Cementitious vs epoxy vs polyester resin in anchoring and grouting
Equipment and machinery which have precise tolerances for alignment or require uniform support cannot be placed directly on finished concrete surfaces. Both the concrete surface and the machine base have irregularities which result in alignment difficulties and bearing load concentrations.
For this reason, machine bases or soleplates are aligned and levelled by shimming or other means and the resulting space between the machine base and the foundation is filled with a load transfer material. The load transfer materials most frequently used are hydraulic cement and epoxy grouts.
In this blog, we’ll talk over various factors that determine which kind of precision grout you’ll need for the project.
What does a precision grout do?
After placement and hardening in the space between a machine or equipment base and the foundation, the grout is expected to perform one of the following functions:
Permanently maintain the original level and alignment of the machinery or equipment and transfer all loads to the foundation when shims and other temporary position devices are removed.
Provide lateral support or corrosion protection for shims which are designed to transfer all loads to the foundation. Some grouts are also required to provide sound deadening and a mechanism for reducing vibration. This can often be achieved by a composite system incorporating elastomeric bearings and a structural adhesive.
The most important requirement for a grout which is intended to transfer loads to the foundation, is for it to have volume change characteristics which result in complete and permanent filling of the space. Plain grout, consisting of cement, aggregate, and water, does not have those characteristics. Several other properties of the grout such as consistency, strength, chemical resistance, and compatibility with the operating environment are also important.
What factors determine which type of grout you need?
1. Size of baseplate
The size of the baseplate determines the method of application:
Small: Pouring is usually fine
Large: Requires pumping
No access for pouring: Requires pumping
2. Loading
Dynamic: This could be impact force such as a hammer press, reciprocating machinery, or repetitive forces such as crane rails or rail systems. Dynamic forces can cause cracking or fatigue in cement grouts. In these cases, use a resin-based grout.
Static: Those that impart less stress (eg. gaps under columns, stanchion bases, holes, cavities, general infill operations). These cases require a cement-based grout.
3. Dimensional stability
Grouts must not shrink:
Cement-based grouts must be non-shrink either in the short term (plastic shrinkage) or the long-term (drying shrinkage).
Resin-based grouts should not shrink on curing.
Grouts should also have thermal movement characteristics sympathetic to their immediate surroundings.
4. Resistance to chemicals
A grout should not react with metal as most bearings and plates are metallic. Some grouts may react with metal and produce corrosion or gases which affect bond and can cause disruption.
Grouts should be resistant to oils, greases, and certain solvent chemicals, as well as salts if used in marine environments.
5. Durability
Grouts must be resistant to creep (deformation over time) so they allow intimate contact between the bearing and foundation, and effective transfer of stress.
A precision grout must have a life expectancy of at least 10 years – most resin grouts are more durable than cement grouts, especially dynamic loading situations. They cost more upfront, but their service life and the fact they don’t need regularly replacing makes them much more cost-effective in the long-run.
6. Effective bearing area
The main function of a grout is to transfer load:
Effective Bearing Area (EBA) is the proportion of grout in intimate contact with the underside of the bearing plate.
A grout must provide an EBA of at least 90%
EBA is affected by poor grout flow, air voids, air entertainment in the grout, and poor design of the grouting operation.
7. Return to service
Engineers calculate the necessary resistance to early stresses in the bearing application. Many grouts are rapid-setting which allows for earlier return to service, but often these generate exotherms that can reduce working time and affect grout integrity especially in thicker sections.
8. Gap size
A grout has to fill the gap under a bearing plate, which can vary due to unevenness of the foundation.
Thin gaps (less than 5mm) should be avoided as most grouts would find it difficult to transfer load over this wafer-thin section.
Some epoxy grouts and structural adhesives are designed for thin sections but require additional considerations.
Most gaps between 15-50mm can easily be grouted by pouring or pumping.
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