Blinding Concrete - Specified Grade

[BigH]

I have a question of all you experienced structural (foundation) and geotechnical engineers. For the project on which I am currently working, there are two grades of PCC (plain cement concrete) specified for blinding mats underneath all foundations. One is called M10 (requiring 10MPa cube strength - 8MPa cylinder strength). The other is M15 (15MPa cube - 12MPa cylinder). The "weaker" one is for light structures - say median drains, toe drains, sign foundations, etc. The "stronger" one is for bridge spread foundations, as an example.

The question: Why, for a mud mat/blinding concrete, do they specify two different grades? Is it necessary?; is it critical? I have my thoughts - just want to hear yours. Hope you all take the bait!!

 

[Rawand]

Dear BigH,,,
Some foundation design techniques take into account the strength of PCC and it's contrebution in speading the load, but this assumption is not helpful unles accompanied by big thickness, as for me i am not using this, as for the stength of PCC, i never specified two, but i always specify the cement content in the blend.
A small cement content is not good for durability cause as u know some building standards allow to reduce the concrete cover to main rebar if PCC exists (Such as BS8110), but the mud mat is relly only to work on clean surface nothing more...

 

[ERV]

BigH,

Here is a direct relation to you're question and may not be related to tensile strength (split-cylinder test), however, is related to compressive strength:

Under sustained compressive loads, concrete will continue to deform for long periods of time. After the intiial deformation occurs, the additional deformation is called creep, or plastic flow. If compressive load is applied to a concrete member, an immediate or instananeous shortening occurs. If the load is left in place for a long time, the member will continue to shorten over a period of several years and final deformation will usually be 2 to 3 times the initial deformation. Perhaps 75% of the total creep will occur during the first year.
Should the long term load be removed, the member will recover most of its elastic strain and a little of its creep strain. If the load is replaced, both the elastic and creep strains will again be developed.
The amount of creep is very dependent on the amount of stress. It is almost directly proportional to stress as long as the sustained stress is not greater than about 1/2 of the compressive strength (f'c). Beyond this level, creep will increase rapidly.
Long-term loads not only cause creep but also can adversely affect the strength of the concrete. For loads maintained on concentrically loaded specimens for a year or longer, there may be a strength reduction of perhaps 15% to 25%. Thus a member loaded with a sustained load of 85% of its ultimate compressive strength (f'c), may very well be satisfactory for a while but may fail later.

See: "Researches Toward a General Flexure Theory for Structural Concrete", H. Rush, 1960, Journal ACI, 57(1), pp. 1-28.

 

[BigH]

ERV - remember, the blinding concrete is only 150mm thick (and usually is only in the order of 50 to 75mm thick). You make good points for a sizeable structure, but . . .

 

[npthao121]

Dear all,
I think, there is no specific reason for specifying two grades of the blinding concrete. We normally call it the lean concrete. its purpose is only for providing a clean surface before we pour concrete for the foundation. In Vietnam this on is specified as 100 mm thk & 10 Mpa grade for the lean concrte. the lean concrete layer is not taken into account during the foundation design process.

In BigH case, the designer may specified two grades due to his own feeling (i guess). Many times I met designers who made specification like this. then i asked them why? their answer is just for more safe - by their own feelings. Especially, for the type of project that they are not familiar before.

It is not necessary to specify two grades of lean concrete. it is not critical at all.
Am i right?

 

[flamby]

I think there is no solid ground for specifying M10 and M15 in this case. Perhaps there were two different designers looking after the structures and provided lean concrete of different specifications as per their 'choice'.

 

[dutchie]

just to expand on some thoughts already mentioned. in my neck of the woods we often spec a "lean" concrete mudslab when designing foundations. it's main purpose from our perspective is to provide a workable surface over what is often a clay or silty clay soil which otherwise would not be suitable for formwork and rebar placing etc. It is never considered in design, and in fact, it will always be better than what you are constructing on, so it's never a weak link either. If the ground is really good, then a mud slab isn't required.

good luck,

dutchie

 

[sc]

I agree with duchie, specifying blinding concrete is purely to get to the required founding depth for a reasonable cost. I consider blinding concrete (whatever the strength) to be better than the soil but worse than the footing concrete. On this basis it has no bearing whatsoever on the design apart from carrying load.

regards
sc

 

[BigH]

For "probable" closure, thanks for all the comments. This different grade for blinding concrete has always baffled me on this job but I think that flame probably hit it on the head - two differnt designers; one for the "bridge" foundations and one for the "culvert" foundations. My first inclination though is the adages: "The bigger the better; the more the better." which I find in many other items on this project - many specifications which are, in fact, over the top.

I've almost always specified mud mats in my geotechnical reports, but I would summize that only 30% of the foundations poured comestically actually ever had it - other than all of my overseas work. For me, the mud mat (blinding concrete) is to prevent foundation disturbance and for a levelling course on which to sit the steel. It has no structural significance (unless you are a client knowing a few buzz words).

and again, thanks to all for their valuable time.

 

[Focht3]

For me, the mud mat (blinding concrete) is to prevent foundation disturbance and for a levelling course on which to sit the steel.

I agree completely. From our other discussions, I'm not surprised with your "encounter."


In reality, the Owner's Representative should have had a "wood shed session" with the two designers before the specs were finalized to make sure that these kinds of really stupid spec book blunders were ironed out in advance. Of course, that assumes the Owner's Rep really gives a damn -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[BigH]

You like a good joke, eh??