What is concrete made of? (Simple explanation)

If you're planning a concrete project in North Carolina—whether a driveway, sidewalk, patio, or foundation—understanding what concrete is made of will help you evaluate contractor proposals and understand why certain finishes, strengths, and maintenance regimens matter. Local Concrete Contractor is a North Carolina–based concrete company in business 15 years, with hundreds of 5-star Google reviews across Charlotte, Raleigh, the Triad, and the Lake Norman area. Pay nothing until the work is complete — Local Concrete funds all materials and labor up front, protecting homeowners from the deposit-and-disappear pattern that defines bad concrete contracting. This guide breaks down concrete's ingredients, explains how they work together, and shows how mix design directly affects your project's longevity.

The four main ingredients in concrete

Concrete is not a single material—it's a composite engineered from four distinct components that work together to create a durable, load-bearing surface. These ingredients are Portland cement, aggregate, water, and optional additives. Understanding the role of each ingredient and how they interact is the foundation for understanding why certain concrete projects last 40+ years while others deteriorate in a decade.

Portland cement acts as the binder. It typically comprises 10–15% of the total mix by weight. When Portland cement powder contacts water, it undergoes a chemical reaction called hydration, producing calcium silicate hydrate—the primary compound responsible for concrete's strength.

Aggregate consists of sand (fine aggregate) and gravel or crushed stone (coarse aggregate). Aggregate accounts for 75–85% of concrete's volume and provides structural support, reduces cost, and controls the concrete's thermal expansion. The size and quality of aggregate particles directly affect workability and strength.

Water makes up 15–20% of the mix by weight. Its primary role is to activate the Portland cement through hydration. However, the amount of water relative to cement—expressed as the water-cement ratio—is one of the most critical variables in concrete durability.

Additives are optional but widely used to improve workability, durability, appearance, or curing speed. Common additives include air-entraining agents, fly ash, fiber reinforcement, accelerators, and plasticizers. In North Carolina's climate—subject to freeze-thaw cycles, salt exposure on roadways, and high humidity—many contractors specify air entrainment to protect against spalling and scaling.

Portland cement: the binder

Portland cement is the active ingredient in concrete. It's not a brand name; it's a product standard defined by ASTM International and the Portland Cement Association. Portland cement is manufactured by heating a mixture of limestone, clay, and silica to approximately 2,700°F in a kiln, then grinding the resulting clinker into a fine powder.

The chemistry of Portland cement involves four main compounds: tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A), and tetracalcium aluminoferrite (C₄AF). When water is added, these compounds hydrate at different rates. C₃S hydrates quickly and contributes early strength (the first 7 days); C₂S hydrates slowly and contributes long-term strength (beyond 28 days). This is why concrete's strength gain is not instantaneous—it's a time-dependent chemical process.

Most concrete in North Carolina uses Type I or Type II Portland cement. Type I is general-purpose cement suitable for most projects. Type II has lower tricalcium aluminate content, making it resistant to sulfate attack—a concern in regions with high soil sulfate levels or near groundwater. According to the American Concrete Institute (ACI), selecting the correct cement type for local soil and water conditions is essential for durability in residential and commercial slabs.

The cement content of a mix directly affects cost and strength. A typical driveway might use 500–600 pounds of cement per cubic yard of concrete. Increasing cement content increases strength but also increases cost and can introduce shrinkage cracks if the water-cement ratio is not managed carefully.

Water and the water-cement ratio

Water is essential to concrete hydration, but more water does not mean stronger concrete. The critical metric is the water-cement ratio (w/c), which expresses the weight of water relative to the weight of Portland cement in the mix.

A w/c of 0.40–0.45 produces high-strength, durable concrete suitable for exposed slabs and freeze-thaw environments. A w/c of 0.50–0.55 produces moderate-strength concrete suitable for non-exposed slabs. A w/c above 0.60 produces weak, porous concrete prone to spalling, scaling, and efflorescence (white powder on the surface).

For example, a concrete mix might contain 500 pounds of Portland cement and 200 pounds of water per cubic yard. That's a w/c of 0.40 (200 ÷ 500 = 0.40). If a contractor adds extra water to make the concrete easier to place, the w/c rises to 0.50 or higher—and compressive strength drops by 10–30% depending on the exact ratio. Lower strength means faster deterioration, especially in North Carolina's freeze-thaw cycle during winter months.

The National Ready Mixed Concrete Association emphasizes that the w/c is the single most important factor controlling concrete durability. Contractors who specify and monitor the w/c protect against long-term cracking, spalling, and premature failure. This is why reputable contractors measure slump (a quick test of concrete workability) and adjust mix design in the truck rather than adding water on site.

Aggregate: sand, gravel, and stone

Aggregate fills 75–85% of the concrete's volume and is the largest component by weight. It serves three critical functions: it provides structural skeleton and load-bearing capacity, it reduces the total cost of the concrete (aggregate is cheaper than cement), and it controls thermal expansion and shrinkage. Understanding aggregate gradation and quality is essential to durable concrete.

Fine aggregate is typically sand with particles smaller than 0.25 inches. Sand fills the voids between coarse aggregate particles and improves workability and finish. Poor-quality sand—containing clay, silt, or organic matter—weakens the bond between paste and aggregate, reducing strength and durability.

Coarse aggregate is gravel or crushed stone typically ranging from 0.5 to 1.5 inches in diameter. Larger coarse aggregate (up to 1.5 inches) reduces the amount of paste needed, lowers cost, and can improve durability if properly graded. However, larger aggregate may be impractical for thin slabs or heavily reinforced sections where rebar or wire mesh spacing is tight.

Aggregate quality matters. Hard, clean, well-graded aggregate produces stronger, more durable concrete. Recycled or low-quality aggregate containing clay films, soft particles, or deleterious materials can introduce weak points and voids. In the Carolinas and across North Carolina, local aggregate sourcing is common; contractors typically use locally available stone and sand to reduce transportation cost and time.

Aggregate gradation—the distribution of particle sizes—affects workability, strength, and appearance. A well-graded aggregate with a continuous range of sizes packs tightly, reducing voids and improving strength. A poorly graded aggregate leaves gaps that the paste must fill, consuming more cement and water, raising cost and reducing durability.

Additives and how they change concrete

Additives are supplementary materials added to the concrete mix in small quantities to modify its properties. They do not replace Portland cement, aggregate, or water; they enhance performance. Common additives in North Carolina projects include the following.

Air-entraining agents introduce tiny, intentional air bubbles—typically 4–8% by volume—throughout the concrete. Each bubble acts as a relief space, allowing water to freeze without generating destructive pressure inside the concrete. This is critical in North Carolina's cold seasons, where freeze-thaw cycles and salt exposure cause scaling (surface spalling). According to NC State Extension, air entrainment is especially important for slabs in the Piedmont and mountain regions where winter temperatures drop below 32°F regularly. The trade-off is a slight reduction in compressive strength (typically 5–10%), but the gain in durability far outweighs the loss.

Fly ash is a byproduct of coal-fired power plants. It contains silica and alumina compounds that react slowly with Portland cement, improving workability, reducing heat generation during curing, and increasing long-term strength. Fly ash also reduces cost and environmental impact. Concrete containing 15–30% fly ash by weight of cement is common and recommended for slabs exposed to sulfate-rich soils.

Fiber reinforcement (plastic, steel, or polypropylene fibers) reduces plastic shrinkage cracks and improves crack control. Fibers do not replace rebar or wire mesh for structural reinforcement; they reduce early shrinkage and increase toughness. Many contractors in the Charlotte metro and Lake Norman area specify synthetic fiber reinforcement for decorative or exposed aggregate finishes to minimize visible cracks.

Accelerators speed hydration and strength gain, reducing curing time. Calcium chloride is a common accelerator, though it can corrode rebar in reinforced concrete. Non-chloride accelerators are safer for reinforced slabs. Accelerators are useful in cold weather, where curing is slow, and time-critical projects.

Water reducers (plasticizers) allow the concrete to achieve the same workability with less water, effectively lowering the w/c and improving strength and durability without additional cost. A typical water reducer might allow a 10–15% reduction in water, raising strength by 10–20% at the same slump.

Concrete strength and PSI ratings

Concrete strength is measured in PSI (pounds per square inch) of compressive strength—the force per unit area required to crush a concrete cylinder or cube. The standard test occurs at 28 days of curing under controlled conditions (73°F and 100% humidity). However, concrete continues to gain strength beyond 28 days, and actual field conditions often differ from laboratory standards.

Typical residential and light commercial projects in North Carolina use the following PSI ratings:

• 3,000–3,500 PSI: Sidewalks, non-structural flatwork, decorative patios. Adequate for foot traffic and light vehicle loads.
• 3,500–4,000 PSI: Driveways, garage floors, parking areas. Suitable for passenger vehicles and light commercial loads.
• 4,000–4,500 PSI: Heavy-duty slabs, commercial floors, structures subject to aggressive chemical or weather exposure.
• 5,000+ PSI: Structural concrete, foundations, and slabs in extreme environments.

Specifying the correct PSI depends on load requirements, environmental exposure, and long-term durability goals. A driveway in Charlotte that supports only personal vehicles may achieve acceptable service life at 3,500 PSI with proper air entrainment and curing. A driveway in a freeze-thaw zone (such as Mooresville or the mountains) that also experiences salt exposure demands 4,000 PSI minimum, plus 5–7% air entrainment, plus a low w/c (0.45 or less) to prevent scaling.

Strength is determined by multiple variables: cement content, w/c, aggregate quality, air content, curing time and temperature, and age. A 28-day PSI rating is a snapshot; the concrete may be stronger or weaker depending on field conditions. Proper curing—maintaining moisture and temperature for at least 7 days—is essential to reaching target strength. Poor curing can reduce 28-day strength by 20–50%.

Curing time and why it matters

Curing is the process of maintaining proper temperature and moisture to allow Portland cement to hydrate and concrete to gain strength. Curing is not the same as drying; in fact, concrete should not be allowed to dry prematurely. Proper curing involves keeping the concrete moist and warm—ideally 73°F and 100% humidity—for a minimum of 7 days, with 28 days being optimal.

Concrete strength develops over time in an S-shaped curve. In the first 7 days, concrete gains roughly 50–70% of its 28-day strength if cured properly. From day 7 to day 28, strength gain slows as hydration continues. After 28 days, strength gain continues at a much slower rate for months or years, depending on cement type and w/c.

What happens during curing affects durability. If concrete is allowed to dry in the first 7 days, the outer layer loses moisture, cement hydration slows, and the surface becomes weak and prone to crazing (fine cracks) and spalling. If curing is disrupted by freezing (below 32°F), hydration stops until the temperature rises, delaying strength gain. In North Carolina's climate, contractors must account for seasonal curing conditions: spring and fall curing is ideal; winter requires heated enclosures or accelerators; summer heat requires shading and moisture retention to prevent rapid drying.

Proper curing methods include the following:

• Wet burlap or plastic sheeting: Covers the concrete to retain moisture. Plastic (white) reflects heat; opaque plastic can trap heat in cool weather.
• Spraying or ponding: Periodic misting or flooding keeps the surface wet for 7–28 days. Effective but labor-intensive.
• Curing compounds: Spray-on membranes that seal the concrete surface and slow moisture loss. Cost-effective and practical for large slabs.
• Heat and humidity control: Enclosed curing (tents, heaters, humidifiers) in cold or dry conditions accelerates strength gain and ensures minimum strength targets are met.

A reputable concrete contractor in Charlotte, Raleigh, or across North Carolina will specify a curing method and monitor curing conditions—especially temperature and surface moisture. This attention to detail directly correlates with long-term durability and fewer callbacks for cracking, spalling, or premature failure.

Frequently asked questions

What are the four main ingredients in concrete?

Concrete consists of Portland cement (10–15%), aggregate (75–85%), water (15–20%), and optional additives (air entrainment, fly ash, fibers). Portland cement acts as the binder, aggregate provides structure and volume, and water activates the cement through hydration. Additives improve workability, durability, or appearance depending on project needs.

What is Portland cement and why is it in concrete?

Portland cement is a fine powder made by heating limestone, clay, and silica at 2,700°F, then grinding the result. It contains calcium silicates that react with water to form a paste that hardens and binds aggregate particles together. Cement typically comprises 10–15% of a concrete mix by weight, though the exact proportion depends on desired strength and application.

How much water should be in a concrete mix?

Water comprises 15–20% of concrete by weight, but the critical measure is the water-cement ratio, which typically ranges from 0.40 to 0.60. A lower ratio (0.40–0.45) produces stronger, more durable concrete, while a higher ratio (0.55–0.60) makes the mix more workable but reduces strength. Too little water prevents proper hydration; too much causes weakness and surface spalling.

What is aggregate and what role does it play?

Aggregate—sand and gravel or crushed stone—makes up 75–85% of concrete's volume and provides structural support and economy. Fine aggregate (sand) fills spaces between coarse aggregate particles (gravel), while coarse aggregate anchors the paste. Aggregate selection affects strength, workability, and cost; larger coarse aggregate (up to 1.5 inches) can reduce material cost and improve durability.

What do concrete additives do?

Additives modify concrete's properties without changing the cement-aggregate-water ratio. Air entrainment introduces tiny air bubbles (4–8% by volume) to protect against freeze-thaw damage in North Carolina winters. Fly ash reduces cost and improves workability; fiber reinforcement adds crack control; accelerators speed curing in cold weather; water reducers increase strength without adding water.

What is concrete strength measured in, and what numbers matter?

Concrete strength is measured in PSI (pounds per square inch) of compressive strength, tested at 28 days of curing. Sidewalks typically require 3,000–3,500 PSI; driveways and patios, 3,500–4,000 PSI; and heavy-duty slabs, 4,000–5,000 PSI or higher. Strength depends on mix design, water-cement ratio, aggregate quality, curing conditions, and time; proper curing over 7–28 days is essential to reach target strength.

How long does concrete take to cure, and why does it matter?

Concrete reaches 50% strength in 7 days and 90% strength in 28 days under proper curing (moisture and temperature control). Some strength gain continues for years, but practical strength benchmarks occur at 7 and 28 days. During this window, temperature, moisture, and traffic avoidance are critical; poor curing leads to surface spalling, crazing, and premature failure.

Why do some concrete projects fail while others last 40+ years?

Durability depends on mix design (water-cement ratio, air entrainment), subgrade preparation, finishing quality, curing, and climate exposure. Salt, freeze-thaw cycles, UV exposure, and tree roots cause scaling, spalling, and settlement if the mix lacks proper air entrainment or the water-cement ratio exceeds 0.50. Reputable contractors in Charlotte, Raleigh, and across North Carolina account for local soil and weather when selecting mixes and specifying finishes.

Key takeaways

• Concrete is 75–85% aggregate (sand and gravel), 10–15% Portland cement, 15–20% water, and optional additives. The exact proportions determine strength, durability, and cost.
• The water-cement ratio is the single most important variable controlling concrete durability. A ratio of 0.40–0.45 produces strong, long-lasting concrete; ratios above 0.60 produce weak concrete prone to spalling and scaling.
• Air entrainment (4–8% intentional air bubbles) is essential for slabs in North Carolina's freeze-thaw climate. It slightly reduces strength but dramatically increases freeze-thaw resistance and extends slab life.
• Concrete requires proper curing for 7–28 days to reach target strength. Curing involves maintaining moisture and warm temperature; poor curing can reduce strength by 20–50% and cause early cracking and spalling.
• Standard driveways and patios use 3,500–4,000 PSI concrete. Specifying the correct PSI depends on load, environmental exposure, and climate; heavy-duty or exposed slabs demand higher PSI and additional protective measures.
• Reputable contractors follow industry standards (ASTM, ACI, PCA) for mix design, material selection, and curing. They avoid adding extra water on site, specify air entrainment for freeze-thaw zones, and monitor curing conditions to ensure durability.

Ready to get started? Pay nothing until the work is complete. Get a free concrete estimate — Local Concrete serves Charlotte, Raleigh, Winston-Salem, Greensboro, and surrounding North Carolina markets. Learn more about how much a concrete driveway costs, the best finish for a concrete driveway, and how to maintain a concrete driveway for maximum longevity. For sidewalk projects, explore concrete sidewalk repair options and sidewalk installation cost. For patios and decorative work, check out stamped concrete cost and design ideas.