"Overage" amount of concrete when estimating concrete quantities 4

[Ron247]

I am trying to get some assistance about how much “overage” do concrete estimators add to their concrete quantities for different types of concrete work (slabs, footings, walls etc.). I have never found any good reference on the subject. I made some estimations of my own years ago based on common problems encountered in construction. I have seen general statements like 5% to 10% as general numbers but I can easily see many examples where these can either be conservative or unconservative.

For me, the harder ones are casting against soil such as footings or slabs on grade. Formed items such as columns, walls and beams have fairly exact dimensions while casting against soil is not so easily controlled. 5% seems reasonable for columns, walls and beams and I tend to use 7%-10% for elevated slabs.
The problems I tend to see with casting against soil are:
[ul]
[li]Measuring, marking and excavating a theoretical straight line that is the width of a string in theory; the paint or chalk used is 1” to 2” wide in reality; just adding 2” to a 20” width is 10%[/li]
[li]Footing widths that do not match somewhat standard backhoe/excavator bucket widths; can easily wind up with footings 2” to 3” wider than specified; as the footing gets wider, this becomes less of any issue[/li]
[li]Soil that sloughs off easily[/li]
[li]Having to use 2” mudsills on foundation excavations due to rain; when the backhoe bucket has 3” teeth, you tend to get 3” mudsills instead of 2”[/li]
[li]Slabs on Grade subjected to rain during fast-track jobs that tend to make contractors muck out the wet stuff but not spend time placing stone, just pour the concrete thicker or pour a mudsill over the entire area (really hard to allow for)[/li]
[li]slab thickness inconsistent due to poor grading; adding 1/2" to a 4" slab is 13%[/li]
[li]Other[/li]
[/ul]
Any info would be appreciated. At this time I am not looking so much at the size of the project although that also affects the number.

 

[dauwerda]

Are you asking in terms of estimating the price of a job or in terms of actually ordering the right amount of concrete for that job? Those are 2 very different things in my opinion - here is my take on both of these situations.

If simply estimating the price of a job, you would use the theoretical amount of concrete based on dimensions on drawings. The actual price of the concrete is very small compared to the price of all the prep work (excavation, forms, fine grading, etc.) and placement and finishing of the concrete.

If actually ordering for a specific pour, it will depend on what it is for and how large the pour is, as you touched on above. For large pours that utilize many trucks, I am used to seeing the "theoretical" amount of concrete ordered initially (rounded up to the nearest yard or truck load), then as the pour is nearing the end, they will do quick measurements of what is left (if any) and get that added on to the order. For smaller pours, that require only one or two trucks I usually see the amount rounded up to the nearest quarter or half yard. However, if you are bank pouring a footing and you know that it was over excavated, you would use average actual dimensions when calculating the required amount of concrete to order.

 

[SlideRuleEra]

As a former bridge contractor... wait, I don't need to say anything.
Dauwerda's explanation covers all the important points.

http://www.SlideRuleEra.net

 

[r13]

For simple estimating purpose, we usually provide 10% to 15% extra on the overall quantity. For onsite estimate for each pour, 5% will usually do the trick, I believe.

 

[BUGGAR]

As said above, the last truck is considered a "hold" load. You'd better have an overage to cover. If you have too much, the super sends it to his house for a new patio.

 

[Ingenuity]

Only thing to add to the above constructive comments is that in some concrete supply markets there is a disposal charge for concrete left in the transit mixer...so you pay for the waste concrete twice.

 

[strucbells]

Isn't the extra commonly used for ecology blocks?

 

[Ron247]

It is predominantly for making the estimate for a hard bid project.

Retired13: I assume when you say for "onsite estimate" you mean you have completed all your excavation and can then more accurately estimate the volume because you can see the actual dimensions of the pour. In that case you use 5%, and to me that makes sense.

Dauwerda: Even on a 4" slab on grade, the concrete can be 20% to 25% of the cost. It gets to be more of the cost for large isolated foundations or strip footings where there is no finishing similar to a floor slab and may not have any forms. I know I am talking about overage, but most jobs will require more concrete than the drawings show. Even 10% of 25% is 2.5% which would have to come out of profit if no overage is included.

Thanks everyone!!

 

[r13]

5% for material ordering estimate during construction, and for very simple structure such as pavement. 10% for project consists of structures with simple geometry, such as earth retaining structure. 15% for the rest, especially buildings, because of variations from the multiple steps of rounding offs, and omission of small items in the calculations.