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?
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 1 st or 2 nd winter What is the problem? What caused it? Who is responsible?
What it looks like
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)
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
Achieving Durable Exterior Flatwork Technical Review Concrete Placing Finishing Curing Sealing & Maintenance
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/m 3 of cement minimum
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)
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
Curing Essential for surface durability! Must balance 3 critical elements Time Temperature Moisture
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 Ponding Method Liquid Curing Membrane
CSA Curing Table 20 Level 1 2 3 Name Basic Additional Extended Description 3 d at > 10 C or for a time necessary to attain 40% of specified strength 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. 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 18/Jun/14 15
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
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
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
Cold Weather Construction Local municipalities require cold weather construction practices for pavement placed after September 30 th 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
Impact of Temperature on Concrete 70 F (21 C) 6 3 2 11 Time to Initial Set 60 F (16 C) 8 3 8 19 Time to Final Set 50 F (10 C) 11 3 15 29 Time to 500 psi (3.5MPa) 40 F (4 C) 14 5 38 57 30 F (-1 C) 19 6 47 72 0 10 20 30 40 50 60 70 80 Hours 18/Jun/14 20
Long Term Strength Development Temperature of Concrete vs Time to 75% Design Strength 70 F (21 C) 8 60 F (16 C) 14 50 F (10 C) 18 40 F (4 C) 25 30 F (-1 C) 55 0 10 20 30 40 50 60 Days 18/Jun/14 21
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
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
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
The Challenge After 1 year of strengthening the links in the chain do you have: This or This 18/Jun/14 25
Questions?
ARMCA Residential Committee Committee reinstated in 2011 Looking for more representation Home Builders Finishers If interested contact Ed Kalis @ ARMCA