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"Post-installed screws for in-situ assessment of mortar strength" presented at ESREL2017 by Shah Nur Sourav

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Abstract: For capacity evaluation, the structural assessment of existing structures is necessary. Concrete strength is an important parameter for such assessment. Non-destructive tests (NDTs) are used along with the traditional approach of core testing for strength assessment of concrete in existing structures. The low reliability of NDT results leads to uncertainty in assessing concrete strength. A new method of non-destructive testing is presented in this paper with the aim of achieving better reliability and reducing uncertainty in the assessment of mortar strength. This approach is based on a modified pullout of post-installed screw anchors. The technique involves a pushing mechanism for a steel screw inside the mortar where a void underneath the screw is left to allow for the uninterrupted movement of the screw inside the concrete. The failure pattern involves local crushing of concrete between the threads of the screw. This paper investigates the load bearing behaviour of threaded screws installed in cement mortar under compressive loading. The results supports the application of the technique in the assessment of compressive strength of mortar. The main parameters affecting the pushing behaviour are presented and their effects are discussed. It is planned to extend the test program to concrete in the future.

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"Post-installed screws for in-situ assessment of mortar strength" presented at ESREL2017 by Shah Nur Sourav

  1. 1. Md Shah Nur Alam Sourav (ARUP/UCD) Dr. Salam Al-Sabah (ARUP) Dr. Ciaran McNally (UCD) Post-installed screws for in-situ assessment of mortar strength This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 642453
  2. 2. Concrete StrengthAssessment
  3. 3. Concrete StrengthAssessment • Assessment of existing structures • Quality assurance • Safety issue • Addressing the changes in loads or regulation • Assessment of material properties e.g. concrete strength • Concrete: Non-homogenous material
  4. 4. Concrete StrengthAssessment Core testing • Direct compressive strength measurement • Reliable method • High cost and time consuming • Damage to structural elements • Technical difficulty
  5. 5. Concrete StrengthAssessment In-situ testing • Low cost and short time • No or partial damage • Additional information • Question of reliability
  6. 6. In-situ Tests: Non-destructive Tests (NDTs) Test methods Reliability Cost Ease of operation Damage Rebound hammer Low Low Easy Unlikely Ultrasonic pulse velocity Low Low Easy None Resonance frequency Low High Difficult None Maturity Medium High Difficult Minor Probe penetration Low Low Medium Minor Pull out Medium High Difficult Minor CAPO Medium High Medium Minor Break off Low Medium Medium Minor Pull off Low Low Medium Minor
  7. 7. Current Practice: EN 13791 True NDT properties True Strength Measured NDT value, Rm=R+error Predicted strength with NDT Predicted strength Approach B Shift calibration Approach A Direct calibration Physicalrelation Measurement error Reference cores Basic Curve Calibration
  8. 8. Bond strength: Mechanism Adhesion and friction along the surface of the rebar Bearing on the ribs of the bar Load
  9. 9. Bond strength: Failure Mendis and French (2000) P. Mendis and C. French, “Bond Strength of Reinforcement in High-Strength Concrete FAILURE,” Advances in Structural Engineering, vol. 3, no. 3, pp. 245–253, 2000. Failure surface Crushed concrete in front of the ribs Load Splitting Failure Complete Pullout Failure
  10. 10. Bond strength: Effect of Compressive Strength Hughes and Videla (1992) B. P. Hughes and C. Videla, “Design criteria for early-age bond strength in reinforced concrete,” Materials and Structures, vol. 25, no. 8, pp. 445–463, Oct. 1992 M. S. Lorrain and M. P. Barbosa, “Estimation of compressive strength based on Pull-Out bond test results for on-site concrete quality control,” Ibracon Structures and Materials Journal, vol. 4, no. 4, pp. 582–591, 2011. Splitting Failure Complete Pullout Failure 0 2 4 6 8 10 0 10 20 30 40 Bondstrength(MPa) Compressive strength (MPa) 0 5 10 15 20 25 30 35 40 0 20 40 60 80 100 Bondstrength(MPa) Compressive strength (MPa) Lorrain and Barbosa (2011)
  11. 11. Post-installed Screw: Mechanism • Screw is installed in a drilled hole • Screw cuts threads in the wall of the hole • Load transfer by bearing mechanism of screw threads • Mechanism is similar to bond action of deformed rebar Screw installed in concrete Wall of the drilled hole Bearing Mechanism of screw in concrete
  12. 12. Post-installed Screw: Failure Failure type • Concrete cone failure • Shallow embedment • High degree of undercut • Complete pullout failure • Deep embedment • Small undercut • Combination of cone and pullout failure • Steel failure Concrete cone failure Combined cone and pullout failure J. H. R. Kuenzlen and T. M. Sippel, “Behaviour and design of fastenings with concrete screws,” in In: RILEM Proceedings PRO 21 “Symposium on Connections between Steel and Concrete”, Cachan Cedex, 2001, pp. 919–929. (Kuenzlen and Sippel (2001) (Kuenzlen and Sippel (2001)
  13. 13. Screw Push-in Test: Mechanism • Push-in mechanism instead of common pullout mechanism • Similar bearing mechanism • Formation of compression strut • Local crushing of concrete under the threads • Concrete shearing off along the outer edge of threads at final stage of loading
  14. 14. Push-in Test: Procedure Drilling Cleaning Installation Set up Loading
  15. 15. Push-in Test: Investigation of failure Failure by mortar shearing • Small undercut • High confinement Splitting failure • High degree of undercut • Low confinement
  16. 16. Push-in Test: Procedure Widened hole up to top 10mm depth Total depth of hole = 65 mm Effective depth =30mm Outer diameter = 10.40 mm Inner diameter = 7.60 mm Pitch of the threads = 9.5 mm Diameter of the drilled hole = 9.5 mm
  17. 17. Push-in Test: Failure Mechanism in Mortar Threads cutting into mortar Stage 1: 2 mm displacement 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Load(KN) Displacement (mm)
  18. 18. Push-in Test: Failure Mechanism in Mortar Crushing of mortar 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Load(KN) Displacement (mm) Stage 2: 4 mm displacement
  19. 19. Push-in Test: Failure Mechanism in Mortar Mortar crushing Sheared off mortar 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Load(KN) Displacement (mm) Stage 3: 8 mm displacement
  20. 20. Push-in Test: Failure Mechanism in Mortar Sheared off mortar under threads No mortar left under the threads after complete shearing off Stage 4: 12 mm displacement
  21. 21. Push-in Test: Mortar Test program 1 • Drilling the hole • Cleaning the hole • Manual Installation of screw • Installation of screw up to 40 mm leaving 25 mm void under the screw • Loading the arrangement Loading arrangement
  22. 22. Push-in Test: Mortar Test program 2 • Same concrete screw • Drilling the hole • Cleaning the hole • Manual Installation of screw • Installation of screw up to the full depth of the drilled hole • Unscrewing operation until the void under the screw becomes 25 mm • Loading the arrangement
  23. 23. Push-in Test: Mortar Test program 1 ---- Test program 2 Test program Mortar mix Age (days) Average Peak load (KN) Co- efficient of variation (%) 1 1 7 10.11 12.84 14 11.03 4.37 28 14.89 6.18 2 7 8.11 7.61 14 8.90 18.57 28 10.32 7.91 2 1 28 12.74 7.63 2 28 10.49 4.69
  24. 24. Push-in Test: Mortar Test No Deviation (MPa) 1 -4.26 2 0.25 3 1.36 4 1.46 5 3.68 6 -2.49 Test program 1
  25. 25. Conclusion • The new test uses the concept of bond strength of deformed bar in concrete • Use of post-installed screw in mortar strength assessment • The test provides reliable estimation of mortar strength • Requires low cost • Has potential to apply in concrete strength assessment • A new technique as an alternative to existing NDTs • Future study includes • Influence of different parameters • Influence of several factors on the test results
  26. 26. Thank you for your attention shah-nur-alam.sourav@ucdconnect.ie Question ?? This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 642453

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