The world of building materials is complex, especially for architects just starting out in their careers. If you’re trying to get a handle on some new products, you might be running up against some terms and properties that you haven’t dealt with before.
This list of material properties, building off one compiled by The Constructor, is a cheat sheet for many physical concepts that you might not be used to thinking about. As you search for materials on Architizer, you may run into these terms when talking to manufacturers, and it’ll make your life a lot easier if you’re comfortable with what they’re talking about. When you hear mentions of “creep,” “specific heat” or even “hygroscopicity”(!) in relation to building materials, referencing this list will help you make well-informed decisions for your projects.
Mechanical properties describe how a material reacts to an applied force. These properties change depending on temperature, the form a material is taking and how the force is applied. For example, wood has higher impact strength when the impact is applied parallel to the grain. Many of these properties are most significant in structural members.
The strength of a material essentially is its ability to not break under stress. Breaking, or ‘failure,’ can be sudden and catastrophic, like glass shattering, or it can be gradual like a wooden roof beam sagging under snow. Loads may be compressive like the pressure pushing down on a column; they may be tensile like the weight of a hanging lamp on a wire; or they may be bending like the weight of a cantilever on a beam. This might be Structures 101, but a material may be very strong in one aspect and very weak in another. Spider silk has incredible tensile strength but virtually no compressive or bending strength.
Hardness is determined by how easily a material can be scratched. Harder materials will scratch a softer material without being damaged; this is essentially why we can drive hard nails through soft wood but not metal. Hardness can be important in choosing between different varieties of concrete or other aggregates. Hardness is measured according to the Mohs scale.
Mohs scale; image via the National Park Service
Elasticity generally is the ability of a material to regain its shape after a stress has been applied to it, but it’s also a measurement of how much a material will temporarily deform relative to how much stress is applied. Concrete doesn’t change much, and it’s inelastic. Steel, relatively, changes a lot and is more elastic.
Plasticity is elasticity’s cousin. Whereas elasticity measures how much a material will temporarily deform, plasticity measures how much force it takes to make a material permanently deform without breaking. A plastic material will bend and stay in that shape, whereas an elastic material will bend and go back to its original shape. Again, something to pay attention to in structural members.
Brittle materials will not bend at all and will just break when a force is applied. Glass is very brittle, and different cladding materials, like ceramic tile or fiber cement, may be brittle. More brittle materials aren’t necessarily worse than less brittle materials as long as the material is strong enough to withstand typical forces.
Bike handlebar fatigue testing, via MaterialsScience2000
Fatigue is the feeling you get while studying for your AREs. Just kidding! Architecture humor … A material that fails after repeated loads gets fatigue. A column that can withstand 100 earthquakes but will snap on the 101st is showing fatigue.
How much a material can withstand an impact. Important for anything that needs to withstand a blast or may have significant environmental threats like broken tree limbs falling.
Abrasion is caused by two materials rubbing up against each other. This is important on pavers and anything that will see heavy traffic.
Creep is more or less what it sounds like. It’s the slow movement of a material over time. Foundations can creep if they are in an unstable place. TLC can creep, given the opportunity.
Mass per volume of the material in its natural state including any pores or voids. If you think of terra-cotta brick, which naturally has air pockets, the bulk density tells you how much a stack of bricks will weigh, as opposed to how much a mound of solid terra-cotta with no bubbles would weigh.
Pure density is a measure of the mass of a solid block of a material as though it included no air pockets. For nonporous materials, the bulk density and pure density are the same.
This is more of a physics term, but if you see it somewhere, it’s a way to measure density. A higher number means a denser material (steel’s specific gravity is 7.82, and aluminum’s is 2.72). It’s technically measured by comparing the density of the substance to the density of water at 4 degrees Celsius.
Porosity is a measurement of the ratio of the volume of the pores to the volume of the solids in a material. A brick with a lot of holes will have a higher porosity, as will coarse sand that doesn’t tightly pack.
Water absorption is the ability of a material to absorb and retain water, as opposed to …
… Water permeability, which is the ability for water to pass through a material, which is different than …
… Hygroscopicity is the ability of a material to absorb water from the air.
Coefficient of Softening
Continuing the water theme, the coefficient of softening is “the ratio of compressive strength of a saturated material to its compressive strength in dry state.” This is particularly important for foundations or any material with a high water-absorption potential.
Fire resistance testing; image via ACImalta
In order to be fire-resistant, materials have to be able to keep their strength and shape in flames and also water so they won’t collapse when they get sprayed with a fire hose.
Freezing can tear apart boulders, to say nothing of vulnerable building components. Frost-resistant materials are dry and dense.
Exterior materials have to be able to resist the corrosive effects of wind, rain and whatever other environmental force may wear at a building’s outside.
Spalling is, according to The Constructor, “the ability of a material to undergo a certain number of cycles of sharp temperature variations without failing.”
Unlike spalling resistance, which is the ability to withstand repeated temperature fluctuations, refractoriness is the ability to withstand prolonged exposure to temperatures over 1,580 degrees Celsius.
Chemical and Thermal Properties
Salt corrosion on aluminum structural elements; image via ATMG
Chemical and Corrosion Resistance
Rust is the most common kind of corrosion, but structures near the ocean are also subject to corrosion from salt spray that can blow relatively far inland.
Specific heat is how much energy it takes to raise a specific mass of a substance (1 Newton, if you’re into the physics) 1 degree Celsius. This sounds similar to …
… Thermal capacity, which is how much energy it takes to change the temperature of a variable amount of a substance. So, to clarify, all brick has the same specific heat, but 1 ton of brick has a higher thermal capacity than one pound of brick. The thermal capacity of a product is important in determining how much the temperature of a building will fluctuate.
Thermal Conductivity and Resistivity
These two properties are the opposite of each other. Highly conductive materials have low resistance and vice versa. Conductivity is represented by the letter U and resistance by the letter R. These properties are important for windows, insulation and anything else that could potentially transmit heat between inside and out.