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Frequently Asked Questions

Why concrete crushes
but can't pull

Concrete's compressive strength is ~10× its tensile strength. Here's exactly why — visually.

REF Wight & MacGregor — Design of Concrete Structures, 15th Ed. §3-3
The core numbers
Compressive Strength f′c
4,000
psi
concrete resists crushing
10×
stronger
Tensile Strength fr
474
psi
fr = 7.5√f′c
Same scale — compression vs tension
Compression
012,000 psi
f′c = 3,000
3,000 psi
f′c = 4,000
4,000 psi
f′c = 8,000
8,000 psi
f′c = 12,000
12,000 psi
Tension fr
same scale ↑
fr @ 3,000
411 psi
fr @ 4,000
474 psi
fr @ 8,000
671 psi
fr @ 12,000
821 psi
What happens inside the concrete
aggregate interlock ✓ ITZ intact
COMPRESSION
aggregates locked — load spreads through bulk
ITZ fails instantly ✗ microcracks propagate
TENSION
paste-aggregate interface separates immediately
Why steel exists — the reinforced beam
LOAD COMPRESSION concrete handles this — f′c ≈ 4,000 psi N.A. TENSION concrete ignored in design — cracks at ~474 psi — STEEL REBAR (fy ≈ 60,000 psi) — carries all tension · 126× stronger than concrete in tension
Ratio by concrete grade — gets worse as grade increases
3,000 psi
3,000
411 psi
7.3 ×
4,000 psi
4,000
474 psi
8.4 ×
6,000 psi
6,000
581 psi
10.3 ×
8,000 psi
8,000
671 psi
11.9 ×
12,000 psi
12,000
821 psi
14.6 ×
fr = 7.5 λ √f′c ACI 318-19 Eq. 19.2.3.1  |  λ = 1.0 normal-weight  |  psi