1

Tong, Stanley

From: Robert Lee <Robert.Lee@txdot.gov>

Sent: Wednesday, February 18, 2015 11:42 AM

To: Tong, Stanley

Subject: Draft WMA Document

Attachments: Warm Mix Asphalt Guidance_GC.doc

Please read and let me know if you have any comments or questions? 

Robert E. Lee, P.E. 

Flexible Pavements Branch 

TxDOT – Construction Division 

(512) 506-5836 (office) 

(512) 771-2129 (cell) 

Drive Smart in Winter Weather 
Guidance & Background on the Use of Warm Mix Asphalt (WMA) 

Warm Mix Asphalt (WMA) has been used successfully in Texas for approximately 5 

years. To date, over 2 million tons of WMA have been used on TxDOT paving projects. 

It has been used in at least 19 districts and over 100 TxDOT projects. WMA provides 

numerous engineering and environmental benefits as listed below. TxDOT administration 

is committed in supporting further implementation of WMA in Texas in order to improve 

pavement performance and be better stewards of our environment. On a national level, 

the FHWA also supports full implementation of WMA through its “Every Day Counts” 

initiative. This document is intended to provide guidance to districts on the use of WMA 

and to encourage them to use a more proactive approach in WMA implementation on 

TxDOT projects where the benefits can be justified. 

Benefits of WMA 

ÿ More Durable Pavement 

o Less oxidized asphalt + less asphalt absorption = better fatigue life 

o Less thermal segregation 

ÿ Better In-Place Density and/or More Effective Compaction 

o Better density leads to better fatigue life 

o Lower permeability 

o Better bonus for contractor 

ÿ Improved Ride Quality 

o Less swelling of rubberized crack seal 

o Less roughness from reduced thermal segregation 

ÿ Wider Paving Window (longer paving season) 

o Paving into winter months 

o Night paving 

o Can accommodate longer haul distances 

ÿ Reduced Emissions, Smoke & Odor 

ÿ Direct Energy Savings ~ $0.25 - $1.00 per mix ton fuel savings

ÿ Better Working Conditions for Paving Crew & Others Around the Paver

ÿ Facilitates the use of RAP & RAS

o Combines a stiffer relatively over-oxidized product (RAP & RAS) with 

WMA which produces a softer and relatively under-oxidized product. 

The following wording pertaining to the use of WMA is contained in the current SS3268, 

Dense-Graded HMA (QCQA), specification and will be contained in all future HMA 

specifications. 

Warm Mix Asphalt (WMA) is defined as HMA that is produced within a target 

temperature discharge range of 215oF and 275oF using Department-approved WMA 

additives or processes. The Department’s Material Producer List of WMA additives and 

processes is located at http://www.txdot.gov/business/resources/producer-list.html.. 
WMA is allowed for use on all projects and is required when shown on plans. The 

maximum placement or target discharge temperature for WMA may be set at a value 

less than 275oF when shown on the plans. 

Department-approved WMA additives or processes may be used to facilitate mixing and 

compaction of HMA produced at target discharge temperatures greater than 275oF; 

however, such mixtures will not be defined as WMA.

As stated in the specification, WMA is allowed on any HMA project. However, there are 

a number of situations where it is advised that the Department require the use of WMA 

and other situations where it may not be beneficial to require WMA. Both of these 

situations are listed below in tabular form.

General Guidelines for Requiring WMA 

ÿ Any time there is not a significant negative financial impact. Example: WMA 

should be specified in most large metropolitan areas because there is enough 

volume of HMA and competition to justify the initial start up costs, etc. 

ÿ On large HMA projects (greater than 10,000 tons). Most of the start up costs can 

be recouped by fuel savings and more effective compaction. 

ÿ In areas that are environmentally sensitive. Example: urban and other similar 

areas where the air quality is of significant concern. 

ÿ On overlay projects where there is a significant amount of crack seal in the 

existing pavement (WMA prevents swelling of the crack seal material). 

General Guidelines When it May Not be Cost Effective to Require WMA 

ÿ In rural areas, if there is not enough HMA production or competition to justify 

start up costs, etc. 

ÿ On small projects (less than 10,000 tons) where the Contractor cannot recoup 

their initial start up costs. For example: if a Contractor paid 80,000 to add a 

foaming unit to an HMA plan and the project only consisted of 10,000 tons of 

mix, then a contractor would have to add $8 per ton to the mix cost to pay for the 

foaming unit. If the Contractor chose to use a chemical system rather than foam, 

the cost would likely be much less than $8 per ton; however, the benefits still 

might not justify the cost. 

The specification wording listed above allows the use of WMA at the Contractor’s 

discretion and allows TxDOT the option to require WMA if desired. The vast majority of 

all WMA used in Texas thus far has been used at the Contractor’s discretion. Many 

Contractors choose to us WMA primarily because it reduces fuel cost, improves mixing 

and compaction characteristics and allows for a longer paving season. It is worth noting 

that the “blue smoke” some times associated with HMA typically occurs at production 

temperatures above 290°F; therefore a maximum production temperatures of 275°F is 

specified for WMA to prevent visible emissions. 
It should also be noted that on most of the projects already placed, a “foaming” system 

was used. Foaming systems are preferred by many Contractors because after the initial 

investment of approximately $80,000, they actually save money because fuel costs are 

reduced and the additive (which is water) is essentially free. Depending on the cost of 

burner fuel etc, Contractor can typically save between 25 cents to $1 per ton when using 

a foaming system. Generally speaking, foaming systems work well at higher production 

temperatures (closer to 275°F); however, their effectiveness diminishes significantly at 

lower production temperatures (250°F or less). 

The other type of WMA technology commonly used is “additives” of which there are a 

number of varieties. Chemical additives are the most common. The chemical additives 

typically have a much lower start up cost than foaming systems; however, there is a 

reoccurring material cost associated with their use. While most chemical additives 

typically add $1 to $3 per ton of mix, much of this cost can be recovered by fuel savings 

and improved mixing and compaction. Chemical systems are preferred by some over 

foaming systems because they tend to be more versatile and can be used over a broader 

production temperature range. Unlike foaming systems, chemical systems are often used 

at production temperatures below 250°F. 

There are two primary reasons why TxDOT has required the use of WMA on the 

remaining projects. The first reason is reducing emissions in EPA non-attainment or 

border line non-attainment areas. The second reason WMA has been required by TxDOT 

engineers for use on overlay projects is where the underlying layer has significant 

amounts of rubberized crack seal. Using WMA to pave at temperatures below 250°F is an 

effective way of reducing the swelling of the rubberized crack seal, therefore improving 

the ride quality of the pavement. This approach requires a plan note that restricts mixture 

production to no greater than 250°F. These plan notes essentially restrict the use of WMA 

to the chemical additive systems since the foaming systems are not as effective at lower 

production temperatures. 

WMA additives or processes are often used to mitigate mixing and compaction issues 

with mixes that contain reclaimed asphalt pavement (RAP) or shingles (RAS). It is often 

necessary for Contractors to bump the top of the WMA production range (275°F) when 

using recycled materials. WMA production with RAP is not as problematic as WMA 

production with RAS. There tends to be a “catch 22” situation when using RAS and 

WMA. The RAS reduces the cost of the mix; however, the RAS does not melt in very 

easily at temperatures much below 275°F particularly if the RAS has a coarser gradation. 

When given the option to use RAS or WMA, Contractors will most often prefer the RAS, 

since there is a more direct cost savings. When WMA is not required, Contractors may 

prefer to use RAS with a WMA additive and produce the mixture at temperatures above 

275°F. For example: a chemical additive or foaming system could be used to produce a 

mixture containing RAS at 290°F. At this discharge temperature, the mix is hot enough to 

melt the RAS yet not so hot that visible emissions are produced. Such mixtures would 

technically not be considered WMA even though a WMA technology or additive is used.