I. Types of cracks
1. Dry shrinkage crack
During the hardening process of concrete, water gradually evaporates, which makes the gel in cement paste dry, shrink and deform. Due to the constraints of surrounding rock and formwork, deformation produces stress, and when the stress value exceeds the tensile strength of concrete, shrinkage cracks will appear. Shrinkage cracks are mostly surface cracks with irregular trend. The main factors affecting concrete shrinkage cracks are: cement variety, cement dosage, water-cement ratio, aggregate particle size gradation, admixture variety and dosage.
2. Temperature cracks
A large amount of heat is generated in the process of cement hydration, which forms a temperature gradient inside and on the surface of concrete and produces stress. When the temperature stress exceeds the binding force inside and outside the concrete, temperature cracks will occur. Temperature cracks are generally wider in winter and narrower in summer. The main factors are the thickness of secondary lining concrete and the variety and dosage of cement.
3. Load deformation cracks
If the virtual slag of inverted arch and side wall foundation is not cleaned up, after concrete pouring, the foundation will settle unevenly, the formwork trolley or plug plate will not be fixed, and the concrete will be hit by a large external force when it is demoulded prematurely. The proportion of load deformation cracks in tunnel lining concrete diseases is increasing year by year.
4. Construction joints
During the construction process, due to power failure, mechanical failure and other reasons, concrete pouring is interrupted or the power failure time exceeds the initial setting time of concrete. When pouring concrete, the surface of the original concrete is not chiseled away or treated incompletely, which leads to cracks at the joint between new and old concrete.
Second, the reasons for the formation of cracks
1. Rough design? The work of construction and supervision units is arbitrary.
Due to various reasons, the survey and design unit can not carry out geological exploration in depth, and the classification evaluation and structural design of tunnel surrounding rock lack scientific basis, which is blind to some extent. Individual construction units are limited to their own management and lack of professional and technical level, and change the design at will. A few projects are supervised by the owner's internal personnel, and the supervision work has lost its independence. However, with the standardization of the construction market, these problems will be gradually solved.
2. The site operation of the construction technology is not standardized.
① The thickness of lining concrete is seriously uneven due to poor tunnel excavation and molding. Underexcavation and overbreak support intrude into the lining limit, resulting in insufficient thickness of lining concrete, and there is void behind the lining concrete of individual tunnels.
② Monitoring and measurement work was not carried out, and the construction time of the secondary lining was determined only by experience, which led to poor safety and reliability, resulting in the secondary lining exceeding the design load and bearing the surrounding rock pressure.
(3) The measurement error of raw materials in concrete production is large, especially the random addition of additives, and the construction water consumption is not adjusted in time according to the actual moisture content of sand and gravel, which leads to the increase of water cement ratio of concrete, and the phenomenon of adding water in concrete transportation and pumping is also common.
(4) The integral steel formwork trolley is used for construction, and the concrete is not vibrated in place or leaked during pouring, resulting in uneven density of concrete.
⑤ Blind pursuit of construction progress, premature demoulding time and imperfect curing conditions make concrete bear load prematurely and destroy the internal stress structure of concrete.
⑥ During construction in summer, sand and gravel are piled up in the open air without cooling measures, and the temperature of concrete is too high when it enters the mold; Thermal insulation measures in winter construction are not effective.
3. Poor quality of raw materials and unreasonable mix design.
Improper selection of cement varieties, insufficient stability, mixing of different batches of cement, poor matching between crushed stone and sand, excessive mud content, large stone powder content in crushed stone and too many needles, which affect the combination of cement and aggregate. In the mix design, the influence of increasing cement dosage on concrete quality is ignored, and it is mistakenly believed that the more cement dosage, the higher the concrete strength. In addition, if the selection of additives and additives lacks the guidance of professional and technical personnel, it often fails to achieve the expected results.
Three. Treatment of concrete cracks
1. Tiny cracks
There are often some small cracks on the concrete surface of tunnel lining, which are stable and can generally heal themselves without affecting the use and durability of the structure. From the aesthetic point of view, the crack surface can be cleaned first, then the epoxy resin slurry can be brushed for two or three times, and finally the concrete surface can be treated with scraper and pigment to make its color consistent with that of the surrounding lining concrete.
2. Perforation
Through cracks are harmful, so effective treatment methods must be taken. Drill a V-shaped groove with a width of 5cm and a depth of 3cm along the direction of the crack. Drill a hole every 0.5m in the groove. The depth of the hole is 1/2 or 2/3 of the lining thickness, generally not less than 15cm. It is not allowed to penetrate the lining to prevent slurry leakage. Rinse the sundries and dust in the tank with clear water, insert the grouting pipe with a diameter of ￠ 10 into the hole, anchor it with epoxy cement mortar, and compact and smooth the mortar with a grey knife. What is the ratio of epoxy mortar to epoxy resin? Cement? Yarn? Ethylenediamine dibutyl ester = 1? 1.6? 3.2? 0. 1? 0. 12。 Wherein ethylenediamine is a curing agent and dibutyl ester is a diluent. After the epoxy resin mortar has a certain strength, the cement-water glass slurry or epoxy resin slurry is pressed under the pressure of 0. 1.5 MPa ~ 0.2 MPa. After grouting, check the grouting effect by pressing water at a pressure of 0.2MPa, and treat the crack surface with scraping and coloring materials.
3. Dense cracks
There are holes behind the lining or dense cracks caused by insufficient lining thickness, so waterproofing and ground reinforcement must be carried out. Staggered points are arranged at intervals of 1.2m ~ 1.5m along both sides of the crack, and a local groove with a depth of 5cm 10cm× 10cm is drilled with a pneumatic rock drill, with a hole depth of 3m. Install WDT25 hollow grouting anchor and inject cement mortar, with water cement ratio of 1? 1? During construction, grouting should be carried out layer by layer from top to bottom, and the grouting pressure is 0.4 MPa ~ 0.6 MPa. After grouting, drill a new hole, press pure cement slurry under the pressure of 0.6 MPa ~ 1.0 MPa, and check the grouting effect. When the specified pressure can not be pressed in, it is considered that the grouting is full. After 24 hours of grouting, install the anchor pad, wipe the square groove with epoxy mortar, and treat the surface with scraping and coloring materials.
Four, how to prevent concrete cracks
1. Improve the design accuracy; Strengthen the geological work in the early stage of the project; Provide detailed engineering geological and hydrogeological exploration data for the design; Improve the design quality.
2. Close the good materials and strictly control the quality of raw materials.
① Cement: Ordinary portland cement is mostly used in the construction site, but the unit cement consumption should be reduced as much as possible. Different brands, specifications and batches of cement cannot be mixed.
(2) Gravel: The gravel standard shall be selected in strict accordance with the design specification of concrete mixture ratio, and the mud content and needle content of gravel shall be strictly controlled.
③ Sand: medium sand with good matching is adopted, the fineness modulus is 2.3 ~ 3.0, the proportion of particles with particle size less than 0.3 15mm is 15 ~ 20%, and the silt content is strictly controlled within 3%, so as to facilitate the transportation and pouring of concrete.
④ Water: Choose drinking water. When using other water, it should be implemented according to the current national standards, and the PH value should be greater than 4. The greater the water-cement ratio, the greater the drying shrinkage of concrete. Strictly controlling the water consumption of pumping concrete is one of the main measures to reduce cracks. During construction, the water cement ratio is generally controlled between 0.45 and 0.55, and the slump is controlled between (12 2) cm.
⑤ Admixtures: The superplasticizer can effectively reduce water consumption, reduce hydration heat, delay the release rate of hydration heat, and thus reduce temperature cracks. However, too much admixture will cause the expansion and cracking of concrete, so the variety and dosage of admixture must be carefully selected during construction.
3. Strictly control the concrete construction technology.
① Control blasting parameters, improve blasting level, strengthen the detection ability of tunnel excavation section size, strictly control over-excavation and under-excavation, and create a good environment and conditions for lining construction.
(2) The construction time of secondary lining should be carried out after the deformation of surrounding rock and primary support is basically stable. When the deformation of surrounding rock is large and the rheological characteristics are obvious, and the secondary lining is needed in advance, the primary support or lining structure must be strengthened.
③ Mixing of concrete
Calculate the dosage of various materials in strict accordance with the grading standards, and check the metering device regularly. Strengthen the detection of aggregate water content and adjust the water consumption of mixture in time. Ensure the workability of concrete.
④ Concrete pouring
It is forbidden to add water to concrete during transportation and transportation. Properly slow down the pouring speed, pour the side walls on both sides symmetrically in layers until the joint between the wall and the arch stops 1h ~ 1.5h, and then pour the arch concrete after the side wall concrete sinks and stabilizes.
Vibrate in time during pouring to improve the compactness and uniformity of concrete, reduce internal micro-cracks and pores, and enhance the crack resistance of concrete.
⑤ demoulding and curing of concrete
The strength of concrete formwork removal must meet the design and specification requirements. It is forbidden to dismantle the formwork in advance in order to catch up with the construction period or improve the turnover of the formwork. When dismantling the formwork, pay attention to | source | test | protect the concrete. After the formwork is dismantled, it shall be maintained in time. The traditional sprinkler maintenance method not only increases the difficulty of civilized construction in the tunnel, but also makes the early strength of concrete not guaranteed because of uneven sprinkler. Therefore, it is suggested to spray concrete curing liquid for curing.