Talking to my concrete guy today. He is forming a foundation for me at the moment. He tells me that on most of his pours lately he is getting surface cracks on the top of the stemwall where the anchor bolts go in–across the wall from one side to the other–and sometimes the same if they run a wall tie within ~6″ of the top of the wall. Apparently these are minor cracks that are not deep, appear quickly, and are covered by the mudsill.
Concrete recipe is 4″ slump, 5.5 sack mix, pea gravel for pumping, poured into panel forms that are oiled. Sounds mysterious to me. He mentioned that the gravel around these parts has either gone from round-ish to square-ish…. or was it vice versa….?
Dunno what to think but wonder if some of you mudheads out there have seen this or know what the hail it is?? It has not happened to me on the few wall pours I have done here. I know that sometimes they level off down inside the form panels rather than building forms to the exact elevation needed, whereas I use 2x and wedge ties and built forms to the height I want. Other than that we use the same mud, the same pump, have the same weather… the water goes down the toilet with the same circular motion… etc.
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Ask him if he sets the bolt in position before the pour or jams it in and does a quick float after the pour.
Also ask him how much extra water he adds to make it easier to push the mud around.
The few stems I have done or had done were always massaged well with the bolt preset and the top of the stem finished with an edger. No cracks.
I don't use bolts any more. All thread drilled, cleaned and epoxied after the walls are up.
You're raising an interesting point I had not thought of... wet set bolts. I don't do that, I hang them from scraps of wood nailed across the forms. It would seem that wet-setting would be OK as long as the vibrating was adequate. Something to consider....
wet set bolts, inadequate vibrating, too much water when poured and too fast a dry time (Forms hot from the sun or simply high temps when the pour is made). I have seen a lot of that cracking right at the bolts here."Poor is not the person who has too little, but the person who craves more."...Seneca
The mud is batched for a 4" slump, no water added at the pour. The walls I have seen these guys do are very, very smooth, zero bubbles or rock pockets, so I assume vibrating is correct although I have not been there when they did it. Their walls look better than mine, I know that. Temps are in the 70s right now, probably about like where you are. I assume that oiled panel forms do a better job of keeping moisture in the concrete than the 2x forms I have used.
Anyone know anything about aggregate shape and what effect that has?
Your concern springs from the cracks you see in the vicinity of holding down bolts. In the other details you describe there are other potentials for a less than highest-strength concrete pour.Using hex or cup-head bolts in concrete for holding down superstructure is extremely poor practice. "L", or "J" shaped bars (appropriately threaded) do an infinitely better job - think 'tornado stress failure'.
These require pre-positioning and good support. Bolts are often inserted as an afterthought, possibly during the initial setting up process of the poured concrete. The probability of come cavitation of aggregate, and the infilling by water rich slurry will certaionly give rise to cracks - such as you describe.The reported high grade finish may also be achieved by using undersized aggregates and over-vibration. Unfortunately you did not mention the results of the crushing tests on samples taken under qualified supervision. This would have told the story.Reference to aggregate size/shape/surface texture is also interesting.
Since the cement crystals need to bond to the total surfaces of all aggregate to achieve maximum strength, the sheared faces of all crushed aggregate are going to present a better 'key' onto which the bond is formed. This does not disqualify rounded river gravel as a satisfactory aggregate - it is just that tested to the ultimate, bond failure is more common in rounded gravel than crushed material.Concrete is still a much misunderstood material - even amongst builders.Hope this helps, Lapun.
>>Using hex or cup-head bolts in concrete for holding down superstructure is extremely poor practice. "L", or "J" shaped bars (appropriately threaded) do an infinitely better job - think 'tornado stress failure'.
All of the bolts are either J bolts or SSTB anchors.
>>Bolts are often inserted as an afterthought, possibly during the initial setting up process of the poured concrete. The probability of come cavitation of aggregate, and the infilling by water rich slurry will certaionly give rise to cracks - such as you describe.
The bolts will be hung into the forms prior to the pour.
>>The reported high grade finish may also be achieved by using undersized aggregates and over-vibration. Unfortunately you did not mention the results of the crushing tests on samples taken under qualified supervision. This would have told the story.
We don't have control of the aggregates per se, aside from ordering pea gravel. I could run over to the batch plant and take a look at their pile of gravel, and I'm sure it would be clean, well-graded 3/8", very unlikely to be crushed.
There won't be any samples taken from this pour. Don't know if he's had others, it is rarely done in residential, i.e. I have had it done twice in 20+ years.
Over-vibration could be an issue. I am going to supervise the pour and make sure that the vibration is the minimum necessary.
>>Reference to aggregate size/shape/surface texture is also interesting.Since the cement crystals need to bond to the total surfaces of all aggregate to achieve maximum strength, the sheared faces of all crushed aggregate are going to present a better 'key' onto which the bond is formed. This does not disqualify rounded river gravel as a satisfactory aggregate - it is just that tested to the ultimate, bond failure is more common in rounded gravel than crushed material.
I've poured plenty of mud from this particular plant and it's not crushed gravel. Curiously there is another plant here that quarries and crushes their own gravel. It is not washed, however, and I have not wanted to buy their concrete. They also use water from their own pond.
Proper vibrating should leave some <=1/8" air bubbles at the form/crete surface. They'er just too small to make rise without over shaking the mix.OTOH, if you see any voids =>1/2", they didn't vibrate enough.Get the vibrator head to the bottom as quick as you can, then pull it out at about 1' per second. For a 10" or stiffer slump, you can slow down to about 10" per second (|:>) For a self leveling slump, speed up to about 14"/sec. Go to an aquarium and see how fast small bubbles rise in clear water. They will move a little slower in concrete.If you drop the head in the mud untill it's almost under the surface, you can see the liquification zone. You want to make sure that you get about a 3" overlap in zones when you vibrate.With air-entrained mud, it's far, far better to see 1/4" bee holes than to not see <=1/8" voids. Over vibrating can drive the entrained air out.SamT
Thank you Sam. Always good to revisit that.
THink of all the new people here who've never seen it (|;<)SamT
Sam gives some good advice. I would also worry about over working the concrete after the bolts are set in the "plastic" stage. The water gets worked out of the surface. Add this to overvibrating and the cracks will come....that's not a mistake, it's rustic