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 1st or 2nd 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/m3 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 evaporating2) 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 MethodLiquid Curing Membrane

CSA Curing –Table 20

A wet-curing period of 7 d. The curing types allowed are ponding, continuous sprinkling, absorptive mat or fabric kept continuously wet.

Extended3

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.

Additional2

Description3 d at > 10 °C or for a time necessary to attain 40% of specified strength

Name

Basic

Level

1

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Note (2) recommends concrete be allowed to air dry for at least one month after curing before exposure to de-icing chemicalsC-2 Exposure requires Level 2 curing

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 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

0 10 20 30 40 50 60 70 80

19

14

11

8

6

6

5

3

3

3

47

38

15

8

2

Hours

Time to Initial Set

Time to Final Set

Time to 500 psi (3.5MPa)

70 F (21 C)

60 F (16 C)

50 F (10 C)

40 F (4 C)

30 F (-1 C)

11

19

29

57

72

Impact of Temperature onImpact of Temperature on ConcreteConcrete

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Long Term Strength DevelopmentLong Term Strength Development

0 10 20 30 40 50 60

55

25

18

14

8

Days

Temperature of Concrete vs

Time to 75% Design Strength

70 F (21 C)

60 F (16 C)

50 F (10 C)

40 F (4 C)

30 F (-1 C)

04/11/23 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…

This Thisor

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After 1 year of strengthening the links in the chain do you have:

Questions?

ARMCA Residential Committee

Committee reinstated in 2011 Looking for more representation

Home Builders Finishers

If interested contact Ed Kalis @ ARMCA