1, project overview
Lancangjiang Bridge in Changdu, Tibet is a 3 17 project connecting Changdu City and National Highway across Zhaqu River. The bridge is a half-through concrete-filled steel tubular arch bridge, with a clear span of 125m and a clear vector of 31.5m. The main arch is a dumbbell-shaped arch rib, and the upper and lower chords are steel tubes with a wall thickness of 20mm and a diameter of 950mm respectively, so it is a long-span thin-walled steel tubular arch bridge. C50 micro-expansive concrete is poured into the steel tube arch rib, and the total consumption of steel tube arch concrete in the whole bridge is about 600m3. The steel tube arch rib of this bridge has an inner flange, so the elastic modulus and fluidity of concrete are very high. Concrete is constructed by underground pumping jacking method, which has the characteristics of high pumping height, long distance, large resistance and long construction time.
2, the technical performance requirements of concrete
The main load-bearing structure of concrete-filled steel tubular arch bridge is concrete-filled steel tubular arch rib, so many factors such as initial setting time, micro-expansion, pumpability, early strength, low air content in concrete and high elastic modulus should be considered comprehensively when designing the mix proportion of concrete-filled steel tubular. The specific requirements for the technical performance of concrete are: ① suitable expansion performance, self-compacting property and initial setting time; ② High elastic modulus, the 28d compressive elastic modulus is greater than 4.0×104mpa; ③ The early strength of concrete is high, and the 3d compressive strength of concrete is greater than or equal to 40MPa；; ④ Good pumpability: large slump, no segregation, no bleeding, good workability and self-compacting, so as to ensure that all parameters meet the requirements.
3. Mix proportion design and performance of 3.C50 steel tube micro-expansive concrete
① In the process of preparing concrete filled steel tube, attention should be paid to the air entraining integer of admixture, because this factor is directly related to the bond between steel tube wall and concrete. Its influence is mainly reflected in the following aspects: the volume fraction of gas introduced into concrete will also increase when the admixture with larger gas introduction number is added, which will easily lead to the debonding of steel pipe wall and concrete during construction and lead to construction accidents. Usually, the pumping pressure of concrete filled steel tubular pipe jacking construction of steel tubular arch bridge is 6 ~ 20 MPa. According to some data, when the air content of concrete filled steel tube is 2.3% ~ 3.0%, about 0.05% of the gas in the concrete volume will be enriched in the inner wall of the steel tube under the action of the above pumping pressure and concrete weight, which will cause an annular gap with a width of 0. 1 1mm between the concrete and the inner wall of the steel tube.
We must attach great importance to the gravity settlement and pumping pressure of concrete near the arch of steel tube arch, because these places are prone to adsorption and enrichment, resulting in the debonding of concrete, which not only affects the mechanical properties of concrete, but also destroys the integrity of concrete-filled steel tube structure. In addition, some data show that when the gas content of core concrete is 1.8%, the gas film produced during the construction of steel pipe pumping jacking is small, and the expansion rate of concrete is greater than 2× 104, which can compensate the gas film gap formed. Therefore, in order to ensure the quality of concrete to meet the relevant requirements, it is necessary to control the air-entraining integral number of admixture in a suitable range. ② Grade I fly ash is selected as the fly ash, and the water volume fraction is less than 95%. ③ the cement is P.O52.5 ordinary portland cement. ④ High energy delayed expansion agent is selected as the expansion agent. ⑤ Fine aggregate should be medium sand, its silt content should be < 1.2%, and its fineness modulus should be 2.6 ~ 3.0. ⑥ Coarse aggregate macadam, 5mm-25mm continuous grading, crushing value < 10%, needle-like mass fraction < 10%, silt mass fraction < 1.0%.
4. On-site construction of 4.C50 concrete filled steel tube
① Preparation of raw materials: In order to ensure the quality of concrete, all raw materials, including graded admixtures, must meet the design requirements and mix proportion requirements, and be prepared in place at one time, without stopping or changing materials in the middle to ensure the completion of one-time pouring. ② Mechanical inspection: Before pumping concrete, the pump and other equipment of the mixing station should be overhauled to ensure the continuous pouring of concrete. ③ Calculation of pumping pressure: Two high-pressure concrete pumps are used to pour at both ends of the arch foot at the same time. The power of concrete pump is ≥75kW, and the pumping pressure is ≥ 16Mpb (double main power 90 pump). In order to ensure concrete pouring at one time and reserve all kinds of equipment, the pumping conduit must be fixed firmly and reliably. ④ Weather selection: In summer construction, considering the influence of the temperature difference between day and night in Changdu on the shape of steel pipe arch, high temperature is not conducive to concrete jacking, and the construction time should be completed before 10: 00 am, and there should be no rain and no wind.
⑤ Take measures to avoid possible lateral displacement due to uneven stress, that is, set two wind-resistant cables at the upstream and downstream of the steel tube arch bridge, adjust them to the arch axis position, and lock the Phoenix cable. ⑥ In the two arches, vault, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib, arch rib. ⑦ Check whether the sealing ring at the joint of conduit and pump truck leaks slurry or air. ⑧ Removing all foreign matters unrelated to construction can not only ensure the cleanliness of the construction site, but also ensure the dense quality of concrete. Pet-name ruby drawings design only set a grouting hole at the arch foot, set an exhaust hole at the top, in order to ensure continuous jacking, in order to prevent catheter blockage, add 1 standby grouting hole, a slag hole, 15m set a small exhaust hole.
Attending pumping sequence: when pumping for the first time, the lifting height of concrete on both sides shall not exceed 1 m. After pumping the upper chord concrete of the steel pipe arch, the lower chord concrete can only be pumped after its strength reaches 80%, so as to ensure the uniform stress on the arch. After the arch rib of the roller is hoisted, the working sequence of self-compacting concrete of steel pipe arch is as follows: building working platform → installing pumping equipment → installing manual stop valve → pressing clean water → disconnecting manual stop valve → discharging sewage → opening vent → pressing mortar → pumping concrete → opening slag outlet after exceeding the pouring opening 1m → closing slag outlet after discharging normal concrete → pumping concrete, and measuring the change of steel pipe arch when it is one third full. Note: When the concrete passes through the flange and suspender, assign a special person to knock it.
5. Concluding remarks
Lancangjiang Bridge in Changdu, Tibet has a long span, dumbbell-shaped arch ribs, small wall thickness and a flange in the middle. Therefore, the performance requirements of concrete are very high. In order to meet the requirements of construction on the working performance and physical and mechanical properties of concrete, higher requirements are invisibly put forward. During the construction, we should scientifically adjust the concrete mixture ratio, reasonably arrange the pumping route and adopt appropriate construction technology to effectively ensure the safety and integrity of the construction. After three months of construction, effective methods should be adopted to detect the compactness of concrete filled steel tube. The results show that the concrete in the steel tube is dense and there is no gap between the concrete and the steel tube.
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