troubleshooting pervious concrete

Presented By:

National Pervious Concrete Pavement Association

Presented By:

National Pervious Concrete Pavement Association Mission

•Expand and improve the use of PC.

•Partner together those who utilize PC.

•Authoritative voice of pervious concrete.

•Promote and advocate common positions.

•Represent before industry, government, and the public.

•Educational Partner of :

W.O.C.

•ISBN Publisher

Design of Pervious Concrete Mixtures 2.0

•Certification & Accreditation

Design for Pervious Concrete Certification (DPC)

Upon completing this session, you should be able to:

Recognize quality PC mixtures;

Control the quality of a PC mix throughout a project;

Identify the cause of failure of PC pavements; and

Provide options for the repair of PC pavement.

Pervious Concrete Mixes

•Interconnected voids are termed “porosity”

•Design Void Content (DVC), typically around 20%

•Quality controlled by the design unit weight for a

corresponding DVC

The primary considerations when determining a mixture design are:

•Strength for loading

•Freeze-thaw resistance

•Porosity

Workability of pervious concrete is primarily controlled by:

• Aggregate angularity and gradation

•Mortar volume

•Sand or fiber content

•w/cm

•Admixtures

•Environmental factors

Aggregate

•Crushed (angular)

•Gravel (rounded)

•S.G. > 2.5

•Absorption < 2.5%

•Well graded

•Clean

•Porosity and permeability decrease

•Unit weight, compressive and

tensile strength increase

•Improves freeze-thaw response

Fibers

•Permeability increases

•Unit weight, compressive and tensile

strength increase

•Improves freeze-thaw response

Cement

•Type I & II Portland

•Fly Ash

•Slag

•Silica Fume

Admixtures used in a pervious concrete mixture:

•Air entraining

•Viscosity modifying

•Water reducing

•Internal curing

•Hydration stabilizing

Generally problems with PC are related to:

•Water content

•Admixture dosing

•Mixture proportions

Proper Yield is Key to Troubleshooting PC

•Cementitious Content

•Paste Content

•Void Content

•w/cm Margins

•Measured by Density Test

(actual vs. theoretical)

Paste Increases to 23%Voids Decrease to 17%

20% Voids

20% Paste

Mix Comparisons with Yield Issues (5%)

Design Actual

Cement 600 632

p/a 0.22 0.23

Unit Weight 128.88 130.42

Voids 20% +- >17%

Allowable w/cm 0.30 0.23

Mix Design

Total Weight in Mix Design= Yield

Unit Weight (UW)

QC in Field

Total Weight Batched / UW = Yield

CY Batched

Water Content

•Extremely low w/cm

•0.27 – 0.40 w/cm

•“Wetter is Better”

•>0.42 full cement hydration

http://youtu.be/lTas3hbsk58

Verifying and Adjusting Workability

•Inverted slump flow test

•Flow mimics discharge

Event Late Fall parking lot placement

Main Issue Sticky mixture required re-tempering with water several time per load to maintain workability.

Mixture Rounded-river gravel with sand and fibers, fly ash, 0.29 w/c, water reducer, hydration stabilizer, and air entrainment.

Observations: Placement occurred during the fall and temperature had dropped between the test pour and actual placement.

Hindsight: Pervious was batched with hot water.

Remediation: Do not use hot water; increase hydration stabilizer dosage; or dose water reducer at the site.

Event Spring parking lot placement.

Main Issue Change in mixture workability as placement progressed.

Mixture Rounded-river gravel with sand and fibers, fly ash, 0.29 w/c, water reducer, hydration stabilizer, and air entrainment.

Observations: At the start of placement workability was correct. As the placement progressed water was added when truck first arrived and halfway through each load.

Hindsight: Aggregate moisture was checked once at the beginning of the placement, but changed throughout the day.

Remediation: When workability began to change aggregate moisture should have been rechecked.

Event Early spring parking lot placement.

Main Issue Discharge rate was slow and the sticky mixture required re- tempering several times per load.

Mixture Rounded river gravel with water reducer, air entrainment, and hydration stabilizer.

Observations: Contractor had been told pervious was supposed to bestiff and accepted the concrete.

Hindsight: The first truck should have been verified before batching additional concrete. Contractor should have rejected the third truck.

Remediation: Do not use hot water; increase hydration stabilizer dosage; or add additional water reducer at the site.

Event Fall parking lot placement

Main Issue Mixture was very dry and had severe next day raveling.

Mixture River gravel mixture with slag and hydration stabilizer.

Observations: Mixture was dry on arrival and required significant water addition. During the second placement raveling occurred and it appeared the previous days pour had not reached sufficient strength.

Hindsight: Combination of slag and hydration stabilizer delayed set time.

Remediation: Check aggregate moisture as the concrete is batched. Reduce the amount of slag and or hydration stabilizer or postpone adjacent placement.

Sticky Mixture

Potential Cause(s) Actions to Consider

Changes in water content Inform driver that no wash water should remain in the truck before batching pervious concrete.

Mixture proportions Check that batched materials followed the prescribed proportions.

Admixture dosages Check ticket for correct admixtures and dosages. Increase water reducer dosage at plant.Add water reducer at the job site.

Sticky Mixture

Admixture effectiveness Use a water reducer with a longer working time Add additional water reducer at the job site.

Haul Time Increase w/c to higher end of allowable limit. Increase hydration stabilizer dosage at the plant. Add water reducer at the job site.

Ambient temperature Check that batch plant is not using warm/hot water.

Workability is Different from Test Placement

Changes in water content Check aggregate moisture contents and absorption.

Make sure batch plant water is adjusted for actual aggregate moisture content.

Make sure excessive free moisture is not lost form the aggregate stockpile and batching location.

Admixture dosages Check batch plant/delivery ticket for correct admixtures and dosages.

Change in admixtures Check that the correct admixtures were batched.

Haul Time Increase w/c to higher end of allowable limit. Increase hydration stabilizer dosage at the plant.

Changes in Workability Within a Load

Insufficient mixing Check that the truck has at least 100 revolutions.

Changes in water content Make mix is adjusted for aggregate moisture content.

Admixture dosage Check ticket for correct admixtures and dosages. Increase hydration stabilizer dosage at the plant.

Admixture effectiveness Use a water reducer with a longer working time. Use a moisture retaining admixture.

Changes in Workability Between Loads

Changes in water content Remove wash water in truck before batching.

Mixture proportions Check that aggregate gradation has not changed(check unit weight).

Ambient temp increase Increase w/c to higher end of allowable limit. Increase hydration stabilizer dosage at the plant.

Ambient temp decrease Check that batch plant is not using warm/hot water.

Adding Water Does Not Improve Workability

Admixture dosages Check ticket for correct admixtures and dosages. Increase hydration stabilizer dosage at plant.Add water reducer at the job site.

Mixture proportions Check that aggregate gradation has not changed(check unit weight).

Cement or Fiber Balling

Too little water Check aggregate moisture contents and absorption. Make sure batch plant water is adjusted for actual aggregate

moisture content.

Admixture effectiveness Add additional water reducer at the job site.

Surface Closes

Check that aggregate gradation has not changed(check unit weight).

Surface Ravels (also caused by poor construction and curing practices)

Too little water Check aggregate moisture contents and absorption Make sure batch plant water is adjusted for actual aggregate moisture

content.

Increased set time Reduce slag content.

www.npcpa.org

For More Information Contact:

Dale FisherExecutive Director, NPCPA

dfisher@npcpa.org

troubleshooting pervious concrete

mixture design

mixture workability

mixture river gravel

hot water

wash water

placementprogressed

design unit weight

mixture roundedriver

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