CN108501172B

CN108501172B - Forming method of large-mixing-amount short-cut synthetic fine fiber concrete

Info

Publication number: CN108501172B
Application number: CN201810350298.7A
Authority: CN
Inventors: 樊俊江; 於林锋; 王琼; 韩建军
Original assignee: Shanghai Building Science Research Institute Co Ltd
Current assignee: Shanghai Nuclear Engineering Research and Design Institute Co Ltd; Shanghai Building Science Research Institute Co Ltd
Priority date: 2018-04-18
Filing date: 2018-04-18
Publication date: 2020-03-24
Anticipated expiration: 2038-04-18
Also published as: CN108501172A

Abstract

The invention discloses a forming method of large-doping-amount short-cut synthetic fine fiber concrete, which is selected from the following three methods according to different fiber doping amounts: 1. the fiber volume ratio in the fiber concrete is less than or equal to 0.2%: uniformly dry-mixing fibers and solid components containing coarse and fine aggregates, and adding liquid components containing liquid additives for uniformly wet-mixing; 2. when the volume ratio is equal, part of the fibers and solid components containing coarse and fine aggregates are uniformly mixed in a dry mode, and water and liquid components of the liquid additive are added to the mixture for wet mixing to obtain fiber concrete with good dispersity and workability; adding the rest fibers in batches, and controlling the adding amount and interval of each time according to the concrete state during feeding; 3. when the volume ratio is more than 0.5%, uniformly dry-mixing the solid component containing the fine aggregate, adding 80-90% of water and all water reducing agents, and wet-mixing to obtain the mortar with good working performance; adding fibers and residual water, and wet-mixing until the fibers are uniformly dispersed in the mortar; then adding the coarse aggregate and wet-mixing until the stones are uniformly distributed in the synthetic fiber concrete.

Description

Forming method of large-mixing-amount short-cut synthetic fine fiber concrete

Technical Field

The invention belongs to the technical field of high-performance concrete preparation, and particularly relates to a forming method of high-doping short synthetic fine fiber concrete.

Background

The synthetic fibers are classified into two types, i.e., crack-resistant fibers and toughening fibers, because the former fibers are fine and are generally called "fine synthetic fibers" (micro synthetic fibers), and the latter fibers are coarse and are generally called "coarse synthetic fibers" (macro synthetic fibers). The coarse synthetic fiber can improve the tensile strength of concrete, play a good toughening role, improve the wear resistance, impact resistance, anti-explosion performance and anti-fatigue life of the concrete, and is widely applied to the engineering fields of pavements, bridge floors, floors of industrial factory buildings, loading and unloading docks, tunnel linings, airport runways, parking ramps, parking lots, industrial and civil buildings and the like. The fine fiber in the concrete mainly plays a role in reducing and inhibiting the plastic settlement cracks and the plastic shrinkage cracks of the concrete, and simultaneously can improve the abrasion resistance and the high temperature resistance of the concrete to a certain extent. Because the fine fiber has no obvious improvement effect on the mechanical property and the bending toughness of the concrete, the application of the fine fiber is greatly limited compared with the application of the coarse fiber.

From the mechanism of fiber toughening, after the concrete is cracked under tensile stress, the fibers play a role in restraining the development of cracks, and external loads are transmitted to the fibers crossing the cracks. If the fiber is not broken or pulled out directly, the load can be transmitted to the upper and lower surfaces of the crack through the adhesive force of the fiber and the cement paste, so that the crack can still continue to bear, and the toughness of the concrete is improved.

Whether the fiber is broken and loses the toughening effect or not depends on the accumulated sectional area of all the fibers at the interface, the larger the accumulated sectional area is, the smaller the tensile stress applied to the fiber is, and the less the fiber is broken, which is not related to the thickness of the fiber but the volume mixing amount of the fiber.

The coarse fibers have larger diameter and lower specific surface area, and have smaller influence on the workability of concrete, so the upper limit of the mixing amount in the concrete is larger. When the mixing amount is larger, the accumulated sectional area of the crack correspondingly can generate obvious toughening effect on the concrete. It was found that even with the coarse fibers, when the volume content thereof in the concrete is less than 0.5%, the toughening effect cannot be exerted. Therefore, it is presumed that when the content of the fine fiber in the concrete exceeds 0.5% (the conventional content is less than 0.2%), the fine fiber can also act as a toughening effect on the concrete.

However, when the volume is the same, the number and specific surface area of the fine fibers are far larger than those of the coarse fibers, so that the influence on the working performance of concrete is obvious, and the traditional fiber concrete forming process can not form even when the volume of the fine fibers is large. Therefore, it is necessary to search a method for forming concrete with a large amount of short synthetic fine fibers, so that the fine fibers can play a role in toughening under specific conditions while playing a role in preventing cracks in the plastic stage.

Disclosure of Invention

The invention aims to solve the technical problem of providing a concrete forming method for applying short synthetic fine fibers in concrete in a large mixing amount.

The technical problem to be solved can be implemented by the following technical scheme.

A forming method of large-dosage chopped synthetic fine fiber concrete is characterized in that the following steps are respectively adopted according to different dosage of chopped synthetic fine fibers (wherein, the parts which are not mentioned in proportion are all added according to the metering proportion):

(1) when the volume ratio of the fibers in the fiber concrete is lower (less than or equal to 0.2%), dry-stirring (1-2) the fibers with solid components such as cement, mineral admixture, coarse and fine aggregate for 1-2 min until the fibers are uniformly mixed, and then adding liquid components such as water, liquid admixture and the like into the mixture for wet-stirring (60-90) s until the concrete is uniformly mixed;

(2) when the volume ratio of the fibers in the fiber concrete is equal (> 0.2%, < 0.5%), uniformly dry-mixing 30-50% of the total fibers (namely the mass percentage of the part of fibers used for concrete mixing in advance is 30-50% of the total fiber amount) with solid components such as cement, mineral admixture, coarse and fine aggregate, adding liquid components such as water and liquid admixture, wet-mixing, and stirring for 2-3 min to obtain the fiber concrete with better dispersibility and working performance. And slowly adding the rest fibers into the concrete in batches, and controlling the volume adding amount of the fibers added each time to be 0.02-0.03% and the time interval to be (10-15) s according to the state of the concrete during feeding. After all the fibers are added, continuously stirring for 1-2 min until the concrete mixture is in a good state;

(3) when the volume ratio of the fiber in the fiber concrete is higher (0.5%), the solid components such as cement, mineral admixture, fine aggregate and the like are firstly mixed uniformly in a dry mode, 80-90% of water and all water reducing agents are added, and the mixture is mixed in a wet mode until the mortar with better working performance is obtained. Adding the fiber and the rest 10-20% of water into the mortar, and continuously wet-mixing until the fiber is uniformly dispersed in the mortar. And then, continuously adding the coarse aggregate into the mortar, and wet-mixing for 1-2 min until the stones are uniformly distributed in the synthetic fiber concrete.

The forming method in the step (1) is a forming method when the fibers are in a conventional mixing amount range, and a dry mixing process of the fibers and solid components such as cement, mineral admixtures, coarse and fine aggregates and the like is added on the basis of a common concrete forming method, the fibers in the mixing amount range have certain influence on the performance of the mixture of the concrete, but the concrete can have good performance of the mixture by adjusting the water consumption or increasing the using amount of a water reducing agent and the like.

The forming method in the step (2) is a forming method when the fiber mixing amount is relatively large, and under the fiber mixing amount, the fibers are mixed and formed at one time, so that the agglomeration and dispersion unevenness of the fibers are easily caused, therefore, the synthetic fine fibers with the volume ratio of 0.2 percent are firstly mixed with other components according to the forming method shown in the step (1) to form fiber concrete with ideal working performance, and then the rest fibers are added in batches, so that the uniform dispersion of the fibers in the concrete can be ensured, and the agglomeration does not occur. This process may be referred to as a post partial fiber process.

The forming method in the step (3) is a forming method when the fiber mixing amount is extremely large, a large amount of fibers need to be dispersed in limited cement slurry under the mixing amount, the cement slurry with the fibers needs to be wrapped with coarse and fine aggregates at the same time, the requirement on water demand is high, the direct adoption of the method in the step (1) or the step (2) for forming can cause the phenomena of large amount of knotting of the fibers, separation of mortar with the fibers and stones and increase of defects of concrete after forming. Therefore, in the invention, the coarse aggregate is separated, and the fiber and the components except for the coarse aggregate are mixed in advance by referring to the forming method of ECC (ultra-high toughness fiber cement-based composite material), so as to obtain the fiber mortar. Then adding coarse aggregate and stirring to obtain the fiber concrete, wherein the method can be called as a post-thickening aggregate method. In order to ensure enough space for dispersing a large amount of synthetic fine fibers, the dosage of the single-component cementing material of the concrete in the method should not be lower than 500kg/m³。

The forming method of the high-doping-amount short-cut synthetic fiber concrete adopting the technical scheme has the following significance: the invention improves the existing concrete forming method on the basis of fully knowing the essence of dispersion of the chopped synthetic fibers in the concrete, adopts a part fiber adding method or an aggregate thickening method, greatly improves the fibers of the chopped synthetic fibers in the concrete, forms a forming method of the concrete with large mixing amount of chopped synthetic fine fibers, and provides a feasible way for the chopped synthetic fibers to play the reinforcing and toughening functions in the concrete.

Detailed Description

The following further describes the embodiments of the present invention in detail.

Example 1:

in order to improve the mechanical property and the bending toughness of the high-strength concrete, high-strength high-modulus PVA fine fibers are adopted to prepare the high-strength concrete with large mixing amount of synthetic fibers, the strength grade of the high-strength concrete is C60, and the volume mixing amount of the high-strength high-modulus PVA fibers is 0%, 0.3%, 0.5% and 0.7% respectively. The traditional fiber concrete forming method and the fiber concrete forming method are respectively adopted to form concrete by adopting different processes according to the fiber mixing amount.

The test result shows that: according to the traditional fiber concrete forming method, when the high-strength high-modulus PVA fiber doping amount reaches 0.3%, the water consumption of single-side concrete and the water reducing agent consumption are required to be increased simultaneously to enable the concrete to reach the designed slump, and meanwhile, the concrete has certain bleeding segregation and the performance of the concrete mixture is poor. When the fiber mixing amount reaches 0.5%, the concrete cannot reach the designed slump no matter the water consumption of the single concrete is increased or the water reducing agent dosage is increased, so that the concrete can be molded, the water reducing agent dosage is greatly higher than the reasonable mixing amount, bleeding is serious, and a certain amount of cavities and slurry leakage defects appear on the surface of hardened concrete. When the fiber content reaches 0.7%, a large amount of agglomeration occurs in the fibers, the slurry with the fibers cannot wrap coarse aggregate, a large amount of defects occur in hardened concrete, and a large amount of stones are exposed.

When the method is adopted for forming, the mixing amount of the high-strength high-modulus PVA fiber is 0.3 percent and 0.5 percent, and the performance of the concrete mixture is better; when the mixing amount of the high-strength high-modulus PVA fiber is 0.7%, the using amount of the water reducing agent of the concrete is increased, the concrete slightly bleeds water, the slump is smaller, but the concrete has formability and no obvious defect on the surface of hardened concrete.

The PVA fiber concrete with large mixing amount, high strength and high modulus prepared by the forming method disclosed by the invention has the following performance indexes:

test piece number	Compressive strength/MPa	Split tensile strength/MPa	Flexural strength/MPa
				GQ-0	66.35	4.13	5.55
GQ-0.3	68.25	4.56	5.79
				GQ-0.5	68.01	4.58	5.87
GQ-0.7	67.20	5.03	6.02

The test result shows that: although the low-dosage high-strength high-modulus PVA fine fiber has no obvious influence on the compressive strength of the concrete, the addition of the high-dosage high-strength high-modulus PVA fiber improves the flexural strength and the compressive strength of the high-strength concrete to a certain extent. When the fiber volume ratio is increased to 0.3 percent and 0.5 percent, the split tensile strength of the concrete is improved by about 10 percent, and the breaking strength is improved by about 5 percent; when the fiber volume ratio is increased to 0.7%, the split tensile strength is improved by 21%, the breaking strength is increased by 10%, and the improvement effect is obvious.

From the load deflection curve of the bending toughness test, when the mixing amount of the high-strength high-modulus PVA fiber is 0.3%, the test piece is instantaneously fractured, the curve is suddenly reduced after reaching the maximum load, the overall shape of the load deflection curve is basically the same as that of plain concrete, and the toughening effect is not generated. When the doping amount of the high-strength high-modulus PVA fiber is 0.5% and 0.7%, the load is rapidly reduced after reaching the peak value, then the load is kept at a lower value, and the load reduction amplitude is gradually reduced along with the continuous increase of the deflection, so that a certain toughening effect is shown.

In general, the forming method and the above embodiments of the present invention verify that the synthetic fine fiber can also play a role in strengthening and toughening to some extent.

Example 2:

the high-doping-amount synthetic fiber concrete is prepared from polypropylene (PP) fine fibers, the strength grade of the concrete is C30, the doping amounts of the PP fine fibers in the concrete are respectively 0%, 0.4%, 0.6% and 0.8%, the concrete is prepared by the forming method, and the strength and the bending toughness of the fiber concrete are researched.

The performance test result of the mixture shows that: the concrete prepared by adopting the large-dosage synthetic fine fiber has good workability when the volume dosage of the PP fiber is 0.4 percent and 0.6 percent, and when the volume dosage of the PP fiber is 0.8 percent, the water consumption and the water reducing agent dosage of the fiber concrete are increased, the bleeding phenomenon is slight, but the surface of the formed concrete is free from defects after hardening.

Because the tensile strength and the elastic modulus of the PP fiber are lower than those of the high-strength high-modulus PVA fiber, and the concrete does not show the reinforcing and toughening effects when the mixing amount of the PP fiber is 0.4 percent and 0.6 percent. When the mixing amount of the PP fiber is increased to 0.8%, the flexural strength and the splitting tensile strength of the PP fiber concrete are improved compared with those of a reference group, and the load deflection curve of the concrete presents a descending section.

Example 2 shows that, for different kinds of synthetic fine fibers, the forming method of the invention can be adopted to improve the mixing amount of the synthetic fine fibers in concrete and exert the reinforcing and toughening effects of the fine fibers.

Claims

1. A forming method of large-doping-amount chopped synthetic fine fiber concrete is characterized in that one of the following three forming steps is selectively adopted according to different fiber doping amounts:

(1) when the volume ratio of the fibers in the fiber concrete is less than or equal to 0.2%, dry-mixing the fibers and solid components comprising cement, mineral admixture and coarse and fine aggregate for 1-2 min until the fibers and the solid components are uniformly mixed, and then adding liquid components comprising water and liquid admixture into the mixture to wet-mix the mixture for 60-90 s until the concrete is uniformly mixed;

(2) when the volume ratio of the fibers in the fiber concrete is more than 0.2% and less than or equal to 0.5%, uniformly dry-mixing 30-50% of the total amount of the fibers and solid components comprising cement, mineral admixture and coarse and fine aggregate, then adding liquid components comprising water and liquid additive, wet-mixing, and stirring for 2-3 min to obtain the fiber concrete with better dispersibility and workability; slowly adding the rest 50-70% of the fibers into the concrete in batches, controlling the volume adding amount of the fibers to be 0.02-0.03% and the time interval to be 10-15 s each time according to the state of the concrete during feeding, and continuously stirring for 1-2 min after the fibers are completely added until the state of the concrete mixture is good;

(3) when the volume ratio of fibers in the fiber concrete is more than 0.5%, dry-mixing solid components including cement, mineral admixture and fine aggregate uniformly, adding 80-90% of water and all water reducing agents, and wet-mixing for 2-3 min to obtain mortar with good working performance; adding fibers and the rest 10-20% of water into the mortar, and continuously wet-mixing until the fibers are uniformly dispersed in the mortar; and then adding coarse aggregate into the mortar, and continuously performing wet mixing for 1-2 min until the stones are uniformly distributed in the synthetic fiber concrete.

2. The method for forming the high-content chopped synthetic fine fiber concrete according to claim 1, wherein the amount of the single-component cementing material in the mixing ratio of the fiber concrete in the step (3) is not less than 500kg/m³。