1. Overview
Grouting is the process of filling the gap between the equipment base (skid, sole plate, or baseplate) and the concrete foundation. Proper grouting ensures uniform load transfer, vibration damping, and long-term alignment stability for rotating equipment.
Load Transfer
100% Contact
Full bearing between skid and foundation
Vibration Damping
Energy Dissipation
Grout absorbs dynamic forces
Alignment Stability
Prevents Shifting
Locks equipment position permanently
Corrosion Protection
Seals Interface
Prevents moisture intrusion at base
Why grout matters: Inadequate grouting is one of the top causes of compressor vibration problems. Voids in the grout allow the baseplate to flex, creating soft-foot conditions that cause misalignment, bearing damage, and seal failures. Grout quality directly impacts machine reliability.
2. Grout Types
Two primary grout families are used for compressor foundations: epoxy (polymer) grout and cementitious (non-shrink) grout. Selection depends on equipment type, loading conditions, and environmental factors.
Grout Comparison
| Property | Epoxy Grout | Cementitious Grout |
|---|---|---|
| Compressive strength | 12,000-16,000 psi | 5,000-10,000 psi |
| Tensile strength | 1,500-2,500 psi | 400-800 psi |
| Bond to steel | 1,500-2,000 psi | 200-500 psi |
| Bond to concrete | 1,000-1,500 psi | 300-600 psi |
| Chemical resistance | Excellent | Good (affected by acids/oils) |
| Vibration resistance | Excellent (viscoelastic damping) | Good (rigid) |
| Temperature limit | 150-200 deg F continuous | 500+ deg F |
| Working time | 30-90 min (temp dependent) | 30-60 min |
| Material cost | $15-$25 per gallon | $3-$8 per gallon |
| Shrinkage | Minimal (0.001-0.003 in/in) | Non-shrink to slight expansion |
Selection Guidelines
| Application | Recommended Grout | Reason |
|---|---|---|
| Reciprocating compressor | Epoxy | High dynamic loads; superior vibration resistance |
| Centrifugal compressor | Epoxy | Alignment sensitivity; excellent bond strength |
| Gas engine driver | Epoxy | Pulsating loads; thermal cycling |
| Gas turbine driver | Epoxy or cementitious | Epoxy preferred but check temperature limits |
| Electric motor | Cementitious or epoxy | Lower dynamic loads; either acceptable |
| Pump baseplate | Epoxy preferred | Alignment-critical; chemical exposure |
| Structural columns | Cementitious | Static load only; cost-effective |
| High-temperature (>150 deg F) | Cementitious | Epoxy loses strength above 150-200 deg F |
Common Grout Products
| Type | Product Examples | Yield (ft3/unit) | Notes |
|---|---|---|---|
| Epoxy | Chockfast Orange, Masterflow 648 | 0.3-0.5 ft3/kit | 3-component mix; aggregate filled |
| Epoxy (precision) | Escoweld 7505, Chockfast Red | 0.1-0.2 ft3/cartridge | For chock pads; pour-in-place |
| Cementitious | Masterflow 713, Five Star | 0.5-0.6 ft3/bag | Mix with water; aggregate filled |
| Cementitious (precision) | Masterflow 928, Embeco 885 | 0.45-0.55 ft3/bag | Extended flow for tight spaces |
Industry standard: API 686 (Recommended Practice for Machinery Installation) requires epoxy grout for all rotating equipment foundations where dynamic loads exceed 25% of static loads or where alignment sensitivity requires less than 2 mils tolerance. Most compressor installations fall into this category.
3. Volume Calculations
Accurate volume calculation prevents shortages (which cause cold joints and voids) and excessive waste. Always calculate net volume, then add a waste factor.
Basic Volume Calculation:
V_grout = (L x W x T) - V_obstructions
Where:
V_grout = Net grout volume (cubic feet)
L = Length of grout area (ft)
W = Width of grout area (ft)
T = Thickness (gap between skid and concrete) (ft)
V_obstructions = Volume displaced by anchor bolts, sleeves, etc.
Anchor Bolt Volume Deduction:
V_bolt = N x pi x (D_bolt/2)^2 x T
Where:
N = Number of anchor bolts
D_bolt = Bolt diameter (ft)
T = Grout thickness (ft)
Bolt Sleeve Volume Deduction:
V_sleeve = N x pi x (D_sleeve/2)^2 x T
Where D_sleeve = Sleeve outer diameter (typically 2-4x bolt diameter)
Conversion: 1 cubic foot = 7.48 gallons
V_gallons = V_grout (ft3) x 7.48
Waste and Contingency Factors
| Factor | Epoxy Grout | Cementitious Grout | Notes |
|---|---|---|---|
| Formwork leakage | 5-10% | 5-10% | Good forms reduce waste |
| Mixing loss | 2-5% | 2-5% | Material left in mixer/buckets |
| Shoulder/chamfer | 5-15% | 5-15% | Grout extends beyond skid edge |
| Surface irregularity | 5-10% | 5-10% | Chipped concrete, uneven surface |
| Total contingency | 20-30% | 20-30% | Order 1.25-1.30x calculated volume |
Complex Geometry
Multi-Section Foundation:
For compressor skids with varying grout thickness:
V_total = Sum of V_i for each section
Section types:
Rectangular: V = L x W x T
L-shaped: Split into two rectangles
Tapered: V = L x W x (T1 + T2) / 2 (average thickness)
Stepped: Calculate each step separately
Chamfer (shoulder) volume:
V_chamfer = Perimeter x (chamfer_width x chamfer_height / 2) x L_chamfer
Typical chamfer: 45-degree, 1-2 inches wide
Prevents edge chipping and concentrates load inward.
Header Box / Forming Volume:
V_header = L_header x W_header x T
Add header box volume if grout fills forming beyond the skid.
Grout thickness limits: Minimum grout thickness is 1 inch (25 mm) to ensure proper flow and consolidation. Maximum thickness for a single pour is typically 4-6 inches for epoxy (exotherm limits) and 6-8 inches for cementitious. Thicker pours require staged placement or aggregate extension.
4. Pour Planning
Grout pour planning ensures continuous placement without cold joints, adequate working time, and proper crew sizing. A failed pour can require complete removal and re-grouting at significant cost.
Pour Sequence
| Step | Activity | Duration | Critical Notes |
|---|---|---|---|
| 1 | Surface preparation | 1-2 days | Chip laitance; SSD condition for cementitious |
| 2 | Formwork installation | 4-8 hours | Seal all joints; allow for head pressure |
| 3 | Pre-pour inspection | 1-2 hours | Check alignment, bolt torque, cleanliness |
| 4 | Material staging | 2-4 hours | Condition to pour temperature range |
| 5 | Mixing and placement | 30-90 min | Continuous pour; no interruptions |
| 6 | Initial cure | 4-24 hours | No vibration; maintain temperature |
| 7 | Form removal | 24-72 hours | After reaching minimum strength |
| 8 | Full cure | 3-7 days | Before equipment operation |
Crew and Equipment
Mixing Rate Estimation:
For epoxy grout:
One mixer (5-gallon capacity) produces ~0.5 ft3 per batch
Batch cycle time: 5-8 minutes (mix, pour, reload)
One mixer crew: 4-6 ft3/hour
For cementitious grout:
One mixer (mortar mixer) produces 2-3 ft3 per batch
Batch cycle time: 5-10 minutes
One mixer crew: 12-18 ft3/hour
Number of Mixers Required:
N_mixers = V_total / (Rate_per_mixer x Working_Time)
Where Working_Time accounts for grout pot life:
Epoxy at 75 deg F: 30-45 minutes working time
Epoxy at 90 deg F: 15-25 minutes working time
Cementitious: 30-45 minutes working time
Crew Sizing:
Per mixer: 2-3 workers (1 mixing, 1-2 placing)
Supervisor: 1 per pour
QC inspector: 1 per pour (verify placement, sample)
Total crew for large pour: 6-12 workers
Pour Direction
| Method | Description | Best For |
|---|---|---|
| One-side pour | Pour from one side; grout flows to opposite side | Narrow baseplates (< 4 ft wide) |
| Center pour | Pour at center; grout flows outward | Wide baseplates with center access |
| Multi-point pour | Pour from several locations simultaneously | Large pours exceeding pot life |
| Head-box pour | Use elevated reservoir to provide head pressure | Long flow distances; tight gaps |
Head pressure rule: For epoxy grout, the grout head (height in the header box above the skid) should be at least 1.5 times the flow distance. For a 6-foot flow distance under a skid, maintain at least 9 inches of head. This ensures complete filling without voids.
5. Temperature Considerations
Temperature is the most critical variable in grouting operations. It affects working time, exotherm, cure rate, and final properties. Both ambient and material temperatures must be controlled.
Temperature Limits
| Condition | Epoxy Grout | Cementitious Grout |
|---|---|---|
| Minimum pour temp | 40-50 deg F | 40-50 deg F |
| Maximum pour temp | 90-95 deg F | 90-100 deg F |
| Ideal pour temp | 65-80 deg F | 60-80 deg F |
| Foundation temp (min) | 50 deg F | 40 deg F |
| Foundation temp (max) | 90 deg F | 100 deg F |
| Exotherm peak (deep pour) | 200-350 deg F | 120-180 deg F |
| Max continuous service | 150-200 deg F | 500+ deg F |
Exotherm Management
Epoxy Grout Exotherm:
The exothermic curing reaction generates significant heat.
Peak temperature depends on:
Pour thickness (thicker = higher peak)
Ambient temperature (warmer = faster reaction)
Grout volume (larger mass retains more heat)
Rules of Thumb:
2-inch thickness: Peak exotherm ~150-180 deg F
4-inch thickness: Peak exotherm ~200-250 deg F
6-inch thickness: Peak exotherm ~250-350 deg F
Maximum Allowable Exotherm:
350 deg F (above this, grout may crack or delaminate)
Mitigation Strategies:
1. Pour in cooler ambient (early morning, evening)
2. Pre-cool materials in air-conditioned storage
3. Use extended-work-time formulations
4. Add aggregate extenders (per manufacturer limits)
5. Pour in multiple lifts (2-3 inch maximum per lift)
6. Pre-cool concrete with water evaporation
Cold Weather Grouting
| Temperature Range | Action Required | Notes |
|---|---|---|
| > 50 deg F | Normal procedures | No special measures needed |
| 40-50 deg F | Pre-heat materials; insulate forms | Slow cure; extend form time |
| 32-40 deg F | Heated enclosure required | Maintain 50 deg F minimum for 72 hrs |
| < 32 deg F | Do not pour | Freeze damage destroys grout integrity |
Hot Weather Grouting
| Temperature Range | Action Required | Notes |
|---|---|---|
| < 85 deg F | Normal procedures | Standard working time available |
| 85-95 deg F | Pre-cool materials; shade work area | Working time reduced 30-50% |
| 95-100 deg F | Early morning/evening pour only | Cool foundation with wet burlap |
| > 100 deg F | Postpone or use special formulations | Exotherm risk; rapid set |
Material conditioning: Store grout materials at 65-75 deg F for at least 24 hours before the pour. Cold epoxy components are viscous and difficult to mix. Hot components have shortened working time and higher exotherm. Air-conditioned storage trailers are standard practice for large grouting projects.
6. Surface Preparation
Surface preparation is the single most important factor in grout bond quality. Poor preparation causes delamination, voids, and eventual equipment vibration problems.
Concrete Surface Preparation
| Step | Method | Acceptance Criteria |
|---|---|---|
| Remove laitance | Chipping hammer, bush hammer, or needle scaler | Expose aggregate; ICRI CSP 5-7 |
| Remove contaminants | Solvent wipe, degreasing | No oil, grease, or curing compound |
| Roughen surface | Mechanical profiling | 1/4-inch amplitude minimum |
| Clean debris | Compressed air, vacuum | No loose particles or dust |
| Moisture condition (cementitious) | Saturated surface dry (SSD) | Dark surface, no standing water |
| Moisture condition (epoxy) | Dry surface | No visible moisture; test with plastic sheet |
Steel Surface Preparation
| Step | Method | Standard |
|---|---|---|
| Remove mill scale | Abrasive blasting or power tool | SSPC-SP6 or SP10 |
| Remove paint/coatings | Abrasive blasting | No coatings in grout contact area |
| Remove oil/grease | Solvent cleaning | SSPC-SP1 |
| Surface profile | Angular abrasive blast | 2-3 mil profile for epoxy bond |
| Time limit | Pour within 4-8 hours of blasting | Before flash rust forms |
Critical inspection point: The concrete surface must be inspected immediately before pouring. Any moisture (for epoxy), oil spots, or loose material will cause bond failure. Use a flashlight at a low angle to check for surface irregularities and contamination. Document the inspection with photographs.
7. Worked Examples
Example 1: Rectangular Skid Grout Volume
Given:
Compressor skid: 20 ft long x 8 ft wide
Grout thickness: 2 inches (0.167 ft)
Number of anchor bolts: 16
Bolt diameter: 1.5 inches (0.125 ft)
Bolt sleeve OD: 4 inches (0.333 ft)
Chamfer: 45 deg, 1.5 inches wide all around
Step 1: Gross volume
V_gross = 20 x 8 x 0.167 = 26.67 ft3
Step 2: Anchor bolt deductions
V_bolts = 16 x pi x (0.125/2)^2 x 0.167 = 16 x 0.00204 = 0.033 ft3
V_sleeves = 16 x pi x (0.333/2)^2 x 0.167 = 16 x 0.01454 = 0.233 ft3
Total deduction = 0.033 + 0.233 = 0.266 ft3
Step 3: Chamfer volume
Perimeter = 2 x (20 + 8) = 56 ft
V_chamfer = 56 x (1.5/12 x 1.5/12 / 2) = 56 x 0.0078 = 0.44 ft3
Step 4: Net grout volume
V_net = 26.67 - 0.27 + 0.44 = 26.84 ft3
Step 5: Apply contingency (25%)
V_order = 26.84 x 1.25 = 33.6 ft3
Step 6: Convert to gallons
V_gallons = 33.6 x 7.48 = 251 gallons
Step 7: Number of kits (assuming 0.4 ft3/kit for epoxy)
N_kits = 33.6 / 0.4 = 84 kits
Example 2: Working Time and Crew Sizing
Given:
Total grout volume: 33.6 ft3 (from Example 1)
Grout type: Epoxy (Chockfast Orange)
Ambient temperature: 85 deg F
Working time at 85 deg F: 25 minutes
Mixing rate per mixer: 5 ft3/hour
Step 1: Required mixing rate
Must complete pour within working time.
Required rate = 33.6 ft3 / (25/60 hr) = 80.6 ft3/hr
Step 2: Number of mixers
N_mixers = 80.6 / 5 = 16.1 -> 17 mixers minimum
This is impractical. Solutions:
a) Reduce ambient temperature (pour in early morning)
b) Pre-cool materials to 65 deg F (extends working time to 40 min)
c) Divide into multiple pours with construction joints
Step 3: Revised plan at 65 deg F material temp
Working time at 65 deg F: ~45 minutes
Required rate = 33.6 / (45/60) = 44.8 ft3/hr
N_mixers = 44.8 / 5 = 9.0 -> 10 mixers
Step 4: Crew sizing
10 mixers x 2 workers = 20 workers + 1 supervisor + 1 QC
Total crew: 22 workers
Alternative: Two-pour approach
Pour 1: Front half (16.8 ft3) with 5 mixers in 40 min
Pour 2: Back half (16.8 ft3) with 5 mixers in 40 min
Total crew: 12 workers per pour (much more manageable)
Example 3: Material Cost Comparison
Given:
Grout volume required: 33.6 ft3 (251 gallons)
Epoxy Grout:
Cost: $20/gallon average (mixed with aggregate)
Material cost = 251 x $20 = $5,020
Kits to order: 84 kits at ~$60/kit = $5,040
Cementitious Grout:
Cost: $5/gallon average
Material cost = 251 x $5 = $1,255
Bags to order: 33.6 / 0.5 = 68 bags at ~$25/bag = $1,700
Cost ratio: Epoxy is ~3-4x more expensive in material.
However, total installed cost difference is smaller because:
Labor cost is similar for both types
Epoxy requires less surface prep (no SSD conditioning)
Cementitious requires longer cure time (higher schedule cost)
Epoxy repairs are less frequent over equipment lifetime
Lifetime value: For a $2M compressor package with
25-year design life, the $3,500 grout cost premium for epoxy
is negligible compared to the reliability benefits.
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