Antenna Bases - Clearing-Up Amateur Misinformation

I recently came across some interesting discussion threads online about the strength of concrete, especially in relation to erecting antenna towers.

The stressful - and expensive - part of antenna tower preparation.

There was an awful lot of nonsense and misunderstanding about.  An exchange in one thread, stated with the characteristic certainty of someone who doesn't have a clue, believed concrete was "as strong after three days as it will ever be."  The overall consensus in that debate was that after 28 days, concrete had reached full strength.

The above views are complete nonsense.

Concrete does not harden, as some think, because of drying out or 'setting' like some evaporative glue.  It hardens as a result of a series of chemical reactions that form an interlocking microscopic mesh of minerals, all based on calcium, that provide the mechanical strength.  If you aren't convinced, mix a small amount yourself and let it harden on a warm, sunny day.  A few days later, you will be able to crush it with your hands, it will be that weak - all because the essential water for crystal growth dried out.

Microscope image of mineral crystals developing in concrete.  The crystals need a considerable time - many years - to fully develop and reach maximum mechanical strength.

After three days, concrete will certainly have 'set', but the crystals that make up the strength have only just begun to grow.  After 28 days, which is a period chosen only for its real-world convenience and not any inherent feature of concrete, it will have reached about 70% of its ultimate strength.  Note that, in the case of very hot and very cold weather, the ultimate strength is greatly reduced.  That's why you should pour structural concrete during reasonable weather.

Reasonable warmth and a good dollop of time are prerequisites for strong concrete.

The full mechanical strength of concrete is never reached for many years.  Five years is about as early as you would want to make the "as strong as it will ever be" claim, though even then, it will still be adding strength for some years again.

So, whilst you may want to put up your tower as soon as that expensive concrete has 'set', don't!  Wait for a month, keep the concrete moist and covered for as long as you can, and then feel sure that the strength is up to scratch.