05 May Contractor Focus

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Contractor FocusRob Burak, P. Eng.ICPI Director of Engineering

Base Construction forInterlocking Concrete Pavements

The previous two editions of ContractorFocusreviewed layout, excavation and

preparation of the subgrade for interlock-ing concrete pavements. This edition ofContractor Focuswill discuss the stepsinvolved in geotextile and base installation.Site preparation, excavation, subgradepreparation and base installation are themost important construction components of aproject. The performance of an interlockingconcrete pavement relies greatly on properexecution. Although unseen when the job iscompleted, these components represent the

foundation for success.Installing GeotextilesGeotextiles are an important part of manypavements. Geotextiles are recommendedover cohesive soils, i.e. clays or silts. They arealso a good choice over soils saturated fora large portion of the year. The fabric sepa-rates fines in soils from the base and preventsthem from migrating upward (carried bywater through capillary action) into the base.The fabric prevents slow movement fines fromweakening the support from the base.

Geotextiles do not typically increase theload bearing capacity of a pavement. Rather,they retain the intended load bearing capac-ity. They can be considered inexpensive insur-

ance for extending the life of a compacted

base. They do not enable decreasing theaggregate base thickness.

Geotextiles are manufactured as wovenor non-woven. Woven geotextiles are recom-mended where the base needs to be sepa-rated from the subgrade. Woven geotextileshave a higher bursting or tear strength thannon-woven, and tend to not stretch or elongateas much. ICPI recommends following manufac-turers recommendations for selecting wovengeotextile as a separation fabric.

When installing geotextiles it is important

to overlap and cover the sides of the excava-tion. Create an overlap in the direction of thegrade, much like shingles on a house. Beginby placing the fabric at the lowest elevationand working upslope. To prevent contamina-tion of the base by soil at the sides of theexcavation, cover the sides of the excavationwith fabric and fasten it to the ground withmetal staples while removing all wrinkles.Excess fabric and exposed staples can beremoved once the job is completed.

When a new section of fabric is placedit should overlap a minimum of 12 in. (300

mm). In weaker soils, it may be necessary toincrease overlap to a minimum of 24 in. (600mm). Geotextile manufacturers can provideguidance on overlap requirements.

Selecting Base AggregatesBase materials for interlocking concretepavements can be crushed stone, cement-treated base, asphalt-treated base, asphaltor concrete. The most common base is com-pacted crushed aggregate. Aggregate basematerials under flexible interlocking concretepavements should follow state DOT or pro-vincial specifications for base used underasphalt. Materials from state or provincialcertified quarries provide some additionalassurance of consistent aggregates andgradations that conform to agency specifica-

tions. The largest aggregates for bases aretypically 3/4in. (19 mm). For pedestrianapplications in residential settings, 1/2in. (13mm) top size aggregate is acceptable.

There are four important properties of anaggregate base material: strength, resistanceto frost heaving, density and drainage. Theyare all influenced by the amount of materialpassing the No. 200 (0.075 mm) sieve. Thekey is balancing these properties to optimizeperformance. Experience shows that 5%to 10% passing the No. 200 sieve givesoptimum performance of these properties.

The foreman should monitor the amount offines by inspecting each truck load of baseand by requesting a sieve analysis from the

Figure 1. Geotextile should cover the sides of the excavation and besecured into the ground with metal staples. Excess fabric can be trimmedafter paver installation. Note that the loader operator is driving into theexcavation over base material and not driving over exposed fabric. Thisprevents wrinkled or damaged fabric.

Figure 2. The depth to the top of a base lift is checked at the edgeof pavement. This provides a good visual check to guide the loaderoperator.

Continued on p. 2


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quarry which will indicate the amount pass-ing the No. 200 sieve.

The foreman should sample a handful ofthe aggregate and visually check the numberof crushed or fractured faces on the aggre-gate. The best base materials have all angu-lar sides from crushing with cubical shapes.

A visual check should confirm very fewflat and elongated particles and almost norounded particles.

Estimating and Job CostingOne ton of loose base generally yields about80 to 100 sf (8 to 10 m2) at 2 in. (50 mm)compacted thickness. The exact area yieldwill depend on the compacted density ofthe base and the weight of the aggregate.Another approach is to use the following for-mula for estimating the tons of base:

Square feet covered by the basexcompacted thickness of

175 base in inches x 1.1

Since base is purchased by the ton, it isbest to measure crew productivity for baseinstallation in tons per person per hour. Thistime includes transferring the base into theexcavation in lifts, raking each lift smooth,compacting it, and repeating this processuntil the base surface is smooth and readyto accept bedding sand. Some contractorsinclude geotextile installation with base pro-duction estimates.

Since each job site is different, productiv-ity varies. Tracking the labor hours for eachsize and type of job will provide a useful wayto view labor hours over time for base place-ment and other job functions in the project.This database will lead to more accurate esti-mates and aid in scheduling projects.

Best Practices for Base InstallationLike soils, the right amount of moisture in thebase material ensures reaching maximumdensity during compaction. Most crushedaggregate bases require about 5% to 6%optimum moisture content to reach 100%Proctor density. If the aggregate is dry,spread and moisten by spraying before com-pacting, letting the water penetrate the fulldepth of base lift thickness.

If the aggregate is not at its optimummoisture throughout the lift thickness, there

is a risk of compacting only the top crust ofthe base lift. As a result, the base lift will notcompact to maximum density and pavementsettlement may occur in the future. Do notadd excessive water as this can create pump-ing of the aggregate under compaction. Thisleads to lower than desired densities. Checkthe moisture content of the aggregate bygrabbing a handful and squeezing it tightlyfor a few seconds. After opening your hand,a good sign of the right amount of moistureis the aggregate sticking together with nowater draining.

When installing the first lift of aggregateit is important to not compromise the integrityof the geotextile with wrinkles. Place the first

Figure 3. When compacting soils or base, over-lap by at least one-third the width of the platecompactor. A best practice is to always startcompaction at the perimeter, work inwards, andwork uphill from the low end of the grade.

Base Construction for Interlocking Concrete PavementsContinued from p. 1

aggregate lift ahead of the loader wheels asshown in Figure 1, ensuring that the equip-ment does not drive directly over the geotex-tile. This also reduces the risk of tearing orpuncturing the fabric.

Compacting the Base

ICPI recommends that the base is installedtypically in 4 to 6 in. (100 to 150 mm) lifts.The thickness depends on the size of compactor. For example, a 5,000 lbf (22 kN) platecompactor will compact up to 4 in. (100 mmthick lifts. A 7,000 lbf (31 kN) machine cancompact up to 6 in. (150 mm). ICPI recom-mends using a minimum 7,000 lbf machineto compact aggregate base. Larger compac-tion equipment will adequately compact liftsthicker than 6 in.

Using the larger compactor for a 10 in.(250 mm) thick base may require only two

lifts of aggregate versus three when using asmaller compactor. Compacting two versusthree lifts saves labor hours and the savingscan be multiplied by total area of base com-pacted over the course of year. When choos-ing a compactor size for rent or purchase,consider the labor and price savings realizedfrom using a larger compactor. The amountof labor hours to excavate, prepare the sub-grade, and install the aggregate base will beat least 70% of the labor expended on jobsthroughout the year. Any reduction in timewithout compromising workmanship contrib-

utes to company profitability.Moving the aggregate into corners

and other difficult or inaccessible places

Figure 4. Choose the right equipment for the job. In difficult areas such as corners, unreach-able by larger plate compactors, use a jumping jack or hand tamper. Remember, theseneglected areas can lead to soil or base settling and callbacks.


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Interlocking Concrete Pavement Magazine | May 2005 3

will depend on the crew. Rake and gradethe base to string lines so that the base willcompact to a uniform thickness and to theplanned slope. This can be done with thebackside of a lute (asphalt rake). The rakehelps prevent aggregate segregation of larg-er particles from smaller ones which reducesthe compacted density. A garden rakeshouldnt be used to spread base aggregate.

Individual base lifts should be installed inuniform thicknesses to prevent waste and helpensure uniform density. ICPI recommends athickness tolerance of +3/4in. (19 mm) to1/2in. (14 mm) for the final base thickness.Too much aggregate wastes time and money.Too little aggregate can create a base withreduced support.

When compacting soils or base, compactin one direction, then compact the entire areawith passes perpendicular to previous ones.Overlap should be about one-third the widthof the plate compactor base (see Figure 3).

The total number of passes to accomplish fullcompaction to the required Proctor densitydepends upon the weight and travel speed ofthe compactor.

Some of indicators that indicate nearlycomplete compaction:

1. When the compactor starts crabbing(moving in a sideways motion)

2. The spike test where it takes at least athree pound (1.5 kg) hammer to drive an8 or 10 in. (200 mm to 250 mm) longspike into the base.

Some experienced contractors knowthat a certain number of passes of a plate orroller compactor yields adequate density inbase materials. However, they may be overcompacting in some jobs and wasting time.This approach to compacting base is usefulif the number of passes is correlated initiallyto base density measurements with a nucleardensity gauge.

The best way to ensure proper density ishaving a soils testing laboratory techniciancheck the base with a nuclear density gauge.They typically charge a minimum fee for a setamount of time for nuking compacted soilor base. For base, the Proctor density (in lbs/ft3or kg/m3) and optimum moisture contentcan be obtained from the quarry rather thanpaying for laboratory testing to determinethese. While density measurements arentneeded for bases in residential pedestrianapplications, they are a good idea for resi-dential driveways. The cost of testing is sub-stantially less than the cost of a crew callbackfor settlement repairs. Density measurements

should be specified and followed in commer-cial and municipal projects.

ICPI recommends minimum base thick-ness for different applications. For pedestrianareas, including patios and walkways, aminimum 4 in. (100 mm) thickness is recom-mended. For residential driveways over welldrained soils a minimum of 6 in. (150 mm)thickness is recommended. These thicknesses

will be increased in colder climates or overcontinually wet or weak soils. Most drivewaysin freezing climates are at least 10 in. (250mm) thick. Northern U.S. and most Canadianregions with cold winters will required drive-way bases well over 1 ft (0.3 m) thick.

Vehicular applications suchas streets and parking lots willtypically have greater basethicknesses or in stiffer bases.ICPI Tech Spec 4 StructuralDesign of Interlocking ConcretePavement for Roads and Parking

Lotsprovides base thicknessguidance for these applications.These should be designed byan engineer familiar with localsoils, local base materials andconstruction practices. The ICPIthickness design procedure isbased upon one developed bythe American Association of Stateand Highway TransportationOfficials (AASHTO) in 1993.Most state and provincial DOTshave adopted this pavement

design method.ICPI recommends a base sur-

face tolerance of 3/8in. (10 mm)over 10 ft (3 m) (see Figure 5).Drainage is achieved through aminimum longitudinal slope of 1%and a minimum cross slope of 2%.Set and check slopes with string-lines, a level or a transit.

Figure 6 shows a string linepulled to the finished elevationand slope. A simple way to checkthe final elevation of the compact-ed base is to run a lute under thestring with a blade as thick as thecompacted pavers and beddingsand (usually about 3 in. or 75mm). This enables a quick visualcheck on the base height andslope as the lute (asphalt rake) ismoved under the string.

ConclusionProper base construction is the

foundation for success. The interlockingpavement industry continues to experiencethe dynamic growth its seen over the past 10years. In order to continue the dynamic growthof the interlocking concrete pavement industryit is critical for contractors to follow industrybest practices. The unseen components of asegmental concrete pavement determine thesuccess of the installation. The next edition of

Contractor Focuswill review bedding sandinstallation and proper construction of edgerestraints two more areas that demand atten-tion to detail.

Figure 5. Checking base surface tolerance: the surfaceshould be 3/8in. (10 mm) over a 10 ft (3 m) straight edge.

Figure 6. Some lutes are as tall as the thickness of compactedpavers and bedding sand. Therefore, a lute can be a conve-nient gauge for comparing the elevation and slope of the fin-ished surface set by the string line, and the elevation and slopof the compacted base surface.

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05 May Contractor Focus