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Everything You Ever Wanted To Know About Concrete* (But Were Afraid To Ask)
In the comments on another post, I mentioned it would be nice if someone talked about concrete — that noble material of the Pax Romana — so I might sound at least slightly knowledgeable on a topic here for once. Sure, it’s not as fun a topic as Same-Sex Marriage or drug legalization, but it may also help me in Dime’s contest this year for worst poster!
A little background: I am one of the owners of a commercial and industrial concrete construction company that does work all across the southeast and as far west as Oklahoma. Yes, it’s exactly as glamorous as it sounds! Concrete is a basically a mixture of cement, aggregate, and water. Yes, cement is an ingredient of concrete, so now you know if someone refers to concrete as “cement” it is appropriate to point and laugh.
Fellow member Captainpower was gracious enough to ask a few questions that I will try to answer adequately here:
Q: Are there different types of concrete?
A: Absolutely! It varies in both compressive and flexural strength, abrasiveness, color, reflectiveness, weight, etc.
Q: What’s the difference between good and bad (or proper and sloppy) concrete mixes and application?
A: I think this question is about the “slump” or “wetness” of concrete when it is first placed. The key measurement in a mix design that many times the “slump” indicates is the water/cement ratio. The concrete is weaker, both chemically and because of material segregation, when that ratio is too high.
Q: Are there any craftsmen left, or is everyone in it for a quick buck?
A: Yes there are some very talented craftsmen out there! It’s actually a pretty tough business to make a quick buck in, though I know residential contracting can attract some unsavory types that will try to take advantage of the unwary homeowner.
Q: Is concrete technology still progressing, or do we pretty much have it mastered since forever?
A: Concrete technology is constantly evolving and advancing, and the Romans would recognize very little about modern concrete, other than that it hardens. There are many chemical admixtures that can make concrete behave in various ways, which means it can be applied in all kinds of different ways. It can have its set time delayed indefinitely, or can be made to set almost immediately. It can be made to flow horizontally like water without losing strength, made waterproof… the list is endless.
Q: What’s better than concrete? What’s worse?
A: It depends on the application. People use concrete in all types of construction. Roadways, precast, structural building elements, kitchen counter tops, even concrete canoes. Does it make good carpet? Not especially, but a polished concrete floor that is colored or stained can look pretty darn good!
Anyone else have questions? The floor is open.
Published in General
Concretevol,
Good post. The Ricochetti come from many backgrounds and it really helps when we share them on the Member Feed. Thanks.
Thanks for this opportunity to ask you questions Concretevol. I expect to see your post promoted to main feed soon.
1. There is a block of 1930’s apartments in front of me, beside the seaside, where reinforcement bars are showing through concrete and rusting. They have been exposed I expect for maintenance. I imagine if this rusting is extensive it causes structural weakness. In what ways are new technologies in reinforcement being used to combat this problem?
2. Why use steel bars as reinforcement and not some other material, like aluminium, stainless steel, tubing, or timber?
3. I’ve heard about concrete cancer in buildings. What is that?
4. Are there any materials that concrete doesn’t get on with? I’m reminded of paint coming into contact with a grease/oil surface, this is a no no. Does concrete have its equivalent of grease that causes it to not set, spoil, or separate?
5. How do you prevent voids when pouring big cross sections of concrete?
6. What is a pre-stressed concrete floor?
7. Does concrete bend?
8. What is the tallest concrete building?
9. What prevents concrete from setting in those big Putzmeister concrete delivery trucks?
Most specs prevent you from broadcasting water on the concrete because it does cause spalling.
I’ll do what I can in Concretevol’s absence.
1. Galvanised and epoxy coated rebar is now used in corrosive environments.
2. Cost is probably an issue.
3. I’ve not heard of that.
4. Chloride ion admixtures are usually forbidden. I’m not sure if this is because of their reaction with the concrete or with the reinforcing. Salt is a no no.
5. Vibrators are used to consolidate the concrete.
6. Prestressed usually means that the panels are poured off-site and set with a crane.
7. Not sure about this, but I do know that concrete floor panels are poured with a camber, which implies that they bend.
8. No idea.
9. Most specs call for concrete to be poured withing 90 minutes of the time it’s first mixed.
Paul above alluded to the amount of water added to the concrete affecting the strength. It’s probably the most important factor. There are chemicals that can be added which give concrete more workability without affecting strength, called plasticizers. In effect, they increase slump without the strength reducing effects of water.
My only real interest in concrete is “How much does it cost?”
Spoken like a true estimator!
When they concreted Bristol Motor Speedway, Dale Earnhardt (blessed be his name) hated it and famously said “Concrete is for sidewalks, asphalt is for race tracks” lol. Of course then he proceeded to kick everyone’s ass on concrete.
I do love driving on a newly paved road. It is much smoother because it doesn’t require the expansion joints which causes that rhythmic bumping. It also doesn’t fare as well in extreme temps, wears and ruts in hot weather and is brittle in cold. While it is many times a good road surface, it is far inferior in parking lots and should never be used in heavy truck traffic areas.
This is all pretty much correct. I was at The World of Concrete expo in Vegas several years ago and was watching a demonstration and the finishers there threw water on the surface during bull floating!! We got up and walked out. lol The biggest problem with that is it drastically increased the water/cement ratio on the surface…..right where all the wear and stresses from weather occurs. The only exception I will make is your point about “tight pour structure”. Actually one of the most important things about an external slab mix is air entrainment. This creates air “bubbles” in the slab that gives the water someplace to go when it expands instead of destroying the concrete. Sometimes residential contractors aren’t aware of this and just get a standard mix for driveways and other exterior slabs. Conversely you neeever want air entrainment on an interior slick finish slab. Sealing the slab off during finishing will not allow the air to leave the slab and massive delamination can occur.
Hmmmmm, this could happen….
Man Larry, don’t get your panties in a bunch! That’s a nice business you have there by the way….hate for something to happen to it!
In my experience concrete is a more noisy road surface cf blacktop. The driving equivalent of nails over blackboard.
Are you sure using cement to refer to concrete isn’t some type of synecdoche? My FIL always uses it that way, except he pronounces it in one syllable – c’ment. ;-)
I am in NO WAY qualified to answer this, but that won’t stop me.
My understanding is that concrete has high compressive strength, but low tensile strength. The reason for reinforcement is to allow longer spans and add some tensile ability. Steel is a material with exceptionally high tensile strength — much greater than aluminum or timber.
Rolling a material into tubes increases compressive strength (try rolling paper into a cylinder and then pressing on the top), but creates weakness along the other axis. And I’m guessing stainless steel is both more expensive and less likely to offer the desired properties than steel rebar.
Hold it. You were in SC and we didn’t get together for a local craft beer? You better mean Charleston, West “By Gawd” Virginia!
Excellent post Concretevol! Your comment about the silliness of calling concrete cement reminds me of one of my fellow engineers and former co-workers who would tie himself up in knots if someone referred to concrete as cement, or referred to soil as dirt. He was easy to mess with.
To clarify the concrete / cement issue to those who weren’t up on their terminology he would point out that referring to concrete as cement, was as incorrect as referring to cake as flour. That usually helped the issue sink in for those who had not soiled themselves in the aggregated knowledge of concrete. In other words, his example cemented the differences in the mind of the public.
Let’s say, hypothetically, that one had an ugly oil stain on one’s concrete garage floor. A stain so ugly that one’s wife couldn’t walk through the garage without frowning and muttering… hypothetically. My question to Dr. Concrete is this, “What is the proper way to clean that concrete surface and restore it to its new-floor state?”
I’ll hang up and listen to your answer on the air.
Perhaps we will start a brewing thread. I am about to brew an IPA with last year’s hop crop, which I dried and froze. Will check out Untappd.
I think Randy answered a lot of these so I will either correct him or tell him where he is wrong ;)
1. Yes rusting is a concern for rebar. The rebar you see exposed in the apartments may be caused by wear to the concrete and/or the rebar was installed too close to the surface. It can be repaired though by sandblasting the rust off of the rebar and using a repair mortar to recover and seal the area. Randy is correct on this on, the most common technique to combat corrosion being epoxy coated rebar. Perhaps you have seen green rebar used in bridge construction for instance…that’s epoxy coated.
2. Randy is partly right but he is an estimator so price is all he is concerned with . lol Steel is extremely strong, flexible, readily available etc etc. Aluminum can be used at times, so can stainless, but both are cost prohibitive compared to their benefits. Fiberglass rebar is used sometimes, say under a piece of medical equipment with powerful magnets. Wood products are unacceptable because of low strength, brittleness and most importantly concrete causes them to deteriorate rapidly.
3. I believe that is just any condition where concrete is deteriorating due to contamination or surface abuse. (chemical, salt, etc)
4. The Chloride admixtures Randy mentioned were used in a lot of accelerators to achieve a faster set time. They don’t have a negative reaction with the concrete but are brutal on any steel reinforcing they touch. I mentioned you don’t want wood embedded in concrete but that is primarily for the wood. Sugar is not great for concrete at all. An old technique to retard concrete was a 2 liter bottle of coke in the mixer and you can use sugar water on the surface to achieve an exposed aggregate finish because it prevents the top layer of paste from setting.
5. Randy is on the money here. Concrete vibrators are used to consolidate the mix during placement. Special attention must be paid also to timing of the concrete delivery, temperatures, rebar spacing, placement procedures…
6. Pre-stressed is exactly that…the concrete is placed around cables or rods that already have been tensioned. They typically poured offsite and installed by crane. An example of this would be concrete bridge beams. Another common technique is post-tensioned concrete. In that case the cables are in a sleeve that the concrete is cast around on site. After the concrete achieves adequate strength the cables are tensioned which puts a compression load on the slab. This is very common in structural concrete buildings like apartments and parking garages.
7. Yes absolutely. Drive across a concrete bridge and feel it bounce. Some amount of flex is necessary to prevent shattering under extreme loads. Different mix designs can allow for more or less flexural strength, it is however a basically rigid material compared to steel.
8. In a sense, whatever the tallest building in the world is that’s the tallest concrete building. That is because steel frame buildings all incorporate concrete both as a surfacing material and to gain structural integrity. As far as the tallest concrete frame only building I would have to google it. lol
9. Several factors there. Delivery time is the biggest thing. No matter what there is a limit to how long that concrete can be on the truck. All sorts of things factor into concrete set time. Temp, overall weather conditions, the cement content of the mix. There are several retarding agents (no I am not one!) that can also slow or delay the start of the hydration process. It can in essence be put to sleep. This is useful if there is an excessively long haul time or a slow placement. Water can also be added before or during discharge if the mix is unworkable but this can be risky on the strength side and must be done within the designed water/cement ratio and be properly mixed. Lastly, the rotation of that drum is key in keeping the concrete mixed and fluid for discharge.
Hope that wasn’t too boring!!
C
Yes those are the expansion joints in the concrete. The material expands and contracts with temperature changes and must have a compressible material installed every so often in it’s length or the slab will actually buckle and crack from the stress. There are some “whitetopping” techniques to minimize this but as a rule blacktop is going to be smoother. It’s a trade off between driveability and durability.
Kim…you know you are talking to a TN hillbilly right?? lol Ok so after googling “synecdoche” I think you may have a point. I don’t think that is a correct use of the term but in some areas they do use c’ment or here it’s SEEment. I’m still ok if you want to point and laugh. :-)
I was waiting for this one.
Heck no I mean Charleston, South Cacalacky! Hello….one of my other 4 total posts was an attempt at an impromptu SC meetup. I will be back many times in the future so clear your calendar! Edmunds Oast here we come!
This is magnificent.
It must be an amazing feeling to be expert in something.
Very well done.
“You can also do some cool acid etching and stains on existing basement or garage slabs or cast your own concrete pavers/stepping stones to make a decorative walkway.”
Could this work on steps? I have concrete steps leading to my front porch that were painted. The paint is peeling and looks just awful. I’d like a better solution than removing the old paint and repainting. The house was built in 1927 and the concrete looks to be in good shape.
This is really really tough. I personally would but a nice piece of cardboard over said spot but perhaps that wouldn’t appear to one’s wife as an improvement…..
The problem with oil stains, as you may know, it can’t be scrubbed off because it isn’t on the surface but down in pores of the concrete. Here is some copied instructions I have given people that seems to have worked pretty well:
Trisodium Phosphate (TSP) will remove any oil stains whatsoever. Available in a powdered form in a cardboard box from your local Lowe’s or Home Depot. (May vary by state.) Mix with water in a 1 cup = 1 gallon strength, then apply to the floor with a mop until the water stops beading on the concrete. Keep a separate bucket of water for rinsing (or a hose) after you’re done; TSP is a bit harsh on skin or dog paws or whatnot.
One variation I haven’t tried is to mix it up along with crushed up kitty litter into a paste , its easier to isolate it over the stain that way. Let it dry and it is supposed to draw the stain up out of the concrete. Either way can still cause some color variations but you can then use a concrete stain or something to color the floor if you wish. Make sure you strip any sealers that may be on the floor first.
That’s cool. I am friends with several brewers…both home brew and commercially. There is so much variety out there now I am too lazy to do it myself however. lol
Sure you good….of course you would have to remove any finishes or paint on them now. If the concrete is in good shape you may even consider sandblasting them. That gives a great non-slip surface and can look really nice when it lightly exposes the aggregate, especially with old concrete which may have colored stone in it like river rocks. Then you can stain or just apply a concrete sealer as is!
Could see it coming right?? Not even kidding though, google is a wonderful thing.