we are currently working on a proje

huangyhg

we are currently working on a proje
we are currently working on a project which will require a mat foundation approx 2m deep x 4m wide x 12m long.  
at what point, as a rule of thumb, should we worry about heat of hydration causing unwanted cracking in the foundation (various publications state anything greater than 1m in depth can be problematic - we are looking for practical recommendations).  also, what measures can be taken to combat this situation (ie: different portland cement in the mix, insulating to protect heat loss and maintain temperature differentials, etc.)
any guidance is greatly apprecieated.
dutchie
if you can get it - aci 207 "mass concrete" would answer most of your questions.
i once placed a large mat - 23' x 23' x 12' (7m x 7m x 3.65m) and we specified chilled aggregate, temperature monitoring, water curing, etc.  you can vent the mat and do a number of other things to control the heat.
dutchie,
type iv cement is a slow hydrating cement for control of temperature in large pours. also admixtures for the retardation of hydration are available, check with local concrete plants for types and availability.
here is good paper that may help:
on electric generating station foundation mats up to 15' thick (4.6m) we had no trouble. this was in south carolina in the summer (air temperature 85+ degrees). used type ii cement (sets more slowly than type i). also replaced concrete mix water with the necessary amount of ice shavings to limit concrete temperature at time of placement to 90 deg or less. finally wet cured the concrete for at least 7 days.
the florida dot thinks that temperature differential and not total heat production is the controlling consideration.  they specify a maximum allowable temperature difference of 35 degrees f between the interior of the concrete and the surface.  this is often accomplished by insulating to prevent heat loss.  other techniques include chilled aggregates and especially mix water.  ice doesn't always melt where you want it to.
fdot also requires the contractor to hire a specality engineer to design the temperature control plan per aci 207.  although i don't recall the number, i thought the "mass" threshold is a function of the volume divided by the surface area.
hope this helps.           
grouser,
wow!  i have never heard of insulating concrete in florida!  i suppose insulation would prevent too rapid of a temp. rise in a hot climate, air temp 110, concrete temp. 95?  i'm used to insulating up here in snow country!
if you replace some of the cement with fly ash you will get less heat. and fly ahs is cheaper than cement. i thought i read that you could replace up to 25%.
lower concrete strength means less cementitous material and therefore less heat.
replacement of opc with pfa also reduces the heat produced.
using a plasticiser to provide workability means the water content can be reduced thereby lowering the cement content while keeping the water cement ratio unchanged.
chilled water (depending on the ambient temperature) may be an advantage to keep the temperature of concrete at the time of placement to below 30 deg.c
zambo
thanks everyone,
we are on the right track now,
dutchie