Concrete: What is it, What do you do with it, and How do you fix it? Jim Grove Federal Highway Administration/Global Consulting Inc. 2012 Alaska Concrete Summit December 5, 2012 Outline How does the hydration process work?
How does it affect what we do in the field? What are some highway uses for concrete? New construction Overlays What do we do with it later? How does the hydration process work?
Whats going on NOT going to talk CHEMISTRY! How does it relate to what I do How does it affect what I do INTEGRATED MATERIALS AND CONSTRUCTION PRACTICES: A State-of-the-Practice Manual IMCP MANUAL http://www.cptechcenter.org/publications/imcp/index.cfm
Cement Chemistry Shorthand Name Chemical Shorthand (Cement Chemistry) Conventional Notation Calcium Silica C
S CaO SiO2 Alumina A Al2O3 Iron Oxide F
Fe2O3 Sulfur S SO3 Five Stages of Hydration Whats in the Bag?
Ground clinker Gypsum (sulfate) Gypsum < < Clinker Cement Particles Jims Axiom Cement chemistry is as
simple AS S Four Main Compounds in Cement Hydration Belite Aluminate Family
B A C Alite C3A s C-3PO Gypsum (Sulfate) Silicate
Family Only 6 Basic Thing You Need to Know 1. There are 2 Families of Reactions Aluminates Silicates 2. There are 4 Compounds that Count
Alites Belites C3A Sulfates AS S AS
A,B,Cs Aluminates Silicates Aluminates Aluminates C3A and sulfate dissolves into solution (silicates begin to dissolve also) Within the solution, C-A-S-H forms and coats the
cement grain / C3A Coating slows the reaction Amount of sulfate in the solution is key If this goes nuts, you are in deep doodoo! (Peter Taylor) Aluminates Why should I care? Affects the Dormancy Stage!
Aluminates Why should I care? Where false set and flash set happens! False set Too much gypsum Can be mixed through? YES NO FlashHeat
setgiven off? Too little gypsum Can be mixed through? Heat given off? NO-Permanent YES Aluminate Family First reaction You wont see it! All over before the truck gets to the paver Contributes
to the coating of the C3A Affects the dormancy period Two critical aspects of the gypsum Amount of sulfate in the solution Form of the sulfates Silicates Silicates
Why should I care? Strength happens here! Determines when you can saw Cracking happens here How soon you can drive on it Permeability happens here! Silicates Strength comes from
Alites (C3S) A Early reaction Belites (C2S) B Later reaction They create: Calcium Silicate Hydrate
C-S-H Causes Stuff to Harden through Contact Stringy Hairs
What Can Atomic Fireballs Tell Us About Cement Hydration? Reaction happens when water is present The reaction gives off heat- exothermic The reaction happens on the surface
+ CH Paste Calcium Carbonate Silicate Reactions Silicates start dissolving when the water hits the clinker-but slowly Contributes calcium ions to the solution When the solution becomes super-saturated, C-S-H forms
Super-Saturated: Calcium Ions Wow!! None Some Silicate Family Alites- early reaction Belites- later/long term reaction Start when water hits them-but slowly Continues during the dormant stage
C-S-H forms when solution becomes super saturated with calcium ionsbeginning of the hardening stage Give concrete its strength Contributes to lower permeability Goal: Long Life Pavements Old US 20 Woodbury Co - 1921 Photo courtesy Iowa DOT Library Old US 20 Woodbury Co - 2000 How do we get back to the future?
Supplementary Cementitious Materials (SCMs)!! How about SCMS? Most concrete has more than cement What is the effect of SCMS? Fly Ash Slag (GGBFS) Natural pozzolans How SCMs Work Calcium Silicate
Hydrate Cement Calcium Silicates + CSH Water Calcium Hydroxide
+ Secondary Reaction Fly Ash / Slag Supplemental Cementitious Material + Water +
CH CH Begins after the cement reaction CSH Composition SiO2 Silica fume
Pozzolanic F Fly Ash Metakaolin C Fly Ash Slag Cementitious Cement CaO Glasser, et. al., 1987
Al2O3 Less Horsepower Secondary Reaction What are some highway uses for concrete? New
construction Full depth new pavement/runways Predominant use in the Upper Midwest Long life pavement First cost competitive Often win in alternate bidding Overlays
Bonded overlay (formerly know as Whitetopping) Iowa Climate Very seasonal Des Moines, Iowa July 7, 2007 Keokuk, Iowa High 100 F Very changeable
January 1996 High 13th 31st Low 60 F -21 F What Is an Overlay? Rehabilitation
Placing a layer of concrete over distressed existing pavement Why Consider Concrete Overlays? Consistently provide cost-effective solutions Utilizes existing pavement structure
Eliminate removal operation Shortens construction time Few pre-overlay repairs are necessary Placed using normal concrete pavement construction practices Can be opened to traffic quickly using standard mixes
Expected Service Life 2 to 6 in. thick > 6 in. thick => => 15 to 25 years >20 years Overlay service life is dependent upon : Sound structural design
Good construction practices Iowa 21 Whitetopping Research 2 thick with fibers 18 years old (1994) Still performing well today Concrete Overlay Guide second edition
Contents 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Overview of Overlay Families Overlay types and uses Evaluations & Selections Six Overlay Summaries (11x17 sheets) Design Section Miscellaneous Design Details Overlay Materials Section Work Zones under Traffic Key Points for Overlay Construction Accelerated Construction Specification Considerations Repairs of Overlays Second Edition September 2008
http://www.cptechcenter.org/publications/overlays/index.htm System of Concrete Overlays Concrete Overlays Bonded Overlay System Thinner Bond is integral to design Composite Asphalt Concrete
Over Concrete Pavements Unbonded Overlay System Thicker Old Pavement is base Over Concrete
Pavements Over Asphalt Pavements Over Composite Pavements Over Asphalt Pavements Over Composite
Pavements Evaluations of Existing Pavements for Overlays When combined with an overlay, can the existing pavement help carry anticipated traffic as: An integrated part of the pavement (Bonded) Or serve as a base or subbase (Unbonbded) Determine:
Required repairs where needed Establish the concrete overlay design thickness What's the Process? Evaluate the pavement in place Choose the type of overlay
If needed repair damage Prepare interface Pave Cut joints
Cure Feasibility HMA pavements with some structural integrity Limited structural (fatigue) cracking No stripping/raveling in HMA layers HMA thickness after milling > 3 in to 4 in. Rutting
in HMA layers ok Non-load associated cracking ok Bonded Concrete Overlay of Asphalt Pavements Spots of distress that arent visible can be determined through evaluation Localized areas of weakness can be strengthen through patching. Milling can remove a number of asphalt surface distresses.
Unbonded Concrete Resurfacing of Asphalt or Composite Pavements Use to restore structural capacity of the existing pavement Used to increase pavement life equivalent to full-depth pavement. Eliminates rutting and shoving problems and results in improved surface friction, noise, and rideability
Normally 4 - 11 thickness Surface Cleaning Power Sweeping Air Blasting Water Blasting Be careful of milling too much! Construction traffic needs adequate support
Longer Lasting Alternative Concrete overlays resist rutting Concrete Placement & Finishing Same as conventional concrete paving Slipform Fixed form Concrete
placed directly on AC or on milled or leveled AC surface Conventional curing and sawing New Technology Stringless Paving for Overlays US 18 Single Lane Paving With Through Traffic Pilot Car
Typically 10% loss of traffic after pilot car starts US 18 Surface Profile Milling US 18 Milled Surface Paving Average Paving per day 6,800 to 7,500 ft. Depends
on distance from plant Cold Weather Concreting Cold Weather Concreting Concrete shall be air entrained Never place on frozen ground Allow for slower hydration
Minimize SCM usage Increase concrete temperature Leave forms in place longer Use maturity to monitor the concrete ACI 306.1-90 Cold Weather Concreting Curing
Maintain a minimum concrete temperature Protect from freezing Protect from drying Dont use a propane burner- carbonation Lengthen curing period ACI 306.1-90
What do we do with it later? Repair isolated areas What do we do with it later? Mill the surface 4 per pass Treatment Sequencing
Jim Grove Federal Highway Administration/Global Consulting Inc. Office of Infrastructure and Transportation Performance 2711 South Loop Drive Suite 4502 Ames, Iowa 50010 Phone: 515-294-5988 Mobile: 515-450-3399 [email protected]
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