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[TowerTalk] Curing Concrete

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Subject: [TowerTalk] Curing Concrete
From: (Chris R. Burger)
Date: Wed, 30 Jul 1997 07:35:29 +200
The Phantom wrote:

> How long do you wait after pouring tower base and guy anchors 
> for the concrete to come to strength before proceeding with the 
> remainder of the installation? 

Concrete strength is specified at 28 days.  Strength is generally 
specified as a pressure, i.e. in MPa (or even in PSI in countries 
that shun international standards).  The concrete continues to 
strengthen while it dries, a process that may continue for years.

The drying (and strengthening) happens rapidly in the early stages, 
after which it slows down.  

Derating curves are available, indicating the percentage of rated 
strength achievable in the first 28 days.  While I don't have them 
with me, let's assume you require 15 MPa strength to erect the tower. 
You only have 10 days to do it in.  Let's also assume that the curves 
show 60% strength at 10 days.  This implies that you have to buy 
15 MPa / 0.6 = 25 MPa concrete.  This concrete is more expensive, as 
the mix contains more cement.  It is also somewhat more difficult to 
vibrate into position.  

The bottom line:  Plan ahead, and save yourself money and effort.

That having been said, I have an admission to make.  90% of the 
antenna work at ZS6EZ is done in the week before a contest.  Hence, 
I've had to overspend on concrete on more than one occasion.  When I 
moved my station from my parents' house, I paid dearly for this poor 
planning.  To remove a concrete block from the ground was almost 
impossible, as I'd had to use the strongest concrete available (I 
think it may have been 35 MPa!) with only three days to get the tower 
up.  The block proved to be almost impossible to break.  A 10 kg (22 
lb) hammer literally could not even dent the surface.  Eventually, 
I had to hire a team with an industrial-grade compressor and a 
jackhammer for a full day to fragment this block to 700 mm below the 
surface.  The full day's work did not even include hauling the rubble 

Chris R. Burger

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