Expansion Joints in Settling Basin

A lot of us reading this are not quite grasping what you are doing. There is a good chance that if we have to ask we might not know the answers. But, could you give a few more details so the average CR4 reader can understand what is going on.

The "3 bays" sounds like you are talking about the floor of a building. The 150*45*10.2 m3 only seems to make sense if you mean 150m x 45m x 10.2m, but wait, that is an awful lot (huge amount) of concrete. What is actually going on here?

I am having the problem on bottom slab only. Its floor is going to be anchored on rock. Since its width is 45m, do i need to provide expansion joint in this direction? Its length is 150m. I have already designed the movement joints along this span at 15/15m. Settling basin is of 3bays i.e. one bay is of 15m span. I have provided the plan and sections

Yes,its the massive concreting. Type of rock is sand rock. Its not the building, its the desander i.e settling basin or sand trap basin of hydropower project.

Cannot fathom in the least what you're up to and want us to do about it? Please provide plans and sections, and for Cripes sake, re-phase your posting language in proper coherent English so that we all can read and understand it!

If the rock is continuous under the entire mat and there are no cracks and concrete is poured with appropriate anchorage to the rock so it becomes monolithic, the co efficient of expansion of the rock and concrete being almost the same, the slab will expand and contract with concrete; however to avoid any thermal stresses induction in the slab, it is a good idea to provide the joint. The cost saving compared to total cost will be very small.

No, that's not a true statement. It all depends on the type of rock.

Pursuant to the AISC Manual, the Coefficient of Linear Expansion (for Degrees F) for 100 Degrees for various rock types and concrete are as follows:

Concrete: 0.00055

Granite: 0.00044

Limestone: 0.0042

Marble: 0.0045

Sandstone: 0.00054

Slate: 0.00044

Additionally, in regards to solids, the Coefficient of Surface Expansion is approximately 2 times the Linear Coefficient, and the Coefficient of Volume Expansion is approximately 3 times the Linear Coefficient. these must also be taken into account when perfoming the concrete slab and wall designs.

Depending on the type and nature of the bedding rock, it may not be advisable to anchor the concrete slab into the rock. If you do, you must take into account of anchor bolt/rock bolt shear due to the thermal expansion/movement of the concrete slab. The slab will most likely drag along the rock surface and cause a whole host of problems. Frankly, if it was me designing this chute, I would place the Thermal Expansion Joints @ no more than 45 to 50 feet o.c. longitudinally and transversely. And yes, you may have to place at least one of these longitudinal joints at the base of the wall. Also, carry through Expansion Joints up into the entire height of each wall. Additionally, I'd place Crack Control Joints spaced at 15 feet o.c each way between the Expansion Joints to control shrinkage cracking. These type of joints feature either a raked or saw-cut kerf at least 1/4 to 1/6 of the slab or wall thickness and are filled with an acceptable flexible joint sealant. All joints must be properly sealed. Make sure that you run steel dowel bars through the Expansion Joints for shear & thermal stress control, much like a concrete highway pavement.

Proper wet curing of the concrete for a specified time period is highly recommended to limit concrete shrinkage cracking. I prefer flooding such a slab for a period no less than 14 continuous days, maybe even more, all depending on the concrete slab mass & thickness.

Please refer to applicable USACE Engineering Manuals (ER) and US Bureau of Reclamation manuals and guidelines for design requirements & details of Civil concrete structures, pavements, dams and spillways. All such manuals and guidelines can be found in the appropriate USACE and USBoR websites. You can them by doing a Google Search.

Good luck with you project Mandy!

===Signed,

CaptMoosie, PE / PhD

Civil, Structural and Environmental Engineer.

I stand corrected. Thank you for the detailed explanation. It is very useful information.

You're very welcome Guest. I am glad to have been of assistance!

Please have a wonderful day!!!

Of course the expansion joint is more of a thermal change in concrete issue, where as contraction joints would address shrinkage cracking following construction. If the water flow is continuous throught eh basin and always about the same temperature, you can assume the temeprarture of the concrete in the basin will be stabilized, as the outside rock temperature will be locally stable after a depth of a few feet. However, the exposed section of the basin above the water line and near the surface soils might be subjected to thermal change. As far as shrinkage control. Obviously you don't want contraction joints as these are just for crack control. It is better to stage concrete pours and pour very dry mixes to control shrinkage. Allow one section of the pour to set for a few days before pouring the nexts adjacent section. This accounts for the majority of water related shrinkage, if staged correctly. You can place an embedded seal in the joints to protect against potential leaks forming at the construction joints, and have concrete around the sealed joint top and bottom cover. The sand is going to be a problem for any exposed surfaces, and operations removal of the sand is an issue to consider also. So be wary.

Thank you all Gurus and Guest. My project is at design phase only yet to start bidding and tendering. Now I have proposed the thermal expansion joints longitudinally at 30m spacing and transversely at 15m spacing. Expansion joints, water bar and sealing material throughout the 150m span increases the cost. Now, project is less attractive due to cost. How can I use minimal joints and make the project cost optimum?

Is it advisable to use expansion joint at 50m spacing?

Mandy, I think you're going to have to possibly re-evaluate the overall scheme of the construct.

First think I'd do if I were to eliminate "Crack Control Joints" and increase the spacing of all "Expansion Joints/Construction Joints" is to look at your proposed concrete mixture first. Sure, you can do what you suggest, but you must keep in mind that you want (or have) to eliminate the shrinkage cracking as much as possible, hence you want a high quality concrete mixture together with proper concrete curing procedure and management over a specific minimum time period.

I don't know where you're located. Possibly the UK (?), so you're possibly using BS concrete standards and metric system. What I'm about to give you is based in the American Concrete Institute (ACI) standards and the English system of measurement.

The following has worked very well for me for large concrete slabs exposed to the elements (very cold winters here in upstate New York USA and moderately hot summers and lots of precipitation of all types). I've had several projects that minimized the number of expansion joints, construction joints and nearly eliminated raked or saw-cut crack control joints to help minimize the overall cost. But, there has to be a trade-off for doing this, and that means you need a higher quality concrete, very strict QA/QC during all phases of construction, concrete curing procedures that require flooding the slab and walls/spraying the walls with water sprinklers etc. and must definitely using a higher minimum steel to concrete ratio, known here in the USA as Shrinkage & Temperature Steel Ratio. The overall aim is to reduce the concrete shrinkage cracking due to the concrete heat of hydration during the curing phase.

First, the concrete mixture: Make sure that is a high quality concrete. The following is a high early compressive strength concrete that has worked very well for me. I've successfully used it in water storage and conveyance structures and large concrete slab and wall constructs, but I want to warn you that it is costly and you must provide adequate S&T steel and curing. Spray-on curing chemicals just won't cut it in my opinion. On one of my stormwater management projects that featured a large underground cast-in-place reinforced concrete storage tank, measuring 36 feet in width x nearly 500 feet in length and 12 feet in height with a central longitudinal wall with openings and a full perimeter wall, I had no visible cracks in the slab and only one shrinkage crack in the central wall near an opening. Forgive me, but I'm doing this off the top of my head as I don't have the specifications in front of me. I like to use a "creamy" concrete mixture having at least 675 pounds of Portland Cement per Cubic Yard, but it can have a maximum Fly Ash Type F content of up to 15-17% substitution of cement by Weight (not volume!); a maximum Water-to-Cement ratio in the vicinity of 0.42; aggregates per the concrete code you're using and final determination of distribution etc by batch lab testing results of various mixture designs; 4,500 psi minimum compressive strength at 28-days age; 3-inch maximum slump without plasticizers; air-entrainment, percentage per your concrete standards; use of mid-range or high-range super-plasticizers recommended, but only added at the job site following acceptance quality testing of the ready-mix truck load.....must have predetermined plasticizer amount per CY as determined by lab testing....added by truck driver after load approvals by Resident Engineer....I advise you to test each truck load for slump, air content and concrete temperature at point of discharge.....make a specified number of concrete compressive test cylinders per X amount of concrete yardage. Under no circumstances is the Contractor allowed to add water to the load or placed concrete unless expressly approved by your firm's Resident Engineer on the job site. Extra water will increase the W/C Ratio and hence increase it's weaknesses, decrease it's compressive strength, and increase the concrete shrinkage cracks.

For water retaining and conveyance structures I like to use a minimum S&T steel reinforcement ratio above 0.0030, to help minimize shrinkage cracking. Here in the states, a minimum S&T Ratio of 0.0018 for 4,000 psi concrete is usually used, but that's for buildings and not what a major Civil project like yours or mine. Of course, if the reinforcement ratio is much higher for structural loading considerations by all means use it! A lot depends on the thickness on your slab and walls.

Proper curing of the slabs and walls is most necessary. I find that flooding a slab with no less than 3 inches depth for a 14-day minimum period is adequate. I myself prefer 21 days minimum. Do not strip off the form work and bulkheads until this period has ended. Maintain the water depth the entire period and utilize sheets of 6-mil minimum poly plastic covering the entire slab using 2-foot minimum sheet overlaps. The key here is to keep the slab from drying out on the surface and keep down the hydration temps.

As suggested by others, place the concrete in a checkerboard fashion throughout to reduce shrinkage and expansion problems. At all concrete construction joints use smooth dowel bars with plastic end caps to transfer shear loads from one slab section to another. Also use a flexible expandable sealant at the construction joints such as "Synko-Flex" and it's compatible concrete primer. this sealant works great in absolutely sealing a joint and hence nearly eliminating water leakage into the slab subbase/subgrade that can cause undermining of the construct.

There's a lot more to this, but I think this information will steer you in the right direction Mandy...also, don't be afraid to ask superiors in your firm for guidance!

No, to answer your question, I wouldn't go over 30 meters (nearly 90 feet) with your expansion joints. This is especially true if the ends of your chute are restrained for in-moveable objects like other concrete slabs, walls, etc that in effect lock-in your new chutes....causing "fixity".

Also, I'm of the opinion not to lock down your slab by anchoring it into the rock substrate, unless your calculation show an absolute need for it, or you have a very steep gradient on you chutes and you must prevent the slab from sliding down the inclined rock face. If you must anchor the slab make sure you take into account the thermal stresses placed on each anchorage bar.

Good luck!

Thank you CaptMoosie Guru for helping me with proper guidance. I have finalized my design now. I have proposed the expansion joints only in the longitudinal direction at 30, spacing. I am using M35 grade concrete and 28mm diameter's steel bar at 150mm spacing in both direction having yield stress 500N/mm2. Bottom slab is going to be anchored in rock. Now the proposed structure is completely stable. Once again, I am very thankful to Gurus for helping me on my design.

GA for RCE!

Yes, I agree with you concerning "staged" concrete placement, especially to limit concrete shrinkage. It'll make more sense to do it that way if there is no project time constraints during the construction phase.

In the event that there is indeed time constraints placed on construction, I still feel that concrete shrinkage can be adequately controlled if the design mix is properly proportioned, has limited slump, lower W/C ratio, and the necessary mid-range to high range plasticizer, etc. as well as the employment of a controlled wet curing of the concrete, ie, by water flooding once the initial concrete setup has occurred.

Well there is always project time constraints on these types of projects, however, in the contract time they usually account for such staged concrete construction for such basins and containment towers also. Plasticizers are almost never allowed for such activities when it is a governmental agency project, because it impacts quality assurance monitoring, and a lack of very precise control on the addition of plasticizers tend to create variability in the mix delivered. This makes it hard to monitor and control water in the mix at the point of placement, and field crews for contractors always favor more water added onsite and drivers tend to favor doing what the concrete foreman wants on the down low from the inspectors. So this tends to lead to a need for inspectors at the plant and inspectors at the project site, more sampling, etc. which no agency ever wants to pay for. In some things we can do in theory to improve concrete quality end up allowing concrete crews to be do things to make the job easier by reducing quality control (as they then know that quality assurance is less able to accurately monitor their performance). a Reduction in W/C content is typical, but again you make it too hard on the concrete crews and you won't get what was specified anyways, (they will just add the water to make the mix more easily workable in the field). Slump is a way to QA for W/C ratio and is susceptible to plasticizers. Wet curing or a good sealer is always a good practice. Also worth considering is the ambient temperature limitations for placement, air entrainment, aggregate quality and size, and type of cement to help control cracking.

You are correct in your statements.

On my own projects I require the Independent Testing Agency personnel to conduct slump tests, temperature readings and air testing on every truck load. When everything is found to be within specs, then the Resident Engineer allows the addition of a measured amount of plasticizer (as determined by previously conducted concrete testing that has been reviewed and approved by the Engineer of Record (me)). Never is the ready mix driver or the concrete crew foreman/contractor's rep allowed to add water to the load during transit or at the site, unless I personally approve of it after discussing matters with my RE. Most times I just won't allow it period. They knew when they bid the project what the stringent concrete specs meant, even if they knew they'd encounter a tough time moving the concrete and finishing it....my response is that "you didn't have to bid the project nor take the award if you knew there would be a hardship, especially your bottom line. Get the proper quantity of personnel and proper equipment on the job site and there will be no problems achieving the specified performance results of the project, all as contained in the Project Manual and in specified standards".

Usually with Contractors and their subs it is a money issue mostly and attaining a huge profit margin versus the Engineer's quality control issues, usually attained by cheating and cutting corners. I usually tell that I will work with them, but if they insist on cutting corners by demanding that water be added to the truck load or to the concrete surface just placed, then I'll get back into their faces and tell them to take a flying leap and leave the jobsite + I'll personally reject the load....and if they counter that they will then I will counter that if they do so under warning I WILL contact their Bonding Company and exercise their Bond(s). Period. that usually shuts them up, gets their attention and I get the resultant & required QA/QC on the project thereafter.

I don't mind being tough, especially when it comes to QA/QC and saving the Taxpayer's $$$$....I really hate to give a GC extras when it isn't necessary....that's why I produce a very high quality level of Project Documents to avoid cost overruns.....boy, I'm a real bastard aren't I, eh?!!! LOL Over 33 years in this Biz you have to grow a thick skin to survive!!!

Yeah most plasticizer if used is added at the plant in the batch now, so you have to have the mix inspection done at the plant. It is very annoying, becasue you also have to have an onsite inspector. Most agencies don't want so many QA personnel on a job, due to cost, and concrete companies seem to dislike adding plasticizers onsite as this affords even less control to the plants own QC. Though enough money seems to overcome their complaints. I have been on projects wher they required the water system to be disabled when trucks are onsite, but this tends to be annoying to enforce as they always send a few out with the water still connected, and sometimes you catch drivers trying to use the hose instead to add water. It is one thing to specifiy no water added,a nd another to stand there monitoring every truck load through the whole placement process, especially when you have a few hundred trucks coming through that day and 2 or 3 boom pumps going with 2 trucks at each pump moving as fast as they can dump out their loads, and municpal clients are always looking to minimize inspection costs and defer more onto contractor quality control (and contractors are always looking to minimize quality control and defer over to QA agents). Most government resident enginners will be strict, but in the end many times get over-ruled by supperior bureaucratic politos, Public Works directors, Assistant PW Directors, Water division Directors, Streets Division Managers, etc.. Hard to be a hard ass on them and come back later for a favor, and these politicos are all about scratching backs, and minimizing potential business complaints to elected officials.