Installation Techniques For Grouting Large, Skid-Mounted Equipment: Avoiding costly mistakes
by charlie rowan
High vibration, unusually high wear part usage, unplanned outages and construction and installation delays are just some of the problems that will happen when a rushed grout job goes bad. The monetary cost impact to the project is huge, but this will also cause a scheduling ripple effect of delays for the other contractors on-site. The costs grow exponentially if the problems (high vibration) are found at commissioning and the unit has to be lifted and regrouted after the plant has been built up around the foundation (see photo 1). Mistakes like this can turn a US$60,000 grout project into US$600,000.
Photo 2 is a good example of excessive foaming of the epoxy grout that was not poured properly. Additionally, the skid beam is not supported due to reduction of aggregate to increase flow, which also reduces the cubic meter yield per unit. The contractor ran short of grout and the skid had to be lifted and rerouted. Because of the low bar loading of skid beams, the complete 100% support of a beam using a lower-strength, high-flow material is normally much better than a partially supported beam on a higher-strength, stiffer material. Partially supported beams will excite and vibrate causing maintenance and operation problems.
Access holes for grouting should be provided in the skid floor plate by the OEM or packager. The end user should specify these when purchasing. Access holes are very important when trying to flow any type of grout 3 to 4.5 m across and 6 to 8 m down the length of the foundation. The grout depth should be 5 to 8 cm minimum after chipping to provide adequate flow (maximum 10 cm for excessive flow distance). Access holes also allow you to pour from the middle cutting the flow distance in half. They also provide inspection points to ensure all of the WF beams are fully supported. Be sure there are no blind or restricted flow areas in the skid design.
Expansion joints for epoxy grout should be installed in advance (after chipping) if a crane is used to lower the equipment in place. If jacks and rollers are used, then the joints should be added prior to lowering the equipment with the jacks. Photo 3 shows special chemical-resistant expansion joint foam that has been glued to the top of the concrete. As the skid is lowered to its final level position, typically 5 to 8 cm above the chipped concrete, the foam should be compressed under the skid beams.
Most grout placement problems are the result of the grout product beginning to thicken as it nears the end of its working time. Knee jerk is to switch to pouring from the opposite side hoping it will meet in the middle. This does not work on baseplates or skid equipment. Air will be trapped and large voids formed under the baseplates, and the center I beams under the skid and the transverse I beams will not be supported.
Proper pre-planning by the installer can avoid problems Based on the average field chipping depth, recalculate the amount of epoxy grout needed. A depth change of 1.27 cm can increase or decrease the quantity needed quite substantially based on the length and width dimensions.
Don’t chance running out of grout.
Shelter for the foundation from rain and excessively hot or cold temperatures must be provided. Typically, this is also required for alignment purposes and should be in place for seven days before grouting. Both the foundation and the material may need to be heated or cooled prior to grouting. Optimum temperature is 21°C. It can take several days to achieve temperature equilibrium, so start early. Removing product from their pallets can speed the material conditioning, but the foundation will take several days to condition, depending on the environment.
The concrete must be thoroughly cured and dry prior to chipping. For new blocks, a 6.8 to 13.6 kg chipping gun with a Moil point provides the best profile (2.5 cm peak to valley) for bonding to the concrete. This also removes all laitance or fine particles that rise to the top of the concrete during placement. This weak layer must be removed (approximately 2.5 cm) to expose broken aggregate.
The aggregate should fracture instead of popping out of the cement paste. Do not use a heavy chipping gun, as this causes excessive micro-fractures below the surface of the concrete. Do not use a bush head, as this will not properly remove the laitance layer, does not create a good bonding profile and it compacts, bruises and crushes the top surface. The edges of the concrete block should be chamfered or radiuses approximately 30°. Rebar wickets or dowels can also be added to provide mechanical locks between the epoxy grout and concrete. Both of these techniques will help prevent edge lifting or a horizontal crack at the grout concrete interface.
Access for mixing and placing of materials must also be considered in advance. The materials must be close to the mixer on the day of the pour. How the material is to be placed either by bucket, wheelbarrow or directly from the mixer must also be considered. Ramps over piping or scaffolding for an elevated block may be needed. The average unit of epoxy grout weighs approximately 97 kg, so providing unencumbered access makes the job much easier. If a scaffold is required, it is best to incorporate an elevated mixing area. The component materials will still need to be lifted onto the deck, but once mixed it can then be easily transported by wheelbarrow or bucket.
Proper ventilation, if environmental conditions require a full enclosure, must be provided during mixing and placing. Any backup mixers or other backup equipment that is critical to the job success should be provided and accessible. Doing a job with only one or two mixers can have a huge downside if one breaks. Most problems can be prevented up front through proper job planning and by following the guidelines available from the American Concrete Institute (ACI) and the product manufacturer.
The following 10 items can help you avoid many of the problems addressed in this article:
- Foundation design should meet American Concrete Institute ACI 351 Guidelines for Foundation Design. Limit grout shoulder width.
- Mechanical locks (rebar dowels or wickets) can be incorporated into the foundation design to prevent edge lifting cracks. These can easily be incorporated at the civil design phase but are costly to field retrofit.
- Proper concrete preparation with a chisel point or Moil point on a 6.8 to 13.6 kg gun per ACI 351. Do not use a bush head or scabbling machine. Provide an average 2.5 cm peak to valley profile. Keep all areas to be grouted bone dry for three to seven days minimum.
- Install expansion joints every 0.9 to 1.2 m before lowering the skid in place. Seal the bottom of the expansion joint to the concrete surface with oil-resistant elastomeric.
- No square corners on jack screw landing plates or skid beam corners or outriggers.
- Cut inspection and grout placement holes for as many sections as accessible.
- Use a high-flow epoxy grout designed and warranted for aggregate reduction. Use a head box.
- Pour from one side only until grout has made it to the opposite side. Pouring from the grout holes is also acceptable but needs to be done by experienced contractors.
- Shade or enclose the skid and work area so the materials and foundation are bone dry and 21°C.
- Pre-planning will ensure a smooth successful job.
Charlie Rowan is the third-generation owner of Robt. L. Rowan & Associates Inc., a Houston, Texas, U.S.A.-based engineering, manufacturing and consulting firm with extensive experience in the design and repair of compressor foundations, and the original inventor of the first commercial epoxy grout. The company holds five patents in the area of machinery support systems and anchorage. Rowan is active in the American Concrete Institute and is one of the instructors for the yearly Gas Machinery Research Council course, “Foundation Design and Repair.”