Cold Weather Concrete Practices

Transcription

1 Cold Weather Concrete Practices The Information below is an outline of common practice for residential foundation placement and winter construction utilize Edmonton and surrounding Area. Conditions in other areas may not be the same and therefore caution and discretion should be used outside of this region. Step 1 Excavation 1. Rip frost Send an excavator to break up the frost that can go down to about 5 feet. 2. The next morning finish the excavation so that footings can be poured that day the earlier the better. This is done so that the builder is not pouring the footing on frozen ground. 3. Main focus gets to unfrozen and undisturbed soil. 4. Watch the forecast to see if the weather is going to turn. If the forecast looks like bad weather is coming the builder may cover the bottom of the excavation with traps and or blankets to keep the frost out. 5. If the bottom of the hole is frozen once excavated the builder will do one of the following. 6. Wait for the ground to thaw 7. Remove the frost by insulating the ground or provide heat to the ground. 8. Excavate below the frost i.e. 12 and place a deeper foundation 9. Or pile the home. Step 2 Concrete Walls Foundation Placement 1. Builders may use insulated concrete forms However this usually is a customer s choice which is governed typically by cost or energy efficiency. 2. Most builders will not pour wall concrete at below -10 degrees Celsius. However with concrete can be poured at colder temperatures provided that proper cold weather concrete practices are utilized to ensure that the concrete will not freeze. Page 1 of 5 1/7/2009

2 3. Leave forms in place as long as possible. 4. Concrete gains very little strength at low temperatures. Fresh concrete must be protected against the disruptive effects of freezing until the concrete attains a compressive strength of about 500 psi. However higher strengths are normally attained this is the minimum set in publications such as National research councils Canadian building digest, RMCAO, and BCRMCA 5. Use of air entrained concrete will also help to protect against freeze thaw cycles. 6. Pour your concrete with lower slumps. 7. During winter months October to May the concrete suppliers are preheating there Aggregates. 8. Hot water improves setting time. 9. The builders are also requesting admixtures to be added with the concrete to ensure shorter initial set up times. 10. Builders will then form the walls most adding two 10 m rebar to the top and bottom control crack. 11. Builders will not pour footings on to frozen ground. 12. There are no builders using a 15 MPA concrete most builder specify 20 MPA or Higher for there concrete work this does a dual purpose first assuming that because of the extra cure time that this gives a higher earlier strength to the concrete i.e. supply a 20 MPA concrete and if only 90 % strength is obtained due to on site conditions this would return a 18 MPA concrete which still exceeds the code requirements. And dependant on sulfate conditions may use concrete as high as 35 MPA. 13. Builder s dependant on the conditions at this point may protect the concrete by use of insulation placed over the wall - in several ways i.e. (Insulated traps, batts of insulation, straw, Foam panels etc.) 14. Builders may wish to use propane or kerosene heaters to keep this space warm. 15. However prolonged use of these will also chemically react with the concrete. 16. Once the initial set is attained the wall will remain dominant until favorable conditions exist. For the concrete to gain strength. 17. The initial set period is what is focused on. Page 2 of 5 1/7/2009

3 18. However if this is not achieved the wall won t have enough strength to backfill let alone construct a house on. Even at 3.5 mpa or 500 psi. 19. The concrete will be soft and crumbles. Or can be easily marred and damaged. Assuming that a builder was not aware of common practice and didn t know that concrete freezes they would not be able to continue with the construction of the home. If for some reason this does occur the wall would have to be removed by the builder and redone. Or engineer s services may be requested. Dependant on the severity. 20. The city inspectors typically would never see this as it is corrected prior to there foundation inspection. However if an inspector sees this they would obliviously voice there concerns. 21. Once the walls are up there is still a 7 to 10 day period where the foundation is exposed to the elements and dependant upon the weather conditions. The ground below the foundation may freeze and expand any where up to 4 inches in extreme cases. 22. Since natural gas and power are not readily available at this point of construction the frost is allowed to penetrate until permanent heat can be established. 23. Some builders May not be permitted with there insurance provider to have open flames. 24. Others may have a difficultly ensuring proper or continuous operation of heating equipment. 25. Hydronic insulating blankets at this point are not a feasible option because of the time they would be required on site to ensure that the ground below the foundation will not be subject to frost heave. 26. Again note: that if concrete does freeze during this process it is noticed as soon as the forms are removed. Concrete that has been frozen doesn t look cured concrete and the builder then would correct the situation. Concrete Cracks 1. The reason concrete will always crack. a. Thermal shock due to temperature change. b. Dimension change due to temperature change (Creep & Shrinkage). c. Movement of the foundation due to frost penetration. Page 3 of 5 1/7/2009

4 2. The Alberta New Home Warranty - Under stands that foundations crack and has workmanship and material standards to follow for these situations. 3. Foundation wall leak wall shall be repaired. a. Acceptable performance conditions Foundation walls shall not allow water penetration 4. Cast in place concrete foundation wall is cracked. a. Acceptable performance conditions Minor cracks resulting from normal shrinkage are acceptable. Cracks excess of 3 mm in width are considered excessive. 5. Other conditions for cracking typically with cracks in excess of ½ professional involvement of an engineer is requested along with any horizontal cracks. 6. Sometimes the use of hydronic heating systems can be used to thaw ground. After Foundation 1. Most of the Edmonton area is comprised of Clays with varying moisture contents and plasticity limits which means when these soils freeze the water will expand approximately 10% therefore if the frost is allowed to penetrate say 6 and the moisture content of the soil is 30% that soil will expand about 3 under the footings. 2. Foundation Garages usually are never intended to be heated and therefore frost penetration is usually deeper and will come out of the ground at a different rate then the home itself. Where as the grade beam system is designed to cushion frost ground movement it will not settle as the frost comes out of the ground. Therefore each is handled a little bit differently. 3. Using the garage Footing and wall system the advantage is that both the house and garage can progress in one pour therefore limiting the time for frost jacking. However a pivot point must be provided to allow for the differential movement other wise the garage will rip apart the house foundation. This system however also has a higher potential of movement once the home is occupied. Since a winter where the conditions are right and the garage is not heated it may move later. 4. Grade beam and pile system once place approximately two week after the foundation was will not change elevation when the frost come out but the home will there fore the use of a slip joint must be utilized other wise the grade beam will rip the foundation wall in half. 5. All the frost must be out of the soil under the home prior to the placement of the basement floor since the loading on the wall is greater than under the pads it does Page 4 of 5 1/7/2009

5 take longer for the frost to come out from the pads and if a floor is placed in a case where there is frost under the pads the floor may have extensive cracking to the slab while the perimeter of the basement hasn t moved at all. 6. If there is snow during the period where to hole is exposed the builder may utilize that snow for its insulating properties until permanent heat can be established. 7. During the defrosting period a builder will check there teleposts daily to ensure that the loads are transferring correctly. 8. If the house is at drywall stage the builder will not board the home until the frost is out however the may load the home with board. General Business practices 1. Allow for soil expansion and contraction in your design. 2. Ensure that the correct admixtures and strengths are for the conditions. 3. Ensure that Employees subcontractors and suppliers are competent to there field of expertise. 4. If something new or unusual is encountered seek out expertise that is relevant for the issue. 5. If someone asks what is the correct method for pouring concrete in cold weather. Response is that they should contact there contractor or builder to find out what there assurances and procedures are for cold weather concrete. Sidebar There have been some cases where Issues have arisen and the builders have conducted core samplings to see if there were bad mixes etc. In those examples the results come back higher typically than originally anticipated. Case in mind 30 MPA concrete pour at C returning with 36 MPA strength. Page 5 of 5 1/7/2009