civil engineering seminar topic, fatigue of submerged concrete under low cycle high magnitude loads

8/4/2019 civil engineering seminar topic, fatigue of submerged concrete under low cycle high magnitude loads

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Avi Mor, 1 Weston T. Hester, 1 and Ben C. Gerwick 1

Fatigue of Submerged Concrete under Low-Cycle,High-Magnitude Loads

REFERENCE: Mor, A., Hester, W. T., and Gerwick, B. C., "Fatigueof Submerged Concrete under Low-Cycle, High-Magnitude Loads,"Journal of Testing and Evaluation, JTEVA, Vol. 17, No. 3, May 1989,pp. 157-166.ABSTRACT: A procedure for testing submerged, reinforced concretebeams subjected to reversible fatigue loading is presented. The proce-dure focused on the use of simplified systems for reversible oading andeffective submersion of the concrete. These systems helped to reducesubstantially the cost associated with fatigue testing of reinforced con-crete beams. Degradation of the beam was measured continuously byanalyzing deflection without halting the cyclic loading. Extensive testson accompanying specimens and post-failure tests were performed. Theprogram also utilized a computerized, highly automatic system for testcontrol, data acquisition, and analysis of data.KEY WORDS: bond, concrete, data acquisition, fatigue, reinforcedconcrete, submerged concrete, testing

NomenclatureHCP

LVDTLWANWA

LWSFNWSF

Hardened Cement PasteLinear Variable Differential (or Displacement) Trans-dueerLight Weight AggregateNormal Weight AggregateLight Weight aggregate concrete with Silica FumeNormal Weight aggregate concrete with Silica Fume

IntroductionFatigue failure occurs when a concrete structure fails after bei ng

exposed to a large n umb er of stress cycles. These stresses may belower than stresses for which the structure was designed, and fail-ure may be sudden and catastrophi c. Fa tigue failure of concrete isa problem which man y designers tend to ignore, since sound designpractice will usually compens ate for fatigue loading. However, un-der severe cyclic loadi ng conditi ons fatigue may become the limit-ing factor in design. Extensive work has been done by other re-searchers on fatigue performance of unreinforced low andmoderate streng th concrete subjected to high-cycle loading, suchas that experienced by machine bases and railroad ties. Limitedresearch has been done on large reinforced/prestressed concretespecimens, high-strength concretes, and submerged concrete. Un-

Manuscript received 3/4/88; accepted for publication 11/18/88.LDoctor of Engineering, Associate Professor, and Professor, Departmentof Civil Engineering, University of California, Berkeley, CA 94720. Dr.Mot is currently with AMMB, 9217 Alcott St., Los Angeles, CA 90035. 1989 by the American Society for Testing and Materials

fortunately, the lack of a well established test procedure for evalu-ating fatigue makes it difficult to correlate or extend these pub-lished test results.It is now particularly critical to develop a consistent approachfor measuri ng and assessing fatigue capability, because it is possi-ble and attractive to design and use concretes with compressivestrength substan tially greater th an that previously considered inearlier fatigue tests and e nvisioned in curre nt codes. Concrete ma-rine structures using these high-strength concretes are being de-signed and built, a nd some are already in use and exposed to severeenvironments. Concretes with compressive strength exceeding 69MPa (10 000 psi) are being used for many applications; in somespecialized areas (mainly high-rise buildings) strengths of up to138 MPa (20 000 psi) have been specified and used. The use ofthese special concretes in the mar ine env ironmen t raises new ques-tions about their ability to resist cyclic loading, since it is difficultto extrapolate previously published dat a to these conditions.

Concrete marine structures, includ ing piers and offshore drillingplatforms, are increasingly using these special concretes as a basicbuilding material; therefore more detailed design criteria areneeded. Structures exposed to ocean waves will experience a largenumb er of high-frequency, reversible, low-to-medium magni tudeloading cycles duri ng their lifetime. They will also experience a rel-atively small nu mbe r of very high-ma gnitud e excitations caused byextreme storms or collisions. Thus a typical structure may see 2 108 loading cycles of relatively low magnitude (up to 50% of ulti-mate strength) to a few thousan ds loading cycles of high magnit ude(over 65% of ultimate strength). These low-cycle, high-magnitudefully reversible loads may cause c rack ing and increasingly severedegradation, especially when these cycles are in the tensile crack-ing range.

Lightweight Aggregate (LWA) would be the material of choicefor most floating mar ine structures, even if the only benefit wouldbe the reduced weight. Other superior properties of LWA concretemake it highly desirable for use under marine conditions. Theseproperties include excellent durabilit y, reduced microcracking dueto the compatibilit y of its components , and high therma l straincapacity.

In this paper the authors describe in detail in apparatus thatmay be used to test reinforced/prestressed concrete, i n air or sub-merged, und er fully reversible loading. The ap parat us is simple toconstruct and operate. The test results fully documen t the fatigueperformance and can be compa red with other previously publisheddata.The aut hors tested high-stre ngth reinforced concrete beams un-der low-cycle, high-magn itude reversible cyclic loading. Half of the

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158 J O U R N A L O F T E S TI N G A N D E V A L U A T IO Nbeams were tested submerged, and the other half were tested inair. Accompanying unreinforced specimens were tested for all ma-jor properties of the concrete; those that affected the fatigue lifewere identified. A model for fatigue of reinforced concrete hasbeen developed. A comprehensive description of the test program,materials, and results may be found in the published report [].The mai n results of that program were:

1. Fa tigue life of high -strength Lightweight Aggregate (LWA)concrete was similar to or better than the fatigue life of NormalWeight Aggregate (NWA) concrete of similar strength.

2. Submersion of high strength (dense) concrete in water did notshorten its fatigue life.

3. The add ition of silica fume to LWA concrete increased its fa-tigue life by 60 to 80% compared to LWA and NWA concrete ofsimilar strength. The same effect was no t evident in NWAconcrete.

4. The variations in fatigue life do not appear to be att ribut ableto variations in compressive strength, tensile strength, flexuralstrength, modulus of elasticity, or density.

5. The a ddition of silica fume to LWA concrete improved itsbond with the bars by up to 100% at a slip of 0.25 mm (0.01 in.).These results agree with data published by Robins and Aust in [2].

6. The fatigue life of high-strength reinforced concrete appearsto be a function of the bo nd between the concrete and reinforcingsteel.

Based on the test results from this program and others, a mate-rial model for fatigue degradation and failure of reinforced con-crete was defined. The model is summarized below:

Step /--Newly cast concrete contains various small voids dis-persed throughout the material, with some local void concentra-tions under aggregates and reinfor cing bars due to bleedi ng.Step 2-- Lim ite d cyclic loading of the concrete in its elastic zoneinduces stress relax ation, possible reduction in void sizes, and tem-porary enhancement of strength properties.Step 3-- Con ti nue d cyclic loading below the ulti mate strength ofthe reinforced concrete element causes microcracks to form andpropagate, gr adually converging into struct ural cracks.Step 4- -Wh ere a structural crack intersects a reinforcing bar allstresses are transferred to the steel at that point, relaxing thestresses in the concrete in the direct vicinity. The stresses are car-ried across the crack by the steel and transferred to the concretethrough the bond between concrete and steel adjacent to thecrack.Step 5-- Con tin ued cyclic loading causes gradual degradati on ofthe bond between the concrete and the reinforcing bar as micro-cracks form and propagate in the interface. The process forces thesteel bar to carry larger porti ons of the stresses as the b ond deterio-rates, and the concrete relaxes away from the bar. On the con-crete's surface this step is recognized by the formation of a fan-shaped area of fine cracks, indica ting debonding next to the majorstructural crack.Step 6- -T he steel reinforcing bar fails suddenly in fatigue whenit reaches its fatigue capacity after the loss of bond causes a maj orportion of the load to transfer to the steel, resulting in completeloss of load carrying capacity at th at location.

BackgroundSeveral methods have been used in test ing the fatigue capacity of

concrete. Flexural elements are usually tested by loading beams in

four-point (third-point) loading, with application of cyclic loadsuntil failure. Fatigue capacity is measured as the n umb er of cyclesto failure under different stress levels. Test methods differ by typeand size of specimen, magnitude and rate of loading, mechanicalcontrols, and analytical procedures.

The prin cipal difficulties with correlating and ext rapolating thefatigue performance of large specimens include:

1. Even at the r apid rate of 10 cycles per second (cps) a speci-men may require more than 107 cycles (or 270 h) of loading, butthis accele rated rat e of loading is not representat ive of the low cycleloading experienced by the structure. Sparks [3] and Hatano andWatanabe [4] found that higher frequencies increased the appar-ent fatigue capacity of concrete tested in air. Art hur et al. [5] foundno frequency effect in air, but fatigue capacity was enhan ced und erlower frequencies in submerged concrete. Waagaa rd [6] found noclear connection between fatigue capacity and load frequencies inthe range 0.2 to 3 Hz for submerg ed concrete. Obviously, the effectis complex and not very well understood; t hus tests should be per-formed at the expected frequencies.

2. Most researchers main tai n the desired maxi mum stresses byloading the specimens to a predetermined deflection. However,specimens degrade over time and make it difficult to maintain aconsistent loading with deflection control techniques. The re sult isa progressive decrease in applied load as the specimen loses stiff-ness. It also results in a varying load when the new stroke (deflec-tion) value is determined by trial and error.

3. The cu mulative numbe r of cycles to the specimen's collapseunder a given load is the commonl y accepted stan dard measure forfatigue capacity. With existing test procedures most researchersinterrupt the cyclic loading periodically to assess the specimen'scondition. This assessment can take the form of measured deflec-tion unde r a static load, or one of the nondestruc tive sonic meth-ods. However, any inter rupti on (rest period) has a n in fluence onspecimen's performance [7]. Therefore it is desirable to measuredeterioration without having to stop cycling.

4. It may be difficult to correlate the results of pure compressionor pure tension with the more severe situation of reversibleloading.

5. The possibil ity tha t fatigue life of submerged concrete is dif-ferent from the fatigue life of air-dry concrete created the nee d totest submerged specimens. Initi al tests were conducted i n big tanksof water (Waagaard [8], Roper et al. [9]) and showed that sub-merged concrete exhibited shorter fatigue life. Furthe r tests in apressure chamber confirme d that depth (water pressure) was not afactor (Waag aard [8], Paterson [0]). Tests by Cornelis sen [11]showed t hat saturat ion of plain concrete was sufficient to reduce itsfatigue life.6. Testing of submerged concrete required th at large tanks wereused to submerge both specimen and loading frame, resulting inan extremely complex and expensive setup.

7. Fatigue testing of large specimens is expensive and long term.The large spread of the results of a limited numbe r of tests makes itdifficult to analyze the data using statistical methods.Experimental Technique

The authors developed an apparatus and test procedure forloading a standardized specimen with a low-cycle, reversible fa-tigue loading. L imitation s of previously described procedures wereminimized by a n umbe r of technologies, including:

1. Specimens were cycled at a rate of 1 Hz, or one complete





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M O R E T A L . O N F A T I G U E O F S U B M E R G E D C O N C R E T E 159compression and te nsion cycle per second. This number, very closeto actual wave frequency, was used in some recent tests (Waagaa rd[6], Muguruma [2]).

2. Specimens were tested under load control (e.g., the auto-matic hydrau lic controller used feed-back fr om a load-cell to applya constant maximum load). No corrections were required once aload level was established.3. Gradu al d egradatio n of the specimen was evaluated by mea-suring deflection of the specimen under load without stopping thecycling; thus unnecessary rest-periods were eliminated.

4. Fully reversible loading was imposed on symmetrically rein-forced concrete beams.

5. A flexible water-jack et was used to simulate fully submergedconditions.

6. Two identical specimens were tested for each test condition.Two specimens may not qualify as a statistical sample, but underthe existing constraints they were used to provide a reasonablygood indication of trends, and to alert us to errors when the resultsfor the two specimens disagreed substantially.

In this program both concrete composition and environmentalparameters were systematically changed and monitored, so as toevaluate the effects of each para meter on the overall behavior. Thec o m p r e s s i v e s t r e n g t h of the concrete was kept constant for all spec-imens by the use of admixtures, several aggregates, and manipu-lating the water/cement ratio. The total amount of c e m e n t i t i o u smaterials in each mix was kept similar, since it was felt that theeffect of quality and quantity of HCP was very important. Exten-sive post-failure evaluation of the concrete in the beams was donewith non-destructive sonic tests, compressive strength tests of ex-tracted cubes, and microscopic evaluation of cracking.

Fatigue Testing ProceduresS p e c i m e n s

Four types of specimens were used: five main reinfo rced concretebeam s (Fig. 1), twelve 74 by 74 by 245 mm (3 by 3 by 20 in.) un rein-forced concrete beams, fifteen standa rd 74 by 147 mm (3 by 6 in.)cylinders, and six standard 147 by 147 by 147 mm (6 by 6 by 6 in.)cubes for the pull-out test.

All molds for the specimens were filled halfway and consolidatedby vibration, th en filled to the top with the same type of vibration.Initial finish was done with wooden trowels, subseque nt finish w i t hsteel trowels.

All specimens were stripped after 48 h and stored in a fog roomfor 60 days. At that time they were moved into the main testingarea and allowed to air-dry in its controlled environment. At least

L o a d#3 Reinforcing ebars #9 ~tirnln~ ~ R n [

8 7 ~ [ ~ 0.5In

IS u p p o r t

two weeks before testing, all specimens that were to be tested sub-merged were immersed in a seawater-filled tank to ensure completesaturation.

L o a d i n g C o n f i g u r a t i o nThe concrete beams were loaded reversibly in flexure under athird-point loading (ASTM C 78; sometimes referred to as "four-

point" loading) (Fig. 2). The 2440 mm (8 ft) concrete beams weretested over a 1830 mm (6 ft) span, so that a length of 305 mm (1 ft)on each side was not subjected to any load during the test and sup-plied additional control data for concrete strength. Figure 3 showsa top view of the testing bay with all the components of the testingset-up.

S u p p o r t F r a m eThe support frame was designed to restrain the concrete beam in

the vertical direction at th e supports to allow for reversible loading,without restraining beam movement in the horizontal direction.Two wide flange steel beams were atta ched with stressing bars intothe floor of the test bay and used for end supports to the concretebeams. At the end supports, steel rollers were stressed against thebeam from top and bottom, and locked in place by the apparatusshown in Figs. 4a and 4b (Section A-A). Small steel plates wereused to spread the load at the line of contact between the roller andthe concrete surface. The plates were placed on the concrete andleveled with Hydrostone (high-strength gypsum) to achieve asmooth transfer of stresses to the concrete. Neglecting to ensuresmooth transfer of loads would result in stress concentration andpremature failure with cracks initiating at that point. The slender,high-strength bolts were hand-tightened with wrenches, and theirflexibility allowed them to bend with the beam and keep loadsvertical.The rollers were centered on the supports with the help of rollerguides, which were welded to the supports perpendicular to theconcrete beam, leaving a space tha t was 3 mm (1/8 in.) wider thanthe rollers. The rollers were not prevented from rolling with themovement of the beam as it was cycled, but remained centered onthe support.

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FIG. l--Detail o f r e i n f o r c e d c o n c r e te b e a m s u s e d i n f a t i g u e t e s t s FIG. 2 - - C o n c r e t e b e a m s w e r e l o ad e d in l e x u r e u n d e r t h e " T h i r d P o i n t "{ 1 i n. = 2 5 .4 mm ) . l o a d i n g c o n f i g u ra t io n (A S T M C 7 8 ) ( l f t = 0 .3 0 5 m) .





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16 0 JOURNAL OF TESTING AND EVALUATION

Steel Columns stressed to the FloorDiI

Load TransferBolts tyingactuator toloading frame

Tie-downbeam

civil engineering seminar topic, fatigue of submerged concrete under low cycle high magnitude loads

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