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    Thread: A long lesson on concrete.

    1. #1
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      A long lesson on concrete.

      I feel that there is a need for people to have a better understanding of what concrete is, why it is made the way it is, and how the various components interact with each other. Similar to water quality, treatment protocols, fish selection, the more we know, the more likely we are to be happy our outcomes.

      Concrete, or more correctly Hydraulic Cement Concrete, is a mixture of Hydraulic Cement, water, aggregates (sand and stone/gravel) and any appropriate admixtures. Cement is the powder that is used in concrete to make it strong.

      First, Hydraulic Cement is a cement that reacts with water. It may be Portland cement, blended cements in which a Portland cement is blended with some slag or pozzolanic mineral admixture, masonry cements, refractory cements, water plug cements, or a whole host of other specialty cements. Portland cement has very specific chemical and physical specifications, and is used in the production of many of the other cements. Masonry cement which is used to make block and brick mortar has hydrated lime added as a means of making the mix sticky so it bonds well to the block or brick. The water plug cements are used to patch cracks and holes in concrete structures to stop the movement of water through the crack or hole. They typically set up within minutes of the addition of water and are expansive so they lock into place.

      The manufacture of Portland cement is done by blending a source of calcium, (generally limestone), a source of silica, (generally shale, or sand), a source of alumina, (generally shale or clay), and a source of iron, (can be mill scale from steel mill). These are proportioned very carefully and the mixture pulverized and then pelletized before being sent down a long rotating kiln operating in the 2500-2800* range which changes the chemical form into reactive crystalline forms. As these crystals react with water, they form what is referred to as calcium silicate hydrate gels, which hold the mix together providing the strength, durability and watertightness. Byproducts of the reaction are the production of heat and calcium hydroxide. The heat is beneficial in cold weather to accelerate the chemical reactions, but can be deleterious in raising the concrete temperature to excess causing thermal cracking. High cement content mixes generate significant amounts of heat and are potentially detrimental, particularly in more massive structures.

      Strength of concrete is generally specified as 28 day moist cured strength, and this is a laboratory condition, not achievable in the field. The temperature is specified as 73.4*F +/- 3*F and either a lime saturated bath or fog room for moisture control. The strength of the concrete is relative to time and temperature. At the above standard temperature and moisture, the strength at 1 day will be about 35% of the strength at 28 days, and at 3 days will be about 60%, and 7 day strengths will be near 75%. As long as moisture is present and temperatures are favorable, the strength will continue to go upward with 90 day strengths being about 120% of design.

      The effect of water on the strength is very significant. Water performs two major functions in concrete. One is the hydration of the cement (the chemical reaction), and this requires about 3 gallons of water per bag of cement. The other need for water is the lubrication (convenience) of the mix needed to place concrete. The water of convenience spreads the cement grains apart and they will not close back very much. This spreading results in a sponge like structure that is weakened by the spread, and it provides increased porosity allowing water to travel more easily through the concrete. A chart in “Design and Control of Concrete Mixtures” published by the Portland Cement Association shows the water to cement ratio in pounds of water per pound of cement needed for different strengths with W/C ratio of 0.33 needed for 7000 psi concrete, and 0.48 for 5000 psi, and 0.82 for 2000 psi. The amount of water, in gallons per cubic yard needed for lubrication is dictated by the size and shape of the aggregates (sand and stone/gravel). Finer size materials have higher surface areas that need to be coated so require more paste (cement and water) to be workable, and so to stay with a fixed water cement ratio, the amount of cement must be increased. That is the reason that you will see mixes specified as 5 bag mix, 6 sack mix, 7 bag mix, 635 pound mix, etc. BTW one bag weighs 94 pounds. A better way to specify concrete is to specify the maximum size of the aggregate and the strength desired. A 5 sack mix may be all that is needed if the aggregate is very large, while a mixture with some sources of fine and course aggregates, due to particle shape and size, may need even an 8 sack mix. Let the expert at the ready-mix company provide a mix that meets your needs.

      The aggregates fall into two major categories crushed and natural. Natural aggregates would be silica sands and river gravels. Crushed aggregates would be more like limestone or granite that has been crushed and graded. It might be course, or fine. Course by definition is the aggregate that is larger than 3/16 inch and fine is that that is finer than 3/16 inch i.e. sand. The larger the coarse aggregate, the less sand, cement and water needed to provide the same strength and slump (fluidity). This has to do with the surface areas that need to be coated with a sufficient film of paste to be fluid and the relative amount of voids between the aggregate particle. But by the same token, the larger the aggregate, the further apart the rebar has to be, the thicker the slabs have to be, the harder it is to finish. So even though, from a technical standpoint large size materials are best, practicality limits the size for most concrete to a maximum size of 1 inch and for a lot of work to a maximum of a half inch. Having aggregates that have a mix of all sizes will provide the most dense packing of the aggregates, and therefore have fewer voids that need to be filled with paste.

      Admixtures that are common are air entraining admixtures, retarders, accelerators, and water reducers. Air entrainment is highly recommended in areas that are exposed to freeze thaw cycles, acting as an internal mechanical antifreeze. Really what it does is provide a pressure relief at close spacing for the ice crystals to expand into. Water expands about 8% on freezing, which is the reason ice floats, and without the expansion areas, the pressure is great enough to internally crack and break concrete, just like it breaks water pipes and automobile blocks. Retarders slow the set of the concrete to allow for longer working times to place and finish the concrete. Most of these include water reducers. Accelerators speed up the set so that concrete can be placed and finished in cold weather or traffic can be placed back on the concrete earlier. Most of these are not particularly good for concrete containing reinforcement, since they contain high percentages of chlorides. Water reducers do just what they say, they reduce the amount of water needed to be able to place the concrete, thereby increasing strength and durability. There are some other admixtures like Xypex which supposedly cut off the pore to pore movement of the calcium hydroxide (pore water).

      The natural pH of concrete is near 13. It is this high pH that provides protection to the reinforcing steel. Steel in this environment has an oxide layer form that prevents corrosion, similar to the anodized surfaces on aluminum items protect them in neutral pH environments. Allowing aluminum in concrete will create a very corrosive atmosphere for the aluminum. The oxide layer that protects the steel can be broken and corrosion will take place if the pH is lowered, or if salt is allowed in. Cover depth and water tightness are the main protective methods for keeping salt away from the steel. When steel corrodes, rusts, it expands and as the saying goes, an inch of steel will make a foot of rust. The expansion will destroy the concrete from the inside.

      The pH of 13 is caused by the calcium hydroxide that is one of the byproducts of the cement reaction. This is soluble and will move to the surface of the concrete. In the air, there can be a reaction wherein the carbon dioxide in the atmosphere reacts with this and creates calcium carbonate, which is essentially limestone, marble, or similar and is essentially insoluble. You see this on brick work as efflorescence, but it is present on concrete but due to color it is not obvious. This so called carbonation, increases the density of the surface of the concrete, improving the watertightness of the concrete and the surface hardness. Under water, this same movement of the calcium hydroxide allows it to get into the water, and with its high pH, it can cause the pH of the water to elevate, particularly if the surface area of concrete to volume of water is large. The rate of flow is dependent on the watertightness of the concrete. Denser concrete will not allow the flow of the high pH material to the surface, whereas porous materials would readily allow movement.

      For small streams, fountains, or other structures the surface area to gallons could be small enough to not even be detected with pH test equipment. It is the surface that provides the reaction area, and a stream with much of the area being stones, and only small areas in between being concrete or mortar, the area is limited. In an all concrete pond, the area is much larger.

      Mixing can be done with a hoe in a mortar pan, but if you are looking at more than one or two bags of premixed hardware store concrete, it is not recommended. It is WORK. For small jobs, the use of small electric powered concrete mixers are available for purchase or rent, and come in very handy. The mix needs some time for the ingredients to absorb the water, or you will end up with a significant slump loss between mixing and placing. The laboratory procedure is mix 3 minutes, rest 3 minutes and mix for an additional 2 minutes. This can be reduced, but if reduced to much less than the first 3 minute mixing, all of the ingredients may not have gotten exposed to the water. The ready-mix trucks mix at a high mixing speed for about 5 minutes before starting down the road to deliver to you, mixing at a slow speed the entire travel time. There is another mix type, wherein the truck arrives on the job with all dry ingredients and mixes on the spot. This type of mixer will produce any amount of concrete that you may need without you having to pay for over orders, but because of the way it proportions, the ability to comply with mix design quantities is not as good as the plant mixed concrete.

      Slump, or fluidity, of the concrete is needed for placement, but the amount differs for different applications. The paving machines do what is called slip form paving and work with very stiff mixes. The pneumatically applied concretes, shotcrete or gunnite, cannot use high fluidity and be able to stand on a near vertical wall. Formed walls are difficult to get the concrete down between the front and back form and around all the rebar if the mix is stiff. Other applications where hand placing, hand finishing, and trying to get concrete to flow into strange form shapes require higher flow. The best concrete is concrete that is placed with the lowest possible flow, since the materials can separate as they flow. Increasing the water will increase the flow, but because of the way water works in concrete, versus some of the admixtures, separation, called segregation, is harder to prevent. If you look at the instructions on the bags of premix, a range of water contents is listed, but based on good concreting practices, the least amount is that you can use is the best practice. Concrete that is delivered by the ready-mix company, the truck carries some water, mostly for washout of the truck, but it can be added on the job site to make final adjustments to the slump. The slump should be specified in the order so the correct amounts of solids can be added for producing the best concrete. Concrete should be placed as close as possible to it final resting place. Pushing concrete around with vibrators, or flowing with high slump results in segregation of the various components.

      Placing concrete can be done by bucket transfer from mixer to location, with the bucket being a small 5 gallon bucket or a crane handled bucket that can hold half a truck load at a time. For most of the applications we will be involved in, getting a truck to the location of the pour would be the preferred method and just run the concrete down the chute and into the forms. If the truck cannot get into the back yard, then pumps are probably the best means. The pumps have may be trailer mounted or truck mounted and are able to push concrete through a 3 or 4 inch line to any place desired. Pump operators tend to like to work with high slump mixes. Whatever method is used, the concrete should be unloaded and put in place within about an hour of mixing. As the concrete sits, it hydrates and the use of the water by hydration reduces the slump, and the addition of more water to extend the placement time is very poor practice. Order smaller loads if the placement is going to be slow. The ready-mix plant will charge you a premium for small loads, but they will also charge you a premium for delayed return of the truck, and the damage done by continuing to mix and handle old partially hydrated concrete is another unseen surcharge.

      Consolidation is the process of getting the concrete to fill as much of the shape or form as possible, reducing the number and size of voids. For the pneumatically applied concrete, it is forced by air into itself closing any gaps. For most other concrete the use of a vibrator is highly recommended. The concrete can be placed without the vibrator, but it requires tapping the sides of the forms, spading the edges near the forms, puddling (using a tamper and bouncing it up and down or the surface), or rodding to get the concrete to flow together and have the air moved to the surface making the concrete a solid mass.

      Finishing is an art. It starts with the strike off of the surface by a process called screeding, wherein the surface level is established. Following this is a process of floating the concrete with generally a wooden float, which helps to close the surface voids, do some consolidation of the concrete, and further refine the final elevations. After the concrete has started to obtain some set, the finishing continues with a trowel to obtain the final smooth finish. Timing is everything. If the finishing is attempted too early, the concrete moves around too much, and if it is attempted too late, it just will not allow any progress. Finishing while bleed water is present mixes the bleed water with the surface cement making for a higher water cement ratio at the surface, weakening the surface, and in extreme cases causing dusting, where the surface is so weak that any activity, (walking, sweeping, etc.) results in the concrete wearing away.

      Curing is one of the most important functions that you can perform to improve the quality of the concrete. Concrete will continue to hydrate, develop strength and watertightness for ever, if it is kept above freezing, and not permitted to dry out. Concrete should be kept wet, and the longer the better. Keeping some form of polyethylene sheeting over the placement for several days to several weeks after the concrete is placed will do more to improve your chances of having the concrete achieve its potential. One of the best methods of curing is ponding. Putting the concrete underwater for some period of time keeps the concrete wet, moderates temperature swings, removes some of the high heat that can cause cracking. Concrete that dries does not have the water available to continue the hydration reactions, and stops gaining strength, stops getting denser, which provides the watertightness.

      Because virtually all of the hydraulic cements, whether in concrete, mortar or grout, utilize Portland cement in some proportion, the above information is applicable to all. If you want strength, durability, and watertightness, use the lowest possible water-cement ratio possible, and cure as long as possible. Reduction of water-cement ratio can be accomplished by the use of more cement (improving the paste), the use of larger aggregates (reducing the amount of paste needed), or the use of certain admixtures.

      The volume of the mixture of cement and water is largest when they are first introduced, and the hydration of the cement by the water results in a smaller volume of the mixture, thus there is a certain amount of shrinkage that is hard to stop, unless some addition is added to make the concrete expansive. Improved curing reduces the effect of this shrinkage by allowing water to be drawn into the concrete by the partial vacuum created by the reaction. Reducing the amount of cement reduces the effect of the shrinkage on the total mix. In addition, there is subsidence, where the solids in the concrete settle in the mix, kind of like gravel dumped in a bucket of water, effectively pushing some of the water out the top in what is called bleed water. There is also drying shrinkage, the most severe occurring during the placing operation in the presence of low humidity and high wind, causing the concrete to crack like the mud at the edge of the pond, but there is also some shrinkage in the hardened concrete that occurs when the pore water is evaporated. Temperature increases cause expansion and temperature drops cause shrinkage. All of these can be additive and without proper reinforcement, and proper joint spacing, the concrete will crack.

      Expansion of concrete is generally accomplished by the addition of something like aluminum powder, which in the presence of the high pH reacts violently producing small hydrogen gas bubbles within the concrete causing expansion while the concrete is somewhat fluid. Expansion can also be caused by sulfates in the soil or water reacting with the calcium hydroxide to make ettringite, a gypsum crystal that expands by adopting additional water into its crystalline structure. If this occurs after the concrete has hardened, it will cause the concrete to crack and deteriorate from the inside out. There are expansive mixtures out there, and some of them are used for grouting where you do not want the mix to shrink from below heavy equipment. At least one type of cement on the market promotes itself as being shrinkage compensating, such that joints are not needed in large slabs.

      Each of the topics has been reduced as much as I know how to reduce them, and any can be expounded on deeper if requested.

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    2. #2
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      This needs to be a sticky......Great info Rich, Thanks
      Susie


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      Rich L is offline Senior Member
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      Yes, it should be a sticky!

      Excellent, Rich! I would like to see more when you can.

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      My old Materials Buddy, its gettin stuck and linked in best threads ever links.

      Thank You Rich
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      Richard who says you know anything about concrete !!! You have only worked with it most of your life so what makes you the expert !! Well...Ok I ll throw in my vote

      Great Post Richard

      Thanks

      Greg

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      Wonderful post Richard I know it will help many
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      This is indeed helpful. Thanks Rich

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      Great read, thanks!
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      A very informative thread indeed.

      Rich, a fellow club member had his new pond gunited a few days ago & he was told the wet the concrete everyday. How many times a day does this need to be done and for how long?

      Also they put in some weep holes in the floor of the pond and the rebar is exposed, will this be a problem when he wets the concrete daily?

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      Rich-
      I am a custom home builder specializing in contemporary architecture. We are asked to do lots of architectural concrete and have been improving our methods for placement and mix designs since 1998. I am currently failing to achieve a satisfactory finish on an interior wall in an entryway and kitchen. The client is unhappy with about 10% of the surface area, primarily at the bottom and near some corners.

      The wall is 9 feet tall and is 12 feet wide and 6" thick with 18" by 24" wings extending as ells off the back side. The architect demanded a thru wall tie layout of 24" vertically and 32" horizontally which prevented us from using a vibrator. We hammered the form every one inch on center both ways. We caulked all seams and corners as much as possible and used a premium form release. The mix design used super plasticizers that achieved a 9" slump with a low water to cement ratio.

      It was my idea to line pump the mix thru access holes that would later be plugged on the backside of the form as the pour progressed. The concrete contractor instead insisted on pouring from the top with 5 gallon buckets which I believe created two issues. We had excessive segregation at the bottom 2' of the wall and a large amount of aggregate was left in the mix box that they chuted the crete into and later shoveled out of into the buckets.

      The other issue which I am baffled by is a dark gray color around many of the thru wall ties and in certain areas at corners especially lower down on the wall. I think we could have accepted the final product if not for the bottom two feet. It is some of the best concrete we have ever placed and in all of Austin TX I have not found a better example of architectural concrete but right inside the front door I have to achieve near perfection with concrete.

      I will try this again and I will this time get my way on the spacing of the thru wall ties and pumping in lifts at different levels from the back side. Do you have any other ideas you might be willing to share that might help us achieve a better finished product? I know it's not a pond but your level of experience and knowledge is not easy to find. Thanks alot for all this great information and any help you can offer with my little problem. Redoing the wall costs in excess of 10k and this will be the third attempt so I'm not going to attempt it until I have more hope than I currently feel.
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      Quote Originally Posted by Tungsten View Post
      A very informative thread indeed.

      Rich, a fellow club member had his new pond gunited a few days ago & he was told the wet the concrete everyday. How many times a day does this need to be done and for how long?

      Also they put in some weep holes in the floor of the pond and the rebar is exposed, will this be a problem when he wets the concrete daily?
      The concrete should be maintained damp or wet continuously. The constant wetting and drying will cause crazing cracks, which are not structural, but show the lack of continuous moisture present for the hydration of the cement in the concrete. If concrete is allowed to dry, the hydration reaction stops, and because the concrete is realatively waterproof, it is hard to get the water back into the concrete to restart the reaction. If the reaction is stopped, then strength development and watertightness improvement stop.

      As for the weep holes, I don't understand why he would put weep holes in a pond. The pH of the concrete creates an environment that prevents steel from rusting. I am assuming the weep holes are going to be filled before putting the concrete in service and the short time will not result in significant rust development. The bars have been out and exposed for longer than that on many jobs before ever being put in the concrete.
      Last edited by RichToyBox; 04-25-2012 at 09:30 PM.

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    13. #13
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      Quote Originally Posted by Drippin' guy View Post
      Rich-
      I am a custom home builder specializing in contemporary architecture. We are asked to do lots of architectural concrete and have been improving our methods for placement and mix designs since 1998. I am currently failing to achieve a satisfactory finish on an interior wall in an entryway and kitchen. The client is unhappy with about 10% of the surface area, primarily at the bottom and near some corners.

      The wall is 9 feet tall and is 12 feet wide and 6" thick with 18" by 24" wings extending as ells off the back side. The architect demanded a thru wall tie layout of 24" vertically and 32" horizontally which prevented us from using a vibrator. We hammered the form every one inch on center both ways. We caulked all seams and corners as much as possible and used a premium form release. The mix design used super plasticizers that achieved a 9" slump with a low water to cement ratio.

      It was my idea to line pump the mix thru access holes that would later be plugged on the backside of the form as the pour progressed. The concrete contractor instead insisted on pouring from the top with 5 gallon buckets which I believe created two issues. We had excessive segregation at the bottom 2' of the wall and a large amount of aggregate was left in the mix box that they chuted the crete into and later shoveled out of into the buckets.

      The other issue which I am baffled by is a dark gray color around many of the thru wall ties and in certain areas at corners especially lower down on the wall. I think we could have accepted the final product if not for the bottom two feet. It is some of the best concrete we have ever placed and in all of Austin TX I have not found a better example of architectural concrete but right inside the front door I have to achieve near perfection with concrete.

      I will try this again and I will this time get my way on the spacing of the thru wall ties and pumping in lifts at different levels from the back side. Do you have any other ideas you might be willing to share that might help us achieve a better finished product? I know it's not a pond but your level of experience and knowledge is not easy to find. Thanks alot for all this great information and any help you can offer with my little problem. Redoing the wall costs in excess of 10k and this will be the third attempt so I'm not going to attempt it until I have more hope than I currently feel.
      The color differences that you describe may be areas or pockets of cement segregated from the aggregate, where the aggregate went around the ties, but without seeing the areas, I don't know.

      The segregation caused by the placement of a supered mix from a high location and allowing it to run, is fairly common. Personally, I would limit the slump of the supered mix to a maximum of 7 inches to prevent the segregation. I know the manufacturers state that you can use higher slumps, but my experience is that they flow too freely, allowing a large pile of rocks at the center and the paste flowing off to the sides. There are some new supers out there that are marketed for flowing concrete, and instead of a slump, the flow is measured by a different method. As you are probably aware, corners are notorious for leaks and it is just the paste that leaks, leaving in some cases exposed aggregate at the edges. It sounds like you are doing what you can to control that.

      I would insist on the concrete contractor using windows in the back of the forms, concrete should not be allowed to drop more than about 5 feet, as segregation is exacerbated. Having the windows will also allow the vibrator to get into the concrete for consolidation and limit the distance the concrete can be "pushed" with the vibrator. Concrete should be placed as close to its final resting place as possible to prevent the segregation associated with flowing the mix.

      When you talk vibration, I assume you are talking internal vibration with the bull, rather than form shakers.

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    14. #14
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      Thanks Rich
      The color problems are like blobs of dark gray and don't seem to be caused by segregation but I may be wrong. I'm really at a loss. Yes, we usually use internal vibration but didn't dare try it with such a tall form and using super P with so few wall ties. Have you ever seen form release react with admixtures to cause discoloration? I can reduce the slump and insist on pour windows with a line pump. I'm thinking the pour windows would need to be every 4 feet each way maximum to limit the distance the mix would have to be pushed with the vibrator. Any thoughts? I'll post a picture of the color tomorrow.
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      I have seen form release agents do a lot of things, causing color differences, but it is generally due to differences in the thickness of application, porosity of form lumber, but not admixtures. Color differences are either just at the surface due to coatings, or go much deeper and the coating won't reach. The admixtures are so thoroughly intermixed, that you would not get different colors due to the admix.

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      mplskoi is offline Senior Member
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      Rich,
      I am looking at replacing my lined pond with a concrete pond this upcoming summer. My subsoil is nearly pure sand. Have some larger rock/pebble pieces in just a few spots. Can I mix this in with cement and aggregate to make concrete? I am thinking that every wheelbarrow that I don't have to haul away is also a wheelbarrow of ready mix that I don't have to haul in to the site. We mixed this way on the farm growing up with good results.
      Also, looking at your recommendation of keeping concrete wet during cure, how soon would you suggest to go ahead and fill a pond?
      If pouring my walls is just as easy as dry stacking and filling is there any reason one is better than the other?

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      You could try using the subsoil, but I would try to get a lab to check the strength of the mix. If the soil has clay in it, one the clay needs a lot of water that it will later permit to evaporate, shrinking and making the concrete weaker, and two, it can coat the sand and gravel particles making bond of the cement to the aggregates be weak. Most concrete sand and gravels is washed to remove the clay, and that process is pretty nasty for a backyard operation.

      If the choice is between a concrete pond and a block pond, and a liner is not going to be incorporated in both choices, I would go with concrete. Absorption of concrete is in the 1 to 3 percent while block is closer to 15 percent, and porosity of a good concrete of 8 inches is negligible, the same cannot be said for block. The addition of Xypex, is said to make both less porous.

      If the concrete is a formed wall type placement, the forms can be removed in 1 or 2 days, and then the wall can be backfilled. This will keep the back somewhat moist and provide support for the pond side to be filled. Footings for bridges in rivers are placed under water, so there is no minimum time to adding water, except having the strength to support the load. If a coating is going to be used that requires the surface to be dry, usually they will state how many days of dry it has to have, and for that I would pump the water out maybe one or two more days longer ahead of coating than they recommend. They probably are not basing their recommendations on submerged wet.

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      tsippel is offline Senior Member
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      effervescence in concrete what causes it .
      same with brick and block.
      tremmy concrete for pooring under water what is in it to do that.

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      Effervescence is the migration of the calcium hydroxide from within the concrete to the surface, and once there it combines with carbon dioxide to become calcium carbonate. The thing with brick and block is that the mortar has additional lime to make it more sticky so it sticks to the brick/block better, and that lime allows more calcium hydroxide to migrate.

      Tremmie concrete is placed using normal concrete, but it is placed through a tube that the end of the tube is kept under the surface of the concrete, basically pushing the water out of its way. The first little bit down there is messed up, but as soon as the tube is buried, the concrete, other than the surface is not exposed to the water.

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      the tremmie concrete the corp eng pored used no tube but did't mix with the water flowing by.
      in the spring i have to pore a boat ramp and can't find the same tremmie mix the corps used.

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