The scheme of a cracked overpass pasting the carbon fiber sheet

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Introduction

Analysis of the cracks’ reasons

According to the relevant inspection report, combined with the structural characteristics, mechanical principle, the cracks of the overpass are mainly divided into contracted crack, temperature crack,construction crack and defective crack.

Classification

Analysis of the cracks’ reasons
Points above are as follows:

1, contracted crack
The cracks of all ordinary steel bar, concrete box and sterna of the ramp are contracted cracks. its characteristics are: cracks are the vertical cracks (if it suffers stress, the cracks should be inclined), most of the parts are distributed in the midspan(inclined cracks that suffer the stress is mainly distributed near the fulcrum).The causes of the cracks in this place are the sterna is relatively thick and shrinkage is larger. at the same time, during the construction side template is removed earlier. maintenance water can’t reach the sterna so that it causes that the sterna’s shrinkage is bigger.This kind of crack has been found during the construction.After nearly three years operation, checking the scene in recent times, the crack has little change.

2, Temperature crack and partial stress crack
Transverse crack that is distributed in the bottom of the beam near the pivot of approach bridge is mainly the temperature cracks, a small number of them are the partial under-anchor cracks.This part of the cracks are the deformation cracks from the point of mechanical view. Stress releases immediately after cracking.From the damage pattern of the limit state of the structure bearing capacity,the damage state around the pivot that reaches the ultimate bearing capacity should be roof cracking.Therefore this part of cracks will not lead to structural damage. at the same time the baseboard of the crack area has no prestressing force steel cable passing. so it will not cause damage to the prestressing force steel bar.

3, Construction crack
This part of the cracks are mainly distributed in the segmental old and new concrete junction surface. it is related with the treatment of the junction surface during the construction and the situation of the concrete casting.

4, Defective crack
The cracks of the baseboard that are distributed around the midspan of approach bridge are mainly defective cracks. the appearance of this kind of cracks is because under the condition of little stock of the structure safety, because of the construction error (showing hollow hole, exposed steel bar,etc.in the report)temperature and other adverse effects causing partial cracking.

Repair methods

Grouting

For the cracks that need grouting, using the method of injection or pressure grouting with the epoxy resin grouting material, the construction measures of the two methods are as follows:

Finding out the crack

Inspecting the conditions of crack carefully before grouting, recording(for example: the number of the ramp of the crack, amphi-position, block number, part, length, width, whether it is penetrated).

Surface treatment

Cleaning the surface of sealant with grinding disc or wire brush before sticking it. if there is oil on the surface, using thinner to erase it. If necessary, using high pressure air to clean it.

Determining the injection point

According to the characteristics of the method of injection construction and the condition of crack, determining the effective injection points.

Fixing the special chassis of syringe

Fixing the special chassis of syringe at the predetermined injection point, paying attention to that the center of chassis should be in accordance with the center of the cracks.

Sealing

Sealing the surface of every crack well with sufficient sealant, and ensuring that the infusion can not outflow.

Dealing with sealant

Before inserting, checking that all sealant has been handled well.

Injecting the resin

According to the specific conditions of construction site, choosing appropriate injection material, installing the injectors on the chassis, and then fitting on the required number of rubber band.The selected injection material will be injected into the cracks through the chasis.

Adding the injection

If the material in the cylinder injector is used up, adding some more. removing the empty syringes, and then installing the another syringe with resin in the original injection point and continuing injecting.

Processing and removing the cylinder syringes

When the injection material is solidified after 24 hours, it can be removed.

Removing the chassis and surface treatment
After discharging the chassis, removing the residual material with pumice stones, making the structure restoring smooth surface.

The method of pressure grouting

Checking the crack

Inspecting the conditions of crack carefully before grouting, recording(for example: the number of the ramp of the crack, amphi-position, block number, part, length, width, whether it is penetrated).

The treatment of crack surface

Cleaning up the mortar and dust sticking on the surface of concrete. within the range of the width of about 6cm, cleaning the foreign body inside the crack of sticky grouting nozzle, then using high pressure air to clean them out. If there is oil, cleaning it with acetone.

Pasting grouting nozzle

Polishing the sticky surface of grouting nozzle’s baseboard with emery cloth, and cleaning it with the acetone, checking whether it is in good condition; And then using adhesive to stick on the surface of the crack that needs grouting, fixing a grouting nozzle parallel to the seam about every 20 cm ~ 50 cm.

Sealing the crack

Sealing tightly the surface of crack and grouting nozzle with well-made epoxy plaster.

Sealing check

After the solidification of the epoxy plaster, checking the effect of sealing seam. if there is leakage, scraping the epoxy plaster again.

Making grouting serosity

According to the temperature and the parts, width, direction of cracks to choose corresponding slurry formulation.

Pressure grouting

The pressure of pressure grouting’s equipment should≥ 0.60 MPa, during the grouting, always observing whether the slurry flows, the velocity of flow, when ending the pressure, the pressure should reach 0.20 MPa~0.60 MPa, and keeping the pressure for 2 ~ 3 minutes. the order of grouting is from the low position to high position successively.

Finishing the sealing and appearance treatment

After grouting, closing the grouting nozzle, after the solidification of slurry the next day, breaking the grouting nozzle and epoxy mastic that has been sealed, then using angle grinder to polish, finally doing the appearance treatment.

Sealing

For the crack’s width is 0.1 ~ 0.2, which can’t grout, then sealing it.The specific requirements of the crack sealing are as follows:

Checking the crack

Inspecting the conditions of crack carefully before grouting, recording(for example: the number of the ramp of the crack, amphi-position, block number, part, length, width, whether it is penetrated).

Crack surface treatment

Removing the mortar and dust sticking on the surface of concrete, within the range of the width that is about 6cm(if the crack is quite dense,increasing the width), cleaning the foreign body in the crack, and then using high pressure air to clean, if there is oil, cleaning it with acetone, brushing the pure epoxy resin slurry evenly.

Making spraying slurry

According to the temperature and parts of cracks in the construction site, choosing corresponding formula of the spraying slurry.

Pressure spraying

The pressure of pressure spraying equipment should be≥0.6MPa. spraying pressure should be controlled between 0.2MPa and 0.4MPa according to different parts. total thickness of spraying should be controlled within 5 mm. it is completed for two times. during the spraying, always observing the concentration of spraying slurry, whether there is slurry drooping, adjusting the concentration and pressure of the slurry at any time.

Pasting carbon fiber sheet reinforcement

For the third and fourth kind of cracks, in addition to adopting the grouting, they should adopt the way of pasting carbon fiber sheet to strengthen. the structure that needs to be improved is the 50m span beam body of the east approach bridge(37 # ~ # 39) and construction joints that have cracks (the construction joints that don’t have the cracks do not need to be strengthened). See the appended drawings about the definite pasting location and size. Carbon fiber materials are PAN-based carbon fiber, fiber sheet 12k carbon fiber silk mold, the weight of the selected carbon fiber is 300 g/m2. the construction steps of pasting carbon fiber sheet are as follows:

The surface treatment

The deteriorated layer of concrete surface(such as weathering, free lime, releasing agent, stripping mortar, paint layer, dirt, etc.) must be removed and ground by grinding machine.removing the dust and loose materials in the area that needs to be strengthened with air nozzle, grinding machine and brush, washing it with water and keeping it fully dry.

The fracture surface repair

Clearing away the bad part of the concrete surface (for example: desquamation,hole, honeycomb, corrosion, etc.).If there are steel bars exposing outside, we must do the anticorrosive processing in advance, and then repairing it with the epoxy adhesive resin mortar material that its strength is equal or greater than the concrete. Cracks are infused by the epoxy resin. when the cracks or removal places have water leakage, we should stop water and deflect water.

Surface modification

Weeding out the bulge of smooth surface(small bumps, etc.) with the cutting machine or grinder and keeping it smooth. filling the concave part (the removal part) with repair adhesive or resin mortar.The corner should be ground up to that the convex Angle R is equal or more than 20 mm (R means curvature radius), filling the concave Angle with resin mortar.

The basement coating

Choosing the appropriate basement paint and coating evenly with a brush and roller.

Pasting of the carbon fiber slices

Cutting the fiber slices with scissors, knives according to the designed size, it is the most appropriate that the size and length of cutting is in the range of 2m according to the usage. after the strengthened fiber pastes on the surface of resin coating, using brush roller and rubber scraper along the fiber direction to make it smooth forcefully, making the resin soak and removing the bubbles. the lap joint length of the fiber direction (longer direction)must be at least 10 cm, the shorter direction don't have to leave. after pasting, placing it for 30 minutes, if the fibers have emerged or off-line situation, planishing and correcting them with roller or rubber scraper.

Usually using the following three methods selectively:

Surface sealing method

Checking

Before grouting, checking and recording the situation of the hollow holes carefully.

The management of the hollow holes’ surface

Removing the foreign bodies in the hollow holes, wiping out the particles sticking on the concrete. During the construction, repairing the hollow holes and cracks of the parts until the surrounding concrete become dense and hard.

Making up the spraying filling epoxy concrete

Making up correspondingly according to the temperature and the parts of the hollow holes in the construction site.

Pressure spraying and filling

The pressure of the pressure spraying and filling equipment should be≥ 0.6 MPa. the pressure of spraying and filling should be controlled between 0.20 MPa to 0.40 according to different parts. the thickness of spraying and filling should be controlled 20 mm for the first time, after the first solidification, doing the next spraying and filling until it is filled up.

Appearance treatment

After the solidification of the spraying and filling materials, grinding it to do the appearance treatment with the grinding machine.

The management of the exposed rebar

Checking

Checking and recording the situation of the exposed rebar carefully before the management.

Surface treatment

Removing the mortar of the parts of exposed rebar and the particles sticking on the concrete, using wire brush to clean out the rust of the steel bar surface, and then using high pressure air to clean it, then cleaning the steel bar and surrounding concrete with acetone, brushing the pure epoxy resin slurry evenly.

Making up the spraying slurry

The steps of pressure spraying are equal to the method of crack sealing.

Conclusion

Matters needing attention of construction

1, please read the check report carefully before the construction, investigating the parts that aren’t detailed in the report again.

2, it is appropriate to choose the season that the temperature is slightly higher for the grouting of the box girder baseboard and sterna crack, and ensuring that the time of grouting should be chosen the afternoon of high temperature, avoiding 22:00 ~ 11:00.

3, the time of pasting carbon fiber sheet should be chosen the period of the lower temperature.

4, for the way of both grouting and sealing that manage the crack, it is not recommended to wide the crack surface so as not to affect the appearance.

5, this report presents that the limit of the seam width of grouting and sealing of the crack is 0.2 mm. if it is conditional, the cracks that are below 0.2 mm should be managed by grouting sealants and fillers.

6, the pasting direction of carbon fiber sheet is along the bridge.

7, for the whole overpass, it is appropriate to start from the fifth line of the approach bridge when managing the defects, the approach bridge firstly, and then the ramp bridge.