Climate Overview:
If you wait for clear days and dry weather to pour exterior concrete in the Pacific Northwest, you may be idle for a long time. The region receives an average of 40 to 90 inches of rainfall annually. The two major weather influences in the Pacific Northwest are the Pacific Ocean and the Cascade Mountains. Moist air from the northern Pacific comes inland and drops significant rainfall on this region as it contacts the coastal mountains. Most rain occurs along the coast. Snow is rare along the coastal areas, but can quickly accumulate as you gain elevation traveling inland. The biggest issue for exterior concrete pours in this region is moisture from rain, standing water, fog and mist. The eastern areas are drier, and take on more of a high-desert climate.

Winter

Climate (December - March): Winters are typically cool and damp for most of this region, with cold and snow only becoming an issue in the higher elevations of the coastal mountains. Long periods of overcast conditions with mist and light rain can occur all winter. The major population centers that run along the coastal valleys rarely see snow, and the ground doesn't freeze until you reach the higher elevations. However, temperatures can hover around freezing for long periods of time, creating freeze-thaw conditions that negatively impact exterior concrete. Exterior concrete is poured year-round in this region.

Mix Designs: This region does not see a hard freeze, but temperatures can fluctuate around the freezing point, causing freeze-thaw cycles to occur many times in a day. This requires the use of cold-weather concrete mix designs, with standard mixes having a minimum compressive strength of 4000 psi. The primary considerations include chemical set accelerators to help the concrete set faster and air-entrainment admixtures to help with freeze-thaw conditions. Air entrainment should be mandatory in all exterior concrete placed in this region. Straight concrete mixes can also be used, as they tend to set faster than hybrid mixes. Using hot water to increase concrete temperature is a common winter practice.

Placement and Curing Practices: The high humidity in this region slows the hydration of concrete, which often minimizes the needs for external curing compounds. Because temperatures below 40 F for prolonged periods can affect concrete strength, cold-weather concreting practices are the norm for exterior pours in the winter, even though temperatures may never get cold enough for a hard freeze. Due to extensive freeze-thaw conditions, this region is prone to spalling and scaling concrete. Proper mix design, placement, and curing are crucial for long-term concrete durability. The primary considerations include blankets for heat retention, heating the mix water, and chemical set accelerators to speed the initial set of the concrete

Special Equipment: Since this region does not see the ground freeze or significant snowfall, there are no special equipment requirements when placing concrete in the winter, with the possible exception of curing blankets.

Spring

Climate (April – May): Spring temperatures are cool and conditions can be wet. The threat of freezing temperatures is usually gone by mid-April, but rain is always a consideration.

Mix Designs: Standard mix designs are the norm. Eliminating accelerators and adding fly ash to mixes is normal as the temperatures start to increase.

Placement and Curing Practices: Traditional placement and curing practices are the norm, as temperatures will dictate cold- or warm-weather practices.

Special Equipment: Plastic sheeting to protect against rain.

Summer

Climate (June – August): Mild temperatures are the norm for this entire region. Extremely hot temperatures are rare. Rain is unpredictable and can occur at any time of the day in the summer. Moderate to high humidity is normal, aiding placement and curing of exterior concrete.

Mix Designs: Standard mix designs are the norm due to the moderate temperatures and high humidity. When warmer than usual conditions exist, hydration stabilizers and water reducers are commonly used to extend the working life of the concrete without affecting performance. Air-entrainment admixtures should be mandatory in all exterior concrete placed in this region.

Placement and Curing Practices: With the high humidity of this region, wet curing of concrete is common practice. When hot, dry conditions exist, surface evaporative control agents should be available. The use of curing compounds and cure-and-seal chemicals is not as common due to the natural high humidity. Planning around rain is often necessary for exterior concrete pours. Due to exposure of the hardened concrete to freeze-thaw conditions in winter, it is prone to spalling and scaling. Proper water content and finishing are critical for long-term durability.

Special Equipment: Plastic sheeting to protect against rain.

Fall

Climate (September – November): Fall temperatures are cool, with the least amount of rainfall. Cold temperatures typically do not become a factor until November. Fall can provide some of the best weather for exterior concrete pours.

Mix Designs: Standard mix designs are the norm. Eliminating fly ash and retarders in mixes is normal as the temperatures start to decrease.

Placement and Curing Practices: Traditional placement and curing practices are the norm, as temperatures will dictate cold- or warm-weather practices.

Special Equipment: Plastic sheeting; curing blankets.

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