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Structural and base materials · Concrete & Cement

Reinforced Concrete (RC)

Explains the composite idea behind reinforced concrete, why the position of the steel and the depth of cover over it matter, and why sizing and adequacy stay with an engineer.

Typical role:StructureSubstrate and base
Commonly met in:Foundations and groundworksStructural frameFloorsExternal walls

Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.

Overview

What reinforced concrete is

Reinforced concrete is not a different concrete but a composite: ordinary concrete with designed reinforcement, typically steel bar or mesh, placed inside it. Concrete resists compression well and tension poorly, so plain concrete cracks and fails where a member is bent or pulled. The steel is positioned to take those tensile forces, and the two materials act together because they bond and expand at broadly similar rates. Fibres dispersed through a mix are a different thing, described under fibre-reinforced concrete rather than here.

Because reinforcement only does its job where the tension actually occurs, position governs everything. Bar size, spacing, laps, anchorage and the depth of concrete cover over the steel are all set by calculation for the specific member and its loads. Reinforcement placed in the wrong face of a slab or beam contributes very little, which is why fixing is checked before concrete is ordered.

Cover has a second job. The alkaline concrete around the steel protects it from corroding, and where that cover is thin, cracked or carbonated, moisture and chlorides can reach the bars. Corroding steel expands and pushes the surrounding concrete off, which is the mechanism behind spalling seen on older exposed structures. Durability and structure are therefore tied together rather than separate topics.

Terminology

Common names and aliases

These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.

  • RC
  • Steel-reinforced concrete
  • Rebar concrete

Applications

Common applications

Where this material is typically encountered. A typical application is not a statement that it suits your project.

  • Foundations, ground beams, rafts and pile caps formed in position below the building.
  • Suspended floor slabs and beams spanning between supports in frames and extensions.
  • Retaining walls and basement walls resisting earth and water pressure.
  • Columns, cores and shear walls carrying vertical and lateral loads in framed buildings.
  • Lintels and support members cast in place over openings in walls.
  • Ground slabs where mesh is used for crack control rather than for load.

Consideration

Where it is commonly considered

  • Where a member has to span, cantilever or resist bending rather than only sit in compression.
  • Where earth or water pressure acts on one face of a wall and has to be resisted.
  • Where a slab or beam has been designed by an engineer for a defined loading.
  • Where an element must be formed in position and tied into surrounding construction.
  • Where shrinkage and movement cracking need distributing rather than concentrating in one place.

Professional review

Where additional professional review is especially important

These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.

  • Anywhere the loading, span or support conditions have not been assessed by a qualified engineer.
  • Where cover cannot be maintained because of congested bars, thin sections or poor fixing.
  • In aggressive or chloride-rich exposure without the durability implications being considered.
  • Where existing reinforced elements are to be cut or cored without knowing where the steel runs.
  • Where reinforcement detailing at joints, laps and corners has not been drawn by the designer.

Properties

Key properties to discuss

Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.

Installation dependency
Fixing reinforcement to the drawing is a distinct trade operation. Spacers hold the cage away from the formwork, ties hold it in position during pouring, and vibration has to compact concrete around the bars without displacing them.
Documentation
Bar bending schedules and reinforcement drawings describe exactly what goes where, and they are the record any future alteration would depend on. Keeping them with the property information at the time is worth doing.
Moisture behaviour
Moisture reaching the steel is what drives corrosion. Cracks, thin cover and water standing on horizontal surfaces are the usual routes, and assessment of an affected element belongs to a qualified professional rather than to a reference page.
Weathering and exposure
Externally exposed reinforced elements weather at the surface, and carbonation slowly reduces the protection the concrete gives the steel. Behaviour varies with mix, cover and exposure and cannot be generalised from one situation to another.
Repairability
Repairing a corroded or damaged reinforced element is a specialist field involving breaking out to sound concrete, treating the steel and reinstating cover. What is appropriate in a given case follows from a structural assessment.
Fire-related questions
Behaviour in fire depends on the whole element, its cover and its design, and is a matter for the designer's information and a qualified professional rather than something a general reference can state.
Substrate dependency
What sits beneath a reinforced slab, including ground conditions, sub-base, membranes and insulation, forms part of how the element behaves and has to be prepared before the reinforcement is set out.

Exposure

Exposure and environmental conditions

  • Is this element exposed to weather, ground water, salts or a chemically aggressive environment?
  • Has the exposure been discussed with the designer when setting cover and choosing the mix?
  • Is water able to stand on any horizontal surface, and where does it drain away to?
  • Are there points where cracks are likely to open and let moisture reach the steel?
  • Does the element sit partly below ground where drainage detailing interacts with it?

Assembly

Substrate and system dependencies

What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.

  • Formwork has to hold position and resist the pressure of wet concrete without deflecting or leaking.
  • Spacers and chairs control cover, and the wrong ones or too few will change what is achieved.
  • Continuity between elements depends on starter bars and laps being placed in advance.
  • Service penetrations, ducts and box-outs have to be coordinated with the reinforcement layout.
  • Any bearing, connection or dowel to adjoining construction forms part of the structural design.

Appearance

Appearance and finish considerations

  • Reinforcement can show as faint lines on a soffit where cover is minimal or where the mix bled.
  • Form-tie positions, joints and panel marks stay visible on elements left as struck.
  • Rust staining on the face is a visual symptom that usually points to something behind the surface.
  • Where a fair-faced result is wanted, the formwork, release agent and pour sequence all need agreeing beforehand.
  • Later drilling and fixing into the face is constrained by where the bars run inside the element.

Durability

Durability questions

Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.

  • What cover has been specified here, and how is it being checked before the pour?
  • How was the exposure classified when the mix and the cover were decided?
  • What crack control is intended, and which cracks are expected as normal behaviour?
  • Is a protective treatment or coating part of the design for the exposed faces?
  • How would the element be inspected in future, and what would be looked for?

Upkeep

Maintenance and repair considerations

  • What signs would suggest the reinforcement is corroding rather than the surface simply weathering?
  • Who should assess cracking or spalling on a reinforced element?
  • Is there a record of where the reinforcement runs, in case of future drilling or coring?
  • Are joints and sealants around the element part of an inspection routine?
  • What would a repair involve, and would it need temporary support while it was done?

Installation

Installation dependencies

What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.

  • Reinforcement is normally inspected against the drawings before formwork is closed and concrete ordered.
  • Compaction has to reach around and between bars, which becomes difficult in congested areas.
  • Pour sequence and construction joints are agreed so that continuity is not compromised.
  • Cutting or bending bars on site to make something fit changes what the engineer designed.
  • Curing matters more where cover is thin, because early surface drying affects the protective layer.

Documentation

Documentation to request

Ask for these in writing, for the specific product being proposed, and keep them with the project record.

  • The structural engineer's reinforcement drawings and bar bending schedules.
  • The concrete mix information supplied for the element and its intended exposure.
  • Records of any pre-pour reinforcement inspection that was carried out.
  • As-built information showing where reinforcement runs, kept for future reference.
  • Any specification for cover, spacers and tolerances that the designer has set.

Conversations

Questions for qualified professionals

Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.

  • Who is designing this element, and what loading has been assumed?
  • Where does the tension occur in this member, and which face carries the main steel?
  • What cover is required here, and how will it be verified before the pour?
  • Has the exposure condition been considered alongside the structural design?
  • Where are the construction joints, and how does reinforcement continue through them?
  • If we later want to drill or form an opening, what would we need to know first?
  • Is temporary propping needed, and for how long after the pour?
  • Who inspects the reinforcement, and is that inspection recorded anywhere?

Blind spots

Commonly overlooked points

  • Mesh in a domestic ground slab is often there for crack distribution rather than to make the slab structural.
  • Reinforcement in the wrong face of a slab does very little, and the error is invisible once concrete is placed.
  • Cover is a durability requirement as much as a structural one, and it is easily lost by careless fixing.
  • Coring or chasing an existing reinforced element without scanning risks cutting bars that are carrying load.
  • Fine cracks are not automatically a defect, but telling normal from significant is a professional judgement.

What this page does not do

  • Sizing, spacing, cover and the adequacy of any reinforced element are engineering decisions for a qualified professional.
  • Nothing here indicates that a reinforced element is sound or suitable, which only a qualified engineer assessing the specific case can address.
  • This page gives no guidance on assessing existing cracks, spalling or corrosion, which needs investigation on site.
  • Build Design Hub does not design, test, certify or endorse structures or any part of them.

Component forms

Building components commonly formed from this material

Elements this material is commonly used to make, named by the position they occupy in an assembly rather than by what they are bought as. This states common practice only — it is not a statement that this material suits any of these components in a particular building.

Backing Wall →Concrete backing gives a substrate that will take heavily loaded fixings.Beam →Cast and precast concrete beams are encountered throughout framed construction.Cantilever →Cast slabs and beams continuing past their support are among the most commonly met cantilevered elements.Coffer →In a coffered or waffle slab the recesses are formed in the concrete itself, so the coffer and the structure are the same element.Column →Cast or precast concrete columns are met across a wide range of building types.Drainage Sump →Cast sumps are formed as part of a concrete slab or basement structure.External Step →Cast steps are used where the flight is large or where the ground beneath is being spanned rather than bedded on.Fence Post →Concrete posts are widely used with slotted panels; they remove the ground-line decay question while introducing handling and fixing constraints of their own.Topping →Structural toppings that tie precast units together are reinforced concrete.Ground Beam →Cast reinforced concrete is the form ground beams are most commonly met in.Chamber →Cast chambers are used where depth or loading requires it.Kicker →Where reinforcement passes through the upstand into the element above, the kicker is part of a reinforced member rather than a plain block of concrete.Light Well →Cast walls are common where the well is formed as part of a concrete basement structure.Head →Cast or precast concrete carries the wall above in many forms of construction.Parapet →Concrete parapets are met in framed buildings, cast with or fixed to the structure.Pile Cap →Caps are almost always encountered as cast reinforced concrete.Pile Shaft →Cast or precast concrete shafts are the most commonly encountered form.Plant Plinth →Cast plinths are met where the base is formed with the structure beneath it and delivers its load directly into it.Tank Wall →A reinforced concrete shell is the form most commonly met where the structure itself contains the water.PT Anchorage →The position is a region of the cast member rather than a fitting alone, which is why it is described as concrete and reinforcement together.Thickening →A thickening is part of the reinforced concrete raft and is cast continuously with it.Ramp Flight →The inclined face of a cast flight is formed with it rather than laid on it.Retaining Stem →Cast reinforced concrete is the most commonly encountered form for engineered retaining stems.Ring Beam →Cast in situ reinforced concrete is the form most commonly met.Shear Wall →Cast reinforced concrete walls and cores are the most commonly encountered form.Silt Trap →Cast traps are used where size or loading requires it.Landing →Cast landings are the common form where the stair is part of a concrete structure and the flights are cast too.Stair Riser →In a cast or precast flight the riser is a formed face of the flight rather than a fitted board.Tread →In a cast or precast flight the tread is not a separate piece at all; it is the formed upper face of the flight.Bearing →Padstones and spreader pieces at bearings are commonly precast or cast concrete.Structural Deck →A cast or precast concrete slab is itself the structural deck.Transfer Beam →Deep reinforced concrete transfer beams and walls are encountered in framed construction.Winder →In a cast flight the winders are formed with the turn rather than fitted into it as separate pieces.

Related entries

Alternatives to consider

Entries that may be considered instead of this one for a similar job. Not a statement that they are interchangeable, or equal in any respect.

Concrete block →Cast concrete masonry units, solid or hollow, used for inner leaves, partitions, substructure and retaining work where a dense and heavy unit is wanted.Gabion walling →Wire mesh cages filled with stone and stacked to form walls, used mostly in external works and landscape for retaining, boundaries and free-draining structures.Structural steel →Hot-rolled steel beams, columns and hollow sections used for primary structure, fabricated off site and connected on site by bolting or welding.

Related entries

Common comparisons

Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.

Fibre-reinforced concrete →Concrete carrying short fibres dispersed through the mix, used mainly to control cracking, and not a replacement for reinforcement a designer has calculated.Glassfibre-reinforced concrete →A thin, light, mouldable cement and sand composite reinforced with alkali-resistant glass fibres, used for panels and features rather than as a structural concrete.

Related entries

Often used alongside

Entries commonly encountered together with this one. Co-occurrence only — never a claim that any combination is compatible or suitable.

Bentonite Panel →Natural swelling clay held between geotextiles or bonded to a carrier, which forms a dense low permeability gel when it hydrates under restraint and closes minor defects as it does so.Cementitious tanking slurry →A rigid cement-based coating bonded to masonry or concrete, used only as one element of a scheme designed by a suitably qualified waterproofing specialist.Damp-proof membrane →A continuous sheet or liquid-applied moisture-control layer associated with floors and slabs, and a different product from the course built into walling.Pre-Applied Bonded Membrane →A composite sheet made for concrete to be cast directly against it, developing an adhesive bond so that water cannot travel along the interface between sheet and structure.Profiled metal decking →Cold-formed profiled steel sheet spanning between supports as permanent formwork to a concrete floor, or as a structural roof deck carrying the build-up above it.Repair mortar →Applied onto the prepared face of an existing element rather than forming part of it.Sprayed concrete →Concrete projected onto a face at speed rather than poured into formwork, compacting on impact, and used where forming a surface would be impractical.Void former →The former is placed beneath or against the element and the concrete is cast over it; it is not part of the element itself.Waterstop →A profiled, hydrophilic or injectable strip cast into concrete joints to interrupt a leakage path inside the structure that no surface-applied membrane can reach.

Related entries

Commonly made using

Entries commonly incorporated into this one. Not a complete bill of materials, and not a statement that any of them suits a particular build-up.

Concrete aggregate →The graded stone and sand that makes up most of a concrete or mortar mix by volume, where grading, shape, cleanliness and absorption decide how the mix behaves.Portland cement →The hydraulic binder that sets concrete, mortar, render and screed. It is one ingredient alongside aggregate and water, not the finished material itself.Recycled aggregate →Mineral aggregate recovered from crushed concrete, demolition arisings or industrial by-products, described by where it came from rather than by what it is.Spacers and chairs →Moulded plastic, cementitious or wire accessories that hold reinforcement at its intended position while concrete is placed, and stay cast into the element.Steel reinforcing bar →Ribbed steel bar cast into concrete to carry tensile forces, protected from corrosion by the alkaline environment of the sound concrete cover surrounding it.Welded reinforcement fabric →Cast into the concrete as the tensile element of the finished composite, in the areas of a construction rather than in its designed members.

Inspiration

Related Ideas Library pages

Design directions where this material commonly appears.

Preparation

Related planning checklists

Owner-side preparation before the conversation where this material comes up.

Go deeper

Related Build Design Hub guides

Planning guidance and neutral comparisons behind the decisions on this page.

Concrete & Cement Materials

Reference entries for cement-based materials — ready-mixed and precast concrete, screeds, levelling compounds, renders of the cementitious family and the admixtures that modify them.

Browse all concrete & cement entries →