APPLICATIONS FOR FGD BY-PRODUCT GYPSU~

• Or . Dona ld Saylak , P. E. •• Tom Scullion, P. E. Texas AlH University Col l ege Stat lon, Texas

"·0 . H. Golden Electr ic Power Re se arch Institute Pa lo Alto , Cal Ho rn ia

ABSTRACT

In acco rda nce wi th the 1977 Clean Ai r Act and more recent legislati on the United States Env i ronmenta l Protection Agency (EPA ) established regulations that required coal-f i red power generat i ng companies to reduce the i r sulfur ox ide (SO.) em i ssions . The most prevalent means of SO. control is accomplished through flue gas desulfurization (FGO). In this process, sulfur laden gasses are scrubbed with a chem ical absorbent (usually a lime slurry). The by-product of th is process is calcium su l fate dihydrate (CaSO, • 2H20) also known as Gypsum. It has been est imated that 20 million tons of FGD Gypsum are being generated annually in the U.S. and has already create~ an inventory of 150 million tons. It has been projected that in 40 years this inventory will quadruple. Some of the commercial appl ications for the use of this by-product include (a ) so il enhancement in agricu lture (b) set retarder in cement produ ction (c ) wallboard products and (d ) the produ ction of un-f i red brick and masonry components. Other uses unde r devel opment include hydra ulic barriers and embankments , synthetic aggregate and stabilized gypsum-bottom ash compos ites .

Th is paper will show how ongoing research at Texas A&H Univers i ty is extend i ng the use of FGO gypsum into other appl ica t ions such as roadbase cons truction . The construction and post-constructi on performance evaluation of 300-foot. experi men ta l test sections us ing cement­stabil i zed Flue Gas Oesulfur ized (FGO) gypsum bases will be presented . It will be shown how the strength of the stabi lized gypsum base materi al can be improved through the addition of bottom ash into the mix .

• Professor - Materials Science - Texas Transportation Institute •• Pavement Research Engineer - Texas Transportation Institute

••• Project Manager - Coal Combustion Systems Divi sion - EPRJ

INTRODUCTION

Gypsum , calcium sulfate dihydrate (C aSO. 2H20) , 1s a natural ly occurr i ng , non-metallic mineral used as a raw mater ia l in t he manufacture of gypsum board, portland cement, plaster products and in agriculture as a soil enhancer (1 ). By-product or synthetic gypsum, includes a fam ily of calcium sulfates including flue Cas Oesulfurization (FGO ) gypsum, Phosphogypsum, Fluorogypsum, Titanogypsum, and Disu lfogypsum. These are names used in the engineering and scientific literature for by -product calcium sulfate to reflect the particular process or ind ustry which produ ced it. For the past 8 years the Texas Transportati on Institute (TTl) has studied virtually all types of by-product gypsums and has be en intimately involved with roadway applications of Phosphogypsum, Flourogypsum and FCO gypsum (I, I, !. ~. 2)'

Flue gas desulfurization (FGO) gypsum is general ly produced from power plants burning lignite or sulfur-contaminated coals . Sulfur is a natural contaminant in some coals, and is almost complete ly converted to sulfur oxides when coal is burned . Consequently, sulfur oxides, being released primaril y in the form of sulfur di oxide, contribute to the format ion of acid rain. For th is reason , the Environmen tal Protection Agency (EPA) has established stringent regulations about discharging sulfur dioxide em issions into the air and the electric power industry has responded by installing flue gas scrubber systems (1). Sulfur dioxide gases are washed with a limestone slurry to produce calcium sulfate, which is pumped to large settling lakes.

Hore than 25 million metric t ons of sulfur oxides (SO.) are emitted annually in the United States (1). This amount is expected to reach 41 mi llion metric tons by the year 2000 . This accounts for approximately 14 per-cent of the total est imated national air pollutant emissions . More than two thirds of all national sulfur oxide em iss ions result from fuel combustion in coal-fired electric power generating stations . The other one third is derived from industrial boilers, copper smelters, petroleum refining, residences, bUS inesses , public institutions, transportati on and other sou rces .

FCO processes result in SO. removal by inducing exhaust gases to react with a chemical absorbent as they ~ove through what is called a scrubber . The absorbent is dissolved or suspended 1n water , form ing a solution or slurry that can be sprayed or otherwise forced into contact with the flue gases. Ninety percent of the FCD systems in use today use limestone (t aCOl ). calc i um hydroxide (Ca(OH)2)' or calcium oxide (CaO ) as the chemical absorbent .

Reactions tak i ng place in flue gas desulfurization can be summar ized as follows:

--"H£.O,--_~.. CaSOJ + CO 2 CaSOl + 502 + H20 --, .. Ca(HSOJh

Ca(HSOJ)2 + O2 + 3H20

1

- --- ,

Rising costs associ ated wi th assuring high quality construction and maintenance of highway systems is spur r ing the continued development of more cost effective construct ion methods and materials. Waste and pollut ion abatement by-products of coal-burn i ng power plants. hydrofluoric and phosphoric acid production industries are currently being given considerable attention in Texas . louisi ana , Florida and other states.

A number of researchers (Z, 1. ! , 2, §) have provided evidence that by-product gypsum can be used as a roadbase or subbase mater ial through st ab ili zat ion with either portland cement, fly ash or combinations of both . When properly mixed, compacted and cured, these materials wi ll develop sufficient strength for field applications . Figure 1 shows 7-day unconf ined compressive strengths for a typ ical FGO gypsum stab ili zed over a range of cement contents. Figure 2 shows that the strength for both 8 and 13 percent cement-stab ilized FGO gypsums continue to increase after one year. Strengths above 350 psi are considered sufficient for light to medium traffic loads. Strengths from 450 t o 650 psi are required for medium to heavy traffic loadings. Figure 1 shows that FGO by-product materials, when sufficiently stabi lized, should qua 1 i fy ·for roadbases and subbases.

Roadbases for ci ty streets, shopping centers. truck terminals , parking lot s and loading platforms have been successfully constructed in the Houston area of Texas using cement and fly ash-stabili zed phosphogypsum and flourogypsum. Personal contacts with two suppliers; Gulf States Mater ia l s (Fluorogypsum) and Mob il Chemical Company near Pasadena, Texas (Phosphogypsum) have revealed a better-than 9S percent success rate on over 700 projects (,8., .2). TTl was involved in the development of the mix design rationale for the base cou rses utilizing both of these by-product gypsums (lQ , 11 , 11).

It is when attempts are made to extend the stabilized-gypsum roadbase concept t o state and federal roads that construc tion difficulties with stabilized-gypsum road bases have been encountered . One project using a ten percent cement-stabilized phosphogypsum base on Texas SH 146 proved unsuccessful (1) . Two other projects in Texas (2) using varying amoun ts of fly ash and cement as stabilizers also had to be replaced after less than a year in service.

In virtually every case, when construction difficulties were encountered, the problems could be related to one or more of the following sources :

(a) Too much mois ture : Overwatering in the field, while either trying to achieve a specified moisture content or to maintain dust control, will weaken the base during its most critical period of strength development . One of the prime locations for this type of damage occurs at transitions from one day ' s work to the next. An improperly prepared transition from one mix design to the next is also potentia lly vulnerable to swelling due to excessive moisture.

3

.. ~ 1200 ~

~ < 1000 • -in • > ,00 .. • • ~ 600 0 C)

~ • 4 00 < < 0 0 < 2 00 ::> > • C ,

00 ~ 5 10 15 20 25 Cement (~:.)

Figure 1. Unconfined Comp ressive Strength versus Cement Content for Typical FGO Gypsum .

2000 • ~

1750 ~

C, < 1500 • in • ., 1250

• • 1000 • C. E 0 750 C)

~ • 500 o 8 ~. Cemenl < -< • 13% Cemenl 0 0 250 :5

a a 50 100 150 200 2 50 300 350 400

Curin g Time (days )

Figure 2. Unconf ined Compress ive Strength versus Cu re Time for a Typical FGO Gypsum Stabilized with 8 and 13 Percent Cement.

4

(b) Qver Stab i l izat ion: Oifferent states allow varying number of days of curi ng t o ach ieve allowable strengths for the i r stabili zed bases. Texas requ i res 6S0 ps i strength after seven days. Illinois permits 6SQ psi after fourteen days . Under these conditions, an inab ility to reach the required seven-day strength is usually compensated by add ing excessive stabilizer. Th is was the case on SH 146 (11) which, al ong with excess ive moisture, produced severe swelling and caused the road to fail.

(c) Inc omp lete Mixing: Blending of mix ingredients can be accompl ished successfully either on the ground (Z) or by preblending in a pug mill (!). The latter is more effi cient and allows for fie ld calibration checks t o establish compl i ance wi th job mix criteria. It can al so be the more labor intensive of the two methods and , thus, more costly , part icular ly on short roadways requir i ng long haul i ng distances from the plant. Achievi ng good mix homogeneity can be a problem when us i ng in-place mixing. This wa s revealed wh en cores from one project in Texas indi cated that the lower 2 inches of an a·i nch base were not mixed (2). This problem is ~sua"y resolved by going to mult iple lifts. However, in most cases, the condit ion may not be discovered until it ;s too late to rect ify.

(d ) Incompatible Stabilizers and Pri me Coats: Cement type and content have a sign i ficant infl uen ce on strength de velopment in s tab i lized by -product gypsum mixtures. Trica lcium aluminate (C)A ) is one of the pr inci pal aluminate compounds in port l and cement . To achieve sulfate res istance in portland cements ASTH CISO recommends that the C)A co ntent in Type II cements to be kept below seven percent. The hydration of C)A in portland cement involves reaction wi th sulfate ions which are suppl i ed by the dissolution of gypsum . The primary initial reaction involving C3A is:

Ettring ite is a stable hydrat ion product only wh i le there is an ample supply of sulfate availabl e . Once Ettr ingite has formed, it continues to grow expansively. If the temperature of the system drops below approximately IS'C (59'F), Ettringite , through a series of intermediate complex reactions is transformed t o Thaumsite , (l! l, a complex calcium- silicate- hydroxide- sulfate- carbonate- hydrate mineral . Both Ettringite and Thaumsite are hydrous minerals. Without an abundance of water or excessive C)A they cannot form . It has been shown (~) that using high sulfate resistant (HSRl cements with C)A contents not exceeding 3 percent will virtually eliminate the swelling problem .

Two prime coats which have shown t o work well as a seal over compacted stabilized bases have been MC30 and RC250 cutback asphalts. Attempts to use emulsions have proven unsuccessful since they tend t o add add itional water t o the mixture . This tends to

5

create a weak shear plane about 0.5 inches bel ow the surface during compacti on subsequentl y comp romis i ng the in t egrity of t he ent i re base when traff ic is introduced or when freeze thaw conditions are encountered.

(e) Insufficj ent Comoaction and Sealing : Degree and type of compaction are critical factors affect ing the ultimate strength ach ieved in stabili zed gyp sum bases. The effect of compaction on both optimum mo isture content an d dry density is shown in Figure 3 . . Similarly the effect of compaction on tensile and unconfined compressive strength is shown in Figure 4. The specification of a field density testing method compatible with gypsum bases should be coord inated wi th t he state hig hway department . Simi la r considerati on should be given to establishi ng state standards for 7-day strengths. Some de vi at ion from standard pract ice should be permitted given that Gyps um is n21 a soil but a crystall ine by ­product wh ich 1s normally used to retard strength development.

For example , the laboratory compaction recommended for stabil ized gyps um is Modi fied Proctor as prescribed by ASTM 01557 wh ich delivers 32 .6-ft.-lb/i nl of energy to the specimen (11). The Texas SOHPT uses its own Modi fied Proc tor test and specimen conf igurat ion under Texas Method 113-E wh ich delivers a maximum of 13 .2 ft - lb/ inl of energy. Figure 4 indicates that the latter would predict the need for a higher optimum moisture content and consequently achieve lower strength than that obta ined us i ng ASTM 01551. Result s from using Tex Method 113- E wil l ult imately produce non-conservative decision criter i a for des ign of the base usual ly ind icating an unnecessary need for more water and/or stabilizer . The procedures recommended in Reference 11 were developed over the seven years TTl has been studying gypsum and stabilized -FGO gypsum base mixtures .

Insufficient sealing of the ba se can make it susceptibl e to premature damage. Two treatments of a standard chip seal surface treatment or a one and a half to two inch th ick highway department approved wear i ng coarse has been found to be effective.

USING FGO GYPSUM IN ROADWAY CONSTRUCTION

As a result of the apparent performance inconsistencies reported from the field with stabilized-gypsum road bases a demons tration project sponsored by TU-Electric and ALCOA was carried out to verify. the mix design rationale and construction practi ces discussed above (4. 12) . The test section would be subjected to at least, IS-months of post­construction evaluation using state-of-the-art pavement performance ~onito ring technology .

In September of 1991, a 2-1ane. 300 foot long test section wa s placed at Texas A&H ' s Riverside Campus located IS miles fr om the ma in campus . The existing road had a 12 in . i ron ore base with a 2-course

6

o o •

1 o o •

7

•

~ Q

~ < , .g

" • ~ ~ • _. I ~ a U

~ ~

.~ o . • ~. ~~ .~ • • • ~ >

• • o • • ~. v~

• E ~ o EU o U~ • -. o •

~

• • ~ • 0 . U 0 • ~. ~o .~ • • .--~o

'" • oE _0 ~. v-_ .~

~~ E O 0 U~ • -. 0 ~

~~ v-_ •• -. -. ~O

M

• • 0 ~

~

Texas Highway Department standard surface treatment. The new section utilized 8M of FGD gypsum base stabilized with 7 percent by dry weight of a high-early-strength (HES), high sulfate resistant (HSR) portland cement.

The gypsum came from the TU-Electric Sandow Power Station at the ALCOA plant in Rockdale, Texas. The chemical breakdown for this materia l is given in Table I. The portland cement was produced by Texas Industries, Inc . (TXI) as their Class C Oil Wel l cement. The chemical and physical analysis shown in Table 2 reveals its high sulfate resistance (low C3A) and high early strength characteristics.

Following the successful construction of the 1991 test section a second demonstration project co-sponsored by the Electric Power Research Institute , TU-Electr ic and ALCOA was conducted to further enhance the performance of gypsum bases through the use of coal-fired power plant bottom ashes (12). In this concept wet bottom ash (boiler slag) and dry bottom ash (cinder ash) were added to the stabilized gyp sum mixture as coarse fractions to improve their compacted density. Different ratios of R, representing the rat io of wet bottom ash (WBAl/dry bottom ash were blended on a 50/ 50 weight bas is with the ALCOA gypsum. For example, for R a 75/25, the aggregate portion of the design mix was comprised of 50 weight percent (w/p ) gypsum, and 50 w/o bottom ash; the latter in a proportion of 75 wl o WBA and 25 w/ o DBA. As in the earlier design (i.e. R· % Control), this mix was also stabilized with 7 percent TXI Class C cement.

The in fluence of the R ratio for a series of mix designs, is shown in Figure 5. which indicates the dry density to increase with the amount of WBA in the mix. The strength as shown 1n Figure 6 appears to maximize for an R of 75/25. It should be noted that the dry density increa sed from 105 pcf for the control mix. (i.e. the mix used in the 1991 test sections) to 115 pcf for the R • 75/ 25 blend. Strength development reflected a significantly higher rate of increase over the 1991 control deSign. On this basis the R • 75/ 25 gypsum/ ash blend wa s selected for the new test section.

The new section was constructed in September 1992 and was identica l in width and length and was placed 1n tandem with the 1991 section. A comparison of the properties of the two roadbase mixtures as shown in Table 3 and Figure 7 reflect a higher 7-day compressive strength and a faster rate of strength development in the 1992 mix design. It should also be noted that field densities were more closely related to the ASTH 01557 laboratory compaction values than those generated by lex Hethod 113E .

ROadbase Construct1on Procedures (1991 Test Section)

The construction of the 1991 test section began by removing the existing roadway to a depth of 8 inches. The gypsum and cement were hauled to the site separately and mixed on the ground using a pulver izer .

8

Table l. Chemical Breakdown for ALCOA' s FGD Gypsum.

Constituent Weight Fract ion

C. 24

SO, 54

CO 3

5,0 2.7

Inert 1.3

H 0 15

pH • 7.0

Table 2. Texas Industries , Inc. Class C Oil Well Cement Chemical Analysis .

Constituents Weililit Fraction Constituents Weillht Fraction SiO, 19.61 PlO" 0.21

Al,O, 4.10 TiO, 0 .21 Fe, O, 5.68 ZnO 0.03 CaD 64.07 MnO 0.32 M,O 0.87 Na,O 0.24 SO, 3.24 K,O 0.42 C,S 64.12 C,S 7.85 C,A 2.34 CAY 17.30

Casa 5.5 Blaine 4600

Strength., psi I~ay 2020 3-day 3974 7~ay 5069 28~av 6193

9

RcW8A1D8A 120 ~~~~~~ ______________________ -, 100/0 ' . - 115

" , 0 .., " > I-en Z w

" > a:

"

110

105

100 010 Control

95

0/ 100 90

65 6 6 10

...... -- -'-.- -

12

--50/50 GypsumlAsh 7% Class C Cement 14 Day Cure

14 16

MOISTURE CONTENT

16

Figure 5. Dry Dens i ty versus Moisture Content for Vari ous Wet Bottom Ash (WBA) / Ory Bottom Ash (DBA ) Bl end s Mixed with 50 Percen t FGD Gyp sum .

" R-WBA/ DBA

..9- 1800

1600

1400

1200

1000

600

600

400 6

Figure 6.

>-75/ 25

~ ...... 50150

7~ Cemenl 6 -0 ,100/0

25 / 75

;; ~ x 01 100

~

010 ContrOl

7 6 9 10 11 12 13 14 15

Cure (days)

Unconfined Compressive Strength versus Cure Time for Var iou s WBA/ DBA Blends Mixed with a 7 Percent Cement- St ab llized f GO Gyp sum.

10

w > ;;

'" w

" a: "- S

'" 0 J: U to Q Z w w za: - >-~ '" Z 0 u Z :::>

Table 3. Compar is on of 1991 and 1992 Test Sect ion Construct ion Data.

Parameter 1991 Section 1992 Section

Thi ckness 8" 8"

Width 18 ' - 0" 18 ' - 0"

lenQth 300 ' - 0" 300 ' . O·

Compaction, pcf

ASTH 105 (10 2'1 120 ( 106%1

lEX 1m 89 (86%1 98 (89%1

Actua 1 103 (l00'1 113 (l00'1

Optimum Mo isture Content , %

., 13 ' J

., 10 . 5 . 2

,

Unconfined Compressive StrenQth, psi

7 - day 500 800

I' · dav 580 950

28 · day 650 1200

56 · day 800 1700

2000 1992 Aoadbase

1500

1000 1991 Roadbase

500

7% C.m.nt

0 7 14 2. " CURE (days)

Figu re 7. Comparison of Unconfined Compressive Strength Oevelopmer: in 1991 and 1992 Mix Design.

11

Water was added to achieve a moisture content of 13 percent . The mixture was spread to grade . A sheeps foot roller provided the initial compaction which was followed by three passes of a 25 ton pneumatic roller . Field density was measured using a Troxler Nuclear Density Heter wh ich was calibrated to allow for the 2 molecules of structural water in the dihydrate gypsum.

One full day was allowed for the roadbase to cure after whi ch a pr ime coat of He-30 cutback was applied at a spread rate of 0. 12 gal / yd' Two days later a 1\ to 2 inch Texas Highway Department Type 0 surface treatment wa s placed over the base. The section wa s open to traffic the next day.

Roadbase Construction Procedures (1992 Test Section)

The method for pl acing the cement-stabilized gypsum/ bott om ash mixture was essentially the same as that used as the 1991 test section except for the sequence in which the base materials were delivered t o the site and placed .

After the subgrade had been prepared the base materials were placed in the following optional sequence (a) wet bottom ash (b) gypsum (c) dry bottom ash and (d) cement . Following placement of the gypsum, each layer was evenly distributed with a grader and pulverized to provide homgeniety. Water was added as required to achieve optimum . The compaction of the base was similar to that performed on the 1991 test section as was the appli cation of the He-30 prime coat and 1\ to 2 inch surface treatment .

POST CONSTRUCTION EVALUATION

The long-term performance of stabilized materials is governed by two factors namely:

1) the ability to withstand shrinkage forces wi thout inducing reflection cracks

2) the ability to withstand traffic loads without fatigue cracking.

In either case cracks in the surface will permit water to seep into both the base and subgrade generally greatly accelerating the rate of pavement failure. To monitor the performance of the research sections three methods of post construction evaluation were used:

a) Visual inspections were made throughout the life of the pavement to detect any shr inkage or load-associated cracking

12

b) Structural evaluat ion using a Falling Weight Deflectometer (FWD) were carried out to non-destructive ly measure in situ layer strengths and

c) Structural evaluat io ns with pavement instrumentation under traffic loads . The multidepth def lectometer (MOD) was used to monitor the pavement ' s response to repeated heavy ax le loads. The results for each of these evaluations are discussed bel ow.

Visual Inspection: At this wr iting, both test sections appear to be sound with no inci pient surface cracks evident over their entire length . A longitudinal . hair line crack about 6 t o 7 feet long has appeared 18 inches in from the outer edge of the eastbound l ane of the 1991 section. However . further examination has shown this to be a subgrade shift and unrel ated t o the roadbase.

The only visual distress detected thus far in the 1992 test section has been in the form of a sl ight transverse expansion bump which has appeared at each end of the roadway (See figure 8). This type of expansive act i on typically occurs at transitions or work stoppage poi nts . as was discussed earlier. due to variations in compaction between the new and existing pavement or with the accumul at ion of excessive water pushed along during grading or rolling. No such bump has shown up on the 1991 secti on. Th is problem is easily corrected by removi ng the bump mater ial down to the subgrade and patching with HMAC. When these two bumps were discovered they were about 1/ 2 to 5/ 8" in height and completely traversed the width of the road . No other bumps have been observed in either section and no further expans ion has occurred 1n the original bumps.

Falling Weight DefJectometer: The FWD is the most common non­destructive testing device used for monitoring the structural integr ity of pavements. The FWD used to monitor the experimental pavements is shown in Figure 9. A load which simulates a truck load is applied t o the pavemen t through a 12 inch diameter load plate . loads up to 20.000 lbs can be app lied. but usually a load of 9,000 lb applied for a duration of 30 ms is used because it simulates a typical legal load limit. The deflect ion of the pavement is measured at the center of the load plate by six geophones located at 12. 24. 36, 48, 60 and 72 inches from the center of the load plate. The seven deflections under a known load produce what is known as a -deflection bowl-. The magnitude of the maximum deflection and the shape of the deflection bowl can be used t o back-cal cul ate the stiffness of each layer in the pavement system .

Typically FWD deflection bowls are measured at regular intervals along the length of the roadway so that the strength and its variat ions can be determined. On both the 1991 and 1992 experimental sections the deflections were taken before construction. soon after construction and at regular intervals thereafter . The parameters of interest are the max imum pavement deflection and its variation along the roadway and the back-calculated modulus of the stabilized base layer. The MODULUS 4.2

13

Figure 8. Expansion Bump Located at Each End of the 1992 Test Section .

Figure 9. Filling Weight Deflectometer Used for Structural Strength Testing.

14

software (11) was used t o perform the back-calculati on. Thi s involves match i ng the measured deflection bowl with theoret ical ly generated bowl s from a layered elastic computer program. These programs require the test load, the layer thicknesses , layer moduli and Poisson ratios as input from whi ch they compute a theoret ical deflection bowl . The layer moduli are changed fr om run to ru n and an error mini mi zation routine is used to arrive at the final layer modul i.

FWD test results from the 1992 gypsum bottom ash project are shown 1n Figure 10 . The lower graph shows the maximum deflection as measured al ong the test sect ion . The or iginal base refers to the deflecti ons in the existing granular base, these being in the range 30 t o 40 mils under a g,OOO l bs load. After construct ion the deflect ions in t he gypsum/ bottom ash base were shown t o be less than 10 mils al ong the entire length of the project except at the ends .

The back-calculated base modul i are also shown in Figure 10. Pr ior to construction, the existing granular base had a modulus of 20 to 40 ksi. The new stabi lized base had a substantially higher modulus we l l in excess of 1,000 ks l. The new base appears to be gaining strength dur i ng the 7-month period from October 92 to April 93 . The modulus appears to be stabilizing at around 2,000 ksi . This is considered to be extremely stiff and equivalent to the strength found in typical cement-stabilized bases. Another encouraging sign is that this strength is being achieved without producing any of the shrinkage cracks normally associated with stabi li zed bases .

In general, the lower the deflections the better is the pavement ' s ability to distribute and carry loads without rutting and cracking . Both the 1991 and 1992 roadbase layers appear to have substantially improved load carrying capabilities compared to the existing road .

Multi Depth Deflectometer Testing; The most important component of any evaluation of a new roadway material is its ability to stand up to multiple passes of fully· loaded trucks. A material wh ich has a low stiffness ~ay be prone to excessive permanent deformation which eventual ly results 1n wheel path rutting . Conversely , a ~aterial whi ch is too soft may fail in flexural or fracture primarily 1n cracking in the wheel paths. To evaluate the experimental roadways, ability to withstand traffic loads both of the 1991 and 1992 experimental test sections were instrumented with multldepth deflectometers (HOD ' s) .

A schematic of the MOO module used for ~easurlng in-situ deflect ions together with a cross-section of a typical HOD installation is shown in Figures 11 and 12, respectively. The location of the MOO modules is in the two experimental test pavements is shown in Fi9ure 13 . The devices and their use are described in greater detailed 1n Reference 18.

The MOO 1s comprised of a series of deflection sensors (lVOT ' s ) which are stacked at various depths in the same hole. The hole size 1s 1 1/ 4 inch in diameter and the sensors are usually located close t o the layer interfaces. The MOO ' s are installed after construction and me.sure

15

1992 GYPSUM/BOTTOM ASH TEST ROAD

2200 April '93 Feb. '93 --...

(4 mont ~l) ,.- 0 - - - - -. -- 2000 It mOI'lIIlI) .' " '----...,-. - -- -0 D ~ 0 0 - I": .--.' en 1500 , I '- Oct. '92

, ::0

, ..J I .' (0 "' O"I ~IJ ::0 , Cl

, \ 0 1000 I ;

I

::E (b) \ \ w en 500

, -< Original Base \ ~ : '" , , .

0 0 25 50 75 100 125 150 175 200 225 250 275 300

BASE LENGTH (feel)

43.0 Original Base ::;;.~ __ - ...... __ ./ •

I • -~ ,.

,

.s 30 .1 z .• '\.-

Feb. '93-: (. ) :

o t; W ..J U. W Cl

o April '93 Nov. '92-},

rl-~--~,~.~ .. ~_~ .. ~_;.;;:::-.;;.~.~.;.~~.~.::._~;=;;~~~~~,r~.;:~-j 7 . 1~ _ •• _ •••••••••••••

o o

Figure 10.

, 50

. , 75 100 125 150 175 200 225

BASE LENGTH (feet)

, , 250 275 300

Comparison of Falling Weight Deflectometer Results for 1992 FGO Gypsum/ Bottom Ash Road Base with Original Sase.

16

1----- CLt._ """

.-,."IOUO;II'I!I -~ " 'o11t~~lH···' H .. .. . r. • lO_ "UHU! STU!,. ,Ul SHU' _(R (otClOS .... '

1,. VOl

lVO' CIIIII:

_U SUI"';

nU11l1 l_

.. nO! (_c, ... -Figure 11 . The MOD Module.

•

CASllHO Co...I'0ut40

........ fLUIIILE 'UflfACE CA,.

17 ...... TIDEf'TH DEfLECTot.IETEft UOOUlE

./SNA .. CONNECTOR

~~'.V5HA" ItE AD II"CSllNE LOCKINGI

/AHCHOR UTEHSION

Nol 10 IIUI,

Figure 12 . The Complete HOD System Incl uding Z HOD Modules In stalled in a Pavement Structure.

_).5" HMAC 1.5" HMAC

[:ii -, - - - .

, ' 2.4" MOOII 2.6" MOOII . '.' " ' ," " , .. "/ , . ~:;?:. 8.0" Base :', '. ";" : 8.0" Base

10.0" MOOn ~~ In.,· MDO#2

Subgrade III! Subgrade

22.1" MOO,3 . 21.8" MOO'3 - 't' L- Anchor

m

Anchor

1991 TEST SECTION 1992 TEST SECTION

III Jndicates MDD Devices

Figure 13 , As Placed HOD Installations for 1991 and 1992 Test Sections .

-~

-.~ E -c: 0 ., 0

~ " 0

, i - f I Iii MDD. '

16 I n- !"I. Rear "fIe !.. .... -A-.L1-. __ .. L A! -.- " ! MDD.2 --" T,A !! ~

14 -____ 4_· ____

H

••

, '*"*""-T '

I B 12~ ······-··-i .................... .

I ! 10" .......... . j ,.

i : i It ,························-+1 __ ······· i :

i 8

6 ... ··· I i

Steeri~g Axle

4

2~ --.---- .. I -~- .• ""--., _ .. , f • __ • __ .'H'_._.",-._

o·

i · 1 I

i ---- ----, --_._._._ ..

- ! .

o 200.314 366.8165 533.3189 699.8214 866.3238 1032.626 Time (msec)

MDD·3

Figure 14. MOO Deflections in 1991 Test Sections After 50 Passes.

N 0

25 I ' , 1" ---, I fli -+­

MOO-'

--'!l E -

I '" i . I I I ' : ~earAxle l

20~ -"'---"~' ··-·-·- 1--·---- .. -... -- . !- ... : "-.. ' ... , ---

I' i ' I , , , ' I I !- I

15~ ' + ............................ ...................... 1 -. 1 ................ i I B

--MOO-2

~

MDO-3

" 0 . ", " " '" " 0

I I I . I '

1 O~ .---r-'-'- ·t·------1-'-'-. , ,

I

---_ .. __ ... __ ._.

Steering Axle

l " 5 .. ·····,······

I/~ I~v ~ I O''6::d:r ::::L ~ o 199.4509 365.9533 532.4558 698.9583 865.4607 1031.963

Time (msec)

Figure 15. MOO Deflections in 199Z Test Sections After 50 P~sses .

both the transient deflections and permanent deformations under truck loads ~ithin the pavement layers relative to an anchor that i s installed at a depth of approximately 10 feet. The MOO system is unique in that it:

• places multiple sensors in a single hole • can be calibrated 1n place and • the sensors are recoverable after testing is complete.

80th test sections ~ere tested ~ith a fully-loaded dump truck. The rear axle we ighed 18,000 lbs which is close to the legal limi t on Texas State roads. The truck ~as driven at approximately 15 mph over each instrument . The resulting deflections for the 1991 and 1992 section are shown in Figures l! and l}, respectively. Each line represents the deflecti ons measured at different depths ~ith i n the structure. The smal l peaks at approximately 200 ms represent the steering axle load and the larger peaks represent the f~11y loaded rear axle. The difference bet~een the two pulses, marked "AM and -8", can be used to calculate the strain induced in each layer and, hence, get an indication of the stiffness of each layer. From Figures 14 and 15, the following can be impl ied :

1) The A value is substantially smaller in the 1992 test section . This indicates that the stiffness of the 1992 base is higher than the 1991 base,

2) The 8 value is substantially higher in the 1992 test section. This implies that a ~eak layer exists at the top of the subgrade just beneath the gypsum{ bottom ash experimental base.

Such weakness could be temporary in nature and caused by prolonged, excessive rainfall. Similar indications were found from an analysis of the FWD data.

In testing each of these sections 50 passes of a fully-loaded truck ~ere made over each section and the following information ~as collected:

I} The change in maximum deflection with number of repetitions

2) The accumulated permanent deformation after each pass (This is measured by the final asymptote of HOD 1. This is the residual deformation remaining once the load has passed. In both cases the deformations were close to zero).

3) The change in shape of the deflection bo~l. Stabilized layers typically are designed to spread the load over a wide distance . However, if the layer should crack under load, the shape of the

21

The SCt is defined as the change 1n deflection (in mils) from the maximum (i.e. when the MOD is directly under load) to the deflection measured when the load i s 1 foot from the HOD . The measured change in maximum deflection with number of repetitions 1s shown bel ow.

Max Deflection (mils)

Number Passes 1991 Section 1992 Section

1 16 .66 21. 2

10 15.87 21.9

SO 17.55 21.5

No significant changes in deflection were observed over the 50 passes. ·The variations indicated in the table were probably attributed to wander of the test truck (i.e. wheels not being directly over the sensor) . It can be concluded that no significant reduction in strength has occurs over the life of the two test sections. The fUnction of a base course 1s to spread the pavement loads so that no permanent deformations occur 1n the subgrade layer . The accumulated sub9rade strains were monitored by observing changes in MOD 2 which was located at the top of the subgrade. The measured deformations are tabulated below .

Accumulated Deformation (mils)

Number of Passes 1991 Section 1992 Section

1 .016 .01

10 . 041 .08

SO .038 .17

Total accumulated permanent deformations after 50 passes for the 1991 and 1992 sections were 0.038 mils and 0.17 mils. respectively. The top of the subgrade on the 1992 site was known to be weak (wet) during this testing . The fact that only 0.17 mils of deformation was induced 1s In indication that the base is doing a good job of spreading the load and .1nimizing subgrade da~age.

The change 1n Surface Curvature Index (SCI) over the 50 passes is tabulated below:

22

SCI mil s 1

Number of Passes 1991 1992

I 5 .31 2.17

10 4.83 2 . 13

50 5 . 57 1.77

If I layer within the pavement cracks it 1s expected that the SCI values will become significantly larger . The results shown above i nd icate that the t ruc k loads employed in this test did not induce any cracks 1n the stab ili zed layers .

From the above discuss ion, it can be conc luded that both the integr i ty of roadbase sections 1s excellent and neither had sustained any noticeable damage during the application of the 50 load passes.

ENVIRONMENIAl IMPACT ANALYSIS

Throughout the course of the program surface water and so i l leachate analyses were conducted on samples generated 1n the vicinity of the job site . In add i tion, all mix ingredients were analyzed to their respective leachate characteristics and compared with EPA Standards for drink i ng water and TClP . The results are shown in Tables 4 and 5. None of EPA ' s allowable heavy metal concentrati ons were exceeded. As expected, sulfate concentrations did exceed the EPA drinking water standard. EPA does not regulate TClP concentrations on sulfates .

CONCLUSIONS

The mix design estimate, construction procedures and post construction evaluation of stabilizer gypsum roadbases was presented . It was shown that when test procedures, ~aterials selected and placement methods are used which are compatible for strength . Development in FGO by·product gypsum mixtures that they will perform adequately as roadway materials . The successful performance of two experimental test sections monitored using state-of-the-art pavement performance evaluation technology would appear to support these conclusions. Based on analysis of mix ingredients and ground water. The conclusion reached after one year is that the stabilized gypsum road base creates a negligible environmental impact, except for areas which could affect drinking water .

1m I •

I .' I .' I k9

CONVERSION FACTORS (ASTM £380-92)

• 0 .039 in . • 3 . 28 (t.

10 .76 fe 1. 307 yd'

• 2 . 2 lb .

23

1 I 1 "Pa 1 GPa 1 kg/ m'

• 0.26 gal . • 145 psi

0.145 x 10' 1.685 lb/ yd1

Table 4. leachate Characteristics.

1:20 l eachate, 24 hr Extract ion

Component , EPA EPA TClP Dry Bottom Rockdale Cement Wet Bott OM Job Hi x Drinking Standards Ash Gypsum Ash Wiler (og/ l) Standards !!!!Il li

As 0.05 5.0 <0.01 <0.01 <0.01 <0 .01 <0 .01

N 8. 1.0 100.0 <0. 50 <0.50 <0.50 <0. 50 <0. 50

~ Cd 0.01 1.0 <0 .01 <0.01 <0.01 <0 .01 <0 .01

Cr 0.05 5.0 <0.05 <0.05 0 . 125 <0.05 <0.05

Pb 0.005 5.0 <0.05 <0.05 <0.05 <0 .05 <0 .05

H9 0.002 0 . 2 <0.001 '<0.001 <0.001 <0.001 <0.00)

Se 0.01 1.0 <0.01 <0.01 <0.01 <0.01 <0 .01

A9 0.05 5 .0 <0.05 <0.05 <0.05 <0. 05 <0.05

S Totll Sulfate Sulfur, ~

S04 O.OlS 0 .055 18 . 84 1.18 0.095 10 . 95

Table 5. Soil leachate and Surface Water Analysis for November 1992 and February 1993 .

EPA November, November February February Drinking 92 92 93 93 Standards (I Surface AI (I

Water

Constituent (mg/ l ) (mg/ l ) (119/1 ) (119/1 ) (og/ 1 )

Arsenic 0.05 <0 .01 <0 .01 <0.0) <0. 01

Barlull 1.0 <0. 50 <0 .50 <0 .50 <0. 50

Cadmium 0.01 <0.01 <0.01 <0.01 <0 .01

Chrolliull 0.05 <0.05 <0 .05 <0 .05 <0. 05 N ~ lead 0.05 <0.05 0.38 <0. 05 <0. 05

Mercury 0.002 ( 0.001 <0.001 <0. DO} <0. 001

SeleniUM 0.01 <0.01 <0 . 01 <0.01 <0.0 ]

Silver 0.05 <0.05 <0.05 <0.05 <0.05

Copper 1.0 <0 .05 <0.05 0.05 <0 .05

A I ulllinum 0.2 <0. 50 <0.50 <0.50 <0. 50

Iron 0.3 <0.04 0 . 18~ O. J8 <0.04

Manganese 0.05 0.031 0.041 0.02 <0.02

Zinc 5.0 0.024 I. 91 0 .026 0.01~

SuI fate 2 SO 0.81 1.32 2.98

Tota l Dissolved 500 2556 ~5.5 Solids

REFERENCES

1. Steffan, P. and Golden, D., "FGO Gypsum Utilization : Sur .... eyof Current Practices and Assessment of Market Potentia'~, 2nd Internationa' FGO Gypsum Utilization Conference, ORTECH Internat ional , Toronto, Canada, May 1991 .

2. C. A. Gregory. D. Saylak and W. B. ledbetter. "The Use of By· Product Gypsum for Road Bases and Subbases", Presented at the Transportation Research Board Meeting , Washington, O. C. , January 1984.

3. D. Little and W. W. Crockford, "Stabilization of Calcium Sulfate", Final Report for the Gulf States Materi als Company by the Texas Transportation Institute, Texas AlK Uni .... ersity, May, 1987.

4. , Project No. 1690 to Texas A&H De .... elopment Foundation from Texas Utilities Generating Company, 1985 .

5. R. A. Taha, ·Utilization of By·Product Gypsum in Road Construction" , Ph .D. Dissertation. Texas A&H Uni .... ersity, December, 1989 .

6. R. Taha, M. Olson and D. Saylak, ·The Utilization of Flue Gas Desulfurization Gypsum in Low Volume Road Construction, Procs ., of the Fifteenth Biennial Low· Rank Fuels Symposium", The Energy and Mineral Research Center, St. Paul, Minnesota, May , 1989 .

7. EPA Controlling Sulfur Oxides, EPA·600/ 8·80·029, August , 1980.

8. Mr . Dale Junghans, Personal Communication, Gulf States Materials, Inc ., La Porte, Texas, July 16, 1990.

9. Mr . Neal Anderson, Personal Communication, Mob i l Chemical Company, Paudena, Texas, March, 1990

10. C. A. Gregory, W. B. Ledbetter and O. Saylak, "Construction and 1n1t1al Performance E .... aluation of Stabilized Phosphogypsum Test Sites", La Porte, Texas, Report for Mobil Chemical Company by the Texas Transportation Institute, May, 1984 .

11. D. Saylak, R. Taha and D. Little, "Recommended Procedures for Sample Preparation and Testing Stabilized Gypsum Mixtures, Procs., of the Second International Syaposiu~ on Phosphogypsum", The Florida Institute of Phosphate Research, MiAmi, Volume 2, January, 1988 .

12. ·Beneficiation of Waste Calcium Sulfate·, TEES Study No . 32131· 70500·C[ sponsored by Texas Higher Education Coordinating Board (Dr. D. Saylak, P.I . ) (on·go1ng).

13. C. Wong and M. K. Ho, ·The Performance of Cement · Stabilized Phosphogypsum as Base State on Highway 146", La·Porte, Texas, State Department of Transportation, Research Section, Austin, Texas, October, 1988.

26

14. D. Hunter, "lime · Induced Heave in Sul fate·Bearing Clay Soils", Journal of Geotechnical Engineering , ASCE, Volume 114. No.2, February, 1988.

15 . Chang , W. F_ and Hantell. H. I . • -Engineering Prooerties and Construction Applications of Phosphogyosum· . University of Hiami. Precs, Coral Gables. Flor ida, 1990 .

16. • ". Utilization of FGD Gypsum and Bottom Ash in Roadway and Building Construction", Texas A&H Research Foundation Contract with the Electri c Power Re search Institu te. Project RF 1211 (on-going).

11 . Uzan, J., Scullion, T., Parades, H. -A Microcomputer Based Procedure to Back-calculate l ayer Mod uli from FWD Data ", TT l Report 1123- 1. Texas A&M University, September 1988 .

18. Scullion, T., -Field Evaluat ion of the Multidepth Deflectometer", TTl Report 1123-2, September. 1989.

27