Untitled Quiz

Material Inputs for Concrete

• Key inputs include water, air, admixtures, sand, aggregate, fly ash, silica fume, slag, and cement.

Storage of Materials

• Aggregates: Must facilitate free drainage, be clearly identified, and prevent uncontrolled mixing of sizes/types.
• Cement: Should be kept dry to avoid contamination and prevent intermingling of different types.

Mixing Equipment

• Batch mixers ensure uniform distribution of ingredients in concrete.
• They should minimize mixing time, offer variable speed options, and not exceed a rated mixing capacity of 65% of the chamber's volume unless proven otherwise.

Concrete Placement

• Aim to deposit concrete close to its final position quickly to avoid segregation and ensure full compaction.
• At low water-cement (w/c) ratios, water enters hydration rapidly, creating fine capillaries that can lead to cracks due to surface tension.
• Keeping the concrete surface continuously wet helps to avoid autogenous shrinkage, especially when casting new against hardened concrete.

Shrinkage in Concrete

• Autogenous shrinkage and drying shrinkage contribute to total shrinkage, resulting in strain.
• Important to consider shrinkage when calculating theoretical thickness based on cross-sectional area and perimeter.

Thermal Expansion

• Materials contract/expand with temperature changes; cement paste and aggregates have different thermal coefficients.
• The coefficient of concrete's thermal expansion is influenced by that of the mortar and aggregates.

Restraint Shrinkage

• Restraint can lead to tensile stress exceeding concrete's tensile strength, resulting in cracks to relieve tension.
• Different scenarios can lead to differing stress effects (short-term vs long-term).

Curing of Concrete

• Curing keeps concrete moist, enhancing the bond between paste and aggregates; improper curing leads to inadequate hardening.
• Curing starts immediately after finishing the surface.
• Proper curing increases compressive strength by 80-100% over uncured concrete, reduces drying, shrinkage, and cracking.

Methods and Effects of Curing

• Multiple methods exist for curing, including wet hessian mats, flooding, plastic sheeting, and controlled lab environments.
• Curing effectively controls moisture loss during cement hydration, directly correlating to increased concrete strength.

Compaction Effects

• Higher compaction results in fewer voids and increased strength; a 5% void content can lead to up to a 25% reduction in strength.

Compressive Strength

• Characteristic strength (f'c) is determined through standard testing at 28 days, where no more than 5% of results should fall below this assessed value.