Complete Foundation Repair
M & M has experience in placing special
regular concrete mixtures or the new fast-curing cretes.
Reciprocating compressor crankshaft alignment determines machine life, downtime outage and costs, wear part usage, and when a regrout or foundation repair is necessary. When a foundation repair becomes necessary, it should be looked upon as a chance to upgrade the old foundation, address the cause of the problem, and prevent it from reoccurring. The foundation should be rebuilt and reinforced with rebar to today's design standards. The chocks should be upgraded to a new chocking system. The anchor bolts should be repaired or replaced with new high strength anchor bolts made per ASTM A-193 specifications. These upgrades can more adequately address typical alignment related foundation problems, such as cracking and foundation integrity and stability than a simple regrout.
A compressor foundation must be able to absorb and transmit the dynamic loads it receives into the mat and subsoil below. To do this it must be monolithic and properly reinforced with rebar. Reinforcement is most important in the upper block area. The best way to achieve this with the materials that are available today is with a concrete repair material reinforced with rebar.
Cracks in the foundation, particularly those found in the lower block, will not be corrected by just chipping out the old grout and repouring more new grout. Chances are the problem will reemerge over time and eventually will have to be corrected. Horizontal and vertical cracks in the foundation, especially those that are weeping oil or fluids must be addressed.
Cracks in the upper foundation block can be repaired by chipping out below the crack line for horizontal cracks or until the crack is gone for vertical cracks. A concrete repair material reinforced with rebar is used to rebuild the foundation. A 3 to 4 inch epoxy grout cap is poured on top as a precision, non-shrink, chemical resistant grout cap. A chock is installed at each anchor bolt to provide an adjustable support system.
Cracks in the lower foundation block become an economic and engineering problem. Economics usually do not permit a complete foundation removal and replacement. A simple regrout will not correct the problem. The solution is to rebuild and reinforce the upper 18 to 24 inches of the block area with a concrete material. The cracks in the lower block are bridged with post-tensioning bolts to bring the lower block area back into a more monolithic state. The highly reinforced upper repair area is post-tensioned to the lower block repair to tie the entire block back together. This provides an economic solution that is not as good as a new block but provides a better engineered, long-term repair than a simple regrout
Standard concrete must cure for 21 to 28 days before epoxy grout can be successfully applied. Because of this downtime problem, many people have tried using epoxy grout as a concrete repair material. When epoxy machinery grout was invented, it was designed to replace cementitious grouts, which were typically, used then as today in a 3 to 4 inch thickness. Epoxy grouts cannot be reinforced with rebar like cementitious materials because of the differences in the thermal coefficient of expansion and modulus of elasticity of epoxy grout and steel. Epoxies, which can have high tensile strengths, do not transfer the tensile loads to the rebar like cementitious materials. This is because epoxies have such a low modulus of elasticity that they stretch rather than transfer the tensile load to the rebar when compared to concrete. Epoxy grouts have a modulus of elasticity about half that of concrete or when loaded equally epoxy grout will compress about twice as much as concrete. This explains many of the field alignment problems and catastrophic failures of crankshafts where the foundation was repaired with a deep pour of epoxy instead of a higher modulus concrete repair material. Epoxy grouts can creep or cold flow over time, further contributing to alignment problems. This problem is exaggerated by both depth and temperature. The greater the depth and temperature the greater the creep. Concrete and Polymer Modified Concrete have virtually no creep when compared to epoxy grout. Most foundations are not chipped to a constant elevation as a crack or oil soaked concrete may create varying demolition depths. Wide differences in depths of pour of epoxy grouts can cause different amounts of compression and creep over time. When rebuilding the foundation with concrete or Polymer Modified Concrete the grout depth is a much more constant depth of 3 to 4 inches, limiting the problems inherent to epoxy grout.
When it is not possible to wait 28 days for regular concrete to cure, Hi-Early or Polimer Concrete is the solution. These concretes are ready for preparation for epoxy grout placement after only 24 to 48 hours of cure time depending on ambient temperatures. These concretes provide all the structural benefits of regular concrete and are usually a lot less cost per cubic foot than epoxy grout. Mixed and placed like regular concrete to any depth needed, these concretes can be used to repair upper block areas or the entire block if downtime and economic conditions warrant a quick repair.
When rebuilding the foundation, the anchor bolts also need to be checked or upgraded. Anchor bolts provide the clamping force necessary to restrain horizontal and vertical loads. This prevents excessive movement and wear on moving parts such as the crankshaft and compressor cylinder rods. The old anchor bolts should be upgraded to high strength two piece bolts like the R-193 series of anchor bolts, which are made from 4140 per ASTM A-193. Many older bolts are A36/307 mild steel bolts. If access or economics doesn't permit complete replacement then the old bolts should be cut off and upgraded to increases holding or pull out to allow for higher capacity of new high strength top sections.
If access is available, the old bolts can be cored or chipped out and completely replaced. If possible the new anchor bolts should terminate in the mat to provide an additional post tensioning benefit.
When replacing Anchor Bolts the top section should be wrapped in foam or enclosed in PVC pipe so it is free to stretch. Additionally the top section should be necked down or have a frangible section per ASTM to allow the bolt to stretch within its elastic limit. This is necessary because many compressor frames and the foundation itself would fail before the bolt when tightened at full diameter would achieve proper stretch. The anchor bolt top sections should be shot peened per mil. spec. for additional surface hardening and to reveal any delamination during the thread rolling process. A 4140 Double Spherical Washer which is hardened per F 436 will prevent point loading of the 2 H nut due to out of level nut bosses or thermal growth of the compressor frame. This will prevent bolts from breaking below the nut due to a stress plane created by point loading.
The coupling nut used to join the top and bottom sections and the Double Spherical Washer should be made from 4140. As softer steel will allow the bolt to relax more and lose its torque. The Double Spherical Washer must be hardened to prevent crush and lose of bolt torque.
A properly engineered foundation repair that incorporates proven, engineered repair designs and appropriate repair materials can provide long term equipment life and cost savings by reducing unscheduled downtime and wear part usage. Remember the bitterness of poor quality is long remembered after the sweetness of low cost is forgotten.
Review of Important Components in the Total Machine Foundation System
Major components that should be considered by the designer are:
As we discuss how to repair or upgrade older foundations, we need to keep in mind that all of the above components are just as important in making a successful repair as they are in a new design. Unfortunately, at the repair stage, we usually can't change the existing condition of the subsoil or the concrete mat. We have to concentrate on what can be accomplished in the upper concrete block, the anchor bolts, the grout, the support system of sole plates, chocks and the machine itself. It should be recognized that good engineering practice would say that most older foundations should be completely replaced by a new more comprehensively designed foundation if the life of the machine is to be extended by 20 years. Since this is rarely economically possible, we should discuss what can be practically done using newer techniques and materials of construction.
Evaluate The Problem:
A. Field inspection to view obvious damage to the concrete, grout and anchor bolts.
B. Review the history and maintenance records of the machine.
C. Review current mechanical condition of the machine.
D. Review original foundation design drawings.
E. Prepare a written scope of work and detailed drawings.
Designing a Repair Procedure
While no two foundation repair/upgrade projects are the same, the following are the tools and techniques are usually considered if a new replacement foundation is out of the question.
A. Rebuild the upper foundation stronger
B. Post tensioning of the upper foundation and the fractured lower foundation.
C. Upgrade the anchor bolts to current industry requirements.
D. Install chocking system so future foundation induced machinery alignment changes can be managed.
E. Re-balance the machine to minimize unneeded stress on the foundation.
F. Establish a follow-up maintenance program.
Contact us for your own specific needs.