[Abstract] Concrete structures play an absolute role in the safety, reliability and durability of Engineering structures. Therefore, the quality control of concrete is very important. Combining with practical experience, the author elaborates the main aspects of concrete quality control in detail.
[keywords] quality control of concrete pouring technology
In the field of Engineering construction, concrete structure occupies an absolute proportion in the construction market because of its low cost, mature design theory and construction technology. However, in the process of concrete construction without pouring, due to improper use of materials, construction technology and management, the following common quality defects often occur:
1. The surface of concrete is linen, bars, honeycombs and holes.
2. Poor combination of construction joints
3. missing edge and missing angle
4. Irregularity of concrete slab
5. Axis Displacement of Concrete Structural Components
6. Shape deviation and surface smoothness of concrete columns, walls and beams
7. Low strength and poor homogeneity of concrete
8. Destruction of concrete protective layer or poor protective performance of concrete
9. Concrete shrinkage cracks, concrete temperature cracks, concrete subsidence cracks, concrete tension cracks, concrete chemical cracks, concrete frost heave cracks, concrete component fabrication, transportation, installation cracks, transverse and longitudinal cracks on the floor surface of pre-stressed circular orifice, pre-stressed circular orifice floor slab bottom cracks
10. Wire breakage, slippage and loosening of steel wire in pre-stressed circular orifice floor slab
11. Poor concrete quality of concrete pile body
How to control the quality of concrete is a very common and important work in quality control. I personally think that in order to control the quality of concrete, we must start from three aspects: raw material quality control, construction technology and maintenance measures. The following three aspects of a brief talk about personal views.
I. Quality Control of Raw Materials for Concrete Casting Construction
In order to control the construction quality of concrete engineering, we must first control the quality of raw materials, and secondly, the scientific mixing ratio of concrete raw materials, which is the prerequisite to ensure the quality. Concrete is a kind of artificial foundation firmware which is made of cement, sand and stone mixed with water and solidified evenly. The quality of concrete directly affects the safety of building structure. Many construction quality accidents are due to Jerry work, not in accordance with the proportion of design drawings and concrete materials, resulting in insufficient strength of concrete structure, resulting in structural defects of buildings, resulting in serious quality accidents. Therefore, in actual construction, mixing concrete materials must be strictly in accordance with scientific mix proportion.
Ordinary concrete is made of cement, chemical admixture, fine aggregate, water, sand, stone and other mixed materials, which are mixed in a certain proportion and poured after uniform mixing. From the concrete expression FCU = as*fce (c/w-ab), it can be seen that the compressive strength of concrete is proportional to the strength FCE of mixed cement. At the same time, the ratio of cement to water is proportional to the strength of concrete. The ratio of cement to water is large, the strength of concrete is high; the ratio of cement to water is small, and the strength of concrete is low. The water cement ratio will affect the rheological properties of concrete and its compactness after hardening. Therefore, under the given conditions of other materials, the water cement ratio is the main parameter to determine the strength, durability and other physical and mechanical properties of concrete. When the cement-water ratio remains unchanged, it is also wrong to attempt to increase the strength of concrete by increasing the amount of cement. When the cement-water ratio remains unchanged, increasing the amount of cement can only increase the workability of concrete and increase the shrinkage and creep of concrete.
2. Quality Control of Concrete Casting Construction Technology
1. Before pouring concrete, the following preparations should be made:
1.1 A reasonable pouring process plan should be formulated according to the situation of the structure to be poured, environmental conditions and pouring quantity. The process plan should clearly stipulate the setting of construction joints, pouring sequence, pouring tools, anti-cracking measures, protective layer and control of settlement joints.
1.2 The bracket, formwork, reinforcing bar and embedded parts should be inspected. The debris, water and dirt on reinforcing bar in formwork should be cleaned up. If there are gaps or holes in the formwork, it should be plugged tightly and without leakage of slurry.
1.3. The uniformity and slump of concrete should be tested.
2. When dumping concrete into formwork from height, concrete segregation should be prevented. When dumping directly, its free dumping height should not exceed 2m; when exceeding 2m, it should fall through the string barrel, chute (chute) or vibration chute (chute) and other facilities; when dumping height exceeds 10m, deceleration device should be installed.
3. Concrete should be layered in a certain thickness, sequence and direction, and the upper concrete should be completed before the initial setting or remodeling of the lower concrete; when the upper and lower layers are concreting at the same time, the distance between the upper and the lower layers should be kept at more than 1.5m; when pouring concrete on the dumping surface, it should be expanded and raised layer by layer from the lower, and the horizontal layering should be maintained. The thickness of layered concrete shall not exceed the requirements of the following table:
Thickness of Layered Concrete Casting
Thickness of pouring layer by vibrating method (mm)
Inserted Vibrator 300
Using attached vibrator 300
250 when using surface vibrator without reinforcement or sparse reinforcement
150 when reinforcement is tight
4. When vibrating concrete with a vibrator, the following requirements shall be met:
4.1 The displacement distance of the insert vibrator should not exceed 1.5 times of the radius of the vibrator, and should be 50-100 mm away from the side formwork, and the depth of insertion into the lower concrete should be 50-100 mm.
4.2 The displacement distance of the surface vibrator should enable the vibrator plate to cover the vibrated part not less than 100 mm.
4.3 The arrangement distance of attached vibrator should be determined by test according to the shape of the structure and the performance of the vibrator.
4.4 The vibration duration of each vibration point is 20-30 s, and the degree is that the concrete stops sinking, no bubbles appear and the surface appears slurry.
5. The concreting of concrete should be carried out continuously. When the interval is interrupted, the interval time should be no less than the initial setting time or the remolding time of the front concrete. The whole time of concrete transportation, pouring and intermittence shall not exceed the stipulations in the table below; when it exceeds, it shall be handled according to the interruption of pouring, and the construction joints shall be retained and recorded.
Full running time of concrete transportation, pouring and intermittence (min)
Concrete strength grade temperature is not higher than 25 and temperature is higher than 25 in concrete strength grade temperature is not higher than 25 and 25 in concrete strength grade temperature is higher than 25 65
Less than C30 210 180
>C30 180150
6. The location of construction joints should be determined before concrete pouring, and should be located in the parts where the shear force and bending moment of the structure are small and easy to construct. Horizontal or vertical planes should be set for construction joints. The treatment of construction joints shall comply with the following provisions:
6.1 The weak layer on the surface of treated concrete should be chiseled out. For the concrete strength of treatment layer, it should reach 2.5 MPa when using water to wash and chisel, 2.5 MPa when using artificial chisel and 10 MPa when using pneumatic chisel. 6.2 Clean water should be used to wash the concrete surface after chipping.
6.3 Anchor bars should be inserted at construction joints in important parts and concrete structures with sparse steel bars required for seismic resistance; concrete with impermeability requirements, i.e., construction joints should be concave, convex or water-stop belts; construction joints should be poured or chiseled into steps when they are inclined.
7. Before pouring concrete, we should carefully check the position, quantity and tightness of the reinforced protective layer cushion blocks, and designate a special person to do repetitive checks. The cushion blocks on the side and bottom of the components should be at least 4/m2, and the wire heads that bind the cushion blocks and reinforcement bars should not extend into the protective layer.
8. When pouring concrete in hot season, formwork and newly poured concrete should be avoided to be directly exposed to sunlight, so as to ensure that the temperature of formwork and reinforcing steel bar before pouring concrete into formwork is not more than 40 C. Concrete should be poured as late as possible to avoid the hot day. When pouring concrete in the environment with low relative humidity and high wind speed in this area, appropriate measures such as wind shielding should be taken to prevent excessive water loss on the surface of concrete.
3. Quality Control of Maintenance after Concrete Casting
1. After general concrete pouring is completed, it should be covered and watered as soon as possible after grouting. For dry hard concrete, high strength and high performance concrete, concrete poured in hot weather and exposed concrete on bridge deck, the initial moisturizing maintenance should be strengthened. If conditions permit, a shed can be installed immediately after completion of pouring, and then be covered and sprinkled after grouting. Covering should not damage or contaminate the surface of concrete. When the concrete surface is covered with formwork, the formwork should be kept wet throughout the maintenance period.
2. The water used for concrete maintenance must be used if it has passed the inspection and may not be taken freely for maintenance. The water spraying and moisturizing maintenance time of concrete should not be less than 7 days. When the temperature is lower than 5 C, measures of heat preservation and maintenance should be taken, and no water should be sprayed on the surface of concrete.
3. Concrete curing adopts measures such as wrapping with film, watering and spraying water to maintain moisture and humidity.
4. In order to prevent the occurrence of non-stress cracks, thermal insulation measures should be taken during concrete curing to prevent drastic changes of surface temperature due to environmental factors (such as sunshine, sudden drop of temperature, etc.). Especially for important structures, the temperature difference between core and surface, surface and environment should not exceed 20 C during curing.
5. When the weather changes abruptly after removing the concrete formwork in winter and hot season, appropriate thermal insulation (winter) measures should be taken to prevent excessive thermal stress of concrete.
6. Before the strength of concrete reaches 1.2 MPa, it shall not be trampled on it; before the strength reaches 2.5 MPa, it shall not be subjected to the load of pedestrians, transport vehicles, formwork, brackets and scaffolding.
epilogue
From the above, it can be seen that the factors affecting the quality of concrete are raw material quality, construction technology and maintenance measures. In the construction of construction projects, materials, measures and management are interlinked. In order to control the quality of concrete, we must deal with each link from three aspects. In practical projects, first of all, we should start with the quality management of personnel, train high-quality personnel, improve the overall construction level of the construction team, strictly follow the design and related specifications, so that every step of the project can be effectively monitored and controlled. Only in this way can the final quality of concrete be guaranteed.