331-348 Materials for the Repair and Strengthening of Concrete Structures the Influence of Material...

8/8/2019 331-348 Materials for the Repair and Strengthening of Concrete Structures the Influence of Material Properties on S

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MATERIALS FOR THE REPAIR AND STRENGTHENING OFCONCRETE STRUCTURES: THE INFLUENCE OF MATERIALPROPERTIES ON STRUCTURAL PERFORMANCE

G C MaysCranfield University, Royal Military College of Science, UK

ABSTRACTThis paper reviews a number of research programmes which have been undertaken incollaboration with industry concerning the selection of materials for the repair andstrengthening of concrete structures. The research has been primarily concerned withthose material properties which affect the structural performance and durability of patchrepaired systems and the selection of appropriate adhesives for strengthening using thetechnique of bonded external reinforcement. The important material characteristics forpatch repair systems are identified and, using appropriate test procedures, the properties ofa range of generically different materials are presented. Similarly, the properties ofadhesives which are relevant to satisfactory structural performance in plate bondingschemes are discussed and the influence of temperature and moisture on these systems isemphasized. Finally, the paper presents the outcome of both analytical and experimentalstudies concerning the structural integrity of patch repaired column and beam elements,including the effect of repairing the structure whilst still under load.

KEYWORDSRepair, materials, strengthening, adhesives, concrete

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A mismatch in coefficient of thermal expansion or subsequent shrinkage of the repairmaterial may result in differential strains between the repair material and the concretesubstrate which eventually lead to interfacial bond failure . A material which exhibits highcreep may become less effective structurally with time under sustained load. Thesignificance of a mismatch in modulus of elasticity will be explained later in this paper byreference to the predicted and observed behaviour of repaired elements.

DURABILITY OF PATCH REPAIR SYSTEMSFurther material characteristics of importance in ensuring satisfactory durability of repairmaterials will include:(a) the effect of curing temperature and temperature in-service on adhesion to the substrate

concrete(b) the effect of thermal, moisture and freeze-thaw cycling on adhesion strength(c) the effect of cyclic loading in-service on adhesion strength(d) the effect of temperature on modulus of elasticity. tensile strength and development of

compressive strength(e) permeability to water, gases and salt solutionThese matters have been investigated by Chandler and Mays (1998) in connection with therepair of concrete pavements, again using a range of generically different repair systems.'t'emperatures in (a) and (d) ranged from 5 C to 30 C whilst the thermal and moisturecycling in (b) involved temperatures up to 40 C and "thunder-shower" conditions.For each property, the repair materials were ranked in order of performance and pointsawarded on a scale of I to 10. The results are summarised in Table 2. The polyester resin,high alumina cement and opc/sand materials performed poorly. particularly with respect toadhesion to the substrate after thermal, moisture and load cycling. Overall, the remainingmaterials performed satisfactorily.

STRUCTURAL PROPERTIES OF ADHESIVES FOR BONDED EXTERNALREINFORCEMENTThe purpose of the adhesive in bonded external reinforcement is to produce a continuousbond between the reinforcing plate and the concrete to ensure that full composite action isdeveloped by the transfer of' shear stress across the adhesive layer. A typical adhesiveshear stress distribution in a plated beam as derived by Mays (1993) is shown in Figure1(a), from which it can be seen that the shear stress increases to a value of about 4 timesthe mean value at the plate ends. This is because the external plate has to be curtailed in atension zone and as a result normal, or peeling, stresses are also generated at the plate endsas shown in Figure 1(b).The magnitude of these stress concentrations is heavily dependent on the modulus ofelasticity of the adhesive, as shown in Figure 2. The modulus may vary from one adhesiveto the next and as a result of temperature variations or plasticisation by water. In order tocontrol the magnitude of stress concentrations a maximum value of 8 to 10 kN/mm2 isnormally recommended. I fowever, at very low values of modulus (


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transfer becomes affected significantly. Since long-term creep effects can be related tofurther decreases in elastic modulus, an initial adhesive elastic modulus of at least2kN/mm2 would seem desirable.Figure 3 shows typical creep relationships between "normalised" stress and time to failurefor steel double overlap joints made with a cold-cure (5 - 30 C) epoxy. It is normallyrecommended that any sustained stress in the adhesive be kept below 25% of the slit '1term joint strength in order to minimise any creep effect . The significant reductiperformance at higher temperatures will be referred to later.If the joint is likely to be subjected to cyclic loads during service , a fatigue asses.must be carried out. A typical relationship between the stress range applied to double .pjoints and the number of cycles to failure is shown in Figure 4. A limit is normally placedon the shear stress range in the adhesive in service (q 4N/mm2). Again , the significanteffect of elevated temperatures should be noted.

INFLUENCE OF TEMPERATURE AND MOISTURE ON STRUCTURALADHESIVES

Not only is the rate of cure of an adhesive sensitive to the ambient temperature andhumidity but so also are its mechanical properties in the hardened state. Of particularsignificance is the temperature at which thermosetting adhesives such as epoxies changefrom a "glassy" to a "rubbery" phase. This Glass Transition Temperature, T,., may be aslow as 40 C for some "cold-cure" construction epoxies and it signifies the temperature atwhich there is a marked reduction in engineering properties.

The effect on creep and fatigue performance of temperatures in excess of Tg (ie testing at55 C) is well illustrated in Figures 3 and 4

. The effect of temperature on the flexuralmodulus of a range of epoxy adhesives is shown in Figure 5. For applications in hotclimates, "warn-cure" (50 - 80 C) variants are available with Tg values increased to 70 -80 C.Many adhesives absorb moisture . in some cases the saturation level reaching 8% byweight . This has the effect of causing plasticisation and the effect on flexural modulus isshown in Figure 6. To limit this effect a maximum water uptake of 3% by weight isnormally specified for structural adhesives. In addition, the long-term stability of theadhesive/substrate interface must be checked since this will he of vital importance to jointdurability .

For this purpose a durability test involving a self-stressed wedge joint issuggested.

STRUCTURAL BEHAVIOUR OF REPAIRED ELEMENTSThree dimensional linear elastic finite element techniques have been used by Embersonand Mays ( 1990 and 1996 ) to develop an understanding of load transfer through simplepatch repairs in reinforced concrete members subject to either axial (Figure 7) or flexural(Figure 8) loading. As might be expected, a low E value for the repair material generatesrelatively high stresses in the substrate concrete and the possibility of high longitudinalinterfacial shear stresses.

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Concentrations of stress also occur in the concrete immediately adjacent to the transverseinterface with the repair material. In the tension zone this may lead to premature flexuralcracking and may also have long-term durability implications.Conversely, a high E value for the repair material causes load to be attracted into therepair, thus placing greater structural demands upon the material. In zones of tension stressthis may result in a requirement for an extremely high bond stress at the transverseinterface between the repair material and concrete. otherwise interfacial failures mayoccur.Experimental programmes have also been conducted on reinforced concrete elementsloaded in either axial tension or flexure. For the specimens loaded axially in tension, atotal of nine generically different repair systems were used. The experimental strain resultsshowed good agreement with the finite element predictions in the elastic range providedthere were no interfacial adhesion failures. Such failures occurred with the polyester resinand the unmodified cementitious mortar.For the flexural specimens six repair systems were used, four of which were common withthe axially loaded prisms. Again, the tests confirmed the accuracy of the finite elementpredictions, particularly as far as mismatch in elastic modulus is concerned for repairs onthe compression face. For repairs on the tension face, the tensile strength of the repairmaterial appears to be a more important parameter, high values generally enhancingflexural performance.Repaired flexural specimens were also subject to creep and fatigue trials. In the former, asustained load of approximately one-third of ultimate was maintained for a period of atleast one year. The results showed that when the patches extend into the shear spans, themechanical properties of the materials start to have an effect on the creep performance.For example, a beam repaired with the acrylic resin mortar exhibited relatively high creepdeflections. a result which reflects the value of creep strain measured in long-term creeptests on the pure material.Load cycling of repaired beams was conducted at 1 Hz over a load range of approximately30% of ultimate. In all cases failure occurred as a result of tensile fatigue fracture of themain reinforcement steel.

REPAIRS UNDER LOADThe experimental work referred to above was based on ideal' specimens fabricated underlaboratory conditions and repaired under zero load. Mays and Barnes (1995) extended thiswork to account for the load state of the structure during repair, the method of removingthe concrete and the interaction of bending and axial loads in members. To this end,reinforced concrete H-frames were designed on a scale of 1:2.5 to represent either a typicalinternal ground-floor frame in a building or a bridge sub-structure. Figure 9 shows aschematic arrangement of a repaired 11-frame within the loading rig.A total of seven frames were constructed, one to act as a control and three with each of tworepair materials. These were a spray-applied micro-concrete and a hand applied acrylic

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modified cementitious repair material. The repairs were applied by a specialist repaircontractor to concrete surfaces which had been prepared using either grit blasting or waterjetting. Three alternative load states were employed during the repair process: no load at repair (representing a propped structure);

equivalent of dead load at repair; equivalent of combined dead and imposed load at repair.A programme of loading and unloading cycles was conducted on each frame to simulateoperation under design loads, before finally loading to failure. Strain and deflectionreadings were taken throughout. The load-deflection curves for frames repaired under zeroload were similar to each other and to that of the unrepaired control frame as shown inFigure 10. Also, there was no significant difference in overall behaviour between the twoframes repaired with different materials under a load simulating dead load only. At lowerlevels of load there was an understandable variation in load-deflection characteristics ascompared with the frames repaired under zero load, because some deflection had occurredbefore repair. However, at loads exceeding 60% of ultimate, the effect of load level at thetime of repair became insignificant. Similar deductions could be drawn for the two framesrepaired under a load simulating dead and imposed load.Examination of the strain readings revealed essentially linear strain distributions across thesection. thus providing strong evidence that full composite action is occurring across therepaired section.

CONCLl-S1O1\SThe results of the programmes of research summarised in this paper have led to thefollowing conclusions with respect to the structural repair and strengthening of concrete:(a) The desirable characteristics of repair systems for satisfactory structural

performance have been identified. European Standards for the specilication andtesting of repair materials are becoming available for the measurement of therelevant mechanical and physical properties of the material. From the results ofsuch tests it is possible to draw some conclusions as to the suitability of a system ofstructural repair.

(b) Further material characteristics of repair materials for satisfactory durabilityperformance have also been identified. Measurement of these properties formaterials proposed for the repair of concrete pavements has enabled their relativeperformance to be evaluated.

(c) Externally bonded reinforcement is a method which can offer time and costadvantages over other methods for the strengthening of concrete structures.However, the selection of a suitable adhesive is a necessary requisite liarsatisfactory structural and durability performance.

(d) The structural properties of an adhesive such as strength. modulus of elasticity.creep and fatigue performance are sensitive to both temperature and moisture in-

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service. Adhesives grades must be selected which are appropriate to theenvironment in which they will be required to operate.

(e) Finite element stress analysis techniques may be used to gain an understanding ofload transfer through patch repairs in reinforced concrete members. The modulusof elasticity of a patch repair material may have a significant influence on thedistribution of stress within a repaired beam, particularly in zones of compressionstress.

(f)

( g )

The flexural performance of a beam repaired in a zone of tension stress is enhancedby the use of a relatively high tensile strength repair material. Under sustainedloads, the creep deflections of beams may be directly related to the creepcharacteristics of the repair materials themselves. The fatigue performance ofrepaired beams is primarily related to that of the steel reinforcement.Experimental studies of structures repaired under load have demonstrated that it ispossible to achieve full composite action across a repaired section. In order toachieve such structural behaviour, care must be taken in the selection of appropriaterepair materials and in their application. Grit blasting and water jetting of thesubstrate concrete are both suitable methods of surface preparation. Given theabove conditions, load levels at the time of repair up to and including the fulldesign load do not appear to influence the ultimate behaviour of the repairedstructure.

ACKNOWLEDGEMENTS"Ihe research reviewed in this paper has been carried out in collaboration with thefollowing organisations whose support is gratefully acknowledged: Balvac Whitley MoranLtd, Department of the Environment. Engineering and Physical Sciences ResearchCouncil, G Maunsell and Partners. Sika Ltd and the Transport Research Laboratory.

REFERENCES

Chandler, J.W.E. and Mays G.C. (1998). The evaluation of tests for repair materials usedon concrete pavements . Transport Research Laboratory Report 303. UK.

Emberson, N.K. and Mays, G.C. (1990). " Significance of property mismatch in the patchrepair of structural concrete : Part l: Properties of repair systems: Part 2: Axiallyloaded reinforced members. Vag. Contr. Res., Vol. 42, No. 152. pp 147-160.161-170.

Emberson, N.K. and Mays G.C. (1996). "Significance of property mismatch in the patchrepair of structural concrete: Part 3: Reinforced concrete members in flexure.Mug. Contr. Res. Vol. 48. No. 174. pp 45-57.

Mays, G.C. (1993), "The use of bonded external reinforcement in bridge strengthening:structural requirements of the adhesive." Bridge Managemen! 2 (Edited by J.E.Harding et al), Thomas Telford, London, pp 672-680.

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Mays, G.C. and Barnes , R.A. (1995), "The structural effectiveness of large volume patchrepairs to concrete structures ." Proc. Instn. Civ. Engrs. Structs I3ldgs, Vol. 110,November , pp 351-360.

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Nominal elastic shearstress distribution

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Figure 6 Moisture dependence of bulk adhesive flexural modulus

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10mm dia. deformedcold-worked bars

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Figure 7 Patch repairs in axially loaded member

Patch repair (shown in compression zone)1760mm (long) x 150mm (wide) x 70mm (deep)

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Figure 9 Schematic test arrangement for H-frames

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331-348 Materials for the Repair and Strengthening of Concrete Structures the Influence of Material...

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