Concrete Strength for Bored cast in place piles and vibration requirements

[geotechguy1]

Hi all, couple of questions regarding bored cast in places pile. Thought I would ask here since I can't get a straight answer out of anyone as to why we specify certain things:

What concrete compressive strength needs to be used, and why? For instance, if I have a bunch of cast in place piles tied into a grade beam or pile cap for foundation, what is the purpose of specifying a 30 MPA or 35 MPA concrete? For a cast in place pile retaining wall, what is the purpose of specifying a 30 MPA concrete? The load on the pile is not remotely approaching that level, so I don't understand why I need to specify this high of strength, and why I should reject piles that only achieve 27 MPA in concrete testing?

The other thing I've noticed is that my firm always specifies that you can't drill adjacent piles (2m spacing typical) during the initial concrete set, therefore requiring the contractor to waste a bunch of time moving back and forth skipping piles. I cannot find any research that supports this requirement...in fact it seems that there is no statistically significant correlation, and in many cases experimental results show that drilling-induced or casing-induced vibration actually causes an increase in the compressive strength as often as it does a decrease in the compressive strength. (for instance this DoT research:

http://mdot.ms.gov/documents/resear... Acceptable Vibrations On Green Concrete.pdf)

And in any case, even if the 'worst case' from this research was true, it only shows that on some occasions applying a continuos vibration greater than that caused by driving a casing within 2m for 30 minutes only leads to a ~7% reduction in strength (and as I mentinoed, it seems to sometimes lead to an increase in strength)...so in the worst case, my 30 MPA concrete is 27.6 MPA concrete, which seems irrelevant since no one can justify why we need 30 MPA concrete in the first place.

Anyway, I'm just wondering if anyone can provide an explanation for these requirements that is based on some factual evidence rather than "this is what we always specify".

 

[jayrod12]

Depending on the class of concrete you're specifying (I'm talking CSA codes here) that dictates the minimum concrete strength. In most cases, we are specifying C class or S class concretes which with their maximum w/c ratio of 0.45 (I think) you're hard pressed to get strengths lower than 35 MPa in reality. So if your strength is coming in at 27 MPa I fear someone may be adding water to the mix.

Regarding the alternating piles. I've always been told it's that they can't have adjacent holes open at any one time, as long as there's concrete in the hole to keep the hole from collapsing in, then that's ok. I haven't seen a spec that you must wait until initial set is complete.

 

[geotechguy1]

The other baffling thing is that we say that the skipping piles requirement is to protect the concrete from vibrations caused by drilling or casing of adjacent piles during the initial set, but we also require the contractor to use a mechanical vibrator directly in the concrete for the upper ~2-3m of concrete.

>So if your strength is coming in at 27 MPa I fear someone may be adding water to the mix.

Would that have a negative impact on the performance of the foundation though? I'm not really an expert in concrete but to me it seems if the compressive strength is 27 MPA or 30 MPA or 35 MPA, why does it matter when I'm putting a warehouse on it. We're telling them to do the pile design based on an end bearing of ~500-1000 kpa allowable, so I can't see how the pile load would ever even remotely approach the concrete strength

 

[cvg]

durability is a primary consideration. A decrease in W/C ratio generally reduces permeability and makes the concrete more durable. It also results in higher strength. This is a relatively cheap method to greatly increase the design life of a critical structural component.

Generally good construction QC will start with a concrete mix design that is guaranteed to meet and exceed the design requirement. There is absolutely no reason to fall short of the design strength unless there was either an attempt to cut corners or just a plain screw up. But my question is why would you ever want to lower the standards and accept work that does not meet the requirements? You work for the owner and they hire you to watch out for their interests, not the contractors.

the jury is still out on pile vibration. not all research shows that is ok. for example:

http://www.cipremier.com/e107_files/downloads/Papers/100/37/100037034.pdf

 

[jayrod12]

cvg has nailed the reasoning why you should not add water. Especially if sulphate attack on the concrete is the main concern (which where I do 99% of my work is the main concern).

And I just read the link that cvg posted. Around here, we don't typically space piles closer than 3d so by that reference we'd be fine.

 

[geotechguy1]

We aren't in sulphate sensitive soils.

The link he posted still seems to jive with the research that I found. Piles drilled 2 to 3 pile diameters away don't have any substantial impact on the concrete. In my case we are essentially requiring 6.

>There is absolutely no reason to fall short of the design strength unless there was either an attempt to cut corners or just a plain screw up.

The question really is why we specify the design strength we do, since I can't justify the compressive strength requirement on the basis of the loading. I want to give an economical and cost effective design.

>But my question is why would you ever want to lower the standards and accept work that does not meet the requirements?

While, why are the requirements what they are? That's the question. I have to justify the standard and 'this is what we specified do it' or 'this is what we always do' isn't really a good response. I need something supported factually with data. I think the durability argument might be a good one, but then, if the building has a 50 year design life, does my concrete have that life even if the water content is out?

 

[jayrod12]

If you don't have a reason to require the higher concrete strength, i.e. exposure to sulphates or chlorides or freeze/thaw, then in theory you could specify whatever strength you need.

But the cost difference between 25 MPa and 30 MPa concrete is almost negligible.

 

[geotechguy1]

Right, so theoretically I could just specify a 15 MPA or a 20 MPA concrete?

Would the lower-strength concrete be more susceptible to damage from freeze-thaw cycles?

I'm aware the cost difference is low, but the area I'm in is in a very intense recession and there is huge pressure to cut costs, even if it is only 10 or 20% on concrete?

 

[jayrod12]

It will be less than that percent reduction in cost. And yes, it would significantly less durable. If you have freeze-thaw issues you also need air-entrainment in your concrete which is actually more of a cost difference than changing concrete strengths (within reason).

 

[SlideRuleEra]

The study involved only one concrete mixture design that nearly matches what MDOT normally uses in shaft foundations. All materials were locally available and approved by MDOT. With a w/c ratio of 0.44, compressive strength at 28 days usually exceeded 5,000 psi (34 MPA).

I appreciate the need to control costs. Placing concrete underground is not a neat, precise operation. Best to be conservative in both structural design and material specs.

Keep in mind that concrete mix design strength (actually what's important is how much cement is in the mix) and susceptibility to vibration while the concrete is green may be interrelated. For example:

If concrete strength is reduced perhaps the delay in working on adjacent shafts is justified, or

If working on adjacent shafts has priority... maybe 34 MPA + is best.

Probably can't have it both ways.

Also, if cast in place piles are used on sites with high water table, a high-cement content mix compensates for some degrading of final concrete strength because of dilution in ground water during placement.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BUGGAR]

It's possible to instrument and record adjacent ground vibrations to assure no damage to freshly placed concrete nearby. Sometimes it is specified. If it isn't specified, you can never get agreement on how much vibration is O.K.
There are established standards for rock blasting.

 

[cvg]

according to the paper I linked, PPV as measured for blasting is not the issue. it is high amplitude, low frequency and especially long lasting vibration caused by the drilling that creates persistent ground movement shown to be damaging to green concrete. you can measure it and you can document it, but you cannot ever assure yourself that it did not cause a problem

 

[Retrograde]

The other thing I've noticed is that my firm always specifies that you can't drill adjacent piles (2m spacing typical) during the initial concrete set, therefore requiring the contractor to waste a bunch of time moving back and forth skipping piles.

This depends on the soil conditions but I have personally seen the concrete level in a freshly cast pile drop 2m due to soil disturbance caused by drilling the next pile too close.

 

[hokie66]

Retrograde,
Where did that concrete go? Into the pile being drilled? Or was the first pile cast with a big void?

 

[EireChch]

Having a lower concrete strength may be ok for axially loaded piles however it would not be ideal for piles subject to lateral loads as concrete strength plays a big part in avoiding brittle failure of the pile. Lower strength would likely increase the the amount of rebar required. This may counteract your 10-20% saving.

 

[Retrograde]

Where did that concrete go? Into the pile being drilled? Or was the first pile cast with a big void?

It went into the surrounding soil. The top layers of the profile were loose alluvial deposits which were disturbed by the drilling process.

 

[Ron]

D U R A B I L I T Y !!!!

Compressive strength is easy to get. Durability...not so much.

If we only consider that we need to meet a strength number for structural purposes, we are not considering the whole picture of the life cycle of the component.

Those piles have to last at least as long at the building is anticipated to last. That's 40, 50 or even 100 years. During that time you might have changes in the groundwater levels, changes in soil pH, changes in sulphate concentrations, changes in chloride concentrations and a bunch of other "stuff".

If you only consider that a strength requirement for structural purposes has to be met, then you are not doing a responsible design for the life of the structure. Durability increases with compressive strength. That has been shown through numerous studies and observations for over a hundred years. We look at our need for structural capacity and we'll call that "x". We then look at our need for longevity and the reliable need for durability for existing and potential changes in the exposure conditions of the concrete and now oudr strength needs to be "x + d"........

Durability is one of the aspects of resisting damage to existing piles when driving piles adjacent to those. There is no direct strength relationship for this, but you do know that if durability is increase, so will the ability to resist damage from adjacent vibration.

Look for a study done by Florida Atlantic University in the 1990's on damage potential for pile driving adjacent to other piles and buildings.