Chapter 10: Concrete Construction

Comprehensive practice flashcards covering material properties, reinforcement methods, casting techniques, structural framing systems, fire performance, and failure mechanisms from Chapter 10: Concrete Construction.

What basic ingredients compose concrete, and what purpose do the aggregates serve?

Concrete is a composite material consisting of portland cement, large aggregate (typically gravel), fine aggregate (typically silica sand), water, and admixtures. Aggregates are inert mineral ingredients that make up a large percentage of total volume and reduce the amount of cement needed.

What specific functions do shale or clinker, superplasticizers, coloring, and fly ash serve as concrete admixtures?

Shale or clinker reduces concrete density; superplasticizers produce a mix that flows more freely (workable with little water); coloring provides aesthetic or safety identification (such as marking buried electrical cables); and fly ash increases workability, decreases water demand, reduces heat of hydration, and improves overall performance.

What are the common specialized types of concrete identified in construction?

Specialized types include ordinary concrete, structural lightweight concrete, insulating lightweight concrete, gypsum concrete, high early-strength concrete, expansive concrete, and water-permeable concrete.

What are the three primary advantages of concrete in building construction?

The three primary advantages are that concrete is fundamentally noncombustible, performs well under fire conditions, and provides good insulating properties.

How does concrete behave when subjected to compressive forces compared to tensile forces?

Concrete is significantly stronger when compressed but weaker when tensioned. Because it is weak in tension, it cannot be used alone where tensile forces occur in a structure unless reinforcement is added.

What materials are embedded in concrete before it hardens to create reinforced concrete?

Reinforcing materials include steel reinforcing bars (rebar), wire mesh, fiber-reinforced plastics, or a combination of these components to resist tensile forces.

What are the three sequential states of concrete from placement to final use?

The three states in order are: 1. Plastic state (wet and poured into forms), 2. Curing state (begins to dry and starts developing strength and durability), and 3. Hardened state (has developed the strength and durability for its intended purpose).

In ordinary reinforcing, what three specifications must the design engineer designate?

The design engineer must specify the number of reinforcing bars to be used, their size (diameter), and the depth of concrete covering the bars.

How is void formation prevented during concrete placement, and how does cured concrete bond to rebar?

Mechanical vibrators are used during placement to ensure wet concrete fills all spaces within the formwork and eliminates cavities. When cured, concrete bonds to the rebar because of textural shaping on the surface of the steel bars.

What are the standard diameter measurements for #3, #5, and #18 rebar?

#3 rebar has a diameter of $3/8\,\text{inches}\ (9.525\,\text{mm})$, #5 rebar has a diameter of $5/8\,\text{inches}\ (15.875\,\text{mm})$, and #18 rebar has a diameter of $2\frac{1}{4}\,\text{inches}\ (57.3\,\text{mm})$.

What is the structural advantage of casting concrete beams in the shape of a tee?

The wider top of the tee permits the concrete to support a greater compressive load, while eliminating concrete below the top reduces dead load. Reinforcing steel is placed in the bottom of the tee to resist tensile forces.

Why is lateral reinforcement used around vertical steel bars in concrete columns?

To prevent vertical reinforcing bars from buckling under compressive forces. Lateral reinforcement (stirrups, ties, and corner bars) resists diagonal tension, supports compressive loads, and resists bending forces caused by wind and settling.

What is the principle behind prestressing concrete, and what limitation of ordinary reinforcing does it solve?

Prestressing tightens (preloads) reinforcing steel to induce compressive stresses in the surrounding concrete that counteract tensile stresses from applied loads, ensuring concrete in tension zones does not remain inactive.

Why is cutting posttensioned reinforcing steel with a torch or saw extremely dangerous during firefighting?

Because posttensioned steel strands are not bonded to the concrete and are stretched under high tension like giant rubber bands. When cut, they can spring out violently, injuring responders and triggering total or partial collapse.

What is 'relaxation' in prestressed concrete and how is it compensated for?

Relaxation is the slow stretching of steel over time (accompanied by concrete shortening), which reduces compressive force and load capacity. It is compensated for by applying slightly higher initial prestressing forces than required for the design loads.

What is the key procedural difference between pretensioning and posttensioning concrete?

In pretensioning, steel strands are stretched between anchors before concrete is poured and released after initial cure; in posttensioning, steel cables are tensioned using jacks only after the concrete has cured to a specified strength.

Why are prestressed concrete systems more vulnerable to early failure in fires than ordinary reinforced assemblies?

Prestressed systems use high-strength steels that yield at temperatures as low as $752^\circ\text{F}\ (400^\circ\text{C})$, causing earlier failure at equal cover depth. Additionally, welded steel embedment plates concentrate forces and are often left exposed.

What is the fundamental structural advantage of cast-in-place concrete over precast concrete?

Cast-in-place concrete provides inherent continuity at intersecting structural members, resulting in fundamentally greater structural rigidity.

What detrimental outcome occurs if concrete is vibrated excessively during placement?

Excessive vibration causes heavy coarse aggregates to settle to the bottom of the forms, while water and cement rise to the top, creating an uneven and compromised mixture.

Why should sand from an ocean beach never be used in reinforced concrete?

Ocean beach sand introduces salt into the mixture, which over time corrodes and deteriorates the embedded reinforcing steel.

How long does properly cured concrete take to reach its normal design strength?

Normal design strength is reached after $28\,\text{days}$ (approximately one month), although concrete theoretically continues to cure indefinitely at a gradual rate.

What is hydration in concrete, and what thermal byproduct does it generate?

Hydration is the chemical process in which water combines with cement particles to form a microscopic gel that hardens the concrete; during this reaction, it releases heat of hydration.

Why is the water-to-cement ratio the single most important factor determining ultimate concrete strength?

Adding excess water beyond what is required for hydration creates microscopic voids as excess moisture evaporates and weakens the cured matrix, preventing the concrete from reaching its intended design strength.

What is the ideal temperature range for concrete curing, and what happens at or above $100^\circ\text{F}\ (37.7^\circ\text{C})$?

The ideal curing temperature is between $50^\circ\text{F}\text{ and }70^\circ\text{F}\ (10^\circ\text{C}\text{ and }21^\circ\text{C})$. Concrete cured at or above $100^\circ\text{F}\ (37.7^\circ\text{C})$ will not reach its proper strength.

What variables influence the fire resistance ratings of concrete structural systems ($1\text{ to }4\,\text{hours}$)?

Variables include concrete density, concrete thickness, concrete quality, supported load, and depth of concrete cover over reinforcing bars.

What is concrete spalling and what is its primary physical cause?

Spalling is the breakdown and chipping away of concrete fragments from the main body. Its primary cause is the expansion of internal moisture into steam or ice, creating internal tensile forces that break the concrete apart.

What is the heat sink effect in concrete and why is it significant during firefighting overhaul?

The heat sink effect refers to concrete's ability to absorb and retain heat from an exposing fire and release it slowly. Concrete will sustain high temperatures for a significant time if not actively cooled, affecting overhaul safety.

How is structural continuity maintained across successive pours at construction joints?

Reinforcement steel rods overlap the construction joints between successive pours (such as intersections of slabs and columns) to ensure continuity of strength and load transfer.

What structural features distinguish a flat-slab concrete frame, and how does a flat plate differ?

A flat-slab frame has slabs $6\text{ to }12\,\text{inches}\ (150\text{ to }300\,\text{mm})$ thick supported by columns, using drop panels or mushroom capitals to resist shear stress under heavy loads. If loads are light and extra column reinforcement is omitted, it is called a flat plate.

What are the structural dimensions and fire characteristics of slab and beam framing (concrete joist construction)?

Slabs can be as thin as $2\,\text{inches}\ (50\,\text{mm})$ supported by closely spaced concrete beams. Because of slab thinness, these systems are highly susceptible to fire damage and frequently require supplemental fireproofing.

What is two-way slab construction (waffle construction) and when is it preferred?

A framing system featuring concrete beams running in two directions forming a waffle-like pattern. It is preferred for spans longer than $40\,\text{feet}\ (12\,\text{m})$ or heavy loads because it increases strength while eliminating unnecessary dead-weight concrete.

Why is a polystyrene core incorporated into precast concrete sandwich panels?

The polystyrene core is sandwiched between layers of concrete to improve the thermal insulating properties of the structural panel.

What are the four standard precast concrete floor slab shapes and their respective span limits?

The shapes are solid slabs, hollow-core slabs, single tee slabs, and double tee slabs. Solid slabs span up to approximately $30\,\text{feet}\ (9\,\text{m})$, while tee slabs can span up to $120\,\text{feet}\ (35\,\text{m})$.

How is tilt-up construction executed and why is the integrity of the roof assembly critical?

Reinforced wall panels are cast on site in horizontal beds and tilted upright by crane. Roof structural integrity is critical because roof failure removes horizontal bracing, causing the tilt-up walls to fail outward from lateral forces.

What is the structural difference between concentric and eccentric stress in posttensioned tendons?

Tendons positioned through the member's center of gravity produce concentric stress; tendons positioned away from the center of gravity produce eccentric stress, which creates upward deflection (uplift) to counteract applied loads.

Why are structural systems in concrete buildings often difficult for firefighters to visually identify from the exterior?

Non-load-bearing curtain walls (composed of aluminum, glass, steel panels, or masonry) conceal internal frames, and exterior insulation finish systems (EIFS) closely mimic the visual appearance of stucco and concrete.

What is the textbook definition and typical dimension of a flat plate concrete system?

A flat plate is a plain floor slab about $8\,\text{inches}\ (200\,\text{mm})$ thick that rests on columns spaced up to $22\,\text{feet}\ (6.5\,\text{m})$ apart, lacking beams and relying on orthogonal and diagonal rebar patterns for support.