Grubb Engineering Services - Ask The Engineer



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			Jaytee wrote: How thick should a concrete slab tank base be to support a water tank holding 5000 gallons of water? Answer: Jaytee, Thanks for the question. The answer depends on the size of the tank's "footprint" and the method of support. Let's assume the weight of the water and tank is around 44,000 lbs. If it is a rectangular tank... say 10'x10'x6' tall, an 8" slab (properly reinforced) would be fine. If it is standing up on end... say 8' diameter and 13' tall and resting on 4 legs, you have an 11,000 lb load at each point. You could then use a thinner slab and just thicken the concrete to around 12" (properly reinforced) at each bearing point. Hope that helps. If you want to convey additional information, I could more specifically answer your question! No Screen Name wrote: Is it better to put #4 bar 18" on center or 6 gauge wire mesh in 8" thick slab that will see heavy industrial traffic? Answer: The answer is NEITHER. You either want to use #4 at 12" on-center or #5 at 18" on-center for an industrial traffic slab. 6 gauge wire is only 0.162 inches in diameter which, when welded at 6" on-center, gives you 0.0412 square inches per foot of slab. You need 5 times that reinforcing! #5's at 18" gives you 0.207 square inches and #4's at 12" gives you 0.200 square inches. Skimping on the reinforcing now will be very costly later when you have to rip out the cracked, shifted slab! AG wrote: I need to replace a portion of a concrete slab that is used occasionally to play shuffleboard & throw a little basketball. I received a quote that included 3500 psi with fiber for a 16' x 20' section with a price of nearly $3800. The original slab has cracked & needs to be removed & a little more fill dirt brought in. My questions: 1. Do I really need 3500 psi with fiber? This slab gets very little traffic. 2. Does this price sound high for North Carolina area? It includes removing & hauling off the original, filling in shallow cracks, & sawing a joint across the concrete. Answer: AG, let's first talk about the concrete strength. 3,500 psi is a bit overkill for a basketball/shuffleboard court. However, the price difference between 3,500 psi and the more standard 3,000 psi should be very small for the amount of concrete you need (approximately 4 cubic yards). Don't consider this a "premium" upgrade. Now on to the reinforcing... Fiber reinforcing will leave you with a "hairy" surface (from the fiberglass fibers that stick up) that may rub smooth in the high traffic areas after continued use. I can't imagine this would be good for shuffleboard. Don't let anybody fool you, sealing the concrete will only delay the "hairy" appearance. I like fiber reinforced concrete in applications where it will be covered with another material such as carpet, tile, VCT, etc. I suggest you use wire mesh (6x6-W1.4xW1.4) that is placed on concrete bricks so that the mesh is in the middle of the slab, not placed on the dirt and "pulled up" by the concrete guy when he pours (that NEVER WORKS). The slab should be a minimum of 4" thick, the fill dirt should be mechanically compacted (fill 8" and compact, fill another 8" and compact, and so on) and the joint should be cut as soon as the concrete is hard enough to walk on (8 hours or so). If this slab is poured on good, tight soil, you should have no problems for many, many years. Pricewise... it may be a bit high, but if you asked ME to cut and remove the slab, haul in dirt, compact it, pour a new slab and screed it... I'd be charging 3X that amount! It's hard work! MJ wrote: I am building a pole barn and am ready to pour a concrete slab. The structure has been built on a slight slope so I had some dirt fill brought in and compacted. The deep end of the fill goes down about 30". The frost line in my area is 4'. Is it possible to drive rebar down below frost line, then bend it over into the concrete to prevent possible settling later on? Answer: Thanks, MJ for the question. Driving rebar into the soil is a big NO-NO (though many contractors do this when placing rebar in footings). Remember, the rust process is electrical (called oxidation). When you drive rebar into the soil, you provide an "electrical connection" to the ground through which electrons may flow. The result... RUST IN THE REBAR. When rust is allowed to continue, the rebar will expand (rust expands to 1,000 times the original steel volume) spalling the concrete and accelerating subsequent oxidation. You've got to get the bottom of the footing below the frost line, not to prevent settling, but to prevent frost heaving the footing upward. If it's interior space, you should also provide some rigid board insulation between the sides of the footing and the soil. This limits cold concrete inside the building that can lead to some real heating problems! Sounds like you need a properly designed concrete sub-wall (30" above the ground and 4' into the ground). You must be way north of the Mason-Dixon line! KC wrote: What is the proper layout to build a concrete patio for a slab house in Georgia with the heat? Does it require 2" inches gravel or no gravel, wire mesh vs rebar and 4" concrete? What is the differences with 3500 psi vs 4000 psi and 5000 psi concrete mix? Answer: Great questions, KC! I apologize now for the lengthy response but that's what you get when you ask 6+ questions at once! For a residential concrete patio, you don't really need a gravel base unless you have perpetually wet soils or other moisture problems on your property and need some drainage. If you are enclosing this patio, make sure you treat the underlying soil with an approved termite chemical, lay down a 6-mil plastic vapor barrier before you pour your slab. The perimeter should transition from a 4" slab to a minimum 12" wide x 12" deep "thickened" slab. Per the building code, the footing should extend a minimum of 12" below the exterior soil level… this keeps the bottom below the "frost line" and will prevent the footing from heaving upward in one of our cold snaps. Don't forget to add (2) #4 (1/2" diameter) reinforcing bars along the length of the perimeter footing. The proper type of wire mesh is perfect for your 4" slab. No need to go with rebar. For your purposes, a standard 3,000 psi concrete mix will be fine. Higher strength concrete would be used for surfaces that are subjected to high wear-and-tear such as in a warehouse, etc. It would have to be one heck of a party to wear out your 3,000 psi concrete patio! Beware of the contractor that adds gallons of water to the concrete mix when the truck arrives! Diluting the concrete to make it "easier to work with" or to make it "go farther" will lead to weak concrete that will crack and wear quickly. That's the gist of it! Call us if you need more information. Big Ben wrote: Wow! This is an amazing web site. Who is your fantastic web designer? I think the "Ask the Engineer" page will be a popular feature. Answer: Thanks, Big Ben. I can't tell you our web designer's name because I don't want you to steal him from us! We only employ the best.


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