Second, the technical requirements of C40 micro-expansive concrete In the design requirements of Magang Bridge, the arch rib of concrete filled steel tube must meet the following points: 1, and the concrete filled steel tube must be poured from the arch foot to the vault with a pump truck; 2. The bleeding rate of pouring steel tube arch rib concrete should be controlled within the relevant range; 3. The pouring sequence of concrete for each part of steel tube arch rib is: arch rib lower tube-arch rib upper tube-arch rib abdominal cavity; 4. The concrete pouring of steel tube arch rib must be completed within the initial setting time of concrete, otherwise the concrete that exceeds the initial setting time is easy to crack under the pressure generated when the pump truck pours concrete, and the cracked concrete is difficult to heal, which will affect the pouring quality of concrete; 5. The concrete filled in the steel tube requires a certain expansion rate to ensure that the inner wall of the steel tube arch rib can be closely combined with the concrete. In order to meet the above design requirements, concrete must have: ① pumpability; ② There should be a long initial setting time; ③ There is a certain expansion rate. Therefore, the design of C40 micro-expansive concrete has the following technical indicators:

Water cement ratio: 0.35 ~ 0.5

Retarder parameter: 0.45 ~ 0.55%

Parameters of expansion agent: 7.00 ~ 9.00%

Slump: 18.0 ~ 22.0 ㎝

Expansion rate of concrete: 0.0 1 ~ 0.04%

Initial setting time of concrete: more than 8.0 hours.

Three. Test results of concrete aggregates and admixtures The main admixtures added to C40 micro-expansive concrete of steel pipe arch rib of Magang Bridge are AEA expansive agent, FDN-500 R retarder and fly ash. The mixing ratio of cement, crushed stone and sand used in C40 micro-expansive concrete is:

Brand of cement: bridge silicate 525R Factory name: Sanshui Hekou Cement Plant

Sand fineness modulus: 2.40 medium sand in Zone II.

Gravel 1 ~ 3 ㎝ continuous grading

Test results of various materials in the mix proportion of C40 micro-expansive concrete;

1, cement inspection results

Standard dispatching water consumption: 26.92%

Setting time: initial setting-2 hours and 50 minutes final setting-4 hours and 05 minutes.

Mortar strength: (Newton/mm2)

age

project

1 day, 3 days, 7 days and 28 days standard actual standard actual standard actual compressive strength 27.038.85 1.552.4 flexural strength 4.904.877. 107.3.

2. Sand screening result: weight of screened sample: 500g.

Sieve size (mm)10.05.02.510.20.60.30.15 Accumulated sieve residue (%) 00.63.612.245.22000

3. Gravel screening results Screening sample weight: 2000g.

Mesh size (mm) 403050 15 1052.5 Cumulative sieve residue (%) 03.45438+09.05443.0000001

4. Test results of fly ash

The fineness (0.045 square hole sieve residue%) of the measured value Ⅲ Ⅲ Ⅲ1of the serial number quality index grade is not greater than 122045 18.92, and the water demand ratio% is not greater than.

951051151003 combustion vector% is not greater than.

58 1264 water content% is not more than

1 1 unstable 15 sulfur trioxide% is not greater than.

3332

All the above materials meet the requirements of mix design, among which the AEA expansion agent and FDN-500 R retarder for approach bridge have passed the factory inspection. The sand in Area II is medium sand, and the gravel is graded continuously at 1 ~ 3 ㎝.

Four. Mix proportion design of C40 micro-expansive concrete The mix proportion design of steel pipe arch rib of Magang Bridge must meet the above technical requirements. Among these technical requirements, the most difficult thing is to measure the micro-expansion rate of C40 micro-expansion concrete. Because there are many additives in this mixture ratio, the compatibility of various additives with cement can meet the design requirements. This requires looking for it in the process of mix design. Design steps of concrete mix proportion of steel tube arch rib of Magang Bridge;

1. Cement used for testing, including standard dispatching water consumption, setting time, volume stability and mortar strength;

2. Check the compatibility between cement and additives;

3, coarse and fine aggregate screening test, the test results meet the design requirements of the mixture ratio;

4. Design the mix proportion of C40 micro-expansive concrete: firstly, make the benchmark mix proportion of this mix proportion, and AEA expansive agent is not added to the benchmark mix proportion; Secondly, adjust the amount of AEA expansive agent in the later concrete mix proportion;

5. Test the initial setting time for each mixture ratio. The concrete operation method is as follows: put the concrete with this mixture ratio into the 15× 15× 15㎝ test mould through a 5 ㎝ sieve hole, and test it with a penetration resistance meter;

6. Test the strength of each mixture ratio for 3 days, 7 days, 2/kloc-0 days and 28 days;

7. Test the micro-expansion rate of each mixture ratio. Cured in water before 14 days, and then put in the air. During the curing process, the expansion rate of concrete is tested every day.

In the process of mix proportion configuration, the most important process is to measure the micro-expansion rate of C40 micro-expansion concrete, because when measuring the micro-expansion rate, as long as there is a little big vibration, it will affect the reading of the dial indicator fixed on the dial indicator frame. In order to measure the micro-expansion rate of C40 micro-expansion concrete, the concrete mix designer has done the following work:

A, C40 micro-expansive concrete micro-expansive rate test site is far away from the construction site and crowded places, and irrelevant personnel are prohibited from entering.

B, establishing another health-care pool for measuring the micro-expansion rate, and fixing the test block for measuring the micro-expansion rate and the fixing iron block of the dial indicator bracket in the health-care pool. The fixed mode diagram is as follows.

C. The micro-expansion rate of C.C40 micro-expansion concrete was measured in water 14 days before measurement, and the subsequent micro-expansion rate values were all measured in air. C40 micro-expansive concrete micro-expansive test block is fixed in the curing pool, and then water is added to the curing pool. When adding water, the action must be light, the water pipe must be inserted into the bottom of the maintenance pool, and the water flow should not be too big. After curing in water 14 days, when draining water outside the curing pool, the action must be light and the water flow should not be too big.

D, when measuring the value of micro-expansion rate every day, the staff must put on rubber shoes, and the pace of walking should be eased, so as not to make the ground vibrate greatly.

In the mix proportion of C40 micro-expansive concrete for steel tube arch rib of Magang Bridge, eight groups of concrete mix proportions were designed, but many groups failed in the test. The following is the successful mixture ratio of C40 micro-expansive concrete, and the dosage of various materials is as follows:

No. cement dosage/m3 fly ash sand dosage/m3 crushed stone dosage-500 RAEA expansion agent water dosage/m3 dosage% dosage/m3 benchmark1420 7016.7635.411.

The slump, compressive strength and longitudinal free expansion rate of these concrete mixture ratios in the test are shown in the following table:

No. slump () compressive strength (Mpa) longitudinal free expansion rate of concrete/kloc-0 /×10-4m 3d7d 28d 3d5d14d21d28d benchmark 1200.290.4 10.490. 5438+0.011.081.531.39 ratio 71933.934.052+0.361.8/kloc-.

When measuring the micro-expansion rate of C40 micro-expansion concrete of Magang Bridge, the fixing forms of test block and dial indicator bracket are as follows:

The test results of C40 micro-expansive concrete mixture ratio of steel tube arch rib of Magang Bridge generally meet the technical requirements of filling concrete with steel tube arch rib, but the slump loss of these groups of concrete mixture ratio is great. After weighing and considering, the mix proportion of filling concrete for steel pipe arch rib of Magang Bridge is selected as "No.7 mix proportion".

Verb (abbreviation of verb) C40 micro-expansive concrete mix construction There is a problem of quick slump loss in the mix design of C40 micro-expansive concrete for steel tube arch rib of Magang Bridge. In the process of pouring concrete for steel tube arch rib of Magang Bridge, corresponding measures are taken to solve this problem. The corresponding measures are as follows:

1, pouring steel tube arch rib concrete can only be carried out in low temperature weather, such as night construction.

2. After the concrete enters the storage hopper of the pump truck, the wringer of the pump truck must keep stirring. If it is found that the slump loss of concrete can no longer meet the construction requirements, a small amount of water can be added for mixing.

3. Due to construction reasons, the pump truck will stop for a long time, and the pump truck will often twitch the piston, that is, the concrete will be pumped slightly backward and then forward.

4. The volume of concrete produced by the concrete mixing station must be consistent with the demand for concrete on site, which means that the produced concrete cannot be exposed to the air for a long time, resulting in excessive slump loss of concrete.

The above points are the measures taken to make up for the deficiency of concrete mix design in the process of pouring concrete filled steel tube of Magang Bridge, and good construction results have been achieved in the whole construction process of concrete filled steel tube arch rib of Magang Bridge.

Conclusion If the concrete in the steel tube arch rib can not be completely combined with the steel tube arch rib, it will have a great influence on the structural stress of the whole bridge. In order to solve this problem, it can only be used in concrete mix design. The design and construction of steel tube arch rib concrete of Magang Bridge have accumulated some construction experience for solving the construction of steel tube arch rib concrete. (Editor: Administrator)