Design requirements for steel fiber mix ratio of concrete

 

Suzhou concrete steel fiber concrete overcomes the shortcomings of ordinary concrete, such as low tensile strength, low ultimate elongation, good brittleness, tensile resistance, bending resistance, shear resistance, crack resistance, fatigue resistance and high toughness. Through its application in bridge deck pavement, the construction method, technical requirements and related precautions of steel fiber concrete are summarized, providing experience for the promotion and application of steel fiber concrete.

 

Quality control of steel fiber mix ratio design

Steel fiber concrete is a cement-based composite material composed of cement slurry. Mortar or concrete is composed of metal fiber reinforced materials. It is a new type of building material, a uniformly dispersed new type of building material with high tensile strength, high ultimate elongation, alkali resistance and other high properties.

 

As the main structure of the bridge, the bridge deck is often overlooked in design and construction. Long-term vehicle loading is the main cause of cracking and damage of the bridge deck, which will affect the quality of the bridge and reduce its service life. The use of steel fiber concrete on the bridge deck will effectively solve the quality problems that are prone to occur during the use of the bridge deck. Suzhou Concrete Steel Fiber Manufacturer

 

I. Steel Fiber Concrete Mix Ratio Design Requirements

The purpose of steel fiber concrete mix ratio design is to reasonably match the constituent materials, namely steel fiber concrete, namely. cement. water. coarse aggregate and admixtures, so that steel fiber concrete can meet the requirements of construction and engineering use to a large extent.

(1) Meet the requirements of compressive strength and flexural strength of highway bridges and improve the durability of bridge decks;

(2) Make the prepared steel fiber concrete easy to use and convenient to meet construction requirements;

(3) Give full play to the characteristics of steel fiber concrete, reasonably determine the amount of steel fiber and cement, and greatly reduce the cost of the project.

 

II. Raw material quality requirements

Steel fiber: The surface should be clean, rust-free, and oil-free. In order to ensure the bonding strength between steel fiber and concrete, there is no bonding. The size and tensile strength meet the technical requirements; the minimum tensile strength of a single steel fiber is 800N/㎜2. The dosage does not exceed 70㎏/M3.

Cement: Use 32.5 or 42.5 grade ordinary Portland cement.

Gravel: Hard stone should be used. Clean. No weathered particles. Rough surface. Gravel is close to cubic particles.

Fine aggregate: Natural coarse sand or machine-made sand should be used. For the cleanliness of fine aggregate, natural sand should be less than 0.075㎜. The percentage of content indicates that the quality of machine-made sand must meet the requirements of the specification, expressed as sand equivalent or methylene blue value.

Water: Unpolluted natural water or tap water.

Admixture: Water reducer should be used. Steel fiber concrete with clear requirements for antifreeze properties should use air-entraining water reducer.

 

III. Steel fiber concrete mix design steps

The steel fiber concrete mix design is the same as the ordinary concrete mix design, and the calculation method is generally used. The following steps can be taken:

(1) Determine the compressive strength and flexural strength of the trial mix according to the strength standard value or design value and the construction configuration strength improvement coefficient.

(2) The water-cement ratio is calculated according to the trial mix compressive strength, which is generally controlled between 0.45-0.50. The compressive strength of ordinary cement concrete can be determined based on the relationship between compressive strength, cement grade and water-cement ratio.

(3) Calculate the steel fiber volume ratio based on the test flexural strength. Generally, the volume ratio is selected to be 1.0-1.5%.

(4) The water consumption per unit volume should be determined through tests according to construction requirements (the influence of admixtures should be considered).

(5) Determine a reasonable sand ratio based on the test (based on the material type). Steel fiber volume ratio. Proper on-site adjustment, such as water-cement ratio), generally controlled between 1.1-1.6%.

(6) The test mix ratio is determined by calculating the material dosage by volume method.

(7) According to the performance of the mixture, adjust and determine the construction mix ratio.

 

IV. Mixed steel fiber concrete

(1) Rolling concrete mixing equipment must be used. When the steel fiber volume ratio is high and the mixture consistency is high, the mixing amount should be controlled, and generally should not exceed 60% of the equipment mixing amount.

(2) Pay attention to the order of placing the mixing materials, generally arranged in the order of cement, steel fiber, fine aggregate, coarse aggregate, and water. First dry mix, then add water to moisten. At the same time, the steel fiber should be placed 2-3 times to ensure that the steel fiber will not clump, bend or break in the mixer.

(3) According to the viscosity of the mixture, the mixing time is reasonable to determine the requirements for uniformity and strength stability. First dry mix, then wet mix. Generally, the dry mixing time is not less than 80 seconds, and the wet mixing time is not less than 100 seconds (the total mixing time must be controlled within 300 seconds).

 

V. Construction and maintenance of steel fiber concrete

(1) Remove garbage, clean the bridge deck, sprinkle water to moisten, and pour cement slurry (cement slurry can be prepared by weight ratio of water = 1:1).

(2) Check the correctness of the placement of the steel mesh for bridge deck paving and the overlap of the steel mesh.

(3) After unloading, the steel fiber concrete should be manually paved and leveled, vibrated, and roughly leveled on the vibration plate (it is not advisable to use a vibration beam to level it), and the vibration plate should overlap by 1/2 each time.

(4) Use a steel pipe to roll the slurry several times, and use the slurry rolling method to avoid exposing the steel fibers.

(5) Use a 3-meter-long aluminum alloy square to scrape (finely flatten) from one side of the steel formwork to the outside. Each time the square should be crossed by more than 1/3.

(6) After the initial setting of the steel fiber, artificial roughening is performed to make the bridge deck rough.

(7) The curing agent can be sprayed at the beginning of the concrete completion. The spraying should be uniform and the surface should have no color difference. After the initial setting, cover with geotextile and sprinkle water to keep the geotextile moist. Cover with geotextile and cure for 7 days, and sprinkle with water for 14 days.

(8) Steel fiber concrete bridge deck pavement C60:00, due to the high concrete grade and fast cement solidification, equipment and personnel should be concentrated on construction, and strive to control the time of steel fiber concrete within 4 hours.

 

VI. Quality control of steel fiber concrete

(1) Quality inspection of steel fiber

First, the length deviation of steel fiber should not exceed 10% of the standard length. At least 10 fibers should be randomly inspected in each batch;

Second, the qualified rate of steel fiber diameter or equivalent diameter should not be less than 90%. The weight method can be used for inspection. 100 fibers should be sampled in each batch. The length should be weighed with a balance and measured with a caliper. The required equivalent average value should meet the requirements;

Third, the tensile strength of steel fiber should be inspected. The tensile strength should not be less than 380MPA;

Fourth, the bending performance of steel fiber. Steel fiber should be able to withstand 3 mm diameter steel rod bending and dismantling 90°. At least 10 fibers should be continuously inspected in each batch;

Fifth, there should be no oil stains, harmful substances or impurities on the surface of steel fiber that affect the bonding between steel fiber and concrete.

(2) Raw material inspection

According to the requirements of the technical specifications for highway engineering construction, the quality control standards of the above raw materials must be met.

(3) Steel fiber concrete inspection

The workability of steel fiber concrete should focus on slump and water-cement ratio. At the same time, the distribution of steel fibers in concrete must be inspected on site. If steel fiber agglomeration is found, the mixing time should be extended.

 

VII. Precautions

(1) Since steel fiber concrete has strict requirements on the control of water-cement ratio during mixing, it is not suitable for construction under rainy or windy conditions. It should be carried out on sunny days. In case of rain, construction must be stopped and the unhardened concrete bridge deck should be covered with geotextile in time. If necessary, a temporary canopy can be built and the remaining work under the canopy should be completed as soon as possible.

(2) Adjust the mixing water in time according to the temperature and wind force to ensure the harmony of the concrete. It is recommended that the construction time be scheduled at a temperature not higher than 22℃.

(3) The amount of health agent sprayed should be adjusted in time under high temperature or strong wind conditions. After spraying the health agent, the geotextile should be covered in time. After the concrete is initially set, water should be sprinkled on the geotextile immediately to prevent the bridge deck concrete from shrinking and cracking.

(4) When widening the steel fiber concrete bridge deck under traffic conditions, in addition to on-site construction, the vibration of vehicles on the old bridge has a great impact on the vibration of vehicles on the steel fiber concrete bridge deck. The width of the bridge will not be paved temporarily, and it will be done after the new grid surface is fully formed.

 

VIII. Conclusion

Steel fiber concrete can better solve the problems of cracks and durability that are difficult to solve with ordinary concrete, which is conducive to improving the quality of bridge deck use and extending the service life of bridge deck. It will be widely used in the reinforcement and reconstruction of old highway bridges, bridge deck repair and bridge deck defect repair.