Arkansas Concrete - Commercial Exam

All references: Pipe and Excavation Contracting

Key Concepts

Quantity takeoff basics. For trench excavation, the quantity of soil to move equals trench length × width × depth; even when the trench depth must be averaged, the formula stays the same (Chapter 9, "Estimating Surface Excavation" introduction). Grading and leveling require different math because the surface may be irregular — hills, gullies and depressions (same chapter introduction).

Soil states. Soil and fill are measured in cubic yards, and you must identify the state: Bank cubic yards (BCY) — soil in its natural, undisturbed condition; Loose cubic yards (LCY) — soil that has swelled after excavation or disturbance; Compacted cubic yards (CCY) — soil that has shrunk after compaction (Pages 60–61). Hauling unit capacities are measured in loose cubic yards (Page 61).

Import and export. Export (spoil) is excess excavated material removed from the site; import (borrow) is acceptable material hauled in for fill. Subtract total fill from total excavation volume: if fill exceeds excavation, the project requires import; if fill is less than excavation, it requires export (Page 61).

Key Concepts

Reading the plans. The plan view is a bird's-eye overhead view of the work area showing the layout; the profile view is a horizontal cut-away view along the alignment showing elevations, grades and depths (Pipe & Excavation Contracting, Page 105, Chapter 3). Most utility site plans are drawn in two scales — one horizontal, one vertical — with the horizontal scale usually ten times the vertical scale (e.g., 1 inch = 40 feet horizontally and 10 feet vertically); slopes appear exaggerated. Match lines align multi-sheet sets (Page 109).

Site grading. Ground immediately adjacent to the foundation must slope away not less than one unit vertical in 20 units horizontal (5-percent slope) for a minimum distance of 10 feet measured perpendicular to the face of the wall. Swales within 10 feet of the foundation: minimum 2 percent. Impervious surfaces within 10 feet: minimum 2 percent. Exception: the slope away from the building may be reduced to 1:48 (2 percent) where climatic or soil conditions warrant (IBC 2015 §1804.4).

Excavation near foundations. Excavation shall not reduce lateral support from any foundation without first underpinning or protecting it (§1804.1). Underpinning must be installed in a sequence identified in the approved construction documents (§1804.2.1). Where excavation reduces support, a registered design professional prepares an assessment and site-specific plans (§1803.5.7).

Key Concepts

What formwork is. A form is a temporary structure or mold supporting concrete while it sets and gains enough strength to be self-supporting. Formwork is the total system of support: the form facing material contacting the concrete plus all supporting members, hardware, and necessary bracing (Chapter 5 – Formwork, Page 54).

Cost significance. Formwork costs range from 35 to 60 percent of the cost of a concrete structure, making it one of the most significant cost components (Page 54).

Lateral pressure variables. The three most important variables affecting the pressure fresh concrete exerts on forms are (1) concrete temperature, (2) rate of placement, and (3) weight of concrete (Page 70).

Unit weight. Normalweight concrete is approximately 150 lb/ft³ (2400 kg/m³) — more than twice the weight of water — and this value drives formwork design loads (Page 70).

Form ties. The usual safety factor for form ties is 2:1; the working load of new ties is limited to one-half the ultimate tensile strength (Page 66).

Form anchors. The minimum safety factor, including live loads and impact, is 3 for form anchors securing formwork to previously placed concrete. The higher factor (versus 2:1 for ties) applies because anchors are often loaded while the concrete they are embedded in is still "green" and has developed only part of its strength (Page 67).

Key Concepts

Bond. The adhesion of concrete to the bar surface, plus the resistance from the bar deformations or ribs, keeps bars from slipping through the concrete and makes the two materials act as one. This gripping of the bars by concrete is known as bond (Placing Reinforcing Bars, Ch. 3 – Concrete and Reinforced Concrete, Definitions).

Minimum cover — cast against earth. For concrete cast against and permanently exposed to earth (such as footings), minimum cover is 3 in. (75 mm) (Contractor's Guide, Table 6.3, p. 100). CRSI states it the same way: 3 inches at sides where concrete is cast against the ground and on bottoms of footings or other principal structural members where concrete is placed on ground (Placing Reinforcing Bars, Ch. 11, Fig. 11-1(a)).

Why cover matters. Cover protects the steel from damage and corrosion where exposed to weather, water, foundation soils or aggressive chemicals; protects steel from fire; and develops strength of bond (Contractor's Guide, Ch. 6 – Concrete Cover).

Bar size designation. For standard American bars, the bar number is the nominal diameter in eighths of an inch — a #8 bar is 1 in. (25.4 mm). For metric bars, the number is the approximate diameter in millimeters (Contractor's Guide, Ch. 6; Placing Reinforcing Bars, Ch. 7).

Key Concepts

Exposure classes and mixture limits (Contractor's Guide, Table 2.5, p. 24) - Class F3 (freezing-and-thawing with frequent exposure to water and deicing chemicals): maximum w/cm 0.40, minimum compressive strength 5000 psi. - Class S2 (dissolved sulfates 1500 to 10,000 ppm): maximum w/cm 0.45, minimum 4500 psi. - Class W1 (concrete in contact with water where low permeability is required): maximum w/cm 0.50, minimum 4000 psi. - Air content: 3/4 in. (19.0 mm) maximum aggregate in severe exposure (continuous moisture before freezing, or deicing salts) — average air content 6 percent (Table 2.2, adapted from ACI 201.2R).

IBC 2015, Chapter 19 - 1901.5, Item 1 — Construction documents must include the specified compressive strength of concrete at the stated ages or stages of construction for which each element is designed. - 1903.4 — ICF material for flat concrete walls shall conform to ASTM E2634. - 1904.1 Exception — Group R-2 and R-3 not more than three stories above grade plane: f′c for basement walls, foundation walls, exterior walls and other vertical surfaces exposed to weather shall be not less than 3,000 psi. - 1904.2 — Nonstructural concrete minimum f′c: 2,500 psi (F0); 3,000 psi (F1); 3,500 psi (F2 and F3); air entrained per ACI 318. - 1906.1 Exception — Group R-3 and other occupancies less than two stories above grade plane of light-frame construction: plain concrete footing thickness may be reduced…

Key Concepts

Subpart C – General safety and health provisions (29 CFR 1926.20) - It is the responsibility of the employer to initiate and maintain such programs as may be necessary to comply with this part — §1926.20(b)(1). - Such programs shall provide for frequent and regular inspections of the job sites, materials, and equipment to be made by competent persons designated by the employers — §1926.20(b)(2) (Page 29). - Machinery, tools, materials, or equipment not in compliance are prohibited; they shall be identified as unsafe by tagging or locking the controls to render them inoperable, or physically removed from the place of operation — §1926.20(b)(3). - The employer shall permit only those employees qualified by training or experience to operate equipment and machinery — §1926.20(b)(4). - PPE standards impose a separate compliance duty for each employee covered, and each failure to provide PPE to an employee may be considered a separate violation — §1926.20(f)(1). The same per-employee rule applies to training — §1926.20(f)(2).

Safety training and education (29 CFR 1926.21, Page 30) - §1926.21(b)(2): the employer shall instruct each employee in the recognition and avoidance of unsafe conditions and the regulations applicable to his work environment. - §1926.21(b)(3): employees handling or using poisons, caustics, and other harmful substances shall be instructed in safe handling and use and made aware of potential hazards, personal hygiene, and personal protective measures required. - §1926.21(b)(4):…