How do martial artists break concrete blocks?

# How Do Martial Artists Break Concrete Blocks?

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March 15 2015 7:23 AM

# How Do Martial Artists Break Concrete Blocks?

Answer by Isaac Gaetz, structural engineer, current Brazilian jiu-jitsu practitioner, past Wushu and childhood Shotokan karate student:

It is really a simple trick when you understand the problem from a structural engineering perspective. The trick consists of a moderate amount of power and speed and a lot of careful selection of the target material and geometry.

The blocks are typically unreinforced concrete “soaps” or pavers with a thickness of about an inch, a width of 8 inches and a length of 16 inches. The blocks are set to span the long way between two supports and are struck as close to dead center as possible. The blow from the martial artist creates bending stresses in the block. From material science we know that when subjected to bending stress, an unreinforced concrete block of this thickness has very low strength. The observable behavior will be a tension crack that develops on the opposite side of the surface being struck. The crack quickly (instantaneously to the naked eye) propagates through the block and you are left with two pieces.

Crunching some quick numbers, I compute that it would, on average, take about 125-175 lbs of static force to break one of these blocks in this manner. Without any other reference, this may sound like an impressive feet, but consider that this is typically less than the athlete's own weight; if he simply stepped onto the block it would potentially snap. Heavyweight boxers have been know to hit with as much as 1,000 lbs of force, in a more difficult, horizontal direction, while martial artists often go so far as to employ a jump prior to striking the blocks. The biggest challenge for the average martial arts athlete to break a single concrete block is psychological, if he mentally commits and strikes with a full-force blow and has good follow-through after impact, he'll easily break the block.

You'll notice that in cases where a thicker concrete piece is broken, it is never one solid concrete block—it is always a stack of the same thin blocks. Furthermore, if you look closely, you often see the martial artist place small pebbles, seeds, or other spacers between the blocks. This allows the force to easily transfer as the blocks break progressively. For this reason, breaking multiple blocks only takes marginally more force than breaking one. They are breaking like a series of dominoes, not like a single superthick block. Breaking an actual thick block would be much, much harder and is rarely, if ever, done. Add even a tiny amount of steel rebar reinforcing to the block and it will be utterly impossible to break by hand.