# Mechanical Characterization of the Self-Healing Effect of Cracks in Ultra High Performance Concrete

Sébastien Granger, Ahmed Loukili, Gilles Pijaudier-Cabot, Gilles Chanvillard

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Sébastien Granger, Ahmed Loukili, Gilles Pijaudier-Cabot, Gilles Chanvillard. Mechanical Characterization of the Self-Healing Effect of Cracks in Ultra High Performance Concrete. International Conference on Construction Materials: ConMat’2005, Aug 2005, Vancouver, Canada. pp.1-9. hal-00362635

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# Mechanical characterization of the self-healing effect of cracks in Ultra High Performance Concrete (UHPC)

S. Granger*, A. Loukili*, G. Pijaudier-Cabot* and G. Chanvillard**

* R&DO, Research Institute of Mechanics and Civil Engineering - Ecole Centrale de Nantes, Université de Nantes, CNRS - France

** Lafarge Research Centre, St Quentin Fallavier - France

## Abstract

Due to time-dependent effects and/or mechanical loading, cracks appear in concrete structures. A few studies have shown that, under special conditions, cracks can heal, without a deliberate external intervention of repair. Healing can take place only in the presence of water because it consists of chemical reactions of compounds exposed at the cracked surfaces. The main result of the phenomenon is the recovery of water-tightness or transfer properties of concrete, initially changed by the presence of cracks. Another interesting consequence should be an improvement of the mechanical behaviour. Self healing was nearly all the time highlighted by water permeability tests. The aim of this study is to propose a mechanical characterization of the phenomenon in an Ultra High Performance Concrete (UHPC). After damaging and ageing in different conditions, beams have been re-loaded under three points bending, in order to highlight the self-healed concrete behaviour. Non destructive testing, including acoustic emission and resonance frequency measurements, have been carried out during experiments. This new approach of characterization demonstrate, as preliminary results, a gain of stiffness after damage, depending on the time of ageing.

**Keywords:** concrete, damage, self-healing, mechanical properties, acoustic emission

Sébastien Granger
R&DO, Research Institute of Mechanics and Civil Engineering - Ecole Centrale de Nantes
1,rue de la Noë BP 92101
44321 Nantes Cedex 3
France

Email: Sebastien.Granger@ec-nantes.fr
Tel: +33 2 40 37 16 63
## 1.0 Introduction

For many concrete structures in the course of their lifetime (or close to their end), the assessment of durability is a key parameter needed in order to know whether safety will be still ensured or not. The presence of cracks, due to mechanical stresses or time dependent effects, is one of the major factor which can influence durability and serviceability, in terms of resistance, permeability and transfer properties. The occurrence of self-healing of cracks and the benefits therefrom are especially significant on the reduction in water transport through cracks or on improving the protection of embedded steel reinforcement from corrosion. A few research works have shown these benefits on concrete made from ordinary Portland cement, which are presented in the first part of this contribution. Thus, some preliminary explanations about the process and the filler materials will be given. The knowledge is, however, scanty, and self healing of cracks is seldom quantified. Mechanical behaviour of self healed concrete have always been mentioned but never quantified. However, this is of great importance, in terms of design, if it can be demonstrated that concrete recovers or improves its mechanical characteristics after being damaged.

This work aims at highlighting self healing of cracks through mechanical tests. Prismatic samples (50x100x500 mm³) of Ultra High Performance Concrete (UHPC) were damaged using three points bending tests, in the post peak regime, and then unloaded to get a crack width between 5 µm and 15 µm. One specificity of UHPC is its small Water/Cement ratio (in this case, close to 0.2). This type of concrete involves a large quantity of unhydrated cement (50 to 60%) [1] and exhibits a very low creep and shrinkage after heat treatment [2,3]. This amount of unhydrated cement supports the self healing of cracks phenomenon. After ageing, i.e. conservation in air or water during a period between 1 and 10 weeks, specimens are retested in the same mechanical conditions, until total rupture, to characterize the behaviour of self healed concrete. Acoustic emission acquisition is carried out during all the steps of the program (damaging and reloading) to localize and qualify the fracture process, prior and after self healing. Measurements of flexural resonance frequency are also carried out, at each step, to show the evolution during ageing. The experimental program and the results will be presented in the paper, prior to the conclusions and what could be expected from future works.

## 2.0 The phenomenon of self healing of cracks: state of art

The process of self healing occurs between opposite surfaces of narrow cracks. Healing can take place only in presence of water because it consists of chemical reactions of compounds exposed at the cracked surfaces. These reactions produce crystals, and the accretion of these from the opposing surfaces of a crack bridges it eventually, so that the continuity is re-established. The essential requirement, with water, is the presence of compounds capable of further reaction. Thus, cement and cementious materials, hydrated or unhydrated, are the essential elements. There are two major hypothesis regarding the reactions of healing: the hydration of unhydrated clinker, or the precipitation of calcium carbonate. The first one requires water only, when the second one requires in addition the presence of carbon dioxide. Silting up of cracks or deposition of debris can contribute to healing but can't provide it by itself. No conclusion, however, can be drawn concerning what really crystallizes in cracks, in specific conditions.

The majority of studies carried out on the subject highlights the phenomenon with water permeability tests. A diminution of the flow rate through cracked specimens is the main method to show the self healing of cracks. Edvardsen [4] performed such tests, conducted on small concrete
specimens, with a single tension crack each, and varying crack width (from 0.10 mm to 0.30 mm) and concrete composition. The typical results show a significantly decreasing rate at the early stages, and then leakage quickly drops to a reduce level. Edvardsen explains the phenomenon by the crystallisation of calcium carbonate CaCO₃ in the cracks exclusively. X-ray diffraction tests from crack borders show CaCO₃ as the main element. The author presents also a model governing the nucleation of calcium carbonate, based on two different crystal growth processes, which explains the two phases of the curves. In the initial phase of water exposure, the kinetics of crystal growth is surface controlled (reactions with crystals on the crack surfaces) and later it is diffusion controlled (reactions with ions diffusing through the calcium carbonate layer already crystallized). The author shows also that neither concrete composition, nor water hardness has influence on the phenomenon. Blocking of flow path by broken concrete particles is also of less importance.

Hearn [5,6] carried out also water permeability tests on mortar (with 78 percent hydrated clinker) and twenty-six years old water cured concrete with insignificant amount of unhydrated clinker. Hearn investigates the chemical effects, like continuing hydration, dissolution and deposition of soluble species (CaCO₃ and others), and physical clogging by concrete particles. The author shows that without drying, especially on virgin never dried 26-years old concrete, there is no self healing of cracks, whereas drying results in an increasing initial permeability followed by self healing. The author carried out chemical analyses of water, inflow and outflow. It points out a significant increase in Ca²⁺ concentration in outflow, which indicates a phenomenon of dissolution-deposition, especially of calcium carbonate CaCO₃. Moreover, self healing was totally absent in tests where water was replaced by propan-2-ol as a permeant. It is the most convincing evidence that physical clogging is not responsible for the phenomenon because its viscosity is closed to that of water. Flow reversal tests also show that self healing is permanent. Concerning continued hydration, no real evidence is established by Hearn, even it is important for recently produced concrete specimens in decreasing permeability, and not for mature ones.

Another type of experiments was carried out by Jacobsen [7,8] to highlight self healing of cracks. It consists of damaging concrete cubes by rapid freeze-thaw cycles, and then storing them in water (20°C during 3 months). Two types of concrete at water/cement ratio of 0.4, one with silica fume and the other one without silica fume, were tested. Deterioration and healing were measured by resonance frequency/dynamic modulus of elasticity and compressive strength. Freeze/thaw cycles led to loss in both resonance frequency and compressive strength. Self healing gave a substantial recovery of the frequency, but only a small recovery in compressive strength, which was explained by the fact that cracks were only partly filled with new products. Indeed, such explanation was possible with SEM observations performed by Jacobsen. It shows that most of the crystals seen in the cracks were newly formed CSH. Portlandite Ca(OH)₂ was also observed locally. The study also demonstrate that self healing is less apparent in concrete containing silica fume than in concrete without silica fume. It points out, in comparison with Edvardsen's works, that no real conclusion can be drawn regarding the filler material.

An important and last pragmatic question is: what is the maximum width of cracks that will be closed by self healing? Different investigators report different maximum cracks width. This is not surprising because the test conditions were rather different from one study to the other. For instance, Edvardsen shows that cracks of 0.3 mm width can heal, whereas Jacobsen works with crack widths of 1 µm to 10 µm. The results depend really on the tests conditions and no general conclusion can be expected.
As a conclusion, these few works have shown that self healing of cracks is a complex chemo-
physical phenomenon, which had been mainly highlighted by water permeability tests. However,
no clear conclusion can be drawn to explain what happens in cracks. Very little results have been
performed regarding the role of self healing in mechanical properties of concrete. A research work
from Pimienta and Chanvillard [9] provides some mechanical results of healed concrete. This
paper aims to provide some new insights.

**3.0 Experimental program**

**3.1 Concrete specimens**

The experimental program is carried out on Ultra High Performance Concrete. Its Water/Cement ratio is close to 0.2. The composition includes silica fume. Concrete is mixed in a conventional industrial concrete mixer. Rheology of the concrete is between fluid ad self compacting. After batching, concrete has been cured for 2 days at 20°C and 100% relative humidity. A thermal treatment is then applied to get the final specimens. The curing conditions are 90°C and 100% relative humidity for 48 hours. Final specimens are beams of 50x100x500 mm³. A notch of depth 20 mm and thickness 1.5 mm is performed in each specimen by placing a plexiglas plate at the midpoint perpendicular to the long direction of the mould before casting. Three identical specimens were cast for each type of test.

**3.2 Mechanical tests configuration**

Specimens are pre-cracked, and then reloaded under three point bending. The test set-up used is an INSTRON machine with 160 kN capacity. The tests are notch opening controlled with a constant rate of 0.05 µm/s. During tests, load and CMOD are acquired. Test configuration is presented in Figure 1.

Pre-cracking is performed in the post peak regime. After having reached the peak (between 3 and 4 kN), unloading is performed when the applied force reaches 2 kN, the CMOD being approximately 30 µm. Residual cracks mouth opening displacement after unloading ranges between 5 µm and 15 µm. Reloading after ageing is performed under exactly the same conditions as loading during the pre-cracking procedure.

<figure><img src="image_2.png" /><figcaption>Figure 1: Mechanical test configuration</figcaption></figure>
### 3.3 Non destructive testing

Non destructive testing is also performed during all the steps of the experimental procedure. Before and just after damaging, and then before reloading, a measure of the resonance frequency of the specimens is performed with a Grindosonic apparatus. Flexural frequencies are measured.

An acoustic emission acquisition is also carried out. The AE system comprises of an eight-channel MISTRAS system (PCI-DISP4) and a PC for data storage analysis. Four piezoelectric transducers (resonance frequency of 150 kHz) are placed on one side of the specimen in a rectangular array (140 x 65 mm²) just above the notch, and coupled to the specimen with silicon (figure 2). The detected signals are amplified with 40 dB gain amplifiers.

<figure><img src="image_3.png" /><figcaption>Figure 2: Acoustic emission instrumentation</figcaption></figure>

### 3.4 Conditions of ageing

Four different periods of ageing have been selected: 1 week, 3 weeks, 6 weeks and 10 weeks. After the pre-cracking phase, and for each period, 3 specimens have been stored in water and 3 others in air.

The water storage conditions are total immersion at 20°C. Regarding air, specimens have been stored in plastic bags, at 20°C. After each periods of ageing, 3 specimens stored in air, and 3 specimens stored in water are reloaded in order to characterize the mechanical behaviour.

## 4.0 Results and discussion

### 4.1 Mechanical highlighting of the self healing effect

Results of mechanical tests are presented in Figures 3 and 4. For all periods and the two conditions of ageing, each curves present two parts, pre-cracking and then reloading. The curves display the applied load versus the crack mouth opening displacement. Figure 3 shows the typical mechanical behaviour of 3 specimens aged in air (3 weeks), to show reproducibility of tests. In the following, only one curve of each type of ageing is presented. Figure 4 presents the typical curves for specimens stored in air and water for 3 different periods of ageing (1, 3 and 6 weeks). It shows there is no gain of stiffness upon reloading for specimens stored in air, and the continuity of all reloading curves with pre-cracking curves is noticeable. These specimens clearly don't exhibit self healing. In comparison, there is a gain of stiffness for specimens aged in water, which signify self
healing. Stiffness tends towards initial stiffness, i.e. that of non cracked concrete. This gain depends on the time of ageing, and is the most important for the longest storage period in water.

<figure><img src="image_4.png" /><figcaption>Figure 3: Mechanical behaviour of 3 specimens aged in air (during 3 weeks) – Reproducibility of tests</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4: Mechanical behaviour of specimens for different conditions of ageing (1,3 and 6 weeks) – Comparison ageing in water and ageing in air</figcaption></figure>
Figure 5 shows the evolution of the gain of stiffness, for the different times of ageing in water. The average curves of reloading for each type of ageing is represented. We can notice that the consequences of self healing are more important for longest periods of exposure to water. Indeed after 10 weeks storage in water, the beams have recovered their initial stiffness. Table 1 presents the evolution of the reloading stiffness with time. These results indicates clearly that new hydrates, whose stiffness is close to that of hydrates from normal hydration, have crystallised in cracks.

The results also show that crack mouth opening displacement at reloading peak decreases with the time of self healing. This result will be confirmed by acoustic emission acquisition. It can be explained by the fact that new products crystallise in the crack, and decreases its length. Nevertheless, these tests show that no flexural resistance improvement is obtained due to self healing of cracks. For all the tests, the peak never exceeds significantly 2 kN, which is the load reached before unloading.

<table><caption>Table 1: Evolution of average reloading stiffness of healed concrete with the time of storage in water</caption><thead><tr><th>Uncracked specimen</th><th>Specimen stored 1 week in water</th><th>Specimen stored 3 weeks in water</th><th>Specimen stored 6 weeks in water</th><th>Specimen stored 10 weeks in water</th></tr></thead><tbody><tr><td>0.307 kN/mm</td><td>0.209 kN/mm</td><td>0.283 kN/mm</td><td>0.301 kN/mm</td><td>0.306 kN/mm</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Figure 5: Reloading behaviour of self healed specimens for different times, compared with non healed specimen (average curves)</figcaption></figure>

These results clearly show that new products have crystallized in cracks. W/C ratio is very low, we can suppose, like Jacobsen [5,6], that new hydrates have been formed. As it will be seen in 4.2, there is also a recovery of flexural resonance frequency, and there is also no gain in resistance. The Figure 5 also shows that self healing do not add ductility to beams.
## 4.2 Measurements of resonance frequency

Average results of flexural resonance frequency are presented in Table 2. Such measurements were carried out with a Grindosonic apparatus, and performed before and after pre-cracking, and after self healing.

<table><caption>Table 2: Evolution of flexural resonance frequency with the time of storage in water</caption><thead><tr><th>Healthy specimen</th><th>Cracked specimen</th><th>Specimen stored 1 week in water</th><th>Specimen stored 3 weeks in water</th><th>Specimen stored 6 weeks in water</th><th>Specimen stored 10 weeks in water</th></tr></thead><tbody><tr><td>1.59 kHz</td><td>1.53 kHz</td><td>1.58 kHz</td><td>1.59 kHz</td><td>1.59 kHz</td><td>1.58 kHz</td></tr></tbody></table>

After a decrease in flexural resonance frequency due to cracking during the pre-loading phase, we can notice a recovery after storage in water. These results confirm the gain of stiffness observed mechanically, due to the crystallisation of new products. The fact we do not really notice an evolution between the different periods, and that the resonance frequency is already recovered after one week of self healing, is probably due to the sensitivity of the apparatus.

## 4.3 Acoustic emission analysis (AE)

The aim of the acoustic emission analysis was clearly the localisation of microcracking events due to new products formed in the cracks. Localization maps of all the process cracking were obtained for all the specimen tests. The first observations, comparing all the tests carried out in the different conditions, do not permit to distinguish acoustic events related to self healing from others. The analysis of these localization maps need to be improved by the analysis of the signal, and its wave form, so as to discriminate typical wave form related to self healing phenomenon. Nevertheless, the evolution of the cumulative AE events number detected with the CMOD tends to confirm the mechanical results (figure 6).

<figure><img src="image_7.png" /><figcaption>Figure 6: Evolution of the cumulative AE events number detected, compared with reloading mechanical behaviour (average curves)</figcaption></figure>
The results show that the beginning in increasing of the AE event number, is related to the reaching of the peak load. Moreover, we can notice that the total number of events tends to increase with the time of healing. This result has to be moderated, because of the disparity in emission properties of the specimens.

## 5.0 Conclusion

In this paper, the results of an experimental study on the mechanical characteristics evolution of previously cracked specimens are presented. Various durations of conservation in water, after pre-cracking, have been performed. After one week of storage in water, it appears that damaged beams tend to recover their initial stiffness. That is confirmed after 3,6 or 10 weeks of conservation. The concrete used in this study has an important quantity of unhydrated cement, and these observations are attributed to self healing of cracks. This gain of stiffness is more important with time of conservation, and we can expect concrete recover its initial rigidity after few time of self healing. Nevertheless, results don't show an evolution of the peak load under reloading after self healing. These mechanical investigations are confirmed by non destructive testing during experiments. Both measurements of flexural resonance frequency and acoustic emission analysis show clearly an evolution of the concrete behaviour with the time of conservation in water. Now, all these investigations have to be corroborated by a more accurate acoustic emission analysis and microscopic observations. The influence of crack width on the phenomenon of self healing has also to be investigated.

## 6.0 References

1. Loukili, A., Richard, P. and Lamirault, J., "A study on delayed deformations of an ultra high strength cementious material", ACI, SP179-59, Recent Advances in Concrete Technology, Vol 179, 1998

2. Acker, P., "Swelling, shrinkage and creep: a mechanical approach to cement hydration", Concrete Science & Engineering, Vol. 37, p.237-243, May 2004

3. Acker, P., "Micromechanical analysis of creep and shrinkage mechanisms", Creep, Shrinkage and Durability Mechanics of Concrete and other quasi-brittle Materials, F.-J. Ulm, Z.P. Bazant, F.H. Wittmann editors, Elsevier, Oxford UK, Aug. 2001

4. Edvardsen, C., "Water permeability and autogenous healing of cracks in concrete", ACI Materials Journal, Vol 96, N°4, July-August 1999, pp. 448-454

5. Hearn, N., "Self-sealing property of concrete – Experimental evidence", Materials and Structures, Vol 30, August-September 1997, pp. 404-411

6. Hearn, N., "Self-sealing, autogenous healing and continued hydration: what is the difference?", Materials and Structures, Vol 31, October 1998, pp 563-567

7. Jacobsen, S., Marchand, J. and Hornain, H., "Sem observations of the microstructure of frost deteriorated and self-healed concrete", Cement and Concrete Research, Vol 25, N°8, pp. 1781-1790, 1995

8. Jacobsen, S. and Sellevold, E., "Self healing of high strength concrete after deterioration by freeze/thaw", Cement and Concrete Research, Vol 26, N°1, pp. 55-62, 1996

9. Pimienta, P. and Chanvillard, G., "Retention of the mechanical performances of Ductal® specimens kept in various aggressive environments", Fib - Symposium 2004, April 26-28, Avignon, France