Ready-Mix Concrete

What is Ready-Mix Concrete

Ready-mix concrete is concrete proportioned and batched at a stationary plant, then transported to the point of placement before it sets. The defining feature is that mixing and quality control happen at the plant, not on the construction site. A central batch plant weighs cement, supplementary cementitious materials, fine and coarse aggregate, water, and chemical admixtures into a recipe, and a truck mixer with a rotating drum keeps the load homogeneous and prevents segregation during the haul. Each delivery carries a ticket recording the mix designation, the batch quantities, the time of batching, and any water added.

The material is a composite. The hardened mass is roughly 60 to 75 percent aggregate by volume (sand plus stone), 7 to 15 percent Portland cement and supplementary binder, 14 to 21 percent water, and a few percent entrapped or deliberately entrained air. Strength comes from the hydration of cement, a chemical reaction with water that forms calcium silicate hydrate gel binding the aggregate into an artificial conglomerate rock. Because hydration is a slow reaction, concrete keeps gaining strength for weeks: the reference value is taken at 28 days, by which point a normal mix has reached about 90 percent of its long-term strength.

Ready-mix is distinguished from a mechanical product like a pump or a motor by being perishable and made to order. There is no inventory: each load is batched minutes before it leaves, and it must be discharged before it stiffens, a window historically capped at 90 minutes by ASTM C94. This perishability shapes the whole commercial structure of the industry. Plants are small and numerous, sited within a short haul of demand, and the product cannot be shipped across regions, so selection is always a local exercise constrained by which plants can reach the site in time.

The industrial history begins in 1916, when Stephen Stepanian, an Armenian American engineer, designed the first motorised transit mixer to replace the horse-drawn mixer and keep concrete agitated en route from plant to site. His patent was first rejected in 1917 on the belief that a truck could not carry a mixer, and he was finally granted it in 1933. The first commercial ready-mix deliveries in the United States date to the 1920s and 1930s, and the transit mixer truck became the icon of the industry that Stepanian is credited with fathering. The post-war construction boom, container-port logistics, and the standardisation of strength classes turned RMC into the dominant way the world places structural concrete.

In scale, concrete is the most-used manufactured material on earth after water, and ready-mix is its principal delivery form for structures. Industry analyses put the global ready-mix market at several billion cubic metres a year, on the order of 5.8 billion cubic metres in the mid-2020s, with China alone accounting for roughly a third of world output. That volume passes through tens of thousands of local batch plants, which is why a procurement engineer rarely chooses a brand and almost always chooses a plant.

Strength and Consistence Classes

Two classifications dominate any RMC order: how strong the hardened concrete must be (strength class) and how fluid the fresh concrete must be (consistence class). Under EN 206, compressive strength is stated as a class such as C30/37, where the two numbers are the characteristic 28-day strength measured on a cylinder and on a cube respectively, both in MPa. Characteristic strength is the value that 95 percent of test results exceed, not the mean, so the plant must target a mean strength comfortably above the class value to allow for production scatter. The cube number is always higher than the cylinder number for the same concrete because the squat cube shape and platen friction inflate the result.

North American practice, governed by ACI 318 and ASTM C94, does not use the dual cylinder-cube notation. It specifies a single value, the specified compressive strength f'c in psi (or MPa), always measured on a standard 150 by 300 mm cylinder. As a rough bridge, 3,000 psi corresponds to about C20/25, 4,000 psi to about C25/30, and 5,000 psi to about C35/45. The conversions are not exact because the two systems define the characteristic value with different statistical confidence levels, so a spec should never silently translate one to the other.

Consistence describes how fluid and workable the fresh concrete is, which determines how it is placed and compacted. The classic measure is the slump test to EN 12350-2 (or ASTM C143), in which a cone of concrete is lifted and the vertical settlement is measured. EN 206 groups slump into classes S1 to S5, while wetter, self-compacting mixes are measured by flow spread instead. The wrong consistence is a common ordering error: too stiff and the crew cannot consolidate it around dense reinforcement, too fluid and it segregates or needs extra water that ruins the w/c ratio.

Consistence is not a free choice independent of strength. A given strength class fixes the water/cement ratio, so once the binder content is set, the only honest way to reach a higher slump class is with a water-reducing or superplasticising admixture, not extra water. This is why a modern RMC order pairs a strength class with a consistence class and lets the plant's chemical admixture programme reconcile the two, rather than adding water in the truck.

Exposure Classes and Durability

Strength alone does not make concrete durable. A high-strength mix can still corrode its reinforcement in a marine environment or spall under freeze-thaw if the wrong mix is used. EN 206 handles this through exposure classes, a system that classifies the environment by its dominant deterioration mechanism and then dictates the minimum mix requirements needed to survive it. Choosing the exposure class is therefore the durability half of the specification, sitting alongside the strength class.

There are five mechanism families. XC1 to XC4 cover carbonation-induced corrosion, where atmospheric carbon dioxide neutralises the alkaline pore solution and de-passivates the steel. XD1 to XD3 cover chloride-induced corrosion from sources other than seawater, principally de-icing salt spray on car parks and bridge decks. XS1 to XS3 cover chlorides from seawater, with XS3 being the aggressive tidal and splash zone. XF1 to XF4 cover freeze-thaw attack, with the higher numbers adding de-icing salt. XA1 to XA3 cover chemical attack, chiefly sulfates in ground or groundwater. A real structure usually carries several classes at once, and the rule is to design for the most severe combination.

Each exposure class fixes three things: a maximum water/cement ratio, a minimum binder content in kilograms per cubic metre, and a minimum strength class. These are durability limits, not strength limits. A buried foundation in non-aggressive ground (XC2) may only need C25/30 strength, but the same C25/30 mix would be illegal in a tidal marine zone, where XS3 forces the w/c ratio down to 0.45, the cement content up to 340 kg/m³, and the strength class up to C35/45 regardless of what the structural calculation alone demands. The exposure class often, not the load case, sets the final mix.

Freeze-thaw classes carry an extra requirement that does not appear in the table above: air entrainment. Concrete exposed to repeated freezing while saturated must contain a network of microscopic air voids, typically a target of about 6 percent (with a tolerance of plus or minus 1.5 percent in ASTM C94), spaced no more than roughly 0.2 mm apart, so that water freezing in the pores has room to expand without cracking the paste. An air-entraining admixture creates these stable bubbles, and the resulting concrete resists hundreds of percent more freeze-thaw cycles than non-air-entrained concrete. The trade-off is a modest strength loss, on the order of 5 percent of compressive strength for each 1 percent of added air.

Durability is ultimately a permeability problem. Carbonation, chloride ingress, and sulfate attack all advance by diffusion through the pore network of the cement paste, and a lower water/cement ratio produces a denser, less permeable paste. This is why every exposure class tightens the w/c limit before it touches strength, and why supplementary cementitious materials, which refine the pore structure, are central to durable marine and de-icing-salt mixes. A structure rated for a 50 or 100 year service life lives or dies on its pore structure, set at the moment the plant chooses the water content.

Mix Constituents and Materials

A ready-mix recipe is built from five constituent families: cement, supplementary cementitious materials, aggregates, water, and chemical admixtures. Each is governed by its own material standard, and each has a defined role in the fresh and hardened behaviour of the concrete. Understanding what each constituent does, and which standard certifies it, lets a buyer read a mix design rather than take it on trust.

Cement is the active binder. The mainstream product is Portland cement (EN 197-1 type CEM I, or ASTM C150 Type I to V), a fine powder of ground clinker that reacts with water to form the calcium silicate hydrate gel that binds the mass. ASTM C150 types are tuned for duty: Type I general purpose, Type II moderate sulfate resistance, Type III high early strength, Type IV low heat for mass pours, and Type V high sulfate resistance. EN 197-1 blends clinker with SCMs at the cement works to produce types CEM II through CEM V, so part of the SCM substitution can happen before the concrete plant ever sees it.

Supplementary cementitious materials (SCMs) replace a fraction of the clinker to cut cost, lower embodied carbon, and improve durability. Fly ash to ASTM C618, a coal-combustion by-product, is typically dosed at 15 to 30 percent of the binder. Ground granulated blast-furnace slag (GGBS) to ASTM C989, a by-product of iron-making, runs at 50 to 55 percent and can go higher for marine work. Silica fume, an extremely fine pozzolan, is used at 5 to 10 percent for high-strength and low-permeability mixes. All three refine the pore structure and reduce chloride penetration, which is why EN 206 lets slag and fly ash count toward the binder through a k-value. Their cost is slower early strength gain and a longer curing requirement.

Aggregates are the inert skeleton, 60 to 75 percent of the volume, and are specified to ASTM C33 (or EN 12620). Fine aggregate is sand; coarse aggregate is crushed stone or gravel, characterised by its nominal maximum size, commonly 10, 20, or 40 mm. Larger maximum aggregate size reduces the water and cement demand for a given workability but needs more clearance between reinforcing bars. ASTM C33 controls grading, deleterious substances, and soundness so that the aggregate does not weaken the matrix or react with the cement alkalis.

Chemical admixtures are dosed in small quantities to modify fresh and hardened properties, and they define a modern mix. ASTM C494 classifies them by type, and matching the right type to the duty is a core selection skill.

Types A, D, and E must reduce mixing water by at least 5 percent; the high-range Types F and G, the superplasticisers, must reduce it by at least 12 percent and in practice cut it by 20 to 30 percent. Separately from C494, air-entraining admixtures to ASTM C260 create the void network for freeze-thaw resistance. Water itself is the fifth constituent and must be potable or tested to EN 1008; it both hydrates the cement and provides workability, which is the source of the central tension in mix design, since the water that makes concrete easy to place is the same water that weakens it.

Key Specification Parameters

Reading an RMC mix design or order sheet is a fundamental skill for the buyer. A delivery may list a dozen parameters, but only a handful truly drive the order: strength class, water/cement ratio, consistence class, maximum aggregate size, binder content, air content, and the delivery time limit. Each is explained below in the terms that appear on a real spec.

Water/cement ratio (w/c) is the mass of water divided by the mass of cement (or water divided by total binder when SCMs are present). It is the single strongest predictor of strength and permeability, a relationship Duff Abrams formalised in 1918: for a given set of materials, strength falls as w/c rises. As a rule of thumb a mix at w/c 0.40 reaches roughly 50 MPa cylinder strength, 0.50 gives about 28 MPa (4,000 psi), and 0.60 drops toward 20 MPa. Every durability exposure class caps the w/c ratio, and the cardinal field sin is adding water in the truck to restore slump, which raises w/c and silently sacrifices both strength and service life.

Compressive strength is verified, not assumed. Acceptance is by casting standard cylinders or cubes from the delivered load, curing them under controlled conditions, and crushing them at 28 days; ASTM C94 and EN 206 both define how many specimens, how they are sampled, and the statistical criteria for acceptance. Because the value is characteristic (95th percentile), the plant targets a higher mean, and consistent low scatter, evidenced by the plant's standard deviation, is itself a sign of good production control.

Consistence (slump or flow) is measured fresh at the point of delivery and carries a tolerance. ASTM C94 sets a slump tolerance that tightens at lower slumps, and EN 206 uses the S1 to S5 class bands. Slump is checked before discharge precisely because it is the parameter most often tampered with by adding water, so the delivery ticket records the as-batched water and any addition.

Air content matters for any freeze-thaw mix. The target is commonly about 6 percent for severe exposure, with ASTM C94 allowing a tolerance of plus or minus 1.5 percent on the ordered value. Too little air loses freeze-thaw resistance; too much air needlessly sacrifices strength, since each added percent of air costs roughly 5 percent of compressive strength.

Delivery and time limits are a parameter, not a logistics afterthought. ASTM C94 historically required discharge within 90 minutes or 300 drum revolutions from the start of mixing; the 2021 revision (C94/C94M-21) replaced the fixed 90-minute rule with a limit agreed between purchaser and producer and stated on the ticket. Concrete temperature is also controlled: ACI 301 and ACI 305.1 commonly cap fresh concrete at about 35 degrees C (95 degrees F) in hot weather, while cold-weather placement needs a minimum temperature and protection from early freezing.

The remaining order parameters are quickly stated. Maximum aggregate size (10, 20, or 40 mm) must clear the reinforcement spacing and cover. Minimum binder content is set by the exposure class and underpins durability and pumpability. Chloride class caps the total chloride from all constituents to protect reinforcement. And the cement or SCM type is specified where sulfate resistance, low heat of hydration, or a carbon target applies. A complete order names all of these so that any qualified plant can batch an equivalent mix.

Selection Decision Factors

To turn the knowledge of the preceding five chapters into an actual order, follow the decision sequence below. Most RMC selection mistakes come not from a single wrong number but from deciding parameters in the wrong order, for example fixing the consistence before the exposure class has set the w/c ceiling. These eight steps double as a fixed RFQ template.

1. Strength class: Take the specified compressive strength from the structural design (C25/30, C30/37, or the f'c in psi). Confirm whether the figure is cylinder or cube based before comparing quotes across standards.
2. Exposure class: Classify the environment to EN 206 (XC, XD, XS, XF, XA) and pick the most severe class that applies. This sets the maximum w/c ratio, minimum binder content, and minimum strength class, which may override the structural strength.
3. Consistence class: Choose the slump or flow class (S1 to S5) for the placement method, whether hand-placed footings, pumped columns, or congested reinforcement. Let admixtures, not water, achieve it.
4. Maximum aggregate size: Select 10, 20, or 40 mm against the reinforcement spacing, cover, and section thickness. Larger sizes cut cement and water demand but need more bar clearance.
5. Cement and SCM type: Specify sulfate-resisting cement for XA ground, low-heat binder for mass pours, and an SCM replacement level for the carbon and durability target, capping replacement where cold-weather early strength is critical.
6. Air entrainment and admixtures: Require air entrainment for XF freeze-thaw (about 6 percent), and the ASTM C494 admixture type (retarder for hot weather or long haul, accelerator for cold weather, Type F superplasticiser for low-w/c high-strength mixes).
7. Delivery logistics: Set the discharge time limit on the ticket per the current ASTM C94 rule, confirm the plant haul time meets it, and fix the fresh concrete temperature window and pour rate so trucks are not held waiting.
8. Plant qualification and cost: Qualify each reachable plant against EN 206 or ASTM C94 production control, review its strength standard deviation and mix-approval records, and only then compare unit price per cubic metre plus delivery and pump charges.

One last commonly overlooked dimension is plant proximity and serviceability: concrete is heavy and perishable, so the deciding factor is usually which plant can deliver within the time limit, in the required volume, at the required pour rate, with a documented quality history. Large vertically integrated producers such as Holcim (LafargeHolcim), CEMEX, Heidelberg Materials, and CRH operate dense local plant networks, while the admixture programme that makes the mix work comes from suppliers such as Sika, Master Builders Solutions, Mapei, GCP, and Fosroc. For a large continuous pour, also confirm the producer can hold the same mix consistent across hundreds of loads and supply batch tickets for the project quality record.