ANTI CRACK FIBER

    Anti crack fiber is a new type of fiber of concrete and mortar which is composed of polypropylene and polyester. It is called secondary reinforcement of concrete. With the development of composite materials, anti crack fiber has been widely used in civil engineering.

   The results show that when the crack resistant fiber with the ratio of 0.05%~0.2% is added into the cement mortar and concrete, it can produce obvious crack resistance, toughening, impact resistance, impermeability,freeze-thaw resistance and fatigue resistance. These excellent properties play an important role in the aspects of crack resistance, toughening and impermeability of plastering mortar of dry mortar, putty of inner and outer walls and joint-filler. Anti crack fiber is widely used in road and bridge engineering, concrete highway and tunnel engineering.

4.3.1 Crack Resistance Fiber Properties

• . High tensile strength
• .Good aging resistance
• .Strong acid and alkali resistance
• .Crack resistance, toughening, impact resistance, impermeability, freeze-thaw resistance
• .Light specific gravity, low dosage and good dispersion
• .Low cost

4.3.2 Application Fields

There are many kinds of anti crack fiber products suitable for dry mortar, mainly including thermal insulation mortar, anti crack plastering mortar, internal and external wall putty, waterproof mortar, joint putty of gypsum board and light concrete board, cement-based and gypsum-based plastering mortar, etc. The application fields include road and bridge, dam, highway, culvert, subway engineering, etc.

4.3.3 Applicable specifications and dimensions

Fiber length	Putty for internal and external walls	Joint filler agent	Cement-based and gypsum-based plastering mortar	Concrete
≦3.5mm	◎	◎	◎
≦7.0mm			◎	◎
≦10mm			◎	◎

4.3.4 Technical Description

 (1) Reduce and eliminate cracks

The research and test at home and abroad show that the crack elimination rate can be almost 100% when 1.2KG anti anti crack fiber is added into 1m3 mortar. In the same volume, the longer the fiber has better crack resistance. The smaller the fiber diameter and the longer the fiber length, the higher the crack resistance and shrinkage cracking resistance of the mortar. When the volume of crack resistant fiber is not less than 0.1%, the synergistic effect of film-forming with redispersible polymer powder can effectively prevent the plastic shrinkage cracking of cement mortar. When the cement mortar shrinks, the polymer cement mortar shows higher tensile strength and elongation at break, and has better flexibility and crack resistance . Figure 4.3-1 shows the effect of crack resistant fiber on the dry shrinkage of concrete.

Meanwhile the dense fiber can prevent the segregation of concrete, improve the overall uniformity of mortar and concrete, weaken the uneven shrinkage, thus reducing the generation of internal cracks of mortar, which is beneficial to the preparation of plastering mortar and calking agent great benefits.

  Picture 4.3-1 Anti crack fiber effect on concrete dry shrinkage

• .Improve the impermeability of cement mortar and concrete

The contrast experiment of the impermeability of 0.05%, 0.1%, 0.3% fiber reinforced concrete and plain concrete under 1.3MPa water pressure shows that the impermeability of 0.05%, 0.1% fiber reinforced concrete is 40% and 48% higher than that of plain concrete, while the increase of 0.3% is less, only 8%. These experiments show that adding a proper amount of anti crack fiber to concrete can increase the compactness of concrete, reduce the water permeability of anti crack fiber concrete, and improve the impermeability of concrete, which has a significant effect on the preparation of waterproof anti crack mortar.

Picture 4.3-2 shows the effect of crack resistant fiber on the impermeability of concrete.

• Improve the impact resistance of cement mortar and concrete

 The drop weight test of anti crack fiber reinforced concrete shows that the addition of fiber helps to improve the impact energy absorption of concrete structures, which is due to the high bond strength between the fiber reinforced network structure formed in the concrete and the concrete matrix, which greatly maintains the overall strength of the concrete. When the stress is transferred from the matrix to the crack resistant fiber, the fiber consumes energy due to deformation. In addition, the crack resistant fiber has good ductility and large ultimate deformation value. Once the concrete cracks, the crack resistant fiber can prevent the rapid development of cracks in the concrete, thus effecitively enhancing its impact resistance.

Picture 4.3-. shows the influence of anti crack fiber on the impact strength of concrete and Picture 4.3-4 shows the influence of anti crack fiber on the toughness of concrete with the dosage.

•  Improve the freeeze-thaw resistance of cement mortar and concrete

The freeze-thaw resistance is an important inex of long-term performance and durability of concrete. Through the experiment of freeze-thew resistance of concrete with different volume of crack resistant fiber, it shows that adding crack resistant fiber can effectively improve the freeze-thaw resistance of concrete.

Picture 4.3-5 shows the effect of anti crack fiber on the freeze-thaw resistance of concrete and Picture 4.3-2 shows the effect of adding anti crack fiber on the compressive strength of concrete after freeze-thaw resistance.

Picture 4.3-2  Comparison of concrete compressive strength changes after 50 freeze-thaw cycles

Item	Plain concrete	Adding 0.05% fiber concrete	Adding 0.1% fiber concrete
Compressive strength change（%）	-0.63	-0.4	+0.6

The anti crack fiber reinforced concrete is especially suitable for the use in the high and cold areas where the positive and negative temperature changes greatly.

   The results show that the fibers are divided into high elastic modulus fibers and low elastic modulus fibers due to different materials. The high elastic modulus fiber can obviously increase the failure strength of the composite , but the improvement of toughness is not great. The low elastic modulus fiber makes the strength of the composite unchanged or slightly decreased, but greatly increases its ductility and toughness. The results show that when the volume of fiber is less than 0.1%, the strength of concrete can not be improved but the flexural toughness can be improved rapidly. When the amount of admixture is high, it can not only reduce the brittleness of composite materials but also increase the ultimate strength of cement matrix, which can significantly improve the flexual strength, tensile strength, impact strength and fatigue resistance of cement mortar and concrete.