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Influence of damage degree on self-healing of concrete
Available online at www.sciencedirect.com
Construction and Building
MATERIALS
Construction and Building Materials 22 (2008) 1137–1142
www.elsevier.com/locate/conbuildmat
Influence of damage degree on self-healing of concrete Wenhui Zhong, Wu Yao
*
Key Laboratory of Advanced Civil Engineering, Materials of Ministry of Education, Tongji University, Shanghai 200092, China Received 27 November 2006; received in revised form 15 February 2007; accepted 23 February 2007 Available online 12 April 2007
Abstract The paper presents experimental results of self-healing process of concrete damaged at different ages. Essentially, the self-healing of damaged concrete is processes of crack closing with re-hydration products of unhydrated or insufficiently-hydrated cementitious particles in damaged regions. Damage degree was measured from decrease in ultrasonic pulse velocity (UPV) before and after loading, and the self-healing effect was deduced from the strength increment after self-healing by introducing a self-healing ratio. The relationship between damage degree and self-healing ratio of concrete was built based upon the experimental results. Analyses of test results show that there exists a damage threshold both for high strength concrete and normal strength concrete. When the damage degree is less than the threshold, the self-healing ratio of concrete is increased with the increase in damage degree; while the damage degree exceeds the threshold, the self-healing ratio is decreased with the increase in damage degree. The damage threshold for normal strength concrete is higher than that for high strength concrete. 2007 Elsevier Ltd. All rights reserved. Keywords: Concrete; Self-healing; Hydration
1. Introduction Concrete in service cracks due to direct stress and substress caused by many kinds of reasons, such as changes of temperature and humidity, inhomogeneous sinking, external loading (dynamic or static loading). Cracks not only influence the service durability of concrete structure, but also are harmful for the structure safety. However, not all initial microcracks develop into harmful cracks or unstable cracks. Since earlier researchers discovered that cracked specimens under compressive testing at 28 days healed autogenously after had been stored outdoors for 8 years, many researchers have been focusing on this subject and done further studies. Wagner [1] tested to investigate whether cracks in seal-coated cement-mortar linings in water pipe and fittings would self-heal by chemical reaction when totally immersed in potable water. Hannant and Keer [2] studied self-healing process of microcracks in concrete. Gray [3] examined the self-healing of interfacial bond *
Corresponding author. Tel.: +86 21 65984191. E-mail address: [email protected] (W. Yao).
0950-0618/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2007.02.006
between steel fibers and mortar matrix. In 1995, Stefan et al. [4] found that concrete after deterioration by freeze and thaw regained almost the all resonance frequencies and the crystals of ettringite and Ca(OH)2 were seen traversing the cracks at several location through scanning electron microscope (SEM). One year later, Stefan [5] studied the deterioration and self-healing on chloride transport in OPC concrete. Nataliya [6] discussed the differences of self-sealing, autogenous healing and continued hydration. Edvardsen [7] gave a comprehensive theoretical discussion of the laws which governed the calcite nucleation and the subsequent crystal growth of water-bearing cracks in concrete, and they also pointed out that the crystal growth responded to two different processes which were determined by the changes in the chemical and physical conditions in cracks. Liu et al. [8] also carried out experiments to study the influences of cement particles diameter distribution and constitute on self-healing performance of concrete. Waterproof concrete structures cracked and leaked gently at the beginning, but after a period of time, it was found that the cracks closed completely and did not leak at all [9]. Wieland and Michaela [10] studied that an autog-
1138
W. Zhong, W. Yao / Construction and Building Materials 22 (2008) 1137–1142
enous healing would take place in water under pressure when up to certain width of the crack. Reinhardt and Jooss [11] established self-healing behavior of cracked concrete as a function of temperature and crack width, and they found that the average crack width measured at the surface showed the fastest self-healing. Romildo et al. [12] studied the crack self-healing tendency after plastic shrinkage cracking of cement mortar and sisal fiber-mortar composite, and got the results that both cracks of two different mortars could be self-healed, but different initial crack width showed different self-healing tendencies. Above-mentioned researches showed that concrete after deterioration could self-heal indeed, the mechanisms and influence factors must be further studied. Relevant researches showed that, although the strength of damaged concrete could be regained in a high degree, the ability of strength recovery was determined by many factors, such as the mixture of concrete, the damaged age, damage degree, conditions of self-healing (humidity, temperature) and curing period etc. Among these factors, the damage degree is the most important factor. It was found that 0.1–0.3 mm wide cracks could be self-healed in laboratory testing, but most structures in service still collapsed finally because of cracks. Assessing the extent of crack growth and self-healing effect is considered in this paper, including issues such as the relationship between the damage degree of concrete and self-healing, the influences of loading ages and concrete strength grades on the effect of self-healing, and the relationship between damage degree and self-healing ratio of concrete was discussed. 2. Experimental 2.1. Specimen preparation The laboratory tests were conducted by using specimens of normal strength concrete and high strength concrete, respectively, for the comparison of the self-healing effects of different strength grades concrete. The materials used in normal strength concrete (series 0#) were ordinary Portland cement (ISO 32.5), Class I fly ash, medium sand, crushed gravel with maximum size of 31.5 mm, and superplasticizer. In high strength concrete (series 1#), they were ordinary Portland cement (ISO 52.5), ground granulated blast-furnace slag (GGBS), silica fume (SF) besides the same fine and coarse aggregates, and superplasticizer used in series 0#. The mix proportions investigated in the study are given in Table 1. The slump of fresh concrete was 200 ± 20 mm. The cubic specimens (100 · 100 · 100 mm) were cast in the oiled steel molds and kept under ambient conditions for 24 h, covered with
a polythene film. The molds were stripped and the specimens were cured in the standard moist room where curing continued at 20 C in accordance with ASTM C 192. 2.2. Testing In order to distinguish the different self-healing effects of concrete damaged at different ages, concrete samples were deteriorated by compression test at 7, 14, and 28 days, respectively, for series 0#, while tested at 3, 14, 28, and 60 days, respectively, for series 1#. All the samples after deterioration were cured for 30 days for normal strength concrete and 60 days for high strength concrete in the standard curing room. Previous studies [13,14] showed that there existed relationship between damage and UPV decrease of concrete by ultrasonic transmission method. Therefore, in this paper, the UPV decrease of the direction which is vertical to loading direction was investigated to assess the damage degree of concrete. 3. Testing results and discussions The mechanical properties and ultrasonic decrease of normal strength and high strength concretes are given in Tables 2 and Table 3, respectively. The number 0#–7–1 means the No. 1 sample of series 0# loaded at the age of 7 days. The UPV of concrete before loading versus compressive strength is shown in Tables 2 and 3. It can be noticed that a higher compressive strength results in a higher UPV, but at relative low strength, the relationship is not obvious, as shown in Fig. 1. The UPV resulted from ultrasonic transmission method gives information about microstructures properties of materials, such as porosity, elastic modulus, microcracks distribution and density. Furthermore, it has also been observed that a relationship established during the developmental stages of the concrete cannot be used to predict the strength of concrete that has already undergone deterioration, but decrease of UPV can be used with more success to assess the deterioration in concrete. After self-healing, not only the mechanical properties of concrete have been recovered, but also the UPV has been regained in a great deal, as shown in Fig. 2. As a result, damage degree could be inferred from the UPV decrease of concrete at loading. Any way, it must be pointed out that the two curves in Fig. 2 are not totally the same, which indicates that there are inherent relationships between damage degree and self-healing effect. Theoretically, concrete can be classified as a composite material which consists three phases: macrocracks, discrete
Table 1 Concrete mixtures (kg/m3) Series
Cement
Fly ash
GGBS
SF
Sand
Gravel
Water
Superplasticizer
0# 1#
280 205
71 0
0 184
0 21
747 706
1126 1153
165 131
3.30 4.92
W. Zhong, W. Yao / Construction and Building Materials 22 (2008) 1137–1142
1139
Table 2 Summary of testing results for normal strength concretes No.
0#–7–1 0#–7–2 0#–7–3 0#–7–4 0#–7–5 0#–7–6 0#–14–1 0#–14–2 0#–14–3 0#–14–4 0#–14–5 0#–14–6 0#–28–1 0#–28–2 0#–28–3 0#–28–4 0#–28–5 0#–28–6
Loading age (day)
7 7 7 7 7 7 14 14 14 14 14 14 28 28 28 28 28 28
Properties before and after peak loading
Properties after self-healing for 30 days
UPV before peak loading (km/s)
Strength (MPa)
UPV after peak loading (km/s)
UPV (km/s)
Strength (MPa)
4.36 4.25 4.52 4.25 4.25 4.31 4.49 4.53 4.64 4.81 4.73 4.76 5.10 4.98 5.15 5.19 5.15 5.12
11.78 11.11 11.78 14.44 13.78 14.00 12.09 10.98 11.87 16.11 16.00 16.22 18.31 19.66 19.65 28.57 32.70 28.54
2.20 3.55 1.63 2.40 2.33 2.72 1.97 2.21 1.49 2.53 1.21 2.33 3.21 2.72 3.45 3.94 — 3.61
4.24 5.08 3.92 4.29 4.40 4.55 3.51 3.22 3.64 4.44 2.72 4.34 4.64 4.08 4.79 4.46 — 4.72
26.99 21.08 26.22 30.00 30.30 29.98 24.99 22.85 26.61 33.23 27.60 33.16 26.90 30.59 29.89 35.14 — 39.32
Table 3 Summary of testing results for high strength concretes No.
1#–3–1 1#–3–2 1#–3–3 1#–3–4 1#–3–5 1#–3–6 1#–3–7 1#–3–8 1#–3–9 1#–14–1 1#–14–2 1#–14–3 1#–14–4 1#–14–5 1#–14–6 1#–14–7 1#–14–8 1#–14–9 1#–28–1 1#–28–2 1#–28–3 1#–28–4 1#–28–5 1#–28–6 1#–28–7 1#–28–8 1#–28–9 1#–60–1 1#–60–2 1#–60–3 1#–60–4 1#–60–5 1#–60–6 1#–60–7 1#–60–8 1#–60–9
Loading age (day)
Properties before and after peak loading
Properties after self-healing for 60 days
UPV before peak loading (km/s)
Strength (MPa)
UPV after peak loading (km/s)
UPV (km/s)
Strength (MPa)
3 3 3 3 3 3 3 3 3 14 14 14 14 14 14 14 14 14 28 28 28 28 28 28 28 28 28 60 60 60 60 60 60 60 60 60
4.78 4.78 4.83 4.67 4.95 4.65 4.78 4.83 4.76 5.18 5.26 5.21 5.35 5.15 5.35 5.26 5.32 5.26 5.43 5.52 5.56 5.52 5.59 5.49 5.49 5.71 5.56 5.43 5.52 5.56 5.52 5.59 5.49 5.49 5.71 5.56
16.70 18.96 17.95 18.65 23.33 18.85 18.98 21.48 17.49 36.01 46.26 40.83 37.70 44.24 42.18 38.60 42.25 37.96 55.98 52.14 54.04 53.39 54.49 55.79 59.13 53.66 55.34 55.98 52.14 54.04 53.39 54.49 55.79 59.13 53.66 55.34
2.00 2.80 2.83 2.60 2.75 2.49 2.65 2.43 2.62 3.34 3.19 3.26 3.77 3.33 3.64 2.70 3.28 2.52 3.85 3.28 3.09 2.56 3.65 3.79 2.09 3.24 2.82 3.85 3.28 3.09 2.56 3.65 3.79 2.09 3.24 2.82
3.46 4.55 4.76 3.88 4.07 4.42 4.31 3.72 4.48 4.22 4.35 4.07 4.26 4.20 4.65 3.82 4.42 3.55 4.76 4.76 4.17 4.22 5.21 5.08 4.05 4.22 4.35 3.42 3.70 3.82 3.40 1.98 2.69 3.06 2.99 3.27
41.30 50.57 49.72 43.35 54.80 47.18 48.06 47.38 46.58 40.79 54.18 48.98 38.71 50.02 47.24 49.34 48.38 47.32 55.12 61.53 61.85 56.54 61.87 60.47 56.79 60.42 64.27 49.73 53.28 41.84 53.43 34.75 52.04 51.67 55.69 53.94
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W. Zhong, W. Yao / Construction and Building Materials 22 (2008) 1137–1142 5.8
microcracks and macrocracks in concrete, in which V3 is the ultrasonic wave velocity in air (340 m/s). V1, V2 are expressed as following:
0# 1#
5.6
UPV(km/s)
5.4
~ 1 ðE0 ; t; K IC ; uÞ V1 ¼m ~ 2 ða0 ; a; 2Þ V2 ¼m
5.2 5.0 4.8 4.6 4.4 4.2 10
20
30
40
50
60
Strength(MPa)
Fig. 1. Relation between UPV and strength
8 UPV of damaged concretes UPV after self healing
7
UPV / km/s
6 5 4 3 2 1 0
2
4
6 8 10 12 Specim e n num b er
14
16
18
Fig. 2. UPV changes of conretes damaged and self-healed
unstable microcracks and matrix filled with stable microcracks. The deformation of matrix under loading could be considered as completely linear elastic, and it has no effect on the acoustic properties of concrete. The closing, opening and propagating of microcracks at uniaxial loading is the main factor which influences acoustic properties of concrete. The macrocracks propagated from initial microcracks is another important factor. The ultrasonic properties of concrete result in the coupling effect of these three factors. Recent studies [14–17] on acoustic properties of quasi-brittle solid materials under loading showed that there was non-linear relationship between UPV and uniaxial stress for linear elastic matrix containing microcracks in random distribution. The UPV Vp could be summarily expressed as a function of mechanical parameters, such as E0, t, KIC, u, which is the elastic modulus, Poisson’s ratio of matrix, material fracture toughness, inner friction angle, respectively, and existing microcrack parameters, such as a0, a, 2, which is half length, shape ratio and density of microcrack, and r1 which is uniaxial stress. The influence of stress on UPV could be neglected, since the UPV of concrete was not measured at loading. The UPV of concrete is a comprehensive effect of matrix, microcracks and macrocracks, which could be simplified as: Vc ¼
1 i1 V1
þ
i2 V2
þ
i3 V3
ð1Þ
where Vc is the UPV of concrete, i1, i2, i3 and V1, V2, V3 are expressed as the volume fractions and the UPV of matrix,
ð2Þ ð3Þ
Increase of E0, KIC leads to increase of V1, while increase of a0, a, 2 leads to decrease of V2. UPV is mostly influenced by V1 before loading, and it is close to that of sound matrix. Higher strength leads to faster UPV. The UPV is not only determined by V1, but also related to V2 (for intact concrete samples, i3 = 0). As a result, the UPV is not totally linear related to strength (as shown in Fig. 1.). When concrete is loaded, the original microcracks in materials propagate into macrocracks, and a great deal of new microcracks derive. The intact matrix ratio i1 decreases greatly, while i2 and i3 increase correspondingly. In the microcracks zone, the propagation of a0, a and 2 at loading results in decrease of V2, thus Vc of damaged concrete decreases greatly. After damaged specimens are cured in moist environment for a period, re-hydration products are recrystalized in the cracks. Stefan [4] found that some re-hydration products clustering along the edges of cracks could be seen in microcracks clearly through Secondary Electron Images (SEI) and Back Scattered Electron Images (BSEI), and the rehydration products filled in the center part were less dense than those along the edges. The width of cracks is narrowed, and i3 is decreased. The bridging of hydration products in microcracks cannot only reduce i2, but also change the inner microcracks and porous structure. Such influences result in the increment of V2, thus Vc of concrete after self-healing increases obviously. It is observed that Vc of some samples after self-healing excesses those before loading, such as samples 0#–7–2, 0#–7–4, 0#–7–5, 0#–7– 6. The results from UPV tests indicate that the UPV of damaged concrete recover almost completely during subsequent storage in moist environment, but the recovery of UPV is influenced by many factors, such as concrete mixture, the destructive age, and damage degree etc. The changes of UPV can reflect the inner damage of materials as the UPV is related to the microstructure in materials. According to studies of Suaris and Fernado [18], it showed that the UPV decrease had a good agreement of secant modulus reduction during cyclic uniaxial compressive loading. So the microstructure changes in concrete may be inferred from decrease of UPV by introducing a damaged degree defined as: v D¼1 ð4Þ v0 where D is the damage degree of concrete, v is UPV after peak loading, and v0 is the UPV before loading. A self-healing ratio of concrete, H, that incorporates compressive strength after self-healing and before damaged can be introduced as:
W. Zhong, W. Yao / Construction and Building Materials 22 (2008) 1137–1142
H¼
Sh S S
ð5Þ
where Sh is the compressive strength after self-healing, S is the compressive strength at the loading. Figs. 3 and 4 show the D–H relations of normal strength concretes (series 0#) and high strength concretes (series 1#) loaded at different ages, respectively. It appears that selfhealing ratio is virtually dependent of the damage degree when the results are presented as a curve. Fig. 3 shows that H decreases slightly when D > 0.6 loaded at 7 days, while H decreases rapidly when D > 0.7 loaded at 14 days. When damage degree is in the range of 0.45–0.55 for the samples loaded at 14 days, self-healing ratio is almost kept constant. It also can be observed that H is increased as D increases, for example, when D is 0.2–0.5 loaded at 28 days in Fig. 3 and when D is 0.3–0.4 loaded at 14 days, 28 days and 60 days in Fig. 4. The results obtained show that there is a damage threshold in the self-healing process. When D is lower than the threshold, H increases as D increases, while D is higher than the threshold, H decrease rapidly with increase in damage degree. The derivation of new microcracks during the damage process results in the exposure of unhydrated cement particles which were wrapped by hydration products before. In a highly moist environment, the unhydrated cement particles re-hydrate, and the fine cracks are bridged and narrowed gradually. If the damage degree is too low, the exposure number of unhydrated cement particles is small
1141
and it is not helpful for re-hydration. However, if the damage degree is too high and excess a limit, the length of new hydration products cannot bridge the cracks, as a result, H decreases. The results in Figs. 3 and 4 show that the damage threshold of normal strength concrete is about 0.6–0.7, while it is about 0.4 for high strength concrete. The higher damage threshold of normal strength concrete might contributed to the milder strength increment compared to that of high strength concrete, and there are much more unhydrated cement grains left in concrete when damaged, so that the normal strength concrete can endure higher damage degree to keep increasing of H. The loading age is another parameter which affects selfhealing of concrete remarkably. For the same damage degree, the earlier specimens are damaged, the better the mechanical and ultrasonic properties are recovered. For high strength concrete damaged at 60 days, the increment ratio of strength is almost close to 0. It is because at later age there are almost no unhydrated cement grains to contribute for the strength increment. It can be noticed that the D–H relation curve of normal strength concrete loaded at 14 days is similar to that loaded at 7 days in Fig. 3, while the D–H relation curve of high strength concrete loaded at 14 days is close to those loaded at 28 days and 60 days in Fig. 4. The results are similar to the development of hydration and strength of normal and high strength concretes. 4. Conclusions
increment ratio of strength
1.3
loaded at 7d loaded at 14d loaded at 28d
1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
damage
Fig. 3. D–H relations of normal strength concretes.
increment ratio of strength
2.0
1.5
Self-healing of concrete is markedly influence by its damage degree. The results obtained from mechanical and ultrasonic properties show that there exists a damage degree threshold both for high strength concrete and normal strength concrete. The self-healing ratio of concrete increases with the increasing of damage degree when the damage degree is lower than the threshold, while the damage degree is higher than the threshold, the self-healing ratio decreases with the increase of damage degree. The threshold level depends on materials. The threshold for normal strength concrete is higher than that for high strength concrete. Acknowledgement
loaded at 3d loaded at 14d loaded at 28d loaded at 60d
This work was supported by National Natural Science Foundation of China (No. 50238040).
1.0
References
0.5
0.0
-0.5 0.24
0.30
0.36
0.42
0.48
0.54
0.60
damage
Fig. 4. D–H relations of high strength concretes.
[1] Wagner EF. Autogenous healing of cracks in cement-mortar linings for gray-iron an ductile-iron water pipe. J Am Water Works Assoc 1974;66(6):358–60. [2] Hannant DJ, Keer JG. Autogeneous healing of thin cement based sheets. Cement Concrete Res 1983;13(3):357–65. [3] Gray RJ. Autogenous healing of fiber/matrix interfacial bond in fiberreinforced mortar. Cement Concrete Res 1984;14(3):315–7.
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[4] Stefan J, Jacques M, Hugues H. SEM observations of the microstructure of frost deteriorated and self-healed concretes. Cement Concrete Res 1995;25(8):1781–90. [5] Stefan J, Jacques M, Luc B. Effect of cracking and healing on chloride transport in OPC concrete. Cement Concrete Res 1996;26(6):869–81. [6] Nataliya H. Self-sealing, autogenous healing and continued hydration: what is the difference? Mater Struct 1998;31(12):563–7. [7] Edvardsen C. Water permeability and autogenous healing of cracks in concrete. ACI Mater J 1999;96(4):448–54. [8] Liu XY, Yao W, Zheng XF, Wu JP. Experimental study on selfhealing performance of concrete. Chin J Build Mater 2005;8(2):184–8. [9] Li HX, Tang CA, Zeng SH, Li SN. Research on self-healing of concrete cracks. Chin J Wuhan Univ Technol 2004;26(3):27–9. [10] Wieland R, Michaela B. Autogenous healing and reinforcement corrosion of water-penetrated separation cracks in reinforced concrete. Nucl Eng Des 1998;179(2):191–200. [11] Reinhardt HW, Jooss M. Permeability and self-healing of cracked concrete as a function of temperature and crack width. Cement Concrete Res 2003;33(4):981–5.
[12] Romildo DTF, Khosrow G, Miguel AS, George LE. Free, restrained and drying shrinkage of cement mortar composites reinforced with vegetable fibers. Cement Concrete Compos 2005;27(5):537–46. [13] Lu QZ, Lin G, Xiao SY, Wang Z. Study on tensile strength reduction of concrete due to triaxial compressive loading history. Chin J Hydraul Eng 2001;1:68–75. [14] Lu QZ, Lin G, Wang Z, Xiao SY. On the reduction of strength of concrete and ultrasonic inspection due to triaxial compressive loading history. Chin J Eng Mech 2002;10:52–7. [15] Zhao MJ, Wu DL. Ultrasonic properties of rock under loading and unloading: theoretical model and experimental research. Chin J Geotech Eng 1999;21(5):540–5. [16] Zhao MJ, Xu R. A calculating method for the crack tensor of the orthotropic rockmass by the elastic wave velocity. Chin J Chongqing Univ Architect 1999;21(2):42–8. [17] Zhao MJ, Wu DL. Ultrasonic pulse velocity and attenuation of rock under uniaxial loading. Chin J Rock Mech Eng 1999;18(1):50–4. [18] Suaris W, Fernado V. Ultrasonic pulse attenuation as a measure of damage growth cyclic loading of concrete. ACI Mater J 1987;84(3):185–93.
Construction and Building
MATERIALS
Construction and Building Materials 22 (2008) 1137–1142
www.elsevier.com/locate/conbuildmat
Influence of damage degree on self-healing of concrete Wenhui Zhong, Wu Yao
*
Key Laboratory of Advanced Civil Engineering, Materials of Ministry of Education, Tongji University, Shanghai 200092, China Received 27 November 2006; received in revised form 15 February 2007; accepted 23 February 2007 Available online 12 April 2007
Abstract The paper presents experimental results of self-healing process of concrete damaged at different ages. Essentially, the self-healing of damaged concrete is processes of crack closing with re-hydration products of unhydrated or insufficiently-hydrated cementitious particles in damaged regions. Damage degree was measured from decrease in ultrasonic pulse velocity (UPV) before and after loading, and the self-healing effect was deduced from the strength increment after self-healing by introducing a self-healing ratio. The relationship between damage degree and self-healing ratio of concrete was built based upon the experimental results. Analyses of test results show that there exists a damage threshold both for high strength concrete and normal strength concrete. When the damage degree is less than the threshold, the self-healing ratio of concrete is increased with the increase in damage degree; while the damage degree exceeds the threshold, the self-healing ratio is decreased with the increase in damage degree. The damage threshold for normal strength concrete is higher than that for high strength concrete. 2007 Elsevier Ltd. All rights reserved. Keywords: Concrete; Self-healing; Hydration
1. Introduction Concrete in service cracks due to direct stress and substress caused by many kinds of reasons, such as changes of temperature and humidity, inhomogeneous sinking, external loading (dynamic or static loading). Cracks not only influence the service durability of concrete structure, but also are harmful for the structure safety. However, not all initial microcracks develop into harmful cracks or unstable cracks. Since earlier researchers discovered that cracked specimens under compressive testing at 28 days healed autogenously after had been stored outdoors for 8 years, many researchers have been focusing on this subject and done further studies. Wagner [1] tested to investigate whether cracks in seal-coated cement-mortar linings in water pipe and fittings would self-heal by chemical reaction when totally immersed in potable water. Hannant and Keer [2] studied self-healing process of microcracks in concrete. Gray [3] examined the self-healing of interfacial bond *
Corresponding author. Tel.: +86 21 65984191. E-mail address: [email protected] (W. Yao).
0950-0618/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2007.02.006
between steel fibers and mortar matrix. In 1995, Stefan et al. [4] found that concrete after deterioration by freeze and thaw regained almost the all resonance frequencies and the crystals of ettringite and Ca(OH)2 were seen traversing the cracks at several location through scanning electron microscope (SEM). One year later, Stefan [5] studied the deterioration and self-healing on chloride transport in OPC concrete. Nataliya [6] discussed the differences of self-sealing, autogenous healing and continued hydration. Edvardsen [7] gave a comprehensive theoretical discussion of the laws which governed the calcite nucleation and the subsequent crystal growth of water-bearing cracks in concrete, and they also pointed out that the crystal growth responded to two different processes which were determined by the changes in the chemical and physical conditions in cracks. Liu et al. [8] also carried out experiments to study the influences of cement particles diameter distribution and constitute on self-healing performance of concrete. Waterproof concrete structures cracked and leaked gently at the beginning, but after a period of time, it was found that the cracks closed completely and did not leak at all [9]. Wieland and Michaela [10] studied that an autog-
1138
W. Zhong, W. Yao / Construction and Building Materials 22 (2008) 1137–1142
enous healing would take place in water under pressure when up to certain width of the crack. Reinhardt and Jooss [11] established self-healing behavior of cracked concrete as a function of temperature and crack width, and they found that the average crack width measured at the surface showed the fastest self-healing. Romildo et al. [12] studied the crack self-healing tendency after plastic shrinkage cracking of cement mortar and sisal fiber-mortar composite, and got the results that both cracks of two different mortars could be self-healed, but different initial crack width showed different self-healing tendencies. Above-mentioned researches showed that concrete after deterioration could self-heal indeed, the mechanisms and influence factors must be further studied. Relevant researches showed that, although the strength of damaged concrete could be regained in a high degree, the ability of strength recovery was determined by many factors, such as the mixture of concrete, the damaged age, damage degree, conditions of self-healing (humidity, temperature) and curing period etc. Among these factors, the damage degree is the most important factor. It was found that 0.1–0.3 mm wide cracks could be self-healed in laboratory testing, but most structures in service still collapsed finally because of cracks. Assessing the extent of crack growth and self-healing effect is considered in this paper, including issues such as the relationship between the damage degree of concrete and self-healing, the influences of loading ages and concrete strength grades on the effect of self-healing, and the relationship between damage degree and self-healing ratio of concrete was discussed. 2. Experimental 2.1. Specimen preparation The laboratory tests were conducted by using specimens of normal strength concrete and high strength concrete, respectively, for the comparison of the self-healing effects of different strength grades concrete. The materials used in normal strength concrete (series 0#) were ordinary Portland cement (ISO 32.5), Class I fly ash, medium sand, crushed gravel with maximum size of 31.5 mm, and superplasticizer. In high strength concrete (series 1#), they were ordinary Portland cement (ISO 52.5), ground granulated blast-furnace slag (GGBS), silica fume (SF) besides the same fine and coarse aggregates, and superplasticizer used in series 0#. The mix proportions investigated in the study are given in Table 1. The slump of fresh concrete was 200 ± 20 mm. The cubic specimens (100 · 100 · 100 mm) were cast in the oiled steel molds and kept under ambient conditions for 24 h, covered with
a polythene film. The molds were stripped and the specimens were cured in the standard moist room where curing continued at 20 C in accordance with ASTM C 192. 2.2. Testing In order to distinguish the different self-healing effects of concrete damaged at different ages, concrete samples were deteriorated by compression test at 7, 14, and 28 days, respectively, for series 0#, while tested at 3, 14, 28, and 60 days, respectively, for series 1#. All the samples after deterioration were cured for 30 days for normal strength concrete and 60 days for high strength concrete in the standard curing room. Previous studies [13,14] showed that there existed relationship between damage and UPV decrease of concrete by ultrasonic transmission method. Therefore, in this paper, the UPV decrease of the direction which is vertical to loading direction was investigated to assess the damage degree of concrete. 3. Testing results and discussions The mechanical properties and ultrasonic decrease of normal strength and high strength concretes are given in Tables 2 and Table 3, respectively. The number 0#–7–1 means the No. 1 sample of series 0# loaded at the age of 7 days. The UPV of concrete before loading versus compressive strength is shown in Tables 2 and 3. It can be noticed that a higher compressive strength results in a higher UPV, but at relative low strength, the relationship is not obvious, as shown in Fig. 1. The UPV resulted from ultrasonic transmission method gives information about microstructures properties of materials, such as porosity, elastic modulus, microcracks distribution and density. Furthermore, it has also been observed that a relationship established during the developmental stages of the concrete cannot be used to predict the strength of concrete that has already undergone deterioration, but decrease of UPV can be used with more success to assess the deterioration in concrete. After self-healing, not only the mechanical properties of concrete have been recovered, but also the UPV has been regained in a great deal, as shown in Fig. 2. As a result, damage degree could be inferred from the UPV decrease of concrete at loading. Any way, it must be pointed out that the two curves in Fig. 2 are not totally the same, which indicates that there are inherent relationships between damage degree and self-healing effect. Theoretically, concrete can be classified as a composite material which consists three phases: macrocracks, discrete
Table 1 Concrete mixtures (kg/m3) Series
Cement
Fly ash
GGBS
SF
Sand
Gravel
Water
Superplasticizer
0# 1#
280 205
71 0
0 184
0 21
747 706
1126 1153
165 131
3.30 4.92
W. Zhong, W. Yao / Construction and Building Materials 22 (2008) 1137–1142
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Table 2 Summary of testing results for normal strength concretes No.
0#–7–1 0#–7–2 0#–7–3 0#–7–4 0#–7–5 0#–7–6 0#–14–1 0#–14–2 0#–14–3 0#–14–4 0#–14–5 0#–14–6 0#–28–1 0#–28–2 0#–28–3 0#–28–4 0#–28–5 0#–28–6
Loading age (day)
7 7 7 7 7 7 14 14 14 14 14 14 28 28 28 28 28 28
Properties before and after peak loading
Properties after self-healing for 30 days
UPV before peak loading (km/s)
Strength (MPa)
UPV after peak loading (km/s)
UPV (km/s)
Strength (MPa)
4.36 4.25 4.52 4.25 4.25 4.31 4.49 4.53 4.64 4.81 4.73 4.76 5.10 4.98 5.15 5.19 5.15 5.12
11.78 11.11 11.78 14.44 13.78 14.00 12.09 10.98 11.87 16.11 16.00 16.22 18.31 19.66 19.65 28.57 32.70 28.54
2.20 3.55 1.63 2.40 2.33 2.72 1.97 2.21 1.49 2.53 1.21 2.33 3.21 2.72 3.45 3.94 — 3.61
4.24 5.08 3.92 4.29 4.40 4.55 3.51 3.22 3.64 4.44 2.72 4.34 4.64 4.08 4.79 4.46 — 4.72
26.99 21.08 26.22 30.00 30.30 29.98 24.99 22.85 26.61 33.23 27.60 33.16 26.90 30.59 29.89 35.14 — 39.32
Table 3 Summary of testing results for high strength concretes No.
1#–3–1 1#–3–2 1#–3–3 1#–3–4 1#–3–5 1#–3–6 1#–3–7 1#–3–8 1#–3–9 1#–14–1 1#–14–2 1#–14–3 1#–14–4 1#–14–5 1#–14–6 1#–14–7 1#–14–8 1#–14–9 1#–28–1 1#–28–2 1#–28–3 1#–28–4 1#–28–5 1#–28–6 1#–28–7 1#–28–8 1#–28–9 1#–60–1 1#–60–2 1#–60–3 1#–60–4 1#–60–5 1#–60–6 1#–60–7 1#–60–8 1#–60–9
Loading age (day)
Properties before and after peak loading
Properties after self-healing for 60 days
UPV before peak loading (km/s)
Strength (MPa)
UPV after peak loading (km/s)
UPV (km/s)
Strength (MPa)
3 3 3 3 3 3 3 3 3 14 14 14 14 14 14 14 14 14 28 28 28 28 28 28 28 28 28 60 60 60 60 60 60 60 60 60
4.78 4.78 4.83 4.67 4.95 4.65 4.78 4.83 4.76 5.18 5.26 5.21 5.35 5.15 5.35 5.26 5.32 5.26 5.43 5.52 5.56 5.52 5.59 5.49 5.49 5.71 5.56 5.43 5.52 5.56 5.52 5.59 5.49 5.49 5.71 5.56
16.70 18.96 17.95 18.65 23.33 18.85 18.98 21.48 17.49 36.01 46.26 40.83 37.70 44.24 42.18 38.60 42.25 37.96 55.98 52.14 54.04 53.39 54.49 55.79 59.13 53.66 55.34 55.98 52.14 54.04 53.39 54.49 55.79 59.13 53.66 55.34
2.00 2.80 2.83 2.60 2.75 2.49 2.65 2.43 2.62 3.34 3.19 3.26 3.77 3.33 3.64 2.70 3.28 2.52 3.85 3.28 3.09 2.56 3.65 3.79 2.09 3.24 2.82 3.85 3.28 3.09 2.56 3.65 3.79 2.09 3.24 2.82
3.46 4.55 4.76 3.88 4.07 4.42 4.31 3.72 4.48 4.22 4.35 4.07 4.26 4.20 4.65 3.82 4.42 3.55 4.76 4.76 4.17 4.22 5.21 5.08 4.05 4.22 4.35 3.42 3.70 3.82 3.40 1.98 2.69 3.06 2.99 3.27
41.30 50.57 49.72 43.35 54.80 47.18 48.06 47.38 46.58 40.79 54.18 48.98 38.71 50.02 47.24 49.34 48.38 47.32 55.12 61.53 61.85 56.54 61.87 60.47 56.79 60.42 64.27 49.73 53.28 41.84 53.43 34.75 52.04 51.67 55.69 53.94
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microcracks and macrocracks in concrete, in which V3 is the ultrasonic wave velocity in air (340 m/s). V1, V2 are expressed as following:
0# 1#
5.6
UPV(km/s)
5.4
~ 1 ðE0 ; t; K IC ; uÞ V1 ¼m ~ 2 ða0 ; a; 2Þ V2 ¼m
5.2 5.0 4.8 4.6 4.4 4.2 10
20
30
40
50
60
Strength(MPa)
Fig. 1. Relation between UPV and strength
8 UPV of damaged concretes UPV after self healing
7
UPV / km/s
6 5 4 3 2 1 0
2
4
6 8 10 12 Specim e n num b er
14
16
18
Fig. 2. UPV changes of conretes damaged and self-healed
unstable microcracks and matrix filled with stable microcracks. The deformation of matrix under loading could be considered as completely linear elastic, and it has no effect on the acoustic properties of concrete. The closing, opening and propagating of microcracks at uniaxial loading is the main factor which influences acoustic properties of concrete. The macrocracks propagated from initial microcracks is another important factor. The ultrasonic properties of concrete result in the coupling effect of these three factors. Recent studies [14–17] on acoustic properties of quasi-brittle solid materials under loading showed that there was non-linear relationship between UPV and uniaxial stress for linear elastic matrix containing microcracks in random distribution. The UPV Vp could be summarily expressed as a function of mechanical parameters, such as E0, t, KIC, u, which is the elastic modulus, Poisson’s ratio of matrix, material fracture toughness, inner friction angle, respectively, and existing microcrack parameters, such as a0, a, 2, which is half length, shape ratio and density of microcrack, and r1 which is uniaxial stress. The influence of stress on UPV could be neglected, since the UPV of concrete was not measured at loading. The UPV of concrete is a comprehensive effect of matrix, microcracks and macrocracks, which could be simplified as: Vc ¼
1 i1 V1
þ
i2 V2
þ
i3 V3
ð1Þ
where Vc is the UPV of concrete, i1, i2, i3 and V1, V2, V3 are expressed as the volume fractions and the UPV of matrix,
ð2Þ ð3Þ
Increase of E0, KIC leads to increase of V1, while increase of a0, a, 2 leads to decrease of V2. UPV is mostly influenced by V1 before loading, and it is close to that of sound matrix. Higher strength leads to faster UPV. The UPV is not only determined by V1, but also related to V2 (for intact concrete samples, i3 = 0). As a result, the UPV is not totally linear related to strength (as shown in Fig. 1.). When concrete is loaded, the original microcracks in materials propagate into macrocracks, and a great deal of new microcracks derive. The intact matrix ratio i1 decreases greatly, while i2 and i3 increase correspondingly. In the microcracks zone, the propagation of a0, a and 2 at loading results in decrease of V2, thus Vc of damaged concrete decreases greatly. After damaged specimens are cured in moist environment for a period, re-hydration products are recrystalized in the cracks. Stefan [4] found that some re-hydration products clustering along the edges of cracks could be seen in microcracks clearly through Secondary Electron Images (SEI) and Back Scattered Electron Images (BSEI), and the rehydration products filled in the center part were less dense than those along the edges. The width of cracks is narrowed, and i3 is decreased. The bridging of hydration products in microcracks cannot only reduce i2, but also change the inner microcracks and porous structure. Such influences result in the increment of V2, thus Vc of concrete after self-healing increases obviously. It is observed that Vc of some samples after self-healing excesses those before loading, such as samples 0#–7–2, 0#–7–4, 0#–7–5, 0#–7– 6. The results from UPV tests indicate that the UPV of damaged concrete recover almost completely during subsequent storage in moist environment, but the recovery of UPV is influenced by many factors, such as concrete mixture, the destructive age, and damage degree etc. The changes of UPV can reflect the inner damage of materials as the UPV is related to the microstructure in materials. According to studies of Suaris and Fernado [18], it showed that the UPV decrease had a good agreement of secant modulus reduction during cyclic uniaxial compressive loading. So the microstructure changes in concrete may be inferred from decrease of UPV by introducing a damaged degree defined as: v D¼1 ð4Þ v0 where D is the damage degree of concrete, v is UPV after peak loading, and v0 is the UPV before loading. A self-healing ratio of concrete, H, that incorporates compressive strength after self-healing and before damaged can be introduced as:
W. Zhong, W. Yao / Construction and Building Materials 22 (2008) 1137–1142
H¼
Sh S S
ð5Þ
where Sh is the compressive strength after self-healing, S is the compressive strength at the loading. Figs. 3 and 4 show the D–H relations of normal strength concretes (series 0#) and high strength concretes (series 1#) loaded at different ages, respectively. It appears that selfhealing ratio is virtually dependent of the damage degree when the results are presented as a curve. Fig. 3 shows that H decreases slightly when D > 0.6 loaded at 7 days, while H decreases rapidly when D > 0.7 loaded at 14 days. When damage degree is in the range of 0.45–0.55 for the samples loaded at 14 days, self-healing ratio is almost kept constant. It also can be observed that H is increased as D increases, for example, when D is 0.2–0.5 loaded at 28 days in Fig. 3 and when D is 0.3–0.4 loaded at 14 days, 28 days and 60 days in Fig. 4. The results obtained show that there is a damage threshold in the self-healing process. When D is lower than the threshold, H increases as D increases, while D is higher than the threshold, H decrease rapidly with increase in damage degree. The derivation of new microcracks during the damage process results in the exposure of unhydrated cement particles which were wrapped by hydration products before. In a highly moist environment, the unhydrated cement particles re-hydrate, and the fine cracks are bridged and narrowed gradually. If the damage degree is too low, the exposure number of unhydrated cement particles is small
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and it is not helpful for re-hydration. However, if the damage degree is too high and excess a limit, the length of new hydration products cannot bridge the cracks, as a result, H decreases. The results in Figs. 3 and 4 show that the damage threshold of normal strength concrete is about 0.6–0.7, while it is about 0.4 for high strength concrete. The higher damage threshold of normal strength concrete might contributed to the milder strength increment compared to that of high strength concrete, and there are much more unhydrated cement grains left in concrete when damaged, so that the normal strength concrete can endure higher damage degree to keep increasing of H. The loading age is another parameter which affects selfhealing of concrete remarkably. For the same damage degree, the earlier specimens are damaged, the better the mechanical and ultrasonic properties are recovered. For high strength concrete damaged at 60 days, the increment ratio of strength is almost close to 0. It is because at later age there are almost no unhydrated cement grains to contribute for the strength increment. It can be noticed that the D–H relation curve of normal strength concrete loaded at 14 days is similar to that loaded at 7 days in Fig. 3, while the D–H relation curve of high strength concrete loaded at 14 days is close to those loaded at 28 days and 60 days in Fig. 4. The results are similar to the development of hydration and strength of normal and high strength concretes. 4. Conclusions
increment ratio of strength
1.3
loaded at 7d loaded at 14d loaded at 28d
1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
damage
Fig. 3. D–H relations of normal strength concretes.
increment ratio of strength
2.0
1.5
Self-healing of concrete is markedly influence by its damage degree. The results obtained from mechanical and ultrasonic properties show that there exists a damage degree threshold both for high strength concrete and normal strength concrete. The self-healing ratio of concrete increases with the increasing of damage degree when the damage degree is lower than the threshold, while the damage degree is higher than the threshold, the self-healing ratio decreases with the increase of damage degree. The threshold level depends on materials. The threshold for normal strength concrete is higher than that for high strength concrete. Acknowledgement
loaded at 3d loaded at 14d loaded at 28d loaded at 60d
This work was supported by National Natural Science Foundation of China (No. 50238040).
1.0
References
0.5
0.0
-0.5 0.24
0.30
0.36
0.42
0.48
0.54
0.60
damage
Fig. 4. D–H relations of high strength concretes.
[1] Wagner EF. Autogenous healing of cracks in cement-mortar linings for gray-iron an ductile-iron water pipe. J Am Water Works Assoc 1974;66(6):358–60. [2] Hannant DJ, Keer JG. Autogeneous healing of thin cement based sheets. Cement Concrete Res 1983;13(3):357–65. [3] Gray RJ. Autogenous healing of fiber/matrix interfacial bond in fiberreinforced mortar. Cement Concrete Res 1984;14(3):315–7.
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[4] Stefan J, Jacques M, Hugues H. SEM observations of the microstructure of frost deteriorated and self-healed concretes. Cement Concrete Res 1995;25(8):1781–90. [5] Stefan J, Jacques M, Luc B. Effect of cracking and healing on chloride transport in OPC concrete. Cement Concrete Res 1996;26(6):869–81. [6] Nataliya H. Self-sealing, autogenous healing and continued hydration: what is the difference? Mater Struct 1998;31(12):563–7. [7] Edvardsen C. Water permeability and autogenous healing of cracks in concrete. ACI Mater J 1999;96(4):448–54. [8] Liu XY, Yao W, Zheng XF, Wu JP. Experimental study on selfhealing performance of concrete. Chin J Build Mater 2005;8(2):184–8. [9] Li HX, Tang CA, Zeng SH, Li SN. Research on self-healing of concrete cracks. Chin J Wuhan Univ Technol 2004;26(3):27–9. [10] Wieland R, Michaela B. Autogenous healing and reinforcement corrosion of water-penetrated separation cracks in reinforced concrete. Nucl Eng Des 1998;179(2):191–200. [11] Reinhardt HW, Jooss M. Permeability and self-healing of cracked concrete as a function of temperature and crack width. Cement Concrete Res 2003;33(4):981–5.
[12] Romildo DTF, Khosrow G, Miguel AS, George LE. Free, restrained and drying shrinkage of cement mortar composites reinforced with vegetable fibers. Cement Concrete Compos 2005;27(5):537–46. [13] Lu QZ, Lin G, Xiao SY, Wang Z. Study on tensile strength reduction of concrete due to triaxial compressive loading history. Chin J Hydraul Eng 2001;1:68–75. [14] Lu QZ, Lin G, Wang Z, Xiao SY. On the reduction of strength of concrete and ultrasonic inspection due to triaxial compressive loading history. Chin J Eng Mech 2002;10:52–7. [15] Zhao MJ, Wu DL. Ultrasonic properties of rock under loading and unloading: theoretical model and experimental research. Chin J Geotech Eng 1999;21(5):540–5. [16] Zhao MJ, Xu R. A calculating method for the crack tensor of the orthotropic rockmass by the elastic wave velocity. Chin J Chongqing Univ Architect 1999;21(2):42–8. [17] Zhao MJ, Wu DL. Ultrasonic pulse velocity and attenuation of rock under uniaxial loading. Chin J Rock Mech Eng 1999;18(1):50–4. [18] Suaris W, Fernado V. Ultrasonic pulse attenuation as a measure of damage growth cyclic loading of concrete. ACI Mater J 1987;84(3):185–93.