Concret Pile Caps

[4wilmar]

As part of the QC team on site I am concerned about thermal cracking on some up-coming pile cap pours which are over 10 ft thick, 15 ft wide & 20 ft long. However the designers advise that these are not really mass concrete pours and no speacial precautions need be taken.

1 If none of the vertical faces are reinforced, how much cracking might we expect if we start out with a concrete temp of 90 degrees???

2 If some of the piles are designated as tension piles how will the tensile load transfer thru the pile cap to the column above if there is no vertical steel in the pile cap???

Any comments would be greatly appreciated as the contractor & engineer of record don't seem very concerned about any of this.

 

[msucog]

it might be good to thumb back through aci 301 and 305. it probably would not hurt to make the team aware that the contractor should probably adhere to hot weather concreting techniques (which may help when the contractor should place the concrete quicker than 90min, is supposed to cool and moist condition it). it also builds your case that appropriate construction techniques were not followed in the event a problem arises later.

seems a little strange to not have any vertical reinforcement for tension piles...but i'm just a dumb ole geotech. a structural might have a better explanation.

i say document, document, document...it ultimately falls on the eor.

 

[civilperson]

Preventing differential temperatures during the cure will minimize if not eliminate the cracking. This is done with a temperature probe in the center of mass, and insulated or heated space at the exterior to keep both locales at the same temperature.

 

[csd72]

Look at the bottom of this page for a free article on mass concrete structures.

http://www.gostructural.com/pg.asp?id=20

 

[DRC1]

ACI does have a definition of mass concrete, which I believe is over 3 feet in the least dimension. If the engineer is not concerned, then the contractor assumes this must not be a problem. Any pour in warm weather should be started erly in he morning. Once the pour has some set we try to put hoses on it to disipate heat.rtarders acn delay set which will reduce heat. Talk to Q/C dept at the plant.

If there is no vertical bars in the cap tension loads must be transfered by concrete shear. How is the uplift tranfered to the pile cap from the mebers that aresupported by the pile cap?

 

[4wilmar]

We have made them aware of ACI's recomendations including the 3 ft criteria, but they don't seem to care. They'll start early in the morning and they do have some fly ash in the mix which should help but they are not going to add any ice or chilled water in the mix.

 

[DRC1]

Chilled water is hard to find, generally involves large expense for a limited no. of pours. Adding Ice is expensive, as it is usually done by hand.
Starting early is good - Late also works. If it is a cap,keep the bar and forms wet all day and pour about 3:30 4:00 so that as the heat builds, the outside temperature is dropping, hopefully. Since you don,t have a lot of exposed area, finishing should not be a big deal. Just have the finishers stay late. They could even come in late.
I would also look into retarder.

 

[Zambo]

As civilperson mentions cracking could occur due to the temperature differential between the centre of the pile cap and the external face.

This temperature differential can be reduced by lowering the temperature of the concrete at the time of placement and/or by lowering the heat of hydration.

Your team aim to reduce the temperature at the time of placement by casting early in the morning. It would be worth monitoring the temperature of the aggregates in case spraying with water the night before can further reduce their temperature (btw casting late in the day, rather than early morning, only reduces the ambient temperature, the aggregates, cement and water will still be hot from the heat of the day).

You also have concrete with fly ash to reduce the heat of hydration. What grade of concrete are you using? From previous test results does it seem possible to reduce the cement content and still reach the required compressive strength.

My feeling is that if you are using a 30N/mm2 concrete there will not be a problem. If using 80N/mm2 concrete there will be cracking. Other grades in between could be a bit worrying.

 

[chicopee]

cracking on piers and their caps happen all the time and what DOT will do is require to drill holes and inject epoxy compound to seal the cracks.

 

[csd72]

The column starters should extend down to the bottom of the pile cap and the pile bars should extend up into the pile cap of a sufficient length so that there an be a compression strut between them.

 

[4wilmar]

Sorry I haven't visited the forum in a while.

Just to let you know, the pile cap was poured with a mix design which included 25% flyash, but no addition of any ice (despite my concerns) to control the initial concrete temp. which went in at about 90 degrees. Day time temps were runing about 100 degrees and night time temps were dropping to around 80 degrees at that time. No exterior insulation was provided to control the temp. differential and when we removed the forms 3 days later, we had a horizontal crack about 10 to 15 mils wide located about 4 to 5 ft from the top, running around the entire pile cap. Crack width increased for another day or two to around 20 to 25 mils and then subsided to around 15 to 20 mils (slighltly over ACIs recommendations but acceptable to the Engs. as there was no rebar in the vertical faces to cause a corrosion problem). Several vertical cracks were also present but these were less than 10 mil wide.

Aside from the flyash, no speacial precautions were taken to control concrete temp. or the temp. differential in very hot weather.

Thanks to all for your comments & advice; hope this info might be helpful to some.

 

[chicopee]

We had cracks on our concrete piers that we had poured in Hartford, Ct. CT DOT had these cracks filled in with some concrete epoxy to seal them from the weather. These cracks were expected probably due to the size of these piers and were not considered a hazard as there was so much rebar in these piers.