How Concrete Works: The Chemistry Behind the World's Most Used Material - Part 2

Strength gain rate depends on cement composition and fineness. High early strength cements with more alite and finer grinding achieve 70% of ultimate strength in 7 days but gain little afterward. Low heat cements with more belite gain strength slowly but continuously. Blended cements with fly ash or slag have slower initial gain but superior long-term strength. Understanding these patterns enables optimization for specific applications. Microstructural evolution explains strength development. Early hydration creates loose, poorly connected gel. Continued hydration densifies gel, strengthens bonds, and fills pores. Large pores segment into smaller ones. The solid phase transitions from discrete particles to continuous network. Crystalline phases grow and interlock. These changes occur over years, explaining concrete's evolving properties. Environmental conditions affect long-term strength. Underwater concrete continues hydrating indefinitely, potentially achieving twice its 28-day strength. Dry concrete stops gaining strength when internal humidity drops. Carbonation—reaction with atmospheric CO₂—can increase or decrease strength depending on conditions. Temperature fluctuations cause microcracking that limits strength gain. Optimal conditions rarely exist in practice. Testing reveals strength development patterns. Cylinder tests at 3, 7, 28, and sometimes 56 or 90 days track early strength. Core samples from structures show actual long-term strength. Non-destructive tests like rebound hammer or ultrasonic pulse velocity estimate strength without damage. These data inform maintenance decisions and validate design assumptions. ### Durability Mechanisms: Chemical Attack and Physical Deterioration Concrete durability—the ability to resist deterioration—determines structure lifespan more than strength. While concrete can last millennia under favorable conditions, aggressive environments cause premature failure. Understanding deterioration mechanisms enables durable concrete design and effective maintenance strategies. Carbonation occurs when atmospheric CO₂ reacts with calcium hydroxide, forming calcium carbonate and lowering pH from 13 to 8. This seemingly benign reaction has serious consequences—pH reduction destroys the passive oxide layer protecting reinforcing steel from corrosion. Carbonation proceeds slowly—typically 1-5 mm per year—but accelerates in low-quality concrete. Once carbonation reaches reinforcement, corrosion begins. Chloride attack, primarily from seawater or deicing salts, causes most reinforced concrete deterioration. Chlorides penetrate concrete through pores and cracks, eventually reaching reinforcement. At sufficient concentration (0.4% by cement weight), chlorides break down steel's passive layer, initiating corrosion. Rust expansion—up to 600% volume increase—cracks and spalls concrete. Marine structures and bridge decks suffer severe chloride damage. Sulfate attack occurs when external sulfates react with aluminate hydrates, forming expansive ettringite. This expansion cracks concrete from within. Sulfates come from soil, groundwater, or seawater. Severity depends on sulfate concentration, concrete permeability, and aluminate content. Sulfate-resistant cements limit aluminate content, while supplementary materials like fly ash bind aluminates in non-reactive forms. Alkali-silica reaction (ASR) is concrete cancer—a slow reaction between cement alkalies and reactive silica in aggregates, forming expansive gel. This gel absorbs water, swelling and cracking concrete. ASR requires three conditions: reactive aggregates, high alkalinity, and moisture. Eliminating any factor prevents ASR. Low-alkali cements, non-reactive aggregates, or supplementary materials prevent this devastating reaction. Freeze-thaw damage occurs when water in concrete pores freezes, expanding 9% and creating pressure. Repeated cycles progressively damage concrete. Air entrainment provides pressure relief, dramatically improving freeze-thaw resistance. Reducing permeability limits water saturation. Salt use accelerates freeze-thaw damage through osmotic pressure and increased saturation. Proper air entrainment and low permeability ensure freeze-thaw durability. Physical erosion from water, wind, or traffic gradually wears concrete surfaces. Abrasion resistance depends on aggregate hardness, paste strength, and surface finishing. Hydraulic structures experience cavitation erosion from vapor bubble collapse. Industrial floors suffer impact and abrasion. Hard toppings, surface treatments, or high-strength concrete improve erosion resistance. ### Reinforced Concrete: The Composite Marvel Reinforced concrete combines concrete's compressive strength with steel's tensile strength, creating a composite that revolutionized construction. This marriage of materials works because concrete and steel have similar thermal expansion coefficients and concrete's alkalinity protects steel from corrosion. Understanding reinforcement principles explains modern construction's possibilities and limitations. The compatibility between steel and concrete is remarkably fortunate. Their thermal expansion coefficients differ by only 15%—close enough to avoid significant stress under temperature changes. Concrete's high pH (12-13) passivates steel, forming a protective oxide layer preventing corrosion. Concrete bonds to steel through adhesion, friction, and mechanical interlocking with ribs. These properties enable effective composite action. Reinforcement placement follows structural engineering principles. Steel resists tension where concrete would crack. In beams, bottom reinforcement resists sagging moments while top reinforcement handles negative moments over supports. Columns use vertical bars for compression and ties for confinement. Slabs employ meshes or bars in both directions. Proper placement is critical—misplaced reinforcement causes structural failure. Concrete cover over reinforcement balances conflicting requirements. More cover improves corrosion protection and fire resistance but reduces structural efficiency. Typical covers range from 20mm for interior elements to 75mm for marine structures. Cover must exceed maximum aggregate size for proper consolidation. Inadequate cover is the leading cause of premature deterioration in reinforced concrete. Prestressing takes reinforcement further by pre-compressing concrete before loading. Post-tensioning applies force after concrete hardens using high-strength steel tendons in ducts. Pre-tensioning stretches tendons before concrete placement. Prestressing enables longer spans, thinner sections, and crack control. However, it requires specialized expertise and equipment, increasing cost and complexity. Fiber reinforcement distributes reinforcement throughout concrete volume. Steel fibers improve toughness and crack control. Synthetic fibers reduce plastic shrinkage cracking. Glass fibers increase tensile strength. Carbon fibers provide exceptional strength and stiffness. Fiber reinforcement can replace traditional reinforcement in some applications or enhance it in others. Each fiber type offers unique benefits and challenges.