ACI cheat sheet

Water-cementitious materials ratio (w/cm) governs strength and durability; lower w/cm yields higher strength and lower permeability, with practical minimums near 0.40 for severe exposure.

Abrams' rule: strength is inversely related to w/c; a common approximation is fc' ≈ A/B^(w/c), so small w/c changes produce large strength changes.

Absolute volume method: V_aggregate = 1 m³ (or 1 yd³) minus volumes of water, cement, and air; aggregate volume is derived, not assumed.

Entrained air (typically 4–8%) improves freeze-thaw resistance; air content increases with maximum aggregate size decreasing and with exposure severity.

Supplementary cementitious materials: fly ash and slag reduce early strength but improve long-term strength, permeability, and sulfate resistance; silica fume greatly increases strength and reduces permeability.

Nominal maximum aggregate size should be the largest that permits adequate workability; it is limited by member dimensions (commonly ≤ 1/5 of narrowest dimension) and by clear spacing between reinforcement.

Slump is a workability indicator only; it does not measure placeability, and high-slump mixtures may segregate unless cohesion is controlled through paste content and grading.

Mixture proportioning starts from target mean strength (fc' plus margin for variability), then selects w/cm, air content, and water content, and finally computes cement and aggregate quantities.

Batching tolerances: cementitious materials commonly ±1% of required mass; aggregates ±2%; water ±1%; admixtures ±3% (per ACI 304 guidance).

Moisture content of aggregates must be measured and free water subtracted from the batch water to maintain the specified w/cm.

Mixing time after all materials are in the drum is typically a minimum of 60–90 seconds for stationary mixers; truck mixers commonly require 70–100 revolutions at mixing speed.

Delivery: concrete should be discharged within 90 minutes of batching (or before 300 revolutions) unless a longer time is approved based on slump and temperature testing.

Truck mixer agitation speed is about 2–6 rpm; mixing speed is higher (typically 8–12 rpm). Adding water at the jobsite is permitted only within specified limits and with records.

Ready-mix tickets must record batch time, mix design, quantities, water added, revolutions, and discharge times; these are the primary delivery quality records.

Volumetric (mobile) mixers allow on-site proportioning and must be calibrated; mix proportions are verified by yield checks.

Concrete temperature at placement commonly should be between 5 °C and 35 °C (40–95 °F); hot-weather limits are often 32 °C (90 °F) or lower.

Free fall of concrete should generally not exceed 1.0–1.5 m (3–5 ft); use drop pipes or chutes to prevent segregation.

Place concrete in horizontal layers of limited thickness (commonly 300–600 mm) and consolidate each layer before placing the next.

Internal vibrators should penetrate the previous layer about 100–150 mm (4–6 in) to knit layers together; avoid over-vibration, which causes segregation.

Vibration is complete when the surface becomes relatively smooth, air bubbles diminish, and the concrete begins to level; typically 5–15 seconds per insertion point.

Vibrator spacing should be about 1.5 times the radius of action (commonly 450–600 mm); withdraw slowly to allow the hole to close.

Never use vibration to move concrete laterally; use hand shovels or chutes to reposition, since flowing with a vibrator causes segregation.

Placement in cold weather requires protection from freezing until the concrete attains sufficient strength (commonly 3.5 MPa or more) and maintaining temperature above 5 °C.

Pumping requires a well-graded, cohesive mix; excessive coarse aggregate or low fines causes blockages, so pump mixes often have higher sand content and adequate paste.

Do not finish concrete while bleed water is present on the surface; finishing too early causes a weak, dusty surface layer.

Bull floating or darbying should follow initial screeding; troweling should begin only after the surface has stiffened and bleed water has disappeared.

Curing must begin as soon as the concrete is finished and kept continuously moist; the curing period commonly is 7 days for normal concrete and 3 days for high-early-strength.

Curing methods include ponding, wet burlap, plastic sheeting, curing compounds, and formwork retention; curing compounds must be applied to a moist surface and form a continuous film.

Evaporation rate above about 1 kg/m²/h (0.2 lb/ft²/h) requires fogging, windbreaks, or evaporation retarders to prevent plastic shrinkage cracking.

Cold-weather protection: maintain concrete above 5 °C (40 °F) until it reaches about 3.5 MPa strength; use insulating blankets, heaters, or enclosures.

Hot-weather precautions include cooling aggregates and water, using ice, shading, and scheduling placement during cooler hours to limit concrete temperature.

Jointing: control joints should be cut or formed to about 1/4 of slab thickness and spaced so that panels are approximately square; saw cutting should occur as soon as concrete can support the saw without raveling.

Slump test measures consistency; typical tolerances are ±25 mm (1 in) for specified slump up to 100 mm, and ±40 mm (1.5 in) for greater slumps.

Air content by pressure meter or volumetric method; typical tolerance is ±1.5% for air-entrained concrete.

Compressive strength test: standard 150 × 300 mm cylinders (or 100 × 200 mm) cured under standard conditions and tested at 28 days; the average of two cylinders is a test result.

Acceptance criteria (ACI 318): no single strength test below fc' − 3.5 MPa (500 psi), and the average of three consecutive tests ≥ fc'.

Temperature test: measure concrete temperature at placement; commonly must be 10–32 °C (50–90 °F) depending on specification.

Sampling frequency commonly requires one set of strength specimens per 110 m³ (150 yd³) or per day of placement, whichever is more frequent.

Cylinders must be molded in layers (2 layers for 100×200 mm, 3 layers for 150×300 mm), rodded or vibrated per standard, and capped before testing.

Quality control includes verifying batch weights, moisture corrections, yield, and slump/air at the point of discharge; statistical evaluation uses standard deviation to determine required average strength.

ACI 318 is the building code requirements for structural concrete, covering design, materials, and construction requirements; ACI 301 covers specifications for structural concrete.

ACI 304 covers measuring, mixing, transporting, and placing; ACI 305 covers hot-weather concreting; ACI 306 covers cold-weather concreting.

ACI 308 covers curing; ACI 302 covers construction of floor slabs; ACI 117 covers tolerances.

Personal protective equipment for concrete work includes hard hat, safety glasses, gloves, boots, and respiratory protection for silica dust; wet concrete is alkaline and can cause chemical burns.

Silica exposure control: cutting, grinding, and drilling concrete require wet methods or local exhaust ventilation and respiratory protection per OSHA 29 CFR 1926.1153.

Formwork and shoring must be designed for loads including concrete weight, construction loads, and impact; reshoring is required until members can support themselves.

Fall protection is required at elevations of 1.8 m (6 ft) or more in general construction; guardrails, nets, or personal fall arrest systems must be used.

Lockout/tagout applies to mixers, conveyors, and pumps; never reach into a mixer drum or hopper while equipment is energized.