Concrete is an engineered stone made by mixing cement, water and aggregates, then allowing chemical reactions to bind the mixture into a hard mass. Search questions such as “what is concrete made of?”, “how does concrete harden?”, “what is the difference between cement and concrete?”, “why is concrete reinforced with steel?”, “why does concrete crack?” and “how long does concrete take to cure?” all point to the same first-principles system: cement reacts with water to form new solid products that glue sand and stone together, while the final performance depends on proportions, mixing, placement, compaction, curing, reinforcement, environment and time.

The simplest model is not “concrete dries and becomes rock.” Ordinary Portland cement hardens mainly because of hydration, a family of chemical reactions between cement minerals and water. Those reactions create binding phases, especially calcium-silicate-hydrate, that fill space and connect particles. Water is therefore both a reactant and a practical ingredient that makes fresh concrete workable. Too little water can make placement difficult and leave voids; too much water can increase porosity and reduce strength after the excess water leaves. This tension between workability and long-term density sits at the centre of concrete technology.

This guide explains concrete from raw materials to decades of service. It covers cement manufacture, clinker minerals, water-cement ratio, sand and coarse aggregate, admixtures, supplementary cementitious materials, mixing, slump, pumping, compaction, hydration, setting, curing, compressive and tensile strength, reinforced concrete, prestressing, shrinkage, creep, cracking, corrosion, carbonation, chlorides, freeze-thaw damage, sulphate attack, quality control, repair, sustainability and common misconceptions. The goal is to make a familiar grey material legible as a chemical, structural and construction system.

Concrete is not cement

Cement is the fine reactive powder that acts as the binder. Concrete is the composite material formed when cement paste binds aggregates such as sand and gravel or crushed stone. Mortar is another related material, usually made from binder, water and fine aggregate without the coarse aggregate typical of concrete.

The distinction matters because aggregates make up most of concrete’s volume. They are not cheap filler with no technical role. Good aggregates provide a stiff skeleton, reduce the amount of paste required, influence workability, thermal movement, shrinkage and strength, and affect durability. A cubic metre of concrete is therefore better imagined as tightly packed particles of several sizes whose voids are filled and bonded by cement paste.

Where cement comes from

Ordinary Portland cement is manufactured mainly from calcium-rich and silica-, alumina- and iron-bearing raw materials. Limestone is a major source of calcium carbonate. Clay, shale or other materials supply silica and alumina. The raw feed is ground and heated in a rotary kiln to temperatures high enough to drive chemical transformations and partially melt the material.

In the kiln, limestone decomposes, releasing carbon dioxide and leaving calcium oxide. At higher temperatures, calcium compounds combine with silica and other oxides to form clinker minerals. The resulting nodules are cooled and ground with a small amount of gypsum, which helps control the rapid reactions responsible for setting.

Cement manufacture therefore uses energy and releases carbon dioxide both from fuel and, importantly, from the chemical decomposition of limestone. This is why reducing the environmental impact of concrete requires attention not only to electricity or transport but also to clinker chemistry and how much clinker is used.

What happens when cement meets water

As soon as water contacts cement, ions begin dissolving from the particle surfaces. Reactions then form new hydration products. The most important binding phase is commonly abbreviated C-S-H, for calcium-silicate-hydrate. It is not one perfectly fixed crystal but a complex nanoscale material that grows around and between cement grains. Calcium hydroxide and other phases also form.

The microstructure evolves from a suspension of grains in water into a connected solid network. At first the mixture remains plastic enough to place. It then loses fluidity and sets. Strength continues developing as hydration proceeds and the internal structure becomes denser.

Hydration can continue for a long time if unreacted cement and suitable moisture remain. Concrete does not suddenly become “fully cured” at one universal age. The familiar 28-day strength is a standard reference point used for testing and design, not a magical completion date.

Setting, hardening and curing are different ideas

Setting is the transition from workable plastic material toward a rigid skeleton. Hardening is the continuing development of mechanical strength after setting. Curing means controlling moisture and temperature so hydration and microstructure development can proceed under favourable conditions.

This vocabulary prevents a common mistake. Concrete can set while still weak. It can harden while losing surface moisture too quickly. It can appear dry while chemical reactions inside are still continuing. Good construction practice therefore protects young concrete instead of assuming that once it looks solid the important work is finished.

The water-cement ratio: one of the most powerful variables

The water-cement ratio compares the mass of mixing water with the mass of cementitious binder. In ordinary concrete, lower water-cement ratios generally enable higher strength and lower permeability when the concrete is properly compacted and cured. The reason is microstructural. Water needed for workable placement occupies space. Water that is not consumed or retained in hydration products can leave capillary pores as concrete hardens. More connected pores make it easier for fluids and aggressive ions to move through the material.

But simply removing water is not a free improvement. A very dry mixture can be difficult to place around reinforcement and may trap air voids. Modern water-reducing admixtures help resolve this trade-off by improving flow without requiring as much extra water.

Worked example: why adding water on site can weaken concrete

A truck arrives with concrete that seems too stiff. Someone adds extra water so it flows easily. Placement becomes faster, so the decision appears helpful. But the additional water raises the water-cement ratio. After hardening, more capillary space remains and strength and durability may fall. The correct solution may instead involve an approved admixture, a mix designed for the needed slump, or better planning of transport and placement. Workability is a real requirement, but uncontrolled water addition solves today’s handling problem by creating tomorrow’s material problem.

Aggregates: the skeleton of concrete

Fine aggregate is usually sand-sized material; coarse aggregate consists of larger gravel or crushed rock. Particle grading matters because a well-graded combination packs efficiently. If particles are all one size, large voids remain between them and more paste is required to fill those voids. A range of sizes can fit together more densely.

Aggregate shape affects workability. Rounded gravel can flow more easily than highly angular crushed stone, while angular particles may interlock differently and influence strength. Surface texture affects the bond between paste and aggregate. Absorption matters because porous aggregates can take up mixing water. Cleanliness matters because clay, organic contamination or weak particles can interfere with performance.

Some aggregates can react chemically with the alkaline pore solution in concrete. Alkali-silica reaction, for example, can form an expansive gel when susceptible silica, sufficient alkali and moisture coexist. Aggregate selection is therefore not merely a question of size.

Fresh concrete and workability

Before hardening, concrete must be mixed, transported, placed and consolidated without segregating. Workability describes how easily this can be done. The slump test is a common field measure: fresh concrete is placed in a standard cone, the cone is lifted, and the amount the concrete subsides gives a practical indication of consistency.

Slump is useful but not a complete description of rheology. Two mixes can show similar slump yet pump differently because particle interactions, paste volume and admixtures differ. Self-compacting concrete can flow through congested reinforcement with little vibration while remaining stable against segregation. High-performance construction therefore treats fresh concrete as a flow system, not simply as “wet” or “dry.”

Admixtures: changing behaviour with small doses

Chemical admixtures are ingredients added in relatively small amounts to alter concrete behaviour. Water reducers improve flow or allow lower water content. Superplasticisers can create highly fluid concrete at low water-cement ratios. Retarders delay setting, useful in hot weather or long transport. Accelerators speed early reactions when rapid strength is needed. Air-entraining agents deliberately create tiny stable air bubbles that improve resistance to freeze-thaw cycles in cold climates.

Admixtures interact with cement chemistry, temperature and dosage. More is not always better. Excess retarder can delay hardening too much; poor air control can reduce strength. Concrete technology therefore depends on tested combinations rather than isolated ingredient folklore.

Supplementary cementitious materials

Concrete can include materials such as fly ash, ground granulated blast-furnace slag, silica fume, calcined clay and natural pozzolans, depending on local availability and specifications. These materials can replace part of Portland cement and alter workability, heat release, strength development, permeability and chemical resistance.

Pozzolanic materials react with calcium hydroxide in the presence of water to form additional binding phases. Slag has latent hydraulic properties and can participate in cementitious reactions when activated by the cement system. The result can be a denser microstructure and lower clinker content, though early strength may develop differently.

Design therefore considers the whole binder system rather than treating every kilogram of powder as identical.

Mixing and transport

Concrete ingredients must be distributed uniformly. Batch plants weigh cement, water, aggregates and admixtures according to a designed recipe. Moisture in aggregate must be considered because wet sand already contributes water to the batch. Ignoring this can shift the actual water-cement ratio.

Truck mixers keep concrete agitated during transport. Time and temperature matter because hydration begins once water and cement meet. Long delays can reduce workability and complicate finishing. Construction logistics are therefore part of material quality. A perfect laboratory mixture can perform poorly if it sits in traffic, segregates during discharge or reaches the formwork after its workable period.

Placement and compaction

Fresh concrete is placed into formwork around reinforcement and embedded components. If trapped air is not removed, voids called honeycombing can form. Internal or external vibration temporarily reduces internal friction and allows the concrete to flow around bars and into corners.

Vibration must be controlled. Too little leaves voids. Excessive vibration of an unstable mix can cause segregation, with heavy aggregate settling and paste rising. The goal is dense, uniform concrete, not maximum vibration time.

Worked example: honeycombing behind reinforcement

A column form is removed and coarse stones are visible with large cavities near a congested reinforcement cage. Painting over the surface hides the appearance but not the defect. Possible mechanisms include low workability, blocked flow paths, inadequate vibration or leakage of paste through formwork gaps. Repair should begin with assessing depth and structural significance, then addressing the process cause so the next pour does not repeat it.

Curing: protecting young concrete

Young concrete needs moisture and a suitable temperature for hydration. If its surface dries quickly under sun, wind or low humidity, water can evaporate faster than it is replaced from within. The surface may shrink while the interior remains wetter, producing plastic shrinkage cracks. Later, inadequate moisture can limit hydration and leave a more porous surface zone.

Curing methods include water ponding, wet coverings, plastic sheets, curing compounds and formwork retention. The appropriate method depends on member geometry, climate and specification. Curing is not cosmetic aftercare. It is part of manufacturing the material in place.

Why concrete is strong in compression but weak in tension

Concrete can carry large compressive stresses because its mineral skeleton and hydrated binder resist being squeezed. In tension, microscopic flaws open and cracks grow more readily. Tensile strength is therefore much lower than compressive strength.

Structural members under bending experience both. A simply supported beam loaded downward is compressed near its top and tensioned near its bottom. Plain concrete would crack and fail relatively early on the tension side. Steel reinforcement is placed where tensile forces need to be carried.

How reinforced concrete works

Reinforced concrete combines concrete and steel so each material contributes where it is effective. Concrete carries compression and protects embedded steel; reinforcement carries tension and controls cracks. The two materials act together because bond transfers stress along the interface.

Steel and concrete also have sufficiently similar thermal expansion over ordinary temperature ranges that they can move together without severe internal stress in normal service. Reinforcing bars have deformations along their surface to improve mechanical interlock with concrete.

Worked example: a reinforced concrete beam

Imagine a beam spanning between two supports. A load in the middle bends it downward. Concrete at the top shortens; material at the bottom wants to elongate. Cracks form in the bottom concrete once tensile capacity is exceeded, but steel bars crossing those cracks carry the tensile force. The cracked concrete still separates and protects the reinforcement, while the uncracked compression zone near the top carries compression. Structural design predicts this cracked behaviour rather than pretending reinforced concrete remains uncracked forever.

Prestressed concrete

Prestressing deliberately puts concrete into compression before service loads create tension. High-strength steel tendons are tensioned either before concrete is cast, called pretensioning, or after the concrete has hardened, called post-tensioning. The compressive force counteracts later tensile stresses.

This allows longer spans, thinner sections and improved crack control. Prestressed bridge girders, floor slabs, tanks and many precast products use this principle. The engineering is sophisticated because designers must account for tendon losses, anchorage, creep, shrinkage and long-term behaviour.

Why concrete cracks

Cracking has many mechanisms. Plastic shrinkage can occur before full hardening when surface evaporation is rapid. Drying shrinkage occurs as hardened concrete loses moisture and contracts. Thermal cracking can occur when different parts of a member heat or cool unequally. Structural overload can create flexural, shear or tensile cracks. Settlement cracks can develop around restraints. Chemical expansion can also cause cracking.

The statement “concrete always cracks” contains some truth but is not a complete engineering answer. The important questions are where the crack is, how wide it is, what direction it follows, when it appeared, whether it is active and what mechanism produced it. Many reinforced concrete structures are designed to tolerate controlled cracking without losing serviceability or safety.

Shrinkage and restraint

Concrete tends to shrink as it dries and as hydration and other processes change its internal volume. If it were completely free to shrink, stress could remain low. In real structures, foundations, reinforcement, adjoining pours and geometry restrain movement. Restraint converts shrinkage strain into tensile stress. When tensile stress exceeds the concrete’s capacity, a crack forms.

Control joints in slabs deliberately create weakened planes or planned locations for contraction. Reinforcement distributes cracking into narrower widths. Mix design, curing and placement sequence also influence outcome.

Creep: slow deformation under sustained load

Concrete under a sustained compressive load continues deforming gradually over time, even when the load does not increase. This time-dependent behaviour is called creep. It is influenced by age at loading, humidity, member size, aggregate stiffness, paste content and stress level.

Creep matters in tall buildings, bridges and prestressed structures because long-term deflection and prestress losses can become significant. A beam that looks acceptable immediately after construction can sag more over years if creep and shrinkage were not included in design.

Concrete strength and why 28 days is used

Concrete strength is commonly measured by compressing standard cylinders or cubes until they fail. Specifications often refer to strength at 28 days because this provides a consistent age for comparison and historically captures a substantial portion of expected strength development for ordinary Portland-cement systems.

But strength can continue increasing after 28 days, especially with slower-reacting supplementary materials. Conversely, poor curing can prevent a mix from reaching its expected potential. A test result reflects the combined effects of ingredients, batching, specimen preparation, curing and testing.

Quality control: from slump to cylinders

Construction teams test fresh and hardened concrete because the material is manufactured on site under variable conditions. Slump checks consistency. Air-content tests may be important for freeze-thaw concrete. Temperature is recorded. Cylinders or cubes are cast for later strength testing. Non-destructive methods such as rebound hammers and ultrasonic pulse velocity can provide additional information, although they require interpretation and calibration.

A low strength result does not immediately prove the whole structure is unsafe. Engineers investigate sampling, curing, test procedure, batch records and in-place strength. Cores may be taken from the structure. Diagnosis proceeds from evidence rather than panic.

Why reinforced concrete corrodes

Fresh hardened concrete is strongly alkaline. This high pH normally creates a thin protective passive film on embedded steel. Corrosion risk rises when that protection is lost, often because carbonation lowers pH or because chloride ions reach the steel.

When steel corrodes, iron converts into rust products that occupy more volume than the original metal. The expanding corrosion products create tensile stresses in surrounding concrete. Cracks form parallel to the reinforcement, the cover delaminates, and pieces may spall away.

Durability therefore depends on slowing the transport of aggressive substances. Low permeability, adequate cover depth, good compaction and curing are critical. Special reinforcement, coatings or corrosion inhibitors may be used in severe environments.

Carbonation

Carbon dioxide from air diffuses into concrete pores and reacts with alkaline hydration products. This process, carbonation, gradually lowers the pH. Carbonation itself can locally densify some pore structure, but if the carbonation front reaches reinforcing steel, the passive protective environment may disappear and corrosion can begin in the presence of moisture and oxygen.

Carbonation progresses faster in porous concrete and under certain humidity conditions. Dense well-cured concrete and adequate cover slow the process.

Chlorides and marine exposure

Chloride ions from seawater, deicing salts or contaminated materials can move through concrete and disrupt the passive film on steel even if the surrounding concrete remains alkaline. Corrosion may then become highly localised.

Marine structures face repeated wetting, drying and salt exposure. Concrete mix design, cover, crack control and curing therefore become central durability tools. Repair that replaces spalled material without addressing chloride-contaminated surrounding concrete can shift corrosion to adjacent regions, so diagnosis must consider the electrochemical system.

Freeze-thaw damage

In cold climates, water in concrete pores can freeze and expand. Repeated cycles can create internal damage and surface scaling, especially when concrete is critically saturated. Air-entrained concrete contains many microscopic bubbles that provide pressure-relief spaces for freezing water.

Freeze-thaw durability therefore depends on both pore structure and exposure. Dense concrete that remains relatively dry behaves differently from saturated concrete. Air entrainment is a deliberately engineered microstructure, not accidental large air voids from poor compaction.

Chemical attack and sulphates

Some soils and groundwater contain sulphates that can react with cementitious phases, causing expansion or loss of cohesion under susceptible conditions. Acids can dissolve alkaline concrete constituents. Industrial wastewater may expose concrete to aggressive chemicals.

Material selection must match the environment. Lower permeability, suitable cement chemistry, supplementary materials, protective linings or alternative materials may be needed. “Concrete is stone, so chemicals cannot hurt it” is a dangerous misconception.

Fire and concrete

Concrete is non-combustible, which is a major fire advantage. But high temperature still changes it. Water in pores turns to vapour, thermal gradients create stress, and cement paste and aggregate expand differently. Severe heating can reduce strength and cause spalling. Dense high-strength concrete can be vulnerable to explosive spalling when steam pressure builds faster than it can escape.

Reinforcement also loses strength as temperature rises. Fire design therefore considers member thickness, cover, load level and duration, not merely whether the material burns.

Why surface finishes can fail

A concrete floor coating can blister or delaminate if moisture vapour moves upward through the slab and pressure or dissolved salts accumulate beneath an impermeable coating. A rendered surface can debond if the substrate is dusty or too smooth. Paint can flake if alkalinity or moisture conditions exceed its tolerance.

Surface failure therefore often reflects what is happening inside the concrete. Testing moisture and understanding vapour paths may be more important than choosing a stronger paint.

Repairing concrete

Concrete repair begins with cause identification. A crack caused by one-time drying shrinkage differs from a crack that continues opening because of foundation movement. Spalling caused by chloride corrosion requires treatment of steel and contaminated concrete, not merely patching the hole.

Repair materials must be compatible in stiffness, shrinkage, thermal movement and bond. Very strong repair mortar can still fail if it shrinks away from the substrate or traps moisture. Durable repair is a system: remove unsound material, treat the cause, prepare the surface, replace material appropriately and protect the repaired zone.

Common misconceptions about concrete

Misconception 1: cement and concrete are the same thing

Cement is the reactive binder. Concrete is cementitious paste plus aggregates and often admixtures and supplementary materials.

Misconception 2: concrete hardens because it dries

Concrete hardens primarily through hydration reactions. Excessive drying early in life can actually harm strength development and surface quality.

Misconception 3: more water makes better concrete

Extra water improves short-term workability but can increase porosity and reduce strength and durability.

Misconception 4: any crack means structural failure

Cracks have many causes and severities. Width, location, pattern, timing and movement must be evaluated.

Misconception 5: reinforcement prevents all cracks

Reinforcement usually controls crack widths and carries tension after cracking. It does not make concrete incapable of cracking.

Misconception 6: concrete reaches full strength in 28 days

Twenty-eight days is a standard test age. Hydration and strength development can continue beyond it.

Diagnostics: reading concrete defects

Map cracking near the surface soon after finishing: investigate plastic shrinkage, overworking, rapid evaporation or surface finishing practices. Long regular cracks in a slab: consider restrained shrinkage and joint spacing. Rust staining and cracks following rebar lines: investigate reinforcement corrosion. Honeycombed pockets: examine workability, vibration and form leakage. Dusting surface: consider weak surface paste, premature finishing, excess bleed water or poor curing. White crystalline deposits: efflorescence indicates dissolved salts carried by moisture to the surface.

The diagnostic sequence is the same used across engineering: identify the symptom, map its location, establish timing, measure severity, test plausible mechanisms and look for the environmental driver. Repair only after the cause is credible.

Practical application: ordering concrete for a slab

Suppose a small commercial floor needs concrete. The owner might focus only on compressive strength, but successful specification requires more: required slump or flow for placement, maximum aggregate size around reinforcement, exposure class, chloride limits, finishing method, curing plan and joint layout. If the floor will receive a moisture-sensitive coating, vapour control and drying expectations matter too.

The lesson is that a concrete mix cannot be separated from how it will be placed and used. The “best” mix is the one that meets structural, durability, construction and finishing requirements together.

Practical application: why curing starts immediately

After a slab is finished, evaporation begins. On a hot windy day, the surface can lose water rapidly even while the deeper concrete remains wet. Waiting until tomorrow to think about curing can therefore be too late. Evaporation control, windbreaks, fogging, coverings or curing compounds may be needed as soon as finishing permits.

This is a good example of a hidden time window in materials engineering. The first hours can influence a surface expected to last decades.

Sustainable concrete and lower-carbon design

Concrete’s global use makes its environmental impact significant. Portland cement clinker has high embodied carbon because kiln fuel is required and limestone releases carbon dioxide during calcination. Reducing clinker content can therefore matter greatly.

Strategies include supplementary cementitious materials, calcined clays, optimised aggregate grading, efficient structural design, lower material volumes, recycled aggregate where suitable, carbon-curing approaches, renewable energy in plants and longer service life. Each strategy has trade-offs. A lower-carbon mix that performs poorly and requires early replacement may not be a real improvement.

Design efficiency can be as important as recipe efficiency. A structural system that uses less concrete, places material only where forces require it, or extends building life can reduce whole-life impact. Reusing existing structures can sometimes avoid more emissions than changing cement formulation alone.

Recycled concrete aggregate

Demolished concrete can be crushed and processed into recycled aggregate. The particles often carry attached old mortar, making them more porous and absorptive than fresh natural stone. This affects water demand, density and shrinkage.

With appropriate processing and mix design, recycled aggregate can be valuable in new concrete or unbound applications. The correct use depends on quality, contamination, structural requirements and local standards. Recycling is not simply crushing old concrete and substituting it one-for-one without testing.

Concrete as a thermal material

Concrete has substantial thermal mass. It can absorb heat as temperature rises and release it later as surroundings cool. Buildings can use this property to moderate indoor temperature swings when concrete is appropriately exposed and ventilation or cooling strategies are coordinated.

Thermal mass is not insulation. Insulation slows heat flow through a building envelope; thermal mass stores heat. A thick concrete wall can still conduct significant heat unless insulation is provided. Confusing these functions leads to poor building design.

Concrete in dams, bridges and skyscrapers

Massive concrete structures illustrate how scale changes the problem. In a large dam foundation or thick raft, cement hydration releases heat faster than the core can lose it. The interior warms and expands. Later cooling can create tensile stress and cracking if the outer zone has already cooled. Mixes for mass concrete may therefore use lower-heat binders, staged placement, cooling pipes or careful temperature control.

High-rise columns may use high-strength concrete to carry enormous compression within limited floor area. Bridges may use prestressing for long spans and crack control. Marine piers prioritise chloride durability. There is no universal concrete; engineering adapts the same material family to different dominant risks.

Frequently asked questions about concrete

What is concrete made of?

Concrete is usually made from cementitious binder, water, fine aggregate, coarse aggregate and often chemical admixtures or supplementary cementitious materials.

What is the difference between cement and concrete?

Cement is a powdered binder that reacts with water. Concrete is the composite material in which that binder glues sand and stone into a hardened mass.

Why does concrete get hot while curing?

Cement hydration is exothermic, meaning it releases heat. The temperature rise is especially important in thick sections where heat escapes slowly.

Why is steel put inside concrete?

Concrete is strong in compression but relatively weak in tension. Steel reinforcement carries tensile forces and controls cracking.

Why does concrete crack?

Cracks can result from shrinkage, temperature change, restraint, settlement, overload, corrosion, chemical expansion or other mechanisms. Pattern and timing help identify the cause.

Does watering concrete make it stronger?

Keeping young concrete appropriately moist supports continued hydration and can improve strength and durability. This is curing, not the same as adding excess water to the fresh mix.

How long does concrete take to cure?

There is no single finish point. Early curing protection may be specified for days, while hydration and strength development can continue for months or longer.

Why is 28-day strength important?

It is a conventional reference age used for specification and comparison. It provides a consistent checkpoint, not a claim that all reactions stop on day 28.

Can concrete be recycled?

Yes. Old concrete can be crushed into aggregate and reused in various applications, although attached mortar, contamination and absorption affect quality.

Is concrete waterproof?

Ordinary concrete is not perfectly waterproof. Water and vapour can move through pores and cracks. Low permeability, joint design, membranes and drainage may be required for watertight structures.

Why does concrete sometimes have white stains?

Efflorescence forms when water dissolves salts inside the material, transports them to the surface and evaporates, leaving crystals behind.

Is stronger concrete always more durable?

Not automatically. Low permeability and good curing often accompany higher strength, but durability also depends on cracks, cover, exposure, binder chemistry, workmanship and specific deterioration mechanisms.

The big picture: concrete is chemistry organised by construction

Concrete is easy to underestimate because it looks inert once hard. In reality its performance is the result of a chain of decisions. Raw minerals become clinker. Clinker becomes cement. Cement and water hydrate. Aggregates create a skeleton. Admixtures control fresh behaviour. Workers place and compact the mix. Curing shapes the microstructure. Reinforcement carries tension. Cover and low permeability protect steel. Joints control movement. Maintenance interrupts deterioration before small defects become major failures.

Once the chain is visible, concrete failures become more understandable. Weak concrete may trace back to excess water or poor curing. Honeycombing points to placement and compaction. Rust cracks point to transport of carbon dioxide or chlorides and the electrochemistry of steel. Random slab cracks point to shrinkage and restraint. A long-lasting structure is not simply “strong concrete”; it is concrete whose composition, geometry, construction and environment were matched.

Useful routes for learning more

Continue with Tell Me About Buildings for how concrete fits into whole structures; Tell Me About Roads for concrete pavements and transport infrastructure; Tell Me About Bridges for loads and spans; Tell Me About Metals for the steel inside reinforced concrete; Tell Me About Chemistry for reaction fundamentals; and Tell Me About Engineering for the wider design process.