Structural Deterioration of Test Pavements: Rigid

F . H . SCRIVNER, Rigid Pavement Research Engineer, AASHO Road Test^

• This material was extracted from AASHO Road Test Report 5 (HRB Special Report 61E) and describes some of the deterioration observed in the portland cement concrete test sections. Well over half of the sections under traffic ex­hibited very little change in condition during the two years of traffic testing. Thus, the data are necessarily restricted largely to the thinner sections in each loop.

For those not familiar with the symbols used in Report 5, the following brief definitions may be helpful:

p = "present serviceability index," a meas­ure of pavement condition depending on surface roughness, cracking and patching.

W = number of axle applications applied to a pavement.

L , = axle load in kips. C = "cracking index," a measure of the

amount of cracking in the pavement surface. "Pumping index" = an approximation of the

accumulated volume of material ejected by pumping action from beneath the pavement and deposited along the edge.

"Design 1 sections" are the group of test sections that furnished the data for the princi­pal Road Test equations relatmg pavement per­formance to pavement design and load.

"Design 3 sections" are a group of test sec­tions involved in a special experiment intended to relate the performance of sections on 6 in. of subbase to that of sections without subbase.

Many of the data were accumulated during the course of weekly inspections made of each of the 368 rigid pavement test sections during the 2-yr period of traffic testing. Pumping data were gathered during inspections of the traffic loops made after each rainfall.

PUMPING

The term "pumping," unless otherwise noted, will refer to the ejection of material from be­neath the pavement and its deposition along the pavement edge. Although fine material often was washed upward through joints and cracks, the amount of material transported in this manner at the Road Test was negligible when compared with the amount deposited along the edge.

' Now, Research Engineer, Texas Transportation Insti­tute, A&M College of Texas.

In cases where pumping occurred in sections with subbase, the material ejected was the sub-base material (usually including the coarse fractions) rather than the underlying soil. Thus, pumping of embankment soil was con­fined to those sections without subbase.

Severe pumping occurred in all loops, but was restricted to the first and second levels of slab thickness in each loop. Some pumping oc­curred in all sections except in Loop 2 none of the pavements subjected to the 2-kip single axle load showed any evidence of pumping.

Occasionally, a considerable volume of the subbase material was deposited along the edge of the pavement in a relatively short period of time. For example. Figure 1 (upper right) shows a windrow containing approximately 3 cu yd of material which was pumped out over­night and deposited adjacent to one 40-ft panel. Other examples of pumping are shown.

Following each rain, all sections under traffic were inspected for signs of pumping, and a rough estimate was made of the volume of ma­terial deposited along the pavement edge. After the inspection, the pumped material was removed from the vicinity of the pavement edge.

From the results of these surveys, a "pump­ing index" was computed. This index approxi­mated the accumulated volume of material ejected per unit length of pavement—averaged over the length of the test section. The dimen­sions chosen for the index were cubic inches of material pumped per inch of pavement length. Thus, a pumping index of 100 cu in. per in. is equivalent to 2.6 cu yd of material pumped from beneath 100 ft of pavement.

A portion of the data is given in Table 1 where the pumping index corresponding to a serviceability level of 1.5 is given for each De­sign 1 section that fell to that level, or at W = 1,114,000 unweighted axle applications if the section survived the traffic test. The average pumping index for failed sections at p = 1.5 was 134; the average for surviving sections at W — 1,114,000 was 34. There was not a clear-cut definition of the value of the pumping index associated with the serviceability level of 1.5. For example, one section failed with a pumping index of 5 while another survived with a- pump­ing index of 209 (Table 1). Nevertheless, the fact that pumping was a very important factor

186

T A B L E 1

P U M P I N G I N D E X AT p = 1.5' OK W = 1,114,000, E X P E R I M E N T D E S I G N 1=

Pumping Index Axle Subbase Load

(kips) Tn^T, T„o^ Ti„„^r.^c. 2.5-In. 3.5-In 5.0-In. 6.5-In. 8.0-In 9.5-In. 11.0-In. 12.5-In. 1.00P Load Ihickness Surface Surface Surface Surface Surface Surface Surface Surface

40T

30S

48T

R N R N R N R N R N R N R N R N

0 3 6 0 25* 11* 8 20 4 4 3 5* 17 10

12 7 5 6

6 6 11 34 7 13 2 2 3 315* 62* 109

90 53 17 22

19 7 18

6 214* 102* 211* 83 63

18 24

17 15 18

9 204* 149* 69 88 12 17 18 18 3 92* 37* 86*

118* 73* 52 37

22 18 24

6 69* 76* 82* 65 106*

51 87

24 23 31

9 88* 103* 101* 146* 45 21 30 27 3

6

189*

116*

191*

91*

47 72 92

48

29 57

19

19 22

24 20 24

13

5

16

20

9 98* 147* 117 209 18 21 6 16 3

6

216*

202*

202*

101*

89 152* 210*

50*

112 86*

26

41 39

35 29 39

34

12

53

28

9 75* 118* 116* 178* 32 30 11 27 3 207* 133* 146*

32 33 27 22

27 11 23

6 104* 301* 63 47 97

20 31

52 18 2

9 193* 203* 79 122* 16 28 4 3 3 91* 108* 127*

35 37 38 29

31 17 35

6 123* 111* 210* 67 47

61 66

113 22 0

9 77* 114* 142 98* 27 84 12 12 3 122* 150* 18

83 32 19 15

19 4 22

6 237* 159 45 29 52

27 31

20 6 20

9 237* 168 120 59 22 12 1 3 3 95* 164 52

44 185 26 25

22 6 53

6 208* 133 41 36 83

60 26

21 20 46

9 123* 105* 228* 40 86 24 3 22

2S

6S

12 S

24T

118* 22 <

18S 3 189* 191* 47 48 19 24 13 16 g w H

H 32T 3 216* 202* 89 50* 26 35 34 53 W

o w

22.4S 3 207* 133* 146* 33 27 22 11 23 g

' Values with asterisk are for p = 1.5. g ' R = reinforced; N = nonreinforced.

C O N F E R E N C E ON T H E AASHO ROAD T E S T

Void in shoulder through which subbase material was ejected from beneath pumping slab.

Subbase material ejected from beneath pavement over­night and deposited along edge.

1 . . . . . .

I-:.

1 Typical pile of subbase material pumped from beneath Embankment material pumped from beneath a pavement pavement, showing change in gradation of material from constructed without a subbase.

fine to coarse in direction of traffic, toward reader.

This void beneath pavement extended more than 5 ft from edge.

Transverse cross-section of a pumping slab showing void beneath pavement.

Figure 1. Examples of pumping.

P A V E M E N T P E R F O R M A N C E 189

T A B L E 2

P U M P I N G I N D E X ' FOR S U R V I V I N G S E C T I O N S . E X P E R I M E N T D E S I G N 1,W 1,114,000

Axle Axle Pumping Index

Type Load 2.5-In. 3.5-In. 5.0-In. 6.5-In. 8.0-In. 9.5-In. 11.0-In. 12.5-In. Slab Slab Slab Slab Slab Slab Slab Slab

Single 2 6 2 1 11 4

12 — 7 9 18 16 18 — 84 21 13 22.4 — 5 9 2 8 10 3 0 164 55 2 1 9

Tandem 2 4 — 65 4 2 2 6 3 2 — 101 34 2 8 4 0 — 66 5 6 16 4 8 149 69 3 6 2 5

*Each value is average for one to eight sections; replicate sections included in averages.

in the performance of rigid pavement sections is demonstrated by the survival curve (Fig. 2).

No consistent trend was found relating the pumping index to subbase thickness, and no significant difference was found between rein­forced and nonreinforced sections. The pump­ing index for surviving sections at the end of the traffic test, however, did show a trend with slab thickness and load. As the data in Table 2 indicate, the amount of material pumped from beneath the same thickness of slab usually in­creased as load increased and under the same load decreased as slab thickness increased.

The permeability of the subbase material, ac­cording to tests made in the project laboratory on 24 specimens-molded at a variety of mois­ture contents and densities, ranged from 6.9 X 10-» to about 8.3 X 10-= ft per min. These tests indicated that within the range of sub-base densities determined in the course of sev­eral trench studies conducted in the traffic loops the permeability of the subbase probably did not exceed 3.5 X 10-' ft per min and may have been lower. Thus, the estimated range of permeability of the subbase in place was, in round numbers, 7 X 10-« to 4 x 10-̂ ft per min. The grading of the material is given in Table 3.

T A B L E 3

GRADATION OF S U B B A S E M A T E R I A L

Size Percent Passing

1-in. sieve J^-in. s i e v e . . . . }4-in. sieve No. 4 s i e v e . . . . No. 40 sieve . . . No. 200 sieve.. Plasticity mdex, minus No. 40 material Max. dry density' (pcf) Field density* (% max. dry dens.) . . .

100 96 90 71 2 5

7 N.P. 138 102

100

60 120 180 2 4 0 PUMPING INDEX, CUBIC INCHES PER INCH

300

» A A S H O T 9 9 - 5 7 . ' Before subgrading.

Figure 2 . Estimated probability that a test section with the indicated pumping index will survive 1,114,000

axle load applications.

190 C O N F E R E N C E ON T H E AASHO ROAD T E S T

Regardless of the permeability of the sub-base it was observed that free water collected under the slab during rains, and that this water did not drain laterally through the subbase material in the shoulder to the side ditches at a rate sufficient to prevent pumping. It was further observed, by means of removing the concrete from a few failed sections and sam­pling the underlying material, that subbase material had apparently been removed by the erosive action of water moving across the top of the subbase, and that the remaining subbase material was relatively undistributed. There was no evidence, either from gradation tests or from visual inspection, that fines from the em­bankment had entered the voids in the subbase layer.

The fact that the great majority of the sec­tions which failed pumped severely prior to

failure leads to the conclusion that had the sub-base material been stabilized in a manner effec­tive in resisting erosion by water, some of the sections which failed would have survived the two years of traffic.

F A U L T I N G , BLOW-UPS Faulting at cracks sometimes occured in the

later "stages of pavement deterioration, but faulting at joints was notably absent through­out the project. One transverse joint faulted seriously, but investigation showed that the joint had been accidentally sawed at some dis­tance beyond the end of the dowels intended to protect it. Over the 2-yr period of the test there were no other cases of measurable fault­ing at joints, all of which were doweled.

The rigid pavements were constructed with-

Figure 3. Examples of the four classes of cracks in rigid pavement at the Road Test: upper left, Class 1; upper right, C 2; lower left. Class 3; lower right. Class 4. Only Class 3 and 4 cracks entered into the determination of the serviceability in

P A V E M E N T P E R F O R M A N C E 191

out expansion joints except at approaches to the test bridges in Loops 5 and 6. One blow-up occurred in Loop 2 at the transverse joint at one end of a structural section with a slab thickness of 2.5 in. At both ends of this section the abutting sections had a slab thickness of 5 in.

Two other blow-ups occurred at transverse joints—one after one-half (12 ft) and the other after three-quarters (18 ft) of the pave­ment adjacent to the blow-up had been removed in the course of maintenance operations.

C L A S S I F I C A T I O N OF CRACKS Cracks were divided into four classes, de­

pending upon their appearance (Fig. 3), as follows:

Class 1 included fine cracks not visible to a man with good vision standing at a 15-ft distance.

Class 2 cracks were those that could be seen at a 15-ft distance but which exhibited only minor spalling such that the opening at the surface was less than 14 in.

A Class 3 crack is a crack opened or spalled at the surface to a width of 1/4 in. or more over a distance equal to at least one-half the crack length, except that any portion of the crack

opened less than 14 in. at the surface for a distance of 3 ft or more is classified separately.

A Class 4 crack is any crack which has been sealed.

During each weekly survey, maps were pre­pared showing the location and classification of each crack, as well as the length of its projec­tion parallel or perpendicular to the centerline of the pavement, whichever projection was greater. Crack lengths in each section were totaled by classes, divided by the area of the pavement, and recorded each index day in units of feet of projected cracks per 1,000 sq ft of pavement surface. Class 3 and Class 4 cracks entered into the determination of the service­ability index of each test section, as described in Section 3.2.1 of Road Test Report 5.

CRACKING I N D E X A statistic useful in studies of cracking apart

from the pavement serviceability concept is the total projected length of all visible cracks, in feet per 1000 sq ft of pavement area, repre­sented by the symbol, C. (In arriving at the value of C for a section that had been patched, the patched area was assigned the cracking equivalent of 1 ft of crack for each square foot of patch.) Figures 4 and 5 show the relation-

CRACKING INDEX. 28 FT PER 1000 SQUARE FEET SERVICEABILITY INDEX 4 3

JUNE 17.1959

1 1 CRACKING INDEX.88FT PER 1000 SQUARE FEET

SERVICEABILITY INDEX 4 I JULY 15.1959

CRACKING INDEX, 152FT PER 1000 SQUARE FEET SERVICEABILITY INDEX 3 4

SEPTEMBER 9,1959

MTCHED AREA

CRACKING INDEX,233 FT PER 1000 SQUARE FEET SERVICEABILITY INDEX I 8

SEPTEMBER 28. 1959

DIRECTION OF TRAFFIC

Figure 4. Progression of cracking in a 3.5-in. nonreinforced section with paved shoulders on 6.0 in. of subbase, 24-kip tandem-axle load.

192 C O N F E R E N C E ON T H E AASHO ROAD T E S T

Y

CRACKING INDEX, 9 FT PER 1000 SQUARE FEET SERVICEABILITY INDEX 4 0

MAY 23. I960

CRACKING INDEX, 42 FT PER 1000 SQUARE FEET SERVICEABILITY INDEX 3 2

JUNE 16, I960

CRACKING INDEX. 62FT PER 1000 SQUARE FEET SERVICEABILITY INDEX 2 9

JULY 25, I960

CRACKING INDEX, 92 FT PER 1000 SQUARE FEET SERVICEABILITY INDEX I 5

AUGUST 1.1960 DIRECTION OF TRAFFIC .

Figure 5. Progression of crackmg in an 8.0-in. nonreinforced section on 3.0 in. of subbase, 30-kip single-axle load.

ship between changes in the value of C and changes in the general appearance of the pavement as cracking progressed with load ap­plications and age. Also, different values of C may occur at about the same serviceability level. For example, when the 3.5-in. pavement (Fig. 4) was nearing failure (p = 1.8) C had the value 233, while at p = 1.5 the 8-in. pave­ment (Fig. 5) had a cracking index of only 92.

Values of C were determined for each sec­tion in the traffic loops on each index day. These data in part are given in Table 4, where C i^ given &i W = 1,114,000 unweighted axle applications for all sections in the main fac­torial experiment that survived the traffic test and sXv = 1.5 for all sections that failed before the end of the traffic test. Table 5 is similar except that C is given at a serviceability level of 2.5 for sections that dropped to that level.

The data in Table 4 permitted 27 independ­ent comparisons between reinforced and non­reinforced section at p = 1.5. From Table 5, 29 such comparisons could be made at p = 2.5. From either table, the relevant data being the same, 77 comparisons could be made at the end of the traffic testing. A summary of the results of these three sets of comparisons—each of which included a test of the statistical sig­nificance of the difference in cracking between

reinforced and nonreinforced section^—is giv­en in Table 6.

From the averaged data in Table 6, the cracking in the reinforced sections with 40-ft panel lengths exceeded that in the nonrein­forced sections with 15-ft panel lengths by 20 to 24 ft per 1,000 sq ft of pavement area. How­ever, the difference was not significant at V = 1.5, and was significant only at the 10 percent level at p = 2.5. At the end of the traffic test, when the average serviceability level of the 154 sections involved in the com­parisons was 4.15, the average difference in cracking was highly significant, although the average values of C were small (30 and 6).

The fact that cracking in surviving reinforced sections was frequently greater than in non­reinforced sections (other factors being equal) [ appears to confirm the usual assumption made by concrete pavement engineers that tensile stresses occurring during periods of decreasing temperature tend to increase with panel length. However, no cracking occurred in the Loop 1 pavements not subjected to traffic; thus, none of the cracks appearing in the traffic loops can be attributed solely to environment (temper­ature changes, moisture changes, subgrade restraint, etc.).

The average value of C at p = 1.5 for the 67|

P A V E M E N T P E R F O R M A N C E 193

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194 C O N F E R E N C E ON T H E AASHO ROAD T E S T

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P A V E M E N T P E R F O R M A N C E 195

T A B L E 6

COMPARISON OF C R A C K I N G I N D E X , C , FOR R E I N F O R C E D AND NONREINFORCED S E C T I O N S

Item Number of Pairs

of Sections

Number of Cases Average C (ft/1,000 sq ft) Item

Number of Pairs

of Sections

C Eeinf. C Reinf. >C' Nonreinf. < C ' Nonreinf.

Reinf. Sections

Nonreinf. Sections Diff.

Comparison at p = 1.5 27 13 14 182 161 21' Comparison at p = 2.5 29 19 10 114 94 20' Comparison at

W = 1,114,000' 77 68 9 30 6 24*

'Not significant. ' Significant at 10 percent level.

' Average p = 4.15. * Significant at 1 percent level.

failed sections in the main factorial experiment (Table 4) was 168, and the standard deviation of individual values of C about their mean was 71. The average value of C at p = 2.5 for the 73 sections which dropped to that level (Table 5) was 103, and the standard deviation was 42.

From the average values of C given in the preceding paragraph and in Table 6 it was con­cluded that a value of C = 100 represented a substantial amount of structural deterioration in most cases, and that the relationships be­tween design (including reinforcing), load and load applications at this level of the cracking index would constitute a useful supplement to the performance curves presented in Section 3.2.2.1 of Road Test Report 5.

A N A L Y S I S OF CRACKING I N D E X

Details of the methods followed in analyzing the cracking index will be found in Section 3.2.3.1 of Road Test Report 5. The resulting equations, and the average absolute residuals, are given below:

For single-axle vehicles on nonreinforced pavements:

log W = 4.70 -I- 0.5 log C - 2.62 log L i + 4.84 log I>2± 0.26 (1)

For single-axle vehicles on reinforced pave­ments:

log W = 4.95 + 0.5 log C - 2.30 log L i + 3.57 log D 2 ± 0.17 (2)

For tandem-axle vehicles on nonreinforced pavement:

log W = 6.61 + 0.5 log C - 4.38 log L i + 6.33 log D2 ,± 0.24 (3)

For tandemruxle vehicles on reinforced pave­ment:

log W = 6.37 -I- 0.5 log C - 3.13 log U + 3.96 log D j i t 0.11 (4)

Graphs of these equations for C = 100 (Fig. 6) may be compared with graphs of the per­formance equation given in Road Test Report 5. Such comparisons of the performance with the cracking index equations show varying degrees of divergence between the two. A dif­ference is to be expected since the service­ability index is heavily weighted by the rough­ness of the pavement whereas the cracking index depends solely on the amount of crack­ing and patching. In addition, analytical proce­dures were not the same in the two analyses.

LONGITUDINAL C R A C K S A T T R A N S V E R S E JOINTS

In the course of the weekly crack surveys, the points at which longitudinal cracks inter­sected transverse joints were recorded in an effort to determine whether there was a tend­ency for such cracks to appear more fre­quently at one location than another. Figure 7 shows histograms of the frequency with which longitudinal cracks appeared within each 6-in. interval of transverse joint. These graphs represent only failed sections and, of those, only the thinnest pavement sections in each loop. To permit a comparison between the sections having 15-ft transverse joint spacing with those having 40-ft spacing, the number of cracks actually observed has been converted to the average number per panel. The data are further summarized in Table 7, which shows the average number of cracks per panel with­out regard to the location of the crack-joint intersection.

Figure 7 shows that there was a pronounced tendency in the 2.5-, 3.5- and 5-in. pavements for longitudinal cracks to appear in the outer wheelpath within 3 in. of the third or fourth dowel located 2.5 and 3.5 ft from the pavement edge. On the other hand, this tendency was reduced or absent in the case of the 6.5- and 8-in. pavements. Table 7 shows that the 40-ft panels tended to crack at transverse joints more often than the 15-ft panels.

196 CONFERENCE ON T H E AASHO EGAD TEST

S I N G L E A X L E L O A D S N O N R E I N F O R C E D RAVEMENT

c' - roo I 11 nil 1—r-TTT

E X T R A P O L A T E D CURVE

A R E A O F E X T R A P O L A T I O N

TANDEM A X L E L O A D S

N O N R E I N F O R C E D P A V E M E N T

c'>ioa

S I N G L E A X L E L O A D S R E I N F O R C E D P A V E M E N T

C ' - l O O

A X L E L O A D A P P L I C A T I O N S IN T H O U S A N D S

T A N D E M A X L E L O A D S R E I N F O R C E D P A V E M E N T

C ' - l O O

h-HJ-rrrt-'-t

A X L E LOAD A P P L I C A T I O N S IN T H O U S A N D S

Jt̂ igure 6. Graphs of equations for cracking index.

T A B L E 7

N U M B E R OF L O N G I T U D I N A L C R A C K S I N E N D S OF P A N E L S I N F A I L E D S E C T I O N S S E X P E R I M E N T

D E S I G N S 1 AND 3'

Slab Panels Average Cracks

(in.) Nonreinf. Reinf. Nonreinf. Reinf.

2 2.5 8 12 0.9 1.7 3 3.5 80 60 1.8 3.3 4 5.0 80 60 0.9 1.6 5 6.5 80 60 1.1 0.6 6 8.0 32 48 0.5 0.7 Avg. 1.0 1.6

'Only sections at lowest level of slab thickness in each loop are included.

'Reinforced panels 40 ft; nonreinforced, 15 ft long.

P R I N C I P A L FINDINGS Pumping of subbase material, including the

coarser fractions, was a major factor in the majority of the failures of sections with sub-base. Pumping of embankment material was

confined to those sections constructed without subbase. The amount of either material pumped through joints and cracks was negli­gible when compared with the amount ejected along the edge.

Faulting occasionally occurred at cracks, never at transverse joints (all joints were doweled).

There was a tendency for the cracking (per unit of surface area) in reinforced sections with 40-ft panel lengths to exceed that in non-reinforced sections having 15-ft panel lengths.

No part of the cracking of pavements in the traffic loops was attributed solely to environ­mental changes, since no cracks were apparent in the non-traffic loop (Loop 1).

From cracking data, equations were derived from which the number of axle applications associated with any given level of cracking can be computed for a given pavement design and load. Graphs of the equations for a selected level of cracking are shown.

Longitudinal cracks tended to originate at transverse joints near dowel bars in 2.5-, 3.5-and 5-in. slabs but not in thicker pavements.

O POSITION OF DOWEL BAR

J J

L i I t :

LOOPZ 2 5" PAVEMENT

LOOP 3 3 5" PAVEMENT

LOOP 4 50 '^ PAVEMENT

-TRANSVERSE JOINT

\ PAVEMENT

LONGITUDINAL CRACK

FREE EDGE

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LOOPS 65" PAVEMENT

o o o o o o o o o o o o

o | o | o | o | o | o | o | o | A | o | o | o | 0 1 2 3 4 5 6 7 8 9 1 0 II 12

X, FEET REINFORCED PAVEMENT

LOOP 6 80'n»AVEMENT I 0 1^ I 6 I 0 I 0 I 0 I

0 I 2 3 4 S 6 7 B 9 I 0 I I I 2 X, FEET

NON-REINFORCED PAVEMENT

IFigure 7. Summary of longitudinal cracks intersecting transverse joints of failed sections at time of removal from test. Data from thinnest sections of each loop, Designs 1 and 3. Shaded bars represent cracks intersecting a joint

within 3 in. of a point directly above a dowel bar.