[TowerTalk] Concrete

Mon Aug 26 17:48:25 EDT 2019

"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 at 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 at 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 at contesting.com ; xdavid at 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
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>    
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk