Eindhoven University of Technology MASTER Behavior of unreinforced concrete-to-concrete interfaces under shear loading Croes, L.

MASTER

Behavior of unreinforced concrete-to-concrete interfaces under shear loading

Croes, L.

Award date:

2019

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Behavior of unreinforced concrete-to-concrete interfaces under shear loading

Graduation research report

Report number A_2019.273

Name L. (Lars) Croes Student-ID no. 0776939

Date 14-05-2019

Supervisors Prof. ir. S.N.M. (Simon) Wijte ir. G.G.A. (Gerrie) Dieteren dr. ir. A.T. (Ad) Vermeltfoort

TU/e TNO TU/e

Eindhoven University of Technology Department of the Built Environment Architecture, Building and Planning Structural design

A BSTRACT

The aim of this research is to re-evaluate which material parameters influence the adhesion shear transfer in an unreinforced concrete-to-concrete interface. The cause of this research is the partial collapse of the parking garage at Eindhoven Airport on the 27^th of May 2017. This partial collapse was caused by the use of a two way spanning composite concrete floor, consisting of precast floor plates and a cast-in-situ compression layer, resulting in a positive moment near the precast floor plate edges. Shear stress capacity of the concrete-to-concrete interface was found to be too low.

At the same time as the four point flexural tests of composite concrete floor systems [13], direct shear tests under compression perpendicular to the interface were performed. Eight test series, consisting of a total of 48 specimens were loaded in compression perpendicular to the interface, then loaded in shear until failure.

Specimens were tested at varying levels of compression perpendicular to the interface, to be able to construct the linear part in the Mohr-Coulomb failure envelope for these interfaces. The found averaged shear strength in the absence of normal loading, 𝜏0 , ranged from 0,57 𝑁/𝑚𝑚² to 1,87 𝑁/𝑚𝑚², the found slope coefficient ranged from 0,50 to 2,94.

The test series were fabricated using different concrete types and precast floor plate surface treatments. Concrete strength was measured by means of cube compressive testing, and interface roughness quantification was performed to document interface roughness parameters.

A correlation analysis was performed on the test results and the obtained material parameters, the relationships described by codes and previous research were investigated.

Test results show a small correlation with both the fib Model Code 2010 and the Eurocode 2

interface shear resistance expressions. The fib Model Code 2010 expression showed a slightly larger correlation with the test results. However, this does not mean the code expressions are suitable to describe the test specimen behavior.

The individual components of these expressions do not show correlation with the found Mohr- Coulomb failure envelope components. The coefficients of cohesion proposed in de codes do not show correlation with the test results, whilst the coefficients of friction proposed in codes show a large negative correlation with the test results. This suggests the proposed coefficients of friction are not suitable to describe the slope of the Mohr-Coulomb failure envelope of the tested unreinforced concrete-to-concrete interfaces.

The influence of differential elasticity moduli between the two concretes was found to have a large negative correlation with the interface shear resistance, suggesting that a large difference in modulus of elasticity between the two concretes may lower interface shear resistance.

No correlation between precast floor plate surface roughness and interface shear resistance, up until adhesive failure, was found.

The code expressions for the shear resistance of an unreinforced concrete-to-concrete interface were compared to the test result 5% characteristic values. This demonstrated that the Eurocode 2 and fib Model Code 2010 expressions are conservative and are sufficiently safe.

T ABLE OF CONTENTS

Abstract ... 2

1 Introduction ... 5

1.1 Background and relevance ... 5

1.2 Research goal ... 6

1.3 Contents ... 6

2 Literature exploration ... 7

2.1 Shear resistance of unreinforced concrete-to-concrete interfaces in codes ... 7

2.1.1 Eurocode 2, EC2, 2011 ... 7

2.1.2 Fédération Internationale du Béton, FIB Model Code, 2010 ... 7

2.1.3 Conclusions ... 10

2.2 Bond strength tests in literature ... 11

2.2.1 Bending test bond analysis ... 11

2.2.2 Splitting tests ... 11

2.2.3 Direct shear tests ... 11

2.2.4 Torsional shear tests... 13

2.2.5 Pull-off tests ... 13

2.2.6 Conclusions ... 14

2.3 Surface roughness quantification in literature ... 15

2.3.1 Surface roughness parameters ... 15

2.3.2 Surface roughness quantification methods ... 16

2.3.3 Conclusions ... 17

2.4 Experimental research review ... 19

2.4.1 Conclusions ... 21

3 Experimental research into interface shear behavior ... 22

3.1 Introduction ... 22

3.2 Test methods and specimens ... 22

3.2.1 Experimental setup... 22

3.2.2 Specimen properties ... 24

3.2.3 Roughness quantification method ... 25

3.3 Experimental results ... 27

3.3.1 Direct shear test under compression ... 27

3.3.2 Roughness quantification ... 32

3.3.3 Concrete strength ... 32

4 Analysis ... 34

4.1 Analysis of shear experiment data ... 34

4.2 Analysis of roughness quantification data ... 35

4.3 Numerical modeling ... 37

4.4 Correlation analysis of experimental results ... 40

5 Conclusions and recommendations ... 43

5.1 Conclusions ... 43

5.2 Recommendations... 45

References: ... 46

Appendices ... 48

Appendix 1 - Specimen properties ... 48

Appendix 2 – Measurement data ... 56

Appendix 3 – Experimental results ... 65

Appendix 4 – Specimen roughness ... 69

Appendix 5 – Correlation matrix ... 71

Appendix 6 – Testing procedure ... 74

Appendix 7 – Eccentricities ... 76

Appendix 8 – Friction shear transfer in pre-cracked situation ... 78

Appendix 9 – Test result comparison with codes... 80

1 I NTRODUCTION

1.1 B

On the 27^th of May 2017, partial collapse of the parking garage at Eindhoven Airport occurred. To find the cause of this partial collapse research was done by two parties, Hageman B.V. and TNO.

The report on the research done by Hageman B.V. ([1] Braam, C.R., Ensink, L.T.H., Meester, J.J., de Vos, W.A.P. and Wijte, S.N.M., 2017)into the cause of the partial collapse of the parking garage at Eindhoven Airport concludes that the moment capacity of the composite concrete floor structure was insufficient. The decrease in moment capacity was caused by the two way spanning of the precast floor plate system that resulted in a positive bending moment on the plate edge. This detail is reliant on the transfer of tensile forces from the precast floor plate to the coupling reinforcement in the traditional concrete (TC) compression layer. This force transfer causes shear stress in the interface between the precast floor plate and the compression layer. Due to the smooth surface finish of the precast self-compacting concrete (SCC) floor plate, experimental research concluded that the average shear strength of the interface varied between 0,45N/mm² and 0,49N/mm². This lower than expected value resulted in a different normative failure mechanism, namely the brittle failure of the interface between the precast SSC floor plate and the compression layer. The extra internal stresses caused by the high temperature on the day in question was the trigger for the partial collapse.

The report on the research done by TNO ([2] Borsje, H. and Dieteren, G.G.A., 2017) into the cause of the partial collapse of the parking garage at Eindhoven Airport also concludes that the partial

collapse was almost certainly caused by a reduced capacity of the interface between the precast SCC and the V causing a reduced moment capacity of the composite concrete floor.

Relevance

The weakness found in the two way spanning composite concrete floor system is not limited to the floor system used, other composite floor systems consisting of a precast floor plate and a VC top layer are also subject to this potentially weak detail. Following the partial collapse the Dutch government, in cooperation with Hageman B.V., released a safety information document to help determine if similar structures built from 1999 to 2017 are subject to this same weakness ([3] Wijte, S.N.M. and Meester, J.J., 2017). If the shear stress at the interface between the precast floor plate and the in-situ concrete exceeds 0,4N/mm², detailed research has to be done and the structure might have to be modified to guarantee safety. The characteristic shear resistance of the interface was taken directly from the research into the partial collapse of the parking at Eindhoven Airport.

Further experimental research into the shear resistance of the concrete-to-concrete interface will gain further knowledge concerning composite concrete floor systems. The goal of this research is to explore which material parameters influence the shear behavior of an unreinforced concrete-to- concrete interface.

1.2 R

In this research, the influence of material parameters on the shear behavior of an unreinforced concrete-to-concrete interface is investigated. The current theories on interface shear resistance are evaluated and compared to the shear behavior of specimens that is found in experimental tests.

The research goal was formulated as:

To quantify the material parameters, that influence shear resistance of an unreinforced concrete- to-concrete interface, and analyze their relationships with the shear behavior of this interface.

The following sub goals are formulated in accordance with the research goal:

- To determine the parameters that influence the shear strength of an unreinforced concrete- to-concrete

- To analyze existing theories that describe shear behavior of an unreinforced concrete-to- concrete interface and describe the relationship between this behavior to the material parameters.

- To determine test methods to investigate the shear behavior of an unreinforced concrete-to- concrete interface and its material parameters.

- To determine and develop non-destructive techniques to quantify the interface roughness parameters.

- To analyze the correlation between the found material parameters and shear behavior of the test specimens.

1.3 C

In this report, experimental research into the shear resistance of a unreinforced concrete-to- concrete interface is summarized. First, expressions of concrete-to-concrete shear resistance in codes are explored. Secondly, experimental studies into the shear resistance of concrete-to-concrete interfaces, roughness quantification methods, and different types of shear tests are explored. Then, previous research into this subject will be summarized. Then, the performed experimental research will be summarized. Lastly, the performed experimental research results are analyzed, and by means of statistical analysis, the relationship between material parameters and the shear behavior of a concrete-to-concrete interface is explored.

2 L ITERATURE EXPLORATION

2.1 S

-

-

The Dutch and European code on precast concrete products and floor plates for floor systems, ([4]

NEN-EN 13747+A2, 2011) states: “The design shear stress at the interface should satisfy 6.2.5 of EN 1992-1-1:2004” ([4], para. D.1 General).

([5] NEN-EN 1992-1-1+C2, 2011) art. 6.2.5 describes the shear resistance of an unreinforced concrete-to-concrete interface as follows:

𝑣_𝑅𝑑𝑖 = 𝑐 ∙ 𝑓_𝑐𝑡𝑑 + 𝜇 ∙ 𝜎_𝑛 ≤ 0,5 𝜈 ∙ 𝑓_𝑐𝑑 Eq. 2.1 Where:

𝑐 is the cohesion factor of the interface

𝑓_𝑐𝑡𝑑 is the design tensile strength of the weakest concrete 𝜇 is the shear friction coefficient of the interface

𝜎_𝑛 is the normal force present perpendicular to the interface ν is a strength reduction factor

𝑓_𝑐𝑑 is the design compressive strength of the weakest concrete

Eurocode 2 (2011) defines the two coefficients, 𝑐 and 𝜇, for different types of surfaces. Four types of surfaces are considered; very smooth, smooth, rough, and profiled.

Table 2.1: Design expressions for interface parameters proposed by EC2, 2011

Surface type cohesion factor (𝒄) shear friction coefficient (𝝁)

Very smooth 0,025 ≤ 𝑐 ≤ 0,10 𝜇 = 0,5

Smooth 𝑐 = 0,20 𝜇 = 0,6

Rough 𝑐 = 0,40 𝜇 = 0,7

Profiled 𝑐 = 0,50 𝜇 = 0,9

The very smooth surface is described as a surface cast against a formwork made of steel, plastic or wood that is specially prepared. The smooth surface is described as being formed by a sliding formwork, formed by extrusion, or a free surface untreated after vibration. The rough surface is described as a surface with a roughness elements of at least 3mm with a spacing of about 40mm. The profiled surface requires cast indentations described in [5] fig. 6.9.

2.1.2 Fédération Internationale du Béton, FIB Model Code, 2010

The FIB Model Code 2010 describes the individual mechanisms that contribute to the shear

resistance of an unreinforced concrete-to-concrete interfaces, namely ([6] fédération internationale du béton, 2010):

- Mechanical interlocking and adhesive bonding

- Friction due to external compression forces perpendicular to the interface

The main parameters decisive for shear resistance of an unreinforced concrete-to-concrete interface that the FIB Model Code 2010 describes are:

- Interface Roughness - Cleanliness of surface

- Concrete strength and concrete quality - Eccentricity of shear force

- State of bond (pre-cracked or not) 2.1.2.1 Interface roughness characteristics

[6] Chapter 6.3.2 describes two interface roughness parameters, the average roughness deviation (Ra) and the mean peak-to-valley height (Rz(DIN)). The average roughness deviation is the most commonly used parameter and represents the average profile deviation from the mean line. While the average roughness deviation is an average along a measurement interval, maximum peak-to- valley height is averaged within a certain number of assessment lengths. [6] formulates the Ra and Rz(DIN) as follows:

𝑅_𝑎= 1

𝑙_𝑚∙ ∫ 𝑦(𝑥) ∙ 𝑑𝑥

𝑙_𝑚

0

≈1 𝑛∑ 𝑦_𝑖

𝑛

𝑖−1

Eq. 2.2

𝑅_{𝑧(𝐷𝐼𝑁)}= 1 𝑚∙ ∑ 𝑧_𝑖

𝑚

𝑖−1

Eq. 2.3

Where:

𝑙_𝑚 is the measurement interval

𝑦_𝑖 is the subsequent height between peaks and valleys 𝑛 is the amount of amplitudes

𝑚 is the amount of assessment lengths (usually one fifth of the measurement interval) 𝑧_𝑖 is the maximum peak to valley height

[6] describes four categories of roughness:

Table 2.2: Roughness category expressions described in FIB Model Code 2010

Category Ra (mm)

Very smooth (e.g. cast against steel formwork) Not measurable Smooth (e.g. untreated, cast against wooden formwork) <1,5 mm Rough (e.g. sand blasted, high pressure water blasted etc.) ≥1,5 mm Very rough (e.g. high pressure water jetting, indented) ≥3 mm

Santos ([8] Santos, P.M.D., 2009) illustrates the average roughness parameter Ra (fig 2.1) and the mean peak-to-valley height Rz(DIN) (fig 2.2).

Figure 2.1: Average roughness deviation of a 2D profile ([8] Fig. III.22)

Figure 2.2: Mean peak-to-valley height of a 2D profile ([8] Fig. III.25) 2.1.2.2 Adhesion

FIB Model Code 2010 describes the interface adhesion (𝜏𝑐) as being reliant on a multitude of parameters. Adhesion only plays part in the interface shear resistance if slip is minimal and the interface is not fully pre-cracked. If the surface roughness is high, a mechanical interlocking effect may occur. This effect decreases with an increase of slip. Note that for smooth surfaces, the adhesion equals zero. The adhesive strength of an interface [7] can be given by:

𝜏_𝑐 = 0,09 ∙ 𝑘_𝑐∙ 𝑓_𝑐^{1 3⁄ Eq. 2.4}

Where:

𝑘_𝑐 is a coefficient for interface roughness, dependent on Ra 𝑓_𝑐 is the cylinder compressive strength of concrete

2.1.2.3 Friction

The interface friction is dependent on compression forces perpendicular to the interface, these compression forces are caused by an external compression force, or clamping forces due to

(activated) reinforcement crossing the interface. [6] Refers to the same friction coefficient 𝜇 as EC2.

[6] Formulates the friction mechanism of the interface shear resistance as:

𝜏𝑓 = 𝜇 ∙ 𝜎𝑛 Eq. 2.5

Where:

𝜇 is the friction coefficient of the interface surface

𝜎_𝑛 is the external compression force perpendicular to the interface 2.1.2.4 Total shear resistance

FIB Model Code 2010 [6] describes the ultimate shear resistance of an unreinforced concrete-to- concrete interface, combining the two mechanisms, as:

𝜏_𝑢= 0,09 ∙ 𝑘_𝑐∙ 𝑓_𝑐^{1 3⁄} + 𝜇 ∙ 𝜎_𝑛≤ 𝛽 ∙ 𝑣 ∙ 𝑓_𝑐 ^{Eq. 2.6} Where:

𝑘_𝑐 is a coefficient for interface roughness, dependent on Ra 𝑓_𝑐 is the cylinder compressive strength of concrete

𝜇 is the friction coefficient of the interface surface

𝜎_𝑛 is the external compression force perpendicular to the interface 𝛽 is a coefficient related to concrete compression struts forming

𝑣 is a reduction factor for the strength of the concrete compression struts

In table 2.3 some collected values for coefficients for the FIB Model Code 2010 design expression (Eq.

3.7) are given [6][7].

Table 2.3: Design expressions for interface parameters proposed by FIB Model Code 2010

Surface preparation Ra kc µ β

fck ≥ 20 MPa fck ≥ 35 MPa

High-pressure water-blasting ≥0.5 2.3 0.8 1.1 0.5

Sand-blasting ≥0.5 0.0 0.7 0.7 0.4

Smooth n/a 0.0 0.5 0.5 0.4

2.1.3 Conclusions

Eurocode 2 (2011) as well as FIB Model Code 2010, describe the shear resistance of an unreinforced concrete-to-concrete interface as a linear function with a constant (depending on a cohesion parameter and the concrete strength), one independent variable (stress perpendicular to the interface), and a slope dependent on a friction parameter (dependent on the interface surface preparation).

To be able to compare these code predictions to experimental research, these three components of the linear function are to be expressed by the experimental research. To be able to re-evaluate the influence of interface surface preparation, a surface roughness quantification method is needed.

Different expressions to describe the shear resistance of a concrete-to-concrete interface are proposed, these propositions are based on experimental research. Shear tests, material tests and roughness quantification methods are required to be able to assess the influence of the material parameters on the shear behavior of a concrete-to-concrete interface.

2.2 B

Santos ([8] Santos, P.M.D., 2009) performs a very complete review of the materials and methods used in a multitude of researches to test the strength of concrete-to-concrete interfaces in shear.

Experiments to test the shear strength of concrete-to-concrete interfaces can be classified in different categories, namely; bending tests, splitting tests, direct shear tests, push-off tests, direct shear tests under compression, torsional shear tests, and pull-off tests. Santos illustrates these types of experiments.

2.2.1 Bending test bond analysis

Different configurations of bending tests have been proposed, [8] notes the complex stress

distribution on the interface plane. Proposed bending tests consist of a three- or four-point bending test, and a variety of material configurations. The stresses perpendicular to the interface is not constant across the interface.

Figure 2.3: Bending test ([8] Fig. V.2) 2.2.2 Splitting tests

Splitting tests are defined in standards to quantify the (tensile) bond strength between two

materials. Wedge splitting tests are an alternative type of splitting test. These tests are also used to measure the splitting tensile strength of materials.

Figure 2.4: Splitting test ([8] Fig. V.18) 2.2.3 Direct shear tests

Direct shear tests consist of two or more concrete elements that are sheared off of each other.

Experimental setups consisting of a symmetrical specimen aim to simplify the stress distribution on the interface plane. Combining a symmetrical direct shear test with external compression

perpendicular to the interface results in a direct shear test under compression. An example of these tests is the triplet-test used in masonry (NEN-EN 1052-3:2002).

Figure 2.5: Direct shear test ([8] Fig. V.9)

Push-off tests, or L-shaped tests, (Fig. 2.6) are an often used variant of the direct shear test. L-shaped tests also minimize the flexural stress on the interface plane. In research by ([9] Bennenk, H.W., 2006), ([10] Hordijk, D.A., Bennenk, H.W., de Boer, S., 2005) and ([11] Lundgren, K., 2007) this test setup is used. However, Lundgren notes that flexural stresses are still present and notices cracking on the outside of the L-shape due to these stresses, when specimens are unreinforced.

Figure 2.6: L-shaped direct shear test ([8] Fig. V.16)

Another often used direct shear test under compression is the slant shear test (Fig. 2.7). The stress perpendicular to the interface is dependent on the geometry of the concrete below the interface, and is not constant across the interface. This test setup is used by Santos in [8]. The stress

perpendicular to the interface increases during the experiment, and cannot be kept constant. The ratio between the shear stress and stress perpendicular to the interface is determined by the angle of the slanted surface.

Figure 2.7: Slant shear test ([8] Fig. V.17) 2.2.4 Torsional shear tests

Torsional shear tests are a relatively simple test to achieve pure (torsional) shear within an interface.

However, production involves the drilling of a core into the specimen, which may influence the specimen. Performing a torsional shear test under compression perpendicular to the interface is very hard.

Figure 2.8: Torsional shear test ([8] Fig. V.11) 2.2.5 Pull-off tests

Pull-off tests are widely used to evaluate the bond strength of concrete repairs (NEN-EN 1542:1999).

However, production involves the drilling of a core into the specimen, which may influence the specimen.

Figure 2.9: Pull-off test ([8] Fig. V.15)

2.2.6 Conclusions

To re-evaluate the linear functions described in ([5] NEN-EN 1992-1-1+C2, 2011) and ([6] fédération internationale du béton, 2010), control over the stresses perpendicular to the interface, separately from the induced shear stress, is required. A suitable test method is the direct shear test with external compression perpendicular to the interface.

2.3 S

The FIB Model Code 2010 ([6] fédération internationale du béton, 2010) describes two roughness parameters that can be derived from a 2D roughness profile. To better characterize the roughness of a surface, Santos ([8] III.4.2.) describes other roughness parameters, that describe other

characteristics of a 2D roughness profile.

2.3.1 Surface roughness parameters

The surface roughness parameters Ra and Rz(DIN), as described in eq. 2.2 and eq. 2.3 do not fully characterize a 2D roughness profile. Fig. 2.10 illustrates different roughness profiles with the same roughness parameter Ra.

Figure 2.10: Different surfaces with same average roughness deviation ([8] Fig. III.23) To characterize differences in roughness profiles, Santos illustrates other roughness parameters defined by standards associations.

2.3.1.1 Mean Peak Height and Mean Valley Depth

The Mean Peak Height (𝑅_𝑝𝑚) and the Mean Valley Depth (𝑅_𝑣𝑚) are given by:

𝑅_𝑝𝑚= 1

𝑚∙ ∑ 𝑝_𝑖

𝑚

𝑖−1

Eq. 2.7

𝑅_𝑣𝑚= 1

𝑚∙ ∑ 𝑣_𝑖

𝑚

𝑖−1

Eq. 2.8

Where:

𝑚 is the amount of assessment lengths (usually one fifth of the measurement interval) 𝑝_𝑖 is the maximum peak height

𝑣_𝑖 is the maximum valley depth

Santos [8] illustrates the 𝑅𝑝𝑚 and 𝑅𝑣𝑚 (fig 2.11).

Figure 2.11: Mean Peak Height and Mean Valley Depth ([8] Fig. III.24)

2.3.1.2 Ten Points Height

The International Organization for Standardization (ISO) proposes the roughness parameter Ten Points Height (𝑅_{𝑍(𝐼𝑆𝑂)}), that is the average of the summation of the five highest peaks and the five deepest valleys within a measurement interval, and is given by:

𝑅_{𝑍(𝐼𝑆𝑂)}=1

5(∑ 𝑝_𝑖

5

𝑖−1

+ ∑ 𝑣_𝑖

5

𝑖−1

)

Eq. 2.9

2.3.1.3 Other roughness parameters

Other proposed roughness parameters are the Maximum Peak Height within a measurement interval (𝑅_𝑝), the Maximum Valley Depth within a measurement interval (𝑅_𝑣), the Maximum Peak-to-Valley Height within one assessment length (𝑅_𝑚𝑎𝑥), and the Maximum Peak-to-Valley Height within one measurement interval (𝑅𝑦). The Root-Mean-Square Roughness (𝑅𝑞) is comparable to 𝑅𝑎, but squares the peak height/valley depth to provide a higher sensitivity to roughness.

2.3.2 Surface roughness quantification methods

Currently, Eurocode 2 (2011) proposes a qualitative visual inspection of a concrete surface to classify the surface roughness. However, a qualitative approach is subjective. A quantitative method to characterize surface roughness is required for this research. Quantitative methods such as the Sand Patch Method (NEN-EN 1766:2017) are described, but are limited in ability to represent the actual roughness profile of a surface. Roughness quantification methods that yield a roughness profile that can be interpreted using the described roughness parameters will be discussed. Thus, indirect roughness quantification methods like the Sand Patch Method, and methods such as microscopy, that study the microscopic characteristics of a surface instead of the macroscopic roughness, will be omitted.

2.3.2.1 2D roughness quantification methods

Santos [8] developed a method to analyze the surface roughness by cutting the specimen perpendicular to the surface to be measured and photographing the resulting surface. This photograph is then processed digitally to acquire a 2D roughness profile (fig 2.12). This method however, is destructive.

Figure 2.12: TDI method ([8] Fig.III.15 d,e)

Nondestructive 2D roughness quantification methods include probing with a mechanical stylus, and laser roughness analysis. Both of these methods consist of measuring the surface height in the Y- direction whilst moving the probe in the X-direction. Probe shape, or laser beam width, does influence the resulting 2D roughness profile, as illustrated in fig. 2.13.

Figure 2.13: Probe shape effect ([8] Fig.III.8)

The suitable probe shape depends on the minimum valley width and maximum slope angle.

However, a 2D roughness profile acquired by profilometry is always an approximation of the actual surface profile.

If using an interval measurement technique instead of continuous measurements, the sampling interval is determined by the surface roughness characteristics of the specimen. Poon ([12] Poon, C.Y., Bhushan, B. , 1995) describes the effects and selection of a suitable sampling interval. When decreasing the sampling interval, the accuracy of the profile approximation increases (fig 2.14). The correlation between two described roughness profiles increases with the number of sampling points per measurement interval, and will increase towards a perfect correlation of 1.

Figure 2.14: Sampling interval effect ([12] Fig.15)

2.3.2.2 3D roughness quantification methods

3D roughness quantification methods mostly consist of scanners that use a striping laser to scan a surface and create a 3D point cloud. This point cloud is an approximation of the 3D relief of the scanned surface. Previously discussed roughness parameters can be derived from this data.

2.3.2.3 Indirect surface roughness quantification methods

Indirect surface roughness quantification methods such as the Sand Patch Method (NEN-EN 1766:2017) classify a surface roughness based on the physical interaction with a measuring device.

Other examples of indirect surface roughness quantification methods are testing by means of an outflow meter, or the air leakage method [7].

2.3.3 Conclusions

A multitude of roughness parameters have been proposed by standards organizations. Which

roughness parameters are most suitable, is a topic of discussion. The most widely adopted roughness parameters are the Mean Peak-to-Valley Height 𝑅𝑧(𝐷𝐼𝑁) and the Maximum Peak-to-Valley height 𝑅_𝑚𝑎𝑥. To derive these roughness parameters, a 2D roughness profile is required.

To describe a 2D surface roughness profile, from which surface roughness parameters can be derived, most used options are mechanical stylus profilometry and laser roughness analysis.

When using mechanical stylus profilometry, suitable stylus shape, sampling interval, assessment length, and amount of measurement intervals is dependent on the surface to be measured.

2.4 E

In ([17] Santos, P.M.D., Júlio, E.N.B.S., 2010), the author summarizes expressions that are milestones in interface shear resistance research. These expressions are based on performed experimental research.

Table 2.4: Milestones in interface shear resistance [17]

Researcher(s) Year Design expression

Birkeland and Birkeland [18] 1966 𝜈_𝑢= 𝜇 ∙ 𝜌 ∙ 𝑓_𝑦

Mattock [19] 1972 𝜈_𝑢= 1,38 + 𝑐(𝜎_𝑛+ 𝜌𝑓_𝑦)

Loov [20] 1978

𝜈_𝑢 = 𝑘√𝑓𝑐(𝜎_𝑛+ 𝜌𝑓_𝑦) Walraven et al. [21] 1987 𝜈_𝑢= 0,882 ∙ 𝑓_𝑐^0,406(𝜌𝑓_𝑦)^{0,159∙𝑓𝑐0,303}

Randl [15] 1997

𝜈_𝑢 = 0,09 ∙ 𝑘_𝑐∙ 𝑓_𝑐^{1 3⁄} + 𝜇(𝜌 ∙ 𝑘 ∙ 𝑓_𝑦+ 𝜎_𝑛) + 𝛼 ∙ 𝜌√𝑓𝑦∙ 𝑓_𝑐

≤ 𝛽 ∙ 𝑣 ∙ 𝑓_𝑐

In ([15] Randl, N., Wicke, M., 2000), Randl describes performed direct shear testing, where new concrete is cast on top of a concrete surface. These surfaces have a roughness ranging from sand blasted to smoothed, the connection is reinforced with dowels. The test setup used is illustrated in fig. 2.15.

Figure 2.15: Direct shear test setup used by Randl ([15] fig. 4)

Randl describes the interface shear transfer, after initial cohesive failure, as having three

components; aggregate interlock, friction and dowel action. And proposes an expression that forms the basis of the FIB Model Code 2010 expression.

In ([16] Santos, P.M.D., Júlio, N.B.S., Silva, V.D., 2007), slant shear tests are performed together with pull-off tests and roughness quantification by means of cross section digital imaging. Roughness parameters are derived from the obtained roughness profiles. Shear stress and tensile stress capacity is are compared with these roughness profiles, and correlation analysis is performed. A large

coefficient of correlation is found, suggesting shear- and tensile stress capacity of an unreinforced concrete-to-concrete interface is highly dependent on surface roughness.

In ([10] Hordijk, D.A., Bennenk, H.W., de Boer, S., 2005), the authors investigate the shear strength of concrete-to-concrete interfaces when SCC is used. A L-shaped direct shear test with reinforcement crossing the interface is used. Five different types of surface treatment are investigated; as cast,

raked, surface film removal with pressurized air, using profiled formwork, and aggregate exposure by use of surface retarder. The authors conclude that roughening by means of raking or surface film removal with pressurized air is most effective. Based on this research, a corrigendum to the EC2 coefficients of cohesion and friction is proposed.

In ([8] Santos, P.M.D., 2009), the author performs more slant shear tests as well as splitting tests. The interface tensile strength and interface shear resistance under compression perpendicular to the interface are determined in testing. The Mohr-Coulomb failure envelope is reconstructed from these test results (fig. 2.16). Surface roughness of the specimens is quantified using 2D Laser Roughness Analysis (2D-LRA). The resulting roughness profile is interpreted using a multitude of roughness parameters. The correlation between these roughness parameters and the shear resistance of a concrete-to-concrete interface is studied. The author proposes to base the cohesion and friction coefficients found in EC2 on the quantified Mean Valley Depth of the primary profile and suggests expressions fitted to the experimental results.

Figure 2.17: Concrete- and interface failure envelope ([8] fig. VI.16)

In ([22] Randl, N., 2013), the author describes the individual shear transfer components found in the fib MC 2010 design approach. Load displacement curves of performed experiments are displayed, comparing unreinforced and reinforced specimens with different interface roughness (fig. 2.17).

Figure 2.17: Load-displacement curves of interface shear failure ([22] fig. 9)

In ([23] Zilch, K., Reinecke, R., 2000), the author describes the influence of the individual shear transfer components during the loading of a reinforced concrete-to-concrete interface. The author notes that adhesion and shear friction contribute immediately, whilst the reinforcement gets activated after interface debonding and loss of adhesive shear transfer.

Figure 2.18: Shear transfer mechanism contribution over time [23]

2.4.1 Conclusions

Different design expressions for concrete-to-concrete interface shear transfer are proposed. These expressions often consist of components describing different shear transfer mechanisms. In

unreinforced concrete-to-concrete interfaces, the adhesion- and shear friction transfer mechanisms are present. Adhesion shear transfer is most often described as being dependent on concrete

strength and surface roughness, whilst the shear friction mechanism is described as being dependent on surface roughness and compression perpendicular to the interface. By performing a number of shear experiments, with varying levels of compression perpendicular to the interface, part of the Mohr coulomb failure envelope of the interface can be constructed.

Figure 2.19: Mohr-Coulomb interface failure envelope

3 E XPERIMENTAL RESEARCH INTO INTERFACE SHEAR BEHAVIOR

3.1 I

During the investigation into the cause of the partial collapse of the parking garage at Eindhoven Airport, experiments have been performed to investigate the behavior of concrete composite concrete floors subject to a positive moment at the seam between two precast floor plates ([13]

Wijte, S.N.M., 2019). At the same time as the production of these specimens, smaller, unreinforced specimens have been produced with the same materials and methods. These specimens consist of a multitude of configurations where the variables are; concrete type of the precast floor plate (self- compacting concrete or traditional concrete [SCC or TC]), the concrete strength, and the precast floor plate surface treatment. The goal of the research is to gain further knowledge into the behavior of an unreinforced concrete-to-concrete interface when loaded in shear, and compression perpendicular to the interface. These experiments have been performed in the same period, as far as possible, as the experiments in [13].

The shear test method used is a direct shear test under compression, motivations for the chosen test setup were; the requirement for independent control over compression perpendicular to the

interface and interface shear loading, the elimination of flexural stresses, and assuring the test specimens are produced similarly to situations in practice (identical method- and direction of casting).

Concurrent with the shear specimen precast floor plate production, larger plates have been produced, these plates are produced with identical materials and methods. These precast floor plates are used to analyze the surface roughness characteristics and are representative for that test specimen series.

To quantify the surface roughness of these precast floor plates, interval mechanical probing is used.

This choice was motivated by accuracy comparison between available roughness quantification methods. Other roughness quantification methods, such as the Sand Patch Method (NEN-EN 1766:2017), have been tested, but were not found suitable for smooth surfaces. A Shadow Profile Photogrammetry method was also developed, but was found too sensitive to light conditions.

3.2 T

The experimental setup of the direct shear tests under compression are described in [14] and illustrated in fig. 3.1. The specimens are produced on their side, the precast floor plate is cast, the compression layer is cast one week later.

The precast floor plate dimensions are 300 mm long and 250 mm wide, thickness varies between 70 mm and 90 mm. The thickness of the compression layer is 200 mm.

One experimental series, consisting of one material configuration, six to nine specimens are tested.

One experimental series consists of two to three different levels of compression perpendicular to the interface (𝜎𝑛). Per level of compression, two to three specimens are loaded in shear until failure. The following displacements are measured:

v1 displacement difference between the precast floor plate and the compression layer in the vertical direction (on both sides of the specimen)

v11 and v12 displacement difference between the precast floor plate and the compression layer in the horizontal direction (on both sides of the specimen)

The following loads are measured:

Load1 Load introduced in the specimen Load2 Load in the precast floor plate support 𝜎_𝑛 Load perpendicular to the interface

Figure 3.1: Direct shear test under compression ([14] Fig.2)

Load1 is spread evenly over the two load introduction points by use of a ball hinge, the load

introduction points consist of steel strips with dimensions: 15mm wide and 250mm long, 95mm o.c.

Where the compression layer is loaded directly adjacent to the interface. Load one will increase until failure of the specimen.

The supports of the experimental setup consist of steel strips with dimensions: 15mm wide and 250mm long, 125mm on center. Where the precast floor plate is supported directly adjacent to the interface. Each support is suspended by two pendulum rods, these pendulum rods inhibit confining forces. Through strain gauges the load in each pendulum rod is documented.

The load perpendicular to the interface, is realized using an enclosure around the specimen, which applies a load that is kept constant during the experiment. In each of the horizontal rods of this enclosure, strain gauges document the loads. Between the horizontal load introduction plates and the specimen, fiberboard is placed to prevent point loads.

The test setup is further illustrated in fig. 3.2.

a) b)

Figure 3.2: a) test setup 3D schematization, b) test setup in practice 3.2.2 Specimen properties

To research the influence of the variables; concrete type and surface treatment, a varied arrangement of specimen series is required (table 3.1). Further specimen properties, including dimensions and images of the surface type, can be found in Appendix 1 – Specimen properties.

During production SCC was not vibrated, whilst traditional concrete was vibrated. The test series are produced by a multitude of different manufacturers, production process of the test specimens was identical to the regular production process of the manufacturers.

Table 3.1: Specimen properties per series Series # precast floor plate

concrete type

Surface treatment Number of specimens 1 Self-compacting concrete Untreated, as cast 9

2 Self-compacting concrete Lightly smoothed 6

3 Traditional concrete Untreated, as cast 6

4 Traditional concrete Untreated, as cast 6

5 Traditional concrete Roughened, raked 3

6 Self-compacting concrete Extremely smoothed 6

7 Self-compacting concrete Roughened, raked 6

8 Traditional concrete Roughened, raked 6

For each experimental series, a number of shear specimens are produced. Concurrently, cubes are produced to test the cube compressive strength of both the precast floor plate and the compression layer, and a larger precast floor plate (600x600 mm) is produced with the same surface

characteristics as the experimental series. These cubes and precast floor plates are used to quantify the material parameters.

3.2.3 Roughness quantification method

To construct a 2D roughness profile that is representative for each series, interval mechanical probing is used. To determine the suitable sampling interval (distance between two samplings), the sampling interval is decreased until correlation between the resulting roughness profiles nears its limit of 1. The suitable sampling interval was be determined to be at least 1 sample per millimeter, but a sampling interval of 2 samples per millimeter is used (one measurement every 0,5 mm).

Figure 3.3: Influence of sampling interval on accuracy of roughness profile portrayal To determine the suitable assessment length, a variety of assessment lengths are studied. Five assessment lengths make up one measurement interval. The suitable assessment length is studied for three different surface types. The assessment length, while using a sampling interval of one measurement per 0,5 mm, is increased until a maximum is found in the corresponding roughness parameter. The suitable assessment length is determined to be at least 50 mm. This is a large assessment length compared to assessment lengths used in literature. The assessment length that was found suitable, 50 mm, is used. With one measurement per 0,5 mm, resulting in 100

measurements per assessment length.

Figure 3.4: Influence of assessment length on roughness parameter Ra 0

0,2 0,4 0,6 0,8 1 1,2

0 0,5 1 1,5 2 2,5

Correlation with original

Amount of samples per 1 mm

0 0,2 0,4 0,6 0,8 1 1,2

0 100 200 300

Average Roughness deviation (Ra)(mm)

Assessment length (mm)

SCC roughened TC roughened SCC untreated

To determine the amount of measurement intervals required to correctly assess the roughness of a given surface, a study is performed to investigate the accuracy of the portrayal of the roughness distribution. Every measurement interval, of length 250 mm, consists of five assessment lengths of 50 mm. To determine the suitable amount of measurement intervals, this amount is increased until the distribution of roughness parameters does not change anymore. The suitable amount of measurement intervals is determined to be at least three.

Figure 3.5: Influence of measurement interval number on distribution of average roughness deviation Ra To conclude, interval mechanical probing is used to assess surface roughness. In total, three

measurement intervals is distributed over the surface to be measured. One measurement interval in the x-direction, one measurement interval in the y-direction, and one measurement interval in the xy-direction. For specimens with a directional surface treatment, the measurement intervals is taken in the intended direction. Each measurement interval consists of five assessment lengths, 50 mm long, the total length of a measurement interval will thus be 250 mm. A depth measurement is taken every 0,5 mm. To probe the surface, a linear gage is mounted into a CNC machine, the CNC machine is then programmed to probe the surface to be measured every 0,5 mm (fig. 3.6). Probing tip dimensions have been chosen as; h = 2,7 mm, r = 1 mm, α = 20°.

Figure 3.6: interval mechanical probing

0 0,1 0,2 0,3 0,4 0,5 0,6

Average roughness deviation Ra (mm)

1 interval 2 Intervals 3 Intervals 4 Intervals 5 Intervals 6 Intervals

3.3 E

3.3.1 Direct shear test under compression

During the direct shear test, the compression stress perpendicular to the interface (𝜎𝑛) is held at a constant level. Using a displacement controlled method, Load1 is increased until failure of the concrete-to-concrete interface occurs, as seen in fig. 3.7. Load1 increases quite linearly until sudden interface failure occurs. Plots of the measurements are displayed (series 3). The measured

displacements and loads of all experimental test series, as described in fig. 3.1, are displayed in Appendix 2 – Measurement data.

Figure 3.7: Load1 during tests

This introduced load, is transferred over the concrete-to-concrete interface, Load2 is the precast floor plate support reaction (fig. 3.8). At failure, no significant difference between the two support reactions was found, the ratio between Load1 and Load2 stabilizes towards the expected ratio during the test. The ratio between Load1 and Load2 during tests is plotted in fig. 3.9. The averaged interface shear stress, Load2 divided by interface area, is plotted in fig. 3.10.

Figure 3.8: Load2 during tests 0

50 100 150 200 250 300

1 101 201 301 401 501 601 701 801

Load1 (kN)

Time (s)

Specimen 1, σn = 0 Specimen 2, σn = 0 Specimen 3, σn = 0.2 Specimen 4, σn = 0.2 Specimen 5, σn = 0.4 Specimen 6, σn = 0.4

0 50 100 150 200 250 300

1 101 201 301 401 501 601 701 801

Load2 (kN)

Time (s)

Specimen 1, σn = 0 Specimen 2, σn = 0 Specimen 3, σn = 0.2 Specimen 4, σn = 0.2 Specimen 5, σn = 0.4 Specimen 6, σn = 0.4

Figure 3.9: Load1 to Load2 ratio during tests

Figure 3.10: Load2 divided by interface area, during tests

During the test, the compression perpendicular to the interface is held at a constant level. At the end of the test, a miniscule increase in 𝜎𝑛 is seen (fig. 3.11).

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8

1 101 201 301 401 501 601 701 801

Load2/Load1

Time (s)

Specimen 1, σn = 0 Specimen 2, σn = 0 Specimen 3, σn = 0.2 Specimen 4, σn = 0.2 Specimen 5, σn = 0.4 Specimen 6, σn = 0.4

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2

1 101 201 301 401 501 601 701 801 Average interface shear stress (N/mm2)

Time (s)

Specimen 1, σn = 0 Specimen 2, σn = 0 Specimen 3, σn = 0.2 Specimen 4, σn = 0.2 Specimen 5, σn = 0.4 Specimen 6, σn = 0.4

Figure 3.11: Interface normal stresses perpendicular to the interface, during tests

During all the tests, displacements were minimal, maximum horizontal displacements found are in the 0,06 mm range. Similarly, vertical displacements were also minimal, maximum vertical

displacements found are in the 0,05 mm range. The horizontal and vertical displacements do

however increase exponentially until failure. Displacements v1, v11 and v12 are plotted in fig. 3.12 to fig. 3.14.

Figure 3.12: v1 - Vertical displacement difference between precast floor plate and compression layer

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45

1 101 201 301 401 501 601 701 801

𝜎𝑛(N/mm2)

Time (s)

Specimen 1, σn = 0 Specimen 2, σn = 0 Specimen 3, σn = 0.2 Specimen 4, σn = 0.2 Specimen 5, σn = 0.4 Specimen 6, σn = 0.4

-0,01 0 0,01 0,02 0,03 0,04 0,05 0,06

0 50 100 150

v1 -Vertical deformation (mm)

Load2 - Precast floor plate support reaction (kN)

Specimen 1, σn = 0 Specimen 2, σn = 0 Specimen 3, σn = 0.2 Specimen 4, σn = 0.2 Specimen 5, σn = 0.4 Specimen 6, σn = 0.4

Figure 3.13: v11 - Horizontal displacement between precast floor plate and compression layer at the top

Figure 3.14: v12 - Horizontal displacement between precast floor plate and compression layer at the bottom The linear part of the interface failure envelope can be constructed using the average shear stress in the interface at failure, and the compressive stress perpendicular to the interface at failure. A linear regression can be used to determine 𝜏₀ and the slope coefficient.

Figure 3.15: Construction of the linear part of the concrete-to-concrete interface failure envelope This linear part of the concrete-to-concrete failure envelope can be constructed for every

experimental series. These test results are plotted in fig. 3.16, Plots of the average shear stress and compressive stress perpendicular to the interface at failure, and the linear regressions can be found

-0,01 0 0,01 0,02 0,03 0,04 0,05 0,06

0 50 100 150

V11 -Horizontal deformation top (mm)

Load2 - Precast floor plate support reaction (kN)

Specimen 1, σn = 0 Specimen 2, σn = 0 Specimen 3, σn = 0.2 Specimen 4, σn = 0.2 Specimen 5, σn = 0.4 Specimen 6, σn = 0.4

-0,01 0 0,01 0,02 0,03 0,04 0,05 0,06

0 50 100 150

V12 -Horizontal deformation bottom (mm)

Load2 - Precast floor plate support reaction (kN)

Specimen 1, σn = 0 Specimen 2, σn = 0 Specimen 3, σn = 0.2 Specimen 4, σn = 0.2 Specimen 5, σn = 0.4 Specimen 6, σn = 0.4

y = 0,5011x + 1,4981

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8

0 0,1 0,2 0,3 0,4 0,5

τ(N/mm2)

σ_n(N/mm²)

in Appendix 3 – Experimental results. The parameters of the found regressions can be found in table 6.1.

Figure 3.16: Test results with linear regressions Table 3.2: Parameters of linear regressions in fig. 6.10

Series 𝝉_𝟎 (𝑵/𝒎𝒎^𝟐) Slope coefficient

Series 1 – SCC untreated 1,85 0,85

Series 2 – SCC lightly smoothed 1,31 2,94

Series 3 – TC untreated 1,50 0,50

Series 4 – TC untreated 0,64 2,16

Series 5 – TC roughened, raked 1,14 0,66

Series 6 – SCC extremely smoothed 0,57 2,25

Series 7 – SCC roughened, raked 1,87 1,32

Series 8 – TC roughened, raked 1,26 1,70

During testing, one specimen in series 6 appeared to be damaged during transport, this specimen is omitted from the research. One specimen in series 8 experienced minor spalling of the precast floor plate edge near the support, this precast floor plate side was deformed during production. No other local failure of the concrete was observed.

A description of the testing procedure described in Appendix 6 – Testing procedure, further analysis of the eccentricities present in the test setup is described in Appendix 7 – Eccentricities during testing. Experiments were performed where test specimens were re-tested after failure, to analyze the friction shear transfer in a situation where no interface adhesion is present. These experiments are described in Appendix 8 – Friction shear transfer in pre-cracked situation.

0 0,5 1 1,5 2 2,5 3 3,5

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7

τ(N/mm2)

σ_n(N/mm²)

Series 1 - SCC untreated Series 2 - SCC lightly smoothed Series 3 - TC untreated Series 4 - TC untreated Series 5 - TC roughened, raked Series 6 - SCC extremely smoothed Series 7 - SCC roughened, raked Series 8 - TC roughened, raked

3.3.2 Roughness quantification

Roughness quantification was performed using interval mechanical probing, consisting of three measurement intervals, each 250 mm in length, with one measurement every 0,5 mm. The resulting 2D roughness profile is an approximation of the measured surface. One of the resulting 2D roughness profiles is illustrated in fig. 3.17. The 2D roughness profile describes the height of every

measurement point in respect to the mean of the profile, and distance to measurement interval starting point. The global slope is corrected to ensure the measured surface is parallel to the CNC x- axis. From the resulting 2D roughness profiles, roughness parameters and their distributions can be derived.

Figure 3.17: 2D roughness profile (series 3, measurement path 1) 3.3.3 Concrete strength

The cube compressive strength of the concretes used was determined at time of testing. For every series, 3 to 9 cubes were tested for both the precast floor plate, and the compression layer.

Table 3.3: Average cube compressive strengths

Series Number of

cubes per concrete

Precast floor plate Compression layer 𝒇𝒄𝒌,𝒄𝒖𝒃𝒆

(𝑵/𝒎𝒎^𝟐)

St.Dev.

(𝑵/𝒎𝒎^𝟐)

𝒇𝒄𝒌,𝒄𝒖𝒃𝒆 (𝑵/𝒎𝒎^𝟐)

St.Dev.

(𝑵/𝒎𝒎^𝟐)

Series 1 – SCC untreated 3 86,20 1,96 44,30 1,03

Series 2 – SCC lightly smoothed 9 56,29 5,49 59,85 2,58

Series 3 – TC untreated 9 40,00 3,74 17,50 0,71

Series 4 – TC untreated 6 70,22 1,15 44,91 2,34

Series 5 – TC roughened, raked 9 56,67 1,79 42,13 2,80

Series 6 – SCC extremely smoothed 9 88,20 1,87 42,86 3,52

Series 7 – SCC roughened, raked 9 93,80 2,99 37,59 1,31

Series 8 – TC roughened, raked 9 66,21 0,63 40,23 3,74

-0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1

0 25 50 75 100 125 150 175 200 225

Position Y-axis (mm)

Position x-axis (mm)

Figure 3.18: Average cube compressive strength of precast floor plate for every series

Figure 3.19: Average cube compressive strength of compression layer for every series 0

20 40 60 80 100

120 Series 1 - SCC untreated

Series 2 - SCC lightly smoothed Series 3 - TC untreated Series 4 - TC untreated Series 5 - TC roughened, raked Series 6 - SCC extremely smoothed Series 7 - SCC roughened, raked Series 8 - TC roughened, raked N/mm2

0 20 40 60

80 Series 1 - SCC untreated

Series 2 - SCC lightly smoothed Series 3 - TC untreated Series 4 - TC untreated Series 5 - TC roughened, raked Series 6 - SCC extremely smoothed Series 7 - SCC roughened, raked Series 8 - TC roughened, raked N/mm2

4 A NALYSIS

4.1 A

To further interpret the test results plotted in fig. 3.16, test series can be grouped with series that are alike in concrete type and roughness (fig 4.1). Test results and linear regression plots of the separate specimen groupings can be found in Appendix 3 – Experimental results.

Figure 4.1: Test results grouped by specimen characteristics

Assuming the average cube compressive strength of the weakest concrete as the interface compressive stress, the concrete-to-concrete interface Mohr-Coulomb failure envelope can be constructed for these experimental results. However, this resulting failure envelope is based on a series average, and is neither a lower nor upper bound. This failure envelope should only be used for inter-series comparison.

Figure 4.2: Construction of the Mohr-Coulomb failure envelope for the concrete-to-concrete interface 0

0,5 1 1,5 2 2,5 3 3,5

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7

τ(N/mm2)

σ_n(N/mm²)

SCC very smooth SCC untreated SCC raked TC untreated TC raked

Table 4.1: Parameters of Mohr-Coulomb failure envelopes constructed for every grouping

Experimentally found Derived

Specimen group 𝝉_𝟎

(𝑵/𝒎𝒎^𝟐)

Slope coeff.

𝒇_{𝒄𝒌,𝒊} (𝑵/𝒎𝒎^𝟐)

𝒇_{𝒄𝒕𝒌,𝒊} (𝑵/𝒎𝒎^𝟐)

𝝉_𝒎𝒂𝒙 (𝑵/𝒎𝒎^𝟐)

𝝈_𝒎𝒂𝒙 (𝑵/𝒎𝒎^𝟐)

SCC very smooth 0,57 2,25 42,86 0,24 20,59 22,28

SCC untreated 1,63 1,66 44,30 0,91 20,88 23,40

SCC raked 1,87 1,32 37,59 1,26 17,30 20,29

TC untreated 1,08 1,31 17,50 0,73 8,21 9,39

TC raked 1,10 1,88 40,23 0,55 19,13 21,10

4.2 A

From the obtained 2D roughness profiles, roughness parameters and their distributions can be derived. Since there is no consensus on which roughness parameter is most suitable, a multitude of parameters are derived and will be analyzed. The distribution of one of the roughness parameters, Ra, is illustrated in fig. 4.3. The distributions of other roughness parameters can be found in

Appendix 4 – specimen roughness, and are presented in table 4.2. No reference precast floor plates are available for series 5 and 6, however series 5 is produced similarly to series 8, and series 5 was smoothed until the surface was perfectly flat.

Figure 4.3: Distribution of average roughness deviation (Ra) 0

1 2 3 4 5 6 7 8 9

0 0,5 1 1,5 2

Average roughness deviation Ra (mm)

Series 1 - SCC untreated Series 2 - SCC lightly smoothed Series 3 - TC untreated Series 4 - TC untreated Series 7 - SCC roughened, raked Series 8 - TC roughened, raked