Concrete is strong in compression and weak in tension, so reinforced concrete puts steel where the tension is, in the bottom of a beam, and prestressed concrete stretches the steel first so the concrete stays squeezed under load. The steel must be covered by enough dense concrete to stay dry and protected, and the concrete must be cured and not loaded early. Lime-pozzolan concrete gains strength slowly, at only 40-60% at 28 days, so do not assume Portland-cement design rules apply.
Part of Composite Materials
Concrete composites are concrete combined with steel, bamboo or fibre so that the reinforcement carries the tension the concrete cannot.
Tension Is The Problem Concrete Cannot Solve Alone
Tensile Strength lists plain concrete at 250-400 psi, effectively zero for design, and puts masonry at 5-15 times weaker in tension than in compression. Compression Strength explains why arches, columns and vaults work: they keep the material squeezed. A beam does not. Its top is squeezed and its bottom is stretched, so a plain concrete beam cracks at the bottom and fails suddenly.
Arithmetic: 250-400 psi is 250 / 145 = 1.7 MPa to 400 / 145 = 2.8 MPa. A steel bar of 345 MPa tensile strength (the low end of mild steel) is 345 / 2.8 = 123 to 345 / 1.7 = 203 times stronger in tension, so a small area of steel carries the tensile load. Never reduce this to a single “times stronger” figure; it changes with the concrete.
Why The Two Materials Cooperate
The Advanced Materials article puts it simply: the concrete surrounds and protects the steel, and the steel carries the tension. Three physical facts make it work:
- Bond. The hardened paste grips the steel. Ribbed or twisted bars bond far better than smooth ones.
- Similar expansion. Steel and concrete expand by nearly the same amount with temperature, so heating does not break the bond.
- Protection. Lime and cement paste are alkaline and keep embedded steel from rusting, while it stays damp-free and the alkalinity lasts.
Where To Put The Steel
Put it in the tension zone: the bottom of a simply supported beam, the top over a support, and across a cracking plane. Keep a layer of dense concrete, called cover, over the bars. Too little cover and moisture reaches the steel; rust is bulkier than the steel it replaces, which cracks and spalls the cover and exposes more steel. For the same reason, compact the concrete well, as Concrete Making instructs for placing in 10-15 cm layers.
Prestressing Starts With Stretching The Steel
In prestressed concrete the steel is pulled tight and anchored so it squeezes the concrete before any load arrives. Under load the squeeze is first relieved before any tension appears, so the concrete never cracks in service. It needs high-strength steel that can be stretched a long way without yielding, and good anchors, because some stretch is always lost. A first settlement should not attempt it. Use arches, vaults and compression forms.
Honest Limits For A Lime-Based Settlement
Concrete Making covers lime concrete, which reaches 40-60% strength at 28 days, may need 3-6 months for full strength, and is the only type achievable without industrial kilns. Roman Concrete notes the Pantheon dome stands unreinforced. Three consequences:
- Do not load for at least 28 days, as the corpus says, and do not remove props until strength has been proven.
- Lime-based concrete is less strong and may protect steel less well than modern cement. Use generous cover, and test.
- Bamboo, as the Advanced Materials article suggests, swells when wet and shrinks when dry, breaking the bond, so keep it for low-load work and seal it.
Reinforced concrete fails without warning when loaded early
A beam or slab that looks hard after a week may have under a quarter of its eventual strength, and a collapse during formwork removal kills. Keep props in place until test cylinders or a trial beam have proved the strength, and never stand under a spanning slab being loaded.
Common Mistakes
- Steel in the wrong place — at the compression face it does little.
- Too little cover — the steel rusts and bursts the concrete.
- Early propping removal — the most common cause of collapse.
What’s Next
- Learn next: Cermet — particles of hard ceramic in metal
- Combine with: Safety Factors — sizing for uncertainty