self-healing behaviour by cementitious ... -...

Proceedings of the First International Conference on Self Healing Materials 18-20 April 2007, Noordwijk aan Zee, The Netherlands Toshiharu Kishi et al.

1 © Springer 2007

SELF-HEALING BEHAVIOUR BY CEMENTITIOUS RECRYSTALLIZATION OF CRACKED CONCRETE

INCORPORATING EXPANSIVE AGENT

Toshiharu Kishi1, Tae-Ho Ahn1, Akira Hosoda2, Shoko Suzuki1 and Hiedaki Takaoka1

1 The University of Tokyo, Department of Civil Engineering, Institute of Industrial Science, Be

407 IIS, 4-6-1 Komaba, Meguro-ku, Tokyo, Japan

2 Yokohama National University, Faculty of Environment and Information Sciences, 79-5, Tokiwadai, Hodogaya-ku, Yokohama City, Kanagawa, Japan

Tel: 81-3-5452-6098 (58093), Fax: 81-3-5452-6395

e-mail : [email protected]

In this study, the self-healing properties of concrete incorporating expensive agent as a partial cement replacement were investigated in terms of recrystallization on cracked concrete as well as the effects of various carbonates for the recrystallization. First, in order to clarify the self-healing mechanism, re-expansion of expansive concrete with a low water to binder ratio under restrained conditions were examined in comparison to normal concrete without expansive agent. Then the three different carbonates, NaHCO3, Na2CO3 and Li2CO3, were used to investigate the effect of cementitious recrystallization. Morphology and the shape and size of precipitated particles in the cracks were examined by microscopy and SEM-EDS. The results show that cracked concrete incorporating expansive agent exhibit much higher self-healing behaviour than in cracked normal concrete, when they are cured in water after cracking. Carbonates seem to be effective in inducing the precipitation of calcium salts in the cracks because of the general increase in the OH- ion and SO4

2- concentration in the pore solution. Expansive agent also seems to play an important role in the formation of various AFm phases such as sulphate AFm(=SO4·AFm), Hydroxy AFm(=OH·AFm) and Hemicarboaluminate for the crystalline phases. From these results, it is considered that the utilization of appropriate dosages of the expansive agent and carbonates for the self-healing of cracked concrete is desirable. Furthermore, self-healing behaviours also have the potential to be useful for underground structures surrounded by groundwater.

Keywords: cementitious recrystallization, re-expansion, carbonates

1 Introduction Autogenous or self-healing of fine cracks in concrete is often mentioned in literature but there is little quantitative data regarding its mechanism. In the literature until now on,1),2) the causes of self-healing are reported to be based on the chemical, physical, and mechanical processes as follows: 1) Swelling and hydration of cement pastes, 2) Precipitation of calcium carbonate crystals, 3) Blockage of flow path by water impurities or by concrete particles broken from the crack surface due to cracking. In particular, this phenomenon is generally attributed to the hydration of previously unhydrated cement grains and may be aided by carbonation since the bonding material so formed containing crystals of calcium carbonate and calcium hydroxide.


2 © Springer 2007

Recently, several researchers have observed the formation of cementitious products such as AFt, AFm and CaCO3 in cracks and calcium hydroxide crystals in air voids in cracked concrete. It was hypothesized that these hydration products had been leached and recrystallized in water that had flown through the crack. However, although it is generally acknowledged that unhydrated cement grains affect the recrystallization of cracked concrete, no detailed examinations have been reported on the healing conditions for this cementitious recrystallization. Therefore, the aim of this study is to evaluate the conditions of the self-healing behavior by recrystallization in cracked concrete incorporating expansive agent, as well as the effect of carbonate type and expansive agent on the self-healing of cracked concrete. Furthermore, in order to understand the precipitation conditions of calcium salts in the cracks, morphology and the shape and size of re-hydration products in concrete by Microscopy and SEM(EDS) analysis were also investigated.

2 Experimental Method

2.1 Materials

Type I Japan Portland cement and expansive agent were used as binding materials in all specimens. For the concrete beam test, two concretes with water/binder ratio of 0.35 were investigated. One used only OPC and the other used OPC incorporating 10wt% expansive agent. Polycarboxylate-based superplasticizer was also used in the preparation of both mixtures. Table 1 shows the mixture proportion of concrete test in this work. For the concrete cylinder test, three types of carbonates, NaHCO3, Na2CO3 and Li2CO3, were selected to investigate the effect of cementitious recrystallization with expansive agent in air voids in cracked concrete. Table 2 and Table 3 show the mixture proportion of concrete cylinder tests and mortar with carbonates in this research. Table 1: Mixing proportions of concrete beams

Unit Weight (kg/m3) Specimen Replacement ratio

W C E S Gs GL SP

EC 10% 673 75

NC 0%

187

748 0

706 357 438 10.5

Table 2: Mixing proportions of concrete cylinders

W/C S/A Air Unit Weight (kg/m3)

(%) (%) (%) W C S G SP

74.2 53.5 1.5 188 257 1007 915 2.57


3 © Springer 2007

Table 3: Mixing proportions of mortar for cylinder tests (W/Binder =40%), (O: used material)

Sample Cement Expansive Agent Sand Carbonates C4A3 S Additives

NaHCO3 O - O O - -

Na2CO3 O - O O - -

Li2CO3 O - O O - -

C4A3 S O - O O

Na2CO3+C4A3 S O O O O

Na2CO3+C4A3 S +Additives

O O O O O

Li2CO3+C4A3 S +Additives

O O O O O

Na2CO3+Exapnaisvie agent + Additives O O O O O

2.2 Performance test

Concrete beams 15 by 15 by 88 cm were prepared for this research. D10 rebar was also used to reinforce the concrete in each specimen. Fig.1 shows induced method of cracks on concrete beams schematically. The specimens were pre-cracked by straining this tool in tension at 28 days after manufacture, in order to clarify the self healing process of load induced cracks at low water to binder ratio. Crack width was controlled between 0.1 mm and 0.3 mm in consideration of maximum tolerable crack widths according to construction codes. Then the specimens were again water cured for one month after cracking. For all concrete cylinder tests, W/B ratio of 0.72 and S/A ratio of 0.53 were applied to cylinders 5Ф x 10 cm in size. These were demolded at 3days and then water cured for 25 days.


4 © Springer 2007

Figure 1: Dimensions of the test specimens in order to induce crack in concrete beam The mortar with various carbonates was spread on surface of concrete after that and crack was induced as Fig. 2.

Figure 2: Experimental procedure for concrete cylinder test

2.3 Microscopy and SEM (Scanning Electron Microscopy)

Microscopy and SEM with EDS-detector were carried out to investigate morphology and the shape and size of re-hydration products and to clarify the mass transport process for recrystallization. All specimens with W/B of 0.40 were analyzed during the healing process of cracked concrete under water supply. Observations were performed at each stop by moving around and following paste/aggregate bond zones and cracks.

3 Results and dscussion

3.1 Effect of expansive agent on self-healing

First, in order to clarify the self-healing mechanism, re-expansion of expansive concrete with a low water to binder ratio under restrained conditions was examined in comparison with normal concrete without expansive agent. Fig. 3 and Fig. 4 show the difference in healing process of cracked concrete under water supply in each case. For concrete beams incorporating expansive agent, a crack with an initial width of 0.22mm was almost healed after one month. Re-hydration products between cracks were observed. However, for the normal concrete beam, the cracks still remained and were only partly closed after the same amount of time. This showed that recrystallization of expansive agent in air voids for self-healing was more effective than that of normal concrete at low water/binder ratio.


5 © Springer 2007

Figure 3: Process of self healing on expansive concrete beam at low water/binder ratio

Figure 4: Process of self healing on normal concrete beam at low water/binder ratio

Fig. 5 shows the XRD pattern of the expansive agent used in this research. It consists of three mineral admixtures, C4A3 S , CaSO4 and CaO, as shown in the XRD result.

Figure 5: Microscopy images of OPC paste and OPC paste incorporating expansive agent at normal water/binder ratio (0.45) [Under water supply until 28 days]


6 © Springer 2007

If these remain unhydrated at low water to binder ratio, they can form AFt and AFm phases based C4A3 S with CaSO4·2H2O or C-S-H phases based both Ca(OH)2, and unhydrated cement particles between cracks easily compare to that of normal concrete without expansive agent. This indicates that expansive agent could be used as a cementitious recrystallization material for self-healing of concrete. However, although expansive agent has the potential to be useful for self-healing of cracked concrete at low water to binder ratio, it has been found that there is a reduction of its self-healing ability due to the sufficient hydration by surplus water for normal water to binder concretes, as shown in Fig. 6. Therefore, in order to utilize the self healing ability at normal water to binder ratio, various carbonates which could be effective precipitations of calcium slats with expansive agent were added to the mixing mortar and concrete. These results are reported and discussed in the following sections.

Figure 6: XRD pattern of expansive agent

3.2 Effect of carbonates on the cementitious recrystallization

The concept of cementitious crystalline waterproofing was applied in this work. Generally speaking, the water-insoluble CaCO3 is evolved from a reaction between the calcium ions Ca2+, derived from the concrete and expansive agent, and the in-water available bicarbonates HCO3

-, or carbonates CO32- from various carbonates such as NaHCO3, Na2CO3 and Li2CO3.

2) Fig. 7 shows the effect of various carbonates and calcium sulphoaluminate (C4A3 S ) on the cementitious recrystallization and precipitated particles in air void of cracked concrete. In case of NaHCO3, most of the cracks still appeared open and empty. However, for the others, cementitious recrystallizaiton and precipitation of calcium salts occurred in the cracks. The SEM and EDS analyses have shown that most of the rehydration products seen in the cracks were newly formed C-S-H, C-A-S-H, AFt, AFm, and Ca(OH)2 were also observed locally. However, it is shown that they couldn’t perform crack bridging by self healing.


7 © Springer 2007

Therefore, in order to strengthen the formation of both AFt, AFm phases based C4A3 S and precipitation of calcium salts based carbonates in the cracks in self healing process, carbonates and C4A3 S were mixed together, and the results were also investigated.

Figure 7: Effects of various carbonates and C4A3 S on the self healing of cracks Fig.8 (a) and (b) show the difference in formation of re-hydration product according to additive. In Fig. 8(a), X-ray spectra obtained from these phases revealed a trend in their chemical composition as the formation of Gehlenite hydrate [C-A-S-H : 2CaO· Al2O3· SiO2

·8H2O] phases during self healing. In particular, it was observed that the silicate peak ratio of C-A-S-H phases increased or decreased due to ion exchanges. However, in case of Fig 8(b) with additives, it was found that re-hydration products in air voids in the cracks or internal pore structure was mainly AFt and AFm phases. This means that additives greatly affected the formation of AFt and AFm phases, as compared to Na2CO3 + C4A3 S . In other words, it can be said that the additives seems to disturb the bonding of C4A3 S and Si ions. Therefore, formation of AFt or AFm phases seems to be profitable during the initial healing stage. Moreover, X-ray spectra taken from re-hydration products in the case of Li2CO3 + C4A3 S with additives as shown Fig. 8(c) also revealed similar trends in their chemical composition as the formation of AFt or AFm phases.


8 © Springer 2007

Figure 8: Various crystal structures of cementitious re-hydration products in crack (a) Na2CO3 + C4A3 S (b) Na2CO3+C4A3 S +Additives (c) Li2CO3+C4A3 S +Additives SEM and EDS analysis of re-hydration products for Na2CO3 + expansive agent incorporating additives are shown in Fig. 9. The microscopy image shows that most of the cracks were fully filled by newly-formed hydration products. Fig. 9 (b) shows Ca(OH)2 phases as well as AFt, AFm and C-A-S-H phases in the cracks also formed. Although it is regarded as an unstable phase chemically over a long-term period, it seems that it could be converted to stable phases the same as Hydroxy AFm(=OH·AFm : 3CaO·Al2O3·Ca(OH)2·10H2O ) or

Hemicarboaluminate (3CaO·Al2O3 ·0.5Ca(OH)2·0.5CaCO3·10.5H2O) by CaCO3 phases based carbonates or expansive agent. Fig. 9 (d) shows that Hydroxy AFm (=OH·AFm: 3CaO·Al2O3·Ca(OH)2·nH2O) without Sulfate ions existed in the system.3),4) From these results, it could be predicted that re-hydration products the same as AFt or AFm phases based carbonates and expansive agent were more effective than that of normal concrete in early stages of self healing. Physical and chemical stability of these re-hydration products and precipitated products as calcite or calcium salts in long term period will be studied in detail through future works.


9 © Springer 2007

Figure 9: Re-hydration products in part of crack in case of Na2CO3 + Expansive Agent + additives (a) Microscopy Image (b) Ca(OH)2 (c) Gehlenite hydrate C-A-S-H (d) Hydroxy AFm =(OH ·AFm)

4 Conclusion 1. The expansive agent considerably affected the self-healing of cracked concrete at low water binder ratio when compared to normal concrete. 2. The addition of carbonates such as NaHCO3, Na2CO3 and Li2CO3 to normal concrete contributes to an increase in self-healing ability in the cracked concrete by the cementitius recrystallization and precipitated particles. In particular, when utilizating the appropriate dosages of carbonates and the expansive agent, self-healing ability in cracks could be increased. 3. SEM-EDS analysis results revealed that re-hydration products were composed of various cementitious crystallization products such as AFt, Sulfate AFm, Hydroxy AFm, Ca(OH)2 and Gehlenite hydrate. Some additives were also efficient in bonding disturbance of C-A-H and Silicate ions at initial healing stage. This appeared to contribute considerably to the formation of AFt or AFm phases the same as crack bridging in cracks.

ACKNOWLEDGEMENTS

The authors would like to thank The New Energy and Industrial Technology Development Organization (NEDO) of Japan for its financial support.


10 © Springer 2007

REFERENCES

[1] Hosoda, T. Shimomura, and T. Kishi, “Self-repairing Function for Cracks of SCC with Expansive Agent”, Proceedings of The Second International Symposium on Self-Compacting Concrete, pp.483-490，2001.

[2] C. Edvardsen, “Water Permeability and Autogenous Healing of Cracks in Concrete”, ACI Materials Journal, pp.448-461, Vol 96, No.4, 1999

[3] F.P. Glasser, A.Kindness, and S.A. Stronach, “Stability and Solubility Relationship in AFm phases Part I. Chloride, Sulfate and Hydroxide”, Cement and Concrete Research, pp.861-866, 1999.

[4] N.Andritsos, A.J. Karabelas, and P.G. Koutsoukos, “Morphology and Structure of CaCO3 Scale Layers Formed under Isothermal Flow Conditions.” Langmuir, Vol.13, No.10, pp.2873-2879, 1997.

self-healing behaviour by cementitious ... -...

Documents