Concrete crack repair
The types of cracks in tunnel lining concrete mainly include: shrinkage cracks, temperature cracks, deformation cracks caused by external load, and stubble joints caused by improper treatment of construction joints.

1. Shrinkage cracks
In the hardening process of concrete, moisture gradually evaporates and dissipates, which causes the condensation colloid in the cement stone to dry shrink and produce deformation. Due to the constraint of surrounding rock and formwork, deformation produces stress. When the stress value exceeds the tensile strength of concrete, dry shrinkage cracks will appear.
Dry shrinkage cracks are superficial and tend to be irregular.
The main factors influencing shrinkage crack of concrete are: cement varieties, dosage and water-cement ratio, aggregate size and gradation, admixture varieties and parameters.
2. Temperature crack
A large amount of heat generated in the hydration process of cement will form a temperature gradient between the inside and the surface of concrete and generate stress. When the temperature stress exceeds the constraint force of the inside and outside of concrete, temperature cracks will occur.
The crack width is wider in winter and narrower in summer.
The temperature crack is related to the thickness of secondary lining concrete and the type and amount of cement.
3. Load deformation cracks
The empty ballast of invert and side wall foundation is not cleaned up, and uneven settlement of the foundation occurs after concrete pouring.
Formwork trolley or plug plate is not fixed firmly, and premature demolding, or demolding concrete by large external force impact and so on are prone to deformation cracks.
The proportion of load-deformed cracks in tunnel lining concrete is increasing year by year.
4. Construction joints (stubble joints)
Due to power outages in the process of construction, mechanical failure interrupted concrete pouring, time, more than the initial setting time of concrete, to continue pouring concrete, the surface of the concrete foundation of the original has not been cut hair, or no rinse after cut hair, nor, cement mortar on the original concrete pouring concrete, causing the cracks between the old and new concrete summer-planting.

I. analysis of the causes of fracture formation

The reason of concrete crack formation is very complicated, which is often the result of a variety of adverse factors.
According to the shape of cracks on the surface of the tunnel door wall and can be determined as dry shrinkage cracks, the concrete production of raw materials measurement error is large, especially the amount of admixtures are arbitrary, not according to the actual moisture content of sand, stone timely adjustment of the construction water consumption, resulting in the increase of concrete water-cement ratio.
During summer construction, sand and stone materials are piled in the open air, without practical and effective cooling measures, and the temperature of concrete entering the mold is high;
Improper selection of cement varieties, poor stability, different batches of cement mixed.
Grading difference of gravel and sand;
The content of mud exceeds the standard, the content of stone powder in the crushed stone is large, the needle, the flake is too much, affected the cement and aggregate cementation.
The phenomenon of adding water in the process of concrete transportation and pumping is also common.
Adopt integral steel formwork trolley construction, concrete pouring time does not vibrate or leakage vibration, concrete homogeneity is poor, no wet curing of concrete after stripping;
These are the causes of lining cracks.

Ii. Concrete crack treatment scheme

As a brittle material composed of many groups of materials, the existence of cracks in concrete is objective.
In the construction of concrete lining concrete construction management should be strengthened to avoid or reduce the concrete cracks;
For the cracks, we should carefully analyze the reasons, distinguish between harmful cracks and harmless cracks, and deal with the harmful cracks, so as to prevent the further development of cracks and affect the stability of lining structure.
According to the present situation of field investigation and observation, grouting and caulking methods were used to repair the cracks in the side wall of tunnel portal.
Through grouting, the cracks are sealed and bonded internally, and special grouting materials are used to repair the grooves 3cm wide and 2cm deep with grooves cut externally, so as to eliminate potential safety and quality hazards and maintain the overall appearance and integrity.

The method of micro damage -- the method of reinforcing epoxy resin with low viscosity modified by pinhole oblique drilling;
High-pressure grouting materials by pinhole method: hk-uw-3 and hk-wg21;
Materials: fast curing solvent-free epoxy resin chemical grouting materials;
Cleaning: clean the surface with a wire brush, then blow the surface clean with an air compressor, check and analyze the cracks in detail, and determine the location, spacing and depth of the drilling hole.
Drilling: using high power drilling tools such as impact hammer drilling cross on both sides along the fissures, pitch in 25 to 30 cm bit 10-14 mm in diameter, hole Angle is 45 ° or less, two-thirds of drill hole depth structure or less thickness, hole must be through the cracks, but shall not be structure through (except structure after grouting), drilling and crack spacing 1/2 or less structure thickness.
Seam washing: blow air into the grouting nozzle with air compressor to clean the fine dust in the seam and observe the cracks.
Insert nozzle: install grouting nozzle (also known as water stop needle) in the drilled hole, with the structure of check valve), tighten it with a special hexagon wrench, make the expansion bolt after the needle expand open, close the crack surface, leave observation hole and pressure relief vent.
Grouting: use a high-pressure grouting machine to inject epoxy resin slurry into the grouting hole (nozzle), and gradually pour it from the bottom up or from one side to the other side. When the grouting starts from the observation hole on the adjacent hole or crack surface, maintain the pressure for 10-30 seconds, observe the grouting situation in the crack by controlling grouting, and then set it as supplementary grouting.
Nozzle removal: after grouting is completed, the exposed nozzle can be removed or knocked out by confirming that the epoxy resin is fully cured.
Groove cutting: use fixed width groove cutting machine to cut the cracks, and then use special grouting material to repair the groove, until its strength and color to meet the same requirements of the second lining concrete surface, with a polishing machine to clean the surface, the basic restoration of the original surface color.

Prevention and control of cracks in tunnel lining concrete

1. Improve design accuracy and strengthen advance geological prediction

Strengthen the geological work in the early stage of the project, provide detailed engineering geological and hydrogeological exploration data for the design, improve the quality of the design, at the same time strengthen the geological prediction work in advance in the construction, provide accurate data basis for the bad location.

2, put good materials into the customs, strictly control the quality of raw materials and technical standards

Cement: cement of different brands, specifications and batches cannot be mixed;
Ordinary Portland cement is often used in construction sites, but the amount of cement per unit should be reduced as far as possible.
Crushed stone: according to the inner diameter of the pumping pipeline, select crushed stone with larger particle size as far as possible;
Strictly control the mud content not more than 1%, needle, flake content not more than 15%, particle size of 5-31.5mm is appropriate, the maximum is not more than 40mm.
Sand: medium sand with good gradation shall be used. The fineness modulus shall be 2.3-3.0, and the particle size shall be less than 0.135mm. The proportion of the content shall be 15%-20%, and the mud content shall be strictly controlled within 3%.
For the convenience of concrete transportation, pumping and pouring, the sand ratio is 35%-45%.
Water: drinking water is the best choice.
Admixtures: in the construction of admixtures must be carefully selected varieties and amount of admixtures, high-efficiency water reducer can effectively reduce mixing water, reduce hydration heat, slow hydration heat release speed, so as to reduce temperature cracks, but too much will cause concrete swelling and cracking.

3. Strictly control the construction operation of each tunnel to improve the construction quality

Strengthen the detection of tunnel excavation section, strictly control the over-excavation and under-excavation, and create good conditions for the later lining construction.
On the premise of ensuring that the excavation section molding meets the design requirements, the initial support should be strictly controlled so that the shotcrete cannot occupy the secondary lining clearance, resulting in the lining thickness not meeting the design requirements.
Before invert and low side wall construction, it is necessary to clean up the foundation dregs, sundries and stagnant water, so as to avoid the influence of different degrees of subsidence and deformation of concrete on the quality of secondary lining in the later stage.
The construction time of secondary lining should be carried out when the surrounding rock and the initial supporting deformation are basically stable.
When the surrounding rock deformation is large and the rheological characteristics are obvious, it is necessary to strengthen the initial support or lining structure when the secondary lining should be carried out in advance.
Secondary lining concrete pouring, the whole process should be technical personnel for the side station, and it is strictly prohibited to add water to the concrete in the process of transportation and pumping.

4. Control and selection of concrete mix ratio

Measure strictly according to the construction batching list, check and correct the measuring device regularly;
Strengthen sand and stone moisture content detection, timely adjust mixing water consumption.
Improve aggregate gradation, add fly ash or high efficiency water reducer to reduce cement consumption, reduce hydration heat.
Concrete with a certain amount of admixtures with water reduction, plasticizing, retarding and other functions, improve the fluidity of concrete mixture, water retention, reduce hydration heat, delay the emergence of heat peak.
Control the temperature of concrete injection;
In construction of the summer, when the temperature higher than that of 30 ℃, ballast, mixing machine should be set-up awnings, rinse with cold water cool gravel;
Try to arrange to pour concrete at night.