CONCRETE MIXES, STRENGTHS, PRICES & USES

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Re: What is "Steng"? Is it a distorted Thai version of "Stre

Postby oil » Sun Jun 15, 2014 2:17 pm

Khunchai wrote:Please excuse my ignorance, what is "Steng"? Is it a distorted Thai version of the English word "Strength"?

In the agreement we are about to sign, our construction team boss wants to specify the concrete mix grade as ปูนซีเมนต์ปอร์ตแลนด์ ประเภท 1 หรือปูนผสมเสร็จกำลังอัด 240 กก./ตร.ซม.ขึ้นไป

Which I translate into English as "ready-mixed cement or mortar with compressive strength of 240 kg / sq.cm. or more"

Is it the same as "Steng" number 240?

So when we say "Steng" 240, do we mean the compressive strength of the dried concrete measured in kilograms per square centimeter?

Is my understanding correct or not :?:

want to know that oo
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby Roger Ramjet » Sun Jun 15, 2014 4:07 pm

To be honest I can't find my old invoices, but steng means compression strength. I seem to recall 1200, 1400, 1600 and 1800 steng though. If you go to this page it has all the data you will need. http://www.google.co.th/search?q=streng ... B550%3B284
The minimum steng for load bearings columns is the strongest, for a slab it can be less depending if you are going to float it or not. Basically it's the amount of cement that is added at the factory to the mix. You can have a company test it after 30 days and tell you if you got the correct mix or not.... a slump test will tell you nothing other than the amount of water added, a "steng" test will tell you what the compression strength is after 30 days.
Most concrete companies will supply all the data you ask for. I had a rep from FastCrete on site for all my big pours, he and my builder both checked the trucks before they would allow them to be poured.
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Re: Superblock vs Double Red Brick (pros and cons of superbl

Postby BKKBILL » Sat Jul 05, 2014 10:23 am

canopy wrote:I find this comment concerning. In order for concrete to reach its proper design strength, it must be properly cured. Stripping off the forms quickly is a good way to assure this doesn't happen. Improperly cured concrete is very weak--maybe just half strength or so. The first week of curing is absolutely critical.


It would be nice to see some evidence that stripping forms the day after a pour has any effect on concrete strength since this is the way most jobs are done. Remove forms wrap columns in plastic or as it was done before burlap with hosing to keep damp. It is not the form giving concrete it's strength it is there to keep concrete in place until set.

Here is a excerpt from a story done on the 40 floor Trump towers in New York. A three day cycle.


The cycle typically starts at one of the city’s roughly 40 concrete plants, all of which are outside Manhattan, before the sun comes up. After concrete is mixed and placed into a truck, it must be poured within about 90 minutes.

New York City traffic sometimes makes this deadline a nail-biter, but William Lyons, president of the Concrete Industry Board, said the batches almost always arrive on time.

As early as 7 a.m., the pouring at construction sites begins. Laborers shovel the cement out of buckets hoisted to the floor that the forms are on, and masons smooth it out. By afternoon, as the cement begins to set, carpenters and engineers are already able to stand on it, to map out and erect the wooden forms for the next level — sometimes while concrete is still being poured at another corner of that floor.

The next day, reinforcement bars, electrical conduits and plumbing pipes that will be encased in concrete are installed on that next level, and the forms are ready for the fresh batch of concrete scheduled to arrive the following morning. When bad weather does not slow things down, “It’s quite an orchestra,” said Robert A. Ledwith, the business manager for Metallic Lathers Local 46, the union whose membership includes people who install rebar.
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Re: Superblock vs Double Red Brick (pros and cons of superbl

Postby BKKBILL » Sat Jul 05, 2014 11:04 am

This really should be in cement, concrete and mortar but a little more on stripping time for forms. It seems Thais are changing over to steel forms so damage to concrete should be minimum, 12 hours @ 20C.

How Early Can Formwork Be Stripped?

By 
Concrete Construction Staff
 
http://www.concreteconstruction.net/con ... ipped.aspx

One way to save money on a repetitive forming job is to cycle the forms more quickly. But anyone in the business knows the risk or producing bad surfaces by removing the forms too early. Balancing such risks against cost savings is the subject of a recent technical report by the Cement and Concrete Association on "Mechanical Damage to Concrete by Early Removal of Formwork." Research was conducted on factors that affect the earliest time that forms can be removed without damaging the concrete. Forming materials, release agents, reuse of forms, concrete strengths and proportions and type of concrete vibration were studied. The main conclusions can be briefly summarized. (1) As might be expected, especially high levels of damage occurred when the concrete was very green. But while it took little time stress from stripping to cause damage at early ages, after approximately 8 hours curing at 68 degrees F or the equivalent, damage during stripping occurred only at relatively high stresses. (2) The probability of causing damage to the concrete by removing forms at any given age was similar for plastic-faced plywood. (3) It was concluded that for exposed high-quality work unsealed plywood formwork may be removed without undue risk of mechanical damage if the concrete has attained an in-place strength of 290 psi as measured by cubes after a curing period equivalent to 12 hours at 68 degrees F. (4) With unsealed plywood treated with release agent, the texture and the number of uses affect the stresses produced in stripping. These peak at about the second use and then taper off. Tests on plywood without release agent showed that initially saturated and air-dried plywood require no difference in striping stresses. (5) Stripping caused less damage when a release agent of any type was used with plywood formwork than if none was used. But for unsealed plywood, an oil containing additives was the most effective of the release agents tested.
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby Mike Judd » Sat Jul 05, 2014 6:01 pm

P1020117.JPG
The finished Silos, You can see the lines of the forms.
P1020118.JPG
What a fine body of men. That's me bottom left.
Re concrete curing, I don't profess to be a Technical expert on the subject, just an observer over many years.What I understand is the whole object is not to have the surface of the concrete dry out quicker than the rest of it. Usually more important with an on ground slab where only the top is open to the air. With beams and columns they are drying out all round, hence the practice, (at least at all the Sydney sites I've worked on ) of stripping the columns any time after the next day. Obviously you don't want to be sticking Pinch Bars etc; in-between the green concrete and the forms damaging it. Beams are usually left with some supports for quite a while as there is a lot of loading with the next floor being formed up and poured. But there are many more important things about concrete strength than exposing it to the air, the mix and placing comes first apart from from the steel and it's placing. Here's some photos of a concrete Sugar Silo in England I worked on. The walls were 300m.m. thick X 40 mts high with a circumference of 80 mts. It had a system of formwork 8mts high that climbed up steel rods on jacks at the max rate of 300m.m. per hour non stop. That meant that 24hrs after the concrete went into the form it came out of the bottom supporting everything above it. That was over 50 years ago , but I believe we reached the top in 6 or 7 days, our job was to keep building the scaffolding ahead of them, that they all worked on, the steel fixers, the concrete guys wheeling the barrows round from the hoist, there were no concrete pumps those days. There was a specially built scaffold fixed under the form on the inside for some guys to float finish smooth the concrete as it was exposed. It was obviously a special mix but curing it didn't come into the equation.
Attachments
P1020119.JPG
Back there months later taking it all down.
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby Sometimewoodworker » Sat Jul 05, 2014 6:57 pm

AAMOF you never want the concrete surface to dry out. You want the correct amount of water in the concrete to stay in the concrete to alow the concrete to cure. Depending on the mix it can be cured enough to walk on within an hour, but you should try to keep the surface from drying for 7 days then a good percentage of the final strength is achieved at about 30 days but from what I understand it is many years (hundreds?) before it stops curing.

FWIW There is some debate as to whether roman concrete has finished curing.
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby Mike Judd » Sun Jul 06, 2014 6:25 am

I would amend my post to say that no special steps were taken to keep the outside of the silo walls wet once the forms climbed up throughout the day. As stated concrete is supposed to go on actually curing for years depending on it's bulk. Test cylinders are about 100m.m. dia which you have 4 for every truck load, they are then crushed in a lab at 7--14--21-- and finally 28 days for the last one to see if they meet the required strength specified of the mix ordered. Personally I am more concerned with correct steel placement and stopping them putting too much water in the mix, I just hose the slab for a few days . Wrapping the columns and beams certainly will not go amiss, but not something that I have found to be essential.
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby canopy » Sun Jul 06, 2014 6:46 am

Just keep in mind once concrete becomes dry the curing and strengthening stops and the process will never start again no matter how wet you make it. Curing impacts every attribute of concrete, not just strength. Different types of concrete usage have different methods of curing such as a slab vs a column. I like to follow ACI 308 for curing and have been very impressed with the results.
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby Sometimewoodworker » Sun Jul 06, 2014 10:03 pm

canopy wrote:Just keep in mind once concrete becomes dry the curing and strengthening stops and the process will never start again no matter how wet you make it. Curing impacts every attribute of concrete, not just strength. Different types of concrete usage have different methods of curing such as a slab vs a column. I like to follow ACI 308 for curing and have been very impressed with the results.

Sorry that is just wrong. You are correct that the surface should be kept from drying out too fast but concrete takes months or in some cases years to cure.
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby BKKBILL » Sun Jul 06, 2014 10:49 pm

Sometimewoodworker wrote:
canopy wrote:Just keep in mind once concrete becomes dry the curing and strengthening stops and the process will never start again no matter how wet you make it. Curing impacts every attribute of concrete, not just strength. Different types of concrete usage have different methods of curing such as a slab vs a column. I like to follow ACI 308 for curing and have been very impressed with the results.

Sorry that is just wrong. You are correct that the surface should be kept from drying out too fast but concrete takes months or in some cases years to cure.


Q:    I was always under the assumption concrete came to a full cure in about 28 or 29 days. Recently I heard a piece of trivia that said Hoover dam has portions in it that won't fully cure for 500 years. What can you tell me about concrete cure time?


A:     Technically speaking, the concrete in Hoover Dam should continue to gain strength forever. This is true for any concrete that has sufficient moisture for the cement hydration process to continue. 
Practically speaking, most concretes gain well over 90% of their total strength in the first few months after placement. After that, the strength gain will continue if moisture is present, but the rate of gain is very slight.

The 28-day strength of concrete is important because that is the basis of acceptance for most projects. For example, if a project specifies 4000 psi concrete, the concrete does not have to reach that level of strength for 28 days. Typically, it is expected that a concrete mix will reach 50% of its design strength in 3 days, 75% in 7 days, and 100% at 28 days, as determined by breaking concrete test cylinders that are made on the day of placement. 
The 28-day requirement is really a holdover from many years ago, as some projects now specify that the design strength be reached much sooner. In Tennessee, ready mix producers have supplied concrete that reaches 3000 psi in only 24 hours, and sometimes in as little as 16 hours. Of course, this concrete will continue to gain strength as long as it properly cured by keeping moisture available, and will often reach ultimate strengths of 8000 psi or more.

http://www.getconcrete.org/index.php?op ... &Itemid=25
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby canopy » Mon Jul 07, 2014 7:05 am

I was always under the assumption concrete came to a full cure in about 28 or 29 daylong time to cure.


Not quite true, but you are onto something. Typical concrete reaches sufficient design strength in that amount of time. That is why in America it is illegal to put weight on a foundation within 30 days of pouring. After 30 days concrete continues to cure and strengthen even more provided you don't let it dry out.

This graph shows how critical curing is to make strong concrete. If it had another test showing what would happen in Thailand if you let your concrete bake in the hot sun and/or be in contact with dry earth the strength would be far worse than the lowest line in this graph:

curing.png
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby Mike Judd » Mon Jul 07, 2014 7:44 am

My whole point with pictures of those Silos was that it's the mix /vibration that is the most important part of any concreting that is carried out, for it to carry all the tons of weight 24 hrs after placement without any steps to keep the concrete moist throughout the process. With slabs and the tops of beams , minor surface cracks that usually occur on very hot days can be fixed with troweling at the right time when initially going off, then the ultimate strength of your concrete is going to depend on how much cement went into the mix , the steel and the correct amount of water. With the steel ,plain round rods are never going to be as efficient as deformed. In western countries ,or at least in Oz ,all the mesh is now made with deformed steel for greater gripping power. In Thailand I couldn't even find any mesh ,rolls or sheets with the steel any bigger than 5m.m. had to make my own every time.
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby canopy » Mon Jul 07, 2014 8:21 am

Couldn't agree more there Mike; there are just so many things you have to do right to get strong concrete. It's a fascinating topic. Due to the lack of building codes / inspections in Thailand it has resulted in builders that are completely oblivious to it all and could hardly care less. What gets me is they just toss some arbitrary quantities of material in a tub and hack it a few times with a hoe and consider it job done. Proper mixing means 7 minutes minimum rotating in the cylinder. Try doing that with a hoe.

As you mention the mesh in Thailand is poor quality and small diameter, but there is a simple way around this. What I do is make "mesh" by laying out normal big diameter rebar used elsewhere in the slab in both directions and making the squares at a wider spacing to get the proper kilograms of steel per square meter of concrete. In so doing it means your mesh is deformed steel and can be full steel (baw law saw in Thai). I hope this tip is useful to someone as it was to me from a professional contractor.
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby BKKBILL » Mon Jul 07, 2014 10:24 am

canopy wrote:Not quite true, but you are onto something. Typical concrete reaches sufficient design strength in that amount of time. That is why in America it is illegal to put weight on a foundation within 30 days of pouring.


Do you have a location as I have never heard of this before and would be interested seeing the law.
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Re: CONCRETE MIXES, STRENGTHS, PRICES & USES

Postby gliffaes » Wed Jul 09, 2014 10:21 am

Mike Judd wrote:I would amend my post to say that no special steps were taken to keep the outside of the silo walls wet once the forms climbed up throughout the day. As stated concrete is supposed to go on actually curing for years depending on it's bulk. Test cylinders are about 100m.m. dia which you have 4 for every truck load, they are then crushed in a lab at 7--14--21-- and finally 28 days for the last one to see if they meet the required strength specified of the mix ordered. Personally I am more concerned with correct steel placement and stopping them putting too much water in the mix, I just hose the slab for a few days . Wrapping the columns and beams certainly will not go amiss, but not something that I have found to be essential.


its England you dont need to keep them wet, we have "weather" for that :lol:
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