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Protection of Exterior Concrete Flatwork in Winter (from ECS)


Posted on January 28th, 2015

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LESSONS LEARNED:

C O N S T R U C T I O N  M A T E R I A L S

OBSERVATIONS AND LESSONS FROM THE SCHOOL OF EXPERIENCE

News Flash – Winter weather is here! That means freezing rain, sleet, snow and ice can accumulate on flat concrete surfaces including sidewalks, driveways, and roads. The easy solution – DEICING SALT. As evidenced by the winter of 2014, deicing salt applications, possibly excessive at times, were the apparent answer to winter accident liability. While salt itself does not react with the concrete, the use of salt to lower the freezing temperature to promote thawing of frozen surfaces can affect the concrete durability.

 

The roller-coaster ride of temperature fluctuations can wreak havoc on exterior flat concrete surfaces due to numerous freeze-thaw cycles in the presence of wet conditions. When the snow or ice melts during the higher temperatures of the day, water can be absorbed into the concrete surface and settle into microscopic pores present in the concrete. When the temperature drops again at night, the trapped water freezes and expands. The expanding ice can exert pressure greater than the strength of the concrete. When the external pressure of the expanding ice exceeds the internal strength of the concrete, scaling and/or spalling occurs on the concrete surface. A damaged surface can eventually result in progressive failure of the  concrete.

 

While air-entrainment in the concrete mix is certainly an advantage to reduce the pop outs from ice damage, it may not be sufficient to resist scaling and spalling. Concrete strength has an effect on the durability of the surface, because a higher compressive strength also means a higher tensile strength that resists the forces being exerted by the ice crystals. Specifying a 4000 psi mix with a sufficient amount of air entrainment for the anticipated exposure would provide greater protection in cold areas than would a 3000 psi mix with no air entrainment. Proper construction techniques such as densifying the concrete surface along with proper curing can improve resistance to scaling. The application of a “breathable” sealer on the concrete surface can reduce the potential for water to enter into the pores of the concrete during  thawing.

 

Concrete flatwork exposed to freezing and thawing cycles is more vulnerable to damage during the first year of exposure.  Deicers are particularly damaging to concrete because the salt mixes with water and salt crystals can grow in the pore spaces within the concrete.  Most deicers come with a warning not to use-orn::oh-crete less than one year old.  Concrete gains strength over time, but the ultimate stength of the concrete  might not be enough to  resist the  pressure from the  ice or salt  crystal formation,  so deicers should be used sparingly.  With proper planning and execution, scale resistant concrete can be achieved for exterior flatwork. A good 2-page resource is CIP 2 – Scaling Concrete Surfaces published by NRMCA.   Sand can be used to help with  slip issues from snowy or icy sidewalks,  driveways,  and roads.

 

If you must use deicing products, sodium chloride based products are generally less damaging to concrete when used in smaller amounts than calcium or magnesium chloride products or calcium magnesium acetate.

Cold Weather Concrete


Posted on November 18th, 2014

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Cold WeatherConcrete: Tips to PourConcrete in Cold Weather

Cold weather concrete can be established as a period of more than three days where some specific conditions occur. The American Concrete Institute under ACI 306 defies that concrete will be exposed to cold weather when the following conditions exist:

  • The average daily air temperature is less than 5°C (40°F) and,
  • The air temperature is not greater than 10°C (50°F) for more than one-half of any 24 hour period.

Cold Weather Concrete Objectives

When concrete is being managed under cold weather, concrete must be protected from freezing shortly after being poured. Also concrete must be able to develop required strength for the safe removal of forms while reducing the circumstances where excessive must be applied to help concrete develop the required strength. Other important factors that must be considered are the proper curing conditions that prevent cracking and provide the intended serviceability of the structure.

Cold Weather Concrete Recommended Tips

Follow these recommended steps to assure that concrete in cold weather will obtain the required design strength.

  • Prior to pour; define the strategies that will be used including materials, forms, testing and other requirements.
  • Schedule and determine the cold weather protection measurement of the concrete mix.
  • Keep a well-define temperature record chart including concrete temperature and exterior temperature.
  • Never pour concrete on frozen ground, snow, or ice.
  • Determine if special considerations and strength requirements must be met; if so, protect concrete at specific temperatures.
  • If heated enclosures are going to be used when placing concrete in cold weather, be sure to know that they must be windproof and weatherproof.
  • If combustion heaters are used, vent outside to prevent carbonation.
  • Cold weather concrete should have the correct amount of air entrained voids that will resist freezing and thawing effects.
  • Concrete in cold weather is recommended to have low slump, and minimal water to cement ratio, to reduce bleeding and decreases setting time.
  • Use concrete curing blankets to prevent freezing and keep the concrete at optimal curing temperature.
  • Use insulation blankets or heated enclosures to maintain concrete temperatures above 50° degrees Fahrenheit for three to seven days.
  • Do not begin final finishing operations while bleed water is present.
  • Request a heated mix or order 100 lbs. of extra cement for each cubic yard of concrete. This extra cement helps develop early strength.
  • Fresh concrete frozen during the first 24 hours can lose 50% of its potential 28 day strength!
  • Maintain the concrete temperature above 40° degrees Fahrenheit for at least four more days after the use of the insulation blankets or heated enclosures.
  • Temperature of the concrete cannot drop faster than more than 40° Fahrenheit in 24 hours.
  • Do not seal freshly placed concrete.
  • It is recommended to place concrete as soon as possible, if the batch plant is too far from the concrete’s final destination, additional steps must be taken to reduce setting problems.
  • Hot water heaters might not be able to withstand hotter temperatures after the initial batches.

Cold Weather Concrete Temperatures

Temperatures for placement and protect concrete in cold weather are recommended under ACI 306. The objective of the ACI 306 is to keep concrete warm, over 5 degrees Celsius, for the first 48 hours, where concrete strength development is critical. When concrete is being placed below 5 degrees, but is not below freezing point, concrete will take longer to develop the required strength. Note that removing formwork when concrete is too cold or hasn’t reached desired strength, could damage concrete strength and surfaces and concrete might collapse.

Using frost blankets and insulated formwork could be necessary to protect concrete. Insulated forms or temporary covers could provide sufficient insulation in beams, columns and walls.

Flyash


Posted on November 18th, 2014

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To our Valued Customers:

Southern Concrete Materials wants to keep you updated as to supply shortages in raw materials used to produce ready mixed concrete. In this case, flyash is currently in very short or no supply at all in many of our market areas.

Flyash is a byproduct of coal fired power plants; these plants have been taken off line by Duke Power for scheduled maintenance or emergency shutdowns. Coal powered plants have been subject to new government regulations and in some cases the utility company is deciding to take these plants off line permanently or to allow conversion to natural gas which burns cleaner than coal. The future of using flyash in concrete could be in question, we urge you to contact your state and federal legislators in support of recycling flyash in concrete.

As SCM as no control of these material sources and we are forced to pass along the increased cost of supplying cement only mixes. This is an additional cost of $4.00 per cubic yard. Our suppliers tell us flyash should be more available after the first of the year and when available the cost savings of using flyash will be passed along to you.

Please take advantage of this notice to make necessary adjustments and notifications to your customers.

Again, we appreciate your business and remain committed to providing you a competitive advantage through superior service, quality and innovative products. Please do not hesitate to let us know how we can be of service to you.

Make a Muscle, Make a Difference


Posted on March 25th, 2013

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Mini Muscle Walk for the Muscular Dystrophy Association at North Asheville Christian School on Monday March 25th

Retaining walls? Are they in your specifications?


Posted on March 20th, 2013

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Why Retaining Wall Connection Strength Matters

Read the full article here.