CO_FTFW_TWPO_900(2)_Geotech_and_Pavements_Tech_Memo.pdf

Subject: Pavement & Materials Recommendations CO FTFW TWPO 900(2) Two Ponds Visitor Parking

From: Mike Voth Pavement & Materials Team Leader, CFLHD

To: John Knowles, Project Manager, CFLHD Melissa Jucha, Highway Designer, CFLHD

INTRODUCTION

This memorandum provides pavement and material recommendations for the Two Ponds Visitor Parking project located adjacent to 80th Avenue in Arvada, Colorado. The existing parking lot provides access to the trails of the Two Ponds National Wildlife Refuge. The purpose of the project will be to rehabilitate the pavement and improve handicap parking. The parking lot and entrance has an area of about 2200 square yards with over 20 parking stalls.

This memorandum also documents the methods used to develop the recommendations. Photos from the scoping site visit are attached.

FIELD INVESTIGATIONS AND DATA REVIEW

The parking lot was constructed in 2001 with 4 inches of asphalt pavement over 8 inches of crushed aggregate base. The expectation was that this pavement structural section would provide over 25 years of service life. While structurally the parking lot has performed well, there are high severity thermal cracks throughout the lot. The premature thermal cracking is likely a result of the following two issues: 1) inadequate asphalt binder grade used during construction, and 2) lack of pavement preservation treatments. No pavement preservation treatments have been placed during the past 18 years. An onsite visual distress survey was completed during the scoping site visit on December 17, 2018. Photos of typical distress are attached.

The subgrade is expected to have clays, but review of the construction records did not indicate that the soil subgrade was problematic from a construction standpoint. The following list includes the key parameters used to support the pavement and materials recommendations:

• Passenger cars are the predominant vehicles using the parking lot. There is the occasional school bus that also uses the lot, but traffic loading will be very low and our minimum ESAL amount of 50,000 was used to verify the rehabilitation recommendations.

Technical Memorandum

Date: June 10, 2019

• Clay subgrade material is prevalent in the area. An R-Value of 10 and resilient modulus of 5000 psi was estimated for the subgrade stiffness. Note that no borings were completed.

• The as-built records indicate 4 inches of asphalt concrete and 8 inches of crushed aggregate base was placed in 2001.

• The existing pavement is not exhibiting fatigue (structural) cracking, but the pavement does have high severity thermal and block cracking throughout the parking lot. The pavement has not received preservation treatments.

DESIGN RECOMMENDATIONS

The pavement rehabilitation and structural section recommendation for the parking lot are as follows:

Asphalt Pavement Rehabilitation Areas Remove existing 4 inches of asphalt pavement. (asphalt removal by milling or other approved methods) Recondition the existing crushed aggregate base (roadbed reconditioning) 4 inches ACP (minor asphalt, state mix, placed in two lifts, less than 5000 tons) Tack coat (tack coat placed in between asphalt lifts)

Asphalt Pavement Reconstruction Areas 4 inches ACP (minor asphalt, state mix, placed in two lifts, less than 5000 tons) 8 inches of aggregate base (minor aggregate, less than 5000 tons) Tack coat (tack coat placed in between asphalt lifts)

Concrete Pavement (PCCP) for Handicap Stalls 8 inches of PCCP (minor concrete pavement) 4 inches of aggregate base (minor aggregate, less than 5000 tons)

See attachment for design information.

The jointing plan for the concrete pavement is not designated on the plans, but the contractor will be required to submit a jointing plan to the CO for approval.

CONSTRUCTION CONSIDERATIONS & SPECIAL CONTRACT PROVISIONS

For construction procurement, a “task order” contract will be used. The project is small in quantities and cost, relative to typical CFLHD projects. The FP-14 will not be used, but all relevant sections will be included in the contract documents. No significant unique or special features are being incorporated. The pavement reconstruction should be relatively straightforward.

Due to the premature block and thermal cracking that occurred with the project built in 2001, an asphalt binder grade of PG 64-28 will be required in the contract.

Attachments

Photo 1: Parking lot exhibiting thermal and block cracking.

Photo 2: Typical thermal cracking in parking lot

Photo 3: General view of parking lot. High severity thermal cracking.

AASHTO FLEXIBLE PAVEMENT DESIGN

SN Determination

Design Inputs W18 = 50,000 ESALs Applications Over Design Period Typ. Range 50k to 10 million

R = 75 % Reliability Typ. Range 75 to 95%

So = 0.49 Standard Deviation AASHTO Standard

MR = 3,600 psi Subgrade Resilient Modulus Typ. Range 3000 to 15000 psi

Pi = 4.2 Initial Serviceability Typ. Range 4.2 to 4.4

Pt = 2.5 Terminal Serviceability Typ. Range 2.0 to 3.0

DESIGN SN = 2.55

AASHTO FLEXIBLE PAVEMENT DESIGN

Layer Thickness Determination Using Layered Analysis Approach

1 Enter applicable information in cells C15 through F22 2 For the first layer, use the E of the next lower layer as the MR in the Inputs Box below this table and enter the resulting layer SN.

3 The minimum layer thickness is calculated in column H. Enter the "practical" layer thickness in column I.

4 Repeat steps 2 and 3 for each layer, always using the E of the next lower layer as the MR in the Inputs Box spreadsheet below this table.

5 Check to ensure resulting SN meets or exceeds required SN 6 Note that this procedure is not valid if any layer beneath layer 1 has an E greater than 40,000 psi.

SN Using E of next Min. Layer Practical Layer Layer Drainage Elastic lower layer in Thickness, D, Thickness, D, Associated

Layer No. Description Coefficient, ai Coefficient, mi Modulus, psi inputs box below inches inches SN (topmost) Layer 1 AC Layer 0.40 1.00 400,000 1.05 2.63 4.00 1.60

Layer 2 Coarse Agg Base 0.12 1.00 30,000 2.55 7.92 8.00 0.96 Layer 3 Gran. Subbase 20,000 0.00 0.00 0.00 Layer 4 Select Borrow 15,000 0.00 0.00 0.00 Layer 5 0.00 0.00 0.00 Layer 6 0.00 0.00 0.00 Layer 7 0.00 0.00 0.00

(bottommost) Layer 8 0.00 0.00 0.00 Subgrade Subgrade N/A N/A 3,600 N/A N/A N/A N/A AC Layer 401 = 0.44 Total Pavement 2.56 Calculated SN

403 = 0.40 Thickness, inches, 10.54 12.00 2.55 SN to Match CAB 301 = 0.14 Design is sufficient Subbase/Minor agg 302 = 0.12

FDR 304 = 0.12

Select borrow 204 = 0.06 to 0.08 (Refer to Exhibit 11.2-C in Chapter 11 of PDDM for additional guidance on coefficients)

Inputs Box W18 = 50,000 ESALs Applications Over Design Period

R = 75 % Reliability

So = 0.49 Standard Deviation

MR = 3,600 psi Subgrade Resilient Modulus

Pi = 4.2 Initial Serviceability

Pt = 2.5 Terminal Serviceability

SN on top of layer = 2.55