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Concrete… maintaining the Quality

Concrete… maintaining the Quality. Achieving durable exterior flatwork in residential construction. Objectives. Review the exterior flatwork problem we face Discuss the issues with achieving durable exterior flatwork Technical summary What can the homebuilding industry do?.

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Concrete… maintaining the Quality

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  1. Concrete…maintaining theQuality Achieving durable exterior flatwork in residential construction

  2. Objectives • Review the exterior flatwork problem we face • Discuss the issues with achieving durable exterior flatwork • Technical summary • What can the homebuilding industry do?

  3. Residential Exterior Flatwork • Concrete work subcontracted to a finishing company • Exterior concrete looks great when home is turned over • Homebuilder hears from unhappy home owner after 1st or 2nd winter • What is the problem? What caused it? Who is responsible?

  4. What it looks like…

  5. What causes it… • Inadequate or no curing and/or • Inappropriate finishing and/or • Inappropriate concrete and/or • Incorrect or no sealing & maintenance In combination with exposure to freezing temperatures & water (& de-icing chemicals)

  6. What do we do? • Must treat achieving durable exterior flatwork as a chain of important events • Each link must be executed correctly to prevent deterioration • Industry partners must work together • Supplier’s cannot make a bullet proof concrete

  7. Achieving Durable Exterior Flatwork…Technical Review • Concrete • Placing • Finishing • Curing • Sealing & Maintenance

  8. Concrete • Alberta Building Code & Canadian Standards Association require: • C-2 Exposure (de-icing chemicals & freeze/thaw) • 32MPa (or 30MPa where indigenous aggregates do not achieve 32MPa) • 0.45 w/cm • 5-8% fresh air (3% hardened air with spacing factor not exceeding 0.23mm) • ARMCA Recommends DURA-MIX • above requirements and; • 300kg/m3 of cement minimum

  9. Placing • Subgrade should be dampened and not frozen • For more consistent set and improved workability • CSA A23.1 requires concrete to be placed within 120 minutes from batching • Prolonged mixing results in loss of air, slump & compressive strength • Water should not be added to increase slump above 100mm • Additional water significantly reduces freeze thaw resistance • Slump over 100mm should be achieved with a high range water reducer (superplasticizer)

  10. Finishing • Minimize handling and do not over vibrate • Allow bleed water to evaporate before finishing • Do not use steel trowels (Fresno) or power trowels on air entrained concrete • CSA A23.1 ‘Steel trowel finish should not be applied to air-entrained concrete’ • ‘Blistering or scaling might occur if trowel finish is applied’ • Use a magnesium float and concrete broom

  11. Time Temperature Moisture Curing • Essential for surface durability! Must balance 3 critical elements

  12. Methods of curing • There are two ways to cure concrete: 1) add water to the surface to replace the water that is evaporating 2) seal the concrete to prevent the water from evaporating • Note: adding water to the surface is NOT adding water that will be worked into the concrete mix--that would weaken it. Wet Curing Blankets Liquid Curing Membrane Ponding Method

  13. CSA Curing –Table 20 Level 1 Name Basic Description 3 d at > 10 °C or for a time necessary to attain 40% of specified strength 2 Additional 7 d at > 10 °C and for a time necessary to attain 70% of specified strength. When using silica fume concrete additional curing procedures shall be used See Clause 1.3.13. 3 Extended A wet-curing period of 7 d. The curing types allowed are ponding, continuous sprinkling, absorptive mat or fabric kept continuously wet. • Note (2) recommends concrete be allowed to air dry for at least one month after curing before exposure to de-icing chemicals • C-2 Exposure requires Level 2 curing 15 10/9/2014

  14. Hot Weather/Severe Drying • When surface evaporation is more than 0.50kg/(m2/h) the concrete must be protected • Hot temperatures &/or windy conditions @ low relative humidity • Evaporation retardant (Confilm, Profilm etc) is recommended • Apply as soon as possible • Do not use as a finishing aid

  15. High Evaporation Days • ARMCA website has a tool to calculate evaporation rate • Input values: concrete temperature, air temperature, relative humidity and wind velocity • July & September 2011, # of days where protection was required • Calgary: July – 29 : Sept – 24 • Red Deer: July – 20 : Sept – 23 • Edmonton: July – 20 : Sept – 26

  16. Cold Weather • Concrete must not freeze before it has reached 3.5MPa! • Concrete needs a significant portion of design strength to withstanding freeze thaw cycles • Concrete must be allowed to dry out & mature before first winter • Use of ‘winter heat’ (warm concrete & accelerators) does not replace required curing

  17. Cold Weather Construction • Local municipalities require ‘cold weather construction practices’ for pavement placed after September 30th • CSA A23.1 ‘Level 2’ curing required • 7 days at a minimum of 10 °C or the time required to attain 70% of 28 day strength

  18. Impact of Temperature onConcrete 11 19 29 57 72 10/9/2014 20

  19. Long Term Strength Development 10/9/2014 21

  20. Sealing & Maintenance • Properly seal the concrete 28 days after concrete placement & before exposure to traffic • Penetrating sealers (silane or siloxane) are recommended and should be reapplied at least every 3yrs • Concrete placed after Sept 15 should be sealed the following spring • Concrete should be cleaned yearly • Snow and ice should be removed as it accumulates

  21. De-icing Chemicals • Avoid whenever possible • All increase the number of freeze thaw cycles & some attack the concrete matrix • Especially damaging in the first winter when concrete hasn’t reached its full strength • Sand is the recommended product to improve traction on ice • Most of the de-icing chemicals present on driveways come from tire transfer from city roads If all the links in the chain are executed correctly exterior flatwork can withstand exposure to standard de-icing chemicals

  22. Achieving Durable Exterior Flatwork…Homebuilder Influence • Insist on DURA-MIX & test concrete before placing • Insist on certified journeyman finishers • Insist finishers use appropriate curing methods for the environmental conditions • Provide home owners with the appropriate information regarding sealing & maintenance

  23. The Challenge… This This After 1 year of strengthening the links in the chain do you have: or 10/9/2014 25

  24. Questions?

  25. ARMCA Residential Committee • Committee reinstated in 2011 • Looking for more representation • Home Builders • Finishers • If interested contact Ed Kalis @ ARMCA

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