Physical and Mechanical Properties of Civil Materials

NCEES FE Civil spec: Materials, C. Physical and mechanical properties of metals, concrete, aggregates, asphalt, and wood. FE Reference Handbook: Mechanics of Materials and Materials Science/Structure of Matter chapters, material property tables.

Civil engineers design with a handful of materials, and the FE Civil exam expects you to know their typical properties and how those properties change with moisture, temperature and loading direction. You also need the basic physical relationships for specific gravity, absorption and moisture content that tie lab data to design.

What the NCEES spec covers

Section C, "Physical and mechanical properties", covers metals, concrete, aggregates, asphalt and wood: strength, stiffness, ductility versus brittleness, density, thermal expansion, and moisture effects.

Key concepts

Material	Typical E	Unit weight	Notes
Structural steel	200 GPa (29,000 ksi)	about 490 lb/ft³	Ductile; α ≈ 12 × 10⁻⁶/°C
Normal-weight concrete	about 25 to 30 GPa	about 145 to 150 lb/ft³	Strong in compression, weak in tension; α ≈ 10 × 10⁻⁶/°C
Aluminum	about 70 GPa	about 170 lb/ft³	One third the stiffness of steel
Wood (parallel to grain)	about 8 to 14 GPa	about 30 to 45 lb/ft³	Anisotropic; moisture sensitive
Asphalt concrete	varies widely	about 145 lb/ft³	Viscoelastic; stiffer when cold

Concrete modulus is commonly estimated from strength: $E_{c} = 57, 000 f_{c}^{'}$ psi (U.S. units) or $4, 700 f_{c}^{'}$ MPa for normal-weight concrete.
Ductile vs brittle. Steel yields and stretches a lot before fracture. Concrete, cast iron and glass fail with little warning.
Wood is strongest and stiffest parallel to the grain. Below the fiber saturation point (about 30% moisture), drying increases strength and causes shrinkage.
Aggregate moisture states: oven-dry, air-dry, saturated surface-dry (SSD) and wet. Absorption is the moisture content at SSD.
Asphalt stiffness depends strongly on temperature and loading rate.

Core formulas

With A = oven-dry mass, B = SSD mass in air and C = mass submerged in water:

G_{bulk} = \frac{A}{B - C} G_{SSD} = \frac{B}{B - C} G_{apparent} = \frac{A}{A - C} Absorption = 100 \frac{B - A}{A}

M C = 100 \frac{m _{wet} - m _{dry}}{m _{dry}} (dry basis)

Where it is in the FE Reference Handbook

Typical properties of common engineering materials (modulus, Poisson's ratio, thermal expansion, density) are tabulated in the Mechanics of Materials and Materials Science/Structure of Matter chapters. Concrete strength charts are in Materials Science. Aggregate specific gravity definitions may not be written out, so know them from practice.

Worked example

A coarse aggregate sample weighs 1,960 g oven-dry, 2,000 g in the SSD condition, and 1,250 g submerged in water. Find the bulk (oven-dry), SSD and apparent specific gravities, and the absorption.

G_{bulk} = \frac{1 , 960}{2 , 000 - 1 , 250} = \frac{1 , 960}{750} = 2.613

G_{SSD} = \frac{2 , 000}{750} = 2.667 G_{apparent} = \frac{1 , 960}{1 , 960 - 1 , 250} = \frac{1 , 960}{710} = 2.761

Absorption = 100 \frac{2 , 000 - 1 , 960}{1 , 960} = 2.04%

The ordering apparent > SSD > bulk always holds, because apparent specific gravity ignores the permeable pores and bulk specific gravity counts them as part of the volume. Use the SSD value for concrete absolute volume calculations.

Common mistakes

Computing moisture content on a wet basis when the convention is dry basis.
Using the apparent specific gravity in mix design, which overstates solid density.
Assuming concrete has useful tensile strength. Its tensile strength is only about 10% of its compressive strength.
Forgetting that wood properties perpendicular to the grain are much lower than parallel.
Mixing GPa and ksi when comparing moduli.

See the mix design lesson for how these values feed concrete proportioning.

Practice Problems

Try these from the free PolyCalc FE Civil problem bank. Each one has a full worked solution.

What is the approximate elastic modulus of structural steel? See the worked solution or practice the Construction Materials deck
A brittle material generally experiences which behavior before fracture? See the worked solution or practice the Construction Materials deck
A piece of wood has a density of 600 kg/m³. Taking the density of water as 1000 kg/m³, its specific gravity is most nearly: See the worked solution or practice the Construction Materials deck
A wood specimen weighs 12.4 kg before oven drying and 10.0 kg after drying. The moisture content (dry basis) is most nearly: See the worked solution or practice the Construction Materials deck
Below the fiber saturation point (about 30% moisture content), as the moisture content of clear wood decreases, its strength generally: See the worked solution or practice the Construction Materials deck
For clear wood, stiffness is generally greatest in which direction? See the worked solution or practice the Construction Materials deck

Related Calculators and Examples

Frequently Asked Questions

What is the modulus of elasticity of structural steel?

About 200 GPa or 29,000 ksi. It is nearly the same for all structural steel grades, even though their yield strengths differ.

What is the difference between bulk and apparent specific gravity?

Bulk specific gravity includes the permeable pore volume in the particle volume, while apparent specific gravity excludes it. Apparent is therefore always the larger value.

How does moisture affect wood strength?

Below the fiber saturation point, about 30 percent moisture content, wood gets stronger and stiffer as it dries. Above that point, extra water sits in the cell cavities and has little effect on strength.

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