at 28 d, where thecorresponding AE source location evolution maps during the ten-sile test are also illustrated. For clarity, the computed AE sourcelocation results are plotted in six sub-figures according to strainlevel. For both kinds of UHPCs, point A and D are chosen at0.010% and 0.046% strain, which represent the elastic stage andstrain-hardening stage, respectively; B and E are the pointsresponding to the elastic ultimate tensile strain (eB-ES = 0.017%,eB-N = 0.020%) and the ultimate tensile strain (eE-ES = 0.155%,eE-N = 0.308%), respectively; C is an inflection point between pointB and D (eC-ES = 0.020%, eC-N = 0.023%); F is a point at the strain-softening stage, whose strain is 1.2 times larger than the ultimatetensile strain (eF-ES = 0.186%, eF-N = 0.370%). Point a, b, c, d, e, f onthe strain-number of AE events curve correspond to point A, B, C,D, E, F on the stress–strain curve, respectively. In Fig. 15(b) andFig. 16(b), each red dot shown in the three-dimensional mapstands for a computed event source attributed to the damageinside the UHPC.

As shown in Figs. 15 and 16, only one AE event can be observedin ES-UHPC and no AE event can be detected in N-UHPC at point A.From point A to point B, a small but noticeable increasing amountof AE events occurred in both ES-UHPC and N-UHPC. The AE eventsin this stage might be motivated by the first UHPC matrix cracking.Such a markable increasing amount of AE events proceeds to pointC. As given in Figs. 15(b) and 16(b) for point C, the distribution ofdamage is relatively concentrated, suggesting the first matrixmicro-crack keep propagating from point B to point C in ES-UHPC and N-HUPC. At point D, more damage dots at differentheight were detected for both ES-UHPC and N-UHPC, indicatingthat multiple microcracks occurred. While these cracks could notbe detected by the crack width measurment with a resolution of0.01 mm during the tensile test. It can be inferred that the micro-cracks formed at this stage should be smaller than 0.01 mm. Atpoint E, the ultimate tensile strain point, the number of AE eventsincreases continuously and more damage dots are concentrated inone region with height of 100–150 mm (ES-UHPC) and 150–250 mm (N-UHPC), corresponding to the location of the tested

Fig. 19. TG and DTG patterns of hyd

specimen’s major crack. This implies that a major crack may haveformed at point E. After point E, the fracture process zone aroundthe major crack tip becomes the main emission source. The AEevent amount continues to increase from point E to point F andthe new damages occurred chiefly at the position of the majorcrack, indicating that it develops into strain softening stage andonly the major crack kept developing.

Figs. 15 and 16 show that with the developing of tensile strain,the number of AE events of ES-UHPC shows the same changingtrend with that of N-UHPC, as well as the damage evolution pro-cess. The AE source locations of both types of UHPCs show a widerdistribution, suggesting that multiple microcracks generatedthroughout the specimen in addition to the major crack. At pointF, the damage dots of N-UHPC distribute more evenly than thatof ES-UHPC, suggesting that the multiple cracking behavior of N-UHPC is more obvious.

After AE source test, some microcracks at different positions ofthe tested ES-UHPC specimen within gage length were observed bydigital camera with micro photo lens, and the corresponding crackwidths ranging from 0.01 mm to 0.05 mm were observed, which isshown in Fig. 17. These detected microcracks further confirm thatES-UHPC exhibits the multiple cracking behavior at 28 d.

3.5. Microstructure

The mineralogical compositions of anhydrate and hydrated CSAcement after 4 h, 1 d, 3 d, 7 d and 28 d are given in Fig. 18. It can beseen that the content of ye’elimite decreases remarkably and thenew phase of ettringite presents after contacting with water for4 h. After thaturing age, the intensity of ye’elimite varies margin-ally indicating the hydration of ye’elimite mainly occurs in the first1 d. Correspondingly, the content of ettringite changes slightlyfrom then on.

The TG-DTG curves of hydrated CSA pastes at different ages areprovided in Fig. 19. The patterns clearly confirm that ettringiteforms as the main hydrates and aluminium hydroxide (Al(OH)3)is also observed as minor hydration product. The water loss atabout 60–180 �C of hardened paste at 4 h, 1 d, 3 d, 7 d and 28 ddue to the decomposition of ettringite is 5.82%, 9.56%, 8.84%,10.74% and 9.54%, respectively. It is obvious that half of the ettrin-gite have generated after 4 h of hydration and the loss of water cor-responding to ettringite gets remarkable from 4 h to 1 d attributedto the persistently abundant creation of the ettringite. Moreover,the content of ettringite from 1 d to 28 d varies slightly, which is

rated samples at different ages.

(a) )b(h4 1d

(c) )d(d3 7d

(e) 28d

Fig. 20. SEM micrographs of hydrated samples at different ages.

778 J.-Y. Wang et al. / Construction and Building Materials 208 (2019) 767–779

consistent with the XRD analysis. In addition, shows that the waterloss at about 180–300 �C confirms the formation of Al(OH)3.

The morphology of hardened cement paste at different curingages is shown in Fig. 20. It can be clearly seen that the ettringiteis detected as main hydration phase. After 4 h of hydration, mas-sive needle like ettringite forms in the paste, and its size variesslightly with curing time.

Strength development of ES-UHPC, especially at the early stage,is mainly governed by the formation of ettringite. The microstruc-ture test results from XRD, TG and SEM confirm that ettringite isthe main hydration products and most of them formed withinthe 1 d With the curing age, in spite of the ettringite and Al(OH)3gel, the hydration of belite would also contribute to further fill

the pores of the matrix and make the structure more compact,which would promote the strength development continuously.

4. Conclusion

To determine the properties of ES-UHPC made with CSAcement, a series of experimental tests have been done involvingelastic modulus, compressive strength, tensile, AE analysis, as wellas microstructure determination. Meanwhile, tensile test has beenconducted on the N-UHPC to compare the tensile property withthat of ES-UHPC. Based on the presented experimental results,the following conclusions can be drawn:

J.-Y. Wang et al. / Construction and Building Materials 208 (2019) 767–779 779

(1) The ES-UHPC exhibited significant tensile strain-hardeningbehavior under uniaxial tensile loading from 4 h to 28 d. Itcan obtain great strain hardening response with high ductil-ity and bonding strength between steel fiber and matrix at1 d. The ultimate tensile strain eUtu of ES-UHPC can reachto 0.199% at 1 d, which is 4.23 times larger than that of N-UHPC at the same testing age. The tensile properties of theES-UHPC trend to be stable at later stage, while the strainhardening response is inferior to the N-UHPC at 28 d.

(2) AE test results show that the two types of UHPCs exhibitmultiple cracking behavior at 28 d. With the tensile straindeveloping, the two types UHPCs show a similar damageevolution tendency and multiple microcracks with crack-width smaller than 0.01 mm generated after reaching0.046% strain. The damage dots of N-UHPC distribute moreevenly than that of ES-UHPC, suggesting that the multiplecracking behavior of N-UHPC is more obvious at 28 d.

(3) Microstructure test results obviously manifest that ettringitegenerates as the main hydration phase. The content ofettringite increases rapidly from 4 h to 1 d, accounting forthe high early strength acquirement of ES-UHPC. In spiteof the ettringite and Al(OH)3 gel, the hydration of belitewould also contribute to the development of strength atlater age.

Conflict of interest statement

We declare that we have no financial and personal relationshipswith other people or organizations that can inappropriately influ-ence our work, there is no professional or other personal interestof any nature or kind in any product, service and/or company thatcould be construed as influencing the position presented in, or thereview of, the manuscript entitled, ‘‘Properties of calcium sulfoa-luminate cement made ultra-high performance concrete: ten-sile performance, acoustic emission monitoring of damageevolution and microstructure”.

Acknowledgements

This work was supported by the National Nature Science Founda-tion of China [grant number 51609172, 51608382], Zhejiang Com-munication Science and Technology Project, and the ShanghaiMunicipal Science and Technology Project [grant number17DZ1204200]. The financial supports are greatly appreciated.Great thanks go to Zhejiang Hongri TENACAL� Innovative MaterialTechnologies Co., Ltd. for providing the UHPC materials.

Data availability

The raw/processed data required to reproduce these findingscannot be shared at this time due to technical or time limitations.

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