What if concrete had no steel inside?

Concrete is the hard gray stuff in sidewalks, walls and bridges.

Inside many concrete bridges there are steel bars. You cannot see them. What if we left the steel out? Would the bridge still hold up a truck?

Concrete is everywhere: sidewalks, walls, tunnels and bridges. Britannica describes it as a kind of artificial stone, made by mixing water, sand, gravel and a binder such as cement. But concrete in a bridge is usually not just concrete. It has steel bars buried inside it, and then it is called reinforced concrete.

Why go to the trouble? In this lab, a 20-tonne truck stops in the middle of a beam bridge 20 meters long. Swap a steel beam for a beam of plain concrete with no steel inside, and see what the model says.

Britannica defines reinforced concrete as concrete with embedded steel (rods, bars or mesh) arranged so that the two materials act together, the steel taking the tensile and shear stresses and sometimes compressive ones too. It notes that plain concrete does not easily withstand tensile and shear stresses and is therefore unsuitable for most structural uses.

This lab tests that claim with a beam model. The geometry stays fixed (a solid beam 20 m long, 1 m deep and 0.5 m wide, supported at both ends, with a 20 t truck at midspan), and only the material changes, from structural steel to plain concrete. Which property decides the result: stiffness, weight or strength in tension?

When a truck drives onto a bridge, the bridge bends a tiny bit.

The top of the beam gets squashed. The bottom of the beam gets stretched.

Britannica explains that a beam carries a load by bending. As it bends, the top of the beam is squeezed, which engineers call compression, and the bottom is stretched, which they call tension.

Start the lab with a steel beam. Under the 20-tonne truck it sags only about 14 millimeters, and the model's safety factor is about 7: the steel could take about seven times the stress it actually feels. Britannica notes that steel is strong in both tension and compression, so the stretched bottom of the beam is no problem.

In a simply supported beam, bending puts the top fibers in horizontal compression and the bottom fibers in horizontal tension (Britannica); the bending stress is largest at the top and bottom faces and grows with the bending moment.

With steel (E = 200 GPa, yield strength 250 MPa), the model gives a maximum sag of 13.6 mm and a peak bending stress of 34.9 MPa, a safety factor of 7.2, even though the solid steel beam itself weighs 78.5 t. The model uses one strength for steel, which Britannica describes as superior to any iron in both tension and compression.

Now Olly makes the beam out of plain concrete. No steel inside!

Oh no, it cracks! It cannot even hold itself up without a truck on it.

Choose concrete with no steel bars and the verdict changes at once: the beam would break. The bending stress is about 18.8 megapascals, but the model's concrete can only take about 3.4 megapascals of stretching before it cracks. The safety factor drops from about 7 to about 0.2.

Now set the truck to 0 tonnes. The empty beam still breaks, with a safety factor of about 0.5, because its own 24 tonnes of concrete is too much for it. Even making the beam 3 meters deep does not save it: the safety factor only reaches 0.9, because a deeper beam is also a heavier one.

With plain concrete (E = 25.7 GPa, cracking strength in bending 3.4 MPa, density 2,400 kg/m³) the peak stress is 18.8 MPa, a safety factor of 0.2. The stress is lower than steel's 34.9 MPa because the concrete beam is lighter (24 t against 78.5 t), but the allowable tensile stress is about 70 times smaller.

Self-weight alone gives 7.06 MPa (safety factor 0.5); the truck alone, with self-weight off, gives 11.8 MPa (0.3). Deepening the beam to 3 m only reaches 0.9: the truck's stress falls as 1/h², but the beam's weight grows with its depth, so the self-weight stress falls only as 1/h (from 7.06 to 2.35 MPa). The model rates the beam safe only for short spans: 2.5 m or less with the truck, and 9.5 m or less with no truck at all.

Concrete is very strong when you squash it. But it is weak when you stretch it.

A bending beam is stretched along its bottom. So plain concrete cracks there.

Concrete is like stone: Britannica says it is strong in compression and weak in tension. The American Concrete Institute puts concrete's tensile strength at only about 10 percent of its compressive strength. So a block of concrete can hold up a great weight sitting on top of it, but it cracks easily when it is pulled or bent.

In a beam, the bottom is always being stretched. That is the weak spot. The model's concrete can be squashed with about 30 megapascals but cracks in bending at only 3.4.

Bending stress σ = M ÷ S is the same in tension at the bottom face as in compression at the top face for a symmetric section, so a beam fails on whichever face its material handles worst. Concrete's compressive strength is about ten times its tensile strength (ACI), so a plain concrete beam cracks from the bottom long before its top is in danger.

The model takes 30 MPa concrete and the ACI 318 estimate for its cracking strength in bending, 0.62√f′c = 3.4 MPa. Its limits: a single solid rectangle, small bends, no check of shear, buckling, the supports or fatigue, and no steel at all. The model reports the result only as a verdict; it does not show how a real crack would grow.

Builders put steel bars inside concrete. Steel is good at being stretched.

The concrete takes the squashing. The steel takes the stretching. Together they make strong bridges!

That is why engineers bury steel bars in concrete. Britannica explains that in reinforced concrete the steel carries the tension and the concrete carries the compression, so together they can span long distances. The concrete also protects the steel inside from rusting and from fire.

Britannica says reinforced concrete was invented in the 19th century, and it made concrete one of the world's most common building materials. A cleverer version, prestressed concrete, uses stretched steel cables to squeeze the concrete before any truck arrives. Britannica adds that modern bridges are almost all built from steel, reinforced concrete and prestressed concrete.

In reinforced concrete, Britannica explains, the tensile strength of steel and the compressive strength of concrete work together so a member can carry stresses over considerable spans; the steel carries all the tension, which also allows less concrete to be used, while the concrete protects the steel from corrosion and fire. Its invention in the 19th century made concrete one of the world's most common building materials.

Prestressed concrete goes further. In post-tensioning, steel tendons run through curved ducts in the beam, are stretched and anchored, and squeeze the concrete into compression while pushing upward against the expected load. Britannica notes this saves steel and concrete and gives lighter designs. The ACI's rule follows directly: use reinforcement wherever tensile stresses would exceed concrete's tensile strength.