The rotating equipment and heavy machinery cannot simply be placed on the concrete foundation since the irregularities in both the machine base and concrete surface will create point loads even if the required alignment could be achieved. Concrete foundations are not designed to withstand that type of loading, impact, or potential chemical exposure.
This article contains excerpts from the paper, “Precision grouting of turbomachinery”, by Fred Goodwin, Rick First, Dan Termunde and Christopher Adams of BASF Construction Chemicals at the 2017 Turbomachinery & Pump Symposia.
Once the critical aligned equipment has been properly leveled, the space between the equipment and foundation is filled with a high quality precision grout to provide linkage and load transfer from the equipment into the foundation and eventually the earth. There are two types of precision grouting material most often used in these applications are: non-shrink cementitious grout and epoxy or polymer grouts.Grout is defined by ASTM C125 terminology as “a mixture of cementitious material and water, with or without aggregate or admixtures that is used primarily to fill voids.” ASTM takes that definition a bit further when describing “grout, non-shrink”: “a hydraulic-cement grout that produces a volume that, when hardened under stipulated test conditions, is greater than or equal to the original installed volume, often used as a transfer medium between load-bearing members.”
Besides hydraulic cement based grout, polymer binders can also be used for a similar purpose to provide improved chemical resistance, faster strength development, higher ultimate compressive strengths, and bond strength between the concrete and steel base plate.
Cementitious grouts (non-shrink) consist of a dry powder containing a hydraulic binder, aggregates and additives to enhance flowability, volume stability, control setting, enhance strength, and maintain homogeneity once mixed with water. Cementitious grouts are capable of being placed over a wide temperature range, require damp substrates, and have similar chemical and physical properties (such as coefficient of thermal expansion and modulus of elasticity) to the substrate concrete. The mixed consistency can be adjusted by varying the amount of mixing water.
Cementitious grouts require moist curing and clean up and disposal of materials is similar to general construction debris. Strength develops over several days to weeks and the material cost is usually significantly less than for polymer grouts. Typically, polymer grouts consist of a resin, hardener, and dry aggregate components packaged separately. Usually the resin and hardener are mixed together, and then the aggregate is immediately mixed into this blend until it is completely wetted, with no dry aggregate is present. Polymer grouts should be placed as close to room temperature as possible. Colder temperatures produce longer working time but also a much higher viscosity resulting in reduced flow of the material.
The placement viscosity of polymer grouts tends to be much higher than most cementitious grouts and changes dramatically with the temperature of the material. The coefficient of thermal expansion and creep of polymer grouts also tends to be significantly higher than either cementitious grouts or concrete, but the modulus of elasticity is usually somewhat lower.
Compressive, tensile, and bond strengths of polymer grouts are higher than their cementitious counterparts. Many polymer grouts are potential allergens or sensitizers so careful attention to the material safety data sheet and application guidance from the material producer is necessary to avoid health and safety issues when handling these materials.
Although polymer grouts and cementitious grouts have these different physical properties, their use as a load transfer linkage between the loads of equipment and the foundation is functionally equivalent. Both materials must be capable of being mixed and placed under the equipment to maintain intimate contact between the metal support structure and the foundation. This intimate contact is generally known as effective bearing area or EBA which combines the placement properties with the volume stability of the grout during and after placement.
The effective bearing area is measured by the area of contact between a baseplate and the hardened grout. Higher compressive strength is usually assumed to imply better load transfer. While compressive strengths are an extremely important physical property to consider, there are severalother performance properties which should be taken into consideration when selecting the best precision grout for a specific application.
Various standardized tests are used to evaluate these properties with different methods used for polymer and cementitious grouts. For cementitious grouts, many of these methods and minimum acceptable values are summarized in the industry specification ASTM C1107. No similar document or guideline exists for polymer grouts. Unfortunately, these different standards do not provide a correlation allowing a direct comparison of the data between cementitious and polymer grouting materials.
Turbomachinery Blog features postings from experts in all areas of turbomachinery, such as: gas turbines, machine diagnostics, materials, repairs, and aftermarket parts, and encourages users to participate, with reader engagement and interaction as its primary purpose.
|There are no products|