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Tackling Winter Tilt-Up With Concrete Sensors

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by Shawn Hickey, CCS., GSC., FTCA

The first question we always hear: “Can you build tilt-up panels in the winter?” Answer: “Of course we can, eh!”

ACI 306R-16 (Guide to Cold Weather Concreting) states: “The conditions of cold weather concreting exist when the air temperature has fallen to, or is expected to fall below, 40°F (4°C) during the protection period.”

This air temperature categorization would include a vast area
in the USA and 99% of Canada. Armed with this standard, we contractors find ourselves being creative with our means and methods during cold weather concrete placements. The following outlines our experience with cast-in-place concrete and the benefits of leveraging new wireless concrete monitoring technology.

First, let’s explore the most popular issues associated with winter tilt-up concrete work:

  • Frozen casting surface
  • Pumping concrete
  • Reduced daylight time
  • Finishing issues
  • Curing in cold conditions
  • Concrete temperature measurements

Familiarity with these issues comes from experience, not textbooks. Addressing each issue is handled through research, technique advancement, materials, tools and equipment. “Experience” and “expensive” are two words separated by a few letters. The more experience contractors have with winter construction, the more costs they have blown trying to perfect these challenging concrete placements.

Here are some simple solutions to these known issues:

  • Frozen Casting Surface: Place ground-source heating piping in or on the casting slab and cover with insulated tarps overnight prior to placement – these are very cheap to install. Ground heaters are becoming popular rental items in most markets.
  • Pumping Concrete: Fluid concrete arrives warm due to the exothermic heat of hydration from the chemical reaction between cement and water. The fluid concrete temperature can be increased using admixtures or batching the concrete with warm water. Talk to your ready-mix supplier about their available and recommended options for your plans and the weather conditions.
  • Short daylight time: Supplementary job site lighting (towers, flood lights, etc.) are readily available.
  • Finishing Issues: Admixtures, tarping, tenting, and casting slab heating will all accelerate the finishing timeframe.
  • Curing: Insulated blankets, heating systems, and heat-of-hydration admixtures will help prevent the concrete from freezing.
  • Concrete Temperature Measurements: This is where this article really begins.

We have been successful in placing and finishing winter tilt-up concrete panels for 26 years in temperatures that keep most polar bears in their dens. In Ottawa, Canada, we often receive large amounts of snow and plummeting ambient temperatures down to -40 degrees Fahrenheit (-40 degrees Celsius) with the wind chill. This is the environment we must work in.

When we are casting in an open (non-tent or uncovered) site, we always cover our panels with insulated tarps following placement. Depending on the forecast, this may include a second layer of tarps, or placing heating lines (piping from a mobile boiler) between the layers of tarps. An issue for us is the temperature of the curing concrete. If the concrete mix we have placed contains winter admixtures (which will increase heat) along with two layers of insulated tarps sandwiching a heating element, we may end up cooking our panels. This process will generate excessive heat and cause unsightly shrinkage cracking. With so many variables coming into play – casting surface temperature, concrete placement temperature, ambient air temperature during and after placement, humidity, admixtures, and the temperature of the rebar – the contractor must develop a suitable placement plan to navigate these challenges. 

Adding to these concerns are the largely unknown stories behind concrete thermal shock. Thermal shock occurs when concrete is rapidly introduced to an opposing temperature. When we are curing our panels and they are warm and toasty under these insulated tarps, a sudden stripping of this insulation layer instantly exposes the panels to the ambient air, which could introduce a delta of 105 degrees Fahrenheit (40 degrees Celsius). Nobody enjoys an ice bath, and neither does our curing concrete.

Additionally, if we leave the curing panels exposed for too long, the film of water that is usually trapped between the concrete panel and the casting bed (the bond breaker waxy film), could freeze, which could create a mild bond, depending on the surface conditions. 

Here’s our latest experience! 

At our recent project, a six-story Hyatt Hotel in Ottawa, we contacted Giatec Scientific Inc. for assistance with monitoring our concrete temperatures. Giatec is a concrete sensor developer that is changing how we monitor concrete elements. These tiny “SmartRock™” sensors are tie-wrapped to the rebar prior to concrete placement with a short lead that acts as a thermocouple. The whole unit is fully embedded in the concrete, so there is no need to take care of wires or data loggers. Anyone with the SmartRock mobile app can scan the monitored area with their smartphone to obtain the real-time internal concrete temperatures. Alternatively, a SmartHub can be placed on the job site for remote monitoring of concrete data. These readings allowed us to monitor the core temperature of the concrete as we finished the panels, gave automated alerts and notifications, and ensured we did not allow the concrete to freeze.  

Another benefit of using a monitor is that the temperature of the concrete can tell us the real-time compressive strength. This is achieved through testing cylinders and flexural beams to generate a maturity graph based on the ASTM C1074 standard specification. This maturity graph is often available from the ready-mix supplier. This calibration data can be easily imported into the SmartRock mobile app. The user simply scans the area and obtains the concrete temperature, and the maturity graph correlates the compressive and flexural values. 

Like most, we needed proof that these digital SmartRock™ sensors work with some degree of accuracy. We hired two independent CSA-certified laboratories to undertake representative concrete batch samples (both cylinders and flexural beams). We compared the measured Giatec sensor results against the concrete cylinder and beam breaks. The results were as hoped! They were within a marginal percentage of each other.  

Here is the calibration curve developed by the independent testing lab. It correlates concrete maturity (the temperature-time factor) to its flexural strength for the mix that was used in our project. 

To measure the flexural and compressive strength of concrete on the job site, we embedded wireless SmartRock sensors in the panels. We also placed SmartRock sensors in the field-cured cylinders to verify the accuracy of the concrete maturity method. These sensors can easily be tagged in the mobile app, and the maturity calibration curve can be assigned to them. The sensor then measures concrete temperature over time (hence concrete maturity) and calculates concrete strength in real-time. Here is an example of what our project manager and site superintendent saw on the mobile app in the field. This data helped us make informed decisions on lifting the panels and avoid delays that are inherent to testing concrete beams and cylinders in the lab. 

What does this mean?

The days of field concrete cylinders and flexural beams are numbered.

We all know field cylinders and beams are not loved and cared for in the field. They are kicked around, left exposed to the elements, and generally have no mass to protect against the extreme temperatures. Without mass, the cold weather affects these slender concrete samples like our fingers without gloves. The mass concrete of panels is generating great heat, while the plastic cylinder is cold and alone within a window opening. Using the SmartRock sensors, we confirmed that the internal temperature of cylinders and beams is lower than that of the panel, even when both are covered under the same insulation tarp (at day one, for example, the beam was at 38.8 degrees Fahrenheit while the panel was 67.2 degrees Fahrenheit). This obviously results in lower strength development in the field-cured cylinder and beam compared to the actual strength of the structural element (e.g. flexural strength at day one: 189 psi in the beam but 372 psi in the panel). Do these neglected field concrete samples reflect the true strength of our mass winter-cast concrete panels? We say, “no”; and there is now an easier, more precise, cheaper solution available to us all.

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TILT-UP TODAY, a publication of the Tilt-Up Concrete Association, is THE source for Tilt-Up industry news, market intelligence, business strategies, technical solutions, product information, and other resources for professionals in the Tilt-Up industry. A subscription to TILT-UP TODAY is included in a TCA membership. Subscriptions for potential TCA members are also available. If you would like to receive a complimentary subscription to the publication, please contact the TCA.