Fiber concrete is based on cement and particle aggregates and uses short fiber fibers to enhance or improve specific properties:strong concrete composite. PP fiber can effectively control micro-cracks caused by factors such as plastic shrinkage, dry shrinkage and temperature changes of concrete/mortar, prevent and suppress the formation and development of first cracks in concrete, and significantly improve the crack resistance and impermeability of concrete/mortar Performance, increase the toughness of concrete, thereby increasing the service life of the pavement. Fibers are generally chaotically distributed in concrete.
Improvement mechanism of PP fiber on concrete performance
Concerning the mechanism by which PP fiber can improve various technical properties, the following three explanations are currently standard.
1. Multi-crack theory
According to the method, after the chaotically distributed fibers are combined with concrete, the performance of the composite matrix after cracking is mainly determined by
The volume ratio of the tissue. When it is higher than the critical volume ratio, the fiber will bear the full load, and it may produce multi-slit cracking, which changes the single-slit cracking and low fracture performance of concrete materials, and the characteristics of pseudo-ductile materials appear. The crack spacing becomes smaller, the number increases, the cracks become more exceptional, and cannot be seen with the naked eye, thereby improving the toughness of the composite material, changing the stress-strain state, and improving the durability.
2. PP fiber cracking theory
This theory is also called "fiber spacing theory," which was proposed by others in the early stage. The method believes that there are holes, micro-cracks, and defects of different sizes and shapes in the concrete. When an external force is applied, stress concentration will occur in these parts, causing crack propagation and causing premature failure of the concrete structure. To reduce this degree of damage,
The size and number of crack sources should be reduced as much as possible; the stress concentration at the crack tip should be relaxed, and crack extension should be suppressed. After a certain amount of PP fiber is mixed in the concrete, the tissue that crosses the crack will transfer the load to the upper and lower surfaces of the break when it is tensioned so that the material at the break can continue to bear. The degree of stress concentration is eased with the PP fiber. As the number increases, the PP fiber spacing decreases and spreads around the cracks; the stress concentration will gradually decrease or disappear.
3. Theory of compound mechanics
The argument is based on linear elastic, homogeneous, and forward-oriented continuous fiber concrete composite materials. The fiber can transfer not only the load but also the bond with the matrix interface. When the tensile force is applied in the direction of PP fiber, the external power is transferred to the thread through the matrix, which increases the tensile strength and elastic modulus of the PP fiber concrete composite, thereby improving the performance of the concrete.
Effect of PP fiber on concrete performance
1. Effect on plastic shrinkage of concrete
Adding an appropriate amount of fine PP fiber to the concrete to form a uniformly distributed three-dimensional grid can withstand the internal stress caused by the reduction of the substrate.PP fiber can effectively inhibit the generation and development of new plastic shrinkage cracks in concrete, And it has the effect of refining the breaks, which improves the structure of the cement stone, reduces the original micro-cracks in the cement stone and reduces the crack width.
2. Impact on concrete shrinkage
Dry shrinkage refers to the reduction and swelling of concrete in dry and wet environments. The addition of PP fiber can help reduce the shrinkage of concrete. The thread will show a better anti-shrinkage effect in mortar. Because of the shielding effect of coarse aggregate in coagulation, its shrinkage value is less than that of ordinary mortar. From the perspective of resistance to shrinkage and cracking, it is more necessary to incorporate PP fiber in the mortar.
3. Influence on concrete compressive strength and bending strength
PP fiber's modulus of elasticity is lower than the matrix's modulus of elasticity, generally only 1 / 10-1 / 12 of the matrix's modulus of elasticity, so its reinforcing effect is weak. The compressive strength has almost no effect. The tensile strength and bending strength can be slightly increased. When the fiber volume ratio is 0.05-0.5%, the increase is generally not more than 15%. When the volume ratio exceeds, the strength of fiber concrete shows a downward trend typically.
4. Effect on concrete bending toughness
The bending toughness of PP fiber concrete has a significant impact. When the volume ratio is 0.05-0.2%, the bending toughness can be increased by 20-80%, and when the volume ratio is 0.5-1.0%, it can be increased by 1-4 times.
5. Impact on concrete impact and fatigue properties
The impact resistance of PP fiber concrete with low content is significantly improved. When the dosage is volume ratio, the impact energy can be increased by 2-5 times.
6. Impact on the durability of concrete
Impermeability
The incorporation of PP fiber improves the impermeability of concrete. Mixing PP fiber with a volume ratio of 0.05-0.1% can increase the permeability of the test piece by more than 40% in terms of seepage pressure and can reduce the seepage height by 30-70% under the same seepage pressure.
Frost resistance
The low frost resistance of PP fiber concrete is significantly higher than that of plain concrete with the same conditions, which can increase the number of freeze-thaw cycles by 50% or more.
Abrasion resistance
The incorporation of PP fiber can improve the wear resistance of concrete. The low content (about 0.1% volume ratio) polypropylene fiber concrete can increase the abrasion resistance by 30-60%, or reduce its wear loss by more than 30%.
Aging resistance
The weakness of PP fiber is that its light aging resistance is not as excellent as inorganic fiber. All kinds of threads have different degrees of aging after long-term light and ultraviolet radiation, of which polypropylene fibers without protective measures are the most serious.
Corrosion resistance
PP fiber is relatively stable in acid-base solution, and its performance is hardly affected. Polypropylene fiber can play a specific role in inhibiting concrete expansion caused by sulfate corrosion and alkali-aggregate reaction.
Preparation of PP fiber concrete
The mix ratio of PP fiber concrete is the same as that of ordinary concrete. There are roughly two methods for the preparation of polypropylene fiber concrete.
1. Dry mix method
Mix the aggregate with PP fiber and stir to make the textures spread out to ensure better adhesion with the mortar and improve adherence. After mixing, mix cement and water into the mixer.
2. Wet mixing method
Put the PP fiber into the water whose water volume meets the designed water-cement ratio, mix it evenly, pour it into the mixer when mixing cement and aggregates, and mix one.
Some experimental studies have shown that the preparation of PP fiber concrete, dry mixing method is more reliable than the wet mixing method.
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