difference between “bm” asphalt mixes & the new kdot...

Difference Between “BM” Asphalt Mixes & the New KDOT

Commercial Grade Specification

A Bit of History….• Point of Supply vs. Point of Usage testing• The Marshall method of mix design• Development of the Commercial Grade spec

A Bit of History….• Before the advent 1990 KDOT Standard

Specifications, materials were tested at the point of supply

• Point of supply testing was changed to point of usage testing from the 70’s through the 80’s by the FHWA

• This change was driven by the results of the AASHO road test and the development of statistically based specifications

A Bit of History….

• By 1990 point of usage testing was commonplace in KDOT and nearly universal

• By 1990, producer process control plans become a specification requirement


• The Marshall method was used by KDOT to select the asphalt content of bituminous pavements

• The contractor selected the aggregates and the DME developed the job mix formula


• Special provision 66P-214 introduced Marshall testing for Type ACA mixes in the field

• 66P-214 was an attempt to provide process control for a bituminous mix in the field using Owner’s testing

• Type ACA was roughly equivalent to a BM-2


• This concept failed due to construction staff misunderstanding the test procedure and the lack of direct contractor responsibility for the mix

Plant Mix Bituminous Mixture –Commercial Grade (1980)

• Introduced with the 1980 Standard Specifications

• Must be composed of a commercial mixture in general use in the area with a satisfactory service record

• A mix design must be submitted for approval


• Generally, the top 2 inches of bituminous mixture shall be a surface mixture similar to a BM-2

• Other approved mixes can be used for the base course

• Production and laydown equipment conforms to Section 601 – Equipment in Standard Specifications


• Compaction accomplished with a minimum of 2 rollers using standard and recognized techniques (no formal density measurement)

• Measurement and Payment – tons of material delivered to the road, measured by commercial scale tickets

In the Meantime…… (1979-1985)

• KDOT implements its Pavement Management System for the entire network

• The system is based on a yearly condition survey and statistically-based models for predicting the effect of different maintenance actions on the pavement network

Plant Mix Bituminous Mixture –Commercial Grade (1990 changes)

• Mixture produced under this spec must meet BM-2 requirements unless another KDOT-type mix is approved by the Engineer

• A complete Marshall mix design either by commercial lab or KDOT must be submitted for approval

Plant Mix Bituminous Mixture –Commercial Grade (1990 changes)

• Contractor process control added • Lot size is 500 tons• Pay for each lot is determined by the deviation of

the cold feed gradations on the #4, #8, #30, and #200 sieves from the design cold feed gradation

• Pay table derived from contractor process data

State of the Practice – Pre-Superpave• Marshall mix design criteria followed to select

asphalt content• No minimum VMA requirement• No limits on VFA• No testing to determine environmental resistance

of mix• Natural sand content limited to 50%

State of the Practice – Pre-Superpave

• Clay fines controlled by PI limits• Maximum recycle content limited to 50%• Mixture control uses cold feed gradations• Road density control by daily field molded

Marshall density and calibrated nuclear measurement or roadway core density

State of the Practice – Pre-Superpave

• A management decision is made to use only Superpave techniques to develop bituminous mix designs

Hot Mix Asphalt (HMA)-Commercial Grade (Post 1998)

• Contractor-KDOT subcommittee set up to rewrite the Commercial Grade specification to use Superpave criteria

• Committee is made up of:Mike and Doug Shilling – Shilling ConstructionRodney Maag and Jeff Frantzen - KDOT

Hot Mix Asphalt (HMA)-Commercial Grade (Post 1997)

• Class A – SM-12.5A or SR-12.5A

Intended for permanent and temporary traffic areas with moderate to high volumes of traffic

Hot Mix Asphalt (HMA)-Commercial Grade

• Class B – SM/SR-9.5A/T, SM/SR-12.5A, SM/SR-19A

Intended for non-traffic areas or short-term trafficked areas with low traffic volumes


• The specification uses most of the best practices for hot mix asphalt


• If the mix contains more than 25% siliciousvirgin aggregates, the addition of anti-strip is required unless the TSR (Lottman) test permits a reduction in anti-strip

• Warm Mix Asphalt is allowed


• Minimum lift thickness is set at 3 times the nominal maximum aggregate size

• Maximum compacted lift thickness for a base course is specified as 4 inches

• Maximum compacted lift thickness for a surface course is specified as 2 inches


• Weather Limitations:

• Pavement smoothness measurement by 10 foot straightedge


• Pay per lot is determined by cold feed gradation and the pay factor tables

• Lot size can be either 500 tons or 750 tons for high production projects

• There can be air void price adjustments on QA/QC-controlled mixes

Conclusions

• The current specification contains most of the recommended “best practices” for HMA mixtures and is targeted to small projects

• Process control and pay factors are streamlined

• Engineering occurs on the front-end, simplifying construction

difference between “bm” asphalt mixes & the new kdot...

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