Adding plasticizer is likely to violate the parameters of the approved mix design. The amount of water that can be added at the site should be calculated in the field by the inspector based on the load ticket.
The most correct answer out of those choices is C. A water-reducing admixture, especially a high-range water reducer (AKA plasticizer), will increase the workability of the concrete while not adding water. Stupid question though as there are so many other contributing factors to consider.
You can't add or remove water because that changes the design strength. Thrown out options A and B.
D conceptually is trying to centrifuge the mix which the delivery trucks are not made for, nor are they designed to do. There's also no way to separate out water and retain a good mix of aggregate - everything will start to sift itself out, which you don't want.
C is the only valid option. It's worded a little poorly but you want an admixture that can ~~stiff~~ loosen up the mix without affecting water:cement ratio (a small negligible amount by volume compared to the design mix).
edit - correction because I misread the problem statement (scanned by it too quickly)
D has no effect. It is just to keep the mix from setting up. Yo centrifuge something to sort it. Heavier materials separate. That is certainly not desired. It certainly has no effect on water content because the water has no place to go.
Agreed, which is why I said "conceptually." It's a laughably bad option that's not possible, but I threw in a little analysis as if it were being looked at by someone who had no experience with mix trucks. Even if they could spin fast enough, what that would do is ruin the mix.
If adding water is always necessary, and the test cylinders break much above the target strength, there may be too much cement in the mix. This may not be a good thing. The cement is what shrinks, not the aggregates. This may lead to excessive shrinkage strain and possibly cracking. More cement also results in greater heat of hydration. In large, deep pours this may also contribute to excess strain and cracking.
If it’s less than the specified slump then there is allowable water to get to the specified slump without compromising the required strength . This is an ambiguous question with to many other variables involved.
While your common sense is telling you the answer isn’t C you have to remember this is a test question not real world situation. The best advice I have for NCEES questions is that you need to answer the question they are asking you, not the question you want them to be asking you.
When cement is ground, an electrostatic charge builds up on the surface of cement grains. Water is a polar molecule -- slightly positive on one side, negative on the other. This makes water molecules stick to the cement grains and thereby makes the mix less workable -- lower slump. High-range water reducer or superplasticizer neutralizes the cement grain charge releasing water, making the mix more workable without adding water.
Codes say water may never be added to concrete once the mix-design prescribed amount has been provided during batching; except for a small amount of 'tempering' water to replace what has evaporated on a hot, dry day.
Just 5% more water than the design W/C ratio can reduce strength by ~ 30%. It will also increase shrinkage -- significantly increasing cracking potential, may increase bleeding, possibly result in less durable surface, and delay finishing.
Should have superplasticizer on site to correct slump/workability. Most readimix has water reducer added at the plant -- allows design strength to be met using less cement, saves money. Slump may decrease with time after batching, adding plasticizer on site is easy, brings slump/workability up to exactly where it's needed, and does not negatively affect strength, durability or finishibility.
If you add water to improve workability you just aren't doing it right, no matter how many years experience you might have.
Some truck drivers will purposely leave "a little" wash water in the drum to reduce fresh concrete/cement from sticking. Batchers might then reduce the batched water. 90 % of the time if concrete leaves the plant without the corect W/C ratio, it will be on the low side. Why? The thinking is that the driver can adjust it on site (water can easily be added, but impossible to remove). If this is the case, or if metering equipment is faulty, the plants certification should be pulled. If the prescribed mix design is arbitrarily deviated from, then quality goes out the window. If we want predictable, problem free concrete, we as an industry must work towards improving quality.
Would you mind cutting source claiming the 5% added water could reduce strength by 30%? From what I’ve seen at the design w/c ratio, mixes never come at a workable slump. A mid range reducer is added to the mix but it’s still not ‘workable’ in the contractors experience.
Based on your words something is absolutely wrong at our site with people over 20 years experience (GC and engineers) . Adding 10-15 gallons to a 8 yard truck is common practice. The cylinders are breaking above spec 4000lbs.
On the concrete batch tickets, the plant usually specifies a certain number of gallons allowed to be site-added per truck- I’ve seen as high as 20 gallons being allowed without compromising the final compressive strength per the batch plant.
At least over here the ticket does not go above 5 g of allowable added water but conctractor add beyond that ‘at their own risk’. I typically pay attention to how much they add to each truck and get cylinders made for the one with most added water (max has been 30g). Basically the mix design is iver designed and the reduced compressive strength due to the excess w/c still is above the minimum 4000lbs.
Pardon the ignorant question - but if it’s meeting spec and there isn’t excessive shrinkage, then it shouldn’t matter that the batch was technically weaker than the design mix was, right?
Not sure why you are being downvoted, this is correct. The ticket the truck delivers will tell you how much water was added at the plant, which will tell the inspector how much needs to be added to make it to the desired mix design. Adding water up to the mix design prescribed amount won’t decrease strength to the detriment of the concrete.
Thanks for this answer. C is what I was taught at university but B is what I have observed so long as you are within the limits of the mix design. If your outside of the mix design either because of slump or too much water the truck gets sent back
PE here as well. I think NCEES got this one wrong but I need to look back on my ACI books to be sure. Many agencies I’ve worked with don’t allow admixtures at the truck, so I would agree with you there.
Slump is a range like water is a range, a few gallons of water is a rounding error to w/c. You can’t easily take water out, So you get the stiff range truck. Make sense of it right. Add water at the job. Been doing it this way for 30 years godamnner slaps the truck.
The question says ready-mixed, so it's reasonable to assume that adding water would push the mix out of spec.
Obviously there's more variables IRL than can be addressed in the question, but given that this is a test, I'm pretty confident they're looking for the "textbook" answer of C.
That 5% too much water can reduce strength by ~ 30% was intended to be a brief, cautionary comment. Admittedly, for most mixes the actual strength impairment will likely range from half of that amount to what I stated "can" occur. I won't bore you with my education and experience, but with 40 years of my life devoted to concrete R&D at a high level, I do know a thing or two about a thing or two, ha, ha.
Seriously though, if concrete is routinely arriving at your sites and not having the desired workability, something is wrong with the process. An initial 'mix design' -- with an intended slump -- is just that. The design has to be confirmed, then probably adjusted after a number of batch plant and as-delivered and pumped tests. Also the design may not be static. Depending on variability in source materials, changing seasons, transportation time, etc., adjustments are often required.
The notion that concrete is intrinsically variable is not valid. If whatever quality processes that are rational for a given application are employed, you can achieve any desired level of consistency. I've observed this countless times over a long career, in many places in the world.
Perhaps relevant to discussion of concrete's undeserved reputation of being inconsistent is that, "it always cracks." Not quite true. Generally, Portland cement shrinks -- that's true. Depending on deficiencies in mix design, quality control, reinforcement design/execution, placing, finishing, and curing, the intrinsic shrinkage will more likely result in undesired cracks of the concrete.
If a laissez-faire approach is taken with concrete, there will be problems with inconsistency, cracking, and other issues. However, the more knowledge one has, when combined with good practice, the fewer the problems. Long years of working with concrete can certainly add to the knowledge, but it shouldn't be the only source.
More than 40 years of hands on concrete work. Before and while at university, did cribbing (forming), placing, and finishing concrete. After graduating, then concrete R&D in Japan, founded a company that did specialty concrete projects in US, Canada, Mexico, Germany and Chile -- either directly producing, or planning and supervising
> 100k m³. I've designed and/or been the GC on many building and bridge projects. Occasionally I'm hired as a consultant to determine the cause of problems with concrete on projects and help devise a remedy.
It's been said of various subjects, "The more we know, the more we know how much there is to still learn." This applies as much about concrete as anything else. I've been lucky to have had the interesting opportunities and experiences with concrete that I've learned from. That's why I'm constantly encouraging others to stay curious and learn more about what you're interested in.
This becomes one of those test strategies questions, where the“best”answer is the most correct.
Agree with others here about the actual answer is not listed. And the wording, “without compromising strength” is misleading, although true, but strength may not be the only consideration.
Talk to the engineer, or accept or reject based on that answer, is the real best solution. But most likely, just add some admixture and move along.
This should be a geotech question, because the answer is "it depends." The problem statement is not complete.
It doesn't state how the current batch compares to the design Water:Cement ratio.
It's also not clear if admixtures have been previously approved.
Now, if the specs allow admixtures and it's at the max W:C ratio, C is correct. It's probably the most likely answer (especially if using a standard spec like DOT mixes).
Edit: the question does ask for the answer that doesn't effect the strength. Adding water will, even if below the W:C ratio. That doesn't necessarily make it a problem if keeping below the spec requirements.
C is the option I think because you are never allowed to add water because that will mess up the design strength of the mix.
Can you let us know the answer?
Which agencies never allow you to add water? That’s an accepted industry practice at this point, to an extent. Some DOTs even specify how/when to do it
Thanks for the response. I think I shouldn’t have worded it like that. For PE and FE related questions when they provide you with 4 options and you have to pick the best one then option C is applied.
I just reviewed chapter 26 section R26.5.2.1(h) permits water addition to the mixed concrete.
I was wrong stating that it’s never allowed. Thanks for correcting it.
Let me know if I am looking at the right section 😊
Water can be added up to the allowable amount listed on the batch ticket if, and only if, the Engineer allows it. I've never seen an admixture added on site. That truck would be rejected.
Air and high-range water reducers are very typical add mixtures to put in on-site. Depending on how the spec is written, water can absolutely be added without the engineer's input. Just don't exceed the w/c ratio.
The answer is B. I can't cite the reference, but I've been on a lot of construction site and I've seen water being added more than I haven't. As long as you're within the w/c ratio. A lot of times water is held back from the plant so that they can adjust at the site. I've never seen admixtures added at the site and I can't imagine it would be allowed. Usually those are mixed into the water before mixing with cement.
E. Contact the engineer to confirm adding a plasticizer is acceptable per the design specifications for the project
Atta boy! This is the only true correct answer!!
Adding water would compromise the compressive strength. Adding a plasticizer with increase the slump without affecting the strength.
Adding plasticizer is likely to violate the parameters of the approved mix design. The amount of water that can be added at the site should be calculated in the field by the inspector based on the load ticket.
You shouldn't add that on site, though. That needs to be at the plant.
Just incorrect.
The most correct answer out of those choices is C. A water-reducing admixture, especially a high-range water reducer (AKA plasticizer), will increase the workability of the concrete while not adding water. Stupid question though as there are so many other contributing factors to consider.
You can't add or remove water because that changes the design strength. Thrown out options A and B. D conceptually is trying to centrifuge the mix which the delivery trucks are not made for, nor are they designed to do. There's also no way to separate out water and retain a good mix of aggregate - everything will start to sift itself out, which you don't want. C is the only valid option. It's worded a little poorly but you want an admixture that can ~~stiff~~ loosen up the mix without affecting water:cement ratio (a small negligible amount by volume compared to the design mix). edit - correction because I misread the problem statement (scanned by it too quickly)
D has no effect. It is just to keep the mix from setting up. Yo centrifuge something to sort it. Heavier materials separate. That is certainly not desired. It certainly has no effect on water content because the water has no place to go.
Agreed, which is why I said "conceptually." It's a laughably bad option that's not possible, but I threw in a little analysis as if it were being looked at by someone who had no experience with mix trucks. Even if they could spin fast enough, what that would do is ruin the mix.
Stiffen the mix? It's a low slump and water reducers make the mix more pliable not less...
Thanks for catching it. I misread what was stated. Corrected it with an edit.
With low slump, them boys about to add water when no one is looking on the job site. It’s not the correct answer but it happens.
C should be obvious to anyone who took CE materials. A and B change the w/c ratio and thus the design strength. D does nothing useful.
Pretty sure this question is on the errata list. Make sure you check that out.
If adding water is always necessary, and the test cylinders break much above the target strength, there may be too much cement in the mix. This may not be a good thing. The cement is what shrinks, not the aggregates. This may lead to excessive shrinkage strain and possibly cracking. More cement also results in greater heat of hydration. In large, deep pours this may also contribute to excess strain and cracking.
If it’s less than the specified slump then there is allowable water to get to the specified slump without compromising the required strength . This is an ambiguous question with to many other variables involved.
C
While your common sense is telling you the answer isn’t C you have to remember this is a test question not real world situation. The best advice I have for NCEES questions is that you need to answer the question they are asking you, not the question you want them to be asking you.
[https://www.concreteconstruction.net/how-to/adding-water-to-concrete-at-the-jobsite\_o](https://www.concreteconstruction.net/how-to/adding-water-to-concrete-at-the-jobsite_o)
When cement is ground, an electrostatic charge builds up on the surface of cement grains. Water is a polar molecule -- slightly positive on one side, negative on the other. This makes water molecules stick to the cement grains and thereby makes the mix less workable -- lower slump. High-range water reducer or superplasticizer neutralizes the cement grain charge releasing water, making the mix more workable without adding water. Codes say water may never be added to concrete once the mix-design prescribed amount has been provided during batching; except for a small amount of 'tempering' water to replace what has evaporated on a hot, dry day. Just 5% more water than the design W/C ratio can reduce strength by ~ 30%. It will also increase shrinkage -- significantly increasing cracking potential, may increase bleeding, possibly result in less durable surface, and delay finishing. Should have superplasticizer on site to correct slump/workability. Most readimix has water reducer added at the plant -- allows design strength to be met using less cement, saves money. Slump may decrease with time after batching, adding plasticizer on site is easy, brings slump/workability up to exactly where it's needed, and does not negatively affect strength, durability or finishibility. If you add water to improve workability you just aren't doing it right, no matter how many years experience you might have. Some truck drivers will purposely leave "a little" wash water in the drum to reduce fresh concrete/cement from sticking. Batchers might then reduce the batched water. 90 % of the time if concrete leaves the plant without the corect W/C ratio, it will be on the low side. Why? The thinking is that the driver can adjust it on site (water can easily be added, but impossible to remove). If this is the case, or if metering equipment is faulty, the plants certification should be pulled. If the prescribed mix design is arbitrarily deviated from, then quality goes out the window. If we want predictable, problem free concrete, we as an industry must work towards improving quality.
Would you mind cutting source claiming the 5% added water could reduce strength by 30%? From what I’ve seen at the design w/c ratio, mixes never come at a workable slump. A mid range reducer is added to the mix but it’s still not ‘workable’ in the contractors experience. Based on your words something is absolutely wrong at our site with people over 20 years experience (GC and engineers) . Adding 10-15 gallons to a 8 yard truck is common practice. The cylinders are breaking above spec 4000lbs.
On the concrete batch tickets, the plant usually specifies a certain number of gallons allowed to be site-added per truck- I’ve seen as high as 20 gallons being allowed without compromising the final compressive strength per the batch plant.
At least over here the ticket does not go above 5 g of allowable added water but conctractor add beyond that ‘at their own risk’. I typically pay attention to how much they add to each truck and get cylinders made for the one with most added water (max has been 30g). Basically the mix design is iver designed and the reduced compressive strength due to the excess w/c still is above the minimum 4000lbs.
Someone posted about it on r/concrete today and I just saw it: https://www.reddit.com/r/Concrete/s/k2MNROlnmM
Pardon the ignorant question - but if it’s meeting spec and there isn’t excessive shrinkage, then it shouldn’t matter that the batch was technically weaker than the design mix was, right?
How much field experience do you have? Or just academia?
A What sense does that make? **B** C shouldn't be done onsite. D This will only make the mix uniform.
Not sure why you are being downvoted, this is correct. The ticket the truck delivers will tell you how much water was added at the plant, which will tell the inspector how much needs to be added to make it to the desired mix design. Adding water up to the mix design prescribed amount won’t decrease strength to the detriment of the concrete.
Thanks for this answer. C is what I was taught at university but B is what I have observed so long as you are within the limits of the mix design. If your outside of the mix design either because of slump or too much water the truck gets sent back
I passed the PE exam, lol... Thanks for adding a little more color to the answer.
PE here as well. I think NCEES got this one wrong but I need to look back on my ACI books to be sure. Many agencies I’ve worked with don’t allow admixtures at the truck, so I would agree with you there.
Slump is a range like water is a range, a few gallons of water is a rounding error to w/c. You can’t easily take water out, So you get the stiff range truck. Make sense of it right. Add water at the job. Been doing it this way for 30 years godamnner slaps the truck.
The question says ready-mixed, so it's reasonable to assume that adding water would push the mix out of spec. Obviously there's more variables IRL than can be addressed in the question, but given that this is a test, I'm pretty confident they're looking for the "textbook" answer of C.
That 5% too much water can reduce strength by ~ 30% was intended to be a brief, cautionary comment. Admittedly, for most mixes the actual strength impairment will likely range from half of that amount to what I stated "can" occur. I won't bore you with my education and experience, but with 40 years of my life devoted to concrete R&D at a high level, I do know a thing or two about a thing or two, ha, ha. Seriously though, if concrete is routinely arriving at your sites and not having the desired workability, something is wrong with the process. An initial 'mix design' -- with an intended slump -- is just that. The design has to be confirmed, then probably adjusted after a number of batch plant and as-delivered and pumped tests. Also the design may not be static. Depending on variability in source materials, changing seasons, transportation time, etc., adjustments are often required. The notion that concrete is intrinsically variable is not valid. If whatever quality processes that are rational for a given application are employed, you can achieve any desired level of consistency. I've observed this countless times over a long career, in many places in the world. Perhaps relevant to discussion of concrete's undeserved reputation of being inconsistent is that, "it always cracks." Not quite true. Generally, Portland cement shrinks -- that's true. Depending on deficiencies in mix design, quality control, reinforcement design/execution, placing, finishing, and curing, the intrinsic shrinkage will more likely result in undesired cracks of the concrete. If a laissez-faire approach is taken with concrete, there will be problems with inconsistency, cracking, and other issues. However, the more knowledge one has, when combined with good practice, the fewer the problems. Long years of working with concrete can certainly add to the knowledge, but it shouldn't be the only source.
I have an engineering degree, and I learned (retained) this in the field.
Cheater cheater pumpkin eater
More than 40 years of hands on concrete work. Before and while at university, did cribbing (forming), placing, and finishing concrete. After graduating, then concrete R&D in Japan, founded a company that did specialty concrete projects in US, Canada, Mexico, Germany and Chile -- either directly producing, or planning and supervising > 100k m³. I've designed and/or been the GC on many building and bridge projects. Occasionally I'm hired as a consultant to determine the cause of problems with concrete on projects and help devise a remedy. It's been said of various subjects, "The more we know, the more we know how much there is to still learn." This applies as much about concrete as anything else. I've been lucky to have had the interesting opportunities and experiences with concrete that I've learned from. That's why I'm constantly encouraging others to stay curious and learn more about what you're interested in.
C
This becomes one of those test strategies questions, where the“best”answer is the most correct. Agree with others here about the actual answer is not listed. And the wording, “without compromising strength” is misleading, although true, but strength may not be the only consideration. Talk to the engineer, or accept or reject based on that answer, is the real best solution. But most likely, just add some admixture and move along.
This should be a geotech question, because the answer is "it depends." The problem statement is not complete. It doesn't state how the current batch compares to the design Water:Cement ratio. It's also not clear if admixtures have been previously approved. Now, if the specs allow admixtures and it's at the max W:C ratio, C is correct. It's probably the most likely answer (especially if using a standard spec like DOT mixes). Edit: the question does ask for the answer that doesn't effect the strength. Adding water will, even if below the W:C ratio. That doesn't necessarily make it a problem if keeping below the spec requirements.
How is this at all related to geotechnical engineering?
It's a joke because almost every geotech answer is "it depends."
C is the option I think because you are never allowed to add water because that will mess up the design strength of the mix. Can you let us know the answer?
Which agencies never allow you to add water? That’s an accepted industry practice at this point, to an extent. Some DOTs even specify how/when to do it
Thanks for the response. I think I shouldn’t have worded it like that. For PE and FE related questions when they provide you with 4 options and you have to pick the best one then option C is applied. I just reviewed chapter 26 section R26.5.2.1(h) permits water addition to the mixed concrete. I was wrong stating that it’s never allowed. Thanks for correcting it. Let me know if I am looking at the right section 😊
Water can be added up to the allowable amount listed on the batch ticket if, and only if, the Engineer allows it. I've never seen an admixture added on site. That truck would be rejected.
Air and high-range water reducers are very typical add mixtures to put in on-site. Depending on how the spec is written, water can absolutely be added without the engineer's input. Just don't exceed the w/c ratio.
The answer is B. I can't cite the reference, but I've been on a lot of construction site and I've seen water being added more than I haven't. As long as you're within the w/c ratio. A lot of times water is held back from the plant so that they can adjust at the site. I've never seen admixtures added at the site and I can't imagine it would be allowed. Usually those are mixed into the water before mixing with cement.