Microfibres and hydrogels to promote autogenous healing in cementitious materials

MICROFIBRES AND HYDROGELS TO PROMOTE

AUTOGENOUS HEALING IN CEMENTITIOUS MATERIALS

D. Snoeck 1, P. Dubruel 2 and N. De Belie 1

1

Magnel Laboratory for Concrete Research, Ghent University, Technologiepark-Zwijnaarde 904, 9052 Ghent, Belgium – e-mail: didier.snoeck@ugent.be, nele.debelie@ugent.be

2

Polymer Chemistry and Biomaterials Group, Ghent University, Krijgslaan 281, 9000 Ghent, Belgium – e-mail: peter.dubruel@ugent.be

Keywords: concrete, self-sealing, self-healing, superabsorbent polymers ABSTRACT

Cementitious materials are sensitive to crack formation and it would be beneficial if the material could stop the crack propagation, repair the damage and reach again the original liquid-tightness and/or strength. Therefore, a cementitious material with synthetic microfibres and superabsorbent polymers (SAPs) is proposed. Upon crack formation, the microfibres will become active and due to the bridging action, they will stop the opening of a crack, forcing the cementitious material to crack somewhere else. There, other fibres will become active. In this way, not one large crack, but several small healable cracks are formed.

Further cement hydration and calcium carbonate precipitation will seal the crack if sufficient building blocks and water are present. The building blocks are available through the well-designed mixture with a low water-to-binder (W/B) ratio and water is available through the inclusion of SAPs. These polymers are able to extract moisture from the environment and to provide it to the cementitious matrix for autogenous healing. This healing will lead to the regain in mechanical properties.

In this paper, the formed products are studied by means of optical and scanning electron microscopy. The healing efficiency was evaluated by reloading cracked and healed specimens and by comparing the new mechanical properties with the original properties.

The crack width was limited to 50 µm at 1% strain. While specimens without SAPs showed a regain of mechanical properties of 40-55% in wet/dry cycles, specimens with SAPs showed a total regain of 80-95%. Even in humid air, those specimens show partial healing of 35-55%. SAP B, a cross-linked potassium salt polyacrylate, showed better healing properties compared to SAP A, a copolymer of acrylamide and sodium acrylate. The smart material with SAP B thus is an excellent material to use in future building applications.

1. INTRODUCTION

Concrete cracks give an unsafe feeling and are aesthetically unwanted. Manual repair of these cracks is not only time-consuming, it is also very expensive. It would thus be beneficial if the material could self-heal. Human bones have the ability to detect damage, to stop its propagation, to repair the damage and to reach again the original strength. These features are also available in cementitious materials with synthetic microfibres and superabsorbent polymers (SAPs) as additives.

2. MATERIALS

The mortar mixtures contained CEM I 52.5 N, Class F fly ash (FA/CEM = 1), fine silica sand M34 from Sibelco (M34/B = 0.35), water (W/B = 0.30), polycarboxylate superplasticizer Glenium 51 (Spl/B = 0.0097), 2 volume-% of Polyvinyl-alcohol (PVA) fibres from Kuraray, and a varying amount of SAP expressed as mass-% (m%) relative to the cement weight. Additional water was used to compensate for the loss in workability. The composition of all mixtures used, is shown in Table 1.

Table 1: Mixture composition of the used test series (kg/m³).

CEM FA M34 W Spl PVA SAP A SAP B

REF 608 608 426 365 11.8 26 - -

0.5A 554 554 388 332 +85 10.7 26 2.77 -

0.5B 590 590 413 354 +26 11.4 26 - 2.95

1B 572 572 400 343 +51 11.1 26 - 5.72

Two types of SAPs from BASF were used: SAP A being a copolymer of acrylamide and sodium acrylate (particle size 100.0 ± 21.5 µm) and SAP B, a cross-linked potassium salt polyacrylate (476.6 ± 52.9 µm). The absorption capacity (Table 2) was determined by means of a filtration test and the moisture uptake by dynamic vapor sorption tests. The cement filtrate was obtained by mixing 10 g CEM I 52.5 N in 100 ml de-mineralized water for 6 h and subsequent filtration. All SAPs were vacuum dried with silica gel in a desiccator with a relative humidity (RH) of 3% prior to testing.

Table 2: Swelling capacities of SAP A and SAP B (g/g SAP).

SAP A SAP B

De-mineralized water (pH = 6.5) 305 ± 4 283 ± 2

Cement filtrate (pH = 12.8) 61 ± 1 58 ± 2

at 60/90/98% RH 0.26/0.83/3.94 0.28/0.84/3.94 3. METHODS

Cracks were created by means of a four-point-bending test (Walter+Bai DB 3) at the age of 28 days until 1% of strain was reached (1 µm/s). After cracking, samples were cured at 20 ± 2°C by applying wet/dry cycles (alternatively 1 h stored in water and 23 h at 60% RH), or by placing them in a climate chamber with a RH > 90% or 60%. After another 4 weeks, the specimens were reloaded under four-point-bending and the mechanical properties during the first and second loading cycle were compared.

 

where  σfc = the first-cracking-strength and σunloading = the residual strength [MPa].

Microscopic observations were performed both on specimens and on thin sections perpendicular to the crack plane, to study the new-formed products. Scanning Electron Microscopy (SEM) equipped with an EDS (Energy Dispersive Spectroscopy) detector was used for elemental analysis of the formed autogenous healing products.

4. RESULTS

The first-cracking-strength of the different series is not significantly different (Figure 1). A higher amount of SAPs, however, would lead to a lower strength [2]. Therefore, in this study, the amount of SAPs was limited to 0.5 and 1% respectively.

Figure 1: First-cracking-strength ( ■ ) and the autogenous regain ( ██▄).

“wd” stands for wet/dry cycles, “90” for a RH > 90% and “60” for a RH = 60%.

Specimens without SAPs showed a regain of mechanical properties of 40-55% in wet/dry cycles, and almost no healing in standard laboratory conditions (RH > 90% and RH = 60%). As the storage condition of 1 h in water and 23 h in air is very strict, this amount of autogenous healing could be anticipated.

Specimens containing SAPs, however, showed a total regain of 55-95%. SAPs hold the water and release it steadily to the cementitious matrix for autogenous healing. Even in humid air, those specimens with SAPs show partial healing of 25-55%. SAPs manage to take moisture out of the humid environment and then they provide it to the cementitious matrix for crack healing. In that way, partial healing occurs.

A higher amount of SAPs, without impairing the strength, leads to a higher amount of healing due to more available water. Series with SAP B showed more healing compared to series with SAP A. This is mainly due to the different particle size.

As the strain was limited to 1%, only small cracks were formed. The crack width was always limited to 50 µm for all test series. These cracks are able to heal completely in wet/dry cycles which caused the regain in mechanical properties.

In the deeper crack region of specimens containing SAPs and stored in standard laboratory conditions (RH > 90% and RH = 60%), further hydration occurred. At the crack mouth, the crack was still open. Only at some distinct places, the formed products were able to bridge a crack. These bridging products were not strong enough and only the inner further hydration was responsible for the measured healing capacity.

Microscopic measurements on a thin section taken from specimens in wet/dry cycles and with SAPs (Figure 2a) revealed that the outer 400 µm of the crack was filled with a whitish product. EDS spectra (Figure 2b) showed that this material was mainly composed of calcium carbonate (CaCO3). In the inner crack less CaCO3 was formed,

but, further hydration occurred, especially at the crack tip. This led to a regain of 0 25 50 75 100 0 2 4 6 8 REF 0.5A 0.5B 1B Regain in properties [%] First-cracking-strength [MPa] w /d 90 60 w/d 90 60 w/d 90 60 w/d 90 60 ICSHM2013_________________________________________________________________________________ 19

tightness and a regain in the available cross-sectional area and thus mechanical properties.

(a) (b)

Figure 2: Thin section of healing products in an autogenously healed crack (a) and an EDS spectrum of the formed healing products at the crack face (b). 5. CONCLUSIONS

The combination of microfibres and superabsorbent polymers enhances the autogenous healing capacity. The material is able to recover its mechanical properties, even in standard laboratory conditions.

SAPs are able to extract moisture from the environment and provide the moisture to the matrix for self-healing. SAP B showed the best healing properties and showed a total strength regain of 80-95% in wet/dry cycles and a partial healing of 35-55% in humid air.

ACKNOWLEDGEMENTS

As a Research Assistant of the Research Foundation-Flanders (FWO-Vlaanderen), D. Snoeck wants to thank the foundation for the financial support. The authors also want to thank Dr. G. Herth from BASF for providing SAP A and SAP B.

REFERENCES

[1] V.C. Li, Engineered Cementitious Composites (ECC) – Material, Structural and Durability Performance, in: E. Nawy (ed.), Concrete Construction Engineering Handbook, CRC Press (2008) 78 p.

[2] D. Snoeck, K. Van Tittelboom, S. Steuperaert, P. Dubruel, N. De Belie, Self-healing cementitious materials by the combination of microfibres and SAPs, Journal of Intelligent Material Systems and Structures, DOI: 10.1177/1045389X12438623.

Na Ca Ca C O MgAl Si_Au K _{Fe Au} Counts [#] 0 1 2 3 4 5 6 7 8 9 10 Energy [keV] crack matrix ICSHM2013_________________________________________________________________________________ 20