Three-Point Bending Behavior of Hybrid Concrete–UHPC Beams

Hybrid concrete members combine Ultra-High Performance Concrete (UHPC) with conventional concrete to balance performance, durability, and cost. UHPC provides superior compressive and tensile strength, fracture resistance, and durability, while conventional concrete offers economy and ease of use.

Such hybrid designs are increasingly used in bridges, high-rise buildings, and precast elements, where local strengthening with UHPC can improve critical regions (supports, joints, or high-stress zones), while the rest of the member is cast with normal concrete.

The critical challenge in these structures lies in the interface between the two concrete grades. Differences in strength, stiffness, and fracture properties can lead to stress concentrations and premature cracking. Finite Element Analysis (FEA) provides engineers with a predictive tool to study cracking mechanisms and reinforcement strategies before physical testing.

Key factors analyzed during FE simulations include:

• Stress distribution in hybrid members under bending.

• Influence of reinforcement on crack initiation and propagation.

• Behavior of the interface zone between UHPC and conventional concrete.

• Comparison of brittle vs. ductile response in reinforced vs. unreinforced beams.

Project Highlights

• Beam configuration: Hybrid beam with C35 concrete (central zone) and UHPC C140 (side supports).

• Material properties:

  • C35: ~55 MPa compressive, ~5.5 MPa tensile strength.

  • UHPC C140: ~97 MPa compressive, ~9.6 MPa tensile strength.

• Reinforcement: Steel bars modeled as line bodies with reinforcement body interaction.

• Loading setup: Three-point bending test with 4.2 MPa compressive stress applied on the top surface of the C35 zone.

• Scenarios:

1. Without reinforcement
2. With reinforcement

Findings:

• The unreinforced model failed earlier due to brittle cracking in the weaker C35 zone.

• With reinforcement, cracking slowed and higher stresses were required to reach failure.

• Cracks consistently initiated and propagated at the interface between C35 and UHPC, highlighting the transition zone as the most critical region.

FE Analysis Tips and Tricks

• Use nonlinear concrete models to capture realistic cracking patterns.

• Explicit dynamic analysis is suitable for simulating crack initiation and propagation in brittle materials.

• Model reinforcement with embedded line elements to balance accuracy and computational cost.

• Ensure fine mesh resolution in crack-prone regions to improve prediction of crack initiation.

Material Selection

• C35 concrete: Used for the central span to represent economical bulk material.

• UHPC C140: Applied at supports to enhance load-bearing and crack resistance.

• Steel reinforcement: Explicitly modeled with line bodies in the reinforced scenario.

Geometry Editing

• Beam geometry divided into two zones (C35 central, UHPC side supports).

• Reinforcement embedded in the beam volume using reinforcement body interaction.

Mesh Generation

• 3D solid elements used for both C35 and UHPC regions.

• Total of 14,208 elements in the model.

• Local mesh refinement applied in high-stress regions near the interface.

Analysis Settings

• Analysis type: Explicit dynamic simulation.

• Material model: Nonlinear concrete behavior included for both C35 and UHPC.

• Elastic–plastic behavior defined for steel reinforcement.

Connection Types

• No special interface elements used between C35 and UHPC; modeled as a continuous part.

• Reinforcement–concrete interaction captured using reinforcement body interaction.

Boundary Conditions

• Both sides of the UHPC region fixed in the vertical and lateral directions, with free displacement allowed in the axial direction.

• Central compressive stress applied as a patch load to replicate three-point bending.

Load Conditions

• Applied load: Equivalent to 4.2 MPa compressive stress at mid-span of the C35 region.

• Support reactions carried by UHPC end zones.

Results Interpretation

• Unreinforced beam: Cracks initiated in the central C35 zone, leading to early brittle failure.

• Reinforced beam: Reinforcement delayed cracking and increased the load capacity, producing a more ductile failure response.

• Interface zone: The transition between C35 and UHPC consistently governed crack initiation and propagation, confirming the importance of interface detailing in hybrid designs.

? This study demonstrates how material interfaces and reinforcement detailing govern the structural performance of hybrid concrete members. Strategic use of UHPC at critical regions can optimize beams for strength, durability, and cost efficiency.