precracking mitigates shrinkage cracks in cemented material · 2014. 5. 15. · shrinkage cracking....

Precracking mitigates shrinkage cracks incemented material

K. P, George, M. Bajracharya & M. GaddamDepartment of Civil Engineering, The University of Mississippi, U.S.A.

Abstract

Despite widespread use of pozzolanic material, such as Portland cement, instructural components, there is concern over possible early shrinkage crackingbecause of desiccation and/or thermal contraction. With the objective ofcontrolling cracks, a field study was undertaken where a variety of treatments orcementing agents were incorporated in 244m test sections in a newly constructedroadway, Among the various test sections, precracking “young” cement-treatedsoil was highly successful in reducing shrinkage cracks. The precracking,accomplished by a vibratory roller in the field, is deemed to have introducedmicrocracks (“damage”), which in turn relieves shrinkage stresses, alleviatingdetrimental shrinkage cracks. Modulus measurements before and afterprecracking substantiate the damage hypothesis. In addition, in situ tests areconducted monitoring recovery of the damaged material. Despite the minordegradation, due mostly to induced microcracks, the material regained structuralattributes closely matching those of the untracked control test section. “Crackhealing” is the mechanism responsible for rejuvenation of the material.Simulating precracking in laboratory specimens, the rate of rejuvenation –measured in terms of material modulus – is investigated employing impactmodal tests. The results show that crack healing takes place over a relativelyshort period, with the recovery complete in a matter of weeks. The level ofprecracking or damage induced in the “young” cemented aggregate governs thelength of recovery. This paper advances a plausible explanation as to howmicrocracks inhibit inherent shrinkage cracks in cemetitious material. Ahypothesis explaining crack healing and resulting recovery of materialcharacteristics is discussed and substantiated.

© 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved.Web: www.witpress.com Email [email protected] from: Damage and Fracture Mechanics VII, CA Brebbia, & SI Nishida (Editors).ISBN 1-85312-926-7

80 Damage and Fracture Mechanics VII

1 Introduction

Transportation agencies are increasingly using cementitious additives, especiallypozzolanic admixtures, to enhance performance of aggregates and/or soils ofvarious gradations. Portland cement is primarily used as an additive in cement-bound material (CBM) in the UK and cement-treated aggregate/soil (CTA) in theUSA for pavement construction. Other pozzolanic materials often utilized inhighway pavement construction include lime, lime-fly ash and waste products,for example, ground granulated blast furnace slag. Yet another material of thiscategory used in pavement construction is roller compacted concrete (RCC).RCC is composed of coarse and fine aggregates, cement, fly ash, water, and insome cases, water reducing additives. Despite widespread use of pozzolanicmaterial, such as Portland cement, in structural components, there is concernover possible early shrinkage cracking because of desiccation and/or thermalcontraction. The cement layer also shrinks owing to drying from “selfdesiccation” (moisture depletion resulting from cement hydration). By virtue ofrelatively large surface area per unit volume, shrinkage cracking is inevitable,especially in pavement layers of cemented aggregates or roller compactedconcrete. Natural crack spacing may be 6 to 10m in CBM/CTA, and a largerspacing in RCC, 10 to 3Om with crack widths attaining nearly 12mm. Mitigationof shrinkage cracks is the topic of this paper.

Several procedures/techniques have been proposed for mitigating shrinkagecracks in cemented pavement layers. A detailed description of these procedurescan be found elsewhere, George [1]. An innovative technique otherwise referredto as “precracking”, which promotes numerous tine cracks (rnicrocracks) incontrast to a few wide cracks are discussed in this paper. Precracking has been asubject of experiments in the past in cement-treated pavement bases. The basicpremise of this technique is that, by precracking (with a vibratory roller),“young” cement base experiences numerous tine or hairline cracks at closespacing. The success of this method depends on inducing cracks while thecement hydration is in progress. The first reported successful experiment ofprecracking by immediate traffic release was conducted in Japan, Yamanouchi[2], with encouraging results. An experimental section built in Mississippi, Teng[3], where the road was opened to traffic immediately, has performed better thana control section where traffic was redirected for a minimum of 7 days. Evenmore encouraging results are reported from Austria, Litzka [4] where the cementbase was subjected to several passes of a 12-ton vibratory roller between 24 and72 hours after construction. A comparison between deflection measurementsbefore and after microcrack initiation showed an increase in the mean values,from 1.09 to 1.32 mm. Nevertheless, this increase of the mean values is reducedin the course of the setting process, suggesting healing of cracks,


Damage and Fracture Mechanics VII 81

2 Objective and scope

This study investigates whether shrinkage cracks can be mitigated byprecracking, This study also addresses whether rnicrocracks induced in thematerial would undergo healing, so the material might gain its strength andstiffness. Whether the crack healing would be partial or complete, and the rate ofhealing rapid or long-term, would also be investigated.

The effect of precracking on alleviating natural shrinkage cracks wasinvestigated in a field trial, George [5]. A 244m section of a newly constructedcement (soil aggregate) base layer was subjected to controlled cracking and theextent of cracks was monitored and compared with those in a control section.The 150mm layer was constructed with a typical sand clay (Class 9, Group C inaccordance with Mississippi Department of Transportation), adding 5.5 percentPortland cement (by weight). One hundred percent of Class 9 soil aggregatepasses through 2.14 mm sieve, with only 15’34.smaller than the 0.074 mm sieve.Structural characteristics, for example, strength and stiffness are also monitoredperiodically in the field study, employing nondestructive load testing. On thepretext that stiffness gain is a surrogate measure of crack healing, stiffness ofcracked and control beams (cast in the laboratory) are measured through theirdynamic characteristics, namely, natural frequencies. Determined by modalanalysis, natural frequencies provide modulus through the theory of beamvibration.

3 Implementing controlled precracking

The speed and frequency of the vibratory roller that was necessary to induceprecracking were arrived at by trial aand error. Requiring impulses to beimparted to the surface at 102rnm apart, we selected a roller speed of 5 kmh(80m) and a roller frequency of 800 vibrations per minute. The issue of numberof passes of the vibratory roller was determined by monitoring the reduction inYoung’s modulus with number of coverage. Five sample locations weremonitored with the geogauge to observe that, on average, the loss in moduluswas nearly 25~0 after four passes of the 8-ton roller. Geogauge is a portable fieldgauge for measuring layer stiffness, and in turn modulus, to a depth of 230 mm.Modulus before and after precracking is plotted in Figure 1. The surface beforeprecracking had a few fine shrinkage cracks, but no more surface cracksappeared even after four passes of the vibratory roller. Therefore, we attributethe decrease in modulus to microcracks induced in the material, rather than tostructural cracks.



500,—Beforecracking ~ ~

~ . After crack)ng I

I

400 ~ ‘

200200 205 210

Station

Figure 1: Modulus before and after precracking.

4 Post-construction field evaluation tests

Field tests were conducted on the control cement section and the adjacentprecracked section addressing the following issues:(i) How effective is precracking in mitigating shrinkage cracks?(ii) Would the micro cracks induced (during precracking) be healed?

4.1 Shrinkage cracks

Manual crack surveys were conducted at three different times: 7, 19 and 28 daysafter the construction of the pavement layer, With effective width affected by acrack taken as one foot, and assigning weight factors based on crack width,cracks of different severity levels were aggregated obtaining the effective crackarea, and in turn, percent crack. Evolution of crack percent for the control andprecracked sections is graphed in Figure 2. Undoubtedly, the precracked sectionoutperformed the control cement section with 28-day percent cracks 4.8% and17.4%, respectively. The slight decrease in crack density between 19 and 28 daysmay be attributed to 9mm precipitation on the twenty-fifth day, just three daysprior to crack surveying. The question now arises how precracking mitigatesshrinkage cracking. The mechanics of shrinkage cracking have been analyticallystudied in the past, George [1], which clearly shows that cracks initiate whenshrinkage stress, primarily tensile, exceeds the strength of cemented material.The microcracks induced in the material simply impede stress build-up, in turnalleviating the tendency for shrinkage cracks. Additionally, the existingmicrocracks could coalesce with increase in shrinkage stress; however, theresulting cracks remain small, and widely distributed.



22

g; ~—~’ ,’,~

P14,,

/’:12, ,

G 10 — Precracked’

+58, Control jg6~

,/’#r

04 - ,’

2jo~o 7 14 21 28 35

Time (days)

Figure 2: Crack density compared in control and sections

4.2 To what extent do microcracks affect material performance?

Since structural performance of material is of utmost importance (for loaddispersion in pavement layers), strength and modulus are perceived to beappropriate assessment measures. Unconfined compressive strength in-situ andon102 mm diameter core samples were determined. Deflection testing by FallingWeight Deflectometer (FWD) was conducted with the deflection basin analyzedfor material modulus.

4.2.1 Strength resultsThe clegg hammer, a nondestructive test device, was employed to estimateunconfined compressive strength at 4, 7 and 28 days, thus assessing the strengthgain of both control cement layer and precracked layer. The results aresummarized in Table 1, with mean and coefficient of variation in columns 2, 3and 4. The number of observations in each category is five or more. The slightdecrease in strength of control cement from 7 days to 28 days could have beenthe result of natural shrinkage cracks occurring in the material from exposure toextremely hot weather (average temperature in the 100° F range). As the resultsshow, the precracked material with relatively low strength at 4 days caught up instrength with the control cement in 28 days, This accelerated strength gain couldbe attributed to crack healing. More evidence to support this hypothesis will bepresented in the latter sections,

Strength of layers was determined from 102mm diameter core samples,extracted by drilling through the pavement layer. The unconfined compressivestrength of samples (height to diameter ratio 2:1), reported in Table 1,conclusively shows that the precracked material surpassed its control counterpartby a wide margin, both in short-term (28 days) and long-term (440 days).



Disturbance caused by core barrel and contamination by water used in the coringoperation are the primary factors in core strength being lower than the Clegghammer strength.

4,2.2 In-situ stiffness/modulusModulus of both control cement and precracked sections were backfigured fromdeflection basins generated by Falling Weight Deflectometer. FWD, byimparting an impulse load on the pavement surface, measures the deflection at 7sensor locations. Emplqying this deflection basin and the MODULUS 5.1program, layer modulus is backfigured and listed in Table 2. With nine or morevalues in each

Table 1: Unconfined compressive strength of cement aggregate, in-situ strengthwith clegg hammer and drilled cores.

Description Strength (kPa)(section) Clegg hammer 102 mm cores

4 days 7 days 28 days 28 days 440 daysMean/CV MeanlCV MeanlCV MeanlCV MeanlCV

Control 1800/19 1820/14 1630/31 700I 5 1660/5(1A,3A)

Precracked 1320122 1270123 1600 /42 880/35 1840/5(2)

Table 2: FWD modulus determined at seven, 28 and 440 days after construction.

Description Moduli (Mpa)(section) 7 days 28 days 440 days

MeanlCV MeanlCV MeanlCVControl 2420130 1850/42 2710/43(1A,3A)

Precracked 1710/53 1380/40 2170/45

category, the mean and coefficient of variation are reported (see columns 2, 3and 4), First, the modulus of the precracked material is 10wer than the controlcement. Second, in both cases modulus decreased during the 7- to –28-dayperiod, attributable to severe weather-related shrinkage cracking and consequentdegradation. Degradation/damage is manifested as stiffness decline. Undercontrolled environment (no further desiccation, as both sections received asphaltconcrete surface on the 35th day), however, their modulus indeed increased(compare columns 3 and 4). Note the modulus increase from 28 to 440 days issubstantial, 46’% and 57Y0, respectively, for control and precracked sections, Thefinding so far is somewhat inconclusive, in that the precracked section outpacedthe control section in strength gain. However, the control section faired well asfar as the modulus gain is concerned. Asserting that modulus gain is severelyhindered by hot weather, a complementary laboratory study was undertaken by



conducting side-by-side tests on beam samples, one group of three precrackedand a companion group set aside as control (or untracked). Results of thislaboratory study comprise the ensuing sections of this paper.

5 Precracking damage investigated employing modal

analysis

Modal analysis is a process whereby we describe a structure in terms of itsnatural characteristics that are the frequent y, damping and mode shapes. In caseof simple structural elements, for example beams, its natural frequency affordsan explicit method for characterizing the dynamic flexural moduli,

As a result of precracking, the cement material is perceived to haveundergone microcracks, with the material suffering a temporary decline instiffness. It is not clear from the field monitoring, if the stiffness decline ispermanent or whether it would recover with time, as cement hydration continuesto produce more cement gel. A laboratory study is designed to substantiate thecrack healing and recovery hypothesis. The experiment will simulatemicrocracks (damage) in the material, with a monitoring plan to track stiffnessgain with time. Duplicate beam specimens, 287mm long and 76mm by 5 Immcross section, were cast from a cement-aggregate mixture, one beam subjected toprecracking and the other preserved as control beam for comparison. The time-dependent stiffness change (gain) of each beam was monitored by modalanalysis, yielding natural frequencies and in turn, modulus. The basic premise ofthis approach is that presence of cracks (damage) results in decline in naturalfrequency, which is directly related to modulus.

5.1 Sample preparation

As indicated, the beams 287mm long with a rectangular cross-section weresubjected to modal impact test. These beams were cast in accordance withASTMD 1632-87 (with slight modification) from soil aggregate, mixedthoroughly with appropriate amount of cement and water. After a 24-hour moistcuring, the beam scheduled to receive precracking was subjected to vibration,while confined in the steel mold, for 7 to 10 minutes. The table, vibrating at10HZ and meeting the specifications of ASTMD 2049 test, was utilized to inducemicrocracks. The 7-minute vibration was repeated after 48 hours, and theprecracked and control beams were moist-cured for a total of 3 days beforesubjecting them to vibration tests, with the tests repeated at 7, 14 and 28 days inorder to monitor the stiffness recovery with time.

5,2 Experimental set-up for vibration study

As illustrated in Figure 3, the beam specimen was suspended in the free-freeconfiguration by thin nylon threads. The point of attachment is one-fifth L from



// /’ // /’ /’

I IICE

powerSupp(y

11INN4M[CSIGWLAWLWR — ~HFuTE% — PRINIER

HP356b5+

Figure 3: Schematic of the experimental set-up,

the free end, a position that is in proximity to the modes of the first and secondflexural modes of a Bernoulli-Euler beam, Note that the exact locations of thesemodes fall at 0.224 IL and O.1322L, respectively, from the free end. Theacceleration response of the sample was monitored by a miniature accelerometerwith the sensitive axis normal to the x-z plane, The accelerometer was halfwayalong the width of the beam so as to minimize the influence from torsionalvibration. The sample was then set into free vibrations in the x-y plane by meansof an impulse along the y-direction, via an impact hammer instrumented with aforce transducer. During the test, the response of the beam and the excitationforce were filtered, amplified and recorded by a dynamic signal analyzer.Discrete Fourier Transform was subsequently performed on the captured signalsto produce the frequency response fimction (FRF). In order to substantiate theresults, the beam was rotated 90° along the x-axis and vibration test repeated. Atypical FRF of a control beam sample is shown in Figure 4. Two peakscorresponding to the first two damped flexural vibration frequencies can beclearly observed. The dampingratios { of the corresponding vibration modes are also indicated, and aredetermined by using the half-power bandwidths.

For a beam of uniform cross section and uniformly distributed load with free-free boundary conditions, the natural frequency (UHis given:



[1

‘/,

LOfl=A EI radlsec (1)~

Ldwhere E = young’s modulus

I = moment of inertia,L = length of beamp = mass per unit length of beam, andA = coefficient, 22.4 and 61.7, respectively for first

and second modesOnce natural frequency is determined from vibration test (modal analysis),Equation 1 maybe employed in calculating Young’s modulus, corresponding tothe firstlsecond mode.

Figure 4: Typical frequency response function.

5.3 Verification of modal analysis test methodology

A finite element (FE) model of the test beam, 287 mm long was developed with44 brick elements employing PATRAN software. The beam was then analyzedby ABAQUS program, Hibbet[6], determining its eigen modes. The analysisrequired two material properties: Young’s modulus and Poisson’s ratio. Themodulus input, of 4400 Mpa, was in fact the experimental modulus derived fromimpulse frequency response test, with Poisson’s ratio assumed to be 0.45. Acomparison of natural frequencies from modal analysis with those fromeigenmode analysis reveals that the FE analysis satisfactorily predicts the firstmode within 7°/0 and 80/0 for control and precracked beams, respectively. Thereis hardly any agreement in the second mode frequency, due primarily to mesh



sensitivityy in the FE analysis. In view of satisfactory agreement of first modefrequency, it will be used in modulus calculation.

5.4 Animation of mode shapes employing ME’scopeVES

The experimental procedure and results were further authenticated by animatingthe measured deflection responses of the beam in slow motion employing aME’scope, Designed to observe and analyze vibration problems in structuresand machines, it utilizes multichannel time or frequency domain data, acquiredduring the excitation of the beam. It displays operating deflection shapes andmode shapes at a moment in time or at a frequency, directly from the measureddata. Though not included here for brevity, the observed mode shapes more orless agree with the theoretical predictions. In addition, the modal frequenciesobtained from FRF plots are indeed verified with those indicated by deflectionresponse of the beam.

5.5 Results and discussion

Inducing different levels of precracking, three sets (control and precracked) ofbeams were tested. For discussion purpose, the ratio of loss in stiffness ofprecracked beam to that of the control beam is referred to as damage.Accordingly, the three beams suffered damages 9, 12 and 18 percent. The trendlines in Figure 5 depict the (damage) recovery of the three beams. The first twobeams receiving 9 and 12 percent damage, by way of microcracks, recovered in7 and 50 days respectively. As expected, the more damage, the longer it takes torecover. Contrasted to these, the beam precracked to a higher damage level (18percent) failed to recover even after 2 months. Note that this beam sufferedsome desiccation (10/0 weight loss) during the two-month period, though kept ina humidity room. It is unclear if the recovery process was at all hindered bymarginal “cure”. Regardless, we assert that there exists a threshold level ofdamage, beyond which full recovery may be unattainable.

The question now arises how the cracked beam with low modulus in thebeginning caught up with its untracked counterpart. As expected, the controlbeam had gained stiffness with time, but the cracked beam in a matter of daysoutpaced the control beam in attaining comparable stiffness. Continued cementhydration for days and even months and resulting bonding of aggregate matrixby calcium silicate hydrate (C-S-H) gel is the primary mechanism subscribing tolong-term stiffness gain of cement treated aggregate. According to Jennings andJohnson [6], the spherical cement particles (tricalcium silicate) are enveloped byhydration shells of C-S-H gel, whose thickness increase over time.



20

16

42o \o 10 20 30 40 50 60 70

Curing time (days)

Figure 5: Healing in material with curing time. Damage is the ratio of loss

in stiffness of precracked beam to the stiffness of control beam.

The C-S-H gel formation is active in both untracked and cracked materials,so also the calcium hydroxide crystal nucleation and growth in the continuumpore space. In precracked material, additionally fresh calcium hydroxide couldpermeate into existing microcracks healing the cracks by bridging crackopenings. It is this additional bonding that brought about the rejuvenation ofcracked beam, resulting in its stiffness attaining a level comparable to that of thecontrol beam. To put it differently, both cracked and untracked materialbenefited from continued gel formation and resulting cementing action, theprecracked material, however, benefited more from nucleation of calciumhydroxide into crack openings.

In CBM and CTA, with relatively small amounts of cement, it is importantthat cracks be small but numerous, which is precisely what is beingaccomplished by vibrating the material while cement hydration is still inprogress. Even with a larger cement additive, as in case of RCC, importance oflimiting crack size cannot be overemphasized for effective healing process.

6 Summary and conclusion

Seeking methods to mitigate shrinkage cracks in cement-treated aggregates, afield test program was implemented in August 2000. A technique, referred to asprecracking, was highly successful in alleviating early shrinkage cracks, In theprecracked section, “young” CTA was subjected to a vibratory roller inducingnumerous fine cracks in the material. Precracking the material was veryeffective in that surface cracks measured at 28 days were 4.8% in contrast to17.4% in the control section. Despite the material suffering a temporary declinein strength, it was regained in a matter of few weeks. The stiffness recovery wasnot as conclusive as strength gain. Therefore, a laboratory study was initiated,



investigating strength/stiffness recovery that is attributable to crack healing.Precracked and control beam specimens (287mm long and76rnmby51 mm crosssection) were subjected to modal analysis, extracting modal frequencies, and inturn, calculating Young’s modulus. The validity of the impulse frequencyresponse technique for determining modal frequency was established bydetermining eigen modes of beams employing finite element modeling, Theanalytical calculations show satisfactory agreement with the experimental data.Mode shapes of the deflected beams were also verified by animation of the beamdeflection under impulse loading. Monitoring of beam stiffness clearly showsthat precracked material regained its stiffness with time, and length of recoverywas governed by level of precracking or damage induced in the “young”material. Indications are that a threshold value of damage exists; beyond whichfull recovery may be unattainable.

Acknowledgment

This paper includes results of a study titled, “Soil Stabilization Field Trial,”conducted by the Civil Engineering Department, University of Mississippi, incooperation with the Mississippi Department of Transportation (MDOT) andFederal Highway Administration (FHWA), USA. Contributions of UpendraJoshi, former research assistant, and Prakash Jadav, research assistant areacknowledged. The opinions, findings and conclusions expressed in this reportare those of the authors and not necessarily those of the MDOT or the FHWA.This does not constitute a standard, specification or regulation.

References

[1] George, K.P.,’’Minimizing cracking in cement-treated materials forimproved performance,” Portland Cement Association, RD 123, p.58, 2001.

[2] Yamanouchi, T,, and Ihido, M., “Laboratory in-situ experiments on theproblem of immediate opening of soil-cement base to general trafjc, “Proceeding of the 4th Australia-New Zealand Conference, 1982.

[3] Teng, T. C., and Fulton, J.P., “Field Evaluation Program of cement-Treatedbases,” Transportation Research Record, 501, pp. 14-27,1974,

[4] Litzka, J., “Cold in-place recycling on low-volume roads in Austria,”Proc,of the 6th International Conference on Low–volume roads,Transportation Research Board, 1999.

[5] George, K. P., “Soil stabilization field trial,” University of Mississippi,Oxford, p. 118,2001.

[6] Hibbit, Karlsson and Sorenson, Inc., “Getting started withABAQUS/STANDARD,” version 5.5,” p,32, 1996.

[7] Jennings, H. M., and Johnson, S. K.,’’Simulation of microstructuredevelopment during the hydration of the cement compound,” AmericanCeram.Society, 69, pp.790-95, 1986.


precracking mitigates shrinkage cracks in cemented material · 2014. 5. 15. · shrinkage cracking....

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