Combined with the construction practice of steel fiber reinforced concrete pavement, the construction technology and quality control measures of steel fiber reinforced concrete pavement are discussed.
Keywords: construction technology quality control measures
Steel fiber reinforced concrete (SFRC) is a kind of composite concrete made by adding steel fiber into ordinary concrete, and it is a new road material. Steel fiber reinforced concrete pavement has good impact resistance, bending resistance, tensile resistance, wear resistance and long fatigue life under dynamic load, and has good ability to prevent and suppress cracks caused by temperature stress. In addition, steel fiber reinforced concrete has good frost resistance. Adding a certain amount of steel fiber into ordinary concrete can obviously reduce the thickness of pavement structure layer, improve road performance and save costs.
1. Selection of raw materials and mix design of steel fiber reinforced concrete
(1) cement
Generally, ordinary portland cement or portland cement can be used. Technical index requirements of cement:
(1) magnesium oxide content: ≤5%
(2) Sulfur trioxide content: ≤3.5%
(3) fineness index: 80μm square hole sieve residue > 10%.
(4) The initial setting time shall not be earlier than 45min.
(5) The final setting time shall not be later than 10h. Due to the special working conditions and small thickness of steel fiber reinforced concrete pavement, cement with high strength, small shrinkage, good wear resistance and frost resistance should be used as far as possible for pavement concrete.
(2) Aggregate
Coarse aggregate (particle size greater than 7mm) should be crushed stone, with hard and clean stone, no weathered particles, rough surface and nearly cubic particles. The maximum particle size shall not be greater than 20mm and 2/3 of the length of steel fiber (the length of steel fiber is generally 3-4cm), the silt content shall not be greater than 1%, and the content of slender flat particles, aeolian fossils and needles shall not be greater than 65438+.
Fine aggregate (particle size less than 5mm) should use natural medium-coarse sand or machine-made sand. Sand must be hard, clean, dry, free of impurities and uniform in particle size. The fineness modulus is 2.0-3.0, and the silt content is not more than 3%. Sand with excessive silt content must be screened. The cleanliness of fine aggregate is expressed by the percentage of natural sand not less than 0.075mm, and machine-made sand is expressed by sand equivalent or methylene blue value, and its quality must meet the specification requirements.
(3) Water and additives
Like ordinary concrete, the concrete water consumption of pollution-free natural water or tap water is about 130- 180kg/m3. In order to ensure sufficient strength and compactness of concrete, the water-cement ratio should be 0.4-0.55. When the water-cement ratio is low, the workability of concrete is poor, and water reducing agent or plasticizer can be added. In order to open the road ahead of time, an appropriate amount of early strength agent can be added to the concrete.
(4) Type and size of steel fiber
The steel fiber used is carbon steel fiber, and the steel fiber used for pavement is suitable for melt shearing. Its tensile strength should be not less than 550Mpa, the fiber diameter is generally 0.4-0.7mm, and the length is generally 50-70 times the diameter.
(5) Mix proportion design
The mix proportion of steel fiber reinforced concrete refers to the proportional relationship between components in steel fiber reinforced concrete. The basic purpose of its design is to reasonably match its constituent materials (including steel fiber, cement, water, coarse and fine aggregates and additives, etc.). ), so that the designed steel fiber reinforced concrete can meet the strength requirements of structural design and quality acceptance. In the practical application of pavement engineering, the mix design of steel fiber reinforced concrete should be directly based on the performance and use of steel fiber reinforced concrete, that is, in the mix design of steel fiber reinforced concrete, the flexural strength of steel fiber reinforced concrete is taken as the mix design index, and the main factors restricting the flexural strength of steel fiber reinforced concrete, such as steel fiber content, length-diameter ratio of steel fiber, the proportional relationship between cement grade and water-cement ratio, are sought, and the flexural strength of steel fiber reinforced concrete is controlled by adjusting the above factors.
When designing the mixture ratio of steel fiber reinforced concrete, we must meet the requirements of pavement design, such as mixing performance and hardening performance, as well as the design requirements of steel fiber reinforced concrete pavement structure, usually compressive strength, flexural strength, bending toughness and so on. Usually, flexural strength and compressive strength are the main strength indexes in pavement structure design. In order to improve the toughness of steel fiber reinforced concrete, steel fiber with high bonding strength with concrete matrix should be selected as far as possible. The strength test of pavement steel fiber reinforced concrete mixture ratio should be carried out according to the pavement grade and engineering requirements.
Second, the construction technology
(1) mixing
Mixing is an important link to ensure the uniform distribution of steel fiber reinforced concrete in concrete matrix. Therefore, mechanical mixing is necessary in the construction of steel fiber reinforced concrete pavement, generally using forced or inverted cone mixer. In order to ensure the uniform mixing of steel fiber concrete and prevent steel fiber from caking, the technology of drying first and then wetting is adopted. The feeding sequence is: sand-steel fiber-stone-cement. Dry mix 1-2 minutes with a blender, and then wet mix with water for about 2 minutes. The total stirring time is controlled within 6 minutes. If the stirring time is too long, fiber clumps will be formed, and the amount of stirring at one time should be controlled below 1/3 of the mixer capacity.
(2) Transportation materials
Steel fiber reinforced concrete is transported by dump trucks. Due to the vibration of steel fiber reinforced concrete during transportation, steel fiber sinking, slump and air content will be lost, which will affect the uniformity of steel fiber reinforced concrete. Therefore, the transportation time and distance should be shortened as much as possible, and the steel fiber reinforced concrete should be prevented from being polluted during transportation. The longest transportation time is limited to the initial setting time of cement provided by the test and sufficient construction operation time. (3) pouring
After the mixture is transported to the designated place, it is generally poured directly into the road groove where the template is installed and leveled manually. When cutting, the same pile should be avoided, and it must be carried out continuously within the specified continuous construction period without interruption, otherwise the steel fibers will be arranged along the joint surface, and the steel bars will not be produced, which is easy to produce cracks.
(4) Vibration
The vibrator of steel fiber reinforced concrete should adopt flat vibrator. If the thickness of the slab is less than 0.2m, it can be paved once, and the vibrating time is generally subject to the surface mortar vibrating until the mixture no longer sinks. No vibration leakage. Then the vibrating beam with two ends placed on the outer mold is vibrated and flattened along the paving direction. In the process of vibration, the excess mixture will be scraped out, and the depressions should be filled at any time. Finally, the seamless steel tube rollers placed on both sides of the formwork are used to roll the cement slurry along the longitudinal direction to ensure the smoothness of the pavement.
(5) surface treatment
In order to prevent the steel fiber from being exposed or protruding vertically from the surface, and to ensure the safety of vehicles and pedestrians, the erected or exposed steel fiber can be pressed in and leveled by other methods before leveling, and the steel fiber should not be taken out when plastering and embossing. The smoothed surface shall be embossed and painted before initial setting, and a rolling brush tool shall be used, and a bamboo broom shall not be used. Use a grooving machine to cut a notch with the required depth, and the grooving time should not be too early or too late. The slotting time of steel fiber plastering is about 12-48 hours, and the compressive strength reaches 5- 10Mpa.
(6) medical care
Like ordinary concrete, steel fiber reinforced concrete should be cured in time. When the concrete is plastered for 2 hours, when the surface has a certain hardness and there is no trace with fingers, the curing can be started. The concrete surface can be covered with straw bags, sacks or wet sand (about 20-30mm), and sprayed with water evenly several times a day to keep it moist, and the maintenance shall be no less than 7 days. Waterproof film can also be pasted on the surface to prevent the water in concrete from evaporating and ensure the normal hydration of steel fiber concrete.
Third, the quality control of steel fiber reinforced concrete
In order to ensure the construction quality of steel fiber reinforced concrete, the following matters should be paid attention to:
1. Each batch of raw materials entering the site must be strictly inspected, and qualified raw materials must be used;
2. The compactness of the soil foundation, the strength, stiffness and uniformity of the base should be tested in advance;
3. Measure the water content of sand and gravel on site, and adjust the construction mixture ratio according to the test mixture ratio;
4. Before paving, check the smoothness of the base, the transverse slope of the road arch and the elevation of the formwork.
5. Specimen making. For every 400 cubic meters of steel fiber reinforced concrete, two sets of bending specimens should be made (for 7-day and 28-day strength tests). For every additional 1000-2000m3 of steel fiber reinforced concrete, an additional set of specimens shall be made for acceptance or later strength inspection. The sample shall be wet cured on site under the same conditions as the pavement. During construction, the strength of 7-day-old specimens should be measured in time to check whether it reaches 70% of the strength of 28-day-old specimens.
Four, steel fiber reinforced concrete pavement joint setting
Because steel fiber has good tensile performance, crack resistance and shrinkage resistance, when construction conditions permit, the pavement with a width of 7-9m generally does not need to be provided with longitudinal joints, and the overall construction can be done, and the transverse shrinkage joints are arranged with the spacing of15-20 m.
Steel fiber reinforced concrete is widely used because of its excellent performance, but the construction of steel fiber reinforced concrete pavement is more complicated than ordinary concrete pavement. If it is not well controlled in the construction process, the problems of steel fiber agglomeration, poor workability of mixture and so on will easily occur, which will reduce the quality of pavement. Therefore, when using steel fiber reinforced concrete, we must strictly control the quality of raw materials, mixing, transportation, pouring, vibrating and curing. Only in this way can we give full play to the superior performance of steel fiber reinforced concrete, improve its social and economic benefits and serve the national development.
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