Grouting and Arresting High Volume Water Ingress
GroutingWhich one to use, When, and Why?
Cementitious and chemical grouts share a common category and name, but the distinct differences far outweigh the common qualities. Both the types of grouts are used in civil applications ranging from sewer pipe rehabilitation to new tunnel or shaft construction, used to fill narrow cavities/cracks, rock fissures or to permeate soils for the control of groundwater, void filling or to increase structural support of the geology. Although cement and chemical grouts can differ in composition, application, and cost, they are complementary products on the same projects. This paper will help you to explore when and why to use the various types of grouts. The practice of injecting grouts and the wide array of grout products available can make choosing the right grout for the project complicated. Because it is difficult to summarize the complexity of the topic into a single paper, some specific topics within this paper have been generalized for simplicity.
Grout Family and Sub types
Grouts used in civil construction and rehabilitation projects can be generally categorized into either a cement or chemical grout. Within each grout family, there are primary grout subtypes. Within the cement grout family, ordinary Portland cement and ultrafine cements define these subtypes. The chemical grout family includes sodium silicate, acrylic gels, and polyurethane expansive foams. Although each parent grout type has primary grout spawns, this is where the commonalities end and the individual grout types split into their own unique characteristics. Cement grouts are considered to be suspended solid grouts, because they have particulates that comprise their composition which is derived from grinding Portland cement clinker. The level of grinding effort applied to the clinker determines the average particulate size. Portland cement grout generally has particulate sizes on average of 15 microns. Microfine cements range from 6 to 10 microns while ultrafine cements can have average particulate sizes of 3 to 5 microns. A grout’s ability to penetrate a rock fissure largely depends on particulate size whereas its ability to permeate a soil is also dependent upon surface tension within the grout. A cement’s rheology, which is the grout’s ability to flow, is accomplished through the control of the water/cement ratio and almost always a superplasticizer is added to reduce viscosity. While additives can be added to slow the cure time, once mixed with water, cement grouts will begin to cure and create a high compressive strength. Once injected, cements are considered long term solutions for either water control or structural improvement having life spans ranging between 100 to 200 years. The primary types of chemical grouts (silicates, acrylics and polyurethanes) are each unique in composition. Although it is truly a suspended solids grout, the particulates are so small, sodium silicates have a high degree of penetrability into soils and rock, very similar to the true solution grouts which have no suspended solids. Sodium silicate is a two component grout that typically has very low viscosity but will often expunge water after gelling, a process called syneresis. Sodium silicates can be sensitive, bordering on unstable, when injected into any groundwater condition. With relatively short gel times, a few minutes to a few hours, sodium silicates are commonly used as temporary solution for water control or structural support with an estimated life span of a few years. Longer life spans can be experienced with silicates depending on the chemistry of the soils. Colloidal silica grout which was developed to reduce the issue of syneresis with sodium silicates, provides better control of gel times, and achieves a lower viscosity. Colloidal silica grout is prepared in a multiple step process where a silicate solution is partially neutralized, leading to the formation of silica nuclei. Colloidal silica has the same expected life span as sodium silicate; however because of the multiple step process for development, prices are considerably higher per mixed liter. Acrylics are defined as "true solution grouts" which are free of suspended solids and have extremely low viscosity - similar to water. The acrylic family consists of acrylamide and acrylates. Each type requires a base resin to be mixed with a catalyst in order to create a gel matrix within a soil or rock with a controllable gel time. Acrylamide changes from a liquid to a solid in a controllable gel time ranging from 3 seconds up to 10 hours. Acrylate gel times range from approximately a minute to one hour. The life span of acrylamide is estimated to be greater than 300 years while an acrylate is estimated to be approximately 50 years. There are two primary types of polyurethane grouts defined as hydrophilic and hydrophobic. Hydrophilic grouts are typically single component systems that react with water and cure to an expansive flexible foam or non-expansive gel requiring moist environment after curing. Hydrophobic expansive foams require little water to react, approximately four percent, and easily withstand wet/dry cycles. True hydrophobic foams, not requiring water to react, also are available. Hydrophilic foams expand 4 to 6 times their original volume, while hydrophobic foams expand up to 20 – 30 times of original volume and may cure flexibly or rigidly. Most manufactures carry polyurethane resins having NSF or UL certifications approving use in potable water applications. The life span of polyurethane foam is estimated to be approximately 75 years. It should be noted that the cost of polyurethane foams reduces drastically when one factors the expansive component of the material into the overall costs.
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