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Publication Number:  FHWA-HRT-14-086    Date:  November 2014
Publication Number: FHWA-HRT-14-086
Date: November 2014

 

The Use of Phosphoric Acid to Stiffen Hot Mix Asphalt Binders

CHAPTER 1. INTRODUCTION

BACKGROUND

An unsolved mystery regarding the premature failure of I-80 in Nebraska led in part to this study. The transverse cracking problems that occurred there, as well as highway performance problems in other States attributed to the use of phosphoric acid but without forensic support, caused State agencies to question the use of phosphoric acid as an asphalt modifier despite its use for 30 years. A number of preconceived objections exist. These include the following:

The American Association of State Highway and Transportation Officials (AASHTO) carried out surveys in December 2005 and again in October 2008. States were asked, "Do you allow the use of acid-modified binders?"

The 2005 survey went out to the 50 States as well as the Canadian provinces. Of 31 respondents, 11 allowed phosphoric acid, 16 banned it, 1 restricted its use, and 3 did not specifically address the issue. Because the Superpave specification is supposed to be blind to additives, it is likely that phosphoric acid would have been allowed in the latter three States.

In the 2008 follow-up survey, five States specifically allowed phosphoric acid, eight States banned it, four placed restrictions on its use, and three had a specification for elastic recovery or phase angle, which would preclude the use of phosphoric acid as the sole modifier. Ten States did not address the issue. Twenty-three States did not respond to the survey.

The more recent 2009–2010 survey conducted by the Asphalt Institute indicated that the use of phosphoric acid is banned by 16 States. Thirty-two States do not specifically address the issue, which would imply that it is allowed, although 20 of these States have either an elastic recovery or phase angle specification, (which would preclude the use of phosphoric acid or merely force the inclusion of polymers), leaving 12 States that would allow phosphoric acid as the sole modifier. One State, Minnesota, has a requirement to carry out the binder tests out after the addition of 0.5-percent liquid amine antistrip additives. Because amines are alkaline, phosphoric acid would be expected to react with the amines and might preclude the use of phosphoric acid. Binder tests are usually carried out before the addition of amine antistrip additives.

The results of these three surveys are summarized in table 1.

Table 1 . History of State regulations on the use of phosphoric acid to modify asphalt.

Status

AASHTO 2005

AASHTO 2008

AI 2009/2010

Allowed

11

5

3

Banned

16

8

16

Not Addressed

3

10

12

Restricted

1

4

0

With Polymers

0

3

20

No Response

21

23

0

Undecided

1

0

0

Total Responses

53

53

51

AASHTO = American Association of Highway and Transportation Officials
AI = Asphalt Institute

SOME PHOSPHORIC ACID CHEMISTRY

Phosphoric acid exists in different strengths or concentrations, with physical forms ranging from a clear odorless colorless liquid or a transparent solid, depending on the concentration and temperature.

The most common form is orthophosphoric acid (H3PO4). It is commonly called phosphoric acid, and is one of the oldest known and most important phosphorous compounds. It is made in vast quantities, usually as an 85-percent acid, by the direct reaction of ground phosphate rock (calcium phosphate or apatite) with sulfuric acid. This yields "green acid," which contains typically 25- to 50-percent H3PO4 and is heavily contaminated with impurities, including anions such as chloride and sulfate (likely an excess of sulfuric acid), which might be expected to cause equipment corrosion problems if used to modify asphalt. Green acid is mainly used to produce fertilizer; some is further refined to phosphoric acid of food-grade quality.

Phosphoric acid is also made by the direct burning of phosphorus and subsequent hydration of the oxide commonly known as P2O5. (Its actual chemical formula is P4O10 but for historical reasons, it is still called P2O5.)

The pure acid is crystalline, and the hydrates have tetrahedral PO4 groups connected by hydrogen bonds. These persist in the concentrated solutions and are responsible for its syrupy nature.

The grades of phosphoric acid (i.e., orthophosphoric acid (H3PO4))) available commercially have acid concentrations of 50, 75, 85, 100, 105, and 115 percent.

Polyphosphoric acids (PPA) exist and have the general formula Hn+2PnO3n+1 for n > 1. They range from pyrophosphoric acid (H4P2O7 (n=2)) through the metaphosphoric acid (large values of n).

Although pyrophosphoric acid may be crystallized in several forms, other grades of phosphoric acid all have equilibrium distributions of molecules and are known only in liquid or glassy form. The viscosity of these acids increases greatly as n becomes large.

The PPA offered commercially is a mixture of orthophosphoric acid with pyrophosphoric acid, triphosphoric acid, and higher acids, and is sold on the basis of its calculated content of H3PO4.

HOW PHOSPHORIC ACID IS USED TO MODIFY ASPHALT BINDERS

There are three uses for phosphoric acid in the modification of asphalt binders:

RESEARCH PLAN

The plan objective is to develop a best practices guide for the use of phosphoric acid as an asphalt modifier and to address the industry perceptions such as effect on aging, moisture damage, and reaction with limestone aggregates. Elements of the plan include the analysis, effect of acid grade and binder types, aging, reaction with limestone aggregates, and moisture, as described below.

Analysis

Analysis will focus on the following tasks:

Effect of Acid Grade and Binder Types

In this area, researchers will determine the following:

Aging

Researchers will determine whether phosphoric acid increases the aging rate of asphalt binders. Some concerns have been expressed on this issue because phosphoric acid is used as a blowing catalyst in the production of roofing asphalt.

Reaction With Lime and Limestone Aggregates

Some State agencies treat their aggregates with lime (calcium hydroxide) to improve the moisture resistance of their asphalt mixes. Lime is a strong alkali and might be expected to react with phosphoric acid in the mix. Limestone (calcium carbonate) is not an alkali but is readily attacked by acids. In either case, if the phosphoric acid is effectively removed from the binder by chemical reaction, it could result in softening of the binder. Testing will determine whether phosphoric acid-modified asphalt will soften if used with limestone aggregates in asphalt mixes.

Moisture

Most liquid antistrip additives are alkaline amines, which might be expected to react with phosphoric acid. This could result in change in binder stiffness. Tests will be carried out to determine whether this is the case.

Because phosphoric acid is very hydrophilic, there is a possibility that it would increase the sensitivity of the mix to moisture damage. Mixes made with aggregates known to be nonstripping and stripping will be tested for moisture damage using the Hamburg wheel tracker.

The team will determine whether the benefits obtain by modification are permanent. In particular, phosphoric acid is very soluble in water-is it leached from the mix by rain?

Table 2 lists the materials used in the study.

Table 2 . Materials used in the study.

Supplier

Name

TFHRC Reference

Description

SHRP

AAD-1

-

Asphalt Binder California Coastal

SHRP

AAM-1

-

Asphalt Binder West Texas Intermediate

SHRP

AAK-1

-

Asphalt Binder Boscan

SHRP

ABM-1

-

Asphalt Binder California Valley

Citgo®

-

B6362

Asphalt Binder Venezuela 94-Percent Bachaquero-13, 6-Percent Merey-16

BP Whiting

-

B6364

Asphalt Binder Canadian Crude and Gulf Coast Sour

Ergon®/Lion Oil

-

B6367

Asphalt Binder Saudi and Arkansas Crudes

Various

Phosphoric Acid

-

Phosphoric Acid 115-, 105-, 75-, and 50-percent grades

Keystone Aggregates MD

-

-

Sandstone Aggregate

Mellot Company MD

-

-

Limestone Aggregate

Arr-Maz

AD-HERE ®
LOF65-00

-

Amine Antistrip Additive

Arr-Mazz

AD-HERE® LA2

-

Amine Antistrip Additive

Innophos

Innovalt®

-

2-Ethylhexyl Phosphate Antistrip Additive

Chemical Lime Company

Lime

-

Calcium Hydroxide

- Indicates not applicable
TFHRC = Turner Fairbank Highway Research Center
SHRP = Strategic Highway Research Program

 

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