Engineer Concrete flashcards
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32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Water/cementitious ratio
- Mass of water divided by mass of cementitious materials in a mix; primary control on strength, permeability, and durability. Lower values give higher strength and lower permeability but reduce workability.
- Absolute volume method
- Mix proportioning method that sums the absolute volumes of cement, water, air, and aggregates to 1 m³, ensuring the mix yields the required volume without excess paste.
- Air entrainment
- Intentional introduction of microscopic air bubbles using admixtures to improve freeze–thaw resistance. Typical content is 4.5–7.5% by volume, reducing strength by about 4–6% per 1% air.
- Alkali–silica reaction
- Chemical reaction between reactive silica in aggregate and alkali hydroxides in pore solution, forming expansive gel that cracks concrete. Mitigated by low-alkali cement, fly ash, slag, or lithium nitrate.
- Slump test
- ASTM C143 test measuring concrete consistency by the vertical settlement of a standard cone. It indicates workability but does not measure stability, placeability, or segregation resistance.
- Segregation
- Separation of coarse aggregate from mortar in fresh concrete, caused by high slump, excessive vibration, or poor grading. Leads to honeycomb and weak zones in hardened concrete.
- Bleeding
- Upward migration of water to the concrete surface due to settlement of solids. Worsened by high w/c, low fines, and overvibration; can cause laitance, weak surface, and plastic shrinkage cracking.
- Internal vibration
- Consolidation method using a poker vibrator inserted at 450–600 mm spacing, penetrating 100–150 mm into the previous lift, and withdrawn slowly to remove entrapped air without segregation.
- Maturity index
- Temperature–time function M = Σ(T − T0)·Δt used to estimate in-place strength. Requires mix-specific calibration relating maturity to compressive strength.
- Plastic shrinkage cracking
- Cracking that occurs while concrete is still plastic, when surface evaporation exceeds bleeding water. Mitigated by fog spray, windbreaks, evaporation retarders, and prompt curing.
- Saw-cut joint timing
- Contraction joints must be sawed when concrete can support the saw without raveling, typically 4–12 hours after finishing. Depth is one-quarter of slab thickness, minimum 25 mm.
- Thermal cracking
- Cracking caused by temperature gradients in thick concrete sections during hydration. Limit differential temperature to about 20°C and use low-heat cement or cooling to control.
- Modulus of rupture
- Flexural tensile strength of concrete, approximated as fr = 0.62 √f'c in MPa for normal-weight concrete. Used in slab design and crack control.
- Rebound hammer test
- Non-destructive test measuring surface hardness to estimate compressive strength. Accuracy is about ±25% and requires correlation with cores; affected by surface carbonation and moisture.
- Core testing
- ASTM C42 method extracting cylindrical cores to verify in-place strength. Core diameter should be at least 3 times maximum aggregate size; strength is corrected for length/diameter ratio and moisture.
- Rapid chloride permeability
- ASTM C1202 test measuring electrical charge passed through a concrete specimen. Ratings: <1000 coulombs very low, 1000–2000 low, 2000–4000 moderate permeability.
- Tension-controlled section
- Reinforced concrete section where tensile strain in steel at nominal strength is at least 0.005, allowing a strength reduction factor φ = 0.90 and ductile failure mode.
- Balanced reinforcement ratio
- Steel ratio at which concrete crushing and steel yielding occur simultaneously. Design ratios are kept below about 0.75 of balanced to ensure ductility.
- Development length
- Embedment length required to develop the yield strength of a reinforcing bar through bond. Depends on bar diameter, concrete strength, cover, and confinement.
- Shear strength of concrete
- Nominal shear resistance Vc = 0.17 λ √f'c bw d in SI units for members without axial load. Stirrups provide additional shear resistance Vs = Av fy d / s.
- Effective prestress
- Prestress force remaining after losses from elastic shortening, creep, shrinkage, relaxation, friction, and anchorage set. Typically 70–85% of initial prestress.
- Pre-tensioning
- Method where strands are stressed before concrete is cast and bonded to it; prestress transfers to concrete at release when it reaches 70–80% of specified strength.
- Post-tensioning friction loss
- Loss of prestress due to friction between tendon and duct, computed as ΔfpF = fpi(μα + KL), where μ is curvature coefficient and K is wobble coefficient.
- Punching shear
- Shear failure around a concentrated load or column in a flat slab, forming a truncated cone. Resisted by slab thickness, shear reinforcement, or stud rails.
- Chloride threshold
- Chloride content at which steel passivity breaks down, about 0.4% by mass of cement. Critical for marine and deicing exposure; control with low w/c and adequate cover.
- Carbonation
- Reaction of CO2 with calcium hydroxide that lowers concrete pH below about 9, depassivating steel. Depth grows with the square root of time; controlled by low permeability and cover.
- Freeze–thaw resistance
- Ability of concrete to withstand repeated freezing and thawing, provided by air entrainment with spacing factor ≤0.20 mm and low w/c. Essential for exterior exposure in cold climates.
- Sulfate attack
- Expansion and cracking caused by sulfate ions reacting with cement hydrates to form ettringite and gypsum. Mitigated by sulfate-resisting cement, low w/c, and supplementary cementitious materials.
- Half-cell potential
- ASTM C876 test measuring corrosion risk of reinforcement. Values more negative than −350 mV versus CSE indicate high probability of active corrosion; used to map anodic regions.
- Epoxy injection
- Repair method injecting epoxy resin into structural cracks wider than about 0.3 mm to restore tensile strength and prevent water ingress. Requires clean crack surfaces and proper injection ports.
- FRP strengthening
- Rehabilitation technique bonding fiber-reinforced polymer sheets or plates to concrete for flexural or shear capacity. Requires proper surface preparation and anchorage to prevent debonding.
- Acceptance criteria for strength
- ACI 318 criteria: average of three consecutive tests ≥ f'c, and no single test below f'c − 3.5 MPa. If criteria fail, cores are extracted and tested to verify in-place strength.