Lightweight concrete field guide: structural and insulating mixes

Lightweight concrete, and the three jobs it does

Lightweight concrete is concrete that weighs less than normalweight because part of the dense stone aggregate is replaced with a lighter, porous aggregate, or because air and foam are folded into the paste. Normalweight concrete runs about 145 to 150 pounds per cubic foot. Lightweight concrete covers a wide band below that, from structural mixes near 110 pcf down to insulating fills that weigh a fraction of normal concrete.

You reach for it for one of three reasons. You want to take dead load off the structure, so the floors, the columns, and the foundations can carry less of their own weight. You want the thermal or fire performance the porous material gives you. Or you want a light, flowable fill that gets placed cheap and carries almost no load. Those are three different materials with three different jobs.

The cement, the water, and the water-cement ratio still set the strength the same way they do in any concrete, the way the mix-design fundamentals lay out. What changes is the aggregate and the density, and the way that aggregate behaves before and during the pour. Everything in this guide comes back to one fact: the light part is the aggregate, and the aggregate drinks water.

How light is lightweight concrete?

Lightweight concrete is anything meaningfully below normalweight, and normalweight sits around 145 to 150 pcf. Below that the material spreads across a spectrum, and ACI 213, the guide for structural lightweight-aggregate concrete, is where the structural bands are defined.

Structural lightweight concrete carries an equilibrium density commonly in the range of about 90 to 120 pcf. Most structural mixes land around 105 to 120 pcf, and a typical composite floor mix sits near 110 pcf. ACI 213 sets the structural floor at a 28-day compressive strength of 2500 psi or higher. Drop below that strength and you have left the structural family, whatever the density says.

Insulating and low-density concrete sits far lower. ASTM C495, the strength test method for these mixes, covers material with an oven-dry density not exceeding about 50 pcf. Perlite, vermiculite, and cellular fills often run in the 20 to 40 pcf range, at strengths measured in the low hundreds of psi rather than thousands. Between the two ends is a middle band of fill and topping mixes lighter than structural but stronger than pure insulation. Treat these as approximate bands, not bright lines. The mix submittal and the engineer set the number that governs your job, and where a mix lands on this spectrum decides almost everything that follows.

The three families: structural, insulating, and cellular

Lightweight concrete splits into three families, and naming the right one is the first decision on any lightweight job. Get this wrong and nothing downstream is right.

Structural lightweight uses a strong, porous lightweight aggregate, usually expanded shale, clay, or slate, to make concrete that carries load at a reduced weight. Insulating or low-density lightweight uses a very light, weak aggregate like perlite or vermiculite, or a high air content, to make a material that insulates and fills but holds little load. Cellular or foamed concrete makes the density with air alone, a stable foam folded into a cement paste or mortar, often with no coarse aggregate at all.

The families overlap at the edges, and a few mixes blend an aggregate with added foam to hit a target density. But the working distinction holds: structural carries load, insulating insulates and fills, cellular is mostly engineered air. A spec that calls for one and gets another is the most expensive mistake in this trade, because it usually is not caught until the material is in place and tested.

What is structural lightweight concrete?

Structural lightweight concrete is concrete made with structural lightweight aggregate that reaches a 28-day compressive strength of 2500 psi or more at an equilibrium density commonly between about 90 and 120 pcf, per ACI 213. It is real load-carrying concrete, just lighter, and it is designed and reinforced like normalweight concrete with a few modifications.

The aggregate that makes it work is expanded shale, clay, or slate, often shortened to ESCS. The shale, clay, or slate is heated in a rotary kiln until it bloats into a hard, ceramic-like particle full of fine pores. That porous structure is what cuts the weight while keeping enough strength to carry a floor. Some structural lightweight also uses other strong lightweight aggregates, but ESCS is the workhorse in North American structural work.

Where it earns its keep is anywhere the weight of the concrete itself is part of the problem. Floors on composite steel deck, parking structures, bridge decks, long-span members, and seismic retrofits all benefit when the dead load comes down. The reduced weight ripples through the design: smaller beams, smaller columns, smaller foundations, and less seismic mass to resist. The trade-off is a stiffer set of placing and curing rules around the aggregate, which the rest of this guide is mostly about.

The lightweight aggregate that makes it work

The lightweight aggregate, the LWA, is the whole game in structural lightweight concrete. Replace the dense crushed stone with a particle that is hard but full of pores and you have cut the weight at the source. The strength and durability of the finished concrete still depend on the paste and the water-cement ratio, but the density and most of the placing behavior trace back to this one ingredient.

Manufactured structural LWA is expanded shale, clay, or slate, made by bloating the raw material in a rotary kiln so it traps a network of fine, mostly closed pores. The result is a strong, angular particle that weighs far less than stone. ASTM C330 is the specification that governs lightweight aggregate for structural concrete, and ASTM C332 governs the lighter aggregates used for insulating concrete, including the perlite and vermiculite group. Natural lightweight aggregates exist too, mainly pumice and scoria from volcanic sources, and they show up regionally.

The pores are the point and also the problem. They are what make the aggregate light, and they are what make it absorb a large amount of water compared to ordinary stone. A normal crushed-stone aggregate absorbs a percent or two of its weight in water. A porous LWA can absorb several times that, and it does not stop the moment it touches the mix. That single property drives the pre-wetting rule, the internal-curing bonus, and most of the pumping and finishing headaches that come with the material.

Why do you pre-wet lightweight aggregate?

You pre-wet lightweight aggregate because the porous particles will pull water straight out of the mix if you batch them dry, and that stolen water is the water your slump, your pump, and your finish were counting on. Pre-wetting, also called pre-soaking or pre-saturating, means soaking the aggregate before it goes into the batch so it arrives already carrying most of the water it wants.

Skip it and the failure is immediate and ugly. The dry aggregate drinks the mix water for the first hour, the slump that looked right at the plant is gone by the time the truck reaches the deck, and the crew is tempted to add water on site to chase workability. Adding water raises the water-cement ratio and weakens the concrete, which is exactly the trade the mix design was built to prevent. A common procedure is to wet the stockpile with sprinklers or soaker hoses for a day or two, then let it drain for a number of hours so the surface moisture is consistent before batching. The supplier sets the actual soak and drain time for their aggregate.

Pumping makes this worse, not better. Under pump pressure the aggregate is squeezed against water it has not yet absorbed, and it takes that water in fast, so an un-pre-wetted mix can lose slump dramatically between the hopper and the boom tip, or plug the line. If anyone tells you a lightweight mix can be batched dry and watered at the truck, that is the person who has not yet owned a failed pour. Pre-wet the aggregate. It is the single rule that prevents the most lightweight-concrete callbacks.

Internal curing: the bonus inside the aggregate

The water you soaked into the aggregate does not just stay put. As the cement hydrates and the paste starts to dry itself out from the inside, the saturated lightweight particles release their stored water back into the surrounding paste, curing the concrete from within. This is internal curing, and it is the upside hiding inside the pre-wetting chore.

It matters because high-cement and low water-cement-ratio mixes are prone to autogenous shrinkage, the self-desiccation that pulls the paste tight and cracks it at early age even when the surface is kept wet. The internal water reservoirs feed hydration where surface curing cannot reach, so internal curing reduces autogenous shrinkage, lowers early-age cracking, and tends to raise strength because more of the cement gets to react. Research on pre-soaked lightweight aggregate consistently shows less early cracking and better long-term hydration.

This is why the practice spread from structural lightweight into normalweight bridge decks and pavements, where a measured dose of saturated lightweight fine aggregate is added to an otherwise normal mix purely to internally cure it. You take the pre-wetting penalty on purpose to get the crack control. On a structural lightweight pour you get the benefit for free, because you had to soak the aggregate anyway.

What you gain from the reduced dead load

The reduced dead load is the reason most structural lightweight gets specified, and the savings compound through the whole structure. Lighter floors mean the beams that carry them can be smaller, the columns under the beams can be smaller, and the foundations under the columns can be smaller. On a tall building those reductions add up to real steel and concrete taken out of the design.

Seismic design gets the largest single benefit. Earthquake force scales with mass, so cutting the weight of the floors directly cuts the seismic demand the structure has to resist. On a retrofit, switching a topping to lightweight can be the move that keeps an existing frame and foundation within capacity instead of reinforcing them. The same logic lets lightweight stretch spans further, because less of the member's capacity is spent carrying itself.

Data centers and other heavy floor-loaded buildings use structural lightweight for the same reason from the other direction. When the live load from equipment is large and fixed, taking dead load out of the slab leaves more of the structure's capacity for the racks and the gear. The weight you do not pour is weight the frame does not have to carry for the life of the building.

Lightweight concrete on composite steel deck

The single most common structural lightweight job is the floor topping on composite steel deck. Lightweight concrete poured over corrugated steel deck, acting compositely with the deck and the supporting beams, is how a large share of multi-story steel buildings get their floors. The lighter the concrete, the lighter the whole floor system, which is exactly what the structural engineer wanted out of the steel frame.

Fire rating is a big part of why lightweight wins here. The deck-and-concrete assembly has to carry a fire-resistance rating, and the lower thermal conductivity of lightweight concrete means a thinner topping can reach the same rated hour as a thicker normalweight slab. The rated assemblies are listed by thickness and concrete type, so the design picks lightweight partly to hit the fire rating without adding depth and weight. Confirm the specific listed assembly the project is built to, because the required thickness and density come from that listing, not from a rule of thumb.

On the deck itself the placement rules from any slab still apply: hold the specified thickness over the flutes, do not let the crew chase a flat finish by adding water, and protect the fresh topping from drying wind. The lightweight wrinkle is the same one as always. The aggregate has to arrive pre-wetted, or the topping that pumped up onto the deck will lose slump and finish hard before the crew is across it.

Strength is fine; the modulus is the catch

Structural lightweight concrete reaches the strengths structural work needs. Mixes at 3000, 4000, and 5000 psi are routine, and higher is achievable with the right aggregate and proportions. Strength is not the limitation people assume it is. The stiffness is.

The modulus of elasticity, the measure of how much the concrete deflects under load, runs lower for lightweight than for normalweight at the same strength. Studies and the design literature put the reduction broadly in the range of 15 to 50 percent depending on the density and the aggregate, with the lighter mixes losing the most stiffness. Lower modulus means more elastic deflection, and lightweight also tends toward higher creep, the slow deflection under sustained load, so long-term deflection deserves a hard look on long spans and thin members. Treat these ranges as direction, not design values; the engineer computes the modulus for the actual mix.

The tensile side drops too, which is why ACI 318 applies a lightweight modification factor, the lambda factor, to shear strength and to development and splice lengths. Lambda is less than one for lightweight, so the code stretches the embedment and trims the shear capacity to account for the weaker bond and lower tensile strength of the lightweight matrix. The designer handles lambda, but the field consequence is real: development lengths on a lightweight job can be longer than the crew expects from normalweight habit, so read the lap lengths off the lightweight drawings, not from memory.

Insulating and low-density lightweight

Insulating lightweight concrete is the opposite end of the spectrum: very light, very weak, and used for its thermal performance and its low weight, not its strength. ASTM C495 covers the strength testing of these mixes at an oven-dry density not over about 50 pcf, and the aggregates, mainly perlite and vermiculite, fall under ASTM C332. Strengths sit in the low hundreds of psi. Perlite or vermiculite insulating concrete commonly carries a minimum compressive strength around 125 psi, well below anything you would call structural.

The classic use is the roof deck. A layer of insulating concrete poured over a structural deck gives thermal value, a fire-resistive layer, and an easy way to build slope to drain across a roof that was framed dead flat. It bonds to the deck, takes the slope the roofer needs, and weighs little enough that the framing does not have to grow to carry it. The roofing membrane goes over the top.

The other use is fill. Lightweight fill levels an uneven deck, builds up a floor to a finished elevation, or fills a void without loading the structure underneath. Geotechnical lightweight fill takes this further, replacing heavy soil over a weak subgrade, behind a retaining wall, or on a settlement-prone site, where the goal is to add volume without adding the weight that would overload what is below. In every case the material is chosen because it is light and cheap to place, not because it carries load.

Cellular and foamed concrete, and flowable fill

Cellular concrete, also called foamed concrete, makes its low density with air rather than with a lightweight aggregate. A preformed, stable foam is generated on site with a foam generator and a foaming agent, then blended into a cement paste or sand-cement mortar. The cured material is shot through with tiny, discrete air cells, and the density is dialed by how much foam goes in. ASTM C796 and C869 cover the test method and the foaming agents for this material.

It flows like a thick liquid and self-levels, which makes it the material of choice for filling things. Cellular concrete fills abandoned tanks, pipes, mine voids, and the space behind tunnel linings. It serves as lightweight geotechnical fill over weak ground and as void fill where you need volume and a little strength but no real load capacity. Densities can run very low, down into the teens of pcf for the lightest pours, with strength to match.

The related family is controlled low-strength material, CLSM, often called flowable fill. CLSM is a deliberately weak, self-consolidating mix used to backfill trenches and utility cuts, and a cellular or air-entrained version is the lightweight form of it. The defining feature is that it is engineered to stay weak, often a few hundred psi or less, so it can be re-excavated later by hand or small equipment. ACI 229 covers CLSM. Do not confuse it with structural concrete that happens to flow; the low strength is the design intent, not a defect.

Mixing lightweight: what changes from a normal mix

A lightweight mix design starts from the same place as any mix, the strength and durability the engineer specified and the water-cement ratio that delivers them, then adjusts for the aggregate. The supplier owns the proportions and submits them, the same as a normalweight submittal, and the field crew protects them. What the field has to understand is what is different.

Lightweight mixes generally carry more cementitious paste than an equivalent normalweight mix, because the lighter aggregate contributes less to strength and the paste has to do more of the work. They are often air-entrained, both for freeze-thaw durability and to help the workability of a mix that does not have heavy stone to lubricate the flow. And the absorbed water in the aggregate has to be accounted for separately from the mixing water, because the porous LWA holds a large reservoir that does not behave like the surface moisture on ordinary sand and stone.

That last point is where lightweight mix design lives or dies. The water-cement ratio that controls strength is the water available to the paste, not the water sitting inside the aggregate pores. If the aggregate moisture is not measured and the batch water adjusted, the effective water-cement ratio drifts and the strength and density drift with it. This is why the supplier specifies a moisture condition for the aggregate at batching and why the field cannot freelance with added water. The mix-design fundamentals on water-cement ratio carry straight over; the aggregate moisture is the lightweight complication layered on top.

Placing and pumping lightweight concrete

Placing lightweight is mostly normal placement with a sharper respect for slump loss. The mix should arrive at the design slump, get discharged and placed without added water, and be consolidated like any concrete. The difference is that lightweight is less forgiving of time and heat, because the aggregate keeps interacting with the mix water the whole way from the plant to the forms.

Pumping is where lightweight separates the crews that prepared from the ones that did not. Under the pressure in a pump line, the aggregate that was not fully pre-wetted absorbs water fast, the mix loses slump between the hopper and the discharge, and in the worst case the line plugs. The fix is upstream, not at the pump: the aggregate has to be soaked and at a stable, known moisture before it is batched. Some lightweight pump jobs also run a slightly higher slump or a richer paste to carry the mix through the line, which the supplier builds into the pump mix. You do not solve a pumping problem by adding water at the truck.

Check slump at the point of placement, not at the chute, on any lightweight pour. A slump that read fine at the truck can be gone at the boom tip if the aggregate is still drinking, and the reading that matters is the one where the concrete actually lands. On a hot day, on a long pump, with marginal aggregate moisture, that gap is where a lightweight pour goes wrong.

Finishing around floating aggregate

Finishing lightweight has its own signature problem: the aggregate is lighter than the paste, so instead of the heavy stone settling, the lightweight particles tend to rise toward the surface. Overwork the surface, especially with a vibrating screed or too much water, and you bring a layer of light aggregate up where you are trying to finish, which makes the surface hard to close and prone to a weak, popping skin.

The technique is restraint and timing. Use the minimum finishing effort that closes the surface, avoid over-vibration that drives the aggregate up, and never add water to the surface to ease troweling. As with any concrete, the worst single finishing mistake is troweling while bleed water is still on the surface, which seals water into the top and gives you a weak, dusting slab. Lightweight bleeds less than normalweight in many mixes, so the window can come up differently than the crew expects.

Wait for the surface to be ready, finish it once with as few passes as the job allows, and cure it promptly. Lightweight surfaces reward a light hand and punish the crew that tries to overwork them flat.

Why lightweight resists fire better

Lightweight concrete carries fire better than normalweight, and the reason is the same porosity that makes it light. The pores in the aggregate and the lower density give the material a lower thermal conductivity, so heat travels through it more slowly and the steel or the space on the far side stays cooler for longer.

This shows up directly in rated assemblies. A composite floor that has to carry a two-hour or three-hour fire rating can often reach it with a thinner topping in lightweight than it would need in normalweight, because the lightweight conducts the heat away from the deck more slowly. The listed assemblies spell out the thickness for each concrete type and rating, so the design uses lightweight to buy the rating at less thickness and less weight at the same time. Use the specific listed assembly for the project; the fire rating comes from that listing, not from the general property.

The fire benefit is a real reason to choose lightweight, alongside the weight savings, and on many floor jobs the two reasons point the same direction.

Shrinkage and cracking control

Lightweight concrete shrinks, and on some mixes it shrinks a bit more than normalweight because of the higher paste content, so crack control still matters. The internal curing from the saturated aggregate helps on the early-age, autogenous side by feeding hydration from within, which reduces the self-desiccation cracking that hits high-cement mixes. That is a genuine advantage lightweight has built in.

Drying shrinkage over the longer term is a separate matter and is handled the usual way: joints placed and timed to control where the slab cracks, adequate curing, and reinforcement to hold cracks tight. The same control-joint and curing discipline from any slab applies here. On a topping, the joints have to match the structure and the bond condition below.

Where the design wants distributed crack control without relying only on joints and bar, fibers come into the conversation. The fiber-reinforced concrete approach, micro fibers for plastic-shrinkage cracking and macro fibers for post-crack toughness, layers onto a lightweight mix the same way it does on normalweight, and lightweight slabs-on-ground and toppings use fibers for the same reasons. Fibers do not replace structural reinforcement, in lightweight any more than in normal concrete.

Durability, freeze-thaw, and carbonation

Lightweight concrete is durable when it is proportioned and cured for the exposure, and the internal curing tends to give it a tighter, better-hydrated paste than its water content alone would suggest. The durability comes from the paste, the same as any concrete. The lightweight aggregate is not the weak link if the paste is sound.

Freeze-thaw resistance depends on proper air entrainment, just as it does in normalweight, and lightweight mixes in freezing exposure are entrained for it. There is a wrinkle: the aggregate's own absorbed water is part of the freeze-thaw picture, so the mix and the curing have to leave the matrix in a state that handles freezing cycles. The supplier and the exposure class drive this.

Carbonation and reinforcement protection follow the paste quality and the cover over the steel. A well-hydrated, low-permeability paste resists the inward march of carbonation that eventually threatens embedded reinforcement, and the internal curing works in favor of that low permeability. Give the steel its specified cover and cure the concrete, and lightweight holds up. The same exposure-class thinking from the mix-design fundamentals applies; lightweight does not get a pass on durability detailing.

What it costs, and what it saves

Lightweight concrete costs more per cubic yard than normalweight, because the manufactured aggregate is more expensive to make than crushed stone and the mixes carry more cement. If you only look at the price of the concrete in the truck, lightweight always loses.

That is the wrong place to look. The savings are in the structure, not the slab. Lighter floors let the steel frame, the columns, and the foundations shrink, and on a multi-story or seismic job the steel and foundation savings can dwarf the premium on the concrete. The cost case for structural lightweight is a whole-building case made by the structural engineer, not a unit-price comparison made at the batch plant.

For insulating and fill uses the economics are different and usually simpler. There the material competes against the cost and weight of the alternative, more structure to carry heavy fill, or a separate insulation and slope system, and lightweight often wins because it does several jobs in one pour. Run the comparison at the level of the whole assembly, not the cubic yard, or you will reject lightweight for the wrong reason.

Testing and QC for lightweight

The defining field test for lightweight concrete is fresh density, the unit weight of the fresh mix. Where slump is the headline test on normalweight, fresh density is the one that tells you whether you got the lightweight mix you ordered. The fresh unit weight is measured the same way as any concrete, by ASTM C138, and it is the fastest signal that the aggregate moisture, the air, or the proportions have drifted off the submittal.

Density on lightweight has two forms that get confused. Fresh density is what you measure in the field on the plastic concrete. Equilibrium density is the in-service density after the concrete has dried to a stable moisture, and it is the number ACI 213 uses for the structural density bands; it is determined by ASTM C567, usually calculated from the mix and the aggregate. Spec compliance is judged on equilibrium density, but the field control test is the fresh unit weight, because it tells you in minutes whether the load is right.

Strength and air round out the QC. Compressive strength is tested by ASTM C39 on standard cylinders, the same as normalweight, with splitting tensile by ASTM C496 where it is called for. Air content is checked because it affects both density and freeze-thaw, though the air test on lightweight has its own method considerations the testing lab handles. The blunt version: if you take one extra test on a lightweight pour that you would not take on a normal one, make it the fresh unit weight, every truck if the spec is tight.

What to document

A lightweight pour with no record of the density and the aggregate condition is a pour you cannot defend if the strength or the dead-load assumption is ever questioned. The record is what proves the floor that was placed is the floor the engineer designed.

Capture the mix identity and the specified strength and density, the family the mix is, the fresh unit weight from each load or as the spec requires, the slump at the point of placement, the aggregate moisture condition or confirmation that the aggregate was pre-wetted, the air content, the cylinders cast and their break results, and the ambient conditions on the day. If the placement was pumped, note it, because the pump is where lightweight slump loss hides. Tie the batch tickets to the placement location so a later question lands on the right concrete.

Common mistakes

• Batching the lightweight aggregate dry, so it steals mix water and the slump is gone before placement.
• Adding water at the truck to chase the lost slump, raising the water-cement ratio and weakening the concrete.
• Pumping un-pre-wetted aggregate, getting severe slump loss or a plugged line under pump pressure.
• Specifying or supplying insulating lightweight where structural was needed, or structural where fill was intended.
• Designing or detailing as if the modulus matched normalweight, then finding more deflection and creep than expected.
• Forgetting the lightweight lambda factor, so development and splice lengths are short for the lightweight matrix.
• Running no fresh unit-weight test, so a drifted mix is not caught until the cylinders break.
• Overworking the surface and pulling floating light aggregate up into a weak, dusting finish.

Field checklist

Standards and references

ACI 213, the guide for structural lightweight-aggregate concrete, is the document the structural side leans on. It defines structural lightweight by an equilibrium density commonly in the 90 to 120 pcf band and a 28-day strength of 2500 psi or higher, and it covers the proportioning and the design properties, including the lower modulus and the internal-curing behavior. ACI 318, the structural concrete code, carries the lambda modification factor that adjusts shear strength and development length for lightweight.

On the materials side, ASTM C330 specifies lightweight aggregate for structural concrete, while ASTM C332 specifies the lighter aggregates for insulating concrete, the perlite and vermiculite group among them. ASTM C495 is the compressive-strength test for lightweight insulating concrete, at oven-dry densities not over about 50 pcf, and ASTM C796 and C869 cover cellular concrete testing and foaming agents. For density, ASTM C567 determines the equilibrium density of structural lightweight, and ASTM C138 gives the fresh unit weight that is the field control test. Strength and splitting tensile follow ASTM C39 and C496. CLSM, the flowable lightweight fill, is covered by ACI 229.

Standard numbers and the values in them shift between editions, so confirm the current standard, the adopted code edition, and any local amendments before citing them on a submittal. The mix submittal from the supplier and the structural engineer's drawings control the actual densities, strengths, and details for the job. Cite the standard that governs the point, and let the project documents override the general guidance where they are stricter.

Units, terms, and conversions

Lightweight concrete carries a small vocabulary that gets used loosely across a drawing set, a supplier sheet, and a spec, so the same idea can read differently depending on who wrote it.

Density is given in pounds per cubic foot, pcf, in US practice and kilograms per cubic meter in metric sources; 100 pcf is about 1600 kg/m3. Structural lightweight is sometimes called structural LWC or sand-lightweight when normal sand is used for the fines. Insulating lightweight goes by low-density concrete or LDC. Cellular and foamed concrete are the same family. LWA is the lightweight aggregate, and ESCS is the expanded shale, clay, and slate that makes up most structural LWA. Strength is in psi or megapascals; 2500 psi is about 17 MPa.

LWA

Lightweight aggregate, the porous aggregate that cuts the density of the concrete

ESCS

Expanded shale, clay, and slate, the kiln-bloated aggregate used in most structural lightweight

Fresh density

Unit weight of the plastic concrete, the primary field QC test for lightweight, per ASTM C138

Equilibrium density

In-service density after drying to a stable moisture, the basis for ACI 213 bands, per ASTM C567

Pre-wetting

Soaking the aggregate before batching so it does not steal mix water or lose slump

Internal curing

Hydration fed by water released from the saturated aggregate, reducing early-age cracking

Lambda factor

ACI 318 modification, less than one, applied to shear and development length for lightweight

Cellular / foamed concrete

Low-density concrete made with engineered air from a foaming agent rather than aggregate

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