8
Bond Strength of Normal-to-Lightweight Concrete Interfaces Hugo COSTA Research Assistant Polytechnic Institute of Coimbra Coimbra, Portugal [email protected] Hugo Costa, born 1977, civil engineering degree (2002), MSc degree (2008) at Univ. of Coimbra. PhD student at Univ. of Coimbra, since 2008. His research area is related to mixture design and structural behaviour of lightweight aggregate concrete. Pedro SANTOS Adjunct Professor Polytechnic Institute of Leiria Leiria, Portugal [email protected] Pedro Santos, born 1977, civil engineering degree (2001), MSc degree (2005), PhD degree (2009) at University of Coimbra. His research interests include pre-casting and strengthening and repair of RC structures. Eduardo JÚLIO Full Professor ICIST & DECivil, IST-UTL Lisbon, Portugal [email protected] E. Júlio is a full professor at the Instituto Superior Técnico of the Technical University of Lisbon. His research interests are in the field of Structural Concrete. He is member of IABSE WC 3, fib Commission 5 and ACI 364 Committee. Summary Structural lightweight aggregate concrete (LWAC) can be used with advantage in new structures, especially in pre-cast components, as well as in the rehabilitation of existing structures. In both situations, the result is a composite structural element, with parts in LWAC and others in normal weight concrete (NWC). Besides material properties, it is also important to characterize the behaviour of LWAC-to-NWC interfaces. With this aim, an experimental study was conducted to characterize the shear strength and the tensile strength of NWC-to-NWC, LWAC-to-NWC and LWAC-to-LWAC interfaces and to compare results with codes’ predictions. A NWC and three different LWAC were considered. Six different types of surface roughness were produced on the concrete substrate and characterized with roughness parameters assessed using the 2D-LRA method, developed by the authors. Slant shear and splitting tests were adopted to quantify the bond strength of the interface in shear and in tension, respectively. Keywords: lightweight aggregate concrete (LWAC); interface; bond strength; roughness parameters; codes. 1. Introduction Structural LWAC presents low density and high performance in terms of both strength and durability. Therefore, this is a competitive material not only for new structures, whenever reduction in dead-weight is aimed, e.g. for the pre-casting industry, but also for the rehabilitation of existing structures. All these situations can give rise to composite structural members, consisting of parts in LWAC and parts in NWC, casted at different ages. In these cases, it is important to assess not only the material properties but also the behaviour of the LWAC-to-NWC interface. The design expressions proposed by codes for concrete structures to predict the shear strength of concrete-to-concrete interfaces are all based on the shear-friction theory [1] and depend mainly on four parameters: (i) compressive strength of the weakest concrete; (ii) normal stress at the interface; (iii) shear reinforcement crossing the interface; and (iv) roughness of the substrate surface. Santos [2] characterized the influence of other parameters on the behaviour of NWC-to-NWC interfaces, namely the differential shrinkage and Young’s modulus, that are not considered by current design codes, particularly by Eurocode 2 (EC2) [3]. Differential shrinkage may not be a relevant parameter in LWAC-to-NWC interfaces (in the case LWAC is the added layer), due to improved internal curing provided by the moisture in the interior of the lightweight aggregates (LWA) leading to reduced shrinkage [4]. However, since the Young’s modulus of LWAC can be quite different from that of NWC [5], depending on the strength and on the density of the adopted LWAC, differential stiffness may influence significantly the behaviour of the interface. The strength of the binding matrix is usually higher than the respective LWAC strength, due to the reduced strength of LWA. Nevertheless, this is also ignored by codes to predict the interface strength.

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Page 1: Bond Strength of Normal-to-Lightweight Concrete Interfacescristina/RREst/Aulas_Apresentacoes... · 2014-02-24 · Bond Strength of Normal-to-Lightweight Concrete Interfaces Hugo COSTA

Bond Strength of Normal-to-Lightweight Concrete Interfaces

Hugo COSTA Research Assistant Polytechnic Institute of Coimbra Coimbra, Portugal [email protected] Hugo Costa, born 1977, civil engineering degree (2002), MSc degree (2008) at Univ. of Coimbra. PhD student at Univ. of Coimbra, since 2008. His research area is related to mixture design and structural behaviour of lightweight aggregate concrete.

Pedro SANTOS Adjunct Professor Polytechnic Institute of Leiria Leiria, Portugal [email protected] Pedro Santos, born 1977, civil engineering degree (2001), MSc degree (2005), PhD degree (2009) at University of Coimbra. His research interests include pre-casting and strengthening and repair of RC structures.

Eduardo JÚLIO Full Professor ICIST & DECivil, IST-UTL Lisbon, Portugal [email protected] E. Júlio is a full professor at the Instituto Superior Técnico of the Technical University of Lisbon. His research interests are in the field of Structural Concrete. He is member of IABSE WC 3, fib Commission 5 and ACI 364 Committee.

Summary Structural lightweight aggregate concrete (LWAC) can be used with advantage in new structures, especially in pre-cast components, as well as in the rehabilitation of existing structures. In both situations, the result is a composite structural element, with parts in LWAC and others in normal weight concrete (NWC). Besides material properties, it is also important to characterize the behaviour of LWAC-to-NWC interfaces. With this aim, an experimental study was conducted to characterize the shear strength and the tensile strength of NWC-to-NWC, LWAC-to-NWC and LWAC-to-LWAC interfaces and to compare results with codes’ predictions. A NWC and three different LWAC were considered. Six different types of surface roughness were produced on the concrete substrate and characterized with roughness parameters assessed using the 2D-LRA method, developed by the authors. Slant shear and splitting tests were adopted to quantify the bond strength of the interface in shear and in tension, respectively.

Keywords: lightweight aggregate concrete (LWAC); interface; bond strength; roughness parameters; codes.

1. Introduction Structural LWAC presents low density and high performance in terms of both strength and durability. Therefore, this is a competitive material not only for new structures, whenever reduction in dead-weight is aimed, e.g. for the pre-casting industry, but also for the rehabilitation of existing structures. All these situations can give rise to composite structural members, consisting of parts in LWAC and parts in NWC, casted at different ages. In these cases, it is important to assess not only the material properties but also the behaviour of the LWAC-to-NWC interface.

The design expressions proposed by codes for concrete structures to predict the shear strength of concrete-to-concrete interfaces are all based on the shear-friction theory [1] and depend mainly on four parameters: (i) compressive strength of the weakest concrete; (ii) normal stress at the interface; (iii) shear reinforcement crossing the interface; and (iv) roughness of the substrate surface. Santos [2] characterized the influence of other parameters on the behaviour of NWC-to-NWC interfaces, namely the differential shrinkage and Young’s modulus, that are not considered by current design codes, particularly by Eurocode 2 (EC2) [3]. Differential shrinkage may not be a relevant parameter in LWAC-to-NWC interfaces (in the case LWAC is the added layer), due to improved internal curing provided by the moisture in the interior of the lightweight aggregates (LWA) leading to reduced shrinkage [4]. However, since the Young’s modulus of LWAC can be quite different from that of NWC [5], depending on the strength and on the density of the adopted LWAC, differential stiffness may influence significantly the behaviour of the interface. The strength of the binding matrix is usually higher than the respective LWAC strength, due to the reduced strength of LWA. Nevertheless, this is also ignored by codes to predict the interface strength.

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In order to characterize the shear and tensile strength of NWC-to-NWC, NWC-to-LWAC and LWAC-to-LWAC interfaces, an experimental study was conducted to evaluate the influence of substrate roughness, differential stiffness and LWAC strengths of both LWA and binding matrix. First, the following concrete mixtures were designed, produced and characterized: a single NWC mixture, with a compressive strength of 50 MPa; and three LWAC mixtures, with densities and compressive strengths ranging between 1.5 and 1.9 and 45 and 75 MPa, respectively. Then, six different texture conditions were considered for the substrate surface: (i) smooth surface (SS); (ii) surface treated with steel wire-brush (WB); (iii) surface left free after vibration (SF); (iv) surface treated by shot-blasting (SB); (v) surface prepared by hand-scrubbing/raking (HS); (vi) surface treated by chemical deactivation (CD). These were all characterized using roughness parameters assessed with the 2D - laser roughness analyser (2D-LRA) method, developed by the authors [1]. Slant shear and splitting tests were adopted to quantify the bond strength of the interface, in shear and in tension, respectively.

2. Concrete mixture design and production The design method used for NWC and LWAC mixtures is based on the methodology initially proposed by Lourenco et al. [6] and further developed by Costa et al. [7], which includes the following steps: (i) prediction of the binding paste strength, through the Feret’s expression; (ii) adjustment of the mixture’s curve to the Faury’s reference curve and definition of the wanted density; (iii) computation of the LWAC strength, considering the strength reduction taking into account the types and dosages of lightweight aggregates.

2.1 Materials

In the study herein described, the following constituents were adopted for the binding paste: cement CEM II-A/L 42.5 R; fly ash addition (in the LWAC mixtures); third generation superplasticizer, polycarboxylates based; water.

The following types of aggregates were used to produce the LWAC used in the study: (i) three types of coarse lightweight expanded clay aggregates (Leca) - structural Leca 2/4 mm (HD2/4), structural Leca 4/12 mm (HD4/12) and structural Leca 4/10 mm (MD); (ii) two types of siliceous fine aggregates - fine sand 0/2 mm (FS) and medium sand 0/4 mm (MS); and (iii) one type of fine Leca sand 0.5/3 mm (XS). In relation to the NWC mixture, besides FS and MS, crushed limestone 6/12 mm (CL) and gravel 4/8 mm (Gr) were also used.

Granulometric analysis was performed for all aggregates and the following properties were characterized for Leca aggregates (Table 1): dry particle density, ρP0; dry bulk density, π0; interior moisture, HP; absorption from natural state, AN; total absorption until saturated state, AS; and crushing strength, fCr.

2.2 Produced concrete

Different binding pastes were defined for the NWC and for the LWAC (Table 2), with different dosages of cement (C), fly ash (FA) and admixture. Thus, these binding paste matrices resulted in different values of the concrete compactness and water / binder (W/B) mass ratio. Consequently, the NWC and the LWAC mixtures have different predicted values for the strength of the binding paste matrix, fbp,p, 50 and 90 MPa, respectively.

In the LWAC mixtures, the granulometric adjustment was performed and combined with the selection and pre-blending of fine and of coarse aggregates, aiming to achieve the desired densities, ρ, and compressive strengths, fc,p, between 45 and 76 MPa.

Table 1: Characterized properties of Leca aggregates.

Type of LWA

ρP0 (kg/dm3)

π0 (kg/dm3)

HP (%)

AS (%)

AN (%)

fCr (MPa)

HD4/12 1,37 0,76 3,0 9,5 5,6 11,7

HD2/4 1,34 0,74 9,5 11,9 1,5 11,0

MD 0,89 0,49 3,1 11,0 7,5 4,0

XS 1,02 0,58 4,2 10,2 5,1 5,3

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Table 2: Concrete references, aggregates arrangement and mixtures parameters.

Concrete reference

Fine aggregate

Coarse aggregate

C

(kg/m3) FA

(kg/m3) W/B fbp,p

(MPa) fc,p

(MPa) ρ

(kg/m3)

N2.4 FS+MS CL+Gr 340 0 0,50 50 50 2350 H1.9 FS+MS HD4/12+HD2/4 420 84 0,27 90 76 1900 H1.7 FS+XS HD4/12+HD2/4 420 84 0,27 90 67 1700 H1.5 FS+XS MD+HD2/4 420 84 0,27 90 45 1500

Typical failure surfaces of the adopted concrete are presented in Fig. 1, being possible to identify the type and distribution of the aggregates in the concrete matrix.

a) N2.4 b) H1.9 c) H1.7 d) H1.5

Fig. 1: Failure surface of the produced concrete.

2.3 Mechanical properties

For all concrete mixtures, the average value of compressive strength (fcm) was characterized in cubic specimens of 150 mm edge [8], at 7, 28 and 90 days of age. The results are presented in Fig. 2, together with the corresponding hardening curve proposed by EC2 [3]. The following properties were also characterized at 28 days of age (Table 3): i) average tensile strength (fctm), assessed with splitting tests using standard cylindrical specimens [8]; ii) average Young’s modulus [9] (Ecm), determined on prismatic specimens of 150×150×600 mm3. For each situation, three specimens were tested.

Table 3: Measured values, at 28 days, of

tensile strength and of Young’s modulus.

Concrete reference

fctm (MPa)

Ecm (GPa)

N2.4 3,89 33,4

H1.9 4,82 27,1

H1.7 3,70 20,8 0

10

20

30

40

50

60

70

80

90

0 7 14 21 28 35 42 49 56 63 70 77 84 91

N2.4H1.9H1.7H1.5EC2

Age (days)

f cm

(MP

a)

H1.5 2,55 14,6

Fig. 2: Evolution of fcm with age.

3. Interface strength

3.1 Surface roughness

The bond strength of the interface was determined in shear, with slant shear tests, and in tension, with splitting tests, at 28 days of age of the added layer. For this purpose, two types of concrete substrate, H1.5 and N2.4, were used. On the specimens with concrete substrate N2.4, four types of concrete (N2.4, H1.9, H1.7 and H1.5) were added and five different roughness conditions of the substrate surface (Fig. 3) were adopted: SS, WB, SB, HS and CD. For specimens produced with concrete substrate H1.5, two types of concrete were added (H1.7 and H1.9) and three different roughness conditions of the substrate surface (Fig. 3) were considered: SS, SF and CD. For each situation, three specimens were tested. These were cured in water at 20 ºC.

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The EC2 [3] proposes a qualitative assessment of the roughness of the substrate surface, only based on a visual inspection. This leads to a classification in four classes: (1) very smooth, (2) smooth, (3) rough and (4) indented. This qualitative approach is subjective since it depends on the inspector’s judgment. Santos et al. [10] proposed a quantitative classification, and therefore more rigorous, of the roughness of the substrate, using roughness parameters, namely [10]: maximum height of peak (Rp) average height of peak (Rpm) maximum depth of the valley (Rv) average depth of the valley (Rvm). These parameters can be assessed using a specific method and equipment, the 2D- LRA, developed by Santos and Júlio [1].

a) N2.4-SS. b) N2.4-WB. c) N2.4-SB. d) N2.4-HS.

e) N2.4-CD. f) H1.5-SS. g) H1.5-SF. h) H1.5-CD.

Fig. 3: NWC and LWAC substrate surface roughness.

The 2D-LRA method gets the texture profile of the surface and computes the corresponding roughness parameters. These values were obtained for each surface condition considered in the study as the average of ten readings (Table 4).

3.2 Interface shear strength

The expression defined by EC2 [3] to determine the design value of the longitudinal shear strength of the interface between concrete layers cast at different ages, νRdi, is given by:

( ) cdydnctdRdi ffcf νααµρµσν 5,0cossin ≤+++= (1)

being c and µ the cohesion and friction coefficients, respectively. According to EC2, the surfaces of Fig. 3, as well as the corresponding values of c and µ, are shown in Table 5.

Table 4: Roughness parameters of the considered surface conditions.

Parameter N2.4 H1.5 (SS)

N2.4 (WB)

N2.4 (SB)

N2.4 (CD)

N2.4 (HS)

H1.5 (SF)

H1.5 (CD)

Rp (mm) 0,57 1,59 2,15 5,11 8,22 3,36 4,37 Rpm (mm) 0,23 0,86 1,38 4,21 4,29 1,97 3,56 Rv (mm) 2,25 2,56 4,00 4,90 9,54 2,31 3,71 Rvm (mm) 0,63 1,24 2,31 3,56 6,88 1,36 2,83

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The interface shear strength was experimentally assessed with slant shear tests [11], using prismatic specimens of 150×150×450 mm3 with the interface at 30 degrees with the vertical (Fig. 4a). The adopted test rate was 5,0 kN/s.

Table 5: Roughness classification and coefficients, of EC2, for the used surfaces.

Substrate surface

N2.4 H1.5 (SS)

N2.4

(WB)

N2.4

(SB)

N2.4

(CD)

N2.4

(HS)

H1.5

(SF)

H1.5

(CD) EC2

Classification Very

smooth Smooth Rough Rough Rough Smooth Rough

c (EC2) 0,25 0,35 0,45 0,45 0,45 0,35 0,45 µ (EC2) 0,5 0,6 0,7 0,7 0,7 0,6 0,7

Two types of failure were observed: (1) cohesive (monolithic failure of the weakest concrete), Fig. 4.b; and (2) adhesive (debonding of the interface), Fig 4.c. The first one occurred mostly in specimens with higher interface roughness (HS, CD, SB) and the second took place mainly in specimens with lower roughness of the interface (SS, WB, SF).

a) Test specimen. b) Cohesive failure. c) Adhesive failure.

Fig. 4. Slant shear test.

The obtained average values of the shear strength are presented in Fig. 5, for each substrate concrete, each roughness of the interface and each added concrete.

0

5

10

15

20

25

SS WB SB HS CD

N2.4H1.9H1.7H1.5

Interface roughness

Shea

r st

reng

th (

MP

a)

0

5

10

15

20

25

SS SF CD

H1.9

H1.7

Interface roughness

Shea

r st

reng

th (

MP

a)

a) Substrate N2.4 b) Substrate H1.5

Fig. 5: Average values of shear strength of the interface.

It was observed that the shear strength increases with the interface roughness. For N2.4 substrate, the use of LWAC with higher strength of the binding paste matrix results, generally, in higher interface strength when compared to NWC. It was also observed that reducing the density of added LWAC, and hence its strength and Young´s modulus, the shear strength of the interface also decreases.

It was also analyzed the correlation between the roughness parameters and the shear strength of the interface. In Fig. 6, the correlation with Rvm is presented. This adjusts well to a power function,

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although the amplitude depends on the concrete strength. Thus, the coefficients of cohesion and friction, c and µ, can also be estimated using this function [2, 9].

0

5

10

15

20

25

0 2 4 6 8

N2.4

H1.9

H1.7

H1.5

Rvm (mm)

Shea

r st

reng

th (M

Pa)

0

5

10

15

20

25

0 2 4 6 8

H1.9

H1.7

Shea

r st

reng

th (M

Pa)

Rvm (mm)

a) Substrate N2.4 b) Substrate H1.5

Fig.6: Comparison between the interface shear strength and the parameter Rvm.

When comparing the values of the interface shear strength, experimentally obtained, with the corresponding EC2 prediction, νRi, according to (1), for nominal values and considering the coefficients of Table 5, it can be observed that this approach leads to too conservative values. In fact, experimental results are approximately twice the predicted values (Fig. 7).

3.3 Interface tensile strength

The tensile strength of the interface, between concretes cast at different ages, was characterized with splitting tests [7], using cylindrical specimens of 150 mm of diameter and 300 mm of height (Fig. 8.a), tested at a rate of 2,0 kN/s. The occurrence of two distinct failure modes was observed, as mentioned before: cohesive (Fig. 8.b) and adhesive (Fig. 8.c).

The cohesive failure was observed on specimens with N2.4 concrete substrate, with high surface roughness and with H1.5 added concrete. This also occurred on specimens with H1.5 concrete substrate, with high surface roughness and with H1.9 added concrete. Interface failure was observed in the remaining situations.

a) Test specimen. b) Cohesive failure. c) Adhesive failure.

Fig. 8: Splitting test.

0,0

5,0

10,0

15,0

20,0

25,0

0,0 2,5 5,0 7,5 10,0 12,5

SSWBSBSFHSCD

ννννRi (MPa)

Shea

rst

reng

th(M

Pa)

Fig.7: Comparison between the shear

strength and the EC2 prediction, νRi

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In Fig. 9, the average values of tensile strength of the interface, for both types of concrete substrate, are presented, for each roughness condition of the interface surface and for each type of added concrete.

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

SS WB SB HS CD

N2.4

H1.9

H1.7

H1.5

Interface roughness

Ten

sile

str

engt

h (M

Pa)

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

SS SF CD

H1.9

H1.7

Interface roughness

Ten

sile

str

engt

h (M

Pa)

a) Substrate N2.4. b) Substrate H1.5.

Fig.9: Average values of the tensile strength of the interface.

Results revealed an increase of the tensile strength with the interface roughness. On specimens with N2.4 substrate, the use of LWAC with higher strength of the binding paste matrix results on higher tensile strength of the interface, when compared to NWC, similarly to what was observed with the shear strength. Results also show that a density reduction of the added LWAC leads, in general, to a reduction of the tensile strength of the interface.

Comparing the roughness parameters with the tensile strength of the interface, it can be concluded that Rpm presents a better correlation. This can again be adjusted to a power function (Fig. 10), although the amplitude also depends on the concrete strength.

0

1

2

3

4

5

0 2 4 6

N2.4

H1.9

H1.7

H1.5

Rpm (mm)

Ten

sile

str

engt

h (M

Pa)

0

1

2

3

4

5

0 2 4 6

H1.9

H1.7

Ten

sile

str

engt

h (M

Pa)

Rpm (mm)

a) Substrate N2.4. b) Substrate H1.5.

Fig.10: Comparison between the interface tensile strength and the parameter Rpm.

4. Conclusions Both the slant shear and the splitting tests proved to be adequate to evaluate the strength of NWC-to-LWAC interfaces, respectively in shear and in tension.

Two failure modes were observed, cohesive (at the weakest concrete) and adhesive (debonding of the interface). For slant shear tests, the failure mode was mainly monolithic, for very rough surfaces (HS and CD), and mainly adhesive, for very smooth and smooth surfaces (SS and WB). For rough surfaces, SF and SB, both failure modes were observed. For splitting tests, the failure mode was mainly adhesive, excepting for the specimens with a very rough substrate and with significant differences in both density and Young’s modulus between the substrate and the added concrete layers.

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The interface strength, both in shear and in tension, increases with the roughness of the substrate surface. Roughness parameters revealed a good correlation with shear and tensile strengths, adopting power functions. However, it seems that an increase of the substrate roughness above certain values, does not contribute to significantly increase the interface strength.

An added LWAC layer with high strength of the binding matrix leads to higher interface strength, when compared to NWC. On the contrary, the addition of a LWAC layer with lower density, hence with lower strength, leads to a lower strength of the interface.

Finally, it was concluded that the LWAC-to-NWC shear strength, predicted by EC2, is too conservative, since measured values were nearly the double.

Acknowledgements The authors acknowledge the financial support of the Portuguese Science and Technology Foundation (FCT) through the PhD Grant number SFRH/BD/44217/2008. This study was developed under the research project “Intelligent Super Skin - Enhanced Durability for Concrete Members” funded by the Portuguese Science and Technology Foundation (FCT) with reference PTDC/ECM/098497/2008.

References [1] SANTOS P., JÚLIO E., “Development of a laser roughness analyser to predict in situ the

bond strength of concrete-to-concrete interfaces”, Magazine of Concrete Research, Vol. 60, No. 5, pp 329-337, 2008.

[2] SANTOS P., Assessment of the Shear Strength between Concrete Layers. PhD Thesis. University of Coimbra, Portugal, 2009.

[3] EN 1992-1-1, Eurocode 2: Design of concrete structures - Part 1.1: General rules and rules for buildings, CEN, 225 p, 2004.

[4] COSTA H., JÚLIO E., and LOURENÇO J., “Lightweight Aggregate Concrete – codes review and needed corrections”, Codes in Structural Engineering - Developments and Needs for International Practice, IABSE-fib, Dubrovnik, 2010.

[5] COSTA H., Mixture Design and Mechanical Characterization of Lightweight Aggregate Concrete, MSc Thesis, University of Coimbra, Portugal, 2008. (In Portuguese).

[6] LOURENÇO J., JÚLIO E., and MARANHA P., Lightweight Concrete of Expanded Clay Aggregates, APEB, Lisbon, 2004. (In Portuguese).

[7] COSTA H., JÚLIO E., and LOURENÇO J., “A New Mixture Design Method for Structural Lightweight Aggregate Concrete”. 8th fib PhD Symposium, Lyngby, Denmark, 2010.

[8] EN 12390, Testing Hardened Concrete, CEN, 2004.

[9] Specification E 397-1993, Concrete-Testing the Young’s modulus, LNEC, Lisbon, 1993.

[10] SANTOS P.M.D., JÚLIO E.N.B.S., SILVA V.D., "Correlation between concrete-to-concrete bond strength and the roughness of the substrate surface", Construction and Building Materials, Vol. 21(8), pp. 1688-95, 2007.

[11] EN 12615, Products and systems for the protection and repair of concrete structures. Test methods - Determination of slant shear strength, CEN, 1999.