Technical requirements for high-performance concrete：
1. Concept of high-performance concrete
High performance concrete is a new type of high-performance concrete with durability as the main technical indicator. High performance concrete must ensure the following properties: durability, workability, applicability, strength, volume stability, and economy. Require a low water cement ratio, use high-quality raw materials, and in addition to cement, water, and aggregates, a sufficient amount of fine mineral admixtures and high-efficiency admixtures must be added. Technical requirements for high-performance concrete with raw materials
1. Cement:
Cement should be Portland cement or ordinary Portland cement, and the mixed material should be slag or fly ash. Concrete with sulfate resistance requirements can also be selected as medium acid resistant Portland cement or high sulfate resistant Portland cement. Early strength cement should not be used. Beijing Shanghai High Speed Railway
The C3A content in clinker is ≤ 8%, C3A≤6%； The alkali content is ≤ 0.80%. When the aggregate has alkali silica reaction activity, the alkali content of cement should not exceed 0.60%. For concrete with an alkali content of C40 and above, the alkali content of cement should not exceed 0.60%
2. Fine aggregate:
Fine aggregate should be clean natural coarse river sand, with reasonable grading, uniform and firm texture, low water absorption rate, low porosity, or artificial sand produced by special units should not use mountain sand. Do not use sea sand. The water absorption rate should not exceed 2%
Fine aggregate should be mainly composed of intermediate fine aggregate. When using coarse and fine aggregates, the sand ratio should be increased and sufficient cement or cementitious materials should be maintained to meet the workability of the concrete. When using fine aggregates, the sand ratio should be appropriately reduced. The fineness modulus requirement is ≥ 2.3%. The alkali activity of fine aggregates should be tested using the mortar rod method, and the expansion rate of fine aggregate mortar rods should be less than 0.10%. Otherwise, technical measures should be taken to suppress the alkali aggregate reaction. The crushing index value of artificial sand and mixed sand should be less than 25%.
3. Coarse aggregate:
Coarse aggregate should be clean crushed stone with reasonable grading, good particle shape, uniform and firm texture, and low coefficient of linear expansion. Gravel can also be used as a substitute for sandstone gravel
Coarse aggregate should be graded in two or more levels, with a loose bulk density greater than 1500kg/m3, a dense porosity of less than 40%, and a water absorption rate of less than 2%. Second level graded crushed stone, C505-10mm, 10-25mm, C30 5-16mm, 16-32.5mm
4. Mineral admixture: Addition
Various types of finely ground mineral admixtures are used to improve the durability of concrete, including fly ash, slag powder, silica fume, iron ash, rice husk ash, and zeolite powder. In high-performance concrete, fly ash and finely ground slag powder are mainly used
Reinforced concrete with a strength grade not greater than C50 should use national standard first or second grade fly ash, but the loss on ignition of fly ash should not exceed 5.0% and the fineness should not exceed 20%; For prestressed concrete with a strength grade not less than C50, national standard first grade fly ash should be selected, but the loss on ignition of fly ash should be controlled at no more than 3.0%
Fly ash is usually mixed with no less than 20% mineral powder. 5. Admixtures
Admixtures should be products with high water reduction rate, low slump loss, and appropriate amount of air added, which can significantly improve the durability and quality stability of concrete. Admixtures and cement should have good compatibility. Admixtures should be identified or reviewed by the Ministry of Railways and tested by the Product Quality Supervision and Inspection Center of the Ministry of Railways. Polycarboxylic acid series products should be used as additives. During the construction process, water reduction rate tests and solid content tests should be conducted.
6. Water
When mixed water can be consumed from other sources, the water quality should meet the requirements in Table 1. Do not use seawater for mixing and maintenance.
Table 1 Mixed Water Quality Index
PH insoluble, mg/L soluble, mg/L chloride (calculated as chlorine), mg/L sulfate (calculated as SO42-), mg/L alkali content (calculated as equivalent Na2O), mg/L prestressed concrete>4.5 4.5 1. stir
Concrete raw materials should be accurately weighed strictly according to the construction mix ratio requirements, and the maximum allowable deviation for weighing should meet the following requirements: 1% for cement and admixtures; 1% additive; 2% in total; 1% mixed water
2. Transportation
The road for transporting concrete should be kept unobstructed to ensure the uniformity of the concrete during transportation. When transported to the pouring site, there should be no layering, segregation, or leakage of the concrete, and it should have the required workability such as slump and air content.
Concrete should be pumped within 60 minutes after mixing and before the initial setting time of 1/2. All concrete should be poured before initial setting in traffic congestion and hot weather, and special measures should be taken to prevent excessive loss of concrete slump.
3. Pouring concrete
(1) Before pouring, a pouring plan should be designed in advance based on the characteristics of the project, construction environment conditions, and construction conditions, including pouring starting point, pouring progress direction, and pouring thickness.; During the concrete pouring process, the predetermined pouring plan shall not be changed without reason.
(2) Before pouring concrete, carefully check the position, quantity, and tightness of the steel reinforcement protection layer cushion blocks. The cushion blocks on the side and bottom of the component should be at least 4 per square meter. The iron wire used to tie the cushion blocks and steel bars should not extend into the protective layer. When using fine aggregate concrete cushion blocks, their corrosion resistance and compressive strength should be higher than the concrete of the component body, and the water cement ratio should not exceed 0.4. When using plastic cushion blocks, the alkali resistance and aging resistance of the plastic should be good, and the compressive strength should not be less than 50 megapascals.
(3) Before pouring concrete into the mold, specialized equipment should be used to measure the workability of the concrete, such as temperature, slump, air content, water cement ratio, and bleeding rate. Only concrete with a mixture performance that meets the design or mix ratio requirements can be poured into the mold
When the design does not require it, the temperature of concrete entering the mold should be controlled between 5-30 ℃
(4) The free fall height during the concrete pouring process shall not exceed 2 meters; when the concrete exceeds 2 meters, it should be transported by means of chutes, chutes, funnels and other equipment to ensure that the concrete does not layer.
(5) Concrete pouring should be carried out layer by layer continuously, with a gap time of no more than 90 minutes. Construction joints should not be left arbitrarily. The paving thickness of concrete should not exceed 600 millimeters (when pumping concrete) or 400 millimeters (when using non pumping concrete). Before pouring vertical structural concrete, 50-100 mm thick cement mortar (with a slightly lower water cement ratio than concrete) should be poured at the bottom.
(6) The initial setting time of concrete shall not be less than 40 minutes, and the final setting time shall not exceed 600 minutes. 4. Vibration
(1) During the process of pouring concrete, it should be vibrated at all times to make the concrete evenly compacted. The vibration should be carried out using a vertical point vibration of an inserted vibrator or a combination vibration of an inserted vibrator and an attached vibrator. When the concrete is sticky (such as concrete poured by bucket method), the vibration points should be densified. Normally,
(2) Concrete should be compacted using an inserted vibrator. The top surface of concrete components, prestressed concrete components, or other thin components can be compacted with a plate vibrator.
(3) The general sign of compacted concrete vibration is that the surface of the concrete does not sink significantly, bubbles do not continuously spray out, and the surface of the concrete is flat after liquefaction grouting.
(4) Do not use a vibrating rod in the template to make the concrete flow or transport it over long distances, which may cause segregation. Do not vibrate the concrete within 1.5-24 hours after compaction. (5)
During the concrete vibration process, repeated vibration should be avoided to prevent excessive vibration. The stability of the formwork support and the tightness of the joints should be checked to prevent leakage of grout during the concrete vibration process
(6) The vibration process should be selected reasonably based on the structural dimensions and spacing between steel bars, and different models and vibration tools should be chosen, such as the diameter and frequency of the vibration rod. To ensure the quality of the reinforced concrete protective layer, small diameter vibrators or manual shovels should be used for special vibration and excavation of the protective layer concrete
(7) After the surface concrete vibration is completed, the exposed surface of the concrete should be promptly repaired and leveled. The slurry to be tested should be re plastered and pressed or roughened. It is strictly prohibited to sprinkle water during plastering to prevent excessive operation from affecting the surface concrete quality, especially for freeze-thaw concrete in cold regions and concrete in arid regions. More attention should be paid to ensuring the quality of the plastering process during construction
maintenance
(1) After the completion of concrete vibration, the exposed surface of the concrete should be promptly and tightly covered to reduce exposure time and prevent surface moisture evaporation. Before the initial setting of concrete with exposed surface protective layer, the covering should be rolled up, and the surface should be rubbed and squeezed at least twice with a trowel to make it smooth, and then covered again. At this point, it should be noted that the covering layer should not directly contact the surface of the concrete before it finally solidifies.
(2) The steam curing of concrete can be divided into four stages: static stop, temperature rise, constant temperature, and cooling. During static parking, the ambient temperature should be maintained at no less than 5 ℃, and the temperature should not rise within 4-6 hours after filling is completed. The heating rate shall not exceed 10 ℃/h. When the temperature is constant, the internal temperature of the concrete shall not exceed 60 ℃, and the maximum temperature shall not exceed 65 ℃. The constant temperature curing time should be determined through experiments based on the requirements of component demolding strength, concrete mix ratio, and environmental conditions. The cooling rate shall not exceed L0 ℃/h.
(3) During the curing period of concrete with formwork, measures such as wrapping with formwork, watering, spraying water or steam should be taken to maintain moisture and moisture
(4) After removing the surface covering or demolding, the concrete should be moisturized and cured by storing water, watering, or covering with water. In addition, when the concrete surface is in a damp state, the exposed surface concrete can be quickly covered or wrapped with materials such as linen and grass curtains, and then the moisturizing materials such as linen and grass curtains can be covered (wrapped) in good condition with plastic cloth or canvas. During the coating (wrapping) process, the coating (wrapping) should be intact and completely overlapped with each other, and the inner surface should have condensed water droplets. Road sections with conditions should extend the curing time of the concrete coating (wrapping) as much as possible
(5) The continuous moisturizing and curing time of concrete after final setting should meet the requirements of Table 2
Table 2 Minimum wet curing time for different types of concrete
Atmospheric humidity (relative humidity ≥ 50%), no wind, no direct sunlight, average daily temperature t (℃) 5 ≤ t (6). During the concrete curing process, attention should be paid to taking insulation measures to prevent drastic changes in the surface temperature of the concrete due to environmental factors (such as sun exposure). During the curing process, the temperature difference between the concrete core and the surface, as well as between the surface and the environment, should not exceed 20 ℃ (when the section is complex, it should not exceed 15 ℃). Before the construction of large volume concrete, a strict curing plan should be formulated to control the temperature difference inside and outside the concrete to meet the design requirements
(7) After the concrete formwork is removed in winter and hot seasons, if the weather suddenly changes, appropriate insulation (cold season) and insulation (summer) measures should be taken to prevent excessive temperature stress in the concrete
(8) During the curing period of concrete, representative structures should be monitored for temperature, and parameters such as core temperature, surface temperature, ambient temperature, relative humidity, and wind speed of the concrete should be measured regularly. The curing system should be adjusted in a timely manner according to changes in concrete temperature and environmental parameters, and the temperature difference between the inside and outside of the concrete should be strictly controlled to meet the requirements.
When the average temperature of day and night is below 5 ℃ or the lowest temperature is below -3 ℃, it should be treated as winter construction
6. Demolition
The concrete strength should meet the design requirements. Only when the concrete strength reaches 2.5 megapascals or above and the surface and edges are not damaged due to formwork removal, can the side formwork be removed. The bottom formwork can only be removed after the concrete strength meets the requirements in Table 3.
Table 3 Concrete Strength Required for Demolition of Bottom Formwork
Structural span (m) ≤ 2 slabs, arch 2~8>8 ≤ 8 beams>8 cantilever beams (slabs)>2 100 ≤ 2 100 75% concrete design strength (%) 50 75 100 75 7
Core molds or internal molds with reserved holes can only be removed when the concrete strength can ensure that the surface of the component does not collapse or crack. In addition to considering the concrete strength during mold removal,
The demoulding time of concrete should also consider that the temperature of the concrete during demoulding (caused by the heat of cement hydration) should not be too high, in order to prevent the concrete from cooling too quickly and cracking when exposed to air, and even to prevent pouring cold water for curing at this time. The formwork shall not be removed before the temperature inside the concrete begins to decrease and when the temperature inside the concrete reaches its maximum.
Under normal circumstances, when the temperature difference between the core concrete and surface concrete of a structure or component and the environment is greater than 20 ℃ (for complex sections, the temperature difference is greater than 15 ℃), the formwork should not be removed in case of strong winds or rapid temperature changes. In cold seasons, if the ambient temperature is below 0 ℃, it is not advisable to remove the mold. In the hot and windy dry season, the demoulding process of segmented demoulding and simultaneous demoulding should be adopted. Demolition should be carried out in reverse order of template installation. Concrete shall not be damaged, and damage to formwork shall be minimized. When the formwork is separated from the concrete, it can be disassembled and lifted. The concrete structure after demoulding can only withstand all design loads after the concrete reaches 100% of the design strength.
4. Concrete quality inspection
1. General Provisions
Concrete quality inspection includes pre construction inspection, construction process inspection, and post construction inspection. Before construction, all inspection items should be qualified before construction; When the inspection items during the construction process are not qualified, the reasons should be analyzed in a timely manner and adjusted. Only after passing the inspection can construction continue. The post construction inspection items should be used together with the pre construction and construction process inspection items as the basis for quality assessment and acceptance.
2.
(1) According to the regulatory requirements, the product qualification certificates and factory quality inspection reports of the main raw materials such as cement, aggregates, mineral admixtures, additives, and water shall be inspected before construction. Concrete should be inspected on-site. Among them, the factory inspection results of the main raw material quality should comply with the relevant requirements of this technical specification. (2) According to the specification requirements,
Cement, aggregates, mineral admixtures, additives for concrete
The main raw materials such as water should be retested and the retest results should meet the relevant requirements of this technical specification
(3) The mix performance of concrete should be reviewed according to the design and construction requirements, and the mixing process and the mechanical properties, crack resistance, and durability test results of related concrete should be checked. The durability of concrete should be inspected by authoritative departments approved by the state and the Ministry of Railways or departments designated by the owner. The inspection results should meet the relevant requirements of this technical specification.
3. Construction process inspection
(1) Conduct daily quality inspections on the main raw materials used for concrete, such as cement, aggregates, admixtures, mineral admixtures, and mixing water, in accordance with regulatory requirements. The inspection results should meet the relevant requirements of this technical specification
(2) The performance of concrete mixtures should be inspected daily according to the specifications, and the inspection results should meet the requirements of the design, construction, and approved construction mix proportion.
(3) The mechanical properties of concrete should be routinely inspected according to the specifications, and the inspection results should meet the design and construction requirements
(4) The durability of concrete should be sampled according to the specifications, and the inspection results should meet the requirements of the design or this technical condition.
(5) In the process of concrete construction, if the types and specifications of main raw materials such as cement, admixtures, mineral admixtures, etc. are replaced, the concrete mix proportion selection test should be re conducted, and the mix proportion performance, mechanical properties, and durability of the test mix proportion concrete should be tested. The test results should meet the relevant requirements respectively
(6) The concrete strength and durability sampling specimens used for construction process control or quality inspection should be taken from the same concrete slab or concrete transported by the same truck
4. According to Appendix J, post construction inspection
The following methods should be used to inspect the quality of solid concrete: 1. Observe the surface of the solid structure with the naked eye or a magnifying glass to see if there are non external cracks. When non external cracks appear on the surface of the concrete, the maximum width of the cracks on the surface of ordinary concrete structures should not exceed 0.20mm, and prestressed concrete structures should not have structural cracks
2. The detection of the thickness of the concrete protective layer should use non-destructive testing methods (when there are doubts about the detection results of the thickness of the concrete protective layer, local damage method can be used for review, and the damaged parts should be repaired in a timely manner after review). The inspection results should meet the design requirements
3. According to the specific requirements for core sampling in TBL 0426-2004, concrete core samples are randomly drilled on cast-in-place concrete solid structures. The electrical flux measurement results of the solid concrete are determined according to Appendix A and should meet the requirements of the design or this technical specification.
4. When the design requires concrete frost resistance, concrete core samples should be randomly drilled from the cast-in-place concrete solid structure according to the specific requirements of TBL 0426-2004 for core sampling. According to dl/t5051-200l, the measurement results of the bubble spacing coefficient of solid concrete should meet the design requirements.
