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Construction and Building Materials 204 (2019) 342–356

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Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Effects of self-healing on tensile behavior and air permeability of highstrain hardening UHPC

https://doi.org/10.1016/j.conbuildmat.2019.01.1930950-0618/� 2019 Published by Elsevier Ltd.

⇑ Corresponding author.E-mail address: 14529@tongji.edu.cn (J.-Y. Wang).

Jun-Yuan Guo, Jun-Yan Wang ⇑, Kai WuKey Laboratory of Advanced Civil Engineering Materials, Tongji University, Ministry of Education, Shanghai 201804, China

h i g h l i g h t s

� Providing quantitative information on recovery of UHPC by self-healing effect.� Tensile strength of new crystals after self-healing was lower than the intact part.� API shows a sensitive response to the tensile damage characterized by AE sources.� Steam re-curing method is more effective for self-healing than water re-curing method.

a r t i c l e i n f o

Article history:Received 10 October 2018Received in revised form 16 January 2019Accepted 27 January 2019

Keywords:Self-healingUltra high performance concreteStrain hardeningTensile stiffnessAir permeabilityAcoustic emission

a b s t r a c t

High strain hardening UHPC was expected to be self-healing due to its multiple micro-cracks in tensionand the low water-to-binder ratio. The direct tensile tests with the acoustic emission (AE) analysismethod and air permeability tests were carried out on high strain hardening UHPC in order to investigatethe self-healing behavior under three re-curing conditions (water, 80%RH and 45%RH). Moreover, the cyc-lic tensile loading was applied to create more severe damage and two re-curing schemes (water andsteam) were used to evaluate the self-healing effects. Experimental results show that the direct tensilestress-strain curves of high strain hardening UHPC re-cured in water and 80%RH conditions have therecovery of first cracking strain. New crystals were observed in the micro-cracks after self-healing, whosestrength was proved to be lower than that of the intact part. The high strain hardening UHPC can recover40%-47% of the first cracking strain after 28 days of water re-curing, where new damages characterizedby AE sources number were found to generate inside the UHPC matrix. Air permeability test shows a sen-sitive response to the damage created by the tensile deformation. Steam re-curing method shows moreeffective and less time consuming for self-healing than water re-curing method.

� 2019 Published by Elsevier Ltd.

1. Introduction

Concrete is the most widely-used construction materials in theworld because of its easy and cheap production and desirablematerial properties as well [1,2]. However, a large number of con-crete structures have caused huge repair costs due to deteriorationin durability, even withdrew from service. Cracks are the one of themajor factors which could influence durability and serviceability ofconcrete structures by increasing the permeability and transportproperties [3].

Ultra high performance concrete (UHPC) is a cementitious com-posite material composed of an optimized gradation of granularconstituents and a high percentage of discontinuous internal fiberreinforcement cementitious materials [4,5]. The water-to-binder

ratio of UHPC is normally less than 0.25 [6]. Owing to its superbpost-cracking ductility under tension and flexure, UHPC has beenattractive for application in civil infrastructure, where bending pre-vails [7]. According to MCS-EPFL recommendation [8], UHPC can becategorized by UO (strain softening), UA (strain hardening, ulti-mate tensile hardening strain is higher than 0.15%) and UB (highstrain hardening, ultimate tensile hardening strain is higher than0.2%). The previous study found that high strain hardening UHPChas excellent crack width control ability [9]. Fig. 1 shows the rep-resentative results of the crack width-strain curves and stress-strain curves of high strain hardening UHPC and strain softeningUHPC. When tensile strain reached 0.2% (around the yield strainof steel bar), the crack width of high strain hardening UHPC is only0.02–0.03 mm. The crack width is less than 0.05 mm in the strainhardening stage, which is smaller than that of engineered cemen-titious composite (ECC) made by PVA fibers, as shown in Fig. 2. Thisis because the elastic modulus of steel fibers is almost 6�7 times

Fig. 1. Crack width-strain curves and stress-strain curves of UHPCs [9]

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larger than that of PVA fiber. On the contrary, the crack width ofstrain softening UHPC is 0.5 mm when tensile strain reached0.2%. Therefore, high strain hardening UHPC was expected to beable to recover during the service period due to the multiplemicro-cracks in tension and the low water-to-binder ratio.

Self-healing of concrete is a combination of complicated physi-cal and chemical processes. The main mechanisms considered iscalcite precipitation and further hydration triggered by penetratedwater through cracks. Further hydration and calcium hydroxidedeposition could result in recovery of tensile strength, compressivestrength and stiffness. The parameters which would influence thisprocess include initial water-to-binder ratio, curing age, relativehumidity and the possibility of accessing water, and of coursethe crack width. Previous studies showed that cracks could besealed completely if they were less than 0.05 mm wide [10,11];while for larger cracks (e.g. wider than 0.1 mm), most of themcan be filled completely [11,12]. Therefore, it can be expected thatthe high strain hardening UHPC has a high potential of self-healingability due to its special multiple micro-cracks patterns (lessthan0.05 mm) during the strain hardening stage.

Most studies investigated the self-healing ability (recovery ofperformance) by means of water permeability and air permeabilitytests. Minoru Kunieda [13] presented autogenous healing behaviorof cracked ultra high performance-strain hardening cementitiouscomposites (UHP-SHCC), and pointed out the recovery of imperme-ability through air and water permeability test results. The studyconfirmed that UHPC-SHCC has the potentially of self-healing.However, little tests can be found regarding the role of self-

Fig. 2. Comparison between high strain hardening UHPC and ECC.

healing on mechanical properties, especially among the UHPC. B.Hilloulin [14] investigated the restoration of UHPC specimensdue to self-healing phenomenon. The finite element analysis wasperformed on a UHPC notched beam. Three-point-bending testswere simulated on the beam before and after healing to simulatethe restoration of the mechanical properties. A hydro-chemo-mechanical model based on micro-mechanical observations wasestablished in this study. Liberato Ferrara [15] presented theresults of the self-healing capacity of high performance fiber rein-forced cementitious composite (HPFRCC). The author used beamspecimens initially pre-cracked in four-point-bending up to differ-ent values of crack opening, which were submitted to differentexposure conditions. In a durability-based design framework,self-healing indices quantifying the recovery of mechanical proper-ties were also defined. The above investigated studies are try toevaluate self-healing behavior through the bending performance.However, the available experiments did not provide informationregarding the direct tensile test after self-healing process. Thedirect tensile tests can provide more scientific information forthe characterization of the high strain hardening UHPC at differenttensile strains related to different loading conditions in the UHPCstructures.

Nowadays, some studies [3,16] have used the acoustic emission(AE) technique to investigate the mechanism of self-healing behav-ior. S. Granger [3] investigated the self-healing of cracks in anUHPC. Mechanical behaviour of self-healed concrete under threepoints bending, and AE analysis of the cracking mechanisms werereported. The mechanical tests demonstrated the recovery of theoverall stiffness, depending on the time of re-curing. The AE anal-ysis was performed in order to determine the mechanical responsecontrolled by the precipitation of new crystals in the crack. Went-ing Li [16] used AE technique in a flexural loading scenario to mon-itor the instant damage and healing of cement paste incorporatingmicroencapsulated adhesive. The effects of the concentration ofmicrocapsules and the level of pre-damage were investigated.The results revealed the distinguished cracking mechanismsaccording to the differentiated feature of the signals in terms ofthe temporal and spectral AE descriptors. The AE technique isproved to be an effective method to investigate the mechanismof self-healing behavior.

In this study, self-healing behavior of high strain hardeningUHPC after being re-cured in three series of conditions (water,80%RH and 45%RH) were investigated. Direct tensile tests and airpermeability tests were used to quantify the recovery of stiffnessand impermeability caused by the self-healing effect. AE analysismethod was performed with the aim of providing some quantita-tive information on the damage mechanism of the tensile behav-iors before and after self-healing. Moreover, the cyclic tensileloading up to strain of 0.1% was applied to create more severe

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damage and two re-curing schemes (water re-curing and steam re-curing) were used to demonstrate the self-healing effects.

2. Experimental program

2.1. UHPC mixture and specimen preparation

The high strain hardening UHPC materials used in this studyconsists of two components, UHPC premixed powder and steelfibers. Table 1 provides the mix proportion of the high strain hard-ening UHPC. The volume fraction of steel fibers in high strain hard-ening UHPC is 2%. A type of straight steel fiber with length of16 mm and diameter of 0.2 mm and with brass coating was used,whose properties are given in Table 2.

Fig. 3 shows the dog-bone shape specimen used for the investi-gation of tensile properties and air permeability. The cross sectionof the dog-bone shape specimen is 50 mm � 100 mm. The width(50 mm) of specimens is longer than three times of the length offibers (16 mm), the influence of fiber orientation distribution couldbe marginal in this work.

Six dog-bone shape specimens were prepared for completedtensile stress–strain curve testing and seven dog-bone shape spec-imens were prepared for self-healing study. All specimens were

Table 1Mix proportion of high strain hardening UHPC.

Unit weight (kg/m3)

Cement Silica fume Ground filler Quartz

733 220 220 982

Table 2Properties of steel fibers.

Fiber shape Tensile strength/MPa Elastic modulus/GPa L

Straight smooth 2500 200 1

Fig. 3. Dimension and photo of

covered with plastic sheets and stored at room temperature for24 h after casting, then cured in the water at a temperature of20 �C until 28-day.

2.2. Air permeability tests

Air permeability tests were conducted using the Autoclam per-meability system, developed by researchers at Queen’s University[17]. The Autoclam has been continually developed and updatedfor the past 25 years by Amphora, being proven as a reliable instru-ment both on site and in the laboratory. It can be used to assess thequality of building materials in terms of measuring the air perme-ability, water permeability and water absorption of concrete struc-tures, both soon after the construction and at any time during theservice life. The set-up of Autoclam permeability system is shownin Fig. 4. The assembly of the Autoclam consists of two parts, theAutoclam body and the base ring. As shown in Fig. 4(a), air perme-ability tests were performed by isolating a test area of 50 mmdiameter using two bolt-on type rings. There were two test areasin each specimen, which were located in the middle of the topand bottom surfaces with the width of 100 mm.

During the air permeability test [18], the pressure inside theAutoclam body increased to 500 mbar and the decay in pressure

sand Water Superplasticizer Fiber

174 13 157

ength /mm Diameter /lm Aspect ratio Density/kg�m�3

6 200 80 7850

UHPC tensile specimen [9]

Fig. 4. Autoclam permeability system.

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is monitored every minute up to 15 min or until the pressurediminishes to zero. A line of natural logarithm of pressure againsttime is obtained by regression and the slope of the regressed linebetween the 5th minute and the 15th minute is used as the air per-meability index (API), and given as a unit of Ln (mbar)/min. If thepressure declines to zero before the time reaches to 15 min, thedata from the beginning of the test is used to determine the slope.

Table 3 gives the criteria for evaluating the protective qualityof concrete specimen based on air permeability index, whichwas proposed by the research organization of the Autoclam per-meability system [18]. In this study, the criteria in Table 3 wereused for evaluating the protective quality of high strain hardeningUHPC.

Table 3Protective quality based on Autoclam air permeability index (API) [18]

Air permeability index Protective quality

Very good Go

API/Ln(mbar)/min �0.1 ＞

Fig. 5. Setup of the direct tensile te

2.3. Damage investigation of UHPC under direct tensile test usingacoustic emission techniques

2.3.1. Test method for direct tensile testThe direct tensile test setup is shown in Fig. 5(a). Both ends of

the specimen have hinge systems to maintain pure tensile loadwithout moment. A universal testing machine (WDW-300 servo-controlled testing system) running in displacement control man-ner was used to conduct the tensile tests. Four linear variable dif-ferential transformers (LVDTs) were attached to the specimen overa gauge length L = 150 mm to measure the average elongation.

The direct tensile test consists of three parts: preloading, load-ing and unloading, and the loading rates for each step were set as

od Poor Very poor

0.1 � 0.5 ＞0.5 � 0.9 ＞0.9

st with AE analysis system [9]

Fig. 6. Experimental procedures (note: e is tensile strain).

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1 mm/min, 0.3 mm/min and 0.3 mm/min, respectively. Loadingwas performed when the applied force reached 0.5kN, unloadingwas performed after the tensile strain reached 0.1%. The stress ofsteel bars at 0.1% strain is about 200 MPa, which is higher than thatin most of the cases of the UHPC structures under the serviceabilitylimit state.

2.3.2. Test method for acoustic emission source locationAcoustic emission (AE) is a micro seismic (elastic) wave gener-

ated from dislocations, micro cracking and other irreversiblechanges in a stressed material [3]. Therefore, the micro crackingof UHPC matrix and the debonding process between UHPC matrixand steel fiber occurred within a UHPC specimen during direct ten-sile loading are the typical AE sources. Locations where AE sourcesoccurred were determined by a three-dimensional (3D) AE analysissystem (Fig. 5). The AE sources are detected by transducers on thesurface of a specimen. Through the preamplifier, all waveforms arerecorded and stored by a windows-based AE data operation pro-gram and the program can be used to extract the AE parametersfor analysis. Fig. 5(b) shows the location of 8 AE transducers onthe specimens. All AE transducers were coupled to the surface ofspecimens by Vaseline. The exhaustion method of 3D AE sourcelocation was described in reference [9].

In order to determine the AE wave propagation velocity and thenoise level of test environment, pencil lead test was conductedaccording to the ASTM E976-99 standard [19]. As shown in Fig. 5(b), the pencil lead was broken 3 times in each location. In theAE source location test, AE wave propagation velocity was decidedas 4700 m/s, and the threshold of the AE system was set as 35 dBfrom the pencil lead test. The stress level of AE activities above thisthreshold are detected by AE transducers. Value for gain of pream-plifier was set as 40 dB, sampling frequency was set as 3 MHz andband characteristic was set as 20 kHz to 200 kHz.

2.4. Self-healing study by three re-curing schemes

As shown in Table 4, three series of specimens stored in differ-ent re-curing conditions were prepared: 1) immersed in water con-dition (20 �C); 2) re-cured in chamber with 20 �C and 80% relativehumidity (RH) condition, created by saturated solution of potas-sium bromide; 3) re-cured in chamber with 20 �C and 45% relativehumidity condition (RH) created by dehumidifier. For each condi-tions, two specimens were prepared. The air permeability testwas conducted on these specimens at 7 days and 28 days of re-curing periods and the reloading was conducted after 28 days ofre-curing period. The detailed experimental procedures can besummarized in Fig. 6(a).

2.5. Self-healing study by the accumulated damage

In order to investigate the accumulated damage caused by cyc-lic loadings (more severe damage condition) and the relevantrecovery degree by two re-curing schemes (water curing or steamcuring conditions), one high strain hardening UHPC tensile speci-men was prepared. An outline of the experiments is given inFig. 6(b). Loading tests consist of three series of cyclic loadings.In the first series of cyclic loadings, load was applied until the

Table 4Three series of specimens by different re-curing conditions.

Specimen types Unloaded strain (First loading)/% Re-curing con

UHPC-Water 0.1 Water(20 �C)UHPC-80%RH 20 �C, 80%RHUHPC-45%RH 20 �C, 45%RH

strain reached 0.13%, and then the load was applied to the strainof 0.1% for the other five times in order to induce more severe dam-age. In the second series of cyclic loadings, load was applied to thestrain of 0.1% for six times again. In the third series of cyclic load-ings, load was applied to the strain of 0.1% for two times. Three ser-ies of cyclic loadings were applied and the stress-strain curveswere recorded. In order to evaluate the recovery of cracks, air per-meability tests were performed in accordance with the testingmethod described in the section 2.2. Water re-curing (28 days)was employed between the first and second series of cyclic load-ings and steam re-curing (3 days) at 90 �C and 100%RH wasemployed between the second and third series of cyclic loadings.

In order to reduce the influence of water content on the air per-meability tests, the specimens re-cured in water should be trans-ferred to a condition with 20 �C and 45% relative humidity atleast 2 days before the tests. As shown in Fig. 7, the weight of

dition Re-curing period Unloaded strain (Second loading)/%

28 days 0.1

Fig. 7. Change in the weight of specimen after water re-curing.

Fig. 8. Tensile stress-strain curves of high strain hardening UHPC.

Fig. 9. Tensile stress-strain curves (unloading at the strain of 0.1%) and r

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the specimens was recorded every 4 h to ensure the water contentof specimens had reached a stable state.

3. Results and discussion

3.1. Mechanical properties of the high strain hardening UHPC

The tensile stress-strain curves of high strain hardening UHPCspecimens are given in Fig. 8, which can be divided into elasticpart, strain hardening part and strain softening part. According toFig. 8, the six tensile stress-strain curves exhibit a high consistency,especially for the elastic part. The ultimate tensile strain (strainhardening part) ranges from 0.35% to 0.47%, exhibiting highductility by forming multiple micro-cracks invisible to naked eyes

elevant AE sources distribution of the specimens re-cured in water.

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(typically below 0.05 mm) in the strain-hardening part, as shownin Fig. 2 [9,20].

3.2. Tensile properties of UHPC under three re-curing conditions

3.2.1. Tensile stress-strain curves of UHPC under three re-curingconditions

To compare the tensile properties of the specimens under threere-curing conditions, the direct tensile stress-strain curves areshown in Figs. 9–11, and each series has two specimens. Thesecurves correspond to the specimens being loaded and re-loadedafter 28 days of re-curing in 1) water, 2) 80%RH condition and 3)45%RH condition. The initial stiffness of each specimens duringthe first loading and second loading are listed in Table 5, wherethe average values and coefficient of variation (CoV) of the initialstiffness are also presented.

As shown in Figs. 9–11, the stress-strain curves of first loadingcan be divided into three parts, namely elastic part, strain harden-ing part and unloading part. As shown in Fig. 9, the tensile featuresof stress-strain curves obtained after water re-curing can bedivided into four stages: 1) the curves start with a steep ascendingportion, which is similar to those of the first loading; 2) the curvesfollow by a branch where a considerable reduction in modulus ofelongation; 3) strain hardening part, which coincided with the firstloaded curve; 4) after the tensile strain reaches 0.1%, there is aunloading part, which leads to descending portion. It can beobserved in Fig. 9 that there is no AE sources generated till the ten-sile strain of 0.009% and 0.011% was reached. This means that thefirst part can be regarded as the recovery of first cracking strain

Fig. 10. Tensile stress-strain curves (unloading at the strain of 0.1%) and releva

without damage generated and accounts for 40%-47% of the firstcracking strain (0.022%, 0.021%). The curves after re-curing in achamber with 80%RH (Fig. 10) also show four parts based on thetensile behavior, while their recovery of first cracking strain isshorter than that after water re-curing, of which accounting onlyfor 13%-19% of the first cracking strain (0.019%, 0.022%). The curvesafter re-curing in 45%RH condition (Fig. 11) exhibit only ascendingand descending parts, where the ascending part shows a significantreduction in modulus of elongation. This indicated that the UHPCspecimens show no recovery under 45%RH condition.

3.2.2. Initial stiffness of UHPC under three re-curing conditionsInitial stiffness is calculated from the strain between 0% and

33.33% of the ultimate tensile strength [8]. The average values ofinitial stiffness during the first loading are 47.96GPa, 45.51GPaand 51.54GPa for the series re-cured in water, 80%RH conditionand 45%RH condition. For the second loading after re-curing, thecorresponding values of initial stiffness reduce to 46.62GPa,29.77GPa and 23.49GPa. It is noticeable that the initial stiffnessof second loading increases as the relative humidity of re-curingcondition increases. This is more remarkable for the specimensre-cured in water, which the initial stiffness of second loadingremained 97% of that of the first loading. This phenomenon pro-vides a support to the important role of water on the self-healingof high strain hardening UHPC with multiple micro-cracks.

The CoV of the second loading initial stiffness of the specimensre-cured in water and 80%RH condition are also presented inTable 5, which were 0.002 and 0.175. The data indicates that the

nt AE sources distribution of the specimens re-cured in 80%RH condition.

Fig. 11. Tensile stress-strain curves (unloading at the strain of 0.1%) and relevant AE sources distribution of the specimens re-cured in 45%RH condition.

Table 5Initial stiffness.

Specimen types Initial stiffness/GPa

First loading Average CoV Second loading Average CoV

UHPC-Water- 1 49.91 47.96 0.058 46.69 46.62 0.0022 46.00 46.54

UHPC-80% RH- 1 46.78 45.51 0.040 26.08 29.77 0.1752 44.23 33.45

UHPC-45% RH- 1 53.26 51.54 0.047 22.79 23.49 0.0422 49.82 24.18

Note: Initial stiffness is calculated from the strain between 0% and 33.33% of the ultimate tensile strength

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recovery are more stable when the specimens were re-cured inwater compared with that stored in 80% RH condition.

According to Figs. 9–11, the self-healing effect on the tensilestiffness of the high strain hardening UHPC is controlled by watercontent in the environment. The higher water content would pro-mote the hydration of anhydrous particles in the UHPC. The car-bonation of concrete have two prerequisite, one is the contactingwith CO2, and the other is suitable relative humidity. If immersethe specimens in the water, the micro-cracks would be filled withwater and no CO2 ingress into the micro-cracks. The main reactionproducts would be calcium hydroxide and calcium siliceoushydrate. The self-healing process is mainly determined by thehydration of anhydrous. If the specimens are exposed to a condi-tion with 80%RH, the self-healing process involves hydration andcarbonation, and these could be slower than the hydration of anhy-drous immersed in water completely.

3.2.3. Observation of micro-cracksFig. 12 shows the photos of micro-cracks at three states, which

was taken by the Canon EOS 5DSR camera (50.6MP) with microphoto lens MP-E65mm f/2.8 1-5x. The micro photos of themicro-cracks after re-curing in water for 28 days were given inFig. 12(a). The figures of the micro-cracks at 0.1% tensile strain insecond loading were given in Fig. 12(b). The figures of the micro-cracks just after unloading of the reloading were given in Fig. 12(c).

As shown in Fig. 12(a), the formed crystals after self-healingwere observed in the micro-cracks and made a bridge betweenthe two interfaces of the micro-cracks and progressively fill it.These crystals might be calcium carbonate or calcium hydroxide[13,21]. These micro-cracks opened up again in second loadingphase. After unloading, the width of these micro-cracks becameinvisible in the micro photos. These micro photos show that crys-tals formed in the micro-cracks by self-healing can bridge the

Fig. 12. Micro photos of micro-cracks at three states.

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micro-cracks but cannot provide the equivalent strength as thematrix have. This is why only partial of the tensile strength andcracking strain could be recovered after self-healing from thestress-strain curves, as shown in Fig. 9.

3.2.4. Damage investigation of the direct tensile tests by AE analysismethod

The results of direct tensile tests have shown the effects of threere-curing conditions on the tensile properties of high strain hard-ening UHPC. The AE analysis was used to describe the damage evo-lution of high strain hardening UHPC under three re-curingconditions. A comparison of AE sources number is made betweenspecimens in three re-curing conditions.

Figs. 9-11 show the distribution of AE source during two directtensile tests, given that the tensile strain reached 0.1%. In the firstloading phase, the number of AE sources ranges from 40 to 61 forthose six specimens. The high strain hardening UHPC specimensshow a homogeneous distribution of AE sources, which is relatedto the multiple micro-cracks. Therefore, equivalent elongation isbalanced in the form of more cracks with crack width at a muchlower level [9]. In the second loading phase, the AE sources num-bers of two specimens re-cured in water are 44 and 50, respec-tively. The numbers are 36 and 24 for specimens re-cured in 80%RH condition, and 14 and 13 for 45%RH condition. It is observedthat the specimens re-cured in water and 80%RH conditionsshowed more AE sources than the specimens re-cured in 45%RHcondition. This phenomenon is more visible for the specimens re-cured in water, the AE sources numbers of the second loading is

very close to those of the first loading. The AE sources are gener-ated when damages occurred inside the high strain hardeningUHPC. In the second loading, for healed micro-cracks, the newformed crystals which make a bridge between the two interfacesof the micro-cracks would crack again, relating to the correspond-ing AE source. However, for unhealed micro-cracks, the AE sourcewon’t be generated if the unhealed micro-cracks open again.Therefore the number of AE sources after re-curing is dependenton recovery of the micro-cracks. The more effective it is, the largerthe number of AE sources presents. The AE analysis method provesthat the water re-curing is more effective than 80%RH and 45%RHcondition for UHPC self-healing.

In addition to the AE sources generated by the re-cracking of thehealed micro-cracks, it is interesting to see that some AE sourcesgenerated in the new positions for all six specimens. The mecha-nismmay be related to these factors: 1) new debonding and slidingprocess in the existing multiple micro-cracks; 2) new formationmultiple micro-cracks, which means new micro cracking of UHPCmatrix and the debonding process between UHPC matrix and steelfiber occurred. After the unloading of the first loading, there wasresidual tensile strain of around 0.05%, which is caused by the gen-eration of the multiple micro-cracks, the debonding and sliding ofpart of the steel fibers which bridged these micro-cracks initially.Sometimes these micro-cracks can be classified as the smalldefects inside the UHPC matrix since they are difficult to bedetected by the equipment.

The AE analysis method is proved to be an effective method toinvestigate the damage degrees of UHPC materials under direct

J.-Y. Guo et al. / Construction and Building Materials 204 (2019) 342–356 351

tensile loading. Furthermore, the AE analysis method could be use-ful to relate the damage degrees of UHPC material at differentdeformation to the reduction of the stiffness of the UHPC structure,

Fig. 13. Results of air p

which could further relate to the different loading conditions at theserviceability state. Definitely, AE analysis is also a very usefulmethod to characterize the damage degree of UHPC structures.

ermeability tests.

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3.3. Air permeability

Fig. 13 shows the air permeability of specimens re-cured inthree conditions. Detailed air permeability indexes of each test sur-face are summarized in Table 7. Based on the criteria for evaluatingthe protective quality of concrete structures as listed in Table 3, thecorresponding protective qualities of all specimens are illustratedin Fig. 13.

3.3.1. Before re-curingThe air permeability indexes of six specimens obtained before

the first loading range from 0.0023 to 0.0068 Ln (mbar)/min, whichshow a small discreteness. According to Table 3, these are all muchless than the threshold of 0.1 Ln (mbar)/min. Therefore, the protec-tive quality of high strain hardening UHPC before loading is ‘‘verygood”. After the first loading (0.1% of the tensile strain and thenunloading), the air permeability indexes of UHPC-Water-1 andUHPC-80%RH-1 are slightly higher than the threshold of 0.1 Ln(mbar)/min, so the protective quality of these two specimens afterfirst loading is ‘‘good”. However, the air permeability indexes ofother four specimens range from 0.0048 to 0.0426 Ln (mbar)/min, which are still less than the threshold of 0.1 Ln (mbar)/min,and the protective quality is ‘‘very good”.

It is observed that although each specimen was loaded to thesame tensile strain in the first loading, the air permeability indexesafter the first loading are highly scattered. In the direct tensile test,damages for example, micro cracking of UHPC matrix and thedebonding process between UHPC matrix and steel fiber generatedrandomly in the specimen. Thus, the number of AE sources in theair permeability test area detected by the AE analysis system weredifferent. According to the literature [22], AE source is directlyrelated to internal damage of materials, and then the accumulatedAE sources number is supposed to be dependent on the damagedegrees of the UHPC. Therefore, different damage degrees lead toa large difference in the air permeability index. As shown in

Fig. 14. Relationship between change in air permeability index and AE sourcesnumber in the test area.

Table 6Recovery effect of self-healing UHPC.

Specimen types First cracking strain (e2nd / e1st)

UHPC-Water- 1 0.402 0.47

UHPC-80% RH- 1 0.132 0.19

UHPC-45% RH- 1 02 0

Note: (1) API1st = Air permeability index after first loading;(2) API2nd = Air permeability index after 28 days re-curing.

Figs. 9–11, the quantities of the AE sources among the air perme-ability tests area are quite different after the first loading. Thus,the air permeability indexes after the first loading are highly scat-tered. The relationship between the air permeability and the AEsources number in the air permeability test area is analyzed inthe section 3.3.4.

3.3.2. After re-curing in three conditionsAfter the first loading, the specimens of UHPC-Water-2, UHPC-

80%RH-2 and UHPC-45%RH-2 showed a similar air permeabilityindex, which ranged from 0.0227 to 0.0426 Ln(mbar)/min. Thesethree specimens could be considered to have same damage degree.After re-curing, UHPC-45%RH-2 showed a similar air permeabilityindex with the state just after first loading, which means that there-curing in 45%RH condition does not improve the impermeabil-ity. On the contrary, the air permeability indexes of UHPC-Water-2 and UHPC-80%RH-2 decreased with increasing re-curing period.After 7 days of water re-curing, the air permeability indexes ofUHPC-Water-2 decreased dramatically and reached as high asthe same level when UHPC specimen was intact and without load-ing. These results confirm that under the same degree of damage,the higher the water content of environment, the more remarkableof the self-healing effectiveness is observed.

As shown in Fig. 13, the average air permeability indexes of thetop and bottom surface of UHPC-Water-1 decreased from 0.118 Ln(mbar)/min to 0.0051 Ln (mbar)/min after 7 days of water re-curing, which is the same level as the state before loading. Theaverage air permeability indexes of UHPC-80%RH-1 maintainalmost the same as that after 7 days re-curing, while the following21 days of re-curing makes the air permeability indexes decreasedslightly. The air permeability index of UHPC-45%RH-1 did notchange because there is no damage occurred in the air permeabil-ity tests area after first loading.

3.3.3. After second loadingThe air permeability indexes of UHPC-Water-2 loading do not

change after the second loading, since there is no damage observedin the air permeability test area. The air permeability indexes of theother five specimens increased slightly after second loading.

3.3.4. Relationship between the air permeability and the AE sourcesnumber in the test area

In order to well understand the highly scattered of the air per-meability indexes, Fig. 14 shows the change in air permeabilityindex and the corresponding AE sources number generated whenthe tensile strain reached 0.1% during the first and second loading.The change in air permeability index as shown in Fig. 14 is calcu-lated as the difference of average air permeability indexes betweenbefore and after loading. To represent the relationship between theair permeability index and the AE sources number more accurate,quadratic polynomial regression method is applied. As shown inFig. 14, there is a positive correlation between the change in theair permeability indexes and the AE sources number in the air

Initial stiffness (E2nd / E1st) Air permeability index (API2nd / API1st)

0.94 0.021.01 0.080.56 0.730.76 0.200.43 0.690.49 0.90

Table 7Air permeability index (API) and AE sources number in the air permeability test area.

Specimen types Before first loading After first loading After 7 days re-curing

After 28 days re-curing

After second loading

APIT APIB APIT APIB NAE APIT APIB APIT APIB APIT APIB NAE

UHPC-Water-specimen-1 0.0023 0.0022 0.1283 0.1084 14 0.0022 0.0079 0.0026 0.0024 0.0504 0.0688 10UHPC-Water-specimen-2 0.0027 0.0022 0.0426 0.0227 6 0.0051 0.0041 0.0031 0.0023 0.0046 0.0038 0UHPC-RH80%-specimen-1 0.0068 0.0067 0.1007 0.0546 11 0.0975 0.0673 0.0796 0.0341 0.0842 0.0372 1UHPC-RH80%-specimen-2 0.0040 0.0042 0.0232 0.0323 6 0.0180 0.0166 0.0061 0.0052 0.0145 0.0113 3UHPC-45%RH-specimen-1 0.0032 0.0032 0.0030 0.0048 0 0.0026 0.0022 0.0031 0.0023 0.0050 0.0393 4UHPC-45%RH-specimen-2 0.0036 0.0042 0.0256 0.0295 7 0.0292 0.0264 0.0219 0.0278 0.0313 0.0433 4

Note: (1) APIT = Air permeability index of top surface; (2) APIB = Air permeability index of bottom surface;(3) NAE = AE sources number in air permeability test area till tensile strain reached 0.1%.

Fig. 15. Tensile stress-strain curves and relevant AE sources distribution of high strain hardening UHPC under cyclic loadings.

J.-Y. Guo et al. / Construction and Building Materials 204 (2019) 342–356 353

Table 8Initial stiffness, air permeability index and AE sources number for UHPC specimen under cyclic loadings.

Specimen states Initial Stiffness /GPa API/ Ln(mbar)/min NAE

APIT APIB

Before loading / 0.0044 0.0039 /UHPC-cycle-1st 45.87 0.1326 0.1275 13UHPC-cycle-2nd 21.42 / 0.1573 3UHPC-cycle-3rd 19.51 / / 1UHPC-cycle-4th 15.95 / / 0UHPC-cycle-5th 16.11 / / 0UHPC-cycle-6th 15.70 / / 0After 7 days water re-curing / 0.0454 0.0083 /After 28 days water re-curing / 0.0024 0.0029 /UHPC-cycle-7th 28.71 0.0029 0.0046 2UHPC-cycle-8th 18.74 0.0033 0.0057 0UHPC-cycle-9th 16.54 0.0036 0.0067 0UHPC-cycle-10th 16.81 0.0059 0.0099 0UHPC-cycle-11th 16.01 0.0067 0.0109 0UHPC-cycle-12th 17.19 0.0076 0.0160 0After 3 days steam re-curing / 0.0025 0.0044 /UHPC-cycle-13th 34.50 0.0030 0.0105 2UHPC-cycle-14th 18.91 0.0034 0.0114 0

Note: (1) APIT = Air permeability index of top surface; (2) APIB = Air permeability index of bottom surface; (3) NAE = AE sources number in air permeability test area till tensilestrain reached 0.1%.

Fig. 16. Tensile stress-strain curves of the specimen under cyclic loading before andafter re-curing.

354 J.-Y. Guo et al. / Construction and Building Materials 204 (2019) 342–356

permeability test area. This relationship can be extended to esti-mate the working states after getting the air permeability indexof UHPC structures.

3.4. Recovery contributed by water re-curing scheme

Table 6 summarizes the recovery of first cracking strain, initialstiffness and air permeability index by self-healing under three re-curing schemes. As shown in Table 6, for specimens of UHPC-water-1 and UHPC-water-2, the first cracking strain recover 40%-47%, the initial stiffness recover 94%-101%, and the air permeabilityindex decrease by 92%-98%. The above test results indicate thatself-healing behavior can recover the performance of UHPC struc-ture under tensile loading in two ways: 1) it can recover the imper-meability of UHPC structure for better durability; 2) it can recoverthe stiffness of UHPC structure in terms of the concept of ‘‘recoveryof first cracking strain”.

3.5. Self-healing behavior by two re-curing schemes of UHPC specimenunder cyclic loadings

Fig. 15 shows the direct tensile stress-strain curves and relevantAE sources distribution of high strain hardening UHPC under cyclicloadings as described in Fig. 6(b). The detailed information on ini-tial stiffness, air permeability index and AE sources number in airpermeability test area till tensile strain reached 0.1% are listed inTable 8. Selected tensile stress-strain curves before and after thetwo re-curing schemes are given in Fig. 16.

As shown in Table 8, the initial stiffness of 7th loading increasedfrom 15.70 GPa to 28.71 GPa after 28 days of water re-curing, andthe initial stiffness of 13th loading increased from 17.19 GPa to34.50 GPa after 3 days of steam re-curing. As shown in Fig. 16,the curves of 7th loading (after 28 days of water re-curing) and13th loading (after 3 days of steam re-curing) in the range of 0–2 MPa are similar to those of 1st loading. The stiffness of curvesin this section (0–2 MPa) of 1st loading, 7th loading and 13th load-ing are 46.17 GPa, 41.96 GPa and 45.12 GPa, respectively. Theresults indicate that both of the 28 days water re-curing and 3 daysof steam re-curing can significantly recover the stiffness in therange of 0–2 MPa, and 3 days of steam re-curing is more effectiveand less time consuming. It would be meaningful to use steamre-curing method for the rapid maintenance of UHPC structures

under service state, for example, UHPC pavement and prefabri-cated UHPC girder. The steam re-curing method can be used torecover the impermeability of UHPC pavement and the stiffnessof the prefabricated UHPC girder within the service life.

Fig. 17. Air permeability index of UHPC specimen under cyclic loading.

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Fig. 17 shows the change of air permeability indexes under cyc-lic loadings. Regarding the first series of cyclic loadings, it can beseen that the air permeability index changed dramatically afterthe first loading. The air permeability indexes of the top and bot-tom surface could not be measured after the second and third load-ing, which means the impermeability of the specimen is ‘‘verypoor” due to the severe accumulated damage. After 7 days of waterre-curing, air permeability indexes of the top and bottom surfacesdecreased to 0.0454 Ln(mbar)/min and 0.0083 Ln(mbar)/min,which is still higher than the state before the first loading. Thiscan be attributed to the higher damage degree in the air permeabil-ity test area of the specimen before re-curing. After 28 days of re-curing in water, the air permeability indexes were recovered to thesame level of those before the first loading. In the second series ofcyclic loadings, the air permeability indexes of the top and bottomsurfaces increased to 0.0076 Ln(mbar)/min and 0.016 Ln(mbar)/min after 12th loading. It seems that little damage was generatedand accumulated in the air permeability test area during the sec-ond series of the cyclic loadings (from 7th to 12th loading). Thisconclusion is supported by the relative AE sources number testedin air permeability test area. After 3 days of steam re-curing, theair permeability indexes recover up to 0.0025 and 0.0044 Ln(mbar)/min, which is the same as the results obtained before thefirst loading.

Fig. 15 (b) shows the AE sources distribution of UHPC specimenunder cyclic loadings. In the first series of cyclic loadings, the num-ber of AE sources were gradually decreased after each loading. Atthe meantime, it can be observed that the residual strain after eachunloading became smaller. These phenomenon can be explainedby the fact that the tensile deformation of specimen was mainlycaused by the elastic deformation of the steel fiber bridging themicro-cracks during the loading. After 28 days of water re-curing,the AE sources numbers increased from 3 to 36 in the 7th loading.In the second series of cyclic loadings, the number of AE sourceswere gradually decreased again. After 3 days of steam re-curing,the AE sources number improved from 2 to 25 in the 13th loading,which is the same as observed before. The AE analysis method pro-vides strong evidence for the self-healing effects by the water re-curing and steam re-curing method.

4. Conclusions

In this study, self-healing behavior of high strain hardeningUHPC after three re-curing conditions (water, 80%RH and 45%RH)were systematically investigated. Moreover, the cyclic tensile load-ing was applied to create more severe damage and two re-curingschemes (water and stream) were used to demonstrate the self-healing effects. The main conclusions can be drawn as following.

(1) The direct tensile stress-strain curves of the high strainhardening UHPC re-cured in water and 80%RH conditionshave the recovery of first cracking strain, which is relatedto self-healing effect. The high strain hardening UHPC re-curing in 45%RH conditions show no recovery.

(2) The self-healing effects on the tensile stiffness and imperme-ability of high strain hardening UHPC are enhancing byincreasing water content in the re-curing conditions.

(3) New crystals were observed in the micro-cracks after self-healing. These micro-cracks opened up again. After unload-ing, these micro-cracks became invisible in the micro pho-tos. These crystals can bridge the micro-cracks but cannotprovide the equivalent strength as the intact part, so thatonly partial of the tensile properties could be recovered afterself-healing. The high strain hardening UHPC can recover40%-47% of the first cracking strain after re-curing in waterfor 28 days.

(4) AE sources generated in the new positions after re-curing,meaning that new cracks generated inside the UHPC matrix.The AE analysis method provides strong evidence for the sig-nificant self-healing effects by the water re-curing andsteam re-curing.

(5) Air permeability test shows a sensitive response to the dam-age created by the tensile deformation. Air permeabilityindexes tested by the Autoclam permeability system couldbe used to characterize the permeability of the damagedUHPC. There is a positive correlation between the AE sourcesnumber generated in the air permeability test area and thechange in the air permeability indexes.

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(6) The cyclic tensile loading up to strain of 0.1% was applied tocreate more severe damage. Steam re-curing method showsmore effective and less time consuming for self-healing thanwater re-curing method. It would be meaningful to usesteam re-curing method for the rapid maintenance of UHPCstructures under serviceability limit state.

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, ‘‘Effects of self-healing on ten-sile behavior and air permeability of high strain hardening UHPC”.

Acknowledgments

This work was supported by the National Nature Science Foun-dation of China [grant number 51609172], Zhejiang Communica-tion Science and Technology Project, and the Shanghai MunicipalScience and Technology Project [grant number 17DZ1204200].The financial supports are greatly appreciated. Great thanks go toZhejiang Hongri TENACAL� Innovative Material Technologies Co.,Ltd. for providing the high strain hardening UHPC materials.

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