Durability of Geosynthetics for Highway Applications

TECHBRIEF Durability of Geosynthetics for Highway Applications

Report Nos. FHWA-RD-97-142, 97-143, 97-144, and 00-157 FHWA Contact: Al DiMillio, HRDI-08, (202) 493-3035

The Structures research and technology program aims to foster increased Nonwoven polyester geosynthetic. durability of new bridges and observ- able increases in the service life of existing structures, placing an emphasis on increasing highway safety while preserving the environment. The program focuses on researching nondestructive evaluation technologies to identify structural deficiencies and sup- port bridge management systems. It also uses high-performance materials to repair and rehabilitate the existing in- ventory of deficient bridges. This find it and fix it program is supplemented by research which examines all aspects of bridges and foundations, including planning, design, construction, management, maintenance, inspection, and demolition.

Specific expertise areas include bridge coatings, bridge infrastructure, bridge Introduction management, nondestructive evaluation, corrosion protection, foundations, The research results described herein are included in four volumes on the scour, geotechnical research, high-per- subject of Durability of Geosynthetics for Highway Applications. Various formance materials, aerodynamics, aspects of geosynthetic durability were addressed in order to develop seismic research, and structures instru- procedures that could be used to predict long-term strength losses of mentation. geosynthetics used in highway applications. This information is essential to designers for allowing tensile capacity for geosynthetics used primar- ily in mechanically stabilized earth (MSE) retaining walls, reinforced soil slopes, and foundation stabilization.

The study was conducted in stages, each stage building on the knowl- edge previously gained. The main objectives addressed were to: (1) develop testing protocols necessary to quantify any strength reduction due Research, Development, and to aging or stress (stress cracking only) mechanisms for polymeric rein- Technology forcement materials (geosynthetics), and (2) develop testing protocols for Turner-Fairbank Highway confined stress-strain testing, which could more accurately characterize Research Center key engineering properties. 6300 Georgetown Pike McLean, VA 22101-2296 The results of the experimental tasks are discussed on the following pages. Stress Cracking Potential of • Stress cracking is a less strin- justification for the currently HDPE Geogrids gent or equal consideration used backfill specifications for (Report No. FHWA-RD-97-142) than creep rupture in devel- HDPE soil reinforcement. oping allowable tensile capacity for intact geogrids. • Provided the first research results on the synergy between Overview • Testing protocol for damaged stress, aging, and construc- geogrids using a Notched tion damage. This study was initiated to al- Constant Testing Load (NCTL) lay voiced concerns (in 1991) procedure was developed, that stress-cracking potential since the reduced stress was not being considered in crack-derived allowable ten- Development of Protocols for developing the allowable sile capacity may be lower Confined Extension/Creep tension load capacity for de- than projected, simply by ap- Testing of Geosynthetics for sign when using high-densi- plying a construction damage Highway Applications ty polyethylene (HDPE) ge- reduction factor. (Report No. FHWA-RD-97-143) ogrids. • Damage to the geogrid can be Stress cracking is a potential significantly limited by using a mode of failure occurring in backfill with a maximum grain Overview thermoplastic materials that size on the order of 20 mm to are under a sustained stress limit damage to levels that are The research was initiated to significantly lower than the not likely to significantly initi- develop a testing protocol to material's room tempera- ate stress-cracking failures at characterize the confined ture yield strength, resulting lower levels than those indi- stress-strain response of in quasi-brittle fracture of cated by creep testing. geosynthetic materials used the material. This is also as tension-carrying reinforce- known as slow crack growth ment for in-ground applica- and environmental stress Further Actions Recommended tions. Current testing meth- cracking (ESC) when in con- ods for stress-strain proper- tact with certain aqueous • The results and engineering ties are conducted in an un- solutions. recommendations from this confined mode, which does narrowly focused study were, not mimic the actual field The extensive laboratory in general, sufficiently clear conditions and is believed to study developed testing and to preclude additional devel- be, for some materials, overly interpretation protocols to opmental studies. conservative. The benefits of measure the potential for using confined stress-strain stress cracking for intact and • The NCTL testing procedure testing to improve characteri- damaged HDPE geogrids. could be submitted to an ap- zation of design properties propriate American Society should allow considerable for Testing and Materials material savings in tensile (ASTM) committee for poten- load applications. Major Conclusions tial adoption as a standard method. A similar testing The detailed laboratory results method for geomembranes is and analyses demonstrated that being considered. Major Conclusions for the one presently available commercial HDPE geogrid: The research, which included de- Impact of Results velopment of a proposed testing • Stress cracking is a potential protocol, concluded that: failure mode for HDPE uniax- • Removed a potential major ially drawn geogrids at their obstacle to the cost-effective • Soil confinement creates ben- nodes only, which are not use of HDPE geogrids for in- eficial effects for the stress- highly drawn. Rib areas, ground reinforcement. strain response of geosyn- which are highly drawn, are thetic materials, particularly not prone to stress cracking. • Provided additional technical for nonwoven geotextiles. • For nonwoven geotextiles, stiff- could be tested as a follow-up, ness and, therefore, modulus are using the protocol testing equip- significantly enhanced as a result ment and the more complex Un- sion load resistance of geosyn- of confinement. Thus, the use of notched Constant Load (UCL) thetic materials over their de- unconfined stress-strain proper- test device developed by Boyle sign life. Prior to the publication ties in designs with nonwoven and the test device developed of this study, no testing or inter- geotextiles appears to be overly by Whittle et al., to assess how pretation protocols were avail- conservative. Increases of 50 to the confined extension test re- able to the profession, and it 400 percent for the modulus sults relate to the actual re- was common practice to assess have been measured. sponse of geosynthetic rein- a large default reduction coeffi- forcement in simulated field cient to determine the allowable • The effect of confined stiffness on conditions. tension load. woven geotextiles and geogrids is considerably smaller, but not • The American Association of State The existing polymer literature necessarily insignificant. Increas- Highway and Transportation Offi- identified oxidation as the prima- es of 5 to 30 percent have been cials (AASHTO) T-15 Committee ry aging degradation mechanism measured. should consider the incorporation for polyolefin thermoplastics of confined stress-strain proper- (polypropylene [PP] and high- • Confined extension testing ties for design, especially in con- density polyethylene [HDPE]) should be performed to deter- junction with the use of nonwo- and hydrolysis for polyester mine the modulus and peak ven geotextiles. (PET) geosynthetics. strength of nonwoven geotextiles and could be used to more accu- The literature further indicated rately determine the properties of Impact of Results that antioxidants are added as all geosynthetic reinforcement stabilizers to polyolefin thermo- materials. Unconfined stress- The results demonstrated that the plastics to protect them during strain testing should be relegated most cost-effective geotextile—the high-temperature processing to Quality Assurance/Quality Con- nonwoven type—has significantly and long-term exposure to trol (QA/QC) functions. stronger stress-strain properties than degradation mechanisms, such previously measured using conven- as exposure to ultraviolet (UV) • Confined creep rupture testing tional unconfined testing methods. radiation and/or oxygen-rich should be performed to deter- This should promote the use of the regimens (such as in-ground). mine the long-term strength of most cost-effective geosynthetic nonwoven geotextiles. available. Existing literature indicated that polyester geosynthetics de- • Confined extension and/or con- grade in any aqueous environ- fined creep rupture testing must ment, the rate of degradation be conducted when calibrating in- Testing Protocols for being more rapid for low molec- strumentation to be used in field Oxidation and Hydrolysis of ular weight (Mn) products in monitoring and for assessing in- Geosynthetics highly acidic and alkaline in situ put parameters for numerical (Report No. FHWA-RD-97-144) regimens. analyses. The use of this testing The research under these tasks technique is essential in calibrat- was divided into phases. ing in situ stress conditions from field instrumentation. Overview The first phase provided base- line chemical and physical char- The research was initiated to de- acteristics for the commercial Further Actions Recommended velop laboratory testing and in- geosynthetics used in the pro- terpretation protocols to assess gram, as well as defining the • The confined extension/creep strength losses of geosynthetics scope of the long-term experi- testing protocol should be sub- due to "aging" phenomena. Con- mental degradation program mitted to the appropriate ASTM sideration and quantification of carried out under Phase 2. It fur- committee for consideration and these losses are required by the ther defined and characterized tentative adoption. AASHTO specifications in order typical in-use environments. to determine the allowable ten- • Select geosynthetic materials value in assessing oxidative which the solution is con- Phase 2 focused on the modifi- degradation resistance. stantly stirred. Again, incuba- cation of existing procedures, tion in excess of 3 years is protocols, and techniques for • Hindered amine light stabilizer necessary to produce signifi- determining thermo-oxidation (HALS)-type antioxidants can- cant degradation at the lesser (for PP and HDPE) and hy- not be monitored by standard required temperatures. drolytic degradation (for PET) OIT methods. The newer High- of commercial geosynthetics Pressure OIT should be inves- and the performance of limit- tigated further in this regard. Major Conclusions, ed preliminary experiments Phase 3 using the developed and/or • High-Performance Liquid Chro- modified techniques to as- matography (HPLC) was • For PP and HDPE, the long- sess potential degradation found to be an ineffective term incubation studies vali- rates and required testing pe- method for routine monitor- dated that the oxidative riods. ing of antioxidant content. degradation process can be divided into main phases as Phase 3 consisted of the im- • Measurement of molecular predicted by the Basic Auto- plementation of a long-term weight (or intrinsic viscosity) Oxidation Scheme (BAS). systematic experimental pro- and Carboxyl End Group During Phase 1—the induc- gram with sufficient expo- (CEG) number are the key tion period—the antioxidants sure variables to permit the polymer tracking methods in are consumed with no appre- calculation of degradation degradation studies for PET ciable tensile strength loss. In rates over usage time under geosynthetics. Phase 2, after the substantial conditions consistent with consumption of the antioxi- end-use environments. • Scanning electron microscopy dants, the oxidative degrada- (SEM) is effective in visually tion process progressively re- examining surface morpholo- duces the tensile strength. gy for evidence of oxidation— Major Conclusions, typically circumferential crack- • The length of the induction Phase 1 and Phase 2 ing or hydrolysis—that is evi- period, which is dependent denced by fiber surface ero- on antioxidant levels and The major aspect of Phase 1 and sion. type, controls the useful life Phase 2 dealt with determining the of the geosynthetic. The de- applicability of available chemical With respect to laboratory incuba- pletion of antioxidants can be and physical characterization tion time/temperature and methods: monitored by OIT measure- methods and the development ments. and/or modification of long-term • Incubation with multiple tem- testing protocols to determine peratures (minimum of three) • Unstabilized polyolefin prod- strength losses attributable to oxi- is necessary for determining ucts have relatively short use- dation and hydrolysis as a func- degradation rates. ful lives. tion of time. The following major conclusions were reached. • Incubation temperatures for • Modified Arrhenius modeling PP and PET geosynthetics techniques have been devel- With respect to polymer character- should generally be less than oped to analyze laboratory ization of geosynthetics: the 80∫C required for HDPE oxidative incubation data and geosynthetics, resulting in an predict tensile strength degra- • Oxidation Induction Time (OIT) even longer incubation time. dation rates. measurements were found to be reasonably effective as a • Oxidative incubations should • Antioxidant depletion is also a measure of oxidative stability, be conducted in circulating air function of the burial regime, but did not provide a quantita- ovens, which provide a more specifically the oxygen con- tive estimate of the concentra- uniform regime. centration and the amount of tion of multi-component an- transition metal present. tioxidant additives. Compara- • Hydrolysis incubations should tive OIT measurements be- be conducted in aqueous me- • Products such as most slit-film tween geosynthetics are of no dia and in heated reactors in PP geosynthetics develop ini- tial cracks during the manu- Factors for Aging are now (GRI), which has conducted facturing process. Oxidation used to determine the allow- some developmental work on studies for such materials able tensile strength of a these methods, should be en- cannot be conducted at ele- geosynthetic in Federal High- couraged to submit revised vated temperatures. way Administration (FHWA) ASTM standards. practices and are being im- • Given the required degrada- plemented worldwide. • Liaison activities with the Eu- tion of incubation to obtain ropean CEN TC189/WG5 meaningful results, accelerat- • Background and recommen- Committee that is preparing ed degradation testing meth- dations were published in an standards and test methods ods using pressure and a full FHWA Geotechnology Tech- for geosynthetics should be oxygen atmosphere to re- nical Note, "Degradation Re- continued. These will be final- duce incubation time should duction Factors for Geosyn- ized for implementation in be developed further. The re- thetics," issued May 15, 1997. the next few years and are search program demonstrat- likely to have a worldwide im- ed the viability of such an ap- • For the FHWA-recommended pact. North American input is proach. material specifications for essential to protect U.S. pro- geosynthetics used as rein- ducers from standards con- • For PET, the long-term incuba- forcements, QA/QC and mini- trary to U.S. specification tion studies validated that mum acceptable polymer practices. conventional Arrhenius mod- characteristics have been up- eling techniques can be used dated with respect to desir- to analyze hydrolysis data able polymer characteristics. and predict tensile strength These requirements have Long-Term Durability of Geosyn- degradation rates. been incorporated into the thetics Based on Exhumed Sam- guideline specifications for ples From Construction Projects • Tensile strength degradation MSE walls and reinforced soil (Report No. FHWA-RD-00-157) rates for PET are accelerated (RS) slopes contained in Re- in pH environments greater port No. FHWA-SA-96-071, than 9 and in acidic environ- Mechanically Stabilized Earth Overview ments of less than 3. Walls and Reinforced Soil Slopes Design and Construc- This study was initiated to de- • PET commercial geosynthet- tion Guidelines. velop a databank detailing ics produced with low molec- the oxidative and/or hydrolyt- ular weight (low intrinsic vis- ic performance based on re- cosity) and/or high CEG will Further Actions trieved geosynthetic materi- degrade at a faster rate. Typi- Recommended als from construction works. cally, these are nonwoven The databank included both products. • The developed protocols for mechanical and polymer oxidative and hydrolytic test- characteristics to potentially • Hydrolytic degradation for ing should be submitted to serve both as a performance PET can be tracked by viscos- ASTM for tentative adoption benchmark for the laborato- ity measurements. as standards. ry-based predictions previously developed and for fu- • The polymer characterization ture retrieval programs. Implementation of the methods, OIT, MI (Melt Flow Research Findings in Index), CEG, and viscosity re- A total of 24 geosynthetic Current Practice quire standardization or mi- samples from 12 locations nor/major revision where were exhumed and tested • The practical effect of the re- ASTM methods are available. for this task. Industry labora- search findings has been to Industry should be advised/ tories working under the quantify, for the first time, encouraged to publish these auspices of the Industrial strength loss as a function of polymer index properties as Fabrics Association Interna- time and environment. part of their Technical Infor- tional (IFAI) (their trade asso- mation Sheets. The Geosyn- ciation) were called on to • Rational test-based Reduction thetic Research Institute acterization be available for the could be inferred. Consistent de- provide measured data for the product used. creases in OIT suggest that not mechanical and polymer prop- all of the antioxidant had been erties of typical current geosyn- • Measurement methods for key consumed in the less than 20 thetics. polymer index properties must be years prior to retrieval. This find- standardized for future compar- ing is consistent with the find- isons. Industry and academic ings in Report No. FHWA-RD-97- polymer laboratories were sel- 144, which suggests that during Major Conclusions dom able to match polymer test the induction period, antioxi- properties from the same sample. dants are consumed with no The results, analyses, and conclu- strength loss. sions may be summarized as follows: • Small decreases in viscosity were measured in all of the retrieved • For significant field degradation PET geosynthetics (all less than Further Actions Recommended to occur (by oxidation or hydrol- 20 years old), suggesting low lev- ysis), retrieval sites need to be 30 els of strength loss due to hy- Additional retrieval programs or more years old. Such sites will drolytic degradation. The rate in- should be initiated within the next not become available until the ferred was consistent with rates few years, focusing on the four to six end of this decade. developed in the study docu- sites sampled under this program mented in Report No. FHWA-RD- where multiple previous retrievals • A successful retrieval program re- 97-144. were made. The standardization of quires that the level of construc- polymer index test methods and tion damage be established dur- • For PP and HDPE geosynthetics, their disclosure in manufacturer lit- ing construction or soon there- no oxidative degradation leading erature and compliance certification after, and that full polymer char- to strength loss was measured or should be promoted.

Researcher—This study was performed by Earth Engineering and Sciences Inc. (E2Si), Baltimore, MD. Subcontractors: GeoSyntec, Atlanta, GA, and Polytechnic University, Brooklyn, NY. Contract No. DTFH61-91-C-00054. Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution is being made to the Resource Centers and Divisions. Availability—With the exception of Report No. FHWA-RD-00-157, which will be available in the near future, copies of the other reports are currently available from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161. A limited number of copies will be available from the R&T Report Center, HRD-11, FHWA, 9701 Philadelphia Court, Unit Q, Lanham, MD 20706, telephone: (301) 577-0818, fax: (301) 577-1421. Key Words—Geosynthetics, geogrid, geotextile, durability, oxidation, hydrolysis, testing protocols for durability, confined extension, confined creep, confined stress-strain, reinforcement, chemical degradation, polypropylene, high-density polyethylene, polyester. Notice—This TechBrief is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The TechBrief provides a synopsis of the study's final publications. The TechBrief does not establish policies or regulations, nor does it imply FHWA endorsement of the conclusions or recommendations. The U.S. Government assumes no liability for the contents or their use.

JANUARY 2001 FHWA-RD-01-050