how safe are txdot bridges with premature concrete...

Project Summary Report 0-1857-S – 1 –

The University of Texas at Austin

C e n t e r f o rTransportation Research

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CENTER FOR TRANSPORTATION RESEARCH

THE UNIVERSITY OF TEXAS AT AUSTIN

Project Summary Report 0-1857-SExplore New Uses for T-Headed Bars in Structural Concrete Reinforcement Applications

Authors: R. E. Klingner and T. J. FowlerCenter for Transportation Research, The University of Texas at Austin

April 2003

How Safe Are TxDOT Bridges with PrematureConcrete Deterioration?

In late 1995 the TexasDepartment of Transportation(TxDOT) noticed prematureconcrete deterioration in asignificant number of in-servicebridge structures around thestate. Most affected structuresare prestressed beams, thoughsubstructures (abutments,columns and bents) at somebridge sites are also damaged.Typical damage to prestressedbeams consists of horizontalcracking on the bottom flanges,longitudinal cracks underneath,and distributed “map cracking,”concentrated at but not limitedto the ends. An example of thiskind of field damage is shownin Figure 1.

This premature concretedeterioration is caused by twoexpansive damage mechanisms:Alkali-Silica Reaction (ASR)and Delayed EttringiteFormation (DEF). OtherTxDOT studies have shownhow these mechanisms workand what causes them to occurin specific cases. Basically,each damage mechanism takesplace within the concrete itself,and is more severe in thepresence of water.

What We Did...Faced with these

deteriorated structures, TxDOT

needed to decide whether toreplace them immediately (atconsiderable inconvenience andexpense), continue to monitorthem, or some combination ofthese. The overall objective ofProject 0-1857 was to provideTxDOT with specific tools fordeciding what to do with in-service structures experiencingpremature concretedeterioration. That objectivewas accomplished using thefollowing work plan:

1) Over a four-year period,field investigations werecarried out on five

TxDOT structures indifferent parts of Texas.In some structures, crackwidths increased veryslowly over time,indicating a very slowdeterioration (Figure 2).In other structures, crackwidths increased morequickly over time,indicating more rapiddeterioration (Figure 3).For a range of affectedstructures, those fieldinvestigations documentedthe increase indeterioration over time,and they gave theinvestigators informationabout the factors thatwould make thedeterioration proceedmore slowly or morerapidly.

2) Using a group ofprestressed girders thathad previously beenrejected by TxDOT,laboratory tests wereconducted to determinethe bending and shearstrength of full-sizedgirders compared withthose of otherwiseidentical but undamagedgirders, and comparedwith the capacitiesassumed in design.Figure 1: Example of field

damage from PrematureConcrete Degradation (PCD)

Project Summary Report 0-1857-S – 2 –

Fatigue loads as well as staticloads were studied.

3) Using cores extracted fromthose damaged specimens,decreases in strength andstiffness due to PCD wereinvestigated, and were relatedto observed and computeddecreases in the capacity ofstructural elements.

4) A simple numerical index ofdamage, referred to as aDamage Index (DI), wasdeveloped to quantify visualobservations of damage. ThatDamage Index is computed as

∑ 2wl for a 12-in. square

defined area, where l is thecrack length and w is thecrack width. Because theDamage Index is relative, theunits are arbitrary. TheDamage Indices obtained inthis study were calculatedusing crack widths inthousandths of an inch, andcrack lengths in inches.

5) Nondestructive evaluation(NDE) techniques, principallyacoustic emission, wererelated to visual observationsof damage to observedstrengths.

What We Found...1) Premature concrete damage,

as measured by decreases inthe compressive strength ofthe damaged concrete, is verystrongly related to the DamageIndex (Figure 4).

2) The Damage Index can beestimated quickly for aparticular structure in termsof the maximum crack widthin that structure (Figure 5).

3) Reductions in concretecompressive strength due topremature concretedegradation can be related to

damaged concrete for itsoriginal strength.

4) Because premature concretedeterioration is more severe inconcrete that is kept wet, fieldstructures show more damageat the ends of girders, under

reductions in the capacity ofdamaged members, usingcurrent design models orstrut-and-tie models, andsubstituting the reducedcompressive strength of the

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Figure 2 Example plot of crack widths over time, showing a fieldstructure with slow deterioration

Figure 3 Example plot of crack widths over time, showing a fieldstructure with faster deterioration

Project Summary Report 0-1857-S – 3 –

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joints in bridge decks.Because those end regions arenormally also subjected to thehighest shear, prematureconcrete damage can becritical for the shear capacityof damaged girders, whichshowed a reduction in shearcapacity of about 14% overthat of otherwise identical butundamaged girders.

5) Because the midspan regionsof beams are normally notsubjected to as much wettingas the end regions, midspandamage due to prematureconcrete degradation isgenerally small, and themoment capacity of damaged

beams is not significantlyreduced.

The ResearchersRecommend...

1) TxDOT should monitor crackwidths in field structures,estimate the correspondingdamage indices usingstructure-specific curves likethat of Figure 5, estimate theloss in compressive strengthusing structure-specificcurves like that of Figure 4,and estimate thecorresponding loss in flexuraland shear strength usingconventional design equations.

For example, the latestmaximum crack widthmeasured on a particularstructure is 0.13 in. (3.4 mm).Using Figure 5, thiscorresponds to a damageindex of 13,097 (UScustomary units). UsingFigure 4, the compressivestrength is estimated as5170 lb/in2. If this is belowthe specified compressivestrength, TxDOT should useshear design equations toestimate the reduced shearcapacity of end regions ofaffected girders. If the shearcapacity estimated in this wayfell significantly below thatused for design, compressivestrengths could be verified bycore tests.

Benefits

1) The primary benefit isconfidence in structuralintegrity of in-servicestructures.

2) Determination of the DamageIndex for a particular girder isrelatively quick and simple.As girders exhibiting PCD aremonitored over time, theDamage Index is determinedand flexural and shearcapacity are computed,assuring TxDOT engineers ofthe structural capacity of agiven bridge.

3) This tool has the potential tosave TxDOT hundreds ofthousands of dollars in girderreplacement costs and costlyload tests for in-servicebridges exhibiting prematureconcrete deterioration.

4) Motorist delay associated withgirder replacement or live loadtesting is also eliminatedthrough use of this innovativeanalysis tool.

Figure 4 Core compressive strength versus Damage Index

COMPRESSIVE STRENGTH VS. DAMAGE INDEX

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DAMAGE INDEX VS. MAXIMUM CRACK WIDTH

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Figure 5 Damage Index (DI) in terms of maximum crack width

The University of Texas at AustinCenter for Transportion Research3208 Red River, Suite #200Austin, TX 78705-2650

Disclaimer

Research Supervisor: Richard E. Klingner, P.E., (512) 471-4582email: klingner@mail.utexas.edu

TxDOT Project Director: Brian Merrill (BRG), (512) 416-2232email: BMERRILL@dot.state.tx.us

The research is documented in the following reports:Research Report 0-1857-1, “Bridges with Premature Concrete Deterioration: Field Observations and Large

Scale Testing,” October 2001.Research Report 0-1857-2, “Nondestructive Testing of Prestressed Bridge Girderswith Distributed Damage,”

August 2002.Research Report 0-1857-3, “Bridges with Premature Concrete Deterioration: Fatigue Testing of Full-Scale,

Prestressed Concrete Box Girders Failing in Shear,” October 2001.

Results from this project were immediately useful in that they allayed concerns regarding the structural capacityof in-service, pretensioned concrete girders exhibiting premature concrete deterioration (PCD). As a result of thetesting performed, immediate or near-term replacement of girders with PCD is not considered necessary. TheBridge Division is considering how best to implement the Damage Index concept developed under this project forlong term monitoring of in-service structures exhibiting PCD. Additional work relating to the Damage Index topercent capacity reduction of girders should be pursued to facilitate easier implementation of the Damage Indexconcept.

For more information please contact Tom Yarbrough, P.E., RTI Research Engineer, at (512) 465-7403 or emailat tyarbro@dot.state.tx.us.

This research was performed in cooperation with the Texas Department of Transportation and the U. S. Department ofTransportation, Federal Highway Administration. The contents of this report reflect the views of the authors, who areresponsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official viewor policies of the FHWA or TxDOT. This report does not constitute a standard, specification, or regulation, nor is itintended for construction, bidding, or permit purposes. Trade names were used solely for information and not for productendorsement. The engineers in charge were Richard E. Klingner, P.E. (Texas No. 42483), and Timothy J. Fowler.

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TxDOT Implementation StatusJanuary 2004

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C e n t e r f o rTransportation Research