Introduction
Underground structures are buried in soil and inevitably affected to varying degrees by groundwater or soil moisture. Therefore, proper waterproofing is essential to ensure the quality and durability of these projects. Here, we summarize the key construction points that you must master for effective waterproofing.
Focus on Self-Defense Prefabricated Houses
Self-waterproofing in prefabricated concrete structures relies on the inherent compactness of the structural material to prevent water ingress. This approach offers several advantages, including simplified procedures, shorter construction times, reduced costs, and improved working conditions for laborers.
For high-rise underground waterproofing, a three-layer defense system is typically employed:
- Concrete structure waterproofing (self-defense)
- External flexible waterproofing membrane
- Lime soil auxiliary waterproofing layer
The structural self-waterproofing layer is the critical line of defense against leaks. Usually, C30 waterproof concrete is used, with an early strength expansion agent (PNC concrete) as an admixture. The mix design follows standard concrete proportions, with cement content not less than 300kg/m³. Use ordinary Portland cement or slag silicate cement of grade 425 or higher.
The mixing process should mirror that of regular concrete, but the PNC admixture must be precisely measured using specialized tools with an error margin under 0.5%, managed by dedicated personnel. Mixing times are extended compared to ordinary concrete: add at least 30 seconds when using a forced mixer, or more than 1 minute when using a self-falling mixer, ensuring thorough and uniform mixing.
Backfill Soil Preparation
Backfilling after waterproofing is a crucial yet often overlooked step. Once water testing of the structure is complete, backfilling must begin promptly. The quality of backfill soil is vital—use of miscellaneous soil or construction waste is strictly prohibited. Instead, cohesive soil or loess with appropriate moisture content should be selected.
Before backfilling, clear all debris and accumulated water from the foundation pit bottom. Backfill should be compacted in layers, with each layer’s thickness between 20-30cm, depending on the compaction equipment used.
Treatment of Construction Joints
For waterproof ready-mixed concrete, continuous pouring is key, minimizing construction joints—especially on top and bottom slabs. Facility joints should be avoided. If horizontal joints on walls are necessary, they must be placed at least 300mm above the bottom slab surface. For arch walls, horizontal joints should lie 150-300mm below the arch wall joint line.
At construction joints, apply a 20mm thick layer of polymer waterproof mortar on the upstream side. Then, either adhere a 3-4mm thick polymer-modified asphalt waterproof membrane or coat with a 2mm thick polyurethane waterproof layer.
If a mid-mounted steel plate waterstop is used, it should be installed vertically at the center of the joint and firmly fixed, with its flange facing upstream to maximize its water-stopping effect.
The interval between pouring concrete on both sides of a joint must be kept short to prevent cracks caused by differential shrinkage. Prior to pouring, joint surfaces should be chiseled, cleaned of loose particles, and washed thoroughly. Keep the surface moist before applying a 20-25mm thick layer of cement mortar or a concrete interface treatment agent. Concrete should be poured promptly afterward. Remove formwork from the joint only after proper curing.
Attention to Waterproofing Details
Waterproofing Deformation Joints
An additional layer of synthetic polymer waterproof membrane must be applied over deformation joints within the external waterproofing membrane. The membrane ends should be fully adhered and sealed to the wall, with a minimum adhesion width of 150mm. Use a composite approach combining a buried waterstop with an external waterproofing layer at deformation joints.
When installing buried waterstops, the following rules apply:
- The waterstop’s hollow circular ring must align precisely with the deformation joint’s centerline.
- The waterstop must be securely fixed; inside the top and bottom slabs, it should be installed in a basin shape.
- During concrete pouring on one side, end formwork must be firmly supported to prevent grout leakage.
- Waterstop joints should be located higher on side walls, never at structural corners, and welded by hot pressing.
- At corners or turning points, the buried waterstop must be formed with a circular arc. The corner radius of steel-edged rubber waterstops should be no less than 200mm and increase proportionally with the waterstop width.
Waterproofing Through-Wall Pipelines
A sleeve must be embedded where pipelines penetrate the waterproof concrete structure, with a waterstop ring welded onto the sleeve. Both sleeve and ring must be cast into the concrete in a single operation, ensuring proper compaction.
The waterstop ring must be fully welded to the sleeve. The concrete thickness at pipe penetration points should be at least 300mm. For rigid waterproof sleeves, pass the pipeline through the pre-embedded sleeve, align it according to the design, and temporarily secure it. Then, weld a sealing steel plate on one end of the sleeve and pipeline, fill gaps with waterproof materials such as waterproof grease or asphalt mastic from the other end, and finally seal tightly with another steel plate.
Article source: Architectural Technology Magazine
Must log in before commenting!
Sign Up