161

SP-224—12

Crack Growth Resistance of Thin Mortar

Layers with Hybrid Fiber Reinforcement

by L. Sorelli, N. Banthia, and G. A. Plizzari

Synopsis: Hybrid fiber reinforcement of cement composites is rapidly emerging as an

innovative and promising way of improving mechanical performance and durability of

cement-based materials.

In the present paper, fracture behavior of medium, high and very high strength mortars

reinforced with hybrid fibers was experimentally studied by using contoured double

cantilever beam specimens. Different combinations of small steel fibers and fibrillated

polypropylene micro-fibers are investigated. These composites are very suitable for thin

sheet products such as roofing sheets, tiles, curtain walls, cladding panels, permanent

forms, etc.

Aim of the paper was to study the influence of matrix strength, fiber type and fiber

combinations on the fracture toughness of the resulting fiber reinforced mortars.

Results indicate that some combinations of fibers and matrix strengths exhibit a higher

resistance to crack growth and evidence the contribution of polypropylene fibers to

mortar toughness.

Keywords: cement; fiber; reinforcement

162 Sorelli et al.

Luca Sorelli received his PhD at University of Brescia in

2003, Italy. His current research consist of hybrid fiber

reinforced concrete, structural applications and finite

element modeling of high performance cementitious

composite.

Nemkumar Banthia, FACI, is a Professor of Civil Engineering

at the University of British Columbia (Canada). He is a

member of several ACI committees and chairs the ACI

Committee 544 on Fiber Reinforced Concrete. His primary

research interests consist of cement-based materials, fiber

reinforced concrete and fiber reinforced polymers,

shotcrete, strain-rate effects and impact, use of fiber

reinforced plastics in repairs.

Giovanni A. Plizzari, ACI Member, is a Professor of

Structural Engineering at the University of Bergamo

(Italy). He is a member of the FIB TG 4.5 “Bond Models” and

the RILEM Committee “Hybrid Fiber Concrete”. His research

interests include material properties and structural

applications of High Performance Concrete.

INTRODUCTION

In the new breed of high performance cement based

materials, there has been great interest lately in the

development of Hybrid Fiber Reinforced Cementitious

Composites (HyFRCC) that combine different types of fibers

in a cementitious matrix [1]. The aim is to take

simultaneous advantages from the material properties of

each fiber type (multi-functionality) and from their

interaction (synergy) to optimize the mechanical and

physical performances of the composite [2-5].

A promising hybrid system of fibers concerns a combination

of steel fibers and polypropylene fibers. The former are

used to enhance strength and toughness properties [6] such

as flexural (modulus of rupture), shear [7], impact [8] and

fatigue strength [9]. The latter are commonly used to

reduce shrinkage cracking [10,11] and permeability [12] of

concrete; in fact, bundles of fibrillated polypropylene

fibers open during concrete mixing and separate into

millions of multistrand filaments that are able to mitigate

crack formation due to plastic shrinkage. Vondran and

Webster [12] found that a volume fraction (Vf) of 0.2% of

Thin Reinforced Cement-Based Products 163

polypropylene fibers markedly reduce both the permeability

and the plastic shrinkage cracking.

Fibers may influence the fracture mechanism in a concrete

structure [3,13]. In fact, small-diameter fibers, here

defined as micro-fibers, may delay the fracture process by

which the micro-cracks coalescence to form large

macroscopic cracks [3,14]. Furthermore, micro-fibers modify

the crack pattern by transforming the macro cracks into a

network of smaller and narrower cracks.

A combination of small synthetic (polypropylene) fibers and

steel fibers could be used to yield a hybrid system that

may prove to be an interesting material for thin concrete

or mortar overlays for structural repair and retrofitting

[15]. The enhanced toughness, the reduced plastic shrinkage

cracking and the lower water-permeability could be highly

advantageous in producing a durable thin repair or product.

The use of short fibers in substitution of conventional

reinforcement (reinforcing bars or welded mesh) may allow a

reduction of labor costs.

In the present work, fracture behavior of thin mortar

layers with a combination of small steel and polypropylene

fibers is experimentally investigated by performing

Countered Double Cantilever Beam test (CDCB) [16].

Furthermore, the research aims to study the influence of

matrix strength on the mechanical behavior of concrete with

hybrid fibers.

The chosen amount of polypropylene fibers was higher than

the amount commonly used for controlling plastic shrinkage

cracking (Vf=0.1-0.2%), with the aim of improving the mortar

toughness. A small thickness of the specimens was adopted

to better reproduce the fiber distribution in thin

cementitious elements.

Specimens with a relatively large size were tested to

reduce the size effects and to allow for a simpler

determination of the mortar toughness.

In order to better understand the fracture behavior and to

determine the constitutive laws for the materials adopted,

the experiments were simulated by Finite Element analyses

based on Non Linear Fracture Mechanics (NLFM) [17].

MATERIALS

The mix compositions include 854, 980 and 1019 kg/m3

of

cement ASTM Type I for Medium (MSM), High (HSM) and

Very High Strength Mortars (VHSM) respectively. The water-

cement-sand (with a maximum diameter size of 5 mm)

proportions, as well as the air entrainer and the

164 Sorelli et al.

plasticizer contents of the three different types of

mortars are reported in Table 1.

Steel fibers (SF) and fibrillated polypropylene micro-

fibers (PP) were combined as reported in Table 2. A

reference concrete without fibers (MSM0, HSM0 and VHSM0)

was also made. Properties of the adopted fibers are

reported in Table 3. The steel fibers have a circular cross

section and a straight shape. They are made from high

carbon steel and are coated with brass for corrosion

protection.

The cylindrical ( = 100 mm, h = 200 mm) compressive

strength (fc) determined after 28 days of curing is reported

in Figure 1; notice that the average compressive strength

for the MSM was approximately 60 MPa, for the HSM was

approximately 95 MPa and for the VHSM was approximately 115

MPa.

SPECIMEN DESCRIPTION AND TEST SET-UP

Figure 2a shows a schematic of the Contoured Double

Cantilever Beam specimen that was adopted for the

characterization of the crack growth resistance. According

to LEFM assumption and a model based on the crack

equivalent, the CDCB specimen is shaped in such a way that,

by using Linear Elastic Fracture Mechanics (LEFM), the

Stress Intensity Factor is independent of the crack length

and the specimen allows for a stable crack propagation

under constant load [18, 19]. The CDCB specimen also leads

to more reliable compliance measurements since the

displacements are large and the critical loads are small

compared with tests on other types of specimen [18]. A

groove reduced the thickness of the middle section from 40

to 15 mm to better control the crack path (Figure 2).

Four CDCB specimens were prepared for each material. The

direction of casting was perpendicular to the surface of

the double cantilever beam specimen and the fresh mortar

matrix was poured while the mould was externally vibrated.

The load was applied vertically by the hydraulic jack of

the Instron machine with a stroke rate of 0.1 mm/min on a

steel wedge placed between two rollers at the top of the

specimen (Figure 3a). The Splitting Load (SL) is the

horizontal components of the total load [19] (Figure 3b).

In order to limit the vertical component of the applied

load that may influence the fracture

behavior of the specimen, the angle of the wedge was chosen

equal to 15° [20]. The coefficient of friction between

the wedge and the rollers was ignored since the wedge

Thin Reinforced Cement-Based Products 165

surfaces were carefully machined (Figure 4). The Crack

Mouth Opening displacement (CMOD) was measured by means of

a resistive displacement transducer (clip gauge) which was

fixed at the level of the loading points (Figure 3a).

The applied load and the CMOD data were acquired using a

digital data acquisition system running at 5 Hz.

RESULTS AND DISCUSSION

The Splitting Load vs. CMOD curves are given in Figure 4

for MSM, in Figure 5 for HSM and in Figure 6 for VHSM

mortars. The plotted curves represent the average curves of

four specimens tested and were obtained by means of the

full least-squares fit Loess procedure (a locally weighted

regression smoothing algorithm).

Notice a significant toughness increase in medium strength

mortars (MSM) due to the presence of steel fibers (along

with a higher residual strength and a more stable behavior

during fracture). Furthermore, the marked differences in

the shapes of curves obtained for the steel fibers as

opposed to those obtained for the polypropylene fibers can

be observed (Figure 4). In the latter case, the curves are

characterized by a lower peak load followed by a steeper

post-peak branch. However, the general enhancement in the

performance of the polypropylene fibers when added to steel

fibers in hybrid materials should be noted.

The same trend is confirmed for the High and Very High

Strength Mortars (Figures 5 and 6). It can be observed that

the peak loads increase with the matrix strength,

especially for the steel fiber reinforced mortars.

The hybrid materials show higher toughness than the mortars

with 0.5% of steel fibers, but lower toughness than the

mortars with 1% of steel fibers. This is in substantial

agreement with other researches carried out on same types

of fibers under bending [5, 21].

While the steel fibers were pulled out from the three

different matrices, part of the polymeric fibers broke

during the fracture. However, the presence of the secondary

polymeric fibers enhanced the fracture energy (GF; defined

as the area under the load-CMOD curves divided by the

projected cracked area) of about 50% for all the

cementitious matrices considered (Figure 7).

This shows that polypropylene fibers in the matrix with

steel fibers allow for appreciable advantages in term of

toughness beside the expected reduction of shrinkage

cracking (synergy).

166 Sorelli et al.

The significant increase of peak load in specimens of HSM

as compared to specimens of MSM can be observed. On the

contrary, no major differences are observed when the matrix

strength further increases; this is probably due to the

bond strength between steel fibers and the mortar matrix

that did not increase in the VHSM (with respect to the

HSM).

Table 4 reports the number of steel fibers counted in the

cross section. In the same table it is also indicated the

expected number of steel fibers assuming either an uniform

3D distribution or a 2D distribution according to [22]:

f

Cf

A

AV

N

⋅

⋅

⋅α=

2

(1)

where N is the expected number of fibers bridging the cross

section, Vf is the volume fraction of fibers, Ac is the

concrete cross section, Af is the fiber cross section area,

is a constant that varies from 0.5 for a 3D distribution

to 0.64 for a 2D distribution [22]. The results show that

the number of fibers counted in the cracked section is

always closer to a 3D distribution.

MODELING

The experiments were numerically simulated by using a 2D

Finite Element model based on Non Linear Fracture Mechanics

(NLFM) to better comprehend the test results and to

identify the fracture parameters of the materials used in

the tests. A discrete crack approach based on the

fictitious crack model was adopted [17].

The Finite Element analyses were performed by using Merlin

[23] that considers the structure as many linear elastic

subdomains linked by interface elements that simulate the

cracks, whose position must be known a priori.

Interface elements initially connect the sub-domains (as

rigid links) and start activating (i.e. cracks start

opening) when the normal tensile stress at the interface

reaches the tensile strength (fct)of the material.

Afterwards, the crack propagates and cohesive stresses are

transmitted between the crack faces according to a stress-

crack opening ( -w) law (Figure 8) which is given as input

for the interface elements.

The CDCB was modeled by adopting 3148 three node triangular

elements (plane stress) for the elastic sub-domains (having

a thickness of 40 mm), linked by means of 67 interface

elements (having a thickness of 15 mm). By assuming a 2D

Thin Reinforced Cement-Based Products 167

model, the stress concentration present at the groove tip

was neglected (Figure 9).

Figure 10 shows the mesh adopted for the CDCB specimens.

The stress-crack opening displacement relationships ( -w)

were approximated with bilinear laws herein (Figure 8). The

tensile strength of this law (fct) was determined from the

experimental compressive strength according to the CEB

Model Code 90 [24]. The experimentally determined fracture

energies (GF) (Figure 7) were used as input data. The other

parameters, namely the stress at the knee point (1), the

crack opening at the knee point (w1) were identified by an

inverse analysis based on the best fitting procedure [25].

Eventually, critical crack opening (wcr) was determined.

Figure 11 shows a typical comparison between the numerical

and the experimental curves for the steel fibers (Vf=1%) in

the High Strength Mortar. The same figure exhibits the

deformed mesh at different loading stages as well as the

distribution of cohesive stresses over the ligament length.

It should be noticed that the crack tip opening

displacement at the peak load is around 0.14 mm and that

the fracture process zone involves most of the ligament

length. The large crack tip opening displacement explains

why, in the adopted specimens, the peak load is more

related to the fiber bridging mechanisms than to the matrix

strength.

The numerical and the experimental curves of the Splitting

Load versus the CMOD are plotted for all the MSM materials

in Figure 12; notice the excellent agreement between the

different curves. The same results are reported in Figures

13 and 14 for the HSM and VHSM mortars, respectively.

The best fitting parameters of the bilinear softening laws

as well as the modulus of elasticity are summarized in

Table 5.

CONCLUDING REMARKS

Splitting tests were carried out on Countered Double

Cantilever Beam specimens. Because of the cross section

thickness of 15 mm, these specimens seem suitable to

characterize the fracture behavior of thin concrete members

made of fiber reinforced concrete.

Experimental results indicated that steel fibers better

enhance the mortar toughness. However, the addition of

polypropylene fibers to a steel fiber reinforced mortar

increases the toughness of the composite for all matrix

strengths considered. In fact, the fracture energy (GF) of

the hybrid materials with 0.5% of steel fibers was improved

168 Sorelli et al.

from 35% to 64% by polypropylene micro-fibers in the three

different matrices strengths (medium, high and very high).

Considering the fact the polypropylene fibers better

control the cracks due to the plastic shrinkage, the

reduced permeability and the lower cost of polymeric

fibers, these hybrid composites seem very suitable for thin

concrete overlays for structural repair and retrofitting.

Furthermore, they can be conveniently adopted for thin

cementitious products, such as roofing sheets, tiles,

curtain walls, cladding panels, permanent forms, etc.

However, although synergy between the two fibers is already

apparent in the hybrids, further optimization attempts are

clearly warranted. Therefore, further research, which

considers plastic shrinkage permeability and thermal

effects, is necessary to optimize the combinations of these

fibers.

The fatigue resistance may also be improved by a hybrid

system where micro-fibers can be active as bridging

mechanism over the micro-cracks surrounding macro-fibers

and cause synergistic effects in the composite. In

addition, in case of a fire, when the free and chemically

bonded water is transformed in vapor, the polymeric fibers

will melt leaving canals through which water vapor can

escape from the boundary zones without spalling off the

concrete covers. This may guarantee the fire protection

required in structural applications.

Non Linear Fracture Mechanics is a satisfactory tool to

model the fracture behavior of these cementitious

composites where the fracture process zone involves most of

the ligament length of the specimen.

Acknowledgements

The authors would like to thank Mr. David Woomk for his

diligence and his enthusiasm in preparing the experimental

tests as well as the helpful support of the technicians of

University of British Columbia (Canada).

Thanks are also due to the Dow Chemical Company and the

Bekaert for supplying respectively the polypropylene and

the steel fibers.

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Thin Reinforced Cement-Based Products 169

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172 Sorelli et al.

Figure 1. Compressive strengths for different materials adopted.

Figure 2. Schematic of a Countered Double Cantilever Beam specimen (a); schematiccrack path (b).

Thin Reinforced Cement-Based Products 173

Figure 3. A CDCB specimen under loading (a); load transmitted by the steel wedge (b).

Figure 4. Splitting Load vs. CMOD curves experimentally determined from MSM fiberreinforced mortars.

174 Sorelli et al.

Figure 5. Splitting Load vs. CMOD curves experimentally determined from HSM fiberreinforced mortars.

Figure 6. Splitting Load vs. CMOD curves experimentally determined from VHSM fiberreinforced mortars.

Figure 7. Fracture energy Gf values for materials adopted.

Thin Reinforced Cement-Based Products 175

Figure 8. Constitutive laws for discrete crack model.

Figure 9. 3D stress distribution due to the groove.

Figure 10. Mesh of the specimen and pre-imposed crack line.

176 Sorelli et al.

Figure 11. Numerical and experimental curves in terms of Splitting Load and CMOD forthe Medium Strength Mortar with 1% of steel fibers.

Figure 12. Experimental and numerical Splitting Load versus CMOD curves for MSMmortars.

Figure 13. Experimental and numerical Splitting Load versus CMOD curves for HSMmortars.

Thin Reinforced Cement-Based Products 177

Figure 14. Experimental and numerical Splitting Load versus CMOD curves for VHSMmortars.

178 Sorelli et al.