Analysis of basic mechanical properties of polypropylene fiber concrete

Li Yinbo

Abstract： With the progress of the society and the development of the national economy， people's demand for building products in daily production and living practice is getting higher and higher， which puts forward higher requirements for building structure strength. In engineering practice， the rational use of polypropylene fiber can effectively optimize the mechanical properties of concrete structures， improve their overall strength and impermeability， and form a good guarantee for the overall quality of construction projects. This paper mainly analyzes the basic mechanical properties of polypropylene fiber concrete.

Key words： polypropylene fiber; concrete; mechanical property

At present， polypropylene fiber has become a very commonly used building material in construction engineering， which has good application performance and plays an important role in improving the overall mechanical properties of concrete. In engineering practice， how to make more reasonable application of polypropylene fiber material to maximize the mechanical properties of concrete and improve the overall quality of concrete structure， so as to promote the improvement of engineering economic and social benefits， is worthy of further exploration by the majority of construction workers [1].

I. material composition of polypropylene fiber concrete

In addition to fibers， polypropylene concrete contains materials such as cement， gravel， sand and water， and also need to add some additives according to the actual needs of engineering construction.

（i） Cement

Concrete structure is the main component of most construction projects， due to the large amount of cement material， so construction projects usually choose ordinary Portland cement. Under normal circumstances， the amount of concrete cement should not be less than 300kg/m3， while the amount of cement in fiber concrete should usually be more than 400kg/m3 to greatly improve the overall strength of fiber concrete.

（ii） Gravel and sand

Adding gravel and sand to the concrete can control the shrinkage and stabilize the volume of the structure. Through the addition of different levels of gravel and sand， and effectively fill the voids， then a dense accumulation effect is formed. In addition， gravel and sand can also play a certain role in cement mixing and water retention， good proportion can avoid segregation and severe bleeding to some extent. In contrast， reasonable addition of gravel and sand can reduce the cost of concrete structures [2].

（iii） Water

Usually use drinking water or pollution-free water sources. When using non-drinking water， its sulfate content should be less than 2.7mg/cm3， its salt content should be less than 5mg/cm3， and pH should be more than 4.

（iv） Polypropylene fiber

Polypropylene is a kind of polymer compound obtained by polymerization of propylene （chemical formula is CH3-CH=CH2）， it is a crystalline polymer with regular structure with milky white， odorless， odorless， non-toxic and of low quality， which belongs to thermoplastic plastics. Polypropylene， with a density of 0.9～0.91g/cm3， is the lightest resin material currently in use. Polypropylene is insoluble in water and has good heat resistance. It can be continuously heat-resistant at 121～160℃， with a melting point of 165～170℃. Polypropylene is a non-polar polymer with excellent electrical insulation properties and its dielectric constant is 2.25， with strong chemical stability. Polypropylene absorbs little water and does not react with most chemicals， such as acids， bases and organic solvents. In addition， polypropylene has good physical and mechanical properties， with tensile strength of 3.3×107～4.14×107Pa， compressive strength of 4.14×107～5.51×107Pa and elongation of 200%～700%， so polypropylene has very high quality processing performance [3].

II. Basic mechanical properties of polypropylene fiber concrete

In construction projects， adding different fibers to cement concrete can improve its mechanical properties to varying degrees， including concrete toughness， durability and strength， which are important indicators to improve concrete load capacity. At present， a wide range of fiber applications are steel fiber， carbon fiber， glass fiber and polypropylene fiber etc. Adding polypropylene fiber into cement matrix can theoretically affect the formation of concrete in the following aspects.

（i） Concrete strength

Compared with ordinary concrete， the tensile and flexural strength of polypropylene fiber is stronger after adding the polypropylene fiber， while the compressive strength is correspondingly reduced. The actual variation is mainly related to fiber addition， concrete mix ratio， test method and other factors. In the specific project construction， specific analysis should be carried out according to the engineering demand， and the cost input should be reduced reasonably on the premise of meeting the mechanical performance demand of concrete.

（ii） Crack resistance effect

Adding polypropylene fiber to concrete can play a certain role in preventing crack， which can be divided into plastic concrete and hardened concrete. The specific mechanism is as follows.

（1） Plastic concrete

In engineering construction， the concrete which presents a plastic state from mixing to pouring and then to hardening can be called plastic concrete. The addition of polypropylene fibers can significantly reduce （or even eliminate） the cracks that occur when concrete is in a plastic state after pouring， while it is difficult to achieve such a good effect by using expansion agent or wire mesh. Firstly， polypropylene fiber is evenly distributed in the concrete， presenting a three-dimensional network structure， which can support the aggregate and prevent the settlement process of coarse aggregate and fine aggregate， that is， the coarse aggregate first sinks， then the fine aggregate sinks. In addition， because of the role of polypropylene fiber， it can reduce the concrete surface moisture precipitation effect. The separation of materials and water in concrete is called segregation， it will not only affect the overall homogeneity of concrete， but also lead to rapid surface water loss due to the large amount of cement slurry on the surface of concrete， or the mortar containing fine aggregate， resulting in a large shrinkage and finally cracks appeared on the concrete surface， this kind of crack is called “settlement crack”. Because the addition of polypropylene fiber can effectively inhibit and prevent the emergence of concrete segregation tendency， it can largely avoid the occurrence of cracks on the concrete surface and ensure the quality of concrete structure. Secondly， polypropylene fiber in the concrete in plastic state are subjected to tensile stress due to dry shrinkage， thereby reducing the number of cracks. The main reason is that the strength of concrete in the plastic state is very low， in the case of water evaporation， concrete will form tensile stress due to contraction， which is very easy to lead to the emergence of cracks. However， the uniform and large amount of polypropylene fiber added into concrete can achieve the resistance to such tensile stress， thus reducing or preventing the occurrence and development of cracks [4].

（2） Hardened concrete

In the process of concrete hardening， there are usually three kinds of shrinkage， namely drying shrinkage， temperature shrinkage and carbonization shrinkage. Firstly， the moisture in the capillary channel of concrete evaporates will form capillary pressure， resulting in the dry shrinkage of cement matrix. When the tensile stress formed in the concrete structure is greater than the tensile strength of the concrete itself， a large number of cracks will appear in the concrete. Under normal circumstances， the drying rate of the concrete after hydration can reach 600×10-8～900×10-8 under the condition of complete water loss. Secondly， concrete structures will chilling shrink due to the environmental temperature quenching heat effect， or the external temperature dropping in the actual use of the process. Under the condition that the concrete is restrained， the temperature change will cause a temperature stress （i.e. tensile stress） inside the concrete to form a lot of cracks. Thirdly， carbonization shrinkage refers to the concrete in the cement after hydration to produce calcium hydroxide， and the carbon dioxide in the air after the concrete internal capillary channel， may react with calcium hydroxide and produce calcium carbonate. Calcium carbonate， by contrast， will decrease in volume by about 12%. And as the volume decreases， concrete will shrink and cracks will appear in the concrete even under constraint conditions. What is worth paying attention to is that the condition of forming carbonization reaction is that the concrete capillary channel contains water， otherwise there will be no carbonization.

Polypropylene fiber can play a certain role in preventing crack in hardened concrete when three kinds of shrinkage result in concrete crack. Adding polypropylene fiber into concrete can reduce the stress concentration at the tip of micro-cracks， so as to ease the expansion of micro-cracks， and to a large extent， avoid the appearance of connected cracks. When cracks occur in concrete structures， the tensile stress will be weakened or eliminated because the front end of the cracks intersects with the fibers. In addition， because polypropylene fibers in concrete are distributed in three-dimensional disorder， they can effectively control the development of cracks， especially reduce the number of cracks with a width of more than 0.05mm， so as to achieve the purpose of crack resistance [5].

（iii） Durability of concrete

Related research shows that adding polypropylene fiber to concrete can effectively improve its durability and greatly enhance the waterproof and impermeable ability of mortar. The addition of a large number of fine fibers can inhibit the formation and development of early dry shrinkage cracks and segregation cracks of concrete， thus reducing the shrinkage cracks of concrete， especially the formation of connected cracks. A large number of fibers are evenly distributed within the concrete and connected with each other， supporting the aggregate， thus reducing the water precipitation on the surface of the concrete and the segregation of aggregates， greatly reducing the pore content inside the concrete 50～100 nanometers and more than 100 nanometers in diameter， so as to improve the overall impermeability of the concrete structure. Engineering practice shows that adding 0.05% polypropylene fiber to concrete can improve its impermeability by about 60%～70%.

In addition， the improvement effect of polypropylene fiber on concrete performance also includes： reducing concrete bleeding， improving concrete surface quality， and significantly enhancing concrete durability indexes such as frost resistance， salt resistance and chemical corrosion resistance.

（iv） Concrete toughness and impact resistance

After concrete is mixed with polypropylene fiber and solidified， high-strength fiber fibers will stick together to form a dense network reinforcement system with disorderly distribution， so as to control the formation and development of cracks and improve the overall toughness of concrete. In addition， because its water permeability is effectively improved， it can provide greater convenience for early maintenance of concrete structures. At the beginning of cement curing， compared with ordinary concrete， concrete mixed with polypropylene fiber can retain more water， make the cement hydration reaction more complete， reduce aggregate segregation， even grading， and increase the surface strength. In the practical application， the surface treatment of polypropylene fiber can make the fiber and cement base material close together， greatly increasing the overall strength of concrete， under the impact of concrete structure， polypropylene fiber can absorb most of the energy， thus reducing the stress concentration effect， preventing the rapid spread of concrete cracks， and making concrete with strong impact resistance. This feature has a very positive effect on concrete structures which are often subjected to impact fatigue， such as roads， airports， warehouse floors， parking lots， and defense facilities.

III. Conclusion

In a word， the reasonable application of polypropylene fiber in construction projects and its incorporation into concrete materials in combination with engineering construction requirements play a very important role in improving the overall mechanical properties of the project. As engineering and technical personnel， we should actively explore in daily work， and draw lessons from some advanced applied technologies and concepts domestic and overseas， then combined with the overall situation of Chinese construction engineering industry， create a set of fiber material application system more in line with Chinese national conditions， injecting a steady flow of vitality for the national economic construction.

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About the author： Li Yinbo （1999- ）， male，