PE Civil (NCEES Principles and Practice of Engineering) — All Questions

For a given set of materials, reducing the water-cement ratio of a concrete mix generally does what to compressive strength?

• a.Has no effect on it
• b.Makes it unpredictable
• c.Increases it✓
• d.Decreases it

By Abrams' law, lower water-cement ratio yields higher compressive strength (down to the point where the mix becomes unworkable). Excess water creates capillary voids that weaken the hardened paste. Admixtures maintain workability at low water-cement ratios.

What is the nominal diameter of a #8 reinforcing bar (U.S. customary sizes)?

• a.0.375 in
• b.0.75 in
• c.0.5 in
• d.1.0 in✓

U.S. rebar bar numbers give the diameter in eighths of an inch: #8 = 8/8 = 1.0 in. Thus a #4 bar is 0.5 in and a #3 bar is 0.375 in. This rule lets you size reinforcement quickly from the bar number.

The slump test on fresh concrete is primarily a measure of what property?

• a.Workability (consistency)✓
• b.Compressive strength
• c.Water-cement ratio
• d.Air content

The slump test measures the consistency and workability of fresh concrete, that is, how easily it flows and can be placed. It does not directly measure strength; separate cylinder tests determine compressive strength, and other tests measure air content.

A concrete batch uses 300 lb of water and 600 lb of cement. What is the water-cement ratio by weight?

• a.0.50✓
• b.0.30
• c.0.60
• d.2.00

Water-cement ratio = weight of water / weight of cement = 300 / 600 = 0.50. The ratio, not the absolute amounts, is the primary control on hardened concrete strength and durability.

For a fixed set of materials, which of these water-cement ratios yields the highest compressive strength?

• a.0.70
• b.0.65
• c.0.40✓
• d.0.55

By Abrams' law, lower water-cement ratio gives higher strength, so 0.40 is strongest here. Excess water leaves capillary voids that weaken the paste; admixtures maintain workability at low ratios.

Air entrainment is added to concrete primarily to:

• a.increase compressive strength
• b.improve resistance to freeze-thaw cycles✓
• c.accelerate setting
• d.reduce workability

Microscopic entrained air voids give freezing water room to expand, greatly improving freeze-thaw durability. Entrained air slightly lowers strength but also improves workability.

A standard 6-in-diameter concrete cylinder fails under a load of 113,100 lb. What is its compressive strength?

• a.3,600 psi
• b.4,000 psi✓
• c.2,000 psi
• d.8,000 psi

Area = pi x (3 in)^2 = 28.27 sq in; strength = load / area = 113,100 / 28.27 = 4,000 psi. Compressive strength is the failure load divided by the cross-sectional area.

In a split-cylinder (Brazilian) test, a 6-in-diameter by 12-in-long cylinder splits at P = 30,000 lb. What is the splitting tensile strength? (T = 2P / (pi L D))

• a.265 psi✓
• b.530 psi
• c.133 psi
• d.177 psi

T = 2P / (pi x L x D) = 2 x 30,000 / (pi x 12 x 6) = 60,000 / 226.2 = 265 psi. The split-cylinder test gives concrete's tensile strength, far below its compressive strength.

For normal-weight concrete with f'c = 4,000 psi, what is the modulus of rupture using fr = 7.5*sqrt(f'c)?

• a.63 psi
• b.948 psi
• c.474 psi✓
• d.7,500 psi

fr = 7.5 x sqrt(4,000) = 7.5 x 63.2 = 474 psi. The modulus of rupture estimates concrete's flexural tensile strength and is used to check cracking.

For normal-weight concrete with f'c = 4,000 psi, what is the modulus of elasticity using Ec = 57,000*sqrt(f'c)?

• a.57,000 psi
• b.7.2 x 10^6 psi
• c.3.6 x 10^6 psi✓
• d.2.28 x 10^5 psi

Ec = 57,000 x sqrt(4,000) = 57,000 x 63.2 = 3.6 x 10^6 psi. This ACI expression relates concrete stiffness to its compressive strength for normal-weight concrete.

The typical unit weight of normal-weight concrete is about:

• a.200 pcf
• b.62.4 pcf
• c.150 pcf✓
• d.100 pcf

Normal-weight concrete weighs about 150 pcf (roughly 145 pcf plain, ~150 pcf reinforced). Lightweight concrete runs 90 to 120 pcf; 62.4 pcf is the unit weight of water.

Proper curing of concrete is important mainly because it:

• a.adds entrained air
• b.increases the water-cement ratio
• c.speeds formwork removal only
• d.retains moisture and temperature for continued cement hydration✓

Curing keeps concrete moist and at a suitable temperature so hydration continues and the concrete develops its designed strength and durability. Premature drying stops hydration and weakens the surface.

The specified compressive strength f'c of concrete is conventionally measured at what age?

• a.7 days
• b.28 days✓
• c.14 days
• d.90 days

Design strength f'c is referenced to a 28-day standard-cured cylinder test, by which age most strength gain of ordinary portland cement concrete has occurred. Early (7-day) tests are used for monitoring.

A fine aggregate has cumulative percentages retained on the standard sieves summing to 270. What is its fineness modulus?

• a.0.27
• b.2.70✓
• c.27.0
• d.3.70

Fineness modulus = (sum of cumulative percent retained on the standard sieves) / 100 = 270 / 100 = 2.70. Sand for concrete typically has an FM between 2.3 and 3.1.

A higher fineness modulus indicates aggregate that is:

• a.coarser✓
• b.of lower specific gravity
• c.more porous
• d.finer

A larger fineness modulus corresponds to a coarser aggregate (more material retained on the larger sieves). Fineness modulus is a single index summarizing overall gradation.

An aggregate sample weighs 510 g at the saturated-surface-dry (SSD) condition and 500 g oven-dry. What is its absorption?

• a.1.96%
• b.2.0%✓
• c.10%
• d.0.5%

Absorption = (SSD weight - oven-dry weight) / oven-dry weight x 100 = (510 - 500) / 500 x 100 = 2.0%. Absorption is referenced to the oven-dry weight and affects mix-water corrections.

An oven-dry aggregate sample of 470.5 g occupies a volume of 180 cubic centimeters. What is its bulk specific gravity? (water = 1.0 g/cc)

• a.2.61✓
• b.2.71
• c.0.38
• d.180

Bulk specific gravity = oven-dry mass / (volume x density of water) = 470.5 / (180 x 1.0) = 2.61. Most natural aggregates have specific gravities around 2.6 to 2.7.

If a stockpiled aggregate carries surface moisture above the SSD condition, the mix should:

• a.increase the batch water added
• b.make no change
• c.reduce the batch water added✓
• d.add more cement

Free surface moisture contributes extra water to the mix, so the batch water is reduced by that amount to keep the effective water-cement ratio on target. Dry aggregate below SSD absorbs water and needs the opposite correction.

Compared with a gap-graded aggregate, a well-graded aggregate generally has:

• a.lower voids and better particle packing✓
• b.a single uniform particle size
• c.higher water demand
• d.a greater volume of voids and therefore a higher cement-paste demand

A well-graded aggregate has a continuous range of sizes so smaller particles fill voids between larger ones, minimizing voids and the paste needed to fill them. This improves economy and workability.

In a sieve analysis of a 1,000-g sample, 150 g is the cumulative amount retained down through the No. 4 sieve. What percent passes the No. 4 sieve?

• a.15%
• b.1.5%
• c.85%✓
• d.150%

Percent passing = 100 - cumulative percent retained = 100 - (150/1,000 x 100) = 100 - 15 = 85%. Gradation curves are plotted as percent passing versus sieve size.

Per ACI limits, the nominal maximum aggregate size should not exceed:

• a.twice the total slab thickness measured through the full depth of the member
• b.three-fourths of the clear spacing between reinforcing bars✓
• c.the concrete cover
• d.any size is acceptable

ACI limits the nominal maximum aggregate to 3/4 of the clear bar spacing (also 1/3 of slab depth and 1/5 of the narrowest form dimension), so concrete flows around reinforcement without blocking.

A soil has D60 = 0.60 mm and D10 = 0.15 mm. What is its coefficient of uniformity, Cu?

• a.0.75
• b.2.0
• c.4.0✓
• d.0.25

Cu = D60 / D10 = 0.60 / 0.15 = 4.0. A larger Cu indicates a wider (better) range of particle sizes; Cu greater than about 4 to 6 is one criterion for well-graded soil.

A soil has D10 = 0.15 mm, D30 = 0.30 mm, and D60 = 0.60 mm. What is its coefficient of curvature, Cc?

• a.0.09
• b.2.0
• c.0.5
• d.1.0✓

Cc = (D30)^2 / (D10 x D60) = 0.30^2 / (0.15 x 0.60) = 0.09 / 0.09 = 1.0. Well-graded soils have Cc between 1 and 3 in addition to a suitable Cu.

In the Superpave binder grade PG 64-22, the two numbers represent:

• a.64 = viscosity, -22 = penetration
• b.both numbers are degrees Fahrenheit
• c.aggregate sizes
• d.64 = maximum pavement temperature (C), -22 = minimum temperature (C)✓

Performance-grade binders are named by the high and low pavement design temperatures in degrees Celsius: PG 64-22 resists rutting up to 64 C and cracking down to -22 C.

The Marshall stability of an asphalt specimen measures:

• a.the percentage of interconnected air voids in the loose uncompacted mix
• b.aggregate gradation
• c.asphalt content
• d.the resistance of a compacted specimen to deformation✓

Marshall stability is the maximum load a compacted asphalt specimen sustains before failure, indicating resistance to deformation; the companion flow value measures the deformation at that load.

The target air-void content for a well-designed dense-graded hot-mix asphalt is about:

• a.about 15%
• b.about 1%
• c.about 4%✓
• d.about 25%

Dense-graded HMA is designed to roughly 4% air voids. Too few voids risk rutting and bleeding; too many allow water and air in, causing stripping and oxidation.

In asphalt mix design, the voids in mineral aggregate (VMA) is:

• a.the total air voids
• b.the intergranular void volume between aggregate particles in the compacted mix✓
• c.the aggregate specific gravity
• d.voids filled with asphalt

VMA is the volume of voids between aggregate particles in the compacted mix, including both air voids and the effective binder. Adequate VMA ensures enough room for a durable binder film.

An asphalt mixture contains 60 g of asphalt binder in a 1,200-g total mix. What is the asphalt content by total weight of mix?

• a.20%
• b.0.6%
• c.5.0%✓
• d.60%

Asphalt content = weight of binder / total mix weight x 100 = 60 / 1,200 x 100 = 5.0%. Dense-graded mixes typically run about 4.5% to 6% binder by total weight.

A compacted asphalt specimen has a bulk specific gravity Gmb = 2.40 and a maximum (Rice) specific gravity Gmm = 2.50. What is the air-void content?

• a.2.4%
• b.0.96%
• c.4.0%✓
• d.10%

Air voids Va = (1 - Gmb/Gmm) x 100 = (1 - 2.40/2.50) x 100 = 4.0%. Air voids compare the compacted (bulk) density to the maximum theoretical (void-free) density.

A tack coat between asphalt layers is applied primarily to:

• a.penetrate and permanently waterproof the underlying soil subgrade before any paving begins
• b.lower the binder grade
• c.increase the aggregate size
• d.bond the new asphalt layer to the existing surface✓

A tack coat is a light asphalt spray that bonds a new lift to an existing pavement or between lifts, preventing slippage and delamination. A prime coat, by contrast, treats a granular base.

Using a stiffer (higher-PG) asphalt binder in a mix generally:

• a.has no effect
• b.causes only low-temperature cracking
• c.increases rutting
• d.reduces rutting at high service temperatures✓

A stiffer binder resists permanent deformation (rutting) at high pavement temperatures. The trade-off is greater risk of low-temperature cracking, which is why binder selection balances both extremes.

The Superpave gyratory compactor is used to:

• a.measure concrete slump
• b.compact specimens simulating field densification under traffic✓
• c.measure asphalt penetration
• d.test steel yield strength

The Superpave gyratory compactor densifies asphalt specimens by combined pressure and gyration to mimic field compaction, producing specimens for volumetric mix-design analysis.

A prime coat differs from a tack coat in that it is applied to:

• a.bond two asphalt lifts
• b.penetrate and bind a granular base before paving✓
• c.increase Marshall flow
• d.serve as the final wearing surface

A prime coat is a low-viscosity asphalt sprayed on an untreated granular base to penetrate, bind fines, and promote adhesion of the first asphalt layer. A tack coat instead bonds asphalt-to-asphalt.

What is the specified minimum yield strength Fy of ASTM A992 structural steel (common for wide-flange shapes)?

• a.36 ksi
• b.29,000 ksi
• c.50 ksi✓
• d.60 ksi

A992 steel has a minimum yield of 50 ksi, now standard for W-shapes. A36 is 36 ksi and Grade 60 rebar is 60 ksi; 29,000 ksi is steel's modulus of elasticity, not a strength.

What is the yield strength of ASTM A615 Grade 60 reinforcing steel?

• a.60 ksi✓
• b.75 ksi
• c.40 ksi
• d.29,000 ksi

Grade 60 rebar has a minimum yield strength of 60 ksi, the value in its grade designation and the standard for most reinforced concrete. Grade 40 and Grade 75 bars also exist.

In the elastic (initial) region of a steel stress-strain curve, Hooke's law states that:

• a.stress is constant
• b.strain is permanent
• c.stress is proportional to strain✓
• d.the material has already yielded

In the elastic region stress is proportional to strain, with the slope equal to the modulus of elasticity, and the deformation is fully recoverable. Yielding and permanent strain begin only beyond the elastic limit.

A steel tensile specimen with a 2.0-in gauge length measures 2.5 in between gauge marks at fracture. What is the percent elongation?

• a.20%
• b.125%
• c.0.5%
• d.25%✓

Percent elongation = (final - original) / original x 100 = (2.5 - 2.0) / 2.0 x 100 = 25%. Percent elongation is a common measure of a metal's ductility.

In the elastic range, a steel specimen shows a stress of 20 ksi at a strain of 0.00069. What is its modulus of elasticity?

• a.0.00069 ksi
• b.29,000 ksi✓
• c.20 ksi
• d.14,500 ksi

E = stress / strain = 20 / 0.00069 = 29,000 ksi. The modulus of elasticity is the slope of the linear elastic portion of the stress-strain curve.

A #8 reinforcing bar (area 0.79 sq in) reaches yield at an axial load of 47.4 kip. What is its yield stress?

• a.75 ksi
• b.60 ksi✓
• c.36 ksi
• d.47.4 ksi

Stress = load / area = 47.4 / 0.79 = 60 ksi, consistent with Grade 60 reinforcement. Yield stress is the axial force at yield divided by the bar's cross-sectional area.

The Poisson's ratio of structural steel is approximately:

• a.1.00
• b.0.30✓
• c.0.50
• d.0.10

Steel's Poisson's ratio is about 0.30, meaning lateral strain is roughly 30% of the axial strain in the elastic range. A value of 0.5 would imply an incompressible material.