Introduction: Waterproofing — the “dam” function of coatings
Essential difference between waterproof coatings and anti-corrosion coatings — anti-corrosion coatings allow trace water permeation
(as long as it does not cause corrosion — such as epoxy coatings — water vapor transmission rate WVTR>10g/m²·day) — waterproof coatings require “zero leakage”
(WVTR0.3MPa, >24h with no water permeation
. Core performance of waterproof coatings — (1) Elongation — >300% (to cover shrinkage cracks in concrete — crack width >2mm / coating does not break under tension); (2) Hydrostatic pressure resistance — >0.3MPa (hydrostatic pressure in basements >10m deep); (3) Adhesion to damp substrates — basements / tunnels — substrates are damp year-round (>95%RH / water film) — coating must be applicable + adhere on damp substrates
. Elastic waterproof coatings — polyurea (SPUA / high performance / high cost), polyurethane (general purpose / mainstream) and acrylic (economical / low-end) — each has its optimal positioning in the three scenarios of roofing / basements / tunnels.
Elastomeric waterproof coating is a type of high-build (>1.5mm DFT) coating with high elongation (>200%) and high elastic recovery rate—capable of remaining continuous and leak-free at cracks (>2mm) occurring in concrete substrates—used for waterproofing of structural areas such as building roofs/basements/bathrooms/tunnels.
I. Comparison of Three Major Elastic Waterproofing Systems
| Performance | Polyurea (SPUA) | Polyurethane (PU/Single-component) | Acrylic (Water-based) |
|---|---|---|---|
| Elongation (%) | 300-600 | 400-800 | 200-400 |
| Tensile Strength (MPa) | 15-25 | 2-8 | 1-3 |
| Surface Dry Time (23°C) | 3-10s | 2-6h | 1-4h |
| Static Water Pressure Resistance (MPa) | >0.5 | >0.3 | >0.2 |
| Construction on Damp Substrate | No (substrate must be dry) | Yes (special formulation/moisture-curing) | Yes (water-based/compatible with dampness) |
| Cost (RMB/m²/mm) | 120-250 | 30-80 | 15-40 |
FAQ
Q1: Why is waterproofing the “back side” (negative pressure water) of a basement more than 10 times harder than the “front side”?
The outer side of the basement concrete wall (front side / in contact with water in the soil) — waterproof layer is on the outside — water pressure presses the waterproof layer against the concrete
“Water helps the waterproof layer cling tightly to the substrate” which is favorable. The inner side (back side / water seeps outward from inside the concrete) — waterproof layer is on the inside — water pressure pushes the waterproof layer away from the concrete
“Water tries to peel off the waterproof layer” which is unfavorable — back-side coating must have extremely high adhesion (>3MPa / pull-off method) + chemical anchoring to damp substrate
— single-component moisture-curing PU (reacts with moisture in the concrete — forms chemical bonds) is the best choice for back-side waterproofing.
Q2: Why are “detail treatment” of waterproof coating at pipe roots/internal & external corners/construction joints—the “weak link of the entire waterproof system”?
At detail areas—(1)substrate is discontinuous—stress concentration—concrete shrinkage/temperature deformation—cracks preferentially occur at details; (2)coating thickness is hardest to be uniform at details
—thin at sharp corners—thick at internal corners—non-uniform—tensile strain of coating at details >5 times that of flat areas—coating cracks first at details—leakage. Reinforcement treatment of details
—(1)at details—substrate grinding—apply primer (>2 coats)
; (2)embed reinforcement fabric (polyester non-woven fabric/60-80g/m²) into coating—total thickness of coating + fabric at details >3mm (flat areas >2mm); (3)separate water ponding test at details (>24h/water level > detail height)
—confirm zero leakage.
Q3: Why is polyurea waterproofing extremely sensitive to the “moisture content” of the substrate?
The reaction of polyurea’s -NCO with water (producing CO₂ bubbles)——>(1) Bubbles in the polyurea coating that cures in >10s are completely unable to escape in time
——frozen within the coating——forming dense microbubbles (>1000 per/cm²)
——waterproofing of the coating = zero; (2) Bubbles destroy the contact between coating/substrate——adhesion drops (>5MPa down to <1MPa). Polyurea construction's “dew point +3°C” red line (substrate temperature > dew point +3°C)
+ substrate moisture content (<4%/CM method)——are the most stringent conditions for polyurea waterproofing construction
——if these conditions are not met——polyurea must never be applied.
Q4: Does the “curing” of one-component moisture-curing PU waterproof coating rely on moisture in the air?
One-component PU——prepolymer contains -NCO end groups——after application——coating absorbs moisture from the air (H₂O)
——NCO+H₂O→NH₂+CO₂↑——NH₂+NCO→urea (-NH-CO-NH-)——forms crosslinking——cures. Therefore, the curing of one-component PU starts from the surface——advances inward
(moisture penetration)——interior of thick coating (>2mm)——insufficient moisture penetration——incomplete curing——interior of coating remains “semi-liquid” (curing degree <80%)——this is the "slow curing of thick coating" issue of one-component PU——recommended single-coat DFT 2 coats.
Q5: Why is the “water resistance” of water-based acrylic waterproof coating inferior to that of PU and polyurea?
The emulsifier (hydrophilic/surfactant) in water-based acrylic remains in the coating—(1) the coating absorbs water and swells
—after immersion >24h—coating thickness expands >5-10%—strength drops >20%; (2) in winter—water in the coating freezes—volume expands—coating “freeze-thaw damage”
—micro-cracks form—snowmelt/rainwater seeps along cracks—repeated freeze-thaw—coating fails within >2-3 years. Water-based acrylic waterproof coating—only suitable for non-immersed/non-freeze-thaw
light waterproofing scenarios (e.g., roofs/non-long-term ponding)—basements and immersed environments—must not be used.
Q6: Does the waterproof coating’s “long-term water pressure resistance” lab test >0.3MPa/24h pass = actual >10 years no leakage?
Lab——constant water pressure/constant temperature/no external force (coating free from tension/compression)——gap from actual working conditions——(1) Actual——water pressure fluctuation (seasonal variation of rainwater/groundwater level)——> water pressure cycle——coating/substrate interface “water pressure fatigue”
; (2) Actual——repeated opening and closing of concrete cracks (<2mm) (temperature/load deformation)
——coating repeatedly stretched/compressed at cracks——fatigue crack growth; (3) “Creep” coating under long-term (>10 years) water pressure slowly creeps and deforms——thickness reduces——eventually ruptures
. The lab’s >24h test can only serve as QC (quality verification)——cannot extrapolate long-term (>10 years)——safety margin of waterproofing works——design thickness of coating > standard requirement > +50% (e.g., grade 1 2.0mm——design 3.0mm)——is the compensation for this “long-term uncertainty”.
Related Reading
Summary
The three major elastic waterproofing systems—polyurea (SPUA/high-performance/high-cost/dry substrate), polyurethane (PU/general-purpose/moisture-curing/mainstream), and acrylic (water-based/economical/light waterproofing)—each have their optimal applications. Reinforcement treatment at details (pipe roots/internal & external corners/construction joints) with reinforcement scrim (+50% thickness) is the key to the success or failure of the waterproofing system. Kexin New Materials provides customers with full-range elastic waterproof coatings and technical support for detail treatment.