1. Introduction

The Pull-Out Test is a commonly used experimental method to evaluate the bond behavior between reinforcement (typically steel bars) and concrete. When using Ultra-High Performance Concrete (UHPC), a class of advanced cementitious composite materials with superior mechanical and durability properties, the bond characteristics can differ significantly from conventional concrete due to UHPC’s dense matrix and fiber reinforcement.

This test becomes particularly important when dealing with ribbed steel bars, which are designed to enhance mechanical interlock with the surrounding concrete. The performance of this bond directly influences structural behavior, load transfer mechanisms, and crack control in reinforced UHPC structures.

2. Purpose of the Pull-Out Test

The test aims to:

• Study failure modes such as pull-out, splitting, or bar rupture.

3. Experimental Setup

• Specimen: Typically, a steel rebar embedded axially into a UHPC cube or cylinder.
• Bond length: Controlled to isolate bond effects (often short to avoid yielding).
• Testing machine: Applies axial tensile load to the bar while restraining the concrete.
• Measurement tools: LVDTs or other sensors measure the relative slip between steel and UHPC.

4. Bond Mechanism in UHPC

The bond between steel and UHPC involves three main components:

1. Chemical adhesion: Initial grip due to chemical bonding.
2. Friction: Resistance from normal pressure after adhesion is broken.
3. Mechanical interlock: Contribution from the ribs on the deformed bars engaging with the matrix.

UHPC enhances these effects due to:

• High compressive strength (>150 MPa).
• Low porosity improves adhesion.
• Steel or synthetic fibers, which may bridge cracks and improve confinement.
• Failure mode: Identified visually and by load-slip behavior:
  • Pull-out failure: Smooth load drop after peak.
  • Splitting failure: Sudden failure due to concrete cracking.
  • Bar rupture: If the bar fails before debonding—rare in pull-out setups.

5. Factors Influencing Results

• Bar diameter and rib geometry
• Embedment length
• Curing and moisture condition of UHPC
• Presence and orientation of fibers
• Confinement level (passive or active)

7. Applications

• Structural design of UHPC elements
• Development of anchorage and lap splice lengths
• Calibration of bond-slip models for numerical analysis