# Self-Consolidated Concrete-to-Conductive Concrete Interface: Assessment of Bond Strength and Mechanical Properties

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## Abstract

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## 1. Introduction and Background

#### 1.1. Surface Preparation and Cleanness

#### 1.2. Differential Shrinkage, Stiffness, and Age

#### 1.3. Fibers and Other Materials Present in the Mix

#### 1.4. Common Bond Strength Tests

## 2. Research Significance

## 3. Experimental Investigation

#### 3.1. Concrete Mixes and Mixing Sequence

#### 3.2. Sample Preparation

#### 3.3. Test Setups and Instrumentation

#### 3.3.1. Compressive, MOE, and Split Tension Tests

#### 3.3.2. Flexural Tests

#### 3.3.3. Slant Shear Tests

#### 3.3.4. Direct Shear Tests

## 4. Results

#### 4.1. Mechanical Properties of CC and SCC

#### 4.2. Concrete-to-Concrete Bond Strength in Compression Test

#### 4.3. Concrete-to-Concrete Bond Strength in Modulus of Elasticity Test

#### 4.4. Concrete-to-Concrete Bond Strength in Flexure Strength Test

#### 4.5. Concrete-to-Concrete Bond Strength in Slant Shear Test

#### 4.6. Failure Modes

#### 4.6.1. Compression and MOE Failure Modes

#### 4.6.2. Flexural Failure Modes

#### 4.6.3. Slant Shear Failure Modes

#### 4.6.4. Split Tension and Direct Shear Failure Modes

## 5. Discussion

#### 5.1. Data Normalization Due to Compressive Strength Variation

#### 5.2. Control Samples

#### 5.2.1. Compressive and MOE Control Samples

#### 5.2.2. Flexure Control Samples

#### 5.2.3. Slant Control Samples

#### 5.2.4. Split Tension

#### 5.2.5. Direct Shear

#### 5.3. Surface Preparation

#### 5.3.1. Surface Preparation Effect on Compression Strength

#### 5.3.2. Surface Preparation Effects on MOE

#### 5.3.3. Surface Preparation Effects on Flexural Strength

#### 5.3.4. Surface Preparation Effects on Slant Shear

#### 5.4. Casting and Testing Directions

#### 5.4.1. Casting and Testing Direction Effects on Compression Strength

#### 5.4.2. Casting and Testing Direction Effects on MOE

#### 5.4.3. Casting and Testing Direction Effects on Flexure Strength

#### 5.4.4. Casting and Testing Direction Effects on Slant Shear

## 6. Comparison of CC and SCC Mechanical Properties with Published Equations

## 7. Conclusions

- The SCC’s compressive strength and stiffness were superior to those of the conductive concrete. The difference in compressive strength was due to the addition of conductive fillers, which replaced parts of the mix. The conductive fillers added to the mix were carbon, graphite, and hybrid steel fibers (HSFs). The compressive strength difference was 19% at 28 days and 13% at 90 days, both in favor of the SCC mix; similarly, the stiffness difference was 35% in favor of the SCC.
- The CC flexural, tensile, and shear strengths were superior to those of the SCC mix. This was due to the presence of HSFs in the mix, which increased the performance. In addition, the HSF allowed the CC mix to be more ductile than the SCC mix, which was observed in the CC samples’ failure modes. The CC’s flexural strength improvement was 4 times that of the SCC’s flexural strength. Similarly, the CC’s tensile and shear strengths were 2 times that of the SCC’s strengths. Moreover, the HSF allowed for better crack growth control and post-peak performance.
- The most suitable surface preparation method to fully utilize the CC mix was the shear key method. The shear key method is based on a groove grid performed initially on a mold, after which the mix is cast into it. However, the rough surface preparation, performed using hand chiseling and a steel wire brush, achieved a similar bond strength to the wet-to-wet surface preparation when the roughness was applied to the SCC’s surface in the slant shear test. However, if the rough surface preparation is applied to the CC mix’s surface, the bond strength would fail. It is important to note that wet-to-wet surface preparation is performed to simulate the maximum bond strength.
- The casting and testing directions played important roles, which could be seen in the flexural strength tests. The most suitable position for conductive concrete is in the tensile region of an element. If the SCC mix is present in the tensile region, it will weaken the sample’s strength and residual capacity. In WNWC, the flexural strength and post-peak performance were similar to those of the CC control. However, in WCWN, the strength dropped significantly, which was due to the SCC’s position. Moreover, the SCC’s presence in the tensile region subjected the sample to a sudden stress due to its initial sudden failure, after which the CC carried the rest of the load.
- An attempt to predict the SCC’s and CC’s mechanical properties based on codes and published equations was made. Most codes and published equations could predict the SCC’s MOE, flexural, and tensile strength. However, care must be taken with CC mixes, since most equations overestimated the MOE and underestimated the tensile and flexural strengths. This issue occurred only due to the presence of conductive fillers, which affected the mechanical properties by either increasing or decreasing performance.

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Disclaimer

## Abbreviations

CC | Conductive concrete |

DN | Dry normal |

DC | Dry conductive |

HSF | Hybrid steel fiber |

LVDT | Linear variable displacement transducer |

M1, M3, M5, M7 | Odd numbers used for CC mixes |

M2, M4, M6, M8 | Even numbers used for SCC mixes |

MOE | Modulus of elasticity |

SCC | Self-consolidated concrete |

SFs | Steel fibers |

SG | Strain gauge |

SK | Shear key |

UHPC | Ultra-high-performance concrete |

UTM | Universal testing machine |

WC | Wet conductive |

WN | Wet normal |

WADB | Wet concrete type A on dry concrete type B |

WAWB | Wet concrete type A on wet concrete type B |

WCWN | Wet conductive concrete over wet normal concrete |

WCDN-S | Wet conductive over dry normal with smooth surface |

WCDN-R | Wet conductive over dry normal with rough surface |

WNDC-SK | Wet normal over dry conductive with shear key |

$A$ | Interface area |

${A}_{s}$ | Expected shearing area |

$b$ | Average width of specimen |

${d}_{0}$ | Cylinder diameter |

$E$ | Modulus of elasticity |

${f}_{r}$ | Flexural strength |

${f}_{s}$ | Shear strength |

${f}_{c}^{\prime}$ | Compressive strength |

$h$ | Specimen depth |

$P$ | Maximum load |

$T$ | Tensile strength |

$\alpha $ | Slant shear angle with respect to the y-axis |

${\epsilon}_{1},{\epsilon}_{2}$ | Longitudinal strain |

${\epsilon}_{{t}_{1}},{\epsilon}_{{t}_{2}}$ | Transverse strain |

${\sigma}_{1},{\sigma}_{2}$ | Stress |

${\sigma}_{n}$ | Normal stress |

${\tau}_{n}$ | Shear stress |

$v$ | Poisson’s ratio |

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**Figure 2.**Configuration used for (

**a**) direct shear specimen, (

**b**) flexural prisms, (

**c**) cylinders, and (

**d**) slant shear prisms.

**Figure 3.**Image summary of applied roughness on concrete bond interfaces. Cylinders are shown in (

**a**,

**b**). Prisms are shown in (

**c**–

**f**).

**Figure 5.**Cylinder experimental tests for (

**a**) compressive strength, (

**b**) modulus of elasticity, and (

**c**) split tension. The strain gauge (SG) locations are shown on the cylinders.

**Figure 6.**(

**a**) Slant shear test preformed on a prism sample in UTM hydraulic machine. (

**b**) Direct shear sketch with dimensions (mm) and strain gauge positioning. (

**c**) Direct shear test setup.

**Figure 7.**Conductive concrete flexural sample results with a visual rep of peak load, L/600, and L/150 position on a load versus deformation graph.

**Figure 10.**(

**a**–

**h**) General failure modes for slant shear samples. (

**i**) Slant shear with SK surface preparation failure modes for (

**i.1**) crushing failure, (

**i.2**) bond failure, and (

**i.3**) combination of both.

**Figure 11.**Split tension failure modes for (

**a**) SCC sample and (

**b**) CC sample. Direct shear failure modes: (

**c**,

**d**) for SCC samples and (

**e**) for CC sample.

**Figure 13.**Flexure (${f}_{r}$) and normalized flexure (${f}_{rn}$) for different concrete mixes and casting types.

**Figure 14.**Shear ($S$) and normalized shear (${S}_{n}$) for different concrete mixes and casting types.

**Figure 16.**Load versus deflection graphs of conductive concrete flexural strength control samples: (

**a**) samples prepared from batches M5 and M7, and (

**b**) samples prepared from batches M1 and M3.

**Figure 17.**Perpendicular stress–strain graph of CC with (

**a**) rough and (

**b**) shear key surface preparations and SCC with (

**c**) rough and (

**d**) shear key surface preparations.

**Figure 18.**Split tension test on (

**a**) four CC samples (S1, S2, S3, S4) and (

**b**) three SCC samples (S1, S2, S3), showing strain gauge (SG) readings of tensile stress vs. horizontal strain.

**Figure 19.**Direct shear test on CC, including (

**a**) load and vertical strain as well as (

**b**) load and horizontal strain graphs, and on SCC, including (

**c**) load and vertical strain as well as (

**d**) load and horizontal strain graphs.

**Figure 21.**Prism results for wet-to-dry casting sequence for (

**a**) WCDN-R, (

**b**) WNDC-R, (

**c**) WCDN-S, and (

**d**) WNDC-S.

**Figure 22.**Perpendicular stress–strain graphs of (

**a**) WCDN, (

**b**) WNDC samples with rough (R1, R2) and smooth (S1, S2, S3) surface preparation, (

**c**) WCDN, and (

**d**) WNDC samples with shear key (SK) surface preparation.

**Figure 24.**Prism results for wet-to-wet casting sequence for (

**a**) WCWN, (

**b**) WNWC from M5, M6 and (

**c**) WCWN, (

**d**) WNWC from M1, M2.

Test | Sample Size (mm) | Testing Code | Sample Prep. | No. of Samples | ${\mathit{f}}_{\mathit{c}}^{\mathbf{\prime}}$ (MPa) |
---|---|---|---|---|---|

Compression | 100 × 200 (S) 150 × 300 (L) | ASTM C39 [42] | Control N, 28D | 2S & 2L | 63.5 |

Control C, 28D | 2S & 2L | 52.5 | |||

Control N, 90D | 1S & 7L | 68.4 | |||

Control C, 90D | 1S & 9L | 60 | |||

WCDN-R, 90D | 4L | 66.5 | |||

WNDC-R, 90D | 3L | 51.3 | |||

WCDN-S, 90D | 1S | 65.1 | |||

WNDC-S, 90D | 2L | 63.8 | |||

WCWN, 90D | 2S & 6L | 66.3 | |||

WNWC, 90D | 1S & 6L | 65.5 | |||

Test | Sample Size (mm) | Testing Code | Sample Prep. | No. of Samples | $\mathit{E}$ (GPa) (v) |

MOE | 150 × 300 | ASTM C39 [42] (Loading) ASTM C469 [43] (Calculations) | Control N, 90D | 4L | 35.1 (0.28) |

Control C, 90D | 6L | 24.7 (0.24) | |||

WCWN, 90D | 3L | 25.5 (0.24) | |||

WNWC, 90D | 3L | 23.9 (0.16) | |||

WCDN-R, 90D | 2L | 30.0 (0.27) | |||

WNDC-R *, 90D | - | - | |||

WCDN-S *, 90D | - | - | |||

WNDC-S *, 90D | - | - | |||

Test | Sample Size (mm) | Testing Code | Sample Prep. | No. of Samples | ${\mathit{f}}_{\mathit{r}}$ (MPa) |

Flexure | 100 × 100 × 500 | ASTM C1609 [44] | WCWN, 28D | 5 | 4.7 |

Including steel fiber | WNWC, 28D | 4 | 12.3 | ||

WCDN-S, 28D | 2 | 2.9 | |||

WCDN-R, 28D | 3 | 3.8 | |||

WNDC-S, 28D | 3 | 7.8 | |||

WNDC-R, 28D | 3 | 13.1 | |||

ASTM C78 [45] | Control C, 28D | 9 full | 15.0 | ||

Excluding steel fiber | Control N, 28D | 12 full | 3.8 | ||

Test | Sample Size (mm) | Testing Code | Sample Prep. | No. of Samples | Bond Shear (MPa) |

Slant Shear | 100 × 100 × 500 Figure 2a | BS EN 12615:1999 [46] loading rate + inclination angle | WCWN, 90D | 3 | 27.2 |

WCDN-S, 90D | 3 | 13.3 | |||

WCDN-R, 90D | 2 | 28.1 | |||

WNDC-S, 90D | 2 | 5.2 | |||

WNDC-R, 90D | 2 | 10.8 | |||

WCDC-R, 90D | 3 full | 15.0 | |||

WNDN-R, 90D | 3 full | 27.1 | |||

WCDC-SK, 28D | 3 full | 19.6 | |||

WNDN-SK, 28D | 3 full | 14.4 | |||

WCDN-SK, 28D | 3 | 16.5 | |||

WNDC-SK, 28D | 3 | 17.7 | |||

Test | Sample Size (mm) | Testing Code | Sample Prep. | No. of Samples | $\mathit{T}$ (MPa) |

Split Tension | 150 × 300 | ASTM C496/C496M-17 [47] | Control N, 28D | 4 | 3.8 |

Control C, 28D | 4 | 7.9 | |||

Test | Sample Size (mm) | Testing Code | Sample Prep. | No. of Samples | Shear Strength (MPa) |

Direct Shear | Figure 2a | Loading based on BS 1881 [48] | Control N, 28D | 3 | 8.4 |

Control C, 28D | 5 | 19.9 |

Sample Preparation | Avg. L/600 (MPa) | Avg. L/150 (MPa) |
---|---|---|

WCWN, 28D | 3.9 | 3.9 |

WNWC, 28D | 11.3 | 6.8 |

WCDN-S, 28D | 2.6 | 2.8 |

WCDN-R, 28D | 2.7 | 3.2 |

WNDC-S, 28D | 6.3 | 4.3 |

WNDC-R, 28D | 11.5 | 8.1 |

Control C, 28D | 14.4 | 11.0 |

Control N, 28D | - | - |

Sample | Failure Criteria * | Avg. Shear Stress (MPa) |
---|---|---|

WCDC-SK-1 | b | 19.6 |

WCDC-SK-2 | a | 19.6 |

WCDC-SK-3 | a | 19.6 |

WNDN-SK-1 | c | 14.4 |

WNDN-SK-2 | c | 14.4 |

WNDN-SK-3 | a | 14.4 |

WCDN-SK-1 | b | 16.5 |

WCDN-SK-2 | a | 16.5 |

WCDN-SK-3 | b | 16.5 |

WNDC-SK-1 | a | 17.7 |

WNDC-SK-2 | a | 17.7 |

WNDC-SK-3 | c | 17.7 |

Test | Equation No. | ACI-318 (MPa) | ACI-363 (MPa) | FIB 2010 (MPa) | Eurocode 2 |
---|---|---|---|---|---|

MOE | (8) | $0.043{w}_{c}^{1.5}\sqrt{{f}_{c}^{\prime}}$ | $320\sqrt{{f}_{c}^{\prime}}+6900;21<{f}_{c}^{\prime}<83$ | $E=25800{\left({f}_{cm}/10\right)}^{1/2}$; basalt, dense limestone aggr. | ${E}_{cm}=9.5{\left({f}_{ck}+8\right)}^{1/3}$ |

(9) | $4700\sqrt{{f}_{c}^{\prime}}$ | $3.385\times {10}^{-5}{w}_{c}^{2.5}{{f}_{c}^{\prime}}^{0.325};{f}_{c}^{\prime}<84$ | $E=21500{\left({f}_{cm}/10\right)}^{1/2}$; quartzite aggr. | - | |

(10) | - | $14495+2176\sqrt{{f}_{c}^{\prime}}$ | $E=19400{\left({f}_{cm}/10\right)}^{1/2}$; limestone aggr. | - | |

(11) | - | $9500{{f}_{c}^{\prime}}^{0.3};25<{f}_{c}^{\prime}<85$ | $E=15100{\left({f}_{cm}/10\right)}^{1/2}$; limestone aggr. | - | |

(12) | - | $3.385\times {10}^{-5}\times {w}_{c}^{2.55}{{f}_{c}^{\prime}}^{0.315}$ | - | - | |

Flexure | (13) | 0.62${f}_{c}^{\prime}$ | $0.94\sqrt{{f}_{c}^{\prime}};21{f}_{c}^{\prime}83$ | - | - |

(14) | - | $0.25{{f}_{c}^{\prime}}^{0.79}$; moist, steam cured | - | - | |

Split Tension | (15) | $0.56{{f}_{c}^{\prime}}^{0.5}$ | $0.59{{f}_{c}^{\prime}}^{0.5};21<{f}_{c}^{\prime}<83$ | ${f}_{ctm}=0.3{\left({f}_{ck}\right)}^{2/3};{f}_{c}^{\prime}\le 50$ | ${f}_{ctm}=0.3{\left({f}_{ck}\right)}^{2/3}$ |

(16) | - | $0.32{{f}_{c}^{\prime}}^{0.63}$ | ${f}_{ctm}=2.12\mathrm{ln}\left(1+0.1\left({f}_{ck}+8\right)\right);{f}_{c}^{\prime}>50$ | - |

Test | Equation No. | Equation | Ref. |
---|---|---|---|

MOE | (17) | ${E}_{c}=3360\sqrt{{f}_{ck}^{\prime}};{f}_{ck}^{\prime}\le 200\mathrm{M}\mathrm{P}\mathrm{a}$ | [64] |

(18) | ${E}_{c}=4700\times 0.5\left(1+{0.7}^{{V}_{f}}\right)\sqrt{{f}_{c}^{\prime}}$ | [65] | |

(19) | ${E}_{c}=4.2\sqrt{{f}_{cy}^{\prime}};30\mathrm{M}\mathrm{P}\mathrm{a}{f}_{cy}^{\prime}75\mathrm{M}\mathrm{P}\mathrm{a}$ | [66] | |

Flexure | (20) | ${f}_{ct}=0.42\left(0.6+{V}_{f}\right)\sqrt{{f}_{c}^{\prime}}$ | [64] |

(21) | ${f}_{ft}=0.39{f}_{cs}^{0.59}$ | [67] | |

(22) | ${f}_{rf}=0.259{f}_{cf}^{0.843}$ | [68] | |

(23) | ${f}_{r}=0.42{{f}_{c}^{\prime}}^{0.68};5\mathrm{M}\mathrm{P}\mathrm{a}{f}_{c}^{\prime}120\mathrm{M}\mathrm{P}\mathrm{a}$ | [69] | |

(24) | ${f}_{fl}=0.79\sqrt{{f}_{cy}^{\prime}};30\mathrm{M}\mathrm{P}\mathrm{a}{f}_{cy}^{\prime}75\mathrm{M}\mathrm{P}\mathrm{a}$ | [66] | |

Split Tension | (25) | ${f}_{spt}=0.21{f}_{cs}^{0.83}$ | [67] |

(26) | ${f}_{spf}=0.188{f}_{cf}^{0.84}$ | [68] | |

(27) | ${f}_{sp}=0.47{{f}_{c}^{\prime}}^{0.56};5\mathrm{M}\mathrm{P}\mathrm{a}{f}_{c}^{\prime}120\mathrm{M}\mathrm{P}\mathrm{a}$ | [69] | |

(28) | ${f}_{spc}=0.57\sqrt{{f}_{cy}^{\prime}};30\mathrm{M}\mathrm{P}\mathrm{a}{f}_{cy}^{\prime}75\mathrm{M}\mathrm{P}\mathrm{a}$ | [66] |

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## Share and Cite

**MDPI and ACS Style**

EL-Afandi, M.; Yehia, S.; Landolsi, T.; Qaddoumi, N.; Elchalakani, M.
Self-Consolidated Concrete-to-Conductive Concrete Interface: Assessment of Bond Strength and Mechanical Properties. *Fibers* **2023**, *11*, 106.
https://doi.org/10.3390/fib11120106

**AMA Style**

EL-Afandi M, Yehia S, Landolsi T, Qaddoumi N, Elchalakani M.
Self-Consolidated Concrete-to-Conductive Concrete Interface: Assessment of Bond Strength and Mechanical Properties. *Fibers*. 2023; 11(12):106.
https://doi.org/10.3390/fib11120106

**Chicago/Turabian Style**

EL-Afandi, Mohammed, Sherif Yehia, Taha Landolsi, Nasser Qaddoumi, and Mohamed Elchalakani.
2023. "Self-Consolidated Concrete-to-Conductive Concrete Interface: Assessment of Bond Strength and Mechanical Properties" *Fibers* 11, no. 12: 106.
https://doi.org/10.3390/fib11120106