In the construction project, the basement structure is a high incidence of quality defects, and the highest frequency is the problem of concrete cracks, and the resulting maintenance costs also account for the vast majority of the total maintenance costs. Therefore, the effective prevention and control of concrete cracks is the key to ensure the construction quality of the basement structure, and even the overall quality of the project to achieve excellent standards. The example project in this paper is a resettlement housing project undertaken by zhejiang peihua construction, and the project department takes countermeasures against the main factors of cracks and achieves the expected results, which is worthy of reference for engineering friends.
At the beginning of the construction, the project department team carried out on-site investigation on the construction quality of the completed and put into use concrete structure basement in the region, combined with the previous construction experience of the company and the project department, as well as the current construction specifications and technical standards, to comprehensively analyze the causes of cracks, and confirmed the end factors.
Through a comprehensive analysis of the end factors, the project team concluded that the main factors responsible for the cracking defects of reinforced concrete basement structures in this region are as follows.
Factor 1: Lack of quality awareness
Factor 2: Non-optimized concrete mix ratio
Factor 3: Excess of same section of joint
Factor 4: Longitudinal tendons are not straight
Factor 5: Steel skeleton, loose nets
Factor 6: Dense concrete vibrations
Factor 7: Inadequate conservation
The project team has developed detailed countermeasures for the above major factors and implemented them during construction, while setting up staff to supervise the implementation of each countermeasure, which has achieved the expected results.
Factor 1 Lack of quality awareness
- Basement before the construction and during the construction process, carry out quality education, technology transfer, group activities, invite the party, design, supervision and other units to technical guidance, improve the quality awareness and ability of the whole staff.
- Establish the reward and punishment mechanism, according to the basement partition block, every completion of a block that is to each team to complete the quality assessment, reward the first and punish the last, so that clear rewards and punishments, reverse the one-sided pursuit of progress and lead to quality defects occur from time to time the status quo.
- The construction site set up process sample area, process flow and operation guide graphic display, including steel, formwork, concrete, installation, waterproof and other professional engineering process samples, as a model for the subsequent construction of large areas, to guide the actual operation.
Conduct quality education and technology transfer
Technical guidance from various participants
Set up a sample area to show the processes and operating instructions for each process.
Determine the effect: Improved quality awareness and the quality of construction in all teams has improved significantly.
Factor 2 Non-optimized concrete mix ratio
- Basement concrete to add hea expander in advanced areas of application mature, but in this region is still the first time, the performance is not familiar with. Hea expander added to the basement wall concrete, if it can not provide sufficient chemical reaction of the water required, not only can not improve the anti-cracking and seepage properties of concrete, but will accelerate the concrete cracking. In view of this property of hea expander, the problem of formulating concrete to compensate for shrinkage is a major factor in the cracking of basement concrete. Before the supply of concrete, we conducted three rounds of test mixes and sample tests with party a, supervisor and commercial concrete company, and adjusted and optimized the construction ratio, so as to reach the standard of concrete strength and at the same time the expansion rate, dry shrinkage rate and seepage resistance to meet the design requirements.
- After determining the concrete proportion, we arranged special personnel to supervise the concrete proportion control in the mixing plant of the company, measure accurately, and arrange for the implementation of sampling inspection of the slump of each truck of incoming concrete, and return any concrete that exceeds the design value.
- In addition to witnessing the retention of the same condition specimens, standard curing specimens and impermeable specimens of the mixed soil, at the same time, the beaker and micro-expansion specimens of the compensating shrinkage mixed soil are retained to observe the expansion effect of the specimens and test the expansion rate. The two types of specimens were used to test whether the compensatory shrinkage concrete poured was “Swollen” And “Not swollen” From “Qualitative” And “Quantitative” Aspects respectively. How much has it expanded”.
Preparation and standard maintenance of specimens for the beaker retention method
Beaker specimens left in each pouring block
Fabrication of micro expansion specimens
Concrete expansion rate specimen testing, expansion rate in line with design requirements
After the expiration of the age, the strength and anti-seepage performance conform to the design requirements, and 3 fine surface cracks are found in the basement at the acceptance stage, which reduces the crack defects of the basement poured with common anti-seepage concrete without swelling agent by more than 80% compared with the previous ones.
Factor 3 Excess of same section of joint
- In the local area, the understanding that the horizontal longitudinal tendons of the basement exterior walls are distribution tendons is deep-rooted, and the joint area of lap joints of the same section is generally controlled at 50 per cent, or even 100 per cent, and cracks are formed artificially at the joint concentrations. In view of this status quo, the steel reinforcement team was specially organized to carry out specification learning and technical transfer, requiring lap length multiplied by factor 1.3, joint area not exceeding 25%, and made clear regulations on joint position, anchoring and so on.
- Strengthened the site inspection, found any violation must be reworked.
Determine the effect: The horizontal longitudinal bar joints of the basement exterior wall are staggered, eliminating the regular vertical cracking of concrete at the joint concentration.
Factor 4 Longitudinal tendon not straight
- Strengthen the protection of raw materials and finished products, raw materials in batches, classified overhead stacking, semi-finished products stored in the isolated temporary site.
- Made of steel skeleton, net, the diameter of 16 and below the main bar is not straight is more common, the bending deformation of steel reduces the ability of the skeleton to resist concrete expansion and dry shrinkage stress. In the construction, strictly in accordance with the design drawings and construction specifications for material, processing, production and installation. The main tendons of the walls and columns are positioned with positioning bars, and the longitudinal bars of beams, plates and walls are calibrated. All steel frames and nets are tied, and jumping is strictly prohibited.
- Production and installation is completed, self-inspection and acceptance requirements can form a solid whole, to achieve horizontal and vertical, spacing, row spacing, anchoring length, protective layer control, etc. To meet the requirements. Each block, each part after three inspections by the supervisor, party a qualified acceptance before sealing or tamping.
Implementation of the three inspections, strengthening quality control
Determine the effect: The skeleton and web reinforcement are positioned accurately and horizontally.
Factor 5 Steel skeleton, loose netting
- The steel dislocation connection, the wall, column vertical steel using electroslag pressure welding connection, the wall ties the horizontal tendons lap joint, plate tendons and the upper tendons of the beam diameter of 22 or more by mechanical connection, plate tendons diameter of 20 or less by lap joint welding connection.
- The steel framework and net are fully tied, and the reinforcement is strengthened by cross-tying with no. 20 wire tie in the areas with intensive reinforcement and areas prone to loose deformation during tamping.
- In the process of reinforcing steel installation, professional engineers of water, electricity and fire-fighting are assigned to work with the holes and pre-buried pipe fittings at the same time, and it is strictly prohibited to change, open holes and cut steel bars afterwards.
Professional team will work together with the construction.
Determine the effect: Steel processing specification, accurate positioning, made of stable skeleton, effective resistance and control of concrete cracking.
Factor 6 Concrete vibrations are not compacted
- Basement before the construction of the basement, carry out the second basement concrete tamping, the second wiping technology transfer, the implementation of the person in charge of the entire standing records, command, to prevent leakage of vibration, over vibration.
- According to the preliminary concrete initial setting time of 2 hours and 45 minutes, the slump of concrete is 20～40mm, which is determined by the test mix with the concrete company, the secondary concrete tamping time should be within 2 hours after the concrete leaves the factory, the concrete slump is 50～80mm, in combination with the temperature change.
- After the second tamping, cover the surface of the second artificial wiping film to prevent water loss and cracks on the concrete surface.
Implementation of secondary tamping and secondary surface coating
Determine the effect: The implementation of secondary tamping improves the concrete density and perception of quality, which can effectively prevent concrete cracking and other defects.
Factor 7 Inadequate maintenance
- The bottom plate and top plate concrete pouring should be carried out with “Two vibrations and two wiping”, after which the film should be covered to keep warm and moisturized, 24 hours for bottom plate and 48 hours for top plate before dismantling the film to measure and release the line, after the completion of measuring and release the line, the blanket should be covered and watered to keep not less than 14d. To prevent the temperature stress of the bottom plate and top plate and the temperature of the top plate. Shrinkage stress generates cracks, and at the same time constrains the wall to produce cracks.
- The time of demolition of side moulds is controlled 24 hours after the completion of concrete pouring, which can reduce the evaporation loss of water in the mould and protect the finished structure, and reduce the damage of human factors to the structure.
- After the demolition of the basement walls and columns, arrange someone to spray the walls and columns for maintenance at the first time after the demolition of the mold to ensure that the concrete surface remains moist. Column timely wrapped film and irrigation maintenance, inside and outside the top of the shear wall on both sides of the installation of the drip system for 24 hours uninterrupted drip moisturizing maintenance, arrange for a 24-hour patrol, maintenance time of not less than 14d.
The second wiping immediately covered with film moisturizing maintenance
Cover with a film and water manually for moisture maintenance
Basement bottom, top plate surface after proofing blanket watering moisture maintenance
Drip irrigation system for moisturizing and maintenance of exterior and interior roofs
Determine the effect: The maintenance measures are effective, the strength of the concrete in the basement blocks meets the design requirements after the expiry of the age, and there are two micro-cracks.
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