Internally curing concrete

Log Out | Topics | Search Moderators | Register | Edit Profile

Author Message Ronald J. Ray, RA, CCS, CCCA Senior Member Username: rjray Post Number: 104 Registered: 04-2004 Posted on Thursday, July 19, 2012 - 05:02 pm:    Does anyone have any opinions/experience using expanded shale, clay, and slate (ESCS) lightweight aggregates in cast-in-place concrete walls or floor slabs? Thanks Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 489 Registered: 10-2007 Posted on Thursday, July 19, 2012 - 09:52 pm:    Lightweight concrete using expanded shale aggregates is common. Why the question? I am confused by the title "internally curing concrete". All concrete cures as a result of a hydration process. The only difference is the concrete on the outside sometime needs help because the necessary moisture may evaporate too fast. Ronald J. Ray, RA, CCS, CCCA Senior Member Username: rjray Post Number: 105 Registered: 04-2004 Posted on Friday, July 20, 2012 - 08:06 am:    Mark, The term “internally curing concrete” comes from the Expanded Shale, Clay and Slate Institute http://www.escsi.org/ContentPage.aspx?id=210 I understand the use of these lightweight aggregates has been used to increase the resistance to water penetration of concrete water tanks. It is being discussed on a project of mine as a possible solution to increasing the water proof qualities of a pool. ken hercenberg Senior Member Username: khercenberg Post Number: 306 Registered: 12-2006 Posted on Friday, July 20, 2012 - 11:28 am:    Actually it sounds like they're saying that because these lightweight aggregate hold water longer, they help hydrate the inboard area of the concrete, allowing a more uniform curing rate. These folks seem to be implying that by curing the concrete more thoroughly, the concrete becomes more resistant to water penetration. This is true to a point, but I would't trust it to hold water without some form of waterproof coating when discussing water tanks or similar structures. I've never heard of these aggregates increasing resistance to water penetration, quite the opposite. It's more along the lines of fly ash and granulated slag that have been touted as increasing the density of the concrete matrix when used as replacement pozzolans in lieu of various amounts of Portland cement, but those also tend to increase the amount of shrinkage if the design mix is not proportioned properly. Most US based structural engineers don't know to redesign the size and spacing of their reinforcement (usually smaller diameter rods spaced more closely together) to accommodate that increased shrinkage. More cracks lead to more leaks, so there has been a backlash effect. Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 490 Registered: 10-2007 Posted on Friday, July 20, 2012 - 06:47 pm:    While I have, and will continue to, specified lightweight concrete with expanded shale aggregate I would not use it as the basis to cut back on waterproofing. I use reinforcing steel to control cracking but the emphasis is on the amount of reinforcing steel. The size of the bars and the spacing is not a big factor in controling cracks. The big issue is more reinforcing steel. This reduces the size of the individual cracks. Also key to controlling cracking is reducing the amount of cement and water in the concrete since shrinkage is strongly associated with more water. What is your structural engineer saying? Richard Hird PE (Unregistered Guest) Unregistered guest Posted on Monday, July 23, 2012 - 10:32 am:    If you are looking for something I have specified micro silica additives to densify concrete and reduce liquid migration through concrete. I have used it for chemical containment basins. It also used for bridge deck toppings. It is pretty expensive, and it is not a substitution for proper reinforcing and detailing to control cracking. Ronald J. Ray, RA, CCS, CCCA Senior Member Username: rjray Post Number: 106 Registered: 04-2004 Posted on Monday, July 23, 2012 - 02:15 pm:    Mark, Our structural engineer began the investigation of the moisture containing lightweight aggregates as a possible solution to control cracking in the floor and walls of the pools. At this point, I don’t think he is at the point of recommending this type of approach. The local distributer of the lightweight aggregates made their sales pitch presentation to the design team and contractor last Friday. A local concrete supplier was also present. The lightweight aggregate sales rep stated that the lightweight aggregates could add between $2.00 and $2.50 per yard to the cost of the concrete. The concrete supplier stated that using Xypex could add around $40 per yard to the concrete cost. I found that difficult to believe. Miles Dee (Unregistered Guest) Unregistered guest Posted on Wednesday, July 25, 2012 - 08:15 pm:    All, First, I do have "a dog in this hunt" as I work for a large ES&C producer. Having said that, yes, the application referred to as Internal Curing has been studied greatly for the past decade. Dale Bentz with N.I.S.T and Dr. Jason Weiss at Purdue are great resources (Google) for information on this subject. As to the use of IC in a large water structure, we provided Expanded Shale Aggregate for IC concrete for a 10mg water tank in Lonetree, Colorado (Denver suburb) for the local water authority, Denver Water Company. Denver Water's Materials Engineer, Erik Holck PE. Design Structural Engineer was Bates Engineering, Mr. Robert Bates PE. Feel free to contact them as they are more than willing to discuss the application. As well, the original application was specified for the tank base and roof only, after the tank base was placed the GC (Garney Const.) requested the IC concrete be purchased for the wall segments as well which was approved due to the far greater performance of the IC Concrete relative to standard concrete. I do agree with other posters here that the IC application should not be confused with "water-proofing" the concrete structure itself, it will simply improve the final concrete product by enhancing the internal hydration of the concrete greatly reducing the potential for shrinkage from autogenous forces created by self desiccation and reduce the potential for plastic shrinkage cracking as well. Once the internal structure of the concrete is improved, sure, you have a lower potential for problems in the concrete i.e. leakage etc, but not waterproofing per se. As well, our industry does not claim to replace conventional curing either. All normal external applied curing applications you normally specify need to continue. A white paper on the Lonetree CO project will be presented at ACI International in Toronto this fall with many great sessions on Internal Curing. This will be a great educational opportunity. Enough spoken here, but feel free to e-mail me and I can send you further information about the application and/or the Lonetree CO project. Also, the ESCSI website mentioned above is a great resource as well. Miles Dee~ John Ries PE FACI (Unregistered Guest) Unregistered guest Posted on Thursday, July 26, 2012 - 01:50 pm:    I’m the technical director for the Expanded Shale Clay and Slate Institute and would just like to give a brief overview of the Internal Curing process and add to some of the things Miles and others have already mentioned. Internal curing is the process of distributing curing water throughout the concrete mixture in a way that does not add to the w/cm. Surface curing does not penetrate very far into the concrete but is still recommended and needed. The IC process involves using an absorbent material that releases its water when the cement/SCM is hydrating/reacting and needs it. By far the most common absorbent material used is lightweight aggregate because it’s readily available and has a long track record of successful use. The process involves replacing about 30% of the finer aggregate with a pre-wetted fine LWA. Realizing all projects and materials are different, in general the design goal is to get about 7 lbs of internal curing water into the mixture for every 100 lbs of cementitious material. ACI defines Internal Curing as: “Supplying water throughout a freshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation.” ASTM just passed ASTM C1761, June 15, 2012 “Standard Specification for Lightweight Aggregate for Internal Curing of concrete” this standard is in the ASTM editorial process and will soon be published. The primary benefits of internal curing are: • Improved Hydration and SCM Reaction thereby providing more efficient use of the cementitious material. Un-hydrated cement is expensive filler. • Improved interfacial transition zone (ITZ) between LWA and hydrating cement paste. This along with better hydration reduces the transport properties and chloride permeability of the concrete • Reduced Early Age and Long Term Shrinkage Cracking • With the overall quality of the concrete being improved the service life is often extended thereby adding to the sustainability of concrete. ken hercenberg Senior Member Username: khercenberg Post Number: 309 Registered: 12-2006 Posted on Thursday, July 26, 2012 - 02:30 pm:    Thanks for the clarification. Is any of this benefit offset by the fact that water will be retained in the concrete long after the initial curing period has achieved target compressive strengths? With all the problems we are already encountering with water vapor and vapor drive, especially in flooring applications, doesn't this further exacerbate the problem by holding moisture in the concrete longer? Doesn't introducing lightweight aggregates cause the concrete to absorb and hold water that comes in contact with the concrete after curing? Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 492 Registered: 10-2007 Posted on Thursday, July 26, 2012 - 03:26 pm:    Normal weight concrete seems to be able to reach the specified strength without the use of an additional interior source of moisture. Mix designs contain more water than is needed to cure the concrete. The added water is there because the concrete needs to be workable. The problem is with surface drying of the concrete not with the lack moisture on the interior of the concrete. It seems that the light weight aggregate manufacturers are trying to take a natural property of light weight aggregates and making it appear as something that fills a problem. They appear to be solving a problem that does not exist. My advice is to not be worried about internal curing of concrete and focus on the basis and I use it a lot. Light weight aggregate has its place but am not convinced that this is it. If your concern is to minimize moisture flow I would think that creating these pockets of moisture does not help. If you want to minimize moisture flow I would consider using fly ash, silica fume, and admixtrures such as Xypex. Benjamin Byard, Ph.D. (Unregistered Guest) Unregistered guest Posted on Friday, July 27, 2012 - 11:08 am:    In theory, there is sufficient water in a mixture to cure all the cement, but this is not true in practice. When the relative humidity around the cement grain falls below about 80%, almost all cement hydration stops. The hydrated cement past is not permeable enough for water to travel very far, because of this there are pockets of desiccation. This is one of the reasons why the degree of cement hydration is usually 60-70% (i.e. 30-40 expensive filler). In addition, this decrease in internal relative humidity causes stress (like that of drying shrinkage, but the water is being consumed by hydration and not the atmosphere). This stress can be in excess of 300psi (in tension) in 4 days, and can lead to additional micro-cracking and increased permeability. When proportioned correctly, internal curing can maintain a higher moisture level that is more uniform across the paste. Thereby increasing the degree of hydration and reducing the stresses due to self-desiccation. The increased degree of hydration will densify the paste structure. The decreased stress will reduce micro-cracking. Both will reduce the permeability. There are numerous journal articles, reports, and field studies that bear these facts out. However, as mentioned above less permeable is not “waterproof”. Autogenous shrinkage has been a reported problem since high-performance concretes came around, prompting millions of dollars in research across the world for the last 25 years. This is not a “problem that does not exist” as stated above. Internal curing also works very with SCMs. The pozzolanic reaction is a long-term reaction, and thus benefits from moisture levels being elevated for longer periods of time. Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 494 Registered: 10-2007 Posted on Saturday, July 28, 2012 - 11:19 am:    Even if the problem can be argued to "exist" I am still comfortable with my recommendations. Take care of the basics. John Regener, AIA, CCS, CCCA, CSI, SCIP Senior Member Username: john_regener Post Number: 565 Registered: 04-2002 Posted on Saturday, July 28, 2012 - 11:17 pm:    This is another case where addressing one concern (curing) could produce another problem (floor covering adhesion failure and mold). We've has several years of presntations at the "CSI Show" on flooring failures/mold and the topic is scheduled again this year in the education program. In my locale, we also have had several presentations on solving excessive moisture migration by using topical application of a sealer that supposedly solves that problem. I know that water in a sand layer under the concrete/over the vapor retarder can make the moisture migration problem worse (but solves a problem of slab curling?). I advise my client, the architect, about this but usually lose 'cause the structural engineer's opinion (interest) trumps concerns about "architectural" stuff like applied finishes. I can only imagine that moisture retained in lightweight aggregate could be similarly detrimental. I don't know the answer to this dilemma. Is the solution a super sealer on the concrete surface (which is warranted except if it is inevitably damaged by construction activities)? The sealer may or may not seal crack control/"expansion" joints and it is not warranted to seal shrinkage cracks subsequent to application. Oh, and it's applied immediately after concrete finishing and right when hydration has just started, instead of using a curing compound that will dissipate or placing a moisture-retaining curing sheet on the concrete and keeping the concrete surface moist for about a week. Contruction managers and contractors like curing compounds because they save money and time, especially when the architect doesn't object and therefore "approves" the method and takes responsibility any problems. Does messing with water:cement ratios and using embedded fibers factor into this? I ain't smart enuff to know how to specify a for-sure, fully-hydrated (shrunk), crack-free and low moisture vapor pressure concrete slab. Does anyone? Is the solution to retaining moisture in concrete simply a matter of applying a curing/sealing compound to the top surface of the concrete? If that alone works, then why spend extra on lightweight/lower strength aggregrate that soaks up water for slow and continuing release after hydration of portland cement has sufficiently completed in the concrete? Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 497 Registered: 10-2007 Posted on Sunday, July 29, 2012 - 04:59 am:    An engineer who insists on sand above the membrane is risking getting pulled into discussion of moisture problems and is out of sync with current good industry practice. ACI 302 clearly endorses locating the membrane on top of the sand. The many engineers who have adopted the ACI criteria have apparently not had problems. Suggest that this engineer is promting out dated orthodoxy. While I believe the moisture problem is overstated and the tests used to determine if there is a problem are close to junk science I also believe that from a liability perspective that it is stupid to ignore the issue because you do not want to become a litigation target. The tests used to measure the moisture flowing through the slab are really measuring the moisture in the surface layer of the concrete. Tests suggest that the moisture flow through good concrete is less than the criteria established by the flooring manufacturers. My recommendation is to: --Follow the recommendations of ACI 302. --Use relatively high reinforcement ratios to control crack size. --Minimize the amount of water and cement in the concrete. Follow the w/cm ratios required by the flooring manufacturers so he cannot avoid liability. --If concrete is in contact with water and low permeability is required ACI 318 reqyures you have a w/cm ratio no greater than 0.5 and f'c = 4,000 psi or higher. --Consider fly ash, silica fume, and xypex if you really believe there is a problem. The use of xypex can cure small cracks. Full hydration might be nice but the tail should not wag the dog. Even with fully hydrated cement concrete will still crack. For practicle purposes you should assume concrete will crack. The trick is to manage the cracks by reducing the amount of water and cement in the mix and by using low shrinkage aggregate. Suggest that this will have more influence than fully hydrated cement. You then put in enough reinforcement to control crack size. My choice is #5 @ 12" oc e.w. in a 5" slab. Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 498 Registered: 10-2007 Posted on Sunday, July 29, 2012 - 01:04 pm:    The following response ws provided by a concrete expert who has worked for a concrete supplier. There have been a number of papers in recent years showing the benefit of internal hydration. Not really a surprise to most of us though. If you can improve curing (internal or otherwise) you can improve the properties of the concrete.The high absorption of lightweight aggregates (20-30%) serves to supply additional moisture (or reduce the rate of drying) and extend the curing period. Lightweight manufacturers have found that they can use what was previously their waste fine products as a replacement of a portion of fine aggregate (usually at 5-8%) and provide this internal moisture for improved hydration. Hence they are promoting the use of a small portion of this waste product in normal weight concrete mixtures as a hydration enhancer. This strategy can enable the lightweight manufacturers to get rid of a product that they previously had to waste. The difficulties are: 1) This very fine sand must be pre-saturated, not always easy. 2) The saturated fine product is very difficult to batch as it hangs up in hoppers and weigh feeders. 3) Most RMX plants don't have the luxury of an additional bunker for the lightweight fines. 4) Distribution infrastructure is very limited and the lightweight producers have had difficulties supplying products during normal business cycles. While they have mountains of this material, they have not inproved their distribution systems to keep a consistent supply into the market. As you point out, there is typically more than sufficient moisture in normal concrete to hydrate the cement. While they have shown improvements under laboratory conditions the real-world benefits are likely marginal at best. After seeing a presentation by one of the lightweight manufacturers Caltrans recognized the difficulties and has chosen not to allow this. Benjamin Byard, Ph.D. New member Username: benbyard Post Number: 1 Registered: 07-2012 Posted on Monday, July 30, 2012 - 11:40 am:    I agree all of your recommendations, they are sound standard steps to minimize cracking and mitigate their effects. As Mentioned above, internal curing maintains higher moisture levels for longer period of time and this is contrary to what is necessary for application of floor finishes. However, for bridge decks, roadways, water tanks, CIP box culverts, and tunnels internal curing has been shown, in real-world project, to perform very well. With structures exposed to de-icing salts and natural elements, it is imperative to limit crack widths and spacing. Internally cured concrete still cracks but, their widths are smaller and spacings longer, thus increasing their service lives in terms of corrosion. Over 2 million yards of internally cured concrete has been place with 1.3 million yards in the form of low slump pavements. Texas State Highway SH 121 incorporated 1300 yards of internally cured concrete on one side of a 5 mile section near Dallas in November 2006. A 500 ft long section of continuously reinforced concrete pavement was evaluated for cracks on February 1, 2007, and again on September 11, 2007. The side with IC showed 21 cracks. In comparison, the normalweight control on the other side of the road showed 52 cracks. In addition, the crack width was considerably smaller on the side with the internally cured concrete. The state of New York has numerous bridge decks throughout the state with internal curing and SCMs. Denver water has just complete several water tanks with internally cured concrete and is specifying internal curing in future tank construction. Indian is currently experimenting with smaller bridges decks, and I am sure other states that I am not involved with are also using the technology. Most of the “difficulties” listed above are reminiscent of the “difficulties” with fly ash from 20 years ago. But, it was evident that fly ash improved the concrete and engineers began specifying it. When ready mix producers began losing jobs because they had did not have ash silos, they got additional silos and learned to adjust accordingly. Sprinklers are most commonly used to pre-wet LWA and are not a concern. As to the distribution issues, I have not heard of that being a concern, but that is above my pay grad as they say.