What Is Lightweight Concrete?

How light can structural concrete get? FHWA says all-lightweight concrete may reach 0.090 kcf, nearly a 40 percent reduction from a typical normal-weight 0.145 kcf. Sources do not agree on one density line, so this guide keeps each definition with the group that wrote it.

What Counts as Lightweight Concrete?

It depends on who is defining it, because the density ceilings differ. ACI’s free FAQ page says structural low-density (lightweight) concrete is made with low-density aggregate having an air-dried density of not more than 115 lb/ft3 and a 28-day compressive strength of more than 2,500 psi.

AASHTO’s definition, as quoted in the FHWA Lightweight Concrete Bridge Design Primer (FHWA-HIF-19-067, November 2021), is concrete with lightweight aggregate meeting AASHTO M 195 and an equilibrium density not exceeding 0.135 kcf, as determined by ASTM C567. A kcf is 1,000 lb per cubic foot, so 0.135 kcf is 135 lb/ft3 (our arithmetic).

The ACI ceiling (115) and the AASHTO ceiling (135) are not the same number. ACI states an air-dried density and AASHTO an equilibrium density. A third source, CIP 36 from the Concrete in Practice series (a hosted copy that does not name its publisher), also asks for strength above 2,500 psi.

How Much Lighter Is It Than Normal Concrete?

Each source gives its own range, so there is no single figure. CIP 36 says structural lightweight concrete has an in-place density on the order of 90 to 115 lb/ft3, against 140 to 150 lb/ft3 for normalweight concrete.

FHWA says bridge lightweight concrete typically runs 0.110 to 0.125 kcf (110 to 125 lb/ft3, arithmetic), and normal-weight concrete 0.140 to 0.155 kcf. ESCSI, the trade association of the makers of expanded shale, clay and slate aggregate, says structural lightweight concrete is typically 25% to 35% lighter, with strengths comparable to normal weight.

FHWA’s lightest case is all-lightweight concrete at 0.090 kcf against a typical 0.145 kcf. By our arithmetic, (145 – 90) / 145 = 37.9 percent, which matches FHWA’s wording “nearly a 40 percent reduction”. The Concrete Weight Calculator multiplies a volume by a density you choose. Its lightweight preset is its own value of 115 lb per cu ft, while the sources above give ranges.

What Are the Types of Lightweight Concrete?

FHWA names three informal types, defined by which aggregate is lightweight. It says the terms may still be used informally but are not used in the specifications.

• All-lightweight. Both the coarse and the fine aggregate are lightweight. FHWA calls it the lightest type of structural concrete.
• Sand-lightweight. The coarse aggregate is lightweight and the sand is normal weight. FHWA says it is currently the most widely used.
• Specified-density. Any blend of conventional and lightweight aggregates, to reach a specified density.

CIP 36 says the lightweight aggregates are typically expanded shale, clay or slate fired in a rotary kiln to make a porous structure, and that air-cooled blast furnace slag is also used. ESCSI, the aggregate makers’ trade association, describes the same shale, clay and slate material, expanded in a rotary kiln.

CIP 36 also mentions non-structural lighter concretes, such as cellular concrete, that are used for insulation. This guide gives no density for them. For plain concrete versus cement and mortar, see concrete vs cement vs mortar.

Where Is It Used, and How Strong Is It?

Sources say structural lightweight concrete can reach strengths similar to normal concrete. CIP 36 says mixtures can be designed for similar strengths, and ACI and CIP 36 both set a structural floor of more than 2,500 psi.

FHWA says all structural lightweight aggregates can produce design strengths above 5.0 ksi (5,000 psi, arithmetic) and 6.0 ksi is widely available. It gives a practical current maximum of 10 ksi, or 10,000 psi.

CIP 36 says the primary use is reducing the dead load of a structure. It lists bridge decks, piers and beams, slabs and walls in frame buildings, parking structures, tilt-up walls and topping slabs. ESCSI, the aggregate makers’ trade association, lists floors on steel deck, parking structures, precast elements, marine structures, and fill and insulating concrete.

FHWA reports one old example: the San Francisco-Oakland Bay Bridge upper deck of 1936 used lightweight concrete at 0.095 kcf (95 lb/ft3, arithmetic), and FHWA says that deck is still in service.

How Is It Handled Differently?

The main differences are pre-wetting, air entrainment and pumping. This is not a mix design, only what the sources say.

CIP 36 says lightweight aggregates generally need wetting before use, and that absorption is typically 5% to 20% by weight of dry aggregate. It also says virtually all lightweight concrete is air-entrained, and that air content must be controlled so the density requirement is met.

For pumping, both CIP 36 and FHWA say to pre-soak the aggregate, because pump pressure drives water into the pores and can cause slump loss. CIP 36 adds that excess water or slump lets the lightweight aggregate separate from the mortar.

CIP 36 also says design generally relies on equilibrium density, which is approximately 3 to 8 lb/ft3 below the fresh density, and that the correlation should be agreed before delivery. For bag weights of other mixes, see how many bags are on a pallet. Masonry units are a separate topic: see concrete block sizes.

What Does a Cubic Yard Weigh? A Worked Example

The weight of a cubic yard is volume times density, and the answer moves with the density you pick. These are labelled arithmetic sums, not weights that any source prints.

1. Convert the volume. 1 cubic yard = 3 x 3 x 3 = 27 cu ft.
2. Normal weight at the tool default. 27 x 150 = 4,050 lb.
3. Lightweight at the tool preset. 27 x 115 = 3,105 lb, which is 4,050 – 3,105 = 945 lb less.
4. CIP 36 lightweight range. 27 x 90 = 2,430 lb to 27 x 115 = 3,105 lb per cubic yard.
5. CIP 36 normalweight range. 27 x 140 = 3,780 lb to 27 x 150 = 4,050 lb per cubic yard.

FAQs About Lightweight Concrete

Sources and Further Reading

Sources define lightweight concrete by density and strength in different ways, so check the definition your project or supplier uses. The sums here are volume-times-density arithmetic, not a mix design. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-10-21.