Scott - here are the replies that I have received as of today. There is a lot of good information here, and I think that it will be a useful addition to your web site. Thanks for suggesting that I put the summary there. 73 - John, W3GOI SUMMARY - HOW LONG SHOULD YOU ALLOW THE CONCRETE IN A TOWER BASE CURE BEFORE PUTTING UP THE TOWER? Original posting: This past weekend my 22-year old son and I (he did the mixing) put in the base for the first tower at my contest station for the future - yes, I will still have lots of wire antennas, too... My question: how long should I let the concrete base cure before I begin sticking up the tower sections? tnx es 73 - John, W3GOI junger@mtn.er.usgs.gov . . . and now the replies: /******************************************************************/ >From rlboyd@CapAccess.org Mon Sep 25 10:52:34 1995 Hi, John. Where do you live? N3ORY is a ham protege of mine -- met him when he was my home improvment contractor and I got him into ham radio 3 years ago. He knows a lot about concrete, has done several projects here involving concrete. You might want to phone him at 301-261-0398. But as I recall, he's made statements like "It reaches 80% of its strength within 24 hours," things like that. Suffice to say, 2-3 days it's almost as strong as it ever will be. 73 Rich Boyd KE3Q /******************************************************************/ >From biss@epg.nist.gov Mon Sep 25 11:00:01 1995 John, According to my son (mason by trade, commercial buildings) it takes 21 days to be considered a full cure....at about 4 days your at 50% if my memory serves me. 73's bob K3ZNV BISS@EPG.NIST.GOV /******************************************************************/ >From pph@dc.dk Mon Sep 25 11:00:41 1995 Hi John, At least 2 weeks, but some concrete (cement) are faster than other. The cement-supplier should know. Time depends also on how cold it is I think. Do not take chances, do it professional. 73, cu Palle, OZ1RH, (OZ9EDR-OZ5W contest team). /******************************************************************/ >From rsamoian@msmail2.hac.com Mon Sep 25 11:08:16 1995 It take 28 days for the concrete to "cure" to it's maximum strength, many people start putting sections up after 1 week, but........ If safety is of prime importantce, 28 days. /******************************************************************/ >From robbins@guid2.dnet.lmco.com Mon Sep 25 11:08:35 1995 i usually wait a week. you should also keep the concrete moist during this time for best curing. dump a bucket of water on it and cover with plastic to slow evaporation. whenever it looks dry water it some more. 73, Dave KY1H Robbins@guid2.dnet.lmco.com /******************************************************************/ >From sawyers@cacd.rockwell.com Mon Sep 25 11:10:00 1995 Depends on the concrete mix that was used. Special formulations for Highway usage are ready in a few hours General purpose concrete continues to cure for 28 days to reach 90-95% of final strength. Generally, in a week you are at 60-70% of final strength. Best thing is to keep it wet for this long to help the curing. This because cement cure is via hydration (the taking up of water into the material). de N0YVY Steve Sawyers PE. My company and I agree on at least one thing: The opinions I express are my own. /******************************************************************/ >From george@epix.net Mon Sep 25 11:22:04 1995 I just had my tower poured by a friend who is a stone mason and he said to let it cure for one month. |And I did. And now the tower is up. GC /******************************************************************/ >From PEMS_ST_DK@noeca.ohio.gov Mon Sep 25 11:47:11 1995 I poured 5 cubic yards of comercail mix for a self supporting tower and was advised to wait 4 weeks or more. Even after the concrete appears"dry" it is setting up and gaining strength. IM NOT AN EXPERT but Id wait a MIN AMUM of 4 weeks or longer depending on the size of the base etc. A call to a local concrete contractor may be your best bet. /******************************************************************/ >From HSL@hgipdx.usa.com Mon Sep 25 12:04:33 1995 > John, As a structural engineer I recommend to my clients and in my company's standard specifications for reinforced concrete that any major structural load carrying element be cured a minimum of 14 days prior to subsequent construction. Concrete by definition and mix design is to reach its design strength in 28 days. The strength to time curve for concrete is not linear it is more asymptotic and follows the 80/20 rule. 14 days provides a good safety margin for starting your tower erection. Mindfull that this assumes good practice in placing and curing. Contact me if you have additional questions 73 ----- Hank Lonberg P.E. KI7YO /******************************************************************/ >From tree@cmicro.com Mon Sep 25 12:30:57 1995 A week is best, but several days are enough. Tree N6TR /******************************************************************/ >From dieven@almaden.ibm.com Mon Sep 25 13:18:50 1995 For my 89 foot crankup (unguyed) I was told to let the concrete cure 28 days. I did wait. I was told that concrete gets most of its strength in the first few days, but it keeps getting stronger until fully cured. I imagine that there are a lot of unnatural stresses on the footing during erection of the tower. In my case, I hoisted it from horizontal to vertical, which resulted in an large upward force on the bolts imbedded in the concrete. I didn't want to ruin an expensive pour by being too quick off the mark putting it up. 73 de Dick, AA6MC /******************************************************************/ From: Rick Niswander John: The longer the better for curing. From a practical standpoint, if the tower is a 72' self supporting jobbie (or similar) leave it go for at least 2 weeks (pref. a month). If it is a base for Rohn 25 or such a week is fine. Rick, K7GM aoniswan@ecuvm.cis.ecu.edu /******************************************************************/ >From w5robert@blkbox.COM Mon Sep 25 18:32:56 1995 John, just saw your posting & on my first self supporting tower operation, I used the 3000# concrete and poured from a truck. Your basic green when poured, as I recall? Anyway, I mounted the Base section around 48 hour mark and proably pulled the tower & 10mtr beam up in under a week. As others said, takes about a month for full cure, but not a linear function. Also: if it's a big pour-- don't forget to water it the first few days. My templete wood board stayed in place for about two days and it was quite warm under the board when removed. -- 73 Robert WB5CRG w5robert@blkbox.com /******************************************************************/ >From k4sb@ix.netcom.com Mon Sep 25 20:01:11 1995 John, I usually let it set for at least a week or more, and I continually keep it wet for that time. You'll be surprised how much water it will absorb, and how much stronger it will be. GL and can I borrow your son this weekend? 73, Ed /******************************************************************/ >From 0006492564@mcimail.com Mon Sep 25 21:55:33 1995 By now someone has told you that the secret to good concrete is if it never dries out it will be good...the act of taking a really long time to dry out (like never) means it is forever curing and consequently forever getting stronger. When I was taking a materials course in college we studied the strength of premixed concrete in the bag...a trick I learned from my Uncle held true: Sakrete (or Quikrete or whatever) like any concrete is a mixture of three basic items: Cement, Aggregate and Sand. My uncle always advised adding a half shovel's worth of Portland cement to an 80 lb bag of the mixes. It makes sense, of the three ingredients in the mixes it is obvious which is the most expensive and which would probably tend to be kept to a minimum by its producer. We did compression tests on the samples in our class and mine came out having the highest strength, thanks to a tip from Uncle Ray. I still have that busted up core on my bookshelf. You may wish to study how much your pour cost you, on something as large as a tower base having a truck come in should be competitive or cheaper - the key is doing bases and guy points simultaneously so you have enough of a job to be considered by the concrete companies...a ham job can be a second stop for them. Get several bids, they may have several different PSI types so make sure you are comparing apples to apples. A bag of the premix stuff is great for a patio stepping stone, but to fill up big holes it takes a lot of them. The going rate for the premix truck in Tampa Bay is mid $50s per cubic yard - dunno what Sakrete runs in W3 but down South it just ain't worth mixing your own, not to mention the sweat equity! GL Jim, K1ZX k1zx@mcimail.com /******************************************************************/ >From g4dbn@cix.compulink.co.uk Tue Sep 26 00:17:34 1995 Hi John, I do some programming for a company that tests concrete, and they do tests at 2 days, 7 days, 14 and 28 days. After 7 days the strength is about half of the ultimate, and after 28 it is 99%, so I guess 10 days or more would be fine if it is a normal mix (ie no entrained air or admixtures, just sand, gravel and cement). These guys test 6inch cubes of concrete cured under water and compress it until it shatters, the 7 day test stuff sort of crumbles and the 28 day stuff goes with a loud and impressive crunch! Good luck. Neil G4DBN/AA1EG /******************************************************************/ >From 0007288678@mcimail.com Tue Sep 26 00:40:29 1995 Hi John, I'm in process of putting up tower myself. I'm wondering with the information you rec'd exactly what did you decide was the time from pour to stacking? i.e. what did you decide was the safe time to "get at it?" Another question(s) I am curious about: 1) What are you putting up (Rohn)? 2) Are you burying your coax/rotor cables? If so are you direct burying or enclosing them in PVC or such? 3)Are you using regular guy cable or phillystran? Just curious, I am putting up 80 feet of 45G with 2 el 40 over 5 ele 20 and I am using phillystran. I understand that it is a good idea to have regular guy cable at bottom end of guy wires and I am just curious how others are splicing the 2 together. I su ppose I should put out a reflector wide inquiry come to think of it. 73 and good luck John. Matt -- AA7BG aa7bg@mcimail.com /******************************************************************/ >From lew@teleport.com Tue Sep 26 00:41:54 1995 Hi John, I've poured bases for5 towers here with #6 to be poured next week. Somewhere along the line someone told me 30 days..... all hearsay with no controlled clinical trials to back me up! Last crank-up though I had the time and mounted the tower after 2 weeks and it has been working fine over the past 2 years... If somebody tells you who really knows... let me know! 73 and I remain, Lew /****************************************************************8/ from: george@peach.epix.net AA3JU@N3IQD.EPA.USA.NA Here is a tip for what it is worth. I have a friend that is a stone mason and after the towers hole had been dug I said to him "Hey Rock (true name) how much for like 3 yards of concrete?" And he said that if I could wait till he had a job on a basement that I could have the left overs real cheep. My cost was 1 pot of coffee and $25 to have Rock and his assistant come over and shovel in rocks from my stone row and about 2 yards of basement floor quality cement. And they even finished it with a nice little hump in the middle so the rain would run away from the tower legs. My advice in short if yer gonna build something get friendly with people who build things! GC ************************************************* * George Cook AA3JU Bangor, PA FN21 * * george@peach.epix.net AA3JU@N3IQD.EPA.USA.NA * ************************************************* /****************************************************************8/ from: Frank, W3LPL, donovanf@sgate.com Earlier this summer N3RR and I discovered that we had been very sloppy about the vital detail George referred to below, and had we not corrected the problem we both may have faced disastrous consequences! On Tue, 26 Sep 1995, George Cook wrote: > And they even finished it with a nice little hump > in the middle so the rain would run away from the tower legs. Bill and I discovered that our lack of attention to water runoff and mud accumulation during the construction and maintenance of our towers had resulted in significant corrosion of the tower legs at the point where they enter the concrete foundation! In both cases, we had discovered it in time and we were able to implement effective corrective measures before it was too late... Another ham in our area was not so lucky! A word to the wise: If you haven't inspected your tower for corrosion at the tower-to-foundation interface, tomorrow wouldn't be too late to inspect it! If all is well, you will find: - - no deterioration of the galvanized surfaces of your tower legs - - Your foundation will have a hump that causes water to run away from the legs - - no cavities in the foundation near the legs for water (or mud!) to accumulate in - - and a foundation that is several inches above the surrounding soil so that mud cannot wash onto the foundation and accumulate While you are carrying out the inspection of your foundation, check your guys for proper tension and integrity of hardware and safety wires too! Remember... safety first 73! Frank W3LPL donovanf@sgate.com ------------- For a large mass of concrete in the base of a tower, letting it dry on its own will not result in significant loss of strength... how many ham towers come down because the concrete base fragmented and allowed the tower to lever over sideways, or sink into the ground? ... for thin flat slabs which are expected to hold up cars, etc., and for building foundations, or machinery foundations, or any use where the concrete is stressed closer to the maximum values, then a controlled cure is necessary...so, in the ham tower world it is not a problem... but, if it is not too much work to wet it a couple of times a day and keep a vapor barrier over it, why not?... Cement plus aggregate becomes concrete through the process of hydration of the lime (a chemical reaction, not simply being physically wetted during the mix)... for every molecule of lime to become properly hydrated into concrete there must be adequate moisture present through out the 28 day curing period... you can measure a 10% to 15% increase in strength for concrete that is kept moist during this time, compared to concrete that is not... And there you have Concrete 101... you are entitled to 0.1 credits towards getting your concrete PHD, for having read this... Denny, K8DO@aol.com