Recently, a staff member in our office was sent out to chronicle the stages of a typical residential foundation installation.  We’re always looking for new images and new ways to look at perhaps the most important part of any home before it all gets covered up.  While we’ve had this interest for several weeks, the coordination didn’t happen and sure enough, winter came.  The temperatures this week in Iowa have been in the high 20s and lows now down into the teens.  It is definitely Cold Weather.  Yet for many professional contractors, this comes with the territory; is the nature of the beast; and is just another project condition to manage.

Some of the questions asked by our staff member were not surprising given the exposure he has had thus far to the industry.  Even more importantly, however, they are quite typical of the questions that get asked many times throughout the fall and winter by home owners, builders, building inspectors and yes, concrete contractors.

1.     How can they pour today?  It is 30 degrees (F) out here.

Easily.  First of all, concrete doesn’t freeze at the same temperature as water.  The concrete mix with far more constituents than potable water has a static freezing temperature that is somewhere between 25 and 27 degrees.  Keep in mind, that is if it were static or at a state where there was no chemical reaction taking place.  The very nature of concrete is a chemical reaction between the water in the matrix and the portland cement particles.  There are four different minerals in the portland cement mix that react at different times with the water.  The reaction is exothermic, which means it gives off heat.  Therefore, there is no static moment for concrete in its early stages, those most susceptible to the freezing temperature conditions as the concrete keeps its own temperature higher than the ambient condition.  A great reference for this reaction process can be read at http://www.understanding-cement.com/hydration.html.

2.     So how cold is too cold?  Is there such a thing?

While technically speaking there is a point or a time where cold might be too cold, it is actually a practical or economic matter.  There is research out there (Korhonen, CRREL 2002) where antifreeze admixtures are used for concrete to be placed in dangerous sub-zero conditions.  The materials are there to permit concrete to be placed at virtually any temperature with little to no protection.  The more aggressive the mix design, the higher the material cost.  Likewise, concrete construction during cold weather can require protection for the concrete mix if it is not designed to sufficiently protect itself beyond the critical maturity stages.  Again, depending on the protection design, the costs can increase for the construction process and the comparison between impact to schedule vs. construction cost become an equation for the contractor and owner to consider.  Additionally, in markets where ready mix producers do not have the equipment to provide heated water to the mix or control the aggregate temperatures, the “How Cold Is Too Cold” point is much higher than the actual concrete construction process dictates.

3.     What do you think this contractor will be doing to protect the concrete placed in these footings?

At 30 degrees and perhaps falling temperatures, since the concrete will be placed in the middle of the day, the best bet is the contractor and ready mix producer will make sure the delivery temperatures for the concrete are between 65 and 70 degrees (F).  This will permit the concrete to be high enough above ambient air temperatures to more than withstand the exposure drop in temperature during placement and finishing.  The footings are the easiest condition to protect.  Once they have stricken off the footings, applied their keyway form and/or dowels, they will then likely throw insulated blankets on the footings for good measure or at least a heavy poly sheet.  At the temperatures anticipated the rest of the day and even into the evening, there is very little concern for these footings freezing if left exposed but this small amount of protection is just smart contracting.  #CFAConcretePros.

4.     Will they continue the project as the temperatures are expected to get colder throughout the week?

It is likely this CFA member will continue the project as the temperatures between the high 20s and lower teens are really not all that bad and protection is easy to provide.  Most contractors will halt or delay a project when they see temperatures that are really to cold for the crews to be out, dangerous for skin exposure, etc.  This happens increasingly in the single digit temperatures to sub-zero.  

5.     They just moved the pour back to tomorrow morning.  Was that a decision you think they reached due to temperatures?

There is a possibility that they are delaying the concrete placement tomorrow due to temperatures but I think it is doubtful at these moderate cold temps.  More likely, they have encountered some preparation delays simply due to the longer time it takes to work in the winter and they don’t quite have the forming complete.  It is also possible that it is late enough in the day that the concrete plant has met their capacity or has started to shut down their process for the day and they won’t be able to get the full quantity of concrete they need.  One of the key points that stems from the research the CFA did on cold weather foundations, however, is pouring earlier in the day is a smart protection decision as it maximizes the natural elevation in ambient temperature conditions in the climate.   This isn’t always the case as we know there are those winter days when a cold front approaches during the day but in general the exposure to the sun is an additional benefit to keeping concrete temperatures elevated.

Much more on smart cold weather practices and the research behind many of the decisions today’s concrete foundation professionals are making can be obtained from the CFA’s Cold Weather Research Report (2004). Online orders and contacting the CFA can be attained at www.cfawalls.org.