Moving cracks are often present in concrete structures and in those circumstances any self-healing technique for concrete must satisfy specific performance requirements, to guarantee its increased durability. These requirements include the capability of withstanding multiple cycles of crack movement without failing to keep healed cracks sealed. This paper shows early results from a testing protocol suggested by the authors to assess the performance of polymers as healing materials for moving cracks. Ultrasound (US) shear waves were used for continuous monitoring of small prismatic mortar specimens containing a single healed crack under a cyclic load. The maximum amplitude of US waves transmitted across healed cracks was correlated to the area effectively healed and the magnitude of crack movement. A decreasing trend of the maximum amplitude during cyclic loading was observed for strain levels on the polymer corresponding to 70% of its strain limit, but soundness at lower strain levels was confirmed after 300 cycles

Couvreur, J

De Belie, Nele

Garcia Feiteira, João Luis

Gruyaert, Elke

English

Ghent University Academic Bibliography

Concrete Solutions – Grantham et al. (Eds.)© 2016 Taylor & Francis Group, London, ISBN 978-1-138-03008-4Resistance to fatigue of self-healed concrete based on encapsulatedpolymer precursorsJ. Feiteira, V. Couvreur, E. Gruyaert & N. De BelieMagnel Laboratory for Concrete Research, Department of Structural Engineering, Faculty of Engineering andArchitecture, Ghent University, BelgiumABSTRACT: Moving cracks are often present in concrete structures and in those circumstances any self-healingtechnique for concrete must satisfy specific performance requirements, to guarantee its increased durability.These requirements include the capability of withstanding multiple cycles of crack movement without failingto keep healed cracks sealed. This paper shows early results from a testing protocol suggested by the authors toassess the performance of polymers as healing materials for moving cracks. Ultrasound (US) shear waves wereused for continuous monitoring of small prismatic mortar specimens containing a single healed crack undera cyclic load. The maximum amplitude of US waves transmitted across healed cracks was correlated to thearea effectively healed and the magnitude of crack movement. A decreasing trend of the maximum amplitudeduring cyclic loading was observed for strain levels on the polymer corresponding to 70% of its strain limit, butsoundness at lower strain levels was confirmed after 300 cycles.1 INTRODUCTIONAs with other emerging concrete technologies, self-healing concrete currently lacks straightforward testmethods to assess its performance. This is particularlytrue for test methods simulating realistic conditionsthat are similar to those found in field structures. Onesuch condition is the occurrence of moving cracksin concrete elements, often present in structures sub-jected to traffic loads or extreme daily temperaturevariation.Under increasing load, self-healed cracks onlyremain sealed if a flexible polymer is used as healingmaterial. Mineral materials would result in the reopen-ing of the healed crack while a new crack would formif very stiff polymers were used. This issue has beenaddressed by the authors in previous publications (e.g.Feiteira et al. 2016) where the strain limit of severalflexible polymers bridging moving cracks was deter-mined. There, the strain limit was defined as the strainafter which failure occurs due to rupturing of the poly-mer or its detachment from the crack walls, resultingin considerable increase of the water uptake throughthe healed crack.If the load on a healed concrete element is cyclic,however, the strain limit of the healing material maybe reduced due to fatigue. This is of great importance,but not studied to date. Even in related research areas,such as injection products for concrete repair, this topicis seldom studied, e.g. Shin et al. (2011). Regard-ing the research on self-healing concrete involvingfatigue performance, only one study by Yang et al.(2011) was found. The differences between the tworesearch areas are essentially limited to the fact thatfor injection products there is the possibility for sur-face preparation before introducing the liquid polymerinto the crack, which is done under pressure. On self-healing concrete, the polymer precursor is releasedfrom embedded capsules immediately upon crackingof the concrete element, flowing into the crack due tocapillary tension.This paper presents early stage data of a testingsetup developed to assess the fatigue resistance of apolymer precursor used as an encapsulated healingagent for self-healing concrete. The proposed test isperformed at a small scale on mortar specimens andincludes a continuous monitoring technique based onthe transmission of ultrasound (US) waves. The use ofultrasound as a monitoring technique in this contextis built upon previous results published by Malm andGroße (2015). US waves are affected by changes in thepropagation medium and are affected by any potentialloss of continuity across the faces of healed cracks.Thus, they have been used to detect fatigue-relatedfailure due to changes in the matrix of the polymerfilling the crack or its detachment at the interface.2 MATERIALSA moisture-curing polyurethane precursor of low vis-cosity (200 mPa.s) was used in this study. From aprevious study (Feiteira et al. 2016) it was known thatthis precursor leads to very effective sealing of cracks585when used as an encapsulated healing agent and hadshowed also good strain capability for this application(approximately 50%). The precursor was encapsulatedin Ø3 mm glass tubes cut to a 50 mm length and sealedwith Poly(methyl methacrylate) glue. Although not fitfor bulk addition during mixing of concrete, the brit-tleness and barrier properties of glass tubes make themideal carriers for laboratory work, where they can bepre-placed into moulds and careful pouring of concreteor mortar can take place.3 PROCEDURE3.1 Specimen preparationA pair of glass capsules was pre-placed in each mouldfor 40 × 40 × 150 mm3 prisms, held by nylon wires,and mortar was poured over them. The mortar mixdesign consisted of cement CEM I 52.5 N, 0/4 mmsand, a cement to sand ratio of 1:3 and a water tocement ratio of 0.5. Mixing and moulding were carriedout according to the procedure described in EN 196-1and the specimens were cured in sealed conditions at20◦C for at least 14 days. After the curing period, a7 × 150 mm2 wide strip of fibre reinforced polymer(FRP) was glued with a structural epoxy glue alongtwo opposing sides of the specimen (see Figure 1).The specimens were then cracked under 3-point bend-ing, with the FRP strips at the compression side. Theywere then used not as conventional reinforcement,but as a way to avoid complete separation of the twohalves of the specimen after cracking.A crack approxi-mately 300 µm wide at the crack mouth remained afterunloading. The final configuration of the specimens isshown in Figure 1. After a period of 3 days, to allowthe healing agent to flow and harden, mortar specimenswith a single healed crack were ready to be tested.3.2 Protocol for cyclic loadingAn LVDT was placed across the crack mouth to con-trol the crack movement during testing. Cyclic loadingoccurred under bending, with each cycle defined as acrack width increase from a starting displacement of0.05 mm to the desired maximum displacement, cor-responding to a certain strain on the polymer fillingthe crack. After each cycle, a waiting period of 5 s wasintroduced at the starting displacement of 0.05 mm, sothat the monitoring equipment could acquire data instable and well defined conditions. A visual represen-tation of these cycles is shown later in this document,in Figure 5. A loading speed of 0.013 mm/s was used,resulting in a frequency of approximately 0.07 Hz, butdepending on the maximum displacement level set forthe test.3.3 Monitoring with ultrasound transmissionequipmentA FreshCon system from Smartmote was used to gen-erate, trigger and acquire ultrasonic pulses of 800Vand a pulse width of 5 µs. Olympus V150-RB shearFigure 1. Specimen configuration with a single crack atmid-span and a reinforcing FRP strip; US shear sensors areattached at opposite tops.Figure 2. Increase of maximum amplitude of US waveduring the healing period.wave sensors were used and monitoring took place at2 s intervals during cyclic loading. The sensors werecoupled to opposing tops of the specimens (Figure 1)with silicone gel and were kept in place with a systemof small supporting metal plates and an elastic bandpressing them against the specimen. The maximumamplitude of the US waves was used as the monitoringparameter.4 RESULTS4.1 Monitoring of the healing processAs shear waves don’t propagate well through liquidphases, the ultrasound shear waves were used to mon-itor the progressive dispersion and hardening of thepolymer precursor inside the crack on one of the spec-imens, after flowing out of the ruptured glass carriers.As shown in Figure 2, while the precursor turns froma liquid compound into a solid polymer, the maximumUS wave amplitude increases progressively.Most of the hardening occurs during the first 24 hafter contact with the moisture present in the mortarmatrix. The slight decrease in maximum amplitudeafter this period could have been due to absorptionof the silicone gel (used to couple the sensors) intothe mortar matrix, otherwise, a slow rate of increase isexpected.The final maximum amplitude after healing isdependent on the area of the crack effectively healed.Figure 3 shows the crack faces of two split specimens586Approachable Approachable Approachable Approachable Approachable Approachabl Positive attitude Positive attitude Positive attitude Downloaded by [GHENT UNIVERSITY LIBRARY], [Nele De Belie] at 02:34 19 October 2016 Figure 3. Crack faces of split specimens, showing a partic-ularly well healed crack (left) and crack with a smaller areaeffectively healed (right); the respective waves transmittedacross the cracks are shown below.with considerably different sizes of the crack surfaceeffectively healed. For these specimens, a fluorescentdye was added to the precursor for easy detection ofthe surface covered with polymer. Although there isa correlation between area healed and the maximumamplitude of the US wave transmitted across the healedcrack, the amplitude is also slightly dependent on theconsistency of coupling between the sensors and thespecimen.4.2 Evolution of US wave amplitude withincreasing polymer strainAs mentioned, the transmission of ultrasonic wavesis affected by changes in the continuity of the prop-agation medium. As a healed crack gets increasinglylarger and the strain on the polymer increases, fail-ure due to detachment or rupturing of the polymermatrix eventually occurs and a decrease of the maxi-mum amplitude of the US wave is thus expected. Thisis shown in Figure 4 for several specimens.4.3 Fatigue assessment: combined cyclicloading and monitoring based onUS wave transmissionAs US wave data was acquired constantly, at 2 s inter-vals, post processing of the data was required to selectonly the data points corresponding to the 5 s waitingperiod introduced in the protocol, at 0.05 mm displace-ment. Only this data, acquired in stable conditions, wasused to monitor the continuity across crack faces. Thedata points of higher wave amplitude occur naturallyat the moments of lower LVDT displacement, corre-sponding to the waiting period, as shown in Figure 5.Figure 4. Progressive reduction of maximum US waveamplitude with increasing crack widening and strain on thepolymer.Figure 5. Approximately 5 cycles of the cyclic loading pro-tocol showing displacement (solid line), amplitude (green)data points and upper envelope of amplitude data (dottedline).The upper envelope of the amplitude data (line cross-ing data peaks) could then be used as the monitoringparameter.Assuming the strain limit of the polymer to be 50%,after which detachment from the crack faces occurs,early tests were performed at different levels of thestrain limit, i.e. 40% of the strain limit and 70% ofthe strain limit. The evolution of the maximum waveamplitude (upper envelope of US wave data acquired)is shown in Figure 6 for specimens at the two strainlevels tested, as a function of the number of cyclesperformed.For a tested strain level of 70% of the strain limit,the maximum amplitude shows a decreasing trend,suggesting that progressive failure may be occurring,which previous results show to be due to detachmentof the polymer from the crack faces. For a lower strainlevel of 40% of the strain limit, there is no considerablechange in the maximum amplitude after 300 cycles,suggesting a sound healed crack and no susceptibil-ity to fatigue failure. A micrograph of a section of thecrack mouth, where polymer could be seen at the sur-face (Figure 7), confirms these results and shows thepolymer still attached to the crack faces.587Approachable Approachable Approachable Approachable Approachable Approachable Approachable Approachable Approachable Approachable Approachable Approachable Approachable Positive attitude Positive attitude Positive attitude Positive attitude Positive attitude Positive attitude Positive attitude Positive attitude Positive attitude Positive attitude Positive attitude Positive attitude Positive attitude Downloaded by [GHENT UNIVERSITY LIBRARY], [Nele De Belie] at 02:34 19 October 2016 Figure 6. Evolution of maximum amplitude of US wavesacquired at a stable deformation of 0.05 mm.Figure 7. Section of 0.38 mm crack mouth where stretchedpolymer could still be seen attached to the crack walls aftermore than 300 cycles.After these early results, the testing protocol wasconfirmed to be accurate and useful for monitoringthe soundness of self-healing cementitious material. Alarger number of tests still needs to be performed, withthe inclusion of water uptake tests after a fixed numberof cycles as additional confirmation of soundness orfailure due to fatigue.5 CONCLUSIONSThe maximum amplitude of US waves transmittedacross healed cracks is a simple parameter that wassuccessfully correlated with both the surface of thecrack that was effectively healed with an encapsulatedpolymer precursor and the strain on the polymer, i.e.the amount of crack movement. Changes in maximumamplitude at stable levels of crack movement were alsosuccessfully used to monitor the soundness of healedspecimens when subjected to multiple cycles of crackmovement. When tested at a maximum strain level of40% of the strain limit, after which detachment of thepolymer from the crack faces is known to occur, theamplitude of the US waves was stable after 300 cycles.This suggests that at levels much below the strain limit,the self-healing technique is resistant to fatigue, whichwas confirmed by microscopic observation of a sectionof the crack mouth. At higher strain levels of 70% ofthe strain limit, there was however a decreasing trendof the maximum amplitude, suggesting that the strainlimits of the self-healing system should be correctedto consider failure due to fatigue.ACKNOWLEDGEMENTSThe research leading to these results has received fund-ing from the European Union Seventh FrameworkProgramme (FP7/2007-2013) under grant agreementn◦ 309451 (HEALCON). The kind help of FabianMalm (TUM) is also acknowledged.REFERENCESFeiteira, J.; Gruyaert, E.; De Belie, N. 2016. Self-healing ofmoving cracks in concrete by means of encapsulated poly-mer precursors. Construction and Building Materials 102:671–678.Malm, F.; Große, C. 2015. Proof of efficiency: Examinationof concrete beams with self-healing properties by non-destructive testing methods. Fifth International Confer-ence on Self-Healing Materials (ICSHM2015). Durham,U.S.A..Shin, H.; Miyauchi, H.; Tanaka, K. 2011. An experimentalstudy of fatigue resistance in epoxy injection for crackedmortar and concrete considering the temperature effect.Construction and Building Materials 25: 1316–1324.Yang, Z.; Hollar, J.; He, X. 2011. A self-healing cementitiouscomposite using oil core/silica gel shell microcapsules.Cement and Concrete Composites 33: 506–512.588Positive attitude Positive attitude Positive attitude Honest and trustworthy Honest and trustworthy Honest and trustworthy Honest and trustworthy Honest and trustworthy Downloaded by [GHENT UNIVERSITY LIBRARY], [Nele De Belie] at 02:34 19 October 2016 

Resistance to fatigue of self-healed concrete based on encapsulated polymer precursors

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Resistance to fatigue of self-healed concrete based on encapsulated polymer precursors

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