Journal of Applied Geophysics 97 (2013) 27–36

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Journal of Applied Geophysics

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GPR abilities in investigation of the pavement transversal cracks

Lech Krysiński ⁎, Jacek SudykaRoad and Bridge Research Institute, ul. Instytutowa 1, 03-302 Warsaw, Poland

⁎ Corresponding author. Tel.: +48 22 39 00 208; fax:E-mail addresses: lkrysinski@ibdim.edu.pl (L. Krysiń

(J. Sudyka).

0926-9851/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.jappgeo.2013.03.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 October 2012Accepted 23 March 2013Available online 6 April 2013

Keywords:GPRPavement crack identificationGPR indications of cracksBituminous pavementConcrete pavement

This paper describes the results of an investigation into the capabilities of the GPR technique within the fieldof pavement crack diagnostics. Initially, laboratory tests were performed on prototypes simulating idealizedcracks. Next, long-term visual observation and repeated GPR scanning were performed, on three roads ofsemi-rigid construction, several hundreds of meters long and subjected to heavy traffic. Furthermore, aroad of rigid construction was tested, having a more than 70-year history of use. In several cases the crackswere probed by drillings, in order to recognize structures responsible for signal generation, or to explain rea-sons of signal lacking.The main result of this work is a list of GPR indications of cracks, which can be noticed on echograms. It wascreated through a correlation of the visually-observed cracks with the corresponding echograms, with deci-meter accuracy. Several types of GPR responses were classified and linked to possible categories of crackstructures, or to processes associated with the presence of cracks (as crumbling, erosion, and lithological al-terations). The poor visibility of cracks was also studied, due to small crack size, or to the blurred character ofthe damaged area, or else to masking effects related to coarse grains in the asphalt mixture.The efficiency of the proposed method for the identification and localization of cracks is higher when along-term GPR observation is performed.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The cracking that occurs in pavements is a fundamental area ofconcern in the field of pavement use and maintenance. Cracks canrepresent some construction failures or some properties of the con-struction materials, but first of all they are a direct manifestation ofthe fundamental process of road deterioration in result of exploita-tion (e.g. alligator cracking developing along the wheel tracks) andenvironmental influence (like transverse cracking due to contractionduring temperature drops). The possible origins of cracks in pavementsare fatigue, shrinkage, movements of subgrade soil, constructional de-fects and ageing. Occurrence of different types of cracks depends onpavement structure and it is different for rigid, semi-rigid and flexiblepavements.

Loss in bearing capacity and subsidence of the subgrade will lead toslab breakage and single transverse line cracks in rigid structures. Thesecracks will be fine or medium-wide and their edges may stagger in thefailure plane direction. For semi-rigid structures, the phenomenon mayresult in cracks of the same type or in block patterns of longitudinal andtransverse cracks. In flexible pavements the affected area will be dam-aged with alligator cracking in the end.

+48 22 814 50 28.ski), jsudyka@ibdim.edu.pl

rights reserved.

Transverse cracks are generally caused by thermally inducedshrinkage at low temperatures. When the tensile stress due to shrink-age exceeds the tensile strength of the asphalt pavement surface,cracks occur. Shrinkage cracks are most often spaced between 5 and15 m apart and are often single-line cracks when they become visibleat the surface, but may develop into double and branched cracksunder traffic (Vanelstraete and Francken, 1997).

Another type of transverse cracks is reflective cracks. Usually, anearlier existing crack is being reflected in a newer overlay. The phe-nomenon is due to the fact that as a result of various factors theedges of an existing crack are subjected to movements which aretransferred to the bottom of overlay, where they induce a concentra-tion of stresses.

Considering the mechanism of transversal cracks development,the types of cracks can be as follows:

• crack with initial vertical propagation — fine or medium-wide,curved, with full bounding between layers, and

• crack with initial horizontal propagation — wide and straight, withdebounding effect occurring with the crack.

GPR is considered to be a viable method of inspection for the pur-poses of studying the mechanisms of cracking within pavements. Themethod is of some use in estimation of crack's propagation depth orlocalization of hidden cracks (Birtwisle and Utsi, 2008; Saarenketo,2009). In particular cases, it helps to clarify reasons of cracking and

28 L. Krysiński, J. Sudyka / Journal of Applied Geophysics 97 (2013) 27–36

to choose propermethods for road treatment. Nevertheless, detailed re-searches on GPR response to cracks in bituminous pavement which tryto formulate explicitly diagnostic criteria (Diamanti et al., 2010) or tostudy the relation with the real crack structure are not numerous(Al-Qadi et al., 2009).

Two general approaches can be distinguished in GPR crack inspec-tion: profile scanning and pseudo three-dimensional (3D) imaging.The profile scanning uses echograms corresponding to verticalcross-section along the profile as the main method of visualization(Saarenketo and Scullion, 2000). The large singular structures (likecracks or local inhomogeneities) inside locally planar and horizontalroad structure are manifested on echograms as diffraction hyperbo-las. The lateral continuation of the structures, being an important fea-ture of cracks, distinguishing them from local inhomogeneities of themedium, can be studied using several mutually parallel profiles. Thetransversal cracks (perpendicular to the road direction) were investi-gated here, as particularly convenient ones for profile scanning. Themethod can be applied to other common crack types (longitudinalcracks or not very dense network) as well. From the profile scanningpoint of view, it is important that crack should be perpendicular tothe profile and then the electric field polarization should be parallelto the crack. Such configuration is necessary in this approach.

The pseudo 3D imaging needs dense data acquisition on the inves-tigated surface area and uses special methods of spatial synchroniza-tion of the large data set (Benedetto et al., 2012). As a result, pseudo3D imaging also allows for the fine visualization of data in the form ofhorizontal slices, essentially enhancing study of the lateral continuationof the singularities of the medium structure (Krysiński and Sudyka,2012a). These advantages are specially useful in studies of systems hav-ing complex geometry, like network of expansion joints in concretepavement. Although the idea of this approach is formally more generaland it has potentially larger possibilities of lateral scan correlation thanthe profile methodology, the 3D imaging has also some practical disad-vantages related to the current state of resolution, efficiency of data col-lection and signal processing quality. Thus the present paper, which isdedicated to detail study of the signals generated by cracks, was basedmostly on profile measurements and the 3D imaging is only mentionedhere.

The use of GPR technique for investigation of pavement transver-sal cracks seems to be promising, but numerous paradoxes occur.Sometimes, well visible cracks more than ten millimeters thickdoesn't generate noticeable GPR response, while ones being thinnerat the surface may produce strong response, but located more deeplyinside the pavement. Thus, some general questions arise. When arecracks detectable by GPR? Which crack features can be extractedfrom GPR images? What can GPR technique tell about cracks? Para-doxically, the object of investigation (i.e. internal crack's structure)is usually very difficult for direct examinations in drillings and out-crops. The most interesting cracks are those occurring in roadssubjected to heavy traffic, where possibilities of drilling are verylimited.

The main aims of the present study are related to the real cracksoccurring in pavements; in particular, the following objectives arepursued:

• recognition of GPR manifestations accompanying visible cracks,with special attention to cases of weak GPR manifestation,

• determination of echogram features types that can be helpful incrack identification or investigation,

• recognition of geometrical and material features of drilled cracks,which can be responsible for generation of a distinctive diffractedsignal, and

• recognition of conditions that can be responsible for the lack of signalor for difficulties in crack detection.

The described problemswere studiedwithin long-termobservation.Repeatable GPR measurements, careful echogram analysis and some

drillings were performed, on several road sections. These field observa-tions were supported also by laboratory tests and some numericalsimulations.

2. Material and methods

The detailed study of the GPR signals characteristic for real cracks,conditions of generation of these signals, and visibility and features ofthe crack structure responsible for the signal generation and visibility,were themain aims of the presentwork. Thus, the high resolutionmea-surementsweremade using High Frequency (2.2 GHz), air-coupled an-tenna collecting data along profile with high density (preferably 50scans/mormore in some cases) and using at least three parallel profiles.Further measurements were performed, using antennas with a lowercentral frequency, for comparison.

2.1. Field GPR measurements and synchronization of GPR data

InGPRmeasurements of roads some specific problemswith distancecorrelation occur, which are related to large length of the profiles, largevelocity of the measuring vehicle, difficulties connected with heavytraffic, latency of the measuring system and parallax effect. The GPRequipment registers its own distance related to rotation of the vehiclewheel, and the position of some anchor points (cracks or some refer-ence objects) can be assigned manually on the echogram during mea-surement. But the effective imprecision of the anchors location interms of the GPR distance is of about several meters (up to 10 m andnever less than 1 m). Difficulties may arise, when we try to confirmthe presence and position of a visible crack on the relevant echogram.In practice, a plenty of different situations occur with visible cracksinterpretation: cracks visible on the surface sometimes have clear GPRresponse, they can have weak response, being interpretable only be-cause of their visibility on the surface, frequently they have no clear re-sponse distinguishing them from the surroundings, etc. Similar problemoccurs when visible GPR manifestations of different intensity do notcorrespond to any visible cracks. Standard GPR anchor points are notsufficient for the identification of crack position or confirmation of itsidentity on echogram. For investigation of crack GPR response, at leastdecimeter precision is necessary.

Several methods of distance correlation were applied in this study.The basic method consists in precise measurement of the distanceswhere the cracks occur along the measuring profile with the use ofother equipment like the measuring wheel or profilograph. The newdistance usually differs from the GPR distance and the differencevaries along the profile. For this reason, the calibration function hasto be constructed by using these rare cracks (or other reference ob-jects), which are well manifested on the echogram.

The main part of the present study consisted of a long-term obser-vation of 6 roads of semi-rigid or rigid construction using GPR scan-ning and visual observations. The echograms were correlated withvisual observations with decimeter precision to find exact spatial cor-respondence between cracks and their potential but not ever visiblemanifestations. The recognized GPRmanifestations allowed to formu-late classification of echogram diagnostic features (section 4). Theclassification is first of all a necessary language for expression of ob-servations being a consequence of the extensive echogram analysis.The proposed classification (name system) can be useful in communi-cation, but it is also helpful in practice, when echogram analysis isdirected to crack identification and investigation, especially in casesof weak crack manifestation. Some provisional scale of the manifesta-tions intensity was defined for purposes of statistical analysis and cor-relation with visual cracks intensity. The list of diagnostic features andintensity scale were tested again to check their efficiency in crack iden-tification on the base of echogram analysis in the investigated 6 roads.There was also one additional, successful test showing a possibility ofidentification of an invisible crack in concrete road covered by a new

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asphalt overlay (Krysiński and Sudyka, 2012b; Sudyka et al., 2011).Some representative cases of cracks were drilled to link GPRmanifesta-tions with structural features of the crevice and to recognize phenome-na related to the long-term activity of the crack.

2.2. Investigated areas

We investigated three road sections, hereinafter referred to as A1,A2, and A3, belonging to a 70-year old road made of cement concreteslabs, 20–22 cm thick. Such slabs consist of two layers carried out in“wet-on-wet” technology. The rare, dispersed reinforcement wasapplied locally only at the base of the lower layer. The slabs areusually 10 m long and dowels are applied on the middle level atexpansion joints, where rests of wood-casing boards can be observed.The construction was built on sand basement. The road is interesting,because it shows a slowly evolving deterioration effects of differentkinds in concrete slabs, which at present are fairly advanced becauseof the long history of the road. Deterioration of the asphalt layers re-lates to the mechanical activity of joints and intra-slab cracks, forwhich various treatment technologies are being tested.

• In the section A1, the slabs were covered by two wearing layers(older and newer) of total thickness of 5–7 cm and the shallowtransversal pavement notches 1 cm thick were carved both at ex-pansion joints and at intra-slab cracks in the newer cover.

• In the section A2, the slabs were covered by one wearing layer ofthickness of 3–4 cm and the transversal pavement notches werecarved at expansion joints only.

• Section A3 has uncovered concrete surface available for direct ob-servation and only local repairs were applied there.

In some places, on heavily damaged expansion joints and cracks,surface patching was applied in the upper part of the concrete slab.The patches are clearly visible on the echogram despite being coveredby latter overlay.

Furthermore, we investigated three more sections, hereinafter re-ferred to as B, C, and D, representing semi-rigid constructions of dif-ferent types and different styles and at different stages of crackingprocess.

• Section B has new, 21 thick package of asphalt layers on the 15 cmthick MCE subbase (mineral cement–emulsion mixture, consistingof recycled asphalt paving with addition of coarse mixed with ce-ment and asphalt adhesive in cold conditions), built on rests ofolder asphalt layer package (4–10 cm of thickness), laying on soilstabilized with cement. The thick but too rigid construction had 7cracks at the middle of observation period and 14 new cracksappeared on this 1000 m long section during the next year.

• In section C — new asphalt overlay, about 10 cm thick, covers olderpavement of variable thickness of 8–15 cm, which lies on 16 cmthick, lean concrete layer. The 1200 m long section has 48 transver-sal cracks and new ones still appear.

• Section D has new 3 layer asphalt package of variable thickness of15–21 cm, built on existing 14 cm thick asphalt layer package,which lies on cement concrete layer about 40 cm thick. The olderpavement had numerous transversal cracks permanently appearingagain despite successive repairs. In the new overlay, only 12 lineartransversal cracks appeared (800 m long section) and this state ofaffairs seems to be relatively stable; no newer cracks appeared intwo last years of observation.

3. Results

3.1. Laboratory tests

The main laboratory model of the crack crevice (Fig. 1 left) repre-sents an idealised form of the crack crevice (filled with a foreign

material) being simulated by an acrylic plate 1.5 cm in width. Thissimplified form generates signal corresponding to scattering on thefiller material only, avoiding formation reflection from the top surfaceof the whole structure, which would introduce additional perturba-tions of the echogram as a result of its removal.

The amplitude of the scattered signal is being compared here tothe levels of noise of at least 3 different kinds.

• Incoherent electromagnetic noise of external origin or related to thesystem own fluctuations, that can be moderated by the use of stack-ing,

• Structural noise related to irregularities of a stochastic character inthe medium being scanned (like masking lattice, section 5) and

• Residual signal of processing procedures removing the maskingbackground.

The two last types of perturbations visible in echograms are usuallydifficult or impossible to be removed. Amplitudes of these three effectsare expressed in some arbitrary units proper to a given GPR system(measuring and processing block) and they can be estimated for differ-ent environments andmeasurement conditions. The idealised laboratorymodel represents extremely purified conditions, which are unavailablein the field situations and such test allows to estimate limits of detectionpossibilities of a given equipment. The results of the laboratory test(Fig. 1 right-hand upper corner) have one important practical conse-quence. According to these observations, the resulting scattered signalthat can be expected in real field situations (moderate permittivity con-trasts, e.g. permittivity ratio less than 3), would have small (not detect-able) amplitude if the crack crevice was of width of about 1 cm or less.Thus, an interesting question arises:what kind of cracks can have notice-able manifestations on echogram and what is the physical reason of thescattering? Some paradoxes occur here, as in some cases field observa-tions demonstrated a significant GPR response at greater depth to crackswhich were thin at the surface and yet in other cases no noticeable GPRresponse was noted for cracks with open crevices several centimetersthick. We investigated this problem in detail.

Numerical simulations are useful in qualitative or semi-quantitativeanalysis of the diffractive signal generated on cracks (Diamanti andRedman, 2012). The example of simulation (Fig. 1 right-hand lowercorner), which comments on the laboratory test, uses single scatteringapproximation. Such modeling allows to study many features of thediffracted signal, which differs visibly from the emitted pulse. Themethod is useful also to analyze the form of echograms correspondingto more complex structures.

3.2. GPR manifestations of cracks

The following classification of echogram features related to cracksis a result of extensive echogram analysis and correlation of theechograms with cracks observed on the road surface. The proposedname system refers to some standard manifestation types commonlyused in echogram description practice, like diffraction hyperbola. Theclassification extends vocabulary to outline the existence of modifiedand compound forms of elementary manifestations as well as a num-ber of essentially different features coexisting with pavement cracks.The systematic classification is useful in practice, because it helps tobring attention to features which can be crack manifestations, espe-cially when the crack is not visible or when these indicators areweak and doubtful. The classification does not have a definitive,closed character, but it is rather a typology which formulates a lan-guage for expression of observations and for discussion of the rela-tions between the manifestation type and the probable mediumstructure.

3.2.1. Elementary manifestationsDiffraction hyperbola (H) is themost elementarymanifestation of the

crack crevice filledwith foreignmaterial (Fig. 1 right-hand upper corner),

Fig. 1. Investigation of the signal scattered by a vertical half-plane, modeling a crack crevice. Left: View of the laboratory system. Right: resulting echogram (upper) and example ofnumerical simulation (lower).

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which would be modeled approximately as a vertical half-plane(Diamanti et al., 2010). Then, the peak point of the diffractive hyperbolacorresponds to the upper edge of the plate. Analogous hyperbola wouldcorrespond to the lower edge of the plate if it existed. Nevertheless, thediffraction hyperbolaswith a clearly defined peak point andwell visiblearms are not very common GPRmanifestations among cracks observedin the field (Fig. 2), with respect to complexity of the cracks structure incomparison to the idealised model. This kind of manifestation is morecharacteristic of expansion joints and reinforcement elements in con-crete slabs.

A supplementary type of elementary manifestation was namedpoint defect (or linear singularity) (D). This feature correlates withvisible cracks (Fig. 3) and as such has been introduced as a necessaryconsideration within echogram analysis. It refers to a localized scatter-ing visible on echogram as a spot or short horizontal line not longerthan 1 m, and it has point character in the sense that this manifestationcould potentially correspond to a formal diffractive point (an object ofsmall diameter in terms of wavelength). This manifestation can be con-sidered as a form of hyperbola reduced to the peak part only. The rea-sons of arms lacking are usually not clear. Sometimes it can be causedby small value of the scattered signal when only the peak part has anamplitude exceeding the noise level. Lack of arms would correspondalso to structures having blurred edges of contrasts decaying whenmoving outside of the centre in horizontal direction. Such structurescan be found in the case of interlayer delamination decaying graduallyat the edges. Delaminations have noticeable coincidence with cracks.It is important that an increased reflection on echogram has diagnostic

Fig. 2.GPR results for road section C: a Christmas tree pattern (Ch) is noticed, consisting of4 hyperbolas (the crack is located at a distance of 3.1 m).

value in pavement crack detection only when it is localized (not longerthan 1 m).

Both types of elementary manifestations would correspond to avery local (point type, i.e. small in diameter) inhomogeneities of themedium. But they give clear suggestion of presence of some structuresingularity of linear shape, if they have lateral continuation (in lateraldirection). Thus, scanning along several parallel profiles placed withina distance of about 50 cm of one from the other was used for tests ofthe method diagnostic ability. It was found that synchronous occur-rence (with distance accuracy to decimeters and at the same level)of a given manifestation on neighboring profiles has strong diagnosticvalue (good correlation with visible cracks). Use of one profile is notsufficient for the purposes of crack identification in general.

Similarly, synchronous occurrence of clear elementary manifesta-tions one above other provides a strong suggestion of some verticalcontinuation of the singular structure. Sometimes these coexistingfeatures enable the formulation of more detailed suppositions as tothe probable depth range and shape of the singular structure. Thereis a strong correlation between vertical groups of manifestationsand cracks. Some representative examples are described below.

Lateral continuation and vertical coincidence of the manifestationsgive strong supposition of crack presence in the case of asphalt pave-ment, because the primary layer structure is relatively simple and sin-gular objects of linear shape are usually not expected there (with

Fig. 3. GPR results for road section C: a vertical group (Gv) of different manifestations isnoticed (the crack is located at a distance of 3.3 m). Somemanifestations can be classifiedas point defects (D, depth 5 and 11 cm). The shape of the most superficial manifestation(at depth 5 cm) is that of a local delamination. Thedeepestmanifestationsweakly resemblethe chi pattern (Chi).

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exception of special roads like in airports). In the case of cement con-crete, joints of different types and some reinforcements are expected;they are usually well visible on echograms and some additional criteriaare used to distinguish them from cracks.

3.2.2. Compound manifestationsThe compoundGPRmanifestations of cracks observed during the in-

vestigations were named in order to outline their diagnostic abilities.Christmas tree pattern (Ch) is the most spectacular example of

crack manifestation. It consists of several diffraction hyperbolas oc-curring directly one above other (Fig. 2). The peaks of hyperbolas cor-respond to the same distance with precision better than decimeterand there is no doubt that they correspond to one singular object ina relatively regular pavement layer system. The diffracting objectcan be treated, in some approximation, as a thin vertical plate cuttingthe system of layers perpendicularly to the profile of scanning. Thepeaks of hyperbolas usually correlate with interlayer horizons.

Vertical group (Gv) is a general name of a set of manifestations oc-curring synchronously one above the other (Fig. 3), when not all ofthem have a form of hyperbola. From this point of view the Christmastree pattern (Fig. 1) is only a special variant of the vertical group ofman-ifestations. Besides hyperbolas, also point defects (D) and non-pointmanifestations listed below (Chi, V, La,…) are very common. In thecase of asphalt pavement – when presence of infrastructure is notexpected – the vertical group has a strong significance in crack identifi-cation especially when it has documented lateral continuation.

3.2.3. Modified forms of manifestations (thick structures)Chi pattern (Chi, χ) has a form of two hyperbolic arms that cross

each other (Fig. 4). In weaker variant of this pattern, the arms donot cross each other, they do not have a common peak point as welland no common hyperbola can be fitted. These properties provethat the singular dispersive structure has non-point horizontal dimen-sions, and distinctive width and shape complexity must be considered.It is also interesting that chi pattern is relatively frequently observed,while a similar form, the double hyperbola (Hd, when arms of bothcrossing hyperbolas are explicitly visible; thicker crevice) occurs ratherrarely. The chi pattern has intermediate features between simple hyper-bola and V-shape subsidence of horizon associated with lithologicalalterations in basement. This pattern also occurs usually at the basal ho-rizon of the pavement.

The drillings made on the cracks having the most spectacular com-pound GPR manifestations described above (road section C, asphaltpavement; Fig. 5) showed, that these cracks have in lower part (olderpackage of layers) large, several centimeter in width, very complicatedand frequently branched shape, and the crevice is filledwithmaterial ofclay lithology. The extensive deterioration (lithological alterations)occurs below the cracks in the subbase, which was built originally as a

Fig. 4. The chi pattern (Chi) on the basal horizon (25 cm depth), visible on echogram(crack located at distance 3 m). Two further weaker manifestations occur above, oninterlayer horizons (at depth 5 and 11 cm); road section C.

rigid one. The crevice in the upper part of the core (overlay) is notwide-ly opened yet and usually does not elicit a clear GPR response.

V-shape subsidence of the basal or interlayer horizon (V) is fre-quently observed on echograms in the vicinity of cracks (Fig. 6).This pattern is visible in the Fig. 6 twice, on the bottom horizon androof horizon of the older layer system. In this case, the deformationof the horizons corresponds to the real, local subsidence of the road(local lack of the basement bearing), because the level failure wasattempted to be corrected during the repair. In a rigid medium likeconcrete, the V-shape subsidence of the basal horizon would suggestpresence of a branched crack inside pavement, which allowed suchdeformation if originally the pavement thickness was constant (dis-cussion of Fig. 10 in section 5).

Lithological alterations in subbase (La) are visible on echograms aschange (usually increase) of reflection at the base of pavement(Fig. 7). They are usually located near crack or dilatation and justthis strong correlation itself suggests that intense lithological changesoccur locally, below the pavement, as a result of loss of sealing, waterpenetration and erosion.

The V-shape subsidence and lithological alterations are character-istic features of the investigated concrete pavement. Fig. 7 is a partic-ular example of the relationship between lithological alterations insandy basements and V-shape subsidence at the bottom of the con-crete slabs, which occur below expansion joints and intra-slab cracks.Because of lack of outcrops, an alternative explanation of the GPRimage is possible in this case. V-shape at the bottom near the expan-sion joint was a common practice in concrete technology in the timeswhen the road was built and the increased bottom reflection can becaused by locally used dispersed reinforcement. But on the otherhand, both features occur also near the intra-slab cracks. TheV-shape subsidence in concrete can be potentially an effect of theconcrete dampening in the vicinity of crack or joint. Another explana-tion suggests that the V-shape subsidence can be a structure in thesubbase, created as a result of water pumping and its associated lith-ological alteration of the subbase material. Also these explanationsare doubtful, because the sharp fold point suggests that the behaviorof sandy basement is not flexible. It is worth to be noted that in any ofthese cases, both manifestations (V and La) as well as another local-ized perturbations of the basal horizon shape are excellent indicatorsof joints and cracks.

Although the investigated cracks occurring in this concrete road sec-tion have a localized form concentrated near the fracture surface, dril-ling of the intra-slab crack hidden below the overlay uncovered theactual feature that allowed the detection. The diffraction (allowing thedetection) was caused by extensive (several centimeters in diameter)lithological changes in the vicinity of the crack at the top of the slab(Fig. 7 left-hand lower corner). Another core drilled on an expansionjoint showed that strongly dispersing changed area at the top of theslab has width of about one decimeter (Fig. 7 right-hand lower corner).These changes can be recognized by heavymechanical degradation of theaffected part of the core during drilling, while the rest of the core andcomparative drill core made in non-cracked neighborhood were notdamaged at all (Sudyka et al., 2011). The 1 cm thick wood casing-boardinside the crevice, steel dowels 3 cm in diameter and patches are otherdiffractive elements responsible for spectacular, exceptional visibility ofjoints there.

3.2.4. Special manifestationsIn the case of section D, among 12 very well developed cracks of

linear shape, only 2 were associated with vertical groups (Gv) wellcorrelated with the crack's position. A special, very precise space cor-relation with decimeter accuracy was performed there. One can saythat there are no clear manifestations of the majority of these crackson echograms (Figs. 8 and 9), while clear manifestations occur morethan 1 m apart of visible cracks. It was also noted that manifestationsof local structure singularities in the upper (newer) package of

Fig. 5. Echograms (from the top: left, central and right profile 2.2 GHz antenna and left profile 1 GHz antenna) and drilling core showing crack structure (crack located at distance 5.5 m),being responsible for generating strong hyperbolas; road section C.

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asphalt layers occur not everywhere but they are concentrated in cer-tain zones only. In this case, these zones of several (up to 20) meterslength have visible coincidence with cracks position. This kind ofcompound crack manifestation was named disturbed zone (Z):agglomerated (several meters long, Za, Fig. 8, dist. 8 to 12.5 m) anddispersed one (about 10 m long or more, Zd, Fig. 9, dist. 7 to 15 m).The disturbed zones are rather not very helpful in identification ofsingle cracks. But this observation gives an insight into the course of

Fig. 6. V-shape subsidence (V) of the basal horizon (depth 20 cm) of the older part of theasphalt layer package (crack located at distance 3.2 m). The deformation of the horizonand width of the structure, exceeding 1 m, are expressed explicitly here. A similar butsmaller deformation is visible above on the boundary between older and newer layers(depth 10 cm); road section C.

the early phase of cracking, possibly before formation of the leadingcrack in a new overlay. Straight linear shape of these cracks wouldsuggest a very local influence of the critical stress in the vicinity ofthe crevice. Nevertheless, the process seems to be not very local, butit has zonal character. The early zonal cracking in this thick overlaycan reflect some zonal problems in heavily fatigued older asphaltlayer package, because significant complexity of the bottom horizonof the overlay is observed locally (intense destruction or specificlocal treatment like networks). Also very intense leveling was appliedduring the repair and hydrological problems are reported in this sec-tion. Nevertheless, the stable situation during several years of obser-vation shows that cracking was significantly slowed down (both inthe cracking zones and in the whole section in general) after forma-tion of the leading cracks, which suggests that the influence of ther-mal effects on early cracking was lowered.

The list of diagnostic manifestations should be supplemented withkey examples — this was not the case in relation to six of the investi-gated roads, but is very useful in diagnostic practice. The asphaltpavement patching called simply patch (P) is a characteristic tectonicform, being a result of replacement of the wearing layer along thecrack, in a zone of some chosenwidth (several centimeters up to severalmeters). Similar technique can be applied at the top of the cement pave-ment, but asphalt mixture is used for patching (Fig. 7, dist. 19, 44, and49 m). Locally changed amplitudes and shifted depths of the interlayerreflections are main characteristic features of this structure traced onechogram (Al-Qadi et al., 2009; Beak et al., 2008). Occurrence of therepair means probable presence of a crack below it and possible

Fig. 7. Upper: Lithological alterations (Lith.alt.) and V-shape subsidence of the bottom horizon accompanying expansion joints (x = 9, 19, 29, 39, and 49 m; characteristic diffrac-tion on dowels at 10 cm depth) and intra-slab cracks (x = 5, 13, 35, and 44 m); section A3. Three patches are visible: on two joints (19, 49 m) and on one crack (44 m). Lower:Examples of drilling cores, showing the strongly diffractive structures (lithological changes; weakened areas) in cement concrete pavement; road section A1. Left: Crack hiddenbelow asphalt overlay. Right: Expansion joint (with wood casing-board and steel dowel) covered by patch and two latter asphalt layers.

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appearance of the crack at the surface in the future. Where the patchwas covered by overlay during amajor repair (hidden patch), the prob-lem of identification is not a trivial one.

3.3. Visibility of cracks in GPR inspection

The cracks manifesting themselves in the most spectacular way onGPR images, corresponding to widespread and extensive pavementalterations, were scanned by using antennas operating at lower fre-quencies, too. Earlier drillings showed that the alterations frequentlyhave blurred character, thus a better visibility is expected in lowerfrequency images. Air coupled antenna of central frequency of1 GHz was used to check the spectacular cracks in asphalt pavementin road section C showing that large structures described earlier arevisible as strong point defects (D, spots) only in low resolutionimage (Fig. 5 lower).

One local road provided a very interesting, additional example ofextensive, branched crack in a concrete base, which is completely in-visible with a 2 GHz antenna (Krysiński and Sudyka, 2012a) on a pro-file 1.5 m distant from the road side. This was a rare case when acrack could be observed directly along the longitudinal cross-sectionoutcropped during the repair of the roadside (Fig. 10). The crack isvisible on the surface as an irregular line, but starting from this line

two major inclined wing fracture surfaces go down in opposite direc-tions. These two wing surfaces delineate area of branched crackingzone. Visibility of cracks in echogram depends on extensive materialalterations expressed in electrical permittivity occurring in the dete-rioration zone due to mechanical destruction and water penetration(Benedetto et al., 2005; Saarenketo and Scullion, 1995; Scullion andSaarenketo, 1995). Lack of the 2 GHz response suggests that alter-ations on the numerous fractures are not sufficient and they do nothave volumetric character. It is also possible that this widespread de-terioration does not continue into lane axis and it occurs at theboundary only, due to some hydrological problems there. The spread-ing downward geometry of the zone (and especially the presence oftwo major wing fractures) gives some potential explanation of phe-nomena observed at the base pavement surface (Chi pattern andV-shape subsidence of the basal or interlayer horizon V) occurringin the vicinity of cracks, especially in cement concrete pavements.

A particular problem with visibility of crack manifestations onechogram is related to the structural noise generated by the irregularmedium structure. An extreme example called “masking lattice” isshown in the Fig. 11. This very regular pattern successfully masksthe manifestation of cracks which are probably numerous in road sec-tion B. The effect is possibly related to the extremely large coarsegrain (comparable with the wavelength used) in the asphalt mixture

Fig. 8. Example of agglomerated disturbed zone (Za; distance 8 to 12.5 m) observed in newer asphalt layer system in the vicinity of crack 10 (marked as C) with no strong evidenceof the crack alone, in the road section D. Much stronger deterioration is visible in the older pavement below depth 25 cm.

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there (Sudyka and Krysiński, 2011). The nature of this phenomenon isnot clear. The typical inclination of the lattice lines has paradoxicalvalue 2.5 ns/m more than twice smaller than inclination of arms inthe case of hyperbola corresponding to a point object (6.7 ns/m).The striking regularity of this pattern is difficult to be modeled nu-merically. The preliminary modeling attempts using stochastic distri-butions of scattering centers were not enough satisfactory. Section B

Fig. 9. Example of dispersed disturbed zone (Zd; distance 7 to 15 m) observed in newer asphcrack alone, in the road section D (three parallel profiles; the lower echogram corresponds

represents a very interesting case of intense ongoing cracking process.The long term observation proved that cracks appeared frequently inplaces where some disturbance on echogram was observed earlier,but they couldn't be clearly interpreted as manifestations of localizeddefects because of masking lattice. For the same reason, several cracksvisible on the surface had no interpretable GPR response (Benedettoet al., 2004; Saarenketo and Scullion, 2000). One core drilled for

alt layer system in the vicinity of crack 12 (marked as C) with no direct evidence of theto the hard shoulder).

Fig. 10. Echogram provided by 2 GHz impulse antenna (upper; profile on lane axis, 1.5 m from the road side) compared with view on the outcrop on the road side during roadrepair (left-hand lower corner) and view of the crack on the surface (right-hand lower corner); additional section on local road. Crack position marked by C.

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comparison on suchweaklymanifested crack showed that the crevice isnot opened (upper layers are not divided) and crack doesn't penetratedeeper than delaminated interlayer horizon (10 cm depth), whichhad some expression on echogram.

4. Conclusions

In this paper, a deep and wide analysis has been performed of GPRsignals observed in the presence of pavement cracks. This study led toa classification of GPR manifestations of cracks.

Elementary manifestations refer to an idealized model of the crackcrevice having the shape of a vertical half-plane with a horizontal edgeor they can be related to local delamination. The shape of the diffractedsignal is a hyperbola (H) with peak point corresponding to the creviceedge and point defect (D), which is a variant of hyperbola reduced tothe vicinity of the peak point only. In the field practice modified formsmust be taken into account, because of visible correlation with the

Fig. 11. Masking lattice observed on the road section B, far away from cracks.

observed cracks: double hyperbola (Hd), chi pattern (Chi), V-shape sub-sidence of the reflection horizon (V), and lithological alterations in sub-base (La). These modified forms are related to the elementary ones, butthey also reflect non-point dimensions and shape complexity of the sin-gular crack area. Some of them (Chi, V, and La) can correspond to largertectonic forms representing phenomena associated with crack presence,likematerial crumbling, erosion and lithological changes related towaterpenetration or pumping both in the pavement and in the subbase.Although cracks occur in large variability of morphological forms sug-gested by these phenomena, it was found that some style is usually typ-ical for a given pavement and several drillings can enable recognizing thecharacteristic features of cracking that can help to interpret echograms.

A single appearance of elementary indicator is not necessarily amanifestation of a crack, but vertical collocation of the elementary in-dicators or their modified variants has a strong diagnostic signifi-cance. Such vertical coincidence is visible in echogram in the formof a compound crack indicator called vertical group (Gv). Its particu-larly impressive variant was called Christmas tree pattern (Ch). Alsothe lateral, horizontal continuation of indicators (their occurrenceon neighboring parallel scans) in the direction perpendicular to theprofile, was found to be very helpful in crack identification. The occur-rence of a vertical group or lateral continuation of somemanifestationwas found to be a strong diagnostic indication of a crack, especially inthe case of asphalt pavement, when any infrastructure or reinforce-ment of linear shape is not expected. Thus, the detailed descriptionand classification of manifestations is helpful in diagnostic practicebecause the manifestations are frequently very weak.

A special type manifestation of the cracking process was noted in aroad section, where well developed transversal cracks, distant onefrom another, have no precise position correlation with any indica-tion of strong diagnostic value. Nevertheless these cracks occur insideseveral zones of up to 20 m each that contain numerous indications of

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the cracking process, while the areas outside of the zones are relative-ly free from crack manifestations. This kind of cracking process man-ifestation was named disturbed zone (Z). This observation can give aninsight into not very local, but zonal character of the early phase ofcracking, before formation of the leading crack in a new overlay.

The cracks having clear manifestations on echogram are generallylarge, developed structures like several centimeter wide crevicesfilled with foreign material or widespread zones of material degrada-tion or lithological changes. The initial, unopened cracks like newones, are not visible using the GPR equipment and the use of largerfrequencies shouldn't make them visible due to masking signal(masking lattice) generated in a granulated medium like the asphaltmixture. Some parts of these widespread structures have blurredcharacter with vague boundaries. It was noted that the use of lowerfrequencies allow a better detection of large elements, while higherfrequencies can outline details. Therefore, using different frequencies(covering several octaves of the electromagnetic spectrum) is veryhelpful in crack diagnostics.

These observations can be useful in identification and localization ofhidden cracks e.g. covered by newer layer, in determination of somegeometrical features of cracks like depth range, width or shape, andsometimes they allow to comment on the course of cracking process.The method is more effective when a long term GPR observation(with constant equipment configuration) is performed; moreover, theavailability of documentation describing the original state of the con-struction is very helpful.

Acknowledgments

These investigations were supported by General Directorate forNational Roads and Motorways (Poland) 2192/2008.

References

Al-Qadi, I.L., Buttlar, W.G., Baek, J., Kim, M., 2009. Cost-effectiveness and performanceof overlay systems in Illinois volume 1: effectiveness assessment of HMA overlayinterlayer systems used to retard reflective cracking. Research Report ICT-09-044, May 2009, Civil Engineering Studies, Illinois Center for Transportation SeriesNo. 09-044, UILU-ENG-2009-2015: 0197-9191.

Beak, J., Al-Qadi, I.L., Buttlar, W.G., 2008. Reflective Cracking Control. (https://engineering.purdue.edu/~ncaupg/Activities/2008/presentations/Al-Qadi%20-%20Reflective%20Crack%20Control.pdf) Special Joint Conference, North Central HotMix Asphalt Conference, Illinois Bituminous Paving Conference, January 9th, (2008).

Benedetto, A., Benedetto, F., De Blasiis, M.R., Giunta, G., 2004. Reliability of radar in-spection for detection of pavement damage. International Journal of Road Mate-rials and Pavement Design 5 (1), 93–110.

Benedetto, A., Benedetto, F., De Blasiis, M.R., Giunta, G., 2005. Reliability of signal pro-cessing technique for pavement damages detection and classification using groundpenetrating radar. IEEE Sensors Journal 5 (3), 471–480. http://dx.doi.org/10.1109/JSEN.2005.846176.

Benedetto, A., Manacorda, G., Simi, A., Tosti, F., 2012. Novel perspectives in bridges in-spection using GPR. Nondestructive Testing and Evaluation 27 (3), 239–251.

Birtwisle, A., Utsi, E., 2008. The use of ground penetrating radar to detect vertical sub-surface cracking in airport runways. Proceedings of the 12th International Confer-ence on Ground Penetrating Radar (GPR2008), Birmingham, UK.

Diamanti, N., Redman, D., 2012. Field observations and numerical models of GPR re-sponse from vertical pavement cracks. Journal of Applied Geophysics 81, 106–116.

Diamanti, N., Redman, D., Giannopoulos, A., 2010. A study of GPR vertical crack re-sponses in pavement using field data and numerical modelling. 2010 13th Interna-tional Conference on Ground Penetrating Radar (GPR). http://dx.doi.org/10.1109/ICGPR.2010.5550224.

Krysiński, L., Sudyka, J., 2012a. Possibilities of the 3D radar in investigation of the roadconstruction: Budownictwo infrastrukturalne, 4, 1 - 2, pp. 65–70 (in Polish, Jan./Feb.).

Krysiński, L., Sudyka, J., 2012b. Identification of the pavement cracks by the use of GPRtechnique. . in Polish Autostrady 07/2012: 1730-0703, pp. 44–50.

Saarenketo, T., 2009. NDT transportation. In: Jol, H.M. (Ed.), Ground Penetrating Radar:Theory and Applications. Elsevier, Amsterdam, Netherlands.

Saarenketo, T., Scullion, T., 1995. Using Electrical Properties to Classify the StrengthProperties of Base Course Aggregates. Research Report 1341–2. Texas TransportationInstitute, College Station, Texas.

Saarenketo, T., Scullion, T., 2000. Road evaluation with ground penetrating radar. Journalof Applied Geophysics 43, 119–138.

Scullion, T., Saarenketo, T., 1995. Ground penetrating radar technique in monitoringdefects in roads and highways. Proceedings of the Symposium on the Applicationof Geophysics to Engineering and Environmental Problems, SAGEEP. Compiled byRonald S. Bell. April 23–26, 1995, Orlando, Florida, pp. 63–72.

Sudyka, J., Krysiński, L., 2011. Radar technique application in structural analysis andidentification of interlayer bonding. International Journal of Pavement Researchand Technology 4 (3), 176–184.

Sudyka, J., Krysiński, L., Harasim, P., Majewski, E., Kusiak, J., Mechowski, T., 2011. Anal-ysis of the pavement cracks propagation by the use of GPR technique. Research Re-port (in Polish). Road and Bridge Research Institute, Department of PavementDiagnostics, Warsaw (November).

Vanelstraete, A., Francken, L., 1997. Prevention of Reflective Cracking in Pavements.RILEM, London (RILEM — French acronym for International Union of Laboratoriesand Experts in Construction Materials, Systems, and Structures).