Lehmann, E. H.

Wang, P. G.

Wittmann, F. H.

Zhang, P.

Zhao, T. J.

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95Restoration of Buildings and MonumentsVol. 20,  No. 2, 95–102 (2014)DOI 10.12900/RBM14.20.2-0009Observation of Water Penetration into Cracked and Water Repellent SHCC after Imposed Strain by Means of Neutron Radiography P. G. Wang1, F. H. Wittmann1,2*, P. Zhang1, E. H. Lehmann3 and T. J. Zhao11Department of Civil Engineering, Qingdao Technological University, 266033 Qingdao, China 2Aedificat Institute Freiburg, 79100 Freiburg, Germany3Neutron Imaging & Activation Group, Paul Scherrer Institute, CH-5232 Villigen, Switzerland*Corresponding author: F. H. Wittmann, e-mail: wittmann@aedificat.deAbstractHuge strain capacity of SHCC under tensile stress can be reached by multiple crack formation, while thecracks remain bridged by fibers. Under protective conditions the full strain capacity can be used in practice.In this contribution it is shown that even at comparatively modest imposed strain cracks are formed, whichmay transport water and ions dissolved in water deep into structural elements made of SHCC. One way toreduce water penetration into cracked SHCC has been investigated, the use of an integral water repellentcement-based matrix. It is shown that in this way water penetration is significantly reduced and hence dura-bility is increased. Keywords: Water penetration; Multi-cracks; Strain Hardending Cementitious Composite (SHCC); Neutron radiography .P. G. Wang, F. H. Wittmann, P. Zhang, E. H. Lehmann and T. J. Zhao961 Introduction Durability of cement-based materials and in partic-ular of steel reinforced cement-based materialsdepends strongly on the penetration rate of waterand ions dissolved in water. It has been shown thatcracks are preferential pathways for water andchloride dissolved in water to penetrate deep intoconcrete [1-7]. It could be shown that NeutronRadiography is a particular powerful method toobserve water penetration into porous materialssuch as concrete and into cracks in particular. SHCC (Strain Hardening Cement-based Composite)is often called to be a pseudo-plastic material. Thatmeans that a huge strain capacity is reached by mul-tiple crack formation while the cracks are bridged byfibers [8-10]. If such a material is applied under pro-tection, without exposure to water or salt solutions,the full strain capacity can be exploited. In caseswhere the material under imposed strain is exposedto natural environment and to contact with seawaterfor instance, the durability becomes an importantissue and service life of a structure may be seriouslyreduced by crack formation. In this case the hugestrain capacity can be used partly only. The maxi-mum allowable strain will not be given by the mate-rials mechanical properties as such but rather by therate of water and chloride penetration. In this contri-bution it is shown that even at an imposed strain of0.5 % water can penetrate deep into the material in ashort time. One way to take advantage of the straincapacity is water repellent treatment. One major aimof this project was to study the influence of integralwater repellent treatment of SHCC on water penetra-tion into the fine cracks, which are formed underimposed strain.2 Materials and Methods / Experimental2.1 Materials and Preparation of SpecimensThe composition of SHCC has been optimized forthese test series by varying the amount of the dif-ferent components in a systematic way. Finally thefollowing composition was chosen: 550 kg/m3ordinary Portland cement Type 42.5, 650 kg/m3local fly ash, 550 kg/m3 sand with a maximumgrain size of 0.3 mm, and 395 kg/m3 water. To thefresh mix 26 kg/m3 of commercial PVA fibres witha diameter of 40 µm produced by Kuraray Com-pany were added. The density and mechanicalproperties of the fibers are shown in Table 1. Inaddition, integral water repellent SHCC has beenprepared by adding 2.0 % (related to the mass ofbinder) silane emulsion to the fresh mix. Aftercompaction in steel forms all specimens wereallowed to harden in the laboratory under wet bur-lap for 24 hours. Then the steel form has beenremoved and the specimens were stored until anage of 21 days in a moist curing room (T  ≈ 20 °C,RH ≥ 95 %).  In this project, dumbbell shaped specimens with athickness of 30 mm and an overall length of330 mm have been cast in steel moulds. The widthat both ends is 90 mm and the centre part with alength of 120 mm has a width of 60 mm. The shapeand the dimensions of the SHCC samples are givenin Fig. 1. The centre part, which has been underuniform tensile stress, had the following size: 120mm × 60 mm × 30 mm. To determine capillaryabsorption of water by means of neutron radiogra-phy, the centre part has been cut out of the dumbbellspecimen with a diamond saw after a certain tensilestrain had been applied. 2.2 Multiple Crack Formation at Differ-ent Levels of Applied StrainThe samples were taken out of the curing room atan age of 21 days. Then the specimens were fixedin a stiff steel frame. In a direct tension test differ-ent strain levels could be imposed. The followingstrain levels were selected: 0.5 %, 1.0 %, 1.5 %and 2.0 %. These chosen strain levels wereimposed to both neat and integral water repellentSHCC samples. The maximum load was kept con-stant for 10 minutes before unloading again. Table 1: Density and mechanical properties of the PVAfibres used in this project.Density,g/cm3Elastic Modulus,GPaUltimate strain,%Tensile strength,MPa1.3 42.0 7.0 1600Figure 1: Geometry and dimensions of the dumbbellspecimens used in this project.  Observation of Water Penetration into Cracked and Water Repellent SHCC after Imposed Strain by Means of Neutron Radiography973 Results and Discussion 3.1 Water Penetration into Neat SHCC after Exposure to Different Strain Levels The process of water penetration into SHCCexposed to different levels of tensile strain hasbeen made visible by neutron radiography. Firstneutron images have been taken on unloaded spec-imens for reference. Selected images taken at 15,45 and 90 minutes of capillary absorption areshown in Fig. 2. As one could have expectedSHCC specimens which have not been under ten-sile stress absorbed very little water even after con-tact with the liquid for 45 minutes. A quantitativeanalysis shows that a small amount of water pene-trates into the porous material nevertheless asshown in Fig. 3. From results shown in Fig. 3 acoefficient of capillary penetration B can be deter-mined. W(t) = B t1/2 (1)B was found to be approximately 0.004 m/h1/2. Itcan be concluded that SHCC has a very low coeffi-cient of capillary penetration as compared to ordi-nary concrete. B was found to be even lower afterdeep surface impregnation of cement-based materi-als with liquid silane.If the type of SHCC, which has been prepared forthis project, is exposed to a maximum tensile strainof 0.5 %, a few cracks can be observed. The widestcracks are quickly water filled up to the height ofthe specimen, which is 60 mm. Some finer cracksbecome visible after 15 minutes only. The moisturedistribution in SHCC in contact with water hasbeen determined quantitatively after 15 and 90 min-utes in the two rectangular areas shown in Fig. 2 onthe neutron image taken after 15 minutes on aSHCC specimen which was exposed to a tensilestrain of 2 %. The moisture profiles along the twolevels are shown in Fig. 4. In this case water pro-files as observed after 15 and 90 minutes of capil-lary absorption on specimens, which were exposedFigure 2:  Neutron images showing water penetration into neat SHCC after different imposed levels of tensile strain.Along the indicated two rectangular areas the water content has been determined.Strain level  Time:  5 min                15 min               45 min                90 min 0 %                       0.5 % 1.0 % 1.5 % 2.0 % Figure 3: Capillary absorbed water profiles in unstrainedSHCC after 15, 45 and 90 minutes of contact with water. 00.020.040.060.080.10 5 10 15 20 25Depth, mmWater content, g/cm3 Reference, 15 minReference, 45 minReference, 90 minP. G. Wang, F. H. Wittmann, P. Zhang, E. H. Lehmann and T. J. Zhao98to a tensile strain of 0.5 % are shown. Obviously thelower part has absorbed more water than the centerof the specimen. The wide cracks, which reach theupper surface, soon contain significantly morewater than fine cracks. From the volume of the pen-etrated water the width of the cracks can be calcu-lated. In Fig. 5 the corresponding moisture profilesof a specimen, which has been exposed to 1 %strain under tension are shown. Again the capillaryabsorption can be observed by the time-dependentrise of the water content in the areas without cracks.The corresponding water profiles as determined onsamples, which have experienced a strain of 1.5 %are shown in Fig. 6. Under these conditions thecracks became wider and they absorb more water.But the damage in the regions near the cracks is alsogradually damaged and these samples absorb muchmore water as compared with the specimen whichwas strained up to 0.5 % only. Finally the waterup-take in SHCC, which has experienced a tensilestrain of 2 % is also distributed over the entire vol-ume and not concentrated on the water filledcracks. These results underline the fact that SHCC has anextreme strain capacity. If the mechanical proper-ties are of interest in a given application exclu-sively, then one can take full advantage of the straincapacity. If durability is an issue, however, the ulti-mate strain, which may be allowed, is very limited.      3.2 Water Penetration into Integral Water Repellent SHCC  By adding silane emulsion to SHCC an integralwater repellent cementitious matrix can be pro-duced. In section 3.1 we have seen that water pene-trates into the fine cracks, which are formed at highstrain due to the so-called pseudo ductility. If thematrix is made water repellent, the water penetra-tion into the cracks should be reduced at least sig-nificantly. Therefore tests have been run with inte-gral water repellent SHCC.Figure 4: Water content in horizontal direction in the center and at the bottom of cracked SHCC after imposed tensilestrain of 0.5%.Figure 5: Water content in horizontal direction in the center and at the bottom of cracked SHCC after imposed tensilestrain of 1 %.00.030.060.090.120 20 40 60 80 100 120Depth, mmWater content, g/cm3 0.5% centre, 15 min0.5% centre, 90 min0.5% bottom , 15 min0.5% bottom, 90 min00.030.060.090.120 20 40 60 80 100 120Depth, mmWater content, g/cm3 1% centre, 15 min1% centre, 90 min1% bottom, 15 min1% bottom, 90 minObservation of Water Penetration into Cracked and Water Repellent SHCC after Imposed Strain by Means of Neutron Radiography99Typical results are shown in Fig. 8. The specimenswhich have not been exposed to strain absorb prac-tically no water during 90 minutes of contact withthe liquid. Even after application of 0.5 % straintraces a small quantity of water can be observed inthe cracks. When a strain of 1.5 % was imposed,cracks begin to absorb more important amounts ofwater.In Fig. 8 the water profile in a specimen after appli-cation 0.5 % strain is shown. Compared with resultsshown in Fig. 4 less water enters the zones withoutcrack and the cracks absorb significantly less wateras well. In Fig. 10 the water profiles as measured onSHCC, which has been strained up to 1.5 % areshown. We can observe that under these conditionsthe cement-based matrix of SHCC still does notabsorb water and the amount of water penetratingthe cracks is significantly less as can be checked bycomparing Fig. 6 and Fig. 10. 4 ConclusionsFrom the results presented in this contribution itcan be concluded that cracks formed by imposedstrain of not more than 0.5 % in SHCC absorbwater by capillary action. This water penetratesdeep into the material. If SHCC is applied as a pro-tective layer, a maximum strain must not beexceeded. This maximum tolerable strain is still tobe determined for different types of SHCC. Integral water repellent SHCC absorbs less waterinto the cement-based matrix and into cracksformed by strain under tension. When cracks areformed, they absorb less water. This is an indicationthat higher tensile strain will be allowable. Surfaceimpregnation with silane after crack formation willbe a promising alternative. Figure 6: Water content in horizontal direction in the center and at the bottom of cracked SHCC after imposed tensilestrain of 1.5%.Figure 7: Water content in horizontal direction in the center and at the bottom of cracked SHCC after imposed tensilestrain of 2.0 %.00.030.060.090.120 20 40 60 80 100 120Depth, mmWater content, g/cm3 1.5% centre, 15 min1.5% centre, 90 min1.5% bottom, 15 min1.5% bottom, 90 min00.030.060.090.120 20 40 60 80 100 120Width, mmWater content, g/cm3 2% centre, 15 min2% centre, 90 min2% bottom , 15 min2% bottom, 90 minP. G. Wang, F. H. Wittmann, P. Zhang, E. H. Lehmann and T. J. Zhao100Figure 8: Neutron images of water penetration into integral water repellent SHCC after imposed different level of tensile strain. Figure 9: Water content in horizontal direction in the center and at the bottom of cracked water repellent SHCC after0.5 % of tensile strain.Figure 10: Water content in horizontal direction in the center and at the bottom of cracked water repellent SHCC after1.5 % of tensile strain.Strain Time:    5 min                15 min                 45 min                90 min Reference 0 %                         0.5 % 1.5 % 00.030.060.090.120 20 40 60 80 100 120Depth, mmWater content, g/cm3 IW0.5% centre, 15 minIW0.5% centre, 90 minIW0.5% bottom, 15 minIW0.5% bottom, 90 min00.030.060.090.120 20 40 60 80 100 120Depth, mmWater content, g/cm3 IW1.5% centre, 15 minIW1.5% centre, 90 minIW1.5% bottom, 15 minIW1.5% bottom, 90 minObservation of Water Penetration into Cracked and Water Repellent SHCC after Imposed Strain by Means of Neutron Radiography101AcknowledgementThe authors gratefully acknowledge substantialsupport of ongoing projects by Cooperative Inno-vation Center of Engineering Construction andSafety in Shandong Blue Economic Zone andNational Natural Science Foundation of China(nos. 51278260, 51008165 and 51108443).References1. F. H. Wittmann, P. Guo and T. Zhao, Influ-ence of cracks on the efficiency of surfaceimpregnation of concrete, Proceedings ofFifth Int. Conf. Water Repellent Treatmentof Building Materials, Hydrophobe V, H. DeClercq and A. E. Charola, editors, Aedifica-tio Publishers (2008) pp. 287-298. 2. P. Zhang, F. H. Wittmann, T. Zhao, E. Leh-mann, P. Vontobel and S. Hartmann, Obser-vation of water penetration into waterrepellent and cracked cement-based materi-als by means of neutron radiography, Int. J.Restoration of Buildings and Monuments 15(2009)  91-100.3. P. Zhang, J. G. Dai, T. Zhao, F. H. Wittmannand S. Hartmann, Water repellent treatmenton surface of cracked concrete under seawa-ter exposure environment, Journal of Build-ing Materials, 12 (2) (2009) 214-218.4. F. H. Wittmann, T. Zhao, Z. Ren and P. Guo,Influence of surface impregnation withsilane on penetration of chloride intocracked concrete and on corrosion of steelreinforcement, Int. J. Modelling, Identifica-tion and Control, 7 (2009) 135 – 141. 5. P. Zhang, F. H. Wittmann, T. Zhao andE. H. Lehmann, Neutron imaging of waterpenetration into cracked steel reinforcedconcrete, Physica B: Condensed Matter,405(7) (2010) 1866-1871.6. F. X. Jiang, F. R. Li, T. J. Zhao andF. H. Wittmann, Study on chloride penetra-tion into the cracked zone of reinforced con-crete and effect of water repellent treatment,Concrete (2) (2011) 21-24, in Chinese.7. P. Zhang, J. Dai, T. Zhao, F. H. Wittmannand Z. Jin, Water absorption of cracked con-crete and efficiency of water repellent treat-ment, Journal of Building Materials 13 (1)(2010) 70-74 (in Chinese).8. G. P. A. G. van Zijl, F. H. Wittmann,B. H. Oh, P. Kabele, R. D. Toledo Filho,E. M. R. Fairbairn, V. Slowik, A. Ogawa,H. Hoshiro, V. Mechtcherine, F. Altmann,M. D. Lepech, Durability of strain-harden-ing cement-based composites (SHCC), Mate-rials and Structures, 45 (2012) 1447-1463.9. G. P. A. G. Van Zijl and F. H. Wittmann(eds.) Durability of Strain-HardeningFibre-Reinforced Cement-Based Compos-ites (SHCC), State-of-the-Art Report, RilemTC 208 HFC, SC 2, Springer Publishers,(2011).10. G. P. A. G. van Zijl, V. Slowik, R. D. ToledoFilho, F. H. Wittmann, H. Mihashi, Compar-ative testing of crack formation instrain-hardening cement-based composites,(to be published) Received March 20, 2014

Observation of Water Penetration into Cracked and Water Repellent SHCC after Imposed Strain by Means of Neutron Radiography

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