"Slump" is a common term in the concrete industry and there is a spec
for it. Usually it is measured by putting a truncated cone of it on a
flat surface and measuring how many inches the top of the cone sags.
And yes, it is essentially a measure of how much water was used in the
mix, although the Portland cement content also has some influence. It
takes VERY little water to facilitate the chemical reaction that creates
hard concrete, but water is also normally necessary to make the stuff
flow where you want it to go so it is a tradoff. I've seen videos of
machines laying concrete for highways, and that stuff is so stiff that
they essentially pack it into place. They can get 10,000 PSI out of it
that way, though.
Excess water in concrete diminishes the ultimate strength of the
concrete. Whatever water that isn't needed for the chemical reaction
ultimately goes away, leaving microscopic voids that diminish the
overall compressive strength since air doesn't hold up much unless it is
moving. It is possible to get concrete with the spec'd amount of
Portland cement that still never meets rated strength simply because
either the driver or the concrete company decided to make their job easier.
Some Google searching can give you charts of compressive strength versus
both water content and amount of Portland cement per cubic yard, but
most of us don't have much ability to check what we're actually
getting. Other than the slump test, that is. You can find charts for
that as well.
73,
Dave AB7E
On 8/26/2019 2:12 PM, Michael OBrien wrote:
When I put in my UST HDX-555 some 20 years ago, I hired a contractor I knew
to dig the foundation with a backhoe and construct the rebar cage as per UST
instructions.
He also ordered the concrete. But when the truck arrived, he insisted upon checking
something that was new to me -- the "slump" of the concrete.
The driver wasn't happy about it, but (as I recall) they placed a hefty blob of concrete
on a slanted board and then timed its travel down the incline. Whatever, the batch didn't
pass the "slump" test.
After conferring with the company office by phone, the driver took that load of concrete
to another construction site. A different truck was dispatched, and the concrete it
delivered was judged to be acceptable, "slump"-wise.
I'm guessing that "slump" has something to do with how much moisture is in the
concrete mix. I had never before, and have never since, heard the term used. But it was
important to my contractor that day.
Mike K0MYW
On Monday, August 26, 2019, 2:26:10 PM CDT, Bob Shohet, KQ2M
<kq2m@kq2m.com> wrote:
Mac,
Thank you for noticing my error. It was not 20 sq. yds, it was 20 cubic yds. That’s what I paid for and
what I received. I actually used ~ 18.5 cubic yds for the two tower bases and six guy anchor holes – the
“extra” was what was dumped in the depression at the edge of my driveway.
I don’t recall the size of the truck or if they had to make more than one trip,
but t was a HUGE truck.
For perspective, because of the boulders and other debris in several of the holes, what would start out as a 3’ x
4’ x 4’ hole became more like 4’ x 5’ x 6’ as a removed boulder or piece of
construction debris (steel cables, sheetrock, broken piece of wood, barbed wire, etc.) would tear out a chunk of the dirt
wall and require re-straightening by the backhoe and enlarging the hole in the process.
Bob, KQ2M
From: Mac
Sent: Monday, August 26, 2019 3:07 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Concrete
Humm ? a 20 Sq yd concrete truck .. ??
that's a very big and very heavy truck , more to the point, they don't
make emm that big ...
mac/mc w5mc
On 8/26/2019 1:15 PM, Bob Shohet, KQ2M wrote:
I used a different method which I will mention here just it case it may be
helpful for a TT reader.
My qth is on irregular and jagged ledge – in some spots the ledge is at or just below the surface (like
an iceberg) in other spots, as little as 1’ away, the dirt extends down for 4’ or more feet.
You can’t predict where it is; you can only dig and see where it actually lies.
My house sits in a small “valley” about 10 – 15’ below the top of the ledge which starts less than 15’ from the
ends of the house. Coming from the street down my driveway facing my house, you could almost imagine this as a “U” with the
middle part of the “U” flat and very elongated.
Both of my towers are on the top of the ledge with no way to get there other than to climb up at a very steep
angle > 30 degrees. This made it impossible to get a large concrete truck, with 20’ sq yds of
concrete, up to the tower base. And even if it made the first base, it could never get through the dense
woods and over the boulders to get to the second base 110’ feet away. It was a challenge.
I decided to hire an experienced person with a SMALL backhoe. We proceeded by having the large concrete truck drive to the base
of the ledge, extend its concrete pouring chute to the top of the ledge and then pick up that chute and place it in the
“scoop” of the small backhoe. (We had discovered that only a small backhoe could navigate the boulders and jagged
ledge (although not without tearing and blowing out two backhoe tires in the process!)) The concrete truck then poured the
concrete through the chute into the scoop of the backhoe filling it ~ 1/2 way. The chute was taken out and the backhoe proceeded
VERY slowly with the sloshing concrete and then poured it into slowly and carefully into the tower base and guy anchor holes (already
strengthened with a cage or rebar and metal ties – previously made by N1MM and myself). At each hole another employee of
the concrete company worked with a concrete trowel, level and other tools to move and spread the concrete and level it properly.
We repeated this process many times over a 2 1/2 hour period so that we could fill in both tower bases and six guy anchor holes.
It was a slow and painstaking process but it produced outstanding results. No concrete
was lost or spilled, the concrete filled all the holes and spread and leveled beautifully and
the highly complex and challenging terrain was overcome. We even had ~ 1 yard of
concrete left over which I had them dump into a depression in the driveway over an underground
stream and which successfully filled and stabilized a mini-sinkhole. :-)
This was in 1998 and cost ~ $1,050 cash for all the concrete, delivery, labor and 1/2 hour of
overtime. I had calculated that doing it all myself with renting a mixer, buying all the concrete mix,
and other chemicals, would have cost at least $400 and consumed at least two full days. I’m not
sure if it would even have been possible to get the mixer through the woods and over the boulders to the two
tower bases, and guy anchor holes on the ledge in the forest, but I do know that the labor would have been
backbreaking – probably almost as bad as the weeks of chainsawing that I had to do in order to clear
the dense forest for the tower locations. I know that I would not have had results as good as having
the professionals do it. There has been no issue with any of the concrete in the past 21 years.
I would also add that the pros know the temperature range that the concrete will set best at as well as the chemical
composition of the concrete to give the best “pour” and “set”. I certainly would
not have known that. For that information and knowledge alone to ensure the safety and the longevity of my
installations, it was worth every penny!
73
Bob, KQ2M
From: k7lxc--- via TowerTalk
Sent: Monday, August 26, 2019 1:11 PM
To: towertalk@contesting.com ; xdavid@cis-broadband.com
Subject: Re: [TowerTalk] Concrete
What size is your tower base?? In my opinion, mixing your own for a
typical tower base is not a cost effective undertaking unless you have some
insurmountable access issue that prevents you from bringing in a mixer truck.
I agree. I've poured dozens of tower bases and every base plan I've ever
seen calls for 3kpsi concrete. The labor involved in buying, transporting, mixing and pouring
a base yourself doesn't usually pencil out compared to having it delivered.
And if there is an access issue, hire a line pump to get the mud in the
hole. It'll pump up to 400 feet which should solve most problems.
Cheers,Steve K7LXCTOWER TECH -Professional tower services for amateur and
commercial
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