The evolution of fluid-applied membranes

Installation of hot rubberized asphalt membrane.
Installation of hot rubberized asphalt membrane.

An asphalt emulsion replaces most of the solvent found in an asphalt cutback with water. Both cutbacks and emulsions contain asphalt cement, a finely ground or milled asphalt mixed with an appropriate solvent. Emulsions suspend the asphalt in the material through chemical emulsifiers and surfactants, and naturally contain very low VOC content. Standard asphalt emulsions remain viable for light-duty waterproofing in areas of low hydrostatic head pressure, commonly referred to as dampproofing. Polymer or rubber-modified asphaltic emulsions provide improved waterproofing protection against hydrostatic head pressure. These products are used primarily due to significantly lower cost compared to higher performing membranes. Applications requiring greater flexibility, improved performance, or faster setting times might be better served with alternate product types.

Modern bituminous asphalt is distilled from crude oil, oil sands, and well sources. It was first synthesized with other polymers into useable rubberized asphalt in the early 1950s. This early ‘polymer-modified’ asphalt contained styrene butadiene rubber (SBR), and found immediate use in the production of the nation’s rapidly growing roadway networks at that time. In the late 1960s, scientific development led to the invention of SBS-rubberized asphalt with markedly higher levels of durability. The SBS asphalt formulation ages better, provides elastomeric qualities and strength while significantly improving abrasion resistance compared to its SBR predecessor.

During the early 1960s, a tire manufacturer commercially marketed SBR and later SBS-rubberized asphalt. Approximately 10 years later, a sheet version of SBS asphalt was developed into modified asphalt roofing, or ‘mod-bit’. This new chemistry includes all the benefits of hot fluid-applied SBS in a polyester or glass-reinforced, factory-controlled thickness sheet good.

Hot-applied SBS membranes are unrestricted by low temperature site conditions. The Blue Cross Blue Shield Headquarters project in Egan, Minnesota, installed a majority of the 8361 m2 (90,000 sf) application in below –18 C (0 F). Hot-applied SBS sets immediately as a fully adhered continuous membrane. In its nearly 60-year track record, hot-applied SBS membranes have one of the lowest installed costs for a waterproofing membrane. They provide robust protection for plaza decks, steam tunnels, vegetated roofing, and parking garage deck applications. Conversely, respirable emissions and unpleasant odors from the melted asphalt should be considered when working in or near densely populated areas. Additionally, the material is nearly 204 C (400 F) when installed. The high temperatures can be dangerous even to an experienced installer.

Polymeric materials

In the latter half of the 20th century, the industry expanded toward new material possibilities, including polyurethanes. However, polyurethanes represent a broad category. Specifying the correct polyurethane requires precise terminology and detailed information regarding the product’s physical property requirements. Subcategories include two-component, single-component, hybridized, unhybridized, modified, and unmodified. Advantages to polyurethane products in general include better elongation and flexibility. Additionally, these products adhere well to a wide variety of substrates. In restoration or maintenance applications, older surfaces may have unknown coatings or pre-existing adhesives. A polyurethane allows the contractor a greater chance of success despite any unknowns over other more substrate-specific products. At a molecular level, urethanes form a spring-like structure that allows for strength and flexibility. This molecular spring also recovers well, which creates high tensile strength, elongation, hardness, and flexibility even at extreme high and low temperatures.

Unfortunately, compatibility between polyurethanes and other waterproofing materials can be tricky. Polyurethanes are often modified with asphalt. The transfer of oils or plasticizers between adjacent asphaltic materials can negatively impact adhesion and create other compatibility challenges. Many injection grouting materials contain polyurethane technology. If injection grouting is used to repair a failing waterproofing system, the compatibly with the existing waterproofing system must be determined before installation. Likewise, transitions, detailing membranes, and protection course should always be confirmed for compatibility prior to installation. Testing information is widely available for polyurethane waterproofing, but modified or partial terminology within the testing criteria can create confusion when comparing products. Due to the various chemistries within this larger group, it can be difficult to establish true ‘equals.’

Two-component polyurethane membranes were introduced in the early 1980s. These solvent-free products cure through a chemical or ‘crosslinking’ reaction. Two-components often have no added solvents, lower VOC levels than subsequent solvent-based, single-component products, less odor, and negligible flammability risk. While they provided a more consistent cure time than the air/moisture-cure single-components, the inconvenience of field mixing led to the development of single-component products later in the decade.

Moisture-cure single-component polyurethanes were developed in the late 1980s to eliminate the onsite mixing required by two-component membranes. Reduced site waste and improved moisture tolerance provide additional advantages over two-component polyurethane products. Single-component polyurethanes are dependent on ambient humidity to initiate the curing reaction, also known as ‘moisture-cure’. The addition of solvent cutbacks assists in workability and extends the recoat time. However, moisture-cure technology presents challenges such as foaming, bubbles, and blisters if installed incorrectly or over the presence of substrate moisture. Solvent content shrinkage may also be observed. Shrinkage is directly related to solids content. The higher the solids content in the liquid membrane, the less shrinkage will occur.

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