5.1 C
ONCLUSIONS Interface shear resistanceThe shear resistance of an unreinforced concrete-to-concrete interface is currently attributed to the concrete strength of the weakest concrete, the compressive stress perpendicular to the interface, and coefficients of cohesion and friction. In an uncracked unreinforced concrete-to-concrete interface, adhesion shear transfer occurs.
These coefficients proposed by EC2 and fib MC 2010 are based on previous experimental results. EC2 relies on qualitative visual inspection, whilst fib MC 2010 proposes these coefficients to be based on roughness parameter Ra. This roughness parameter is derived from a 2D roughness profile.
Three different shear transfer mechanisms can be identified: adhesion-, friction- and reinforcement shear transfer. In an unreinforced concrete-to-concrete interface, adhesion- and friction shear transfer determine shear resistance.
Direct shear experiments
To re-evaluate expressions for this shear resistance, direct shear tests under compression
perpendicular to the interface were performed. The test setup used, satisfied the requirements. The independent control over shear loading and loading perpendicular to the interface was as intended.
The test setup also allowed for a realistic production method, reduced flexural stresses, and provided opportunity to minimize and check eccentricities. No unexpected specimen behavior occurred during testing.
The found 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. No apparent relationship between interface roughness and shear resistance was found. The test series can be grouped with series that are alike in concrete type and roughness (table 5.1).
Table 5.1: Parameters of linear part in the Mohr-Coulomb interface failure envelope
Specimen group 𝝉𝟎
(𝑵/𝒎𝒎𝟐)
Slope coefficient
𝒇𝒄𝒌,𝒊 (𝑵/𝒎𝒎𝟐)
SCC very smooth 0,57 2,25 42,86
SCC untreated 1,63 1,66 44,30
SCC raked 1,87 1,32 37,59
TC untreated 1,08 1,31 17,50
TC raked 1,10 1,88 40,23
Roughness quantification
The adopted method, interval mechanical probing, was found suitable to describe a 2D roughness profile of a given surface. 2D roughness profile interpretation using roughness parameters is sensitive to changes in assessment length. Roughness parameters Ra and Rz(DIN) were found to be least susceptible to changes in measurement technique. Using specialized roughness quantification equipment may improve accuracy of the derived roughness profile.
Material properties testing
The cube compressive strength of the concrete was measured. However, measurement of the splitting tensile strength and the modulus of elasticity of the materials used may be valuable in the
interpretation of the experimental data. In this research, additional pull-off tests were performed according to fig. 2.9. Lack of rest results proved these tests inconclusive, but the testing method performed well.
Numerical modeling
Near the load introduction, a tri-axial state of compression occurs. The specimen also experiences some splitting tensile stresses perpendicular to the interface. Peak shear stresses occur that are higher than the assumed interface average, actual shear stress capacity might be larger. A large difference in modulus of elasticity between the two concretes further increases peak stresses, which may reduce interface shear resistance. Shrinkage stresses might further influence the stress
distribution, but this was not studied due to lack of temperature- and relative humidity data for the manufacturer production locations.
Correlation analysis
The fib Model Code 2010 expression is a better predictor of the experimentally found shear resistance than the Eurocode 2 expression. The Pearson correlation coefficient between the fib Model Code 2010 expressions and test results is 0.4133, whilst the Pearson correlation coefficient between Eurocode 2 expressions and test results is 0.3420.
The found shear strength in the absence of normal loading, 𝜏0 , was found to have no correlation with concrete strength parameters, unlike assumed in codes. Neither has 𝜏0 any correlation with the shear resistance at 𝜎𝑛= 0 described by both Eurocode 2 and fib Model Code 2010.
The regression slope coefficient has a large positive correlation with the strength of the weakest concrete. The regression slope coefficient has a large negative correlation with the ratio of concrete strengths, suggesting that a large difference in modulus of elasticity between the two concretes may lower interface shear resistance. The regression slope coefficient has no correlation with surface roughness, contrary to code suggestion. The regression slope coefficient has negative correlations with the Eurocode 2 and fib Model Code 2010 predictions, suggesting codes do not describe this behavior correctly.
Surface roughness has no correlation with interface adhesion shear transfer, however, it will
influence the shear transfer when the interface has debonded (appendix 8). A rough interface might increase the probability that the cast concrete adheres better to the old concrete, but this was not the topic of research, since pre-cracked specimens rejected. A rough interface might increase the robustness of the cast joint.
Code expression safety
The code expressions for shear resistance of an unreinforced concrete-to-concrete interface are compared to test result characteristic values (Appendix 9). This demonstrates that the Eurocode 2 and fib Model Code 2010 expressions are conservative and are sufficiently safe.
5.2 R
ECOMMENDATIONSThe test setup used is the cause of peak stresses, additional studies utilizing a different test setup may be done to evaluate found relationships. The limited size of the test specimens might influence the found shear resistance, further experimental research might include larger scale direct shear tests to minimize the influence of the peak stresses at the load introduction sites.
To investigate the influence of differential elasticity moduli on shear behavior, further experimental research may be done. Material tests to determine the modulus of elasticity of each concrete are advised.
In this research, macroscopic roughness characteristics were quantified. However, microscopic roughness and porosity might have a large influence on interface shear behavior. These
characteristics are highly sensitive to production methods and environmental parameters. In future research this aspect could be investigated.
The influence of differential shrinkage stresses was not studied due to lack of temperature- and relative humidity data for the manufacturer production locations. These stresses may however influence the interface stress distribution. In future research, this aspect could be analyzed.
During post-test inspections, it was noted that the roughness characteristics of the cracked surface differ from the precast floor plate surface roughness characteristics. To be able to predict shear friction transfer, it might be beneficial to perform roughness quantification on a surface, then cast concrete on top, load it until interface failure, then perform roughness quantification again on the broken surface.
Unreinforced concrete-to-concrete interface robustness might be investigated using cyclic load testing. Interface roughness might prove more significant in this aspect.
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