CEE 311L PC Concrete

What are the four main ingredients of concrete?

Portland cement, water, fine aggregate, and coarse aggregate.

What is paste?

Portland cement + water.

What is mortar?

Portland cement + water + fine aggregate.

What is concrete?

Portland cement + water + fine aggregate + coarse aggregate.

What factors affect concrete quality?

Chemical composition, aggregate, water, admixtures, proportions, mixing, transporting, hydration, placing, vibrating, and curing.

List the order of operations for concrete production

Mix design, trial mixes/testing, batching, mixing, transporting, placing, vibrating, finishing, curing, maintenance.

What are the goals of mix design?

Economy, practicality, use of available materials, workability, durability, strength, and appearance.

What are the three common concrete mix design methods?

Arbitrary volume method, weight method, and absolute volume method.

Which concrete mix design method is most accurate?

Absolute volume method.

Which concrete mix design method is easiest?

Weight method.

What is the 1:2:3 concrete mix method?

1 part cement, 2 parts sand, 3 parts coarse aggregate.

What is the first step in concrete mix design?

Determine strength requirements.

What is f'c?

Specified compressive strength used by the design engineer.

What is f'cr?

Required average compressive strength used for mix design.

What equation is used when standard deviation is less than 500 psi?

f'cr = f'c + 1.34s

What equation is used when standard deviation exceeds 500 psi?

f'cr = f'c + 2.33s - 500

What does s represent in strength calculations?

Standard deviation of compressive strength results.

Why is required average strength greater than specified strength?

To ensure most concrete exceeds the specified strength.

Approximately what percentage of concrete is expected to exceed the specified strength using f'cr = f'c + 1.34s?

About 90%.

What is Step 2 in concrete mix design?

Determine the water-cement ratio.

What is the water-cement ratio (w/c)?

Weight of water divided by weight of cement.

How does increasing w/c ratio affect strength?

Strength decreases.

How does increasing w/c ratio affect permeability?

Permeability increases.

Which w/c ratio should be used if multiple exposure conditions apply?

The lowest applicable w/c ratio.

What exposure condition generally requires the lowest w/c ratio?

Reinforced concrete exposed to chlorides.

What is Step 3 in mix design?

Determine coarse aggregate requirements.

Why are larger aggregates desirable?

They improve workability and reduce cement and water demand.

What aggregate shape improves workability?

Round, smooth particles.

Where is maximum aggregate size specifically used in mix design?

Step 3: Coarse aggregate requirements.

What size is maximum aggregate usually compared to nominal maximum aggregate size?

One sieve size larger.

How does increasing coarse aggregate volume affect slump?

It reduces slump.

How does decreasing coarse aggregate volume affect slump?

It increases slump.

What is Step 4 in mix design?

Determine air-entrainment requirements.

Why is air entrainment used?

To improve freeze-thaw durability.

What is Step 5 in mix design?

Determine workability requirements.

What is slump?

A measure of ease of placing, consolidating, and finishing concrete.

What causes segregation?

Coarse aggregate settles while water rises.

How can slump be increased?

Using admixtures or rounded aggregates.

What is Step 6 in mix design?

Determine water content.

What factors affect water demand?

Aggregate size, shape, and desired slump.

Why should water not be added at the jobsite?

It changes the w/c ratio and reduces quality.

What moisture condition is assumed when determining water content?

Saturated surface dry (SSD).

What is Step 7 in mix design?

Determine cement content.

How is cement content calculated?

Cement weight = Water weight ÷ w/c ratio.

What minimum cement content is required for severe exposure?

564 lb/yd³ (334 kg/m³).

What minimum cement content is required for underwater placement?

650 lb/yd³ (385 kg/m³).

What is Step 8 in mix design?

Determine admixture requirements.

What is an admixture?

A material added to concrete to modify its properties.

What is Step 9 in mix design?

Determine fine aggregate requirements.

How is fine aggregate volume determined?

By subtracting water, cement, air, and coarse aggregate volumes from the total volume.

What is Step 10 in mix design?

Apply moisture corrections.

Why are moisture corrections necessary?

Aggregates may contain moisture different from SSD condition.

What happens to aggregate weight when moisture is present?

It increases.

How is free moisture handled in calculations?

It is subtracted from batch water requirements.

What is Step 11 in mix design?

Prepare and evaluate trial mixes.

What properties are checked during trial mixes?

Air content, slump, and compressive strength.

What is batching?

Measuring and combining materials in correct proportions.

Why is batching by weight preferred?

It is more accurate than batching by volume.

What is the purpose of mixing concrete?

To produce a uniform mixture.

What percentage of water is commonly added first during mixing?

About 10%.

What is ready-mixed concrete?

Concrete completely mixed at a central plant and delivered by truck.

What is shrink-mixed concrete?

Partially mixed at the plant and completed in the truck.

What is truck-mixed concrete?

Concrete mixed entirely in the mixer truck.

What is the maximum typical time from start of mixing to discharge?

90 minutes.

What tests are commonly performed on fresh concrete?

Slump and air-content tests.

What test measures workability?

Slump test.

How is a slump test performed?

Fill cone in three layers, rod each layer 25 times, lift cone, and measure slump.

What does a high slump indicate?

Greater workability.

What test measures total air content?

Air-content test.

What types of air are measured by the air-content test?

Entrained and entrapped air.

What are common methods of air-content testing?

Pressure, volumetric, gravimetric, and Chase Air Indicator methods.

What are standard concrete test cylinder dimensions?

6 in. diameter × 12 in. height.

How are strength cylinders prepared?

Three layers with 25 rods per layer.

How long are cylinders cured onsite initially?

24 hours.

List common methods of placing concrete

Chute, conveyor, wheelbarrow, bucket, and pump.

What is the maximum recommended drop height during concrete placement?

Less than 3 feet.

Why should excessive horizontal movement be avoided?

To prevent segregation.

What is vibration used for?

Concrete consolidation and compaction.

When must vibration be completed?

Before initial set.

What is the most common type of concrete vibrator?

Internal (poker) vibrator.

What is a typical vibration duration at one location?

Less than 10 seconds.

What is finishing?

Smoothing and texturing the concrete surface.

When must finishing be completed?

Before final set.

What is screeding?

Striking off concrete to the required elevation.

What is bullfloating?

Removing high and low spots and embedding large aggregate particles.

What is curing?

Maintaining moisture and temperature to promote hydration and strength gain.

Why is curing important?

It improves durability, strength, watertightness, and freeze-thaw resistance.

What are the three basic curing approaches?

Maintain water, seal the surface, or apply heat and moisture.

What is ponding?

A curing method where water is retained on the concrete surface.

What are examples of wet coverings?

Burlap, cotton, and rugs.

How can the surface be sealed during curing?

Plastic sheets, impervious paper, membranes, or forms left in place.

When is steam curing commonly used?

For precast concrete and cold-weather construction.

When should curing begin?

Immediately after final set.

What is the minimum typical curing period?

7 days.

What is plastic shrinkage?

Shrinkage of fresh concrete due to evaporation before setting.

What is drying shrinkage?

Shrinkage after setting caused by moisture loss.

What is creep?

Long-term deformation under sustained load.

How does w/c ratio affect permeability?

Higher w/c ratios greatly increase permeability.

Why is high permeability undesirable?

It allows water and chemicals to penetrate concrete.

How does increasing w/c ratio affect strength and stiffness?

Both decrease.