Influence of hooked-end steel fibres on the fresh and hardened properties of concrete using Dreux–Gorisse mix design method

The Big Picture: Making Concrete Tougher

Imagine concrete as a very strong but brittle cookie. It's great at holding up heavy weights (compression), but if you try to bend it or pull it apart, it snaps easily like a dry cracker. This is because standard concrete is made of sand, gravel, cement, and water.

This study asked a simple question: What happens if we mix in tiny, hooked metal wires (steel fibers) to turn that brittle cookie into something more like a tough, flexible bagel?

The researchers wanted to see if adding these metal "hooks" would make the concrete stronger without making it impossible to pour. To make sure the results were fair, they used a very strict recipe book called the Dreux–Gorisse method. Think of this method as a precise baking scale that ensures every batch of concrete has the exact same amount of sand and gravel, so the only thing changing is the amount of metal wire added.

The Ingredients and the Recipe

The team used local materials from Morocco:

• Sand and Gravel: They tested these thoroughly, like a chef tasting ingredients before cooking. They checked if the sand was clean (free of clay) and if the rocks were the right shape.
• The "Secret Weapon": Hooked-end steel fibers. These aren't straight needles; they have little hooks at the ends, like tiny fishhooks. This shape helps them grab onto the cement paste and hold tight.

They created four batches of concrete:

1. The Control: No metal fibers (the plain cookie).
2. Batch A: 20 kg of fibers per cubic meter.
3. Batch B: 30 kg of fibers.
4. Batch C: 40 kg of fibers (the maximum amount tested).

What Happened When They Mixed It? (Fresh Properties)

When they first mixed the concrete, they noticed a change in how "flowy" it was. This is called workability.

• The Analogy: Imagine trying to pour a bucket of smooth sand. It flows like water. Now, imagine mixing in thousands of tiny, tangled fishing hooks. Suddenly, the sand gets stuck on the hooks, and it becomes hard to pour.
• The Result: As they added more fibers, the concrete got "thicker" and harder to move.
  • The plain mix flowed down 6 cm (a nice, smooth pour).
  • The mix with the most fibers (40 kg) only flowed down 1.5 cm. That's a 75% drop in flow.
  • Takeaway: More fibers mean the concrete is "stiffer" and harder to pump or spread, but the researchers noted this is manageable if you use special water-reducing additives (like a lubricant).

What Happened After It Hardened? (Hardened Properties)

Once the concrete dried and set, they tested two things: how heavy it was and how strong it was.

1. The Weight (Density)

• The Analogy: Steel is heavy, like a lead weight. Concrete is light, like a sponge. If you replace some of the light sponge with heavy lead, the whole thing gets heavier.
• The Result: The more fibers they added, the heavier the concrete became. The 40 kg mix was noticeably denser than the plain mix because steel is much heavier than sand or cement.

2. The Strength (Compressive Strength)

• The Analogy: Think of the concrete as a wall of bricks. Without fibers, if a crack starts, it runs straight through the wall and the wall collapses. With the hooked fibers, imagine the cracks trying to run through a net. The fibers act like safety nets or staples that bridge the gap. When a crack tries to open, the hooks grab the other side and hold it together, stopping the crack from spreading.
• The Result: The concrete got significantly stronger.
  • The plain concrete could handle a certain amount of pressure.
  • The mix with 40 kg of fibers could handle 32.7% more pressure before breaking.
  • Even the smallest amount of fiber (20 kg) made the concrete stronger, but the most fibers made the biggest difference.

Why Did This Work?

The paper explains that the "hooked" shape of the fibers is the hero here.

1. Anchoring: Because they are hooked, they don't slip out of the cement easily. They are mechanically locked in place.
2. Crack Bridging: When the concrete starts to crack, the fibers stretch across the crack and hold the two sides together, preventing the crack from getting bigger.
3. Uniform Strength: The fibers are scattered in all directions, so the concrete is strong no matter which way the pressure comes from.

The Final Verdict

The study concludes that adding hooked steel fibers is a great way to make concrete stronger and tougher.

• The Trade-off: You lose some "flow" (it gets harder to pour), but you gain a lot of "strength" (it holds up better under pressure).
• The Sweet Spot: The researchers found that 40 kg of fibers per cubic meter was the best balance, giving a massive 32.7% boost in strength while still being workable enough to use in construction.

Important Note: The paper strictly tested how well the concrete holds up under squeezing (compression). It did not test how well it handles bending or pulling apart, nor did it test how it holds up over many years. It simply proved that for making concrete stronger against crushing forces, these metal hooks are a very effective tool.