Fiber Pull out: 3 Major Facts You Should Never Miss

Fiber pull-out relates to the toughness of the fiber reinforced concrete. It is a failure mode in which the anchor pulls out of the concrete without development of the full steel or concrete capacity. Pull-out tests are typically performed to assess the anchorage of synthetic fiber no matter it is steel fiber or pp fiber. Fiber pull out capacity is important when it comes to retaining walls, slopes, and bridging over voids, where the synthetic fiber is anchored into stable ground that is outside the zone of failure.

We put synthetic fibers into the reinforced concrete to increase its energy absorption capacity, the toughness as well as the tensile and flexural strength of concrete. A better command of the concept of fiber pull-out helps you make the fullest use of the scattered synthetic fibers in the cement fluid before curing and sealing.

What Causes Fibre Pull out?

The cause of fiber pull-out is the weak bond between cement components.

Surely it is related to fiber matrix debonding. After a certain load is applied, a series of debond cracks can be detected in the fiber–matrix interface. Different shapes of the fibers creates various bonding strength between the materials like the rock, synthetic fiber, and sand in cement slurry. The shaped fibers forms a stronger mechanical bonding in the concrete mixture while smooth fiber with less friction is easily “pulled out”.

Fiber pull-out is one of the failure mechanisms in fiber-reinforced composite materials. Other forms of failure include delamination, intralaminar matrix cracking, longitudinal matrix splitting, fiber/matrix debonding, and fiber fracture.

To demystify the mechanical bond of fibers and other components inside the concrete, let’s figure out how the pp fiber works after entering the concrete.

When the concrete reinforced fiber is mixed into the fluid concrete, the soil paste of the concrete will soak the surface of the concrete synthetic fibers. Immediately after the concrete hardens and is finished, the fibers are mechanically held in tight bond. This phenomenon is similar to the hardening effect of cement slurry around gravel. Sand, rock and fibers become essential components of fiber reinforced concrete composites. All these components are irregular and frictional.

These characters help them form a tight mechanical bonding in the concrete mixture. Concrete fibers (steel fibers or macrofibers) can be smooth, but the smooth fibers can grab very limited volume of cement slurry and hardly form locking bonds like other irregularly shaped components. Fibers with smooth surface may experience “fiber pull-out” rather than effective load transfer.

How to Prevent Fibre Pull Out

(1)Create a chemical bond establishing a stronger interface and greater resistance to fiber pullout. To this end, you can coat the fiber with organic material that creates with other components of concrete hydration.

(2)Add new features or properties to the fiber by changing the superficial chemistry of the fiber to make it more conducive to concrete hardened properties.

(3)Breaking the fiber to create a better absorption environment forcing the fibers to “drink more water”. As the concrete paste cures and hardens, it grabs tightly the fiber into the concrete. While this method damaged some of the structural integrity of the fiber it still conform to our aim of gaining a greater interaction between the fiber and concrete.

(4)Manufacturing your fibers into various shapes applies to steel and plastic fiber alike. The principle is similar to the previous breaking principle but done so with a specific design concept and accuracy.

(5)By processing them into different shapes, the interlocking ability between individual fiber is stronger, thus a higher strength. The fibers can exert a stronger adhesion, and can get more compatible bond force inside the concrete. This rule also applies to polypropylene fibers, mesh fibers, and macrofibers.

Why Fiber Shapes Matters for Fiber Pull Out?

We see various shapes exist on the synthetic fiber market: waved, corrugated, end hooked, multiple anchor points, and milled. Of course the synthetic fibers with smooth surface also exists and is widely used. But since the smooth fiber is strong enough to tackle the tensile and flexural problems of the concrete, why bother to manufacture the fiber into various shapes?

Take steel fibres as an example. When producing the steel fiber, we press a groove on the steel wire with a pinch roller to change its shape. In this way, the produced steel fiber has a better fitting force and can better bond with the concrete. It can also be better embedded in the steel fiber mass, thereby greatly strengthen the strength of the concrete.
Another method is to bend and press the steel fibers into a wave-like shape. This method allows a pinch roller to press and bend the steel fibers into the shape of water waves, which can make the bonding force of the steel fibers even stronger.

There is a more important method, which is to see the steel fiber pressed into a hooked shape. In this way, we can maximize the bonding force of the steel fiber. After adding steel fiber in to the reinforced concrete, the strength is far superior than the former ones.