11.5 Concrete Materials & Mix Design
Key Takeaways
- Concrete is portland cement, water, fine aggregate, and coarse aggregate, with optional admixtures and supplementary cementitious materials
- The water-cement ratio is the primary determinant of compressive strength — adding water at the truck to improve workability lowers strength
- ASTM C150 cement types run I (general), II (moderate sulfate resistance), III (high early strength), IV (low heat), and V (high sulfate resistance)
- Air entrainment protects against freeze-thaw damage and improves workability, but every percent of entrained air costs roughly 5% of compressive strength
- Type V cement is specified where sulfate-bearing soils and groundwater attack concrete, a condition found in parts of Arizona
The Heaviest Subject on the Exam
Quick Answer: Concrete is 14 of 100 items on the AZ ROC B-1/B-2/KB-1/KB-2 outline and 17 of 100 on the residential outline — the largest single area on both. Mix design questions concentrate on the water-cement ratio, cement types, admixtures, and aggregate.
Concrete is four ingredients doing four different jobs:
| Ingredient | Typical volume | Job |
|---|---|---|
| Coarse aggregate | 40–45% | Bulk, stability, dimensional stability, cost |
| Fine aggregate (sand) | 25–30% | Fills voids between coarse particles; workability |
| Portland cement | 7–15% | Binder — reacts with water to form paste |
| Water | 14–21% | Hydrates the cement and provides workability |
Air, entrained or entrapped, occupies the remainder. Everything else — admixtures, fly ash, slag, fibers, pigments — modifies that base.
The Water-Cement Ratio
This is the concept the exam returns to more than any other. The water-cement ratio (w/c) is the weight of mixing water divided by the weight of cementitious material. Lower ratio, stronger and more durable concrete.
| w/c ratio | Rough strength character | Typical use |
|---|---|---|
| 0.40 | High strength, low permeability | Structural, water-resistant, sulfate exposure |
| 0.45 | Strong, durable | Most structural work |
| 0.50 | Moderate | Standard slabs and footings |
| 0.60+ | Weaker, more permeable | Non-structural fill only |
The chain of consequences the exam wants you to trace:
More water → higher w/c → more capillary voids in the hardened paste → lower compressive strength, higher permeability, more drying shrinkage, more cracking.
That is why a mix must never be "watered up" at the truck to make it easier to place. Adding roughly one gallon of water per cubic yard raises the slump about an inch and can drop compressive strength by several hundred psi, while also increasing shrinkage cracking. The correct tool for workability is a water-reducing admixture or a superplasticizer, which increases slump without adding water.
Worked example. A mix contains 564 lb of cement per cubic yard (six sacks) and 254 lb of water. The w/c ratio is 254 ÷ 564 = 0.45. Add 8 gallons of water per yard (about 67 lb) and the ratio becomes 321 ÷ 564 = 0.57 — a mix specified for structural work has been converted into something closer to non-structural fill, with no change on the delivery ticket.
Cement Types (ASTM C150)
| Type | Name | Use |
|---|---|---|
| I | Normal | General-purpose; most work |
| II | Moderate sulfate resistance / moderate heat | Structures in moderate sulfate soils; larger placements |
| III | High early strength | Cold-weather work, fast form turnover, early post-tensioning |
| IV | Low heat of hydration | Mass concrete such as dams; rarely produced today |
| V | High sulfate resistance | Severe sulfate exposure in soil or groundwater |
Types II and V matter in Arizona. Sulfate-bearing soils and groundwater occur across parts of the state, and sulfate attack expands and disintegrates ordinary concrete over years. Where the geotechnical report flags sulfate exposure, the fix is a combination of Type II or Type V cement, a low water-cement ratio, and sometimes supplementary cementitious materials. Reading the geotech report is the general contractor's job, not the ready-mix supplier's.
Supplementary cementitious materials — fly ash, slag cement, and silica fume — replace part of the portland cement. They reduce heat of hydration, improve long-term strength and sulfate resistance, and lower cost and embodied carbon, at the price of slower early strength gain.
Admixtures
| Admixture | Effect | Typical use |
|---|---|---|
| Air-entraining | Creates microscopic air bubbles | Freeze-thaw exposure; improves workability |
| Water-reducing (plasticizer) | Same slump with less water, or more slump at the same w/c | Nearly universal |
| High-range water-reducer (superplasticizer) | Large slump increase without added water | Congested reinforcement, pumped mixes, high-strength work |
| Accelerator | Faster set and early strength | Cold weather, fast turnaround (non-chloride near reinforcement) |
| Retarder | Slower set | Hot weather and long hauls — the Arizona workhorse |
| Corrosion inhibitor | Protects embedded steel | Marine, deicing salt, aggressive soils |
Air entrainment deserves its own note. It is the standard defense against freeze-thaw damage, which matters at Flagstaff and Show Low elevations even though it is irrelevant in Yuma. Typical target contents run about 4½ to 7½ percent depending on exposure and aggregate size. The trade-off is quantitative: each 1% of entrained air costs roughly 5% of compressive strength, so an over-aerated mix fails cylinders. Air-entrained concrete should also never be steel-troweled to a hard finish on exterior flatwork — troweling drives the air out of the surface and causes delamination.
Aggregate and Hot-Weather Mix Practice
Aggregate must be clean, sound, well-graded, and free of organics and clay coatings. Good gradation — a spread of particle sizes — minimizes voids, which minimizes the paste required, which lowers cost and shrinkage. The nominal maximum aggregate size is limited by geometry: generally not more than one-third the slab depth, one-fifth the narrowest form dimension, or three-quarters of the clear spacing between reinforcing bars.
Hot-weather concreting is the Arizona default from roughly May through September. ACI treats it as a distinct condition because high ambient temperature, low humidity, direct sun, and wind combine to accelerate hydration and evaporation:
| Problem | Consequence | Control |
|---|---|---|
| Rapid slump loss | Placement and finishing difficulty | Retarder; shorter haul times; schedule early morning or night pours |
| High concrete temperature | Lower ultimate strength; faster set | Chilled mixing water, ice substitution, shaded aggregate stockpiles |
| High evaporation rate | Plastic shrinkage cracking | Fog spray, evaporation retarder, windbreaks, immediate curing |
| Hot subgrade and forms | Water loss from the bottom of the placement | Pre-wet subgrade and forms before placing |
The evaporation rate — driven by concrete temperature, air temperature, relative humidity, and wind speed — is the number to watch. When it approaches roughly 0.2 lb/ft²/hr, plastic shrinkage cracking becomes likely and protective measures are mandatory rather than optional.
Exam tip: hot-weather questions usually turn on when to act. The controls belong before and during the pour, not after the cracks appear.
A ready-mix truck arrives with a 4-inch slump on a mix specified at w/c 0.45. The crew wants an easier placement. What is the correct action?
A geotechnical report identifies severe sulfate exposure in the soil and groundwater. Which cement type is specified?
A mix is over-aerated at 9% air against a 5% target. What is the most direct consequence?
A 6-inch slab is being placed in July in Phoenix with low humidity and steady wind. Which control most directly prevents plastic shrinkage cracking?