Performance Analysis Report Of Steel Fiber Reinforced Concrete

Performance Analysis Report Of Steel Fiber Reinforced Concrete

Cut off steel fiber reinforced concrete has been widely used in various fields such as logistics, cold storage, industrial plants, parking lots, and other building floors. Its advantages are that it significantly improves the cracking phenomenon of building floors and has higher construction efficiency compared to traditional processes. However, a simple design method for calculating the content of steel fiber reinforced concrete has not yet been established.

Relevant foreign institutions have published information on the mix proportion of steel fiber reinforced concrete, and even proposed some reference tables and sharing of experience mix proportions. Domestic teams in related fields have also proposed to use flexural strength as an indicator for the design of steel fiber reinforced concrete mix proportions, and through experiments, establish the quantitative relationship between flexural strength and various main influencing factors, which is conducive to mix proportion design. But most of them still follow the mix design method of ordinary Portland cement concrete, determining the mix proportion of the mixture based on the compressive strength of the concrete, only adjusting the sand ratio, water consumption, and cement dosage appropriately. When determining the mix proportion based on this, it is unnecessary to increase the compressive strength in order to achieve higher flexural strength. The design of the mix proportion of steel fiber reinforced concrete should be based on the requirements for the use of steel fiber reinforced concrete and the characteristics of the mix proportion of steel fiber reinforced concrete.

Requirements and characteristics of mix design for steel fiber reinforced concrete

一、 The requirements for the mix design of steel fiber reinforced concrete The purpose of the mix design of steel fiber reinforced concrete is to reasonably mix its constituent materials, such as steel fibers, cement, water, coarse and fine aggregates, and admixtures, so that the prepared steel fiber reinforced concrete should meet the following requirements:

1. Meet the required strength and durability of the project. Construction projects should generally meet the requirements of compressive strength and tensile strength, while pavement projects should generally meet the requirements of compressive strength and flexural strength.

The workability of the steel fiber reinforced concrete mixture prepared should meet the construction requirements.

3. Economically reasonable.

Under the condition of meeting the engineering requirements, fully utilize the reinforcing effect of steel fibers, reasonably determine the amount of steel fibers and cement, and reduce the cost of steel fiber reinforced concrete (note: the analysis object of this study is the Popper cut steel fibers).

二、 The main characteristics of the mix design of steel fiber reinforced concrete compared to ordinary Portland cement concrete are:

1. Adding steel fibers to the mix of ordinary Portland cement concrete can significantly improve the bending, tensile, and fatigue resistance and toughness of the concrete. Therefore, strength control is carried out through mix design. When there is a requirement for compressive strength, in addition to controlling according to compressive strength, the mix proportion of the mixture should also be determined according to the engineering properties and requirements, and controlled according to flexural strength or tensile strength respectively, in order to fully exert the reinforcing effect of steel fiber concrete. Ordinary Portland cement concrete is generally controlled by compressive strength (road concrete is controlled by flexural strength) to determine the mix proportion of the mixture.

2、When designing the mix proportion, it is necessary to consider the ideal uniform dispersion of steel fibers in the concrete (note: Bourbon wire cut steel fibers have a secondary dispersion effect, and the coefficient of dispersion effect was not considered in this study), and the surface of the steel fibers should be filled with mortar to ensure the quality of steel fiber concrete.

3、After adding steel fibers to the mixture, its workability decreases. In order to obtain suitable workability, it is necessary to reduce the content of coarse aggregate appropriatelyDesign principles and 

methods of steel fiber reinforced concrete mix proportions.

The basic method for designing the mix proportion of steel fiber reinforced concrete is based on the characteristics of the steel fiber reinforced concrete mixture and its strength after hardening. Its main purpose is to reasonably determine the four basic parameters of water cement ratio, steel fiber volume ratio, unit water consumption, and sand ratio of the mixture according to the usage requirements. From this, the dosage of each component material can be calculated.

When determining the basic parameters, it is necessary to meet the requirements of compressive strength, toughness, flexural strength, tensile strength, as well as workability and economy.

The experimental data shows that the compressive strength, flexural strength, and tensile strength of steel fiber reinforced concrete are related to the concrete grade; The mix ratio, steel fiber volume ratio and length to diameter ratio, maximum particle size of aggregate, and tensile strength of steel fiber are related factors, among which the steel fiber content, length to diameter ratio, and concrete grade have the greatest impact on the compressive strength of the matrix. Therefore, the steel fiber content is determined by the relationship between toughness and length to diameter ratio, and the steel fiber content. Then, the mix ratio of the matrix is determined by compressive strength (note: in this experiment, the SF80/50BP cut steel fiber is bonded into rows of steel fibers using Bolpei wire, and there may be differences in experimental data for other types of steel fibers).

The mix proportion and steel fiber content determined from this can meet both compressive strength requirements and flexural strength or tensile strength requirements. After preliminarily determining the mix proportion and volume ratio (note: volume ratio is the relevant parameter of steel fiber content, the larger the volume ratio, the higher the steel fiber content), the gradation of steel fiber concrete is determined according to the workability requirements (note: in this experiment, Bolpei wire is used to bond steel fiber SF80/50BP cut steel fibers into rows. Due to the differences in the variety and type of raw materials used for preparing steel fiber concrete and the different construction conditions, the design of the mix proportion in actual engineering is generally based on preliminary calculations, through experiments and combined with construction sites. The condition adjustment is determined.

一、 Determination of water cement ratio

 Due to the compressive strength of steel fiber reinforced concrete, it mainly depends on the strength of ordinary Portland cement concrete and the bonding force between steel fibers and orthopedics. The strength of concrete and its adhesion to aggregates mainly depend on the cement grade and water cement ratio, while the volume fraction and aspect ratio of steel fibers have little effect on compressive strength (which can increase compressive strength by 5% to 10%). Therefore, the water cement ratio of steel fiber reinforced concrete can be obtained by using the relationship equation (1-1) between the compressive strength of ordinary Portland cement concrete and the cement grade and water cement ratio.

ƒ fcu=ARC (C/W-B) formula (1-1)

ƒ fcu -- Steel fiber reinforced concrete trial mix tensile and compressive strength (Mpa)

RC - compressive strength of cement measured at 28 days (Mpa)

C/W - The required cement water ratio for steel fiber reinforced concrete

B -- Empirical coefficient. (When the coarse aggregate is crushed stone, A=0.46, B=0.52; when it is gravel, A=0.48, B=0.61.)

The compressive strength of steel fiber reinforced concrete can be determined according to equation (1-2);

ƒ fcu=ƒ fcu+Z σ Formula 1 (1-2)

ƒ fcu -- Design compressive strength of steel fiber reinforced concrete (Mpa)

Z -- Guarantee rate coefficient

σ 1-- Standard deviation of compressive strength (Mpa)

The size of the guarantee rate coefficient should be determined according to the importance of the project and the requirements of the guarantee rate, which can be determined according to the table below. The standard deviation of strength can be determined by the statistical data of the construction unit. If there is no statistical data, the strength grade of steel fiber reinforced concrete is CF25-30, σ 1=5.0 (Mpa); When CF35-CF60, σ 1=6.0Mpa.   

Relationship between Guarantee Rate and Guarantee Rate Coefficient A-1

By substituting the tensile and compressive strength of the trial mix, the condition of coarse aggregates, and the grade of cement into equation (1-1), the water cement ratio can be obtained. Usually, when the compressive strength requirements are met, its durability can also be met (note: in severely cold and frozen areas, the maximum water cement ratio, minimum cement dosage, etc. should be executed according to relevant specifications).

When determining the water cement ratio, the water cement ratio required for strength or durability should be compared, and the smaller one should be selected as the design water cement ratio. The water cement ratio of steel fiber reinforced concrete should not exceed 0.50 (according to the relevant requirements of GB0037-2013 "Code for Design of Building Floors").

 二、Determination of steel fiber volume fraction

 1. When there are requirements for compressive strength and flexural strength, the determination of the volume fraction of steel fibers (note: the volume fraction is a related parameter of the steel fiber content, and the larger the volume fraction, the higher the steel fiber content). According to the comprehensive regression analysis of 145 sets of test data from the Air Force Engineering Design and Research Bureau and other units, the flexural strength of steel fiber reinforced concrete is related to the water cement ratio, steel fiber volume ratio, length to diameter ratio, and the flexural strength of cement as follows:

 ƒfcu=Rtm(0.12C/W + 0.31 + βtmρfιτ/df )  (1-2) 

ƒ fcu -- flexural strength of steel fiber reinforced concrete trial mix (Mpa)

Rtm - measured flexural strength of cement at 28 days (Mpa)

C/W - The required cement water ratio for steel fiber reinforced concrete

β TM -- Cutting type 0.30 32 1.004 0.126 0.125

Formula (1-3) is applicable to cement grades M4.25 and M5.25, with a water cement ratio of 0.4-0.6, medium second, sand ratio of 40% -60%, crushed stone particle size of 5-20mm, steel fiber volume ratio of 0.5-2.5%, and aspect ratio of 40-100.

The flexural strength of steel fiber reinforced concrete trial mix can be calculated according to equations (1-4):

ƒfcu= ƒfcu + Zσ2   （1-4） 

ƒ fcu -- Design flexural strength of steel fiber reinforced concrete (Mpa)

Z -- Guarantee rate coefficient

σ 2-- Standard deviation of flexural strength of steel fiber reinforced concrete

The trial strength can also be calculated by multiplying the design flexural strength by an improvement coefficient of 1.10-1.15 according to relevant specifications.

According to equations (1-3), when the flexural strength, water cement ratio, cement flexural strength, and variety of steel fibers of steel fiber reinforced concrete are known, the volume fraction of steel fibers can be obtained. When determining the volume ratio, while meeting the strength requirements, economic efficiency and ease of construction must be considered, and the amount of steel fibers used should be minimized as much as possible. If necessary, the water cement ratio can be adjusted appropriately to obtain an appropriate volume ratio.

(2) The volume fraction of shear steel fibers can also be determined according to equations (1-5) based on the flexural strength, water cement ratio determined by compressive strength, and cement compressive strength.

According to the experimental results of Southeast University, the flexural strength of shear steel fiber reinforced concrete is related to the cement ratio, volume ratio, aspect ratio, and compressive strength of cement as follows:

ƒ fcu=Rc (0.0802C/W+0.08 ρ F ιτ/ Df-0.0801) (1-5)

ƒ fcu -- flexural strength of steel fiber reinforced concrete trial mix (Mpa)

Rc - measured compressive strength of cement at 28 days (Mpa)

C/W - The required cement water ratio for steel fiber reinforced concrete

If the flexural strength, water cement ratio, cement compressive strength, and steel fiber length to diameter ratio of steel fiber reinforced concrete are known, the volume fraction of shear steel fibers can be obtained according to equation (1-5). 2. When there are requirements for compressive strength and tensile strength, the determination of steel fiber volume fraction is based on literature. The tensile strength of steel fiber reinforced concrete is related to the tensile strength of its matrix concrete, steel fiber volume fraction, and aspect ratio, as follows:

ƒ ft=ƒ t (1+ α T ρ F ι F/df) (1-6)

ƒ ft - Design tensile strength of steel fiber reinforced concrete (Mpa)

ƒ t -- Design tensile strength of steel fiber reinforced concrete (Mpa)

α T -- The coefficient of influence of steel fibers on tensile strength.;

δ――― Discretization coefficient

三、Determination of water consumption per unit volume and cement dosage for steel fiber reinforced concrete

Under the condition of maintaining a certain water cement ratio, the unit volume water consumption and steel fiber volume ratio are the main factors controlling the workability of the mixture. The determination of water consumption should ensure that the mixture meets the required workability and is easy to construct. The workability of steel fiber reinforced concrete is controlled according to the Weibo consistency, generally suitable for 15-30 seconds. Due to various factors affecting the water consumption per unit volume and differences in the selection of raw materials, the water consumption also varies. In practical applications, it can be determined through experiments or based on existing experience. It can also be selected according to the material variety specifications, steel fiber volume ratio, water cement ratio, and consistency reference table.

四、Determination of Air Content per Unit Volume of Steel Fiber Reinforced Concrete in Rows

(1) Prepare a flat glass plate and weigh W2 (accurate to 0.05kg) together with an empty measuring bowl; Fill the container with water and slide a glass plate along the upper edge. Occasionally use a water dispenser to add water until the measuring bowl is filled with water and there are no bubbles under the glass plate. Weigh the measuring bowl filled with water together with the glass plate to obtain the weight W1 (accurate to 0.05kg). The volume of the measuring bowl V: W=W1-W2 (W, W1, W2-kg) V=W1-W2 (W, W1, W2-kg) V=W/r (V-L) r - The specific gravity of water at the calibration temperature (r-kg/L)

(2) Fill the measuring bowl with ordinary Portland cement without adding bonded steel fibers, and connect the straight pipe to the bottom of the exhaust valve under the bowl cover. Connect the U-shaped pipe to the upper part of the exhaust valve. Open the inlet valve, gently place the bowl cover on the measuring bowl, clamp it with a clamp, and check the level of the instrument with a spirit level. Open the exhaust valve, use a water injector to add water from the inlet valve until water flows out at a uniform speed from the U-shaped pipe outlet, and then close the inlet and exhaust valves. Use a hand pump to inflate and pressurize the air, causing the gauge pressure to slightly exceed -3. After stopping for 5 seconds, adjust the pressure with a fine adjustment valve to accurately stop at -3. Gently tap the dial, the gauge pressure remains at -3. Then press the pressure valve 2-3 times to balance the chamber pressure with the bowl pressure. Read the pressure gauge value, which is equivalent to 0% gas content

(2) Carefully mix steel fibers into the prepared ordinary Portland cement concrete according to the designed dosage, mix thoroughly, and ensure that the binder is fully decomposed. Tap the container for preparing steel fiber reinforced concrete in a clockwise direction to remove any external air that may have entered during the addition of steel fibers. Repeat step (1).

The subtraction of the data from two experiments results in the gas content of steel fibers in the growth of concrete (note: in this experiment, Bolpa wire was used to bond SF80/50BP cut steel fibers into rows of steel fibers, which were bonded with environmentally friendly adhesives and did not react with concrete. The gas content can be considered zero, and there may be differences in the experimental data of other types of steel fibers).