Plain-language definitions for the terms you’ll see in specs, drawings, and bid sheets — explained with field context from the DBO team.
Structural & Load Terms
HL-93 Load Rating
A modern highway load standard (AASHTO) that accounts for semi-truck axles and lane loads.
Think of it like: Designing for a loaded semi parked over your duct bank.
Field tip: Always check if the duct bank will cross roadways or heavy equipment paths — HL-93-rated structures are built for that.
HS-20 Load Rating
The older standard many drawings still reference — roughly equivalent to older truck load assumptions.
Why it matters: Most modern projects have moved to HL-93; if a supplier still designs to HS-20, verify their structural calcs.
Compressive Strength (psi)
How much load the concrete can handle before crushing — usually 4,000–6,000 psi.
For context: 5,000 psi is roughly 2× the strength of sidewalk concrete.
Rebar / Reinforcement
Steel bars or mesh inside the concrete that give it tensile strength.
Visualize: The skeleton inside the concrete.
DBO note: Depending on load, DBO uses traditional rebar or fiber reinforcement to handle both flex and point loads.
Shear Key
A tongue-and-groove feature molded into precast sections to keep them from shifting under load.
Think of it like: Puzzle-piece edges that lock sections together underground.
Thermal & Electrical Terms
Thermal Resistivity (ρ Value)
Measures how well concrete and soil carry heat away from cables. Lower = better.
Example: DBO targets ρ < 60 °C-cm/W to keep cables cooler and extend life.
Ampacity
The maximum current a cable can carry without overheating.
Connection: Thermal resistivity directly affects ampacity — poor backfill or concrete mix can drop it fast.
Thermal Backfill / CLSM
Specialized soil or flowable fill designed for low thermal resistance.
Field note: It’s not always required, but if cables are running hot, thermal fill around the duct bank helps dissipate heat.
Conduit & Material Terms
Schedule 40 / Schedule 80 PVC
Wall-thickness ratings for PVC conduit. Schedule 80 is thicker, used where added strength or abrasion resistance is needed.
Analogy: Schedule 80 is like heavy-duty EMT; Schedule 40 is standard spec-grade conduit.
HDPE Conduit
High-density polyethylene pipe — flexible and fused instead of glued. Common in long directional-bore runs.
Difference: PVC is rigid (good for encasement); HDPE is flexible (good for bores).
Fiberglass (RTRC) Conduit
Reinforced thermoset resin conduit — extremely strong and lightweight.
Where used: Specialty sites where corrosion or temperature are a concern.
Conduit Bank
A group of conduits encased together in concrete — the “duct bank” itself.
Visualize: The multi-lane highway your power and data cables run through.
Bell End / Coupler
Factory-flared or attached fitting that connects conduits end-to-end.
Why it matters: Misalignment here causes cable damage — DBO uses fixed jigs to lock every conduit before the pour.
Design & Drawing Terms
Section View
Cross-section drawing showing conduit layout, spacing, and encasement thickness.
Field tip: Always confirm section matches plan view — misalignment is a common coordination miss.
Plan View
Top-down view of the duct bank route — shows offsets, bends, and vault locations.
Elevation
Vertical profile showing depth and grade changes along the duct bank run.
Bend Radius
The minimum curve a conduit or duct bank can take without damaging cables.
Analogy: Like bending a garden hose — too tight, and you kink it.
DBO note: DBO builds prefabricated sweeps to spec to maintain correct radii in precast systems.
Encasement Thickness / Concrete Cover
The amount of concrete around the outside of conduits.
Why it matters: Protects from loads and corrosion; typically 3″ minimum for electrical duct banks.
Installation & Field Terms
Guide Pins / Alignment Keys
Pins or dowels cast into precast sections to ensure perfect alignment during installation.
Think: The dowel pins that line up engine parts — essential for smooth conduit continuity.
Watertight Joints / Gaskets
Rubber or neoprene seals between precast sections to keep water out.
Why it matters: Prevents cable corrosion and extends service life.
Backfill / Bedding
Material placed under and around duct banks.
Field note: Use controlled fill that matches design density and thermal properties; avoid rocky soil that can damage conduits.
Setting / Placement
The act of lowering precast duct bank sections into the trench with lifting anchors and alignment guides.
Crew snapshot: Usually an operator and a few laborers or electricians — far fewer than a field pour.
Cure Time
How long concrete needs to reach strength after pouring — typically 7–28 days.
Why it matters: In precast systems, this happens offsite. On cast-in-place, it holds your trench open.
Testing & Quality Terms
Load Test
Verifies that the duct bank meets its rated strength — often by applying HL-93 equivalent loads during design validation.
DBO practice: DBO performs proof testing on new designs to certify performance under real-world conditions.
Dimensional Tolerance
Allowed variation in conduit spacing and alignment.
In context: Field-built banks can vary up to an inch; DBO’s precast units hold within a few millimeters.
QC / QA
Quality Control (plant side) and Quality Assurance (project side).
Quick rule: QC makes sure it’s built right; QA verifies it performs right.
Project & Procurement Terms
Submittal Package
The shop drawings, mix designs, and test data the supplier submits for engineer approval.
DBO edge: Submittals are fully engineered — no “redlines later” guesswork.
Lead Time
Time between order and first delivery.
Reality check: Some suppliers quote fast then stall; always verify actual production capacity and backlog.
Staging / Sequencing
How materials are stored and delivered in line with trench progress.
DBO advantage: Integrated logistics that match delivery order to construction flow.
Quick Visual Analogies (for Non-Engineers)
| Concept | Analogy | Key Takeaway |
| HL-93 | Weight of a semi-truck | Duct banks must handle real traffic loads |
| Thermal ρ | Breathability of fabric | Lower = better heat flow, cooler cables |
| Guide Pins | Lego® connectors | Keeps precast sections perfectly aligned |
| Conduit Schedule | Pipe wall thickness | Higher number = thicker, stronger |
| Shear Key | Puzzle-piece joint | Locks precast segments together |
| Encasement | Armor shell | Concrete that protects the conduit bundle |
When in Doubt, Ask for Data
The best suppliers won’t just define these terms — they’ll show test reports, mix designs, and dimensional checks to prove compliance. DBO provides full documentation for every duct bank produced, including load testing, concrete ρ values, and conduit alignment verification, so field teams and engineers are never guessing what’s under their site.
