Natural Wood Fibres for Self Healing Concrete

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Concrete Repair, Rehabilitation and Retrofitting II Alexander et al (eds) 2009 Taylor & Francis Group, London, ISBN 978-0-415-46850-3

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Using natural wood fibers to self heal concrete

M.R. de Rooij, S. Qian, H. Liu, W.F. Gard & J.W.G. van de KuilenDelft University of Technology, Delft, The Netherlands

ABSTRACT: Changing from damage prevention to damage management, thereby allowing some cracks in astructure as long as the cracks self-repair over time, is the basic concept behind self-healing materials. Follow-ing nature, where wood fibers allow for both transport and bonding possibilities, this paper describes variousoptions to apply wood fibers. Preliminary results on how to obtain single wood fibers and initial experimentson concrete crack width are presented. It is shown that the boundary conditions for a successful application ofwood fibers as self-healing carriers can be met. However, it is also shown that many very practical obstacles in

the manufacturing of self healing concrete still need to be cleared.

1.2 Application to concrete materials

In all modesty, it should be mentioned that self-healing is not something very new in materialsscience. The self-healing capability has been a prop-erty which was sometimes there by coincidencerather than intentionally imposed. A well knownexample is the mortar used by the Romans for theconstruction of their buildings and large infra-structural works (Riccardi, 1998). The exceptionaldurability of these structures is the consequence ofmicro-cracks closing spontaneously due to a chemi-cal reaction between the mortar and the moisture inthe air leading to controlled dissolution and repre-cipitation (Sanchez-Moral, 2004).

Hence, it is fair to say that concrete-like materi-als have a good track record for self-healing to startfrom. However, the previous example also highlightsone of the key parameters for successful self-healing:repair material should be transported from the bulkmaterial to the place of damage (the crack). It is in

this process part that some of the developments inself-healing materials are taking place. Glass vialswith repair agent, for example, have been a reportedoption (Dry, 1994). Another option is pursued inthis paper focusing on the transport properties ofwood fibers. The paper describes various optionsto choose wood fibers, preliminary results on howto obtain single wood fibers and initial experimentson crack width. First however, some boundary con-ditions for concrete are discussed based on earlierresearch work.

1 CONCEPT INSTRUCTIONS

1.1 The concepts of self-healing

Materials science has improved qualities of materialstremendously by dedicated engineering work. When,for example, a material is not strong enough, thecause will be identified, composition and processingof the material altered, until it results in a material

being stronger, hence having improved properties.Such success stories can be identified for almostevery property of a material. However, in this lineof improvement the basic starting point has almostalways been to raise the levels, set a new record (inour example make it stronger).

Width hindsight, all strategies to improve thestrength and reliability of materials developed over the

past 20 centuries are ultimately based on the paradigmof damage prevention, i.e. the materials are designedand prepared in such a way that the formation andextension of damage as a function of load and/or time is

postponed as much as possible (Van der Zwaag, 2007).Damage is defined here as the presence of micro- ormacroscopic cracks not being present initially.

However, in recent years it has been realized thatan alternative strategy can be followed to make mate-rials effectively stronger and more reliable throughdamage management, i.e. materials have a built incapability to repair the damage incurred during use.Cracks are allowed to form, but the material itself iscapable of repairing the crack and restoring the func-tionality of the material. The material is self-healing.


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2 EARLIER RESEARCH WORK

2.1 Healing of early age cracks

The first research work on concrete material that wasrelated to self-healing materials at Delft University of

Technology, consisted of crack healing of early agecracks in concrete (Ter Heide, 2005; 2007). In thisstudy, three point bending tests were performed on

prismatic concrete specimens to fracture the speci-mens. These tests were performed between 20 to72 hours after casting, producing crack (mouth) open-ings of the crack between 20150 m. After crackingthe samples were stored in different relative humidity(RH) environments: under water or in climate cham-

bers with 95% or 60% RH. Furthermore, the influ-ence of compression stresses closing the crack wasinvestigated. Stresses of 0.0, 0.5, 1.0 and 2.0 N/mm2

have been applied.After two weeks the samples were taken from

their different conditions and tested again under threepoint bending, this time until failure. The results ofthese tests were compared with reference samples not

being fractured. These reference samples were loadedin two steps: first (after two weeks) a predeterminedcrack was made, similar to the fractured specimen atage of one day. Next, the specimens were unloaded.Then, immediately following this, the specimenswere loaded again until failure. By now comparingthe results of the healed specimens with the unhealed

references samples, the following conclusions couldbe drawn:

Cracks do heal under the conditions that the cracksare made at an early age and the cracks are closedagain (a compressive stress is applied) and thespecimens are stored under water.

The amount of compressive stress does not seemto influence the strength recovery. The results indi-cate that a compressive stress is needed to closethe crack, but once the two crack faces touch eachother, or the distance between the crack faces issmall enough, crack healing can happen.

With increasing age of the specimen at the momentof cracking, a decrease in strength recovery isfound. This seems to be related to the degree ofhydration at the age of the specimen when the f irstcrack is made.

The width of the crack does not seem to have aninfluence on the strength recovery, as long as thecrack faces can come close enough to each other.

Crack healing is only observed when the crackedspecimens are stored under water.

From this research it became clear that self-healingis possible, but the crack faces should be close

together. In other words, the crack width should notbe too large.

2.2 Keeping crack width small

Concrete does not posses much capacity to take uptensile forces. Therefore, the concrete normally usedin practice is almost always reinforced concrete. Thesteel reinforcement bars are meant to take up tensile

forces, but the amount and location are also chosen todistribute cracks and thus keep crack widths small.

Keeping crack widths small becomes more andmore an objective rather than an important side issue,a next level can be reached by using fiber reinforcedconcrete. Research has shown that the most funda-mental property of a fiber reinforced cementitiousmaterial is the fiber bridging property across a matrix(Lin & Li, 1997). This is the average tensile stresstransmitted across a crack with uniform crack open-ing as envisioned in a uniaxial tensile specimen.

Naturally there will be a point of optimization

between the requested different properties and e.g.the amount, dimensions and properties of fibers nec-essary to fulfill these requirements. A group that has

performed much research in materials optimizationby microstructural tailoring is the group of prof. Li(Li, 2003). Through his research he has developedEngineered Cementitious Composites (ECC) overthe last decade since its invention in the early 1990s.ECC is a component of microstructure tailoring toits extreme, showing tensile strain capacity of 5%(Li, 2001), approximately 500 times larger than thatof normal concrete, or even regular fiber reinforcedconcrete.

When an ECC structural element is loaded (flex-ure or shear) to beyond the elastic range, the inelasticdeformation is associated with microcracking. Themicrocrack width is dependent on the type of fiberand interface properties. However, it is generally lessthan 100 micron when PVA fiber is used.

Figure 1. Crack width development in ECC with 2%REC15 fibers (Li, 2003).


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cherry, koto, maple, oak, utile/mahagony and walnutwood. Based on the amount of fibers present, the firstchoice has been made for Oregon pine to study fur-ther. The determined size of lumen in this wood isabout 40 m. A cross-section of this wood is shownin Figure 3. Tests are now set up to investigate thisfiber further.

3.3 Parallel preliminary experiments

While searching for the wood fiber type to be usedin future research, preliminary experiments on pro-ducing concrete with natural fibers were under-taken in a parallel research program. The first testsdescribed here were set up with sisal fiber, becausethat was readily available in large enough quantitiesto produce mortar specimens. These tests should giveinsight in the workability problems that could occur,as well as distribution of fibers through the mortar.Furthermore, it would also give a first idea on crackdistribution and crack width.

Again tests were designed from an ECC starting

point. The specimens should be cracked in a 4 pointbending test. The crack width should be kept small by

so called bridging fibers. For this purpose we chosePVA fibers, because of the good experience in ECC.The self-healing fibers should be represented by sisalfor the time being. The produced mortar mix compo-sition is given in Table 2.

Specimens were demoulded after one day and then

cured in air (20C/65% RH). At 28 days the speci-mens were fractured using 4-point bending tests.After fracturing the number of cracks and the averagecrack width was determined. The tests were repeatedfive times.

Results showed that the number of cracks rangedfrom 3 to 7, with an average crack width rangingfrom 120 m to 7080 m. Most noticeable was thatthe fibers were well distributed, but that they had not

been broken, see Figure 4. When these fibers haveto repair the cracks in concrete they should break infuture experiments, in order to be able to deliver therepair agent. Further research is moving to establish

how weak the healing fibers should be in order tobreak upon fracture of the sample.

Figure 3. Cross-section of Oregon pine wood.

Table 2. Mix composition of preliminary mortar tests withnatural fiber.

Amount

Component GramByvolume

Portland cement 500

Sand 400

Fly ash (type F) 600

Water 300Superplasticizer 20

PVA fiber 1% (v/v)

Sisal fiber 2% (v/v)

Table 1. Overview of first inventory on physical andmechanical properties of possible fibers.

FibersLength(mm)

Diameter(m)

Tensile strength(MPa)

Synthetic

PE 12.7 38 2700

PVA 812 39 1620

PP 6 12 770880

Stem fibers

Flax 27.436.1 17.821.6 500900

Hemp 8.314.1 17.022.8 310750

Jute 1.93.2 15.920.7 250350

Ramie 60250 28.135.0 870

Hibiscus 1601500 40350

Sugarcane 0.82.8 6.626 170290

Bamboo 2.8 1040 350500

Deciduouswood

0.32.5 1045

Coniferouswood

1.09.0 1560 700

Seed-hairfibers

Cotton 1265 1220 300600

Coir 0.91.2 16.219.5 130175

Leaf fibers

Sisal 1.83.1 18.323.7 250550

Banana 2.25.5 1830 530750


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Figure 4. Sisal fibers in crack appear to be unbroken.

3.4 Future steps: filling fibers

As a future outlook on this very early research path,the next mark is getting extracted fibers from Oregon

pine filled with water and then sealed with a coatingto keep the water inside. Water is used as filling mate-rial as a first basis. The focus will be on application

of a coating that can contain the future self-healingagent inside the fiber. This coating should be strongenough to contain the healing agent during the vig-orous mixing process of concrete, but should on theother hand also be or become weak enough to frac-ture when needed for repair is at hand. In addition, the

pull-out strength should remain larger than the f iberbreaking strength. It is our current philosophy that byapplying a coating we alter the fiber properties in the

process so much, that the fiber will break upon finaluse in concrete when a crack passes along.

4 CONCLUSIONS

In this article a very first inside look has been pre-sented on the possible concepts of using naturalfibers to self-heal concrete. Preliminary literatureresearch has shown that the boundary conditions fora successful application of wood fibers in concretecan be met. However, it has also been observed thatmany and also very practical obstacles in the research

still have to be cleared. To name a few that have beenmentioned throughout the article:

obtaining (bundles of) fibers in enough quantity toperform the research,

making a mixture composition in which the cracks

remain small enough for self-healing to have achance,

producing fibers that break even with these verysmall crack widths,

producing a coating that keeps the repair agent inthe fiber during mixing of the concrete and duringundamaged service life of the concrete,

producing a coating that is not so strong, that it willprevent the fiber to break.

As we know it is very well possible to manipulatethe properties of the natural fibers as much as it is

possible to tune the properties of the future coating on

the fiber, we are looking forward to the challengingresearch in the new field of self-healing materials.

REFERENCES

Dry, C. 1994. Matrix cracking repair and filling using activeand passive modes for smart timed release of chemicalsfrom fibres into cement matrices. Smart Materials andStructures3(2): 118123.

Li, V.C., Wang, S. & Wu, C. 2001. Tensile strain-hardingbehavior of PVA-ECC. ACI Materials Journal. 98(6):

483492.Li, V.C. 2003. On Engineered Cementitious Composites

(ECC)A review of the material and its application.J. ofAdvanced Concrete Technology.1(3): 215230.

Lin, Z. & Li, V.C. 1997. Crack bridging in fiber reinforcedcementitious composites with slip-hardening interfaces.J. meachics and Physics of Solids. 45(5): 763787.

Riccardi, M.P., Duminuco, P., Tomasi, C. & Ferloni, P. 1998.Thermal, microscopio and X-ray diffraction studieson some ancient mortars. Thermochim Acta 321(12):207214.

Sanchez-Moral, S., Garcia-Guinea, J. & Luque, L. 2004.Carbonation kinetics in roman-like lime mortars.Materi-

ales de Construction54(275): 2337.Ter Heide, N., Schlangen, E. & Van Breugel, K. Experimentalstudy of crack healing of early age cracks. In Proc. KnudHgjaard conf. on Advanced Cement-Based Materials,

June 2005.Denmark.Ter Heide, N. & Schlangen, E. 2007. Selfhealing of early

age cracks in concrete. In Proc. of the First Int. Conf. onSelf Healing Materials, Noordwijk aan Zee, 1820 April

2007. Dordrecht: Springer.Van der Zwaag, S. 2007. Self Healing Materials, an Alter-

native Approach to 20 Centuries of Materials Science.Dordrecht: Springer.


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Natural Wood Fibres for Self Healing Concrete