Tensile strength determination is one of the most critical quality control tests for cement-based waterproofing coatings, directly reflecting a membrane's ability to resist cracking, substrate movement, and sustained water pressure. This article provides a comprehensive technical guide to tensile strength testing of polymer-modified cementitious coatings, covering applicable standards such as GB/T 23445-2009, GB/T 16777-2008, EN 14891, and ASTM D412, along with specimen preparation, conditioning, test procedures, and result evaluation. It serves as a practical reference for manufacturers, contractors, testing engineers, and building owners involved in the production, acceptance, and certification of cementitious waterproofing materials.
Technical Overview
Cement-based waterproofing coatings, commonly referred to as polymer cement or JS coatings, are two-component systems consisting of a liquid polymer emulsion and a powder blend of cement, graded fillers, and functional additives. When the two components are mixed and applied, the cement hydrates while the polymer film forms simultaneously, producing a flexible yet durable elastoplastic membrane that bonds strongly to concrete and masonry substrates. Because these membranes must accommodate thermal movement, substrate shrinkage cracking, and structural vibration while remaining watertight, their mechanical performance under tension is a decisive quality indicator.
Tensile strength testing measures the maximum tensile stress a free film of the coating can withstand before rupture. It is typically evaluated together with elongation at break, and both parameters define the product classification and fitness for purpose. In the Chinese national standard GB/T 23445-2009 for polymer cement waterproofing coatings, three types are distinguished: Type I targets high deformability applications with a minimum tensile strength of 1.2 MPa and elongation at break of at least 200 percent; Type II requires at least 1.8 MPa with a minimum elongation of 80 percent; and Type III requires at least 1.8 MPa with a minimum elongation of 30 percent for applications where mechanical strength is prioritized. Equivalent international frameworks, such as EN 14891 for liquid-applied waterproofing membranes beneath tile installations, verify tensile performance after exposure to water, alkaline media, heat, and freeze-thaw cycles, classifying cementitious membranes into deformability classes.
The governing test methodology for Chinese cement-based coatings is GB/T 16777-2008, the standard test methods for building waterproofing materials, which prescribes specimen molding, conditioning, dumbbell cutting, and tensile execution. Internationally, tensile properties of elastomeric films are frequently determined following ISO 37 or ASTM D412 using dumbbell specimens, while pull-off tensile adhesion of coatings applied to a substrate can be assessed per ASTM C1583. Accredited third-party laboratories routinely apply these methods for type testing, factory production control, and conformity assessment.
Testing Samples
Two-Component Polymer Cement Waterproofing Coating Films
The primary test object is a laboratory-molded free film prepared from the liquid and powder components mixed at the manufacturer-specified ratio. The mixed paste is applied onto a release substrate such as polyethylene film, polytetrafluoroethylene sheeting, or a release-agent-treated glass plate, in two or three coats, to a final film thickness of 1.5 mm with a permitted deviation of plus or minus 0.2 mm as required by GB/T 23445-2009. These free-film specimens represent the intrinsic cohesive strength of the cured membrane independent of substrate effects.
One-Component Cement-Based Flexible Coatings
Premixed single-component cementitious coatings, which only require the addition of water or are ready-mixed, are sampled and molded in the same manner. The film-forming and curing schedule follows the product standard and the manufacturer instructions, ensuring that the evaluated tensile performance corresponds to the material as delivered to site.
Conditioned and Aged Specimen Sets
To evaluate durability, parallel specimen sets are subjected to defined aging treatments before tensile testing. Typical conditioning regimes include immersion in water at 23 plus or minus 2 degrees Celsius for 168 hours, exposure to heat at 80 plus or minus 2 degrees Celsius for 168 hours, and immersion in saturated calcium hydroxide solution for 168 hours to simulate long-term contact with moist, alkaline concrete environments. Testing these aged sets allows calculation of tensile strength retention rates, which must commonly remain at or above 80 percent of the unconditioned values according to GB/T 23445-2009.
Testing Items
A complete tensile performance evaluation of cement-based waterproofing coatings generally encompasses the following items, tested on identical dumbbell specimen geometries:
- Tensile strength of the unconditioned (as-cured) film
- Tensile strength after water immersion treatment
- Tensile strength after heat aging treatment
- Tensile strength after alkaline treatment with saturated calcium hydroxide solution
- Tensile strength retention rate for each aging condition
- Elongation at break, determined simultaneously with tensile strength
- Optional tensile adhesion strength of the coating applied to a concrete substrate per ASTM C1583
Testing Methods
Specimen Preparation and Curing
The liquid and powder components are conditioned at 23 plus or minus 2 degrees Celsius and 60 plus or minus 15 percent relative humidity for at least 24 hours before mixing. The components are combined at the specified ratio, mechanically stirred for approximately five minutes, rested briefly to release entrained air, and stirred again to uniformity. The paste is then cast onto the release substrate in multiple coats, with each coat applied after the previous layer has surface dried, until the target thickness of 1.5 plus or minus 0.2 mm is achieved. The film is cured under standard laboratory conditions for 96 hours, carefully demolded, turned over, and further conditioned for at least 72 hours so that both faces complete hydration and polymer film formation, following the schedule of GB/T 23445-2009.
Specimen Cutting and Measurement
Dumbbell-shaped specimens conforming to Type 1 of GB/T 528, identical to ISO 37 Type 1, are die-cut from the cured film. This geometry provides a central parallel section 6 mm in width with a 25 mm gauge length, forcing failure to occur within a defined zone. At least five valid specimens are prepared for each test condition. The thickness of each specimen is measured at the center and both ends of the narrow section using a gauge accurate to 0.01 mm, and the median thickness is used in the strength calculation. Specimens containing bubbles, pinholes, nicks at the edges, or non-uniform thickness are rejected.
Apparatus and Test Execution
Testing is performed on a calibrated universal tensile testing machine fitted with self-tightening wedge grips and a load cell whose capacity is matched to the expected failure load, with force measurement uncertainty within plus or minus 1 percent. The crosshead speed is set to the value prescribed by the governing standard, commonly 200 mm/min for polymer cement coating films tested under GB/T 16777-2008, while highly elastic membrane systems tested under other frameworks may require speeds up to 500 mm/min. Gauge marks are applied for elongation measurement, either with a contact extensometer or by crosshead displacement calibrated against the initial gauge length. The specimen is loaded at constant speed until rupture, and the maximum force together with the extension at break is recorded.
Calculation and Evaluation of Results
Tensile strength is calculated as the maximum force divided by the original cross-sectional area of the narrow section, that is, the die width multiplied by the measured median thickness, and is expressed in MPa. Elongation at break is calculated as the increase in gauge length at rupture relative to the initial gauge length, expressed as a percentage. Retention rates after each aging treatment are obtained by dividing the aged tensile strength by the unconditioned tensile strength. Results from specimens that slip in the grips or fracture outside the parallel gauge zone are discarded and replaced. The median of at least five valid determinations is reported as the characteristic value for each test condition, rounded to 0.1 MPa. The results are then compared with the classification limits of GB/T 23445-2009 or the applicable EN, ISO, or ASTM criteria, with minimum tensile strengths of 1.2 MPa for Type I and 1.8 MPa for Types II and III, together with the corresponding elongation and retention requirements.
Applications
Tensile strength determination of cement-based waterproofing coatings supports quality assurance and decision-making throughout the entire product and construction lifecycle. Typical applications include:
- Factory production control and batch release testing by coating manufacturers
- Type testing and conformity certification of products by accredited third-party laboratories
- Construction acceptance testing for waterproofing works in bathrooms, kitchens, balconies, basements, podium decks, swimming pools, water tanks, and tunnels
- Formulation research and development, including optimization of polymer-to-cement ratio, filler grading, and additive dosage
- Comparative evaluation of competing materials during tendering and specification writing
- Failure analysis and condition assessment of installed waterproofing membranes
FAQ
Question: Why is tensile strength tested on free films rather than on coatings applied to a substrate? Answer: Free-film testing isolates the intrinsic cohesive and elastic properties of the membrane itself, eliminating the influence of substrate roughness, adhesion quality, and concrete surface strength. Adhesion-related performance is assessed separately through pull-off tensile adhesion tests such as ASTM C1583.
Question: What is the difference between unconditioned tensile strength and tensile strength after water immersion? Answer: The unconditioned value characterizes the material as cured under standard laboratory conditions, whereas the immersed value reflects performance in permanently wet service environments such as pools and basements. Standards require the immersed strength to retain a minimum proportion, typically 80 percent, of the original value to confirm water resistance of the polymer network.
Question: How many specimens are required and how is the final result reported? Answer: At least five valid dumbbell specimens are tested for each condition. Invalid failures, such as breaks outside the narrow section or grip slippage, are excluded, and the median of the valid results is reported in accordance with GB/T 16777-2008 practice.
Question: Why must the film thickness be controlled to 1.5 plus or minus 0.2 mm? Answer: Standardized thickness ensures comparability between laboratories and between products. Excessive thickness slows hydration and polymer coalescence in the interior, while insufficient thickness increases sensitivity to local defects, both of which distort tensile results.
Question: What are common causes of unacceptably low tensile strength results? Answer: Frequent causes include incorrect liquid-to-powder mixing ratio, incomplete mixing, inadequate curing time or humidity, film application below minimum thickness, cement paste beyond its shelf life or partially carbonated powder, and improper specimen cutting that introduces edge notches acting as stress concentrators.