Nanjing Yangtze River Bridge 2 Information

1. Brief Introduction of the Second Nanjing Yangtze River Bridge

The Second Nanjing Yangtze River Bridge is located at 1 1 km downstream of the Nanjing Yangtze River Bridge. The south branch bridge is a steel box girder cable-stayed bridge with inverted Y-shaped pylon, including lower, middle and upper pylons and beams. The tower column adopts asymmetric hexagonal hollow thin-walled section, and the upper tower column is the anchorage area of stay cable, which adopts circumferential prestressed concrete structure; The beam adopts box section, and the upper, middle and lower beams are prestressed concrete structures. Beicha Bridge is a long-span continuous beam bridge with post-tensioned bonded prestress. The span arrangement of the main bridge is: 90+3x 165.

(m), five-span prestressed concrete box-section continuous beam, box girder adopts three-dimensional prestressed system; The superstructure of the south and north approach bridges are two prestressed concrete box girder bridges with equal cross section, and the box girder adopts bidirectional prestressed system.

The whole secondary prestressed concrete has a large amount of work, and the layout of prestressed channels is complex (super-long, round, arc, etc. ), the construction quality requirements are high and the construction is difficult.

2. The importance of grouting

In the post-tensioned bonded prestressed concrete structure, the pressure grouting is used to fill the gap between the pre-embedded channel of prestressed tendons and prestressed tendons, which solves the problem of corrosion protection of prestressed tendons and the problem of working with structural concrete. When the post-tensioned prestressed tendons are in non-horizontal inclined state, multi-span bending state and vertical state, the exudation of cement slurry evaporates to form a space without cement slurry, which makes the prestressed tendons there lose protection. However, prestressed arrows are very sensitive to corrosion damage (stress corrosion) under high stress conditions (in modern prestressed structures, the stress of prestressed tendons is usually above 1000MPa), resulting in cross-section defects in the corroded parts of prestressed tendons, which affects the safety and durability of prestressed concrete structures. Therefore, the grouting quality directly affects the corrosion resistance of prestressed tendons and the safety and durability of prestressed structures.

At present, quality problems often occur in the grouting construction of prestressed tunnels: the cement slurry in the tunnel is not full and there are cracks; Shrinkage of hardened cement paste and separation of pore wall; After hardening, the strength of cement slurry does not meet the specification requirements.

3. Brief introduction of JM-Ⅲ, a special additive for prestressed concrete channel grouting of the Second Bridge.

JM-Ⅲ (anti-crack and impermeability) concrete high-efficiency reinforcing agent is a multifunctional product developed by Jiangsu Institute of Building Materials. It not only has high crack resistance and impermeability, but also has the characteristics of water reduction, moderate setting time, good plasticity, good construction workability, frost resistance and corrosion resistance. It is a multifunctional crack resistance and impermeability material.

Second, the indoor mixture ratio test of grouting cement slurry

1. Test method for main properties of cement slurry

Test method for fluidity of cement slurry

The flow cone method is used to test the fluidity of cement slurry, and the flow cone time is tested according to the American standard CRD-C79-58, which is determined by measuring the time when a certain volume (1725mL) of cement slurry flows out of the flow cone of standard size. Any cement slurry should be tested at least twice.

(2) Test method of bleeding rate of cement slurry

With 1000mL measuring cylinder, inject about 800mL of prepared cement slurry into the measuring cylinder, write down the volume value and put it into the measuring cylinder.

Cover and seal the upper opening. From the time the cement slurry is injected into the measuring cylinder, open the upper opening cover every hour, tilt the measuring cylinder and suck it out with a straw.

Water, recorded, divided by the volume of exudation by the water content of the sample slurry, is the Qinshui rate. The calculation formula is as follows:

Bleeding rate (%) = bleeding volume/(sample slurry weight (g)* slurry water content (%))* 100%.

(3) Test method of cement slurry expansion rate

Testing the expansion rate of cement slurry is divided into two parts: one is to test the expansion rate of cement slurry before setting, and the other is to test the expansion rate of cement slurry in the middle and late stage. The measurement of expansion rate before coagulation is combined with the measurement of bleeding rate, that is, after the cement slurry that has undergone bleeding test continues to stand for 2 1h (in fact, there are still 24 hours before pulping), the height of the swollen slurry surface is determined. The volume after expansion divided by the original volume of cement slurry is the expansion rate. The calculation formula is as follows:

Expansion rate = (height of cement slurry after expansion-initial grouting height of cement slurry)/initial grouting height of cement slurry * 100%.

The method of measuring the expansion rate in the middle and late period is to soften the mortar triple die with 4cm * 4cm* 16cm cement and insert copper at both ends for measurement.

First, 24 hours after the cement slurry enters the mold, dismantle the mold and measure the length of the specimen as the initial length of the specimen. The experiment was carried out under the standard condition of 20℃

Ok, the first 14d is water conservation, and 14 ~ 18d is wet air conservation. The length of the sample was measured at 2d, 3d, 7d, 14d and 28d respectively.

Degree. The expansion length divided by the base length of the specimen is the expansion rate, and the calculation formula is as follows:

Expansion rate = (expansion length-initial length)/test block foundation length * 100%

(4) Test method of cement slurry strength

A set of (6) test blocks were prepared for each cement slurry with mortar test block mold, and the standard curing time was 28 days, and their compressive strength was measured.

2. Fix the dosage of JM-Ⅲ at 8%, adjust the water-binder ratio, and test the performance of cement slurry.

(1) See Table 1 for the test results of cement slurry fluidity.

Result analysis:

① The fluidity of cement slurry with water-binder ratio of 0.34 ~ 0.35 meets the specification requirements;

(2) After the cement slurry is left standing for 20min minutes, the fluidity loss is great, so it is required to be grouted as soon as possible after preparation; The grouting time of each channel should not be too long;

(3) No interruption is allowed in the grouting process, and it must be discharged from the slot after one-time uninterrupted hole formation.

(2) See Table 2 for the test results of bleeding rate of cement slurry.

Result analysis:

① With the increase of water-binder ratio (W/A), the bleeding rate increases;

(2) Prepare cement slurry with various proportions, pour it into the measuring cylinder and let it stand, and the cement slurry will be layered (the upper layer is foam, the middle layer is Qinshui, and the lower layer is solid slurry, only the slurry with w/a = 0.33 has no Qinshui layer). With the increase of w/a, the Qinshui layer will gradually increase, while the foam layer can basically remain unchanged;

(3) Suck out the exudate and foam from the measuring cylinder (which can only be done in actual operation) and re-inject it into the measuring cylinder. After standing for 24 hours, the mixture of foam and Qinshui is layered more obviously, but the slurry deposited in the lower layer can still be washed with water after 24 hours, indicating that there is no strength;

④ When the water-binder ratio is 0.33 ~ 0.35, the bleeding rate is less than 2%.

(3) See Table 3 for the test results of cement slurry expansion rate.

Result analysis:

① The expansion of cement slurry within 3h (that is, the early expansion of cement slurry before solidification) cannot be measured, because the cement slurry and foam on its surface are gradually separating and layering. Moreover, when the stagnant water and foam on the surface are absorbed, the volume of the slurry has not changed, which only shows that this method can not measure its change, but it can not show that it has not expanded in the early stage.

② It was found that with the increase of water-binder ratio (in the range of 0.33 ~ 0.37), the expansion rate of cement paste increased, which indicated that the reaction between JM-ⅲ and cement was more complete with the increase of mixing water.

(3) With the increase of time (within 28d), the expansion rate of cement paste increases, indicating that the reaction between JM-Ⅲ, cement and water is gradual, and the expansion rate increases rapidly in the first few days, and then gradually slows down;

④ The expansion rate of 28d is generally less than that of 14d, indicating that the expansion growth rate of cement paste caused by JM-Ⅲ is less than its own shrinkage growth rate around 28d.

(4) See Table 4 for the test results of cement slurry strength.

According to the data in Table 4:

① The strength of the test block can fluctuate around 70MPa due to the enhancement of water reduction of JM-ⅲ;

② When the water-binder ratio is between 0.33 and 0.37, the strength of the test block increases slightly with the increase of water-binder ratio, indicating that the hydration reaction of cement is more complete with the increase of water consumption;

③ When the water-binder ratio is 0.36, the strength of the test block is the highest.

3. Fix the water-binder ratio and adjust JM-Ⅲ to measure the performance of cement paste.

According to the test results of "fixing JM-ⅲ content and adjusting water-binder ratio", it is known that the comprehensive performance index of cement paste is the best when the water-binder ratio is 0.35. Therefore, the water-binder ratio is 0.35, and the content of JM-Ⅲ is adjusted to 6%, 8%, 10%, 12% and 15% to determine the performance index of cement paste.

Through a series of tests (the test data are not listed in detail, but the test results are summarized), we found that with the increase of JM-ⅲ content:

① The flow cone time of cement slurry decreases gradually. However, when the dosage is 15%, the flow cone time is shorter than that when the dosage is 12%. At this time, there is cement precipitation at the bottom of the stirring pot;

(2) With the increase of the thickness of foam layer on the surface of cement paste, the phenomenon of delamination and segregation becomes more and more serious, and the seeping rate increases;

③ The expansion rate of cement paste increases gradually, but when the content exceeds 12%, the expansion rate is no longer obvious;

(4) The strength of cement slurry 28d test block fluctuates around 70MPa.

4. Optimize the combination and determine the admixture content and water-binder ratio of cement slurry.

Through the above two series of tests, we have determined that the optimum proportion of cement slurry for grouting is: JM-III content 8%, water-binder ratio 0.35, and its comprehensive performance is shown in Table 5.

Third, the simulation test

1. Comparative test of grouting methods and proportions in the construction site of Nancha Hunan Road and Bridge Company.

(1) grouting tissue

The contrast test between vacuum-assisted grouting and ordinary grouting was carried out.

The comparison test of vacuum grouting and pressure injection of different cement pastes shows the matching of two different cement pastes, as shown in Table 6.

In the comparative test, the channel material, channel length and the number of steel strands passing through the channel are all equal, and grouting is carried out at the same time. After grouting for 3 days, the pipe is cut for inspection.

(2) Analysis of test results

(1) For the samples grouted by ordinary press-fitting technology and mixed with JM Ⅲ cement slurry, plastic corrugated pipe Road can be well filled with cement slurry, and the pits caused by a small amount of bleeding are mainly concentrated on the 5mm high convex waveforms in plastic corrugated pipe, with only a few nail pits with depressions less than 2mm and bubbles between the two waveforms. It shows that if ordinary grouting technology is adopted, plastic corrugated pipe tunnel can be well compacted as long as it is strictly controlled.

(2) The samples grouted by vacuum-assisted grouting technology, mixed with the cement slurry provided by VSL Company, can also fill plastic corrugated pipe well. However, the early strength of grout with real auxiliary grouting is higher than that with ordinary grouting.

③ When cement slurry mixed with JM-Ⅲ is used, vacuum-assisted grouting and ordinary grouting are used to make comparison samples. After the comparative inspection of pipe cutting, it is found that the plumpness of vacuum-assisted grouting in the upper part of plastic corrugated pipe is slightly better than that of ordinary pressure loading, which is due to the short slurry leakage time at the slurry outlet and the failure to eliminate the gas-water mixture in the pipe. Therefore, the conditions for judging the success of vacuum-assisted grouting should be: the consistency of slurry at both ends is the same, there is no obvious bubble, and the pressure (0.7MPa) is kept for 2min as the standard.

2. Grouting test in Beicha site of Shandong Communications Engineering Company.

(1) grouting tissue

The four bellows are divided into two groups, one round and one flat, which are called group A and group B for short.

The proportion of group A is: JM-Ⅲ 8% by weight, water-binder ratio 0.36 (considering that the pulping effect on site is not as good as that in the laboratory), grouting will be carried out in the afternoon of February 1999+ 18, and grouting will be checked in the morning of February 65438+29.

The proportion of group B is: JM-Ⅲ dosage 8%, water-binder ratio 0.40, grouting at noon on February 24th, 65438+February 29th, and cutting the pipe to check the grouting effect.

(2) Analysis of test results

(1) The cement slurry in the annular corrugated pipe of Group A basically fills the whole channel, and there is no phenomenon of prestressed tendons exposed; There is a layer of dark red toner block with a thickness of about 0.5mm on the top surface of the arch, which is dispersed by hand. This is caused by foam accumulation and water loss in the cement slurry, which shows that the slurry discharge time of the slurry discharge hole is too short and the foam in the cement slurry is not excluded. At the same time, the cement slurry is frozen, which means that the assembly cannot be frozen within 48 hours after pouring, otherwise it will lose strength or cause strength loss.

(2) The slurry in the flat corrugated pipe of Group A completely fills the whole channel, and no prestressed tendons are exposed.

(3) due to man-made reasons in grouting, there is no cement slurry pressed to the vault in a large range, resulting in no cement slurry wrapped steel strand in this range. This shows that in the process of grouting, any lack of sense of responsibility and failure to operate according to the process will lead to loose grouting and form hidden dangers of accidents.

(4) In Group B, the local bulge of the flat corrugated pipe has a depression of about 5 10mm, which is most obvious in the range of 400mmn, with a width of 60mm and a depth of about 20mm, indicating that the water-binder ratio of the cement slurry is too large and a gap is formed after bleeding.

3. Grouting Test in Prestressing Test Center of Southeast University

(1) grouting tissue

The mixing ratio of cement slurry is JM Ⅲ 8%, and the water-binder ratio is 0.35.

Grouting was carried out in the afternoon of 25th, October, 2000, and the pipe was opened for inspection on 9th, February, 2000.

(2) Analysis of test results

(1) The cement slurry in the circular corrugated pipe is full and dense, and there are no cracks and pores during peeling; Due to the limited cement slurry during grouting, the slurry can not be fully discharged, so there are bubble-like nail-sized pits closed by thin cement slurry at the top of hardened slurry, but the number is small; After the peeled corrugated tube was placed in the air for 15d, transverse cracks in accordance with the mechanical principle occurred, indicating that the hardened slurry exposed to the air would form cracks due to drying shrinkage.

(2) The slurry in the flat corrugated pipe is full and dense, and there are no cracks and pores during peeling; After the hardened slurry of the peeled bellows was placed in the air for 15d, the transverse cracks conforming to the mechanical principle were visible to the naked eye.

Four. conclusion

1. According to the test, the water reducing rate of JM-ⅲ admixture is high, reaching about 20%; When 8% of cement is added, the later expansion rate of cement paste is above 0.2%. The appropriate dosage is 8% ~ 10%, and the water-reducing effect is not obvious when it is greater than 10%, and there are a lot of bubbles on the surface, and the water-reducing effect is not enough when it is less than 8%. When the temperature is below -2℃, the setting time of cement slurry mixed with 8% JM-ⅲ is about 30 ~ 40 hours, which can be used in winter construction with early strength agent.

2. During grouting construction, the dosage of JM-Ⅲ should be accurate, and the mixing time should be no less than 3min. According to the experiment, the fluidity loss of cement paste is great when it is left standing for more than 20min minutes, so the on-site slurry storage tank should be continuously stirred manually to prevent the fluidity of cement paste from decreasing.

3. Construction considerations

① The storage time of ①JMⅲ Ⅲ should not exceed three months. When the powder in JM-ⅲ is in the form of block or granule, its activity will be greatly reduced. Therefore, JM-ⅲ containing blocks or particles should be regarded as invalid and should not be used in engineering.

② The cement for grouting in summer should be non-early strength ordinary portland cement, which is beneficial to keep the fluidity of cement paste.

(3) After the cement slurry mixed with JM-Ⅲ is made, there is a foam layer with a certain thickness on the surface, which must be fully discharged during grouting, and the slurry outlet should be blocked after no foam slurry is discharged from the slurry outlet.

(4) In the actual construction process, attention should also be paid to properly adjusting the cement slurry ratio according to the actual conditions of prestressed concrete channel length, temperature and humidity during construction, construction equipment and mixing water.

⑤ Before each construction, the raw materials of cement slurry shall be inspected and the fluidity of cement slurry shall be determined.

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