Full-scale testing – load to break

Problem statement
Traditional calculation models are often conservative and cannot fully describe the load-bearing capacity and deformation capacity of new design and material solutions. Steel fiber reinforced concrete (SFRC) is increasingly used as individual reinforcement for concrete structures such as slabs on grade and pile supported slabs. However, there is uncertainty about design methods, ductility, crack development and residual capacity in service and ultimate limit states.

The aim of this project was to carry out a unique full-scale load test of a multi-field SFRC pile-supported slab to study cracking, deformations and ultimate load, and to collect detailed measurement data to improve future design models and standards.

Results
The results of the test can be summarized in four main points:

1. Cracking (SLS): The first cracks were observed at load levels corresponding to about 30% of the calculated ultimate load. The cracks developed in a controlled manner and the crack widths were kept below 0.6 mm in the serviceability limit state (SLS), which is in line with the Eurocode requirements.
2. Deformations (SLS): The plate deflections were lower than L/500 at SLS. At higher load levels, the plate showed good ductility, with gradually increasing deformations without sudden collapse.
3. Ultimate Limit State (ULS): The slab exhibited stable behavior up to 60 kN/m². Final failure occurred only at about 77.8 kN/m², corresponding to about 30% above the design load. The failure occurred through a combination of flexural failure and local punching around some piles.
4. Measurement data: Fiber optic strain gauges and DIC (Digital Image Correlation) analysis provided high-resolution data on cracking and strains. They confirmed that the steel fibers contributed to load transfer after cracking and contributed significantly to the toughness of the structure.

In conclusion, the test showed that SFRC has the potential to be used as a single reinforcement in multi-span slabs on piles. The results provide a basis for improved design models and future standardization.

Implementation
The full-scale test was conducted according to the following steps:

Construction: A full-scale slab with dimensions of 20 × 16 m (5×4 compartments) and thickness of 250 mm was cast on a pile system with Ø700 mm piles. The slab was reinforced with steel fibers only (≈48 kg/m³).

Materials testing: The compressive strength of the concrete varied between 35-43 MPa. Tensile and flexural tests on prisms and beams verified the contribution of fiber reinforcement to the residual capacity. Verification of fiber quantity by sampling of fresh and hardened concrete.

FEM analysis: A 3D model in AxisVM was used to analyze load cases and determine critical load areas for the test. The analysis formed the basis for the placement of measuring equipment and load zones.

Loading: Loading was performed with big bags filled with steel slag (density ≈3500 kg/m³). Loads were applied sequentially from about 40 to 80 kN/m². Five load steps were applied: initial SLS, then gradually up to ULS and final failure. The load was maintained between the different load steps for 4-7 days.

Instrumentation: The plate was equipped with 13 LVDT sensors for vertical deformations

Fiber optic strain gauges (Luna ODiSi) for continuous strain measurement. Digital image correlation (ARAMIS) for 3D deformation measurement. Crack microscope and ultrasound (Pundit Lab) for crack width and depth. Temperature sensors (thermocouples) and cameras for continuous recording of load paths and for temporary strain measurement.

Data analysis: The results were analyzed focusing on crack development, load-deformation curves and energy dissipation. They were compared with the FEM analysis for calibration and future improvements of computational models.

The methodology ensured a very detailed basis that can be used for standardization and development of new guidelines for the SFRC.

Equipment

• FEM software (AxisVM)
• Load material: steel slag in bags (3500 kg/m³)
• 13 LVDT deformation sensors
• Fiber optic strain gauges (Luna ODiSi)
• Digital image correlation (ARAMIS)
• Scanning microscope and ultrasound (Pundit Lab)
• Thermocouple
• Cameras for documentation and analysis

Standards

• EN 1990: Eurocode – Basic rules
• EN 1991-1-1, 1991-1-3, 1991-1-4: Loads on structures
• SS-EN 1992-1-1: Concrete structures
• ACI 437.2-13: Load testing of concrete structures
• ISO 16311: Condition assessment of concrete structures

FIB Bulletin 80: Proof loading of concrete structures