There are the following differences between underwater concrete pouring and ordinary construction concrete pouring: construction method, particle size, amount and ratio of materials, the composition of concrete, corrosion resistance grade, etc.
In this article, the Hong Kong-Zhuhai-Macao Bridge(HZMB) will be analyzed as an example.
Difference between underwater concrete pouring and common concrete pouring
Underwater concrete pouring means pouring concrete below the horizontal surface, while not actually pouring concrete into the water.
Underwater concrete pouring needs to be in the air after taking measures (Cofferdam, caisson, caisson, concrete filling, drainage, etc.) to isolate the seawater. Not to mention the problem of corrosion, pouring concrete into the water, the water-cement ratio will not be able to control, and instantly no strength. If using concrete drainage, this part of the concrete is to be scrapped.
In addition to the concrete poured on site, many piers for the HZMB were prefabricated onshore and then assembled at the bridge site location.
There is a difference between concrete used at sea and regular construction concrete. Concrete used at sea is what we call offshore concrete.
In addition to the strength and mix consistency of offshore concrete should meet the design and construction requirements, it should also have the required seepage resistance, frost resistance, and corrosion resistance, to prevent corrosion of reinforcing steel and resistance to ice impact performance. In addition, reinforcement should also be made special protection.
Underwater concrete pouring generally requires an increase in the amount of cement and the proportion of cementitious materials. It needs to be poured with a conduit so it requires a high degree of concrete fluidity. Stone particle size and sand ratio have to be controlled.
Underwater concrete is commonly used in the construction of infill piles. The raw material ratio is somewhat different from ordinary concrete, which requires good fluidity, as well as effective control of the slump. During construction, concrete reaches underwater through a conduit, which works on the principle that its density is greater than the specific gravity of water or mud. The concrete surges upward due to its fluidity. After the construction, the top part of the structure in contact with water or mud is generally chiseled away.
For water-reducing agent blending, the focus should be on low water reduction and high slump-proof collocation methods. Fully ensure that the concrete has good workability. The setting time should not be too short.
Seawater contains chloride ions. It will gradually erode the concrete, and eventually enter and corrode the steel reinforcement resulting in steel mixed piers that can not meet the load-bearing standards. This process will have an impact after 20-40 years. The design life of the HZMB requires 120 years. So the concrete used in the HZMB is added with special ingredients to prevent the penetration of chloride ions. It is said that the concrete soaks for 120 years, the chloride ions can not enter much. There will be no problem with the corrosion of steel.
The reinforced concrete soaked in seawater is completely different from that of ordinary buildings and freshwater. For this anti-corrosion problem, in the early 90s when the HZMB was first proposed, there were damage experiments and material pre-research. It is an important technical point of the bridge project.
Information about the East and West artificial islands of the HZMB and the prefabrication of immersed tubes.
East-West Artificial Island
The brief construction steps.
1 Dredging and backfilling of the foundation trench of the west island and the three adjacent sections.
2 The sinking of isolated sheet piles in the west island area to form a water stop weir and foundation support.
3 Backfilling sand on the small island.
4 Construction of the concealed section of the tunnel on the west island.
The above picture is the construction drawing. It can be seen that there are sheet pile cofferdams around the construction of the artificial island. The construction conditions, in this case, are basically the same as those on land. Only the environment is much more difficult. Later, the steel sheet pile cofferdam has to be removed, and the corrosion resistance of the concrete structure is still relatively high.
The prefabrication of immersed tubes
Like bridges, immersed tubes are mostly prefabricated on land and floated by tugboats for underwater installation. After the immersed tubes are installed in place underwater, they will be protected by backfilling sand and soil around them. The precast concrete structure of the immersed tubes emphasizes more on durability control compared to the ordinary concrete structure, which is low permeability and high durability concrete. The precast concrete of the immersed tubes mainly adopts a comprehensive temperature control scheme of cooling water with ice. Sections are maintained by means of conservation shed insulation and moisturization. No harmful cracks are allowed in the precast concrete of tube sections, and the maximum width of cracks allowed is ≤ 0.2mm.
For prefabricated hydraulic structures used underwater, a higher grade of corrosion resistance is required. For directly cast structures generally also form weirs to separate them from seawater. It is still necessary to improve the grade of durability and other aspects accordingly.
There is a method of pouring submerged concrete for underground diaphragm walls. Underground diaphragm wall concrete is poured underwater by the conduit method. According to the subdivision of the underground diaphragm wall in this project, all wall widths were filled with concrete using two conduits. The conduits were lifted by the grouting frame and the underground diaphragm wall grouting method is shown in the figure below.
1. Underwater concrete properties
The strength grade of concrete for the wall was C40. The slump of concrete was controlled at 20±2cm when the concrete was put into the hole. Ordinary silicate cement was used for cement, and a special concrete ratio was designed to make the concrete meet the requirements of underwater infusion.
2. The use of tubes
The tube should be inserted at an elevation of 0.3m~0.5m from the bottom of the tank ( appropriately for releasing the water barrier ball) before pouring concrete. Before pouring concrete, a concrete water barrier ball should be placed in the conduit near the mud surface.
Check the installation length of the conduit and keep records. Measure the rise height of the concrete surface once per truckload of concrete and fill in the records. The depth of the conduit insertion into the concrete should be maintained at 2 to 4 meters.
Test splicing and test pressure before the use of the conduit. The test pressure is 0.6~1.0Mpa. The conduit should be as close to the joint as possible.
3. Adopt an airbag water trap.
References
Underwater construction
Offshore concrete structure
What is the Best Concrete Sand Ratio?
Immersed tube