is this an issue?

[delagina]

I'm checking concrete mix submitted by contractor. I don't really do this match. only thing not matching our notes is estimated density = 140pcf, not 150 pcf per our notes. any problem with this? f'c is met.

 

[RDMESH]

I usually dont think that the reduced density will affect your concrete final strength by that much...I usually watch the water cement ratio and the size of aggregate the contractor is planning to use which will affect the strength of concrete.....Hope that helps...

 

[SlideRuleEra]

Depending on the aggregate used in the mix, 140pcf is reasonable. "Typical" concrete actually weight about 144 to 145pcf. The 150pcf is conservative round number that has been used for decades. Round numbers were more often used in the days before computers and electronic calculators... it made the mental mathematics simpler.

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[EngineeringEric]

I always thought the 150 accounts for the additional weight when steel is added. Steel weight 490pcf and assuming concrete has 1% steel makes it 150 pcf...

Also keep an eye on Fly Ash percentage and AE. other than that durability may be an issue but only if it is rough/demanded use.

 

[mike20793]

I agree with Eric. I've always been told the 150 pcf includes the weight of reinforcing; 140 pcf is for concrete only.

 

[Archie264]

Eric, what number do you watch for on fly ash?

 

[EngineeringEric]

For Fly Ash, it depends on the season. When it is winter months and a contractor wants to place Fly-Ash > 10-15% i have had problems with the reaction not getting hot and not working. When it is summer i am more forgiving but I still look at it, when higher than 20% i flag it and just state that it is the supplier's responsibility to ensure that the mix reaches compressible requirements


I had a slab once with high Fly-Ash placed in middle of winter. Cold weather curing used. and the slab ended with like 1000psi instead of 4000psi. The plant had record of the mix used and video of components being added so it isn't like someone forgot to hit the 'Cement' Button at the batch plant. Fly Ash took the blame and i have been conscious since, maybe incorrectly? You guys probably know better than I!

 

[SlideRuleEra]

EngineeringEric - As you mentioned, the 150 pcf number does make a reasonable allowance for the weight of rebar. But 150pcf has other relevance, too. Per ACI 347, the hydrostatic pressure of fresh concrete on horizontal wall and column forms is calculated using the 150 pcf value. Weight of rebar is not an issue for the wall & column form pressure. It is considered to compensate for any possible temporary, minor effects from vibrating the fresh concrete.

For unreinforced or concrete with minimal reinforcement, there are a couple of situations where the 150 pcf number is not conservative. If the dead weight of concrete is used to resist uplift, better to use its true weight (or less) of the concrete than 150 pcf. The same reasoning applies if the concrete is submerged or below the water table and is being used to resist floating - such as the seal of a cofferdam.

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[SlideRuleEra]

Gee, got my words twisted... should be: "Per ACI 347, the hydrostatic pressure of fresh concrete on vertical wall and column forms..."

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[Ron]

The unit weight of concrete depends on the specific gravity of the aggregates and the cement content. For most concretes, the fine aggregate will have a specific gravity of 2.63 to 2.65. The coarse aggregate will have a specific gravity of 2.45 to 2.65 depending on the source....for example, a crushed limestone aggregate will have a lower specific gravity...typically 2.45 to 2.55; whereas, a crushed granite aggregate will have a specific gravity of around 2.65. This has little to do with the strength of the concrete as higher strength concrete may be made with either.

 

[Archie264]

Eric, thanks for the info.

And batch plants now video tape their mixes? Wow...that sounds like yet another way to move towards putting the blame at the foot of the engineer. I specify f'c but I'm no chemist; I assume they know their mixes better than I do. But if they're providing something that comes in at 1,000 psi instead of 4,000 psi and then providing a videotape to play "gotcha!"...well, I'll think long and hard before I ever "approve" another mix design submittal. Surely they must have had some inkling that it would come in low? After all, that's their entire line of work.

 

[UpNorthStruct]

Be aware that the use of fly ash can be very helpful in mass pours to keep the maximum temperature of the concrete down while curing. In a mass pour mixes with 25% fly ash or more can be used.

 

[Ingenuity]

The plant had record of the mix used and video of components being added

This does not surprise me in the world that we live today...I know from experience that all telephone calls to the batch plant dispatcher of one of the largest concrete suppliers in Sydney (Australia) are recorded and time stamped, and a play back is often used to verify/deny customer vs dispatcher conversation.

 

[EngineeringEric]

My video tape conversation was not shown to me. The plant told me this after i told them that they may have forgot to add cement. The plant may have blamed the fly ash in order to not take the full blame. They were able to pass some of the blame to the contractor by saying the concrete was not cured properly... left me with a warm fuzzy feeling of trust as you can imagine.

I am intrigued by the phone conversations being recorded.

 

[steellion]

Unit weight of lightweight concrete can affect fire rating. I wouldn't worry too much about normal-weight concrete. Even for uplift/overturning, you are only counting on 60% of the dead load by code so it would just eat into your factor of safety a little bit. As previously stated, if you're specifying 150 PCF, that's probably too heavy because that includes reinforcement. I usually assume 145 PCF for plain concrete.

 

[hokie66]

The density of concrete in the range of 140-150 is normal, as long as it is consistent on a given project or component. When there are issues with apparently low strength test cylinders, the density is the first thing I look at. Variation in density usually indicates poor cylinder preparation.

 

[Archie264]

Steellion,

Lightweight concrete affects fire rating? Is that in the IBC or some place similar? I'd be interested to learn about it. In my experience lightweight concrete performs better in a fire. But then, that's with the view of looking towards the concrete itself, not as the structure as a whole.

Hokie66,

I can add an anecdote in support of what you said. As a very junior engineer I watched the tester rod the cylinders a total of 10 times vs. the 25 times required. I was too green to feel confident speaking up and sure enough, the concrete cylinders broke low. To check the concrete's strength we had to conduct Swiss Rebound Hammer testing or some other type of in-place testing which name I can't remember at the moment. It involved shooting powder actuated studs into the concrete and measuring how far they stuck out, if I'm remembering correctly.

 

[EngineeringEric]

That sounds like a swiss hammer. I do believe for accurate results you must compare the result in Question Area to and area of onsite concrete of known/tested capacity.

 

[Ingenuity]

some other type of in-place testing which name I can't remember at the moment. It involved shooting powder actuated studs into the concrete and measuring how far they stuck out

Windsor Probe test.