What is Bendable Concrete?

Concrete is inherently brittle. To enhance its load-bearing capacity and resistance to bending stress, reinforcement is added, resulting in reinforced cement concrete (RCC). However, RCC still tends to fracture in a brittle manner under flexural stress.

Engineered Cement Composites (ECC), commonly known as bendable concrete, offer a superior solution to this limitation. Bendable concrete incorporates ultra-ductile fiber reinforcement, which significantly improves its bending capacity and flexibility.

Unlike conventional cement concrete, bendable concrete exhibits high flexibility and resistance to bending stress, earning it the name "flexible concrete."

Due to its high demanding ductile nature, bendable concrete is gaining wide application and future scopes in various fields. Let's discuss in detail the features, properties, and significance of bendable concrete in the era of sustainable construction.   

Evolution of Bendable Concrete

The development of bendable concrete, also known as Engineered Cementitious Composites (ECC), began in the 1990s with the pioneering work of Dr. Victor Li and his research team at the University of Michigan. Their goal was to create a concrete material that could withstand tensile stress without cracking. 

By incorporating high-performance fibers into the cement matrix, they succeeded in producing a material with exceptional ductility and flexibility. The development process focused on optimizing the fiber content, dispersion, and matrix composition to achieve a balance between strength and flexibility. This breakthrough led to the creation of ECC, a revolutionary material that has significantly improved the durability and resilience of concrete structures.

Composition of Bendable Concrete

The bending nature of ECC is achieved by altering its material composition, different from that of conventional concrete. 

The design mix of bendable concrete primarily eliminates the coarse aggregate. The leading ingredient that contributes flexibility to the material is fibers. 

ECC contains high powder content, with cementitious materials like silica fume, blast furnace slag, and fly ash used alongside cement. It incorporates low amounts (typically 2% by volume) of short, discontinuous fibers like Polyvinyl Alcohol (PVA) fibers.  So, the main ingredients of ECC include:

1. Cement/Pozzolans
2. Fibers
3. Sand
4. Water
5. Superplasticizers

Working of Bendable Concrete

Bendable concrete consists a major amount of fibers which play the role of imparting bending property for concrete. Under heavy load, ECC deforms more than normal concrete without fracturing. Bendable concrete is 60 times more flexible and 40 times lighter than lighter than traditional concrete. 

Lets understand why an ECC bends so remarkably:

• Strain Hardening: Unlike conventional concrete, which exhibits brittle failure, ECC undergoes strain hardening. This means it can sustain increasing amounts of strain without a significant increase in stress after initial cracking.
• Multiple Cracking: Under tensile load, instead of forming one large crack, ECC develops many fine cracks. Each micro-crack is bridged by fibers, which transfer the load across the crack and prevent it from widening.
• Fiber Pull-Out: As the concrete bends, the fibers within the matrix undergo a pull-out mechanism rather than breaking. This fiber pull-out process dissipates energy and contributes to the material’s overall ductility.

Features of Bendable Concrete

1.Ductility and Flexibility: ECC are highly ductile compared to normal concrete. Under flexural loads, ECC can achieve very high curvature, similar to a ductile metal plate yielding. Extensive inelastic deformation is achieved via multiple microcracks, analogous to plastic yielding in metals.
ECC can undergo distributed damage throughout the yield zone, enhancing its ductility.

2. Crack Resistance and Damage Tolerance: ECC has a tensile strain capacity of 3-5%, significantly higher than the 0.01% for normal concrete. This high strain capacity leads to excellent crack resistance and tight crack width control. ECC deforms more than normal concrete without fracturing, preventing large cracks and enhancing durability.

3. Capacity for Self-Healing: ECC can self-heal hairline fractures through the reaction of extra dry cement with CO2 and water to form calcium carbonate. Experiments at the University of Michigan demonstrated that one to five wet-dry cycles could heal cracks up to 60 micrometres wide.

Read More On: Self-Healing Concrete

4. Reduced Water Permeability: Unlike traditional concrete, which requires sealants for waterproofing, ECC resists moisture inherently. The fine aggregates and waterproof fibers in ECC dramatically reduce permeability, enhancing its crack-resistant properties.

5. Resilience Under Tension: ECC is highly ductile, and capable of deforming over five percent under tension without losing strength. This superior performance is due to the incorporation of various fibers that enhance the material's ductility.

6. Strength Parameters: ECC exhibits tensile strength ranging from 10 to 15 MPa and can achieve compressive strength up to 70 MPa. Its ultimate tensile strain can reach 3 to 5%, with a strain capacity three hundred times greater than that of conventional concrete.

Applications of Bendable Concrete

• Seismic Zones: ECC is particularly useful in areas prone to earthquakes due to its ability to deform without losing its load-bearing capacity.
• Repair and Retrofitting: Its bendable nature makes ECC ideal for repairing and retrofitting existing structures. In 2003, ECC has been used for repairing of the 60 years old Mitaka dam at Hiroshima, Japan. The surface of the dam was badly damaged and repaired with ECC of which was sprayed over 600 m2 surface area.
• Infrastructure: ECC is used in bridges, pavements, and buildings where flexibility and durability are critical. ECC link slab, which replaced a conventional expansion joint on a Michigan bridge deck, has lasted over a decade without repair or maintenance. Victor Li, CC BY-ND.

Other project references include, Glorio-tower Roppongi (Japan) and - Kitahama Building (Japan).

Also Read: What is Light Emitting Cement?