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Compressive strength of concrete
Last Post 07 Apr 2011 05:34 PM by jonr. 16 Replies. |
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parvizb
New Member
Posts:1
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| 05 Jan 2009 02:31 AM |
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Dear All
I am new user. I would like to learn if there is any way or formula to calculate concrete compressive strength theoretically before concrete preparation (by means of characteristics of materials used for concrete). |
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Bruce Frey
Basic Member
Posts:429
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| 05 Jan 2009 04:29 PM |
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Concrete supply companies do this all the time (along with experience) and confirm the results with trial batches. I had a course that touched on concrete mix design back in college and it is not exactly "cookbook".
You can get books about it through ACI (american Concrete Institute) and technical bookstores.
Bruce |
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Manfred
Basic Member
Posts:200
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| 06 Jan 2009 08:40 AM |
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Concrete is a very complex material mix together with several diverent aggregates using only one thing in common, portland cement. Right now portland cement is the only binder tested worldwide that will allow and provide structural strenght. There are other binders available, but their are non-cementitious and not tested with code approval.
In my travels I have found a recurring measurement regardless of location, whether it be Kenya, Germany or the US. Per cubic yard of concrete (27 cubic feet) you will use so many bags of cement in order to achieve this strenght:
5 bags = 2500psi
5 1/2 bags = 3000psi
6 bags = 3500psi
6 1/2 bags = 4000psi
This will get you started. Other than Canada, US and the Caribbean most measurments are metric. You then are dealing with cubic meters but the measurment ratio remains the same. |
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| Manfred Knobel<br>Moss Pointe Builders, Inc. |
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Birdman
Basic Member
Posts:179
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| 06 Jan 2009 10:56 AM |
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Since this is the green building talk it should be noted that the process of manufacturing cement is pretty un-green. Although it produces a durable material (which is a green concept) the process is very energy intensive. Manfred states that cement is the only binder that will provide strength. It should be noted that up to 35% of the cement in a mix can be replaced with flyash and this actually improves the finished concrete product in a number of ways.
I certainly don't advocate for eliminating concrete, however, in doing designs one should work toward using concrete as efficiently as possible (as with all materials). For example, a typical house sits on an 8" continuous concrete foundation wall. If one were to engineer the foundation to use minimal concrete and still provide generously adequate strength to support a house about 2/3 of the concrete could be eliminated. This would involve more formwork and more strategic steel placement and therefor more cost - but it would save on an energy intense material. In essence most of the concrete in a typical foundation is just there to take up space and simplify forming.
In answer to the original question, yes, mix design is a highly predicable thing and has been studied to the nth degree. You can research it however deeply you care to go - many PhD's have.
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Manfred
Basic Member
Posts:200
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| 07 Jan 2009 07:16 AM |
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To add to Birdman's good answer:
Look at geopolymers, there are binders available out there that are "very green" yet untested, hence not allowable for structural support. There are theories even that the Egyptians used geopolymers to cast in place the heavy sand stones that make up the pyramids - they did not drag them up there from a quarry - again a very interesting theory and revelation. |
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| Manfred Knobel<br>Moss Pointe Builders, Inc. |
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Summers
New Member
Posts:40
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| 14 Jan 2009 05:12 PM |
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In " Theory " the compressive strength of concrete is dictated by the % of cement content. Additives such as flyash are substituted for cement in some cases without affecting compressive strentgh. Probably the only point left out for a discussion on strentgh has been actual placement, curing and protection of a concrete structure.
The mix is the mix ! After finishing, care should be taken to insure proper hydration as the concrete cures out. Common methods involve curing compounds and or evaporation retarders that hold the moisture in, preventing premature hydration. This should assure you of the desired strentgh called for in the mix design.
A Swiss Hammer test is usually the easiest way to determine strength. Fast, accurate and inexpensive to accomplish.
Birdman, made the point that cement is "un-green" due to the manufacturing process ! He goes on further to state " one should work toward using concrete as efficiently as possible". Very true statements !
The best placed and cured concrete, immediately begins to deteriorate when it is exposed to the earths atmosphere. Carbonation caused by Co2 on the surface affects the the first 3-5 mm in weeks. Higher concentrations will be deeper. A carbonated surface allows moisture and contaminates into the matrix and a loss of pH is experienced. Once below 11.5 pH any steel in the concrete looses the Passivating Layer and corrosion cells begin to form on the re-bar. At this point concrete really becomes "un-green", and commences a short life cycle due to the corrosion! The speed of the corrosion will be determined and exacerbated by porosity, contaminates induced, chemical migration out of the matrix and internal moisture content of the concrete.
Once you have determined what strentgh is required, "Make Your Concrete Green" by application a of liquid penetrating corrosion inhibitor as soon as possible. Don't be fooled by ones containing sodium silicates or silanes! Not only will it serve to cure and stop evaporation for a suprior cure, it also Waterproofs and gives Chemical Resistance to a surface in one application. Does not need to be removed like a curing compound and because it forms a solid Inside the gel pore structure, it densifies and adds compressive strentgh. As the years pass, keeping contaminates and moisture out, will also avoid low pH corrosion cells on the steel.
Manfred described concrete as a "very complex material". In actuality it is a "Thermodynamically Unstable Material" and until we have a full understanding of that and begin protecting it from Day One, it won't be as Green as we'd like to think or hope ! |
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| The Cost Effective Answer to Concrete Corrosion |
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Farmboy
Basic Member
Posts:356
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| 15 Jan 2009 11:29 AM |
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Summers, good info. I'm planning to build an ICF home with a 5" slab on grade reinforced with an 18" grid of #4 grade 60 rebar and 2" insulation. We will use the slab as our finished floor.
Is a "corrosion inhibiter" the same as a "sealer" that our concrete supplier recommended for our shed floor?
Usually an acidic liiquid is used to prep the surface and washed off prior to applying the stain. This opens the pores so the stain will take more uniformly. If we fill and seal those "pores" do we lesson deterioration and maintain compressive strength?
Also is there much advantage to painting the rebar with a red oxide to cover surface rust?
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Summers
New Member
Posts:40
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| 18 Jan 2009 04:59 PM |
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Farmboy,
Corrosion Inhibitors, usually refers to any number of materials used to prevent oxidation of metals. Passive, Anodic and Cathodic inhibitors are intended for specific uses. My reference to an inhibitor in the last post was for a passive penetrant that protects the steel, by waterproofing the concrete, by giving chemical resistance to a surface, by maintaining high pH around the steel and promoting a high ohms resistance of concretes that slow electron flow from anodic/cathodic areas. [ corrosion cells] You can PM me to discuss further if you like.
I doubt, what I recommend would be the same as your concrete supplier. Volumes of material have been published on "Sealers". Some good and some not worth putting on. Points to consider when choosing: What is the warranty? Does the spec caution not to allow the product to come in contact with metals or glass? Does the spec provide for a Re-Application? [ if so, what happened to the first coat ]? Are there adhesion issues? That should get you started in the right direction! Suffice to say, there are Sealers! and Sealers?
I cringe at the thought of purposefully putting an acid on a concrete, since we specialize preventing acidic attack! Yes, it opens the gel-pore structure for better stain penetration, at the same time lowers the pH and causes the limestone binder to dissolve. I recommend a very mild acid wash, followed by a clean water rinse, then an application of a wash to restore [at least back to nuetral] the pH of a surface. After your stain application, when you waterproof, as I recommend, you will stop deterioration and add compressive strentgh, with no change in appearance of your stain.
Several studies have been commisioned regarding coated re-bar. FDOT learned a lesson with Seven Mile Bridge going to Key West. A nick or a pinhole in even Epoxy coated steel causes the entire electrochemical reaction to focus all the milivolt current at the compromised site. As opposed to surface oxidation dispersed evenly over say a ten foot length, it is concentrated at the exposed site, corroding completely through the steel. Then you have a structural failure instead of rust on the re-bar. Doesn't hurt to paint, however it is now understood that maintaining a pH above 11.5 on the steel, keeping chloride thresholds Low and maintaining a high resistive low moisture matrix is the answer.
Adopt this mind set and we can truly," Make Concrete Green" and get the design life cycle we intend. |
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| The Cost Effective Answer to Concrete Corrosion |
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Bruce Frey
Basic Member
Posts:429
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| 19 Jan 2009 07:37 AM |
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Posted By Farmboy on 01/15/2009 11:29 AM
Summers, good info. I'm planning to build an ICF home with a 5" slab on grade reinforced with an 18" grid of #4 grade 60 rebar and 2" insulation. We will use the slab as our finished floor.
Is a "corrosion inhibiter" the same as a "sealer" that our concrete supplier recommended for our shed floor?
Usually an acidic liiquid is used to prep the surface and washed off prior to applying the stain. This opens the pores so the stain will take more uniformly. If we fill and seal those "pores" do we lesson deterioration and maintain compressive strength?
Also is there much advantage to painting the rebar with a red oxide to cover surface rust?
I think in your specific application, the need for a "sealer" is more related to aesthetics and maintenance than for corrosion protection, which is an issue, personally, I would worry about in your situation. If you start mixing products, make sure there are no compatibility issues. Summers raised valid concerns about concrete deterioriation and for a concrete structure, especially an outdoor stucture, a corrosion inhibitor is something to think about. If you use epoxy covered rebar ("greenbar"), you absolutely do have to be meticulous in patching damaged areas for the reasons he mentioned. Painting normal rebar, however, is generally a waste of time and money. Normal light surface rust is not a problem. If it is heavy rust or scaling, it should be wire brushed. Bruce |
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Summers
New Member
Posts:40
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| 21 Jan 2009 09:44 PM |
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Farmboy,
If you read the "compatability issues" comment from Bruce Frey, take it to heart! That is why I mentioned, points to consider when choosing a sealer.
If you go to www.StableCrete.com you'll see more information and Q&A regarding compatability. Once had a G.C. who after applying StableCrete, called to tell us he had catastophic coating failures on balconey and walkways of a high rise condominum concrete restoration, [ 18 story building ]. Investigation revealed, a positive waterproofing of all surfaces, that was intended. Problem was, neither the G.C. or the painting contractor followed the engineers specification.
An inexpensive Latex deck coating was applied, that didn't withstand long term exposure to water. Because the surfaces were waterproofed, all areas that had standing water after the first rain [depresions and low spots] demonstrated a coating that actually peeled of the substrate. Problem was a poor quality paint that adhered very well on dry surfaces, but didn't have a binder in it to withstand the water that ponded until it evaporated from the low spots.
Sandblasting the building was necessary, with the cost being borne by the coating supplier. Best to follow a spec, check compatability and ask the questions before you jump!
Glenn |
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| The Cost Effective Answer to Concrete Corrosion |
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Bruce Frey
Basic Member
Posts:429
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| 22 Jan 2009 03:36 AM |
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Farmboy,
I made a rather serious typo in my previous response. The paragraph should read:
I think in your specific application, the need for a "sealer" is more related to aesthetics and maintenance than for corrosion protection, which is an issue, personally, I would NOT worry about in your situation. If you start mixing products, make sure there are no compatibility issues.
Corrosion is not a likely problem for your situation in my opinion.
My humble apologies for being in a hurry and not adequately proofreading my response.
Bruce |
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Farmboy
Basic Member
Posts:356
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| 24 Jan 2009 03:42 PM |
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Glenn and Bruce, my awareness of concrete strength and durability is much higher now thanks to your inputs. No apology required!
Just returned from the Int'l Home Builder's Show in Las Vegas where I discussed floor staining with a Kemiko rep. They recommend using a non-acid wash to prep the concrete, however, their STAIN which does have an acid in it must be rinsed well (they emphasize this) prior to waxing and sealing. Given my slab will be insulated at bottom and edges, it won't be exposed to the exterior elements that can "corrode" and weaken it.
Your cautions about compatibility of products is taken seriously. Your example, Glenn, points this out well.
We'd like to stain and stamp our exterior flatwork, so as long as we take measures to properly cure, finish and seal it we should expect long-lasting stucturally sound products. Dave
PS If anyone attends an event at the Las Vegas Convention Center, be prepared to walk on a lot of concrete. Those halls are huge!!
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pie2mats
New Member
Posts:9
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| 06 Apr 2011 08:23 PM |
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hello everyone!
well, that's a good way to put it. nice information resource works well for my future endeavor of the service while keeping it environmentally friendly. a good way to lessen the carbon footprint we leave behind.
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jonr
Senior Member
Posts:5341
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| 06 Apr 2011 09:12 PM |
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Proper curing (ie, moisture) accounts for more than 1/2 of concrete's strength. Good design and things like post-tensioned steel can greatly reduce the amount of concrete used.
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McFish
New Member
Posts:77
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| 07 Apr 2011 12:54 PM |
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Please be aware of an alternative to steel rebar. Basalt rebar is stronger, lighter, and does not corrode. It is less effected by PH changes. It is more expensive, but getting cheaper all the time. Since it doesn't rust, there is no spalling, and concrete will last a lot longer. There may still be issues with code officials, but performance data is available on many websites. It may not be cost effective in your application, but it will be before too many years. As with many of the geopolymers, it is a much greener material than steel or concrete. As mentioned earlier, concrete takes a lot of energy to create; the magnesium geopolymers much less. Basalt rebar is made from what is basically rock, so the raw material is much cheaper than steel. |
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Alton
Veteran Member
Posts:2164
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| 07 Apr 2011 04:08 PM |
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There may also be issues with the structural engineer since some do not like to get out of their comfort zone. I think it will take some time for the existing engineers to accept something new like Basalt rebar. I am all for this new rebar but I will still need to have the plans wet stamped by an engineer. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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jonr
Senior Member
Posts:5341
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| 07 Apr 2011 05:34 PM |
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If salt isn't present then I don't know that non-steel rebar is any improvement wrt corrosion. |
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