Bob Engberg, K0MVL phone: 907-552-1895 e-mail: engberg@ctis.af.mil URL: http://www.ctis.af.mil/~engberg/ Listed below are the responses to my question on tower cement. Thanks everyone for the good advise and references. ____ ____ ____ ____ ____ ____ ____ ____ Here's the question: I have a 3x3x5 ft hole for my Rohn 25G tower. Locally a pea-gravel mix is available. Just add water. Is this the "right stuff"? I sorta recall using a sand mixture last time - about 15 yrs ago. [I forgot to mention in the original question that the tower will be 60-70 ft. with my old TH6DXX.] _____ ____ ____ ____ ____ ____ ____ ____ Here are the answers I received: Bob, for guyed towers boring a hole proably isn't needed. I didn't on my guyed towers with a TH6 on top. Worked fine. Had the hole bored for the self supporting CUZ a back hole makes too big a mess and you CAN NOT BACK FILL and make things work on self supporting. Must be "virgin" soil around the cement base. The cost can get high as they charge a flat rate to show up then a charge per hole, but the rate to show is the $$$$ part. Best check your friends and see who works at the hole borers before buying a self supporter!! HI! -- 73 Robert WB5CRG w5robert@blkbox.com _____ ____ ____ ____ ____ ____ ____ ____ Hi Bob - For what it's worth (and KL7RA has a proven track record, obviously), Rohn does not agree with his idea of using no rebar in the base. Granted, they don't recommend very much - just 4 verticals inside the corners of the form, tied together with wire to hold their positions as the concrete is poured. Also, most folks use gravel in the bottom of the hole, rather than sand - probably depends on how well your soil "perks." Finally, Rohn does not recommend over-smoothing the insides of the hole - in fact they specify that it should be left rough. Fact is, all of these Rohn recommendations doubtless represent a lot of liability-proofing, and you can probably do with a lot less. My ME son tells me that the only forces on the base are vertical, once the first guy set is in place - the forces produced by the wind are translated by the guys into compressive forces on the tower, straight down. Ditto the weight of beam, rotor, owner, etc. So a 4000 lb concrete block is probably overkill HI 73, Pete N4ZR@netcom.com "Better, faster,cheaper -- choose any two" "No no no -- it's WEST Virginia" _____ ____ ____ ____ ____ ____ ____ ____ Friend of mine says the Rohn book specifies square OR round holes. Round is easier if you have a tractor with PTO or other piece of gear to use -- or you can rent a power augur I guess. Rich Boyd KE3Q _____ ____ ____ ____ ____ ____ ____ ____ Bob (& others interested in tower concrete) I read the comments and the one on using a square hole so it would not rock in the wind? My tower is self-supporting and at the previous QTH we bored a 42" ROUND hole 7ft down and at the current QTH we bored a 48" round hole 7.5 FT down. I notice no rocking of the cement and was always told the round hole (bored) was the way to go.... It always appeared the square holes were used as they were hand done & hence fit the budget better - I'm just not getting into a hole 7ft deep.... -- 73 Robert WB5CRG w5robert@blkbox.com _____ ____ ____ ____ ____ ____ ____ ____ I suggest that you use a batch -plant prepared mix. It is a stronger mix than you can buy in a bag and mix with water. I would assume from the tower you are installing that you will be using guy wires. To me it makes little difference. Use quality products and prevent things liked a cracked base. I would suggest a 6 sack, pea gravel mix which will test to 6,000 psi at 28 days of curing and it can be pumped into place for a modest price. Pumping the base makes the next to last phase a real piece of cake. Good luck with your project. I will work you in the contest under my call of 6E2T. 73, Harv KD6QK hhiller@cts.com ____ ____ ____ ____ ____ ____ ____ _____ The Book (Rohn) says to guy at 35 and 70 using 500 ft of 3/16 with the anchors 55 feet from the base. The base should be 2 by 2 by 4. [they have different size bases and guying depending on wind load. This is for 80mph. Conservative numbers.] But here is what I do: I guy just below the top section. [That allows me to add to the tower in the future. All I have to do is pop off the top section and add tower sections.] I dig the hole making it square. I don't go down 4 feet. Three feet is okay for Fairbanks. I went 5 feet for the 190 foot and 4 foot for 150 ft. The 125 foot towers are 3 feet. [The hole is made square by using a hoe on the sides. I even get into the hole and polish the sides to make it square as possible. The idea is you don't want it (the concrete block) to rock back and forth in the wind. ie round hold is bad.] I build a concrete form out of 2 by 4 boards and place it over the hole. I hand pack around the outside of the form with dirt. This will give you a square block that is 3 1/2 inches about ground. You trowel this off away from the tower for water run off. [no standing water] I add sand to the hole about 5 inches. When I put the tower into the hole it is pushed into the sand. [This is to allow water to escape the tower legs if any gets in to it. [no standing water]. [the tower section acts like rebar- no other added] I place the tower section into the hole. Usually two sections. Guy the top off with rope to near by stakes. I use a level and make the section straight up/down. [this is very hard to do after you get much concret into the hole. Also make sure the tower angle is correct to the guy points. Trying to turn a tower when its in only a small amount of mud is very very hard.] An easy way to keep the tower straight is to get a long board and just spike it to the ground and have it resting against the tower. This keeps pressure on the tower to keep it straight. It doesn't take long to "set" the position. I rent a mixer. Add 5 shovels pea gravel#2 and one shovel cement and water to mixer. Let it mix for few seconds then pour into hole. Notice that the first pour didn't do much. Just a wad of grey mud at the bottom. I fill the hole completely with cement. I do not fill mostly with sand then top off or do I throw rocks, beer cans etc into the hole. I thought about throwing my daughter in one time when she ran off with the hose to squirt her brother. Thats it. I had a dumptruck load of #2 dumped in the woods near my house and have been feeding off that for last 10 years. The concret is about $8 bucks a bag and need only 2 or 3. I throw away the unused stuff because frezzing ruins it and I dont trust it any more. To rent a mixer is $45 a day, but I get the big one. Rich kl7ra@icefog.gcgo.nasa.gov ____ ____ ____ ____ ____ ____ ____ _____ Bob, I am NOT the expert on this but I've done 3 25G tower holes and here's what I understand: (I an EE, not Civil E) The cement does only 1 thing, and that is provide the weight to keep the first 30 feet vertical. After that, the guy wires equalize the forces and keep the tower vertical. So, you don't need any special cement or rebar in your hole. Just make sure you have plenty of cement, not too much aggregate (sp?... you know, rocks/gravel/etc) and let it get good and hard (48 hours curing) before you start adding the other sections. Works for me, and is what I'll do on the new tower when I move next month. 73, Tom WB4iUX Tom.Skelton@ClemsonSC.ATTGIS.COM ____ ____ ____ ____ ____ ____ ____ _____ You didn't say what you were going to put on the tower, nor did you say how high it would be, but my advice is to use the "standard" strength concrete that is meant for steel towers and that is 3000 psi. That mix would be a minimum of 5-1/2 sacks of cement mixed with less than 6-1/2 gallons of water/sack and coarse aggregate (max size 3/4 inch)[I used exactly 3/4 in stone] and average size sand. Reference: page 10-23,24 of: Physical Design of Yagi Antennas by David B. Leeson, W6QHS, Published by ARRL, 1992. 73... Bill, n3rr@cais.com _____ ____ ____ ____ ____ ____ ____ ____ The premix is really expensive to use. One bag only does 1 cubic foot. Best to get pea gravel #2 and mix it 5 to one. I have seven towers 190 feet on down up for years in Fairbanks. So far the base of each tower looks good. No cracking etc. 73 Rich kl7ra@icefog.gcgo.nasa.gov ___ ____ ____ ____ ____ ____ ____ _____ 1. I am not a civil engineer. that being said... 2. Concrete is usually specified in terms of P.S.I. and stone size. 3000 psi is the common garden variety stuff, and should be good enough for tower work. If you want to be on the safe side, you can go to higher numbers. I think I used 4000 psi (or better) for my rohn 45. -Tony, K1KP, fisher@hp-and.an.hp.com ____ ____ ____ ____ ____ ____ ____ ____ I believe the sand and cement mixture you speak of is mortar, used to bond brick and blocks. Concrete, or ready-mixed is a mixture of sand, gravel and cement. That is what you want for your tower base. And what it takes to fill that 3X3X5 hole is a lot of hand mixing, for that matter, it's a lot of mixing with a machine. If you have a ready-mix company near by, they might be able to deliver to you on an over flow basis when they have a nearby delivery. Good Luck, 73, K8Joe"Palooka" jpontek@aol.com ____ ____ ____ ____ ____ ____ ____ _____ 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 * ************************************************* ____ ____ ____ ____ ____ ____ ____ _____ For whatever it's worth, here's what the Rohn drawings have to say about the concrete to be used -- you may be able to find some of the cited specs, etc. on the bag. Alternatively, Dave Leeson has some words on the right concrete to use in his book on physical yagi design - unfortunately it's at home. >From Rohn: "7. Concrete materials shall conform to the appropriate state requirements for exposed structural concrete." ... 8. Proportions of concrete materials shall be suitablefor the installation method utilized and shall result in durable concrete for resistance to local anticipated aggressive actions (sic). The durability requirements of ACI 318 shall be satisfied based on the conditions expected at the site. As a minimum, concrete shall develop a minimum compressive strength of 3000 PSI in 28 days." 9. Maximum size of aggregate shall not exceed 3/4 inch; size suitable for installation utilized; or one-third clear distance behind or between reinforcing. ... 13. Minimum concrete cover for reinforcement shall be 3 inches unless otherwise noted." This would seem to indicate that your pea-gravel stuff would be OK. Good luck 73, Pete N4ZR@netcom.com