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queetz@home
Apr 10, 2006, 11:11 PM
Here is a skill testing question(s) for you engineer types out there. I am obsessively been observing this new high rise under construction in Vancouver and one thing that bugs me is I notice that they pour the concrete for new floors and columns when the weather is kinda rainy.

As a matter of fact, we have had three days of sunny weather in which one of them (Friday), they poured one half of the floor. However, the concrete pour does not happen on the weekend(even though the website said they would) as there was practically no activity on the site (that I can see). And today, when it started to rain, the concrete pour continues.

I was under the impression that for any concrete pour, it is not wise to them then during wet weather.

1) Is it normal for construction projects to pour while its raining?

2) or does it really matter if the concrete pour occurs while raining?...i.e., the concrete settles regardless of the conditions outside?

Le1000
Apr 10, 2006, 11:42 PM
The more water is in the concrete the longer it will take for it to dry, but it will not affect the solidity of it. The drying process will ‘cause the water molecules to evaporate because the concrete is made of tiny crytals which prevent it from bonding with the water. The only issue I can see with pouring concrete in the rain is that the water droplets might dent the smooth surface.

Eddy_A
Apr 11, 2006, 12:02 AM
The more water is in the concrete the longer it will take for it to dry, but it will not affect the solidity of it. The drying process will ‘cause the water molecules to evaporate because the concrete is made of tiny crytals which prevent it from bonding with the water. The only issue I can see with pouring concrete in the rain is that the water droplets might dent the smooth surface.

Yeah the longer is takes to cure(not dry) that better the different compenents bond.

Queetz: Concrete gets poured under water all the time and cures...why would some rain stop it?

queetz@home
Apr 11, 2006, 1:47 AM
I dunno....that's why I'm asking for input...

Below is the article, which shows the harmful effects of rain to freshly finished concrete. This is why I'm wondering why despite the information below, they still pour concrete on the construction project while its still raining...

http://www.cement.org/tech/faq_rain.asp

Frequently Asked Questions
Cement & Concrete Technology Home > FAQs > Rain on fresh concrete

Q: How does rain affect fresh concrete?

A contractor placed a concrete driveway for me last night. Before they could start the finishing work, it rained for several minutes. We covered everything we could but the concrete still got quite wet. In the end, they were able to finish (floating and edging), but I am really concerned how this will hold up long term.

A:Rainfall during placement of concrete flatwork can present challenges to achieving a quality concrete. Potential outcomes range from no damage to a weakened nondurable surface. Only time will tell at which end of the range your specific situation will fall. Descriptions of a best case scenario and a worst case scenario follow:

Plastic sheets used to protect the freshly placed concrete from rain.

Best case: The concrete is protected as much as possible from the falling rain. After the rain has stopped, the water that has fallen on the surface is allowed to evaporate just as bleed water from the original concrete mixture must be allowed to evaporate prior to proceeding with finishing operations. To substantially change the water-cement ratio (w/c) of the concrete at the surface of the slab, energy must be added to the system, typically in the form of troweling passes with excess water on the concrete surface. If the water is allowed to evaporate, the w/c remains reasonably low, and since w/c governs the strength of the concrete there is no substantial damage to the finished surface. In extreme cases it is not uncommon to physically remove excess water from the slab surface by dragging a garden hose or a broom across the concrete surface to lower the volume of water that must evaporate. With proper timing and process, the durability of the concrete is not affected.

Typical scaling of concrete slab due to rain on nondurable paste surface

Worst case: The concrete is not protected from the rain; the water is not allowed to evaporate from the slab surface; and multiple passes of the floats and trowels used to finish the surface are made with the surface moisture in place. The energy supplied by the finishing operations mixes the excess water into the slab surface creating a high w/c ratio in the near surface of the concrete reducing its strength and thus its durability. In the worst situation, the damage to the concrete surface is readily apparent since the texture of the surface is easily damaged or removed after the initial curing period. (If the surface is dusty after 14 days of curing there a likely to be a problem.) If the surface strength is only slightly affected, the long term durability of the concrete may be reduced as evidenced by a general loss of the surface mortar (scaling) after the concrete has been through a winter season of freezing and thawing cycles; however, the concrete strength and durability below the surface would not be affected.

In most cases concrete is warranted for one year, which will allow you to assess the potential durability of the concrete surface, and in instances similar to this one many contractors are willing to extend the warranty for an additional period of time (an extra year or two) to settle the doubt.

--------------

Note that I did see them cover the freshly poured concrete a few times, most specially on January when it was raining non-stop in Vcr for 28 days. But other than that, including today, there is no covering...

Le1000
Apr 11, 2006, 2:50 AM
I dunno....that's why I'm asking for input...

Below is the article, which shows the harmful effects of rain to freshly finished concrete. This is why I'm wondering why despite the information below, they still pour concrete on the construction project while its still raining...

http://www.cement.org/tech/faq_rain.asp

Frequently Asked Questions
Cement & Concrete Technology Home > FAQs > Rain on fresh concrete

Q: How does rain affect fresh concrete?

A contractor placed a concrete driveway for me last night. Before they could start the finishing work, it rained for several minutes. We covered everything we could but the concrete still got quite wet. In the end, they were able to finish (floating and edging), but I am really concerned how this will hold up long term.

A:Rainfall during placement of concrete flatwork can present challenges to achieving a quality concrete. Potential outcomes range from no damage to a weakened nondurable surface. Only time will tell at which end of the range your specific situation will fall. Descriptions of a best case scenario and a worst case scenario follow:

Plastic sheets used to protect the freshly placed concrete from rain.

Best case: The concrete is protected as much as possible from the falling rain. After the rain has stopped, the water that has fallen on the surface is allowed to evaporate just as bleed water from the original concrete mixture must be allowed to evaporate prior to proceeding with finishing operations. To substantially change the water-cement ratio (w/c) of the concrete at the surface of the slab, energy must be added to the system, typically in the form of troweling passes with excess water on the concrete surface. If the water is allowed to evaporate, the w/c remains reasonably low, and since w/c governs the strength of the concrete there is no substantial damage to the finished surface. In extreme cases it is not uncommon to physically remove excess water from the slab surface by dragging a garden hose or a broom across the concrete surface to lower the volume of water that must evaporate. With proper timing and process, the durability of the concrete is not affected.

Typical scaling of concrete slab due to rain on nondurable paste surface

Worst case: The concrete is not protected from the rain; the water is not allowed to evaporate from the slab surface; and multiple passes of the floats and trowels used to finish the surface are made with the surface moisture in place. The energy supplied by the finishing operations mixes the excess water into the slab surface creating a high w/c ratio in the near surface of the concrete reducing its strength and thus its durability. In the worst situation, the damage to the concrete surface is readily apparent since the texture of the surface is easily damaged or removed after the initial curing period. (If the surface is dusty after 14 days of curing there a likely to be a problem.) If the surface strength is only slightly affected, the long term durability of the concrete may be reduced as evidenced by a general loss of the surface mortar (scaling) after the concrete has been through a winter season of freezing and thawing cycles; however, the concrete strength and durability below the surface would not be affected.

In most cases concrete is warranted for one year, which will allow you to assess the potential durability of the concrete surface, and in instances similar to this one many contractors are willing to extend the warranty for an additional period of time (an extra year or two) to settle the doubt.

--------------

Note that I did see them cover the freshly poured concrete a few times, most specially on January when it was raining non-stop in Vcr for 28 days. But other than that, including today, there is no covering...

That's very interesting. Now that I think about it would weaken the concrete if the water molecule dried after the concrete cured, because it would leave air gaps.

Liz
Apr 11, 2006, 3:37 AM
as long as fresh concrete's covered when its raining hard, there shouldn't be many problems with it. while some driveway contractors might mix rainwater into concrete, an experienced contractor isn't gonna make that mistake. The water may affect some of the concrete and create more voids on the top of the surface, but since that's the compression zone and concrete flooring is more vulnerable in tension and the steel has adequate cover, it wont undermine the strctural integrity.

as for them not working on weekends, I don't know about canada but around here even if a crew is authorized to work on weekends the contractor will not do it unless it is needed since workers must be compensated more for it.

that said different mixes cure at different rates when the temperature and humidity change, so using maturity testing would be the preffered method for determining when the concrete is stong enough for the next level. accoustic testing can also determine if there's damage and how deep on top of the slab.

Kelvin
Apr 12, 2006, 8:44 PM
The more water is in the concrete the longer it will take for it to dry, but it will not affect the solidity of it. The drying process will ‘cause the water molecules to evaporate because the concrete is made of tiny crytals which prevent it from bonding with the water. The only issue I can see with pouring concrete in the rain is that the water droplets might dent the smooth surface.

Portland Cement absolutely requires water in order to hydrate (dry & harden). This process will happen regardless of the external moisture content or RH. In fact, portland cement will hydrate under water (e.g. Tremie Concrete). The amount of water in the mix does not change the time to achieve complete hydration.

Hydraulic cements require a certain amount of water to initiate and complete hydration. Excess water in the mix then does one of two things: it creates better workability and it reduces the ultimate strength. In the bad old days, we had a basic water-to-cement ratio of 0.4 to 0.6 that allowed for a balance between the two. Today, we have superplasticisers that allow us to mix superflowable (and ultraflowable) concretes that permit us only to mix the absolute minimim water (w/c of 0.2 to 0.4) and maximise strength potential.

Kelvin
Apr 12, 2006, 8:54 PM
To answer the question, and as discussed in the PCA Q&A avove, rain would only be expected to affect the top most portion of the concrete (the so-called paste layer approx. 5mm to 10mm deep) and have no structural concern beyond that.

Where large pieces of flatwork (driveways or slabs) are being poured, it may cause scaling, dusting and pitting. Often however, structural slabs are often topped with "nonstructural" surfacing once the building is closed in, so a high quality slab is eventually achieved.

On vertical pours (columns and walls for example), falling rain is esier to control (with a few simple tarps) and would be of negliable concequence to the pour anyway.

KevinFromTexas
Apr 26, 2006, 11:25 AM
Isn't there a type of concrete that will actually set up under water? I think I heard something about that on some dam construction project or else a tunnel or something like that. Or am I just smoking too much?

Eddy_A
Apr 26, 2006, 8:17 PM
Isn't there a type of concrete that will actually set up under water? I think I heard something about that on some dam construction project or else a tunnel or something like that. Or am I just smoking too much?

There different concrete classes/types.

II
IIA
III
etc.

Can't remember all the different types and what they do.
But yes there are specific types and variations that cure better underwater, some which are ideal curing exposed to cold weather etc.

Kelvin
Apr 27, 2006, 12:02 AM
There are variations in basic cement composition that derive the classes noted above. PCA (portland cement assoc.) uses roman numerals, while in Canada we use Type 10, 20, 30, etc. Other countries have their own designators as well.

All Portland cements are in a class of cements which are "hydraulic", meaning that they require water to set. No other physical processes are required, therefore Portland cement concrete (PCC) will harden under water (although for reasons previously discussed, it may of questionable strength).

EtherealMist
May 3, 2006, 3:30 PM
Isn't true that concrete that is mixed with a higher water ratio have a stronger final strength? Does rain / wet weather add to this?

Chicago3rd
May 3, 2006, 4:00 PM
My mom watered out new houses concrete slab in Texas daily so it wouldn't crack.

Also, I heard that Hoover Dam is still not competely dry in the center.

Liz
May 3, 2006, 4:23 PM
Isn't true that concrete that is mixed with a higher water ratio have a stronger final strength? Does rain / wet weather add to this?
nope, additional water will only cause the cement to lose strength, a good w/c ratio is in the .4-.6 range, increasing the water too much will reduce the overall strength and create curing issues. In the field you really have to keep an eye on contractors to make sure they don't add too much water to a batch that's been sitting around for a while.

My mom watered out new houses concrete slab in Texas daily so it wouldn't crack.

Also, I heard that Hoover Dam is still not competely dry in the center.

as for watering the driveway it is good to keep most concrete mixtures moist while they are curing, this ensures that the sun (eapecially in texas) wont evaporate all of the water from the surface and cause scaling. you really only need to do this for the first week though.

all concrete will continue to cure and gain strength over time, the 28 day strength used for design purposes with probably about 90% of the strength concrete will achieve over time. so I'm sure that the hover dam is still curing today, even though it was set so long ago. as to whether the actual concrete is still wet or not, I don't know.

EtherealMist
May 5, 2006, 6:18 AM
nope, additional water will only cause the cement to lose strength, a good w/c ratio is in the .4-.6 range, increasing the water too much will reduce the overall strength and create curing issues. In the field you really have to keep an eye on contractors to make sure they don't add too much water to a batch that's been sitting around for a while.



as for watering the driveway it is good to keep most concrete mixtures moist while they are curing, this ensures that the sun (eapecially in texas) wont evaporate all of the water from the surface and cause scaling. you really only need to do this for the first week though.

all concrete will continue to cure and gain strength over time, the 28 day strength used for design purposes with probably about 90% of the strength concrete will achieve over time. so I'm sure that the hover dam is still curing today, even though it was set so long ago. as to whether the actual concrete is still wet or not, I don't know.


thanks for the info :tup:

Line_and_Grade
Aug 9, 2006, 1:33 AM
Ugly finish most of the time. If it is all schedualed it has to pour. Once you start you can't stop.

Gahrok
Aug 15, 2006, 3:22 AM
Here is a skill testing question(s) for you engineer types out there. I am obsessively been observing this new high rise under construction in Vancouver and one thing that bugs me is I notice that they pour the concrete for new floors and columns when the weather is kinda rainy.

As a matter of fact, we have had three days of sunny weather in which one of them (Friday), they poured one half of the floor. However, the concrete pour does not happen on the weekend(even though the website said they would) as there was practically no activity on the site (that I can see). And today, when it started to rain, the concrete pour continues.

I was under the impression that for any concrete pour, it is not wise to them then during wet weather.

1) Is it normal for construction projects to pour while its raining?

2) or does it really matter if the concrete pour occurs while raining?...i.e., the concrete settles regardless of the conditions outside?

Queetz, I have not read all replies to your question, so excuse me if I repeat anyone here,

Some clues are within your post. You say this is a highrise, which probably makes it a prestige project being constructed by a recognized professional company and subject to proper controls (compared to a one man band drive laying job where cutting corners is probably inevitable). Also being in Canada probably makes it a big bucks project.
I guess also that we are discussing floor slabs which being tension members generally will indicate use of a comparatively higher structural concrete grade 280~300kn or thereabouts, maybe even higher.
Strong concrete’s get hot whilst curing. If a member is allowed to dry too quickly, it will impair the ultimate strength.
Drying out is also a function of surface area since a concrete member will dry rapidly where it is exposed to the air, and one thing floor slabs have in abundance is exposed surface area.
Surface area as a ratio of their usual nominal thickness of say 200~250 mm, means they dry out rapidly, and being a highrise the higher you go the greater the drying effect of the wind. This is real problem in hot climates.

Consequently mechanical control of this drying is essential and normally mandatory under building codes.

There are many ways to achieve this but the traditional and simplest method is to cover the concrete with Hessian and keep it wet for several days. This retards the rate at which the concrete can relinquish its heat and internal moisture content and allows it to develop more slowly.

I note your post was made 10 April, and although I have no idea of the temp in Vancouver in the spring you mentioned 3 sunny days, so if one puts together the clues I suspect the reason for purposely concreteing in the rain could be far more rudimentary.

The contractor is utilizing the weather to do his curing for him, and thus avoiding additional costs that he would have to incur otherwise.
Nothing wrong with this, your contractor is commercially smart and I would venture probably successful at winning contracts.
Cheers,
G.

Kelvin
Aug 15, 2006, 1:40 PM
What is 280 - 300 kN (structural) grade concrete?

Also, slabs are primarily flexural elements not axial. This is esp. true in thin slabs and, depending on the geometry, potentially "two-way" elements meaning that bi-axial flexural conditions exist. This type of behaviour can be somewhat complex but not insurmountable.

Gahrok
Aug 16, 2006, 4:48 AM
Yeah got me units confounded in my typing frenzy, should have been kg/cm2 o’course, well spotted.;)

vid
Aug 16, 2006, 5:16 AM
Here, they were rebuilding a concrete reinforcement wall near my house, and they wouldn't pour on rainy or even humid days, which is why it took them about a month and a half to pour a 40 yard long reinforcement wall. :P

Kelvin
Aug 16, 2006, 1:09 PM
Well again that is their choice. I have been to pours in driving rain, snow, hot, dry, humid, etc. In the end, it all comes down to how the concrete is cured (treated in the days following placement). Admixtures and curing regeime can do alot to affect the final quality of the material.

For those who want the conversion, 300 kg/cm2 is ~30 MPa (normal) concrete or 4,300 psi. However is is not necessarily immediatly comparable to North American cylinder tests (6x12 or 4x8) if it was a cube test.

bsereny
Oct 25, 2006, 2:19 AM
If water was an issue, nothing would get built in Florida where it rains all the time!

http://www.bryansereny.com/construction-photography/loft-donwtown4.jpg

Xelebes
Nov 30, 2006, 4:08 AM
If water was an issue, nothing would get built in Florida where it rains all the time!

http://www.bryansereny.com/construction-photography/loft-donwtown4.jpg

He's talking about Vancouver. I know with a certainty that Vancouver has more rain than Miami, unless Miami gets hit with more than its fair share of hurricanes.

Anyways, I work at a cement factory. Water makes not the difference if it is already mixed within ratio (25:4 (kg)). Once it is in, it has a harder time being broken and the sort. If there is too much water in the mixing - then yes, it does become a problem, not when it is pouring.

texcolo
Nov 30, 2006, 4:33 AM
Concrete does not dry it cures. It is a chemical reaction of the lime in the portland cement and does not loose any water content.

If you are pouring a large slab in the rain you have to worry about the raindrops potmarking the slab and about pooling. Water can and does have a negative effect on the PSI of a slab. But, the concrete ordered is always stronger than necessary, and the rebar and/or post-tension cables are always over-engineered for reasons such as rain effecting the PSI.

Cylinders are cast to break on different days as the slab cures. The cylinders give an indication of how strong the concrete is at the time they are broken. Approximatly 99% these cylinders pass there break requirements - NO MATTER WHAT HAPPENS!!! Concrete is pretty amazing stuff.