CARING FOR CONCRETE

COLD AND HOT WEATHER CONDITIONS

Code Precast Products, Inc.

Talking Points

Before pour actions Segregation in concrete After pour actions Cold weather caring Hot weather caring Curing process after pour

Before the Pour

Cleanliness. Not only the area around the pour, but the forms themselves. Buildup of excess oil, concrete, and dirt not only makes it harder to strip, but leaves the finished product looking bad.

Make sure form oil is applied. All inserts, anchors, knockouts, cups, or any other extra product are

properly secured and in the correct spot. Rebar is in the form. Vibrator is working. Make sure someone who knows how to finish is finishing the product.

Segregation in Concrete

Segregation in Concrete

Segregation occurs when aggregates separate themselves from the rest of the concrete.

Usually occurs when too much water is added, causing coarse aggregates to sink and separate.

After pouring, water and admixtures will leak from the form, greatly reducing strength in the finished product.

Water may also rise to the top, causing a weaker finished product (higher w/c ratio).

If segregation occurs, stop the truck and mix. If there is still segregation, have the truck go back and add aggregates to even out the mix.

If there is still segregation, contact myself or Ed about dumping the mix.

Segregation in Concrete

Segregation can greatly reduce the strength in our finished product. It also leads to honeycomb and sand lines, causing extra labor to fix

the finished product. Segregation can also be caused by over vibrating. If the mix is good

and flows well, a dead blow hammer will suffice in a vibrating method.

Should a vibrator be needed, make sure the person vibrating knows what they are doing.

After Pour Actions

After the pour, regardless of temperature, cover the product. Clean all tools and materials used for the pour. Should steel trough finish or stamping be needed, set a reminder. Keep product covered until time to strip, regardless of temperature. Sometimes, loosening forms while the product is still covered is

acceptable in certain temperatures.

Cold Weather Caring

According to ACI 306, “A period when for more than three successive days the average daily air temperature drops below 40 degrees and stays below 50 degrees for more than one half of any 24 hour period.”

During cold weather, it may be necessary to heat the water or aggregates. Using hot cement is not an effective method.

Colder temperatures will extend the time needed to reach initial set. To combat, more cement or accelerator can be used.

Supplementary heat, enclosures, or curing blankets may be used as well.

Cold Weather Caring

When temperatures drop to less than 40 degrees, freshly poured concrete is susceptible to becoming compromised.

All concrete must be protected from freezing until it has reached a minimum strength of 500 pounds per square inch (psi), which typically happens within the first 24 hours.

If concrete freezes while it is still fresh or before it has developed sufficient strength to resist the expansive forces associated with the freezing water, ice formation results in the disruption of the cement paste matrix causing an irreparable loss in strength.

Cold Weather Caring

Concrete should not be allowed to freeze during the first 24 hours after it has been poured.

Cement hydration is an exothermic reaction, meaning it releases energy by light or heat.

Concrete mixture produces some heat, so protecting that heat from escaping is key.

The use of polyethylene sheeting or insulating blankets are necessary to keeping that heat and retaining moisture.

Hot Weather Care

When the temperature of freshly mixed concrete reaches near 77 degrees, adverse site conditions can impact its quality.

Ambient temperatures above 90 degrees and the lack of a protected environment (a building) can relate to difficult in producing quality concrete.

Cooling aggregates and water added to the mixture is vital. Protecting the surfaces of which concrete is being poured. A higher water to cement ratio is necessary for flow ability and faster

set time.

Hot Weather Care

20 degrees in temperature will reduce the setting time of a mixture by as much as 50%.

As the concrete temperature increases the setting time is further reduced.

Increased rate of cement hydration at elevated temperatures are the main causes of problems associated with the hot weather.

Cracking, even with a good w/c ratio, can happen in extreme hot weather.

Drying Cracking occurs when water is added to a mix design without adjusting the amount of cement.

Thermal Cracking occurs when temperatures drop and increase day to day.

Hot Weather Care – Preventing Moisture Loss Moisture loss is the leading

cause of decreased strength is concrete.

High temperatures in concrete and outside, low humidity, solar radiation, and wind can all cause moisture loss.

The leaves less water in the w/c ratio that called for, therefore not allowing the proper amount to hydrate the cement.