Analysis of Common Technical Indicators for Dry-Mix Mortar
Release Time:
2016-04-27
Source:
Analysis of Common Technical Indicators for Dry-Mix Mortar
Dry-mix mortar is now widely used; however, during its application, some manufacturers, users, and regulatory authorities often confuse certain concepts and performance indicators related to dry-mix mortar. The following section clarifies the key concepts:
Viscosity and strength: The higher the viscosity, the greater the strength. Strength generally refers to the physical performance characteristics of mortar after it has hardened, whereas viscosity typically describes the state of the mortar immediately after mixing and is an indicator relevant to the construction process. There is no inherent relationship between viscosity and strength.
Bond strength and compressive strength: Bond strength refers to the maximum adhesive force exerted by mortar on the substrate. It is categorized into shear strength and tensile (pull-out) strength. Tensile strength is the ability of the mortar surface to resist tensile forces acting perpendicular to that surface, while shear strength is the strength determined by applying parallel shear forces.
Compressive strength refers to the maximum stress at which a mortar specimen fails under applied compression; it is generally regarded as the mortar’s cohesive strength. High bond strength does not necessarily imply high compressive strength, and low compressive strength does not necessarily imply low bond strength.
Waterproofing and Water Resistance: The concepts of waterproofing and water resistance are primarily relevant to putty products. Water resistance means that the material’s performance does not undergo significant changes in a wet environment, whereas waterproofing means that water cannot penetrate the material. Putty is merely a leveling material applied prior to painting and neither possesses nor should it be expected to possess the properties of a waterproofing material. Standards JG158 “Putty for Interior Use in Buildings” and JG157 “Exterior Wall Putty for Buildings” only specify the water resistance of putty, requiring that the cured putty be immersed in water for a specified period and then inspected for any changes in the putty layer. However, in actual applications, some manufacturers engage in exaggerated marketing by claiming that their products are waterproof putties, while some end-users frequently inquire whether exterior wall putties are waterproof.
Thermal insulation and thermal resistance: Thermal insulation refers to the ability of masonry wall materials or products to prevent heat loss in winter and maintain a stable indoor temperature. It generally denotes the capacity of building envelopes—including roofs, exterior walls, windows, and doors—to impede heat transfer from indoors to outdoors during the winter, thereby sustaining an appropriate indoor temperature. Thermal resistance, on the other hand, refers to the ability of masonry wall materials or products to block heat gain in summer and maintain a stable indoor temperature. It typically describes the capacity of building envelopes to shield interior surfaces from solar radiation and high outdoor temperatures in summer, thus keeping the interior surface at an appropriate temperature. China’s Building Energy Efficiency Design Code stipulates that energy efficiency targets vary by region; the distinction between thermal insulation and thermal resistance is as follows:
(1) The heat transfer processes differ. Thermal insulation refers to heat transfer during winter, typically analyzed as steady-state heat transfer while also accounting for certain transient effects; thermal resistance, on the other hand, pertains to heat transfer in summer and is usually considered as periodic heat transfer over a 24-hour cycle.
(2) The evaluation criteria differ. Thermal insulation performance is typically assessed using the thermal transmittance coefficient or the thermal resistance value. Thermal retardation performance, on the other hand, is usually evaluated by the maximum inner-surface temperature of the building envelope under summer outdoor design conditions (i.e., in hot weather). If, under the same conditions, the maximum inner-surface temperature does not exceed that of a 240-mm-thick brick wall (i.e., a one-brick wall), the envelope is considered to meet the thermal retardation requirements.
(3) Different structural measures are employed. Since thermal insulation performance primarily depends on the thermal transmittance coefficient or thermal resistance of the building envelope, lightweight envelopes constructed from porous, lightweight insulating materials exhibit lower thermal transmittance coefficients and higher thermal resistances, thereby delivering superior thermal insulation. However, due to their light weight and poor thermal stability, such envelopes are highly susceptible to solar radiation and fluctuations in indoor–outdoor temperature, which can cause the inner-surface temperature to rise readily, resulting in relatively poor heat-shielding performance.
Hydrophobicity and Waterproofing: According to the “Terminology Definitions” in the test method for hydrophobicity of thermal insulation materials, hydrophobicity is defined as a performance indicator that reflects the material’s resistance to water penetration. It is quantified as the volume percentage of the portion of the test specimen that remains impermeable after being subjected to a specified water flow rate under controlled spray conditions. Hydrophobic materials typically exhibit relatively low water pressure resistance, whereas the waterproofing performance of waterproofing materials is usually assessed under specific dynamic water pressure conditions. Moreover, waterproofing materials do not necessarily possess hydrophobic properties (such as the beading effect), and hydrophobic materials do not always demonstrate excellent waterproofing performance.
