Construction and Building Materials 27 (2012) 91–96

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Construction and Building Materials

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Use of polymer modified mortar in controlling cracks in reinforced concrete beams

S. Ahmad a, A. Elahi a, S.A. Barbhuiya b,⇑, Y. Farid a

a University of Engineering and Technology, Taxila, Pakistanb University of the West of Scotland, Paisley, United Kingdom

a r t i c l e i n f o

Article history:Received 26 March 2011Received in revised form 8 August 2011Accepted 8 August 2011Available online 3 September 2011

Keywords:CracksPolymer modified mortarReinforced concrete beamsDeflection

0950-0618/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.08.023

⇑ Corresponding author. Address: School of EngineerScotland, Paisley PA1 2BE, United Kingdom. Tel.: +44

E-mail address: salim.barbhuiya@uws.ac.uk (S.A. B

a b s t r a c t

This paper presents the results of an experimental investigation on the strengthening of existing crackedRC members. The proposed technique consists of applying locally available polymer modified mortar incracked beams to increase the load carrying capacity. A total of six full scale RC beams were constructedwith the same material using the same mix and water–cement ratio. Initially, beams were tested until thedevelopment of cracks with width reached a limiting value of 1 mm. The beams were then repaired withthe application of polymer modified mortar technique. After 3 days of water curing the beams weretested again and loaded till the failure. An improvement in the load carrying capacity was observed inthe beams after the retrofitting. Results clearly demonstrate the effectiveness of the proposed techniquein repairing the RC members for strengthening the existing structures.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Reinforced concrete (RC) is the most frequently appliedstructural material because of its durability, which has been usedfor many years to build a wide variety of structures from housesto bridges. Cracking and spalling are the most common phenome-non of deterioration of concrete. Cracks in concrete may occur inboth the plastic state as well as hardened state owing to the inter-nal stresses which arise from the response of the constituent’smaterials to the external excitation as well as their environment[1]. Beyond certain limit, cracks not only destroy the serviceabilityof the structure, but also lead to exposure of reinforcement to theenvironment resulting possible corrosion. Therefore, cracks maycause considerable structural distress or a deficient in durability[2–5]. Repair and rehabilitation of deteriorated concrete structuresare essential not only to use them for their intended service life butalso to assure the safety and serviceability of the associated com-ponents so that they meet the same requirements of the structuresbuilt today and in future. The repair of cracks depends not only onthe understandings of the causes, but also on the selection of asuitable repair technique that takes the causes into account. Suc-cessful long-term repair procedure must address the causes ofthe cracks in addition to mitigate the cracks themselves [6,7]. Con-ventional upgrading techniques often lead to heavy demolition,lengthy construction time, reconstruction and relocation of inhab-itant with all the related costs (direct and indirect). Huge indirect

ll rights reserved.

ing, University of the West of1418483451.arbhuiya).

costs, the environmentally hostile approach and the hassle associ-ated with conventional techniques are some of the major reasonsthat discourage owners and custodians of buildings from theirany commitment regarding retrofitting.

A number of retrofitting strategies for RC structures have beenimplemented in the past [8–10]. Concrete or steel jacket provisionto defective RC members improves the strength in addition to thestiffness enhancement. The use of a shotcrete overlay is also acommon technique. This technique involves adding an extra rein-forcement and a layer of new concrete around the existing ones[11–13]. Although this technique may increase the strength, stiff-ness and ductility several deficiencies are also encountered. Uncer-tainty between the bond of new and existing concrete surface isthe major drawback. These are also proved to be time consuming,labour intensive and may create a disruptive situation. In order toavoid these possible situations, steel cage is used as an alternativeto the complete jacketing [14–16]. Non-shrink grouts are normallyfilled in the available spaces between the steel cage and the exist-ing concrete. A grout concrete or shotcrete cover may be providedto ensure resistance against corrosion and fire. An improvement inshear and flexural strength can also be achieved by the implemen-tation of steel plate adhesion [17]. However, there is a need ofsound understanding of both the short and long-term behaviourof the adhesive used in this case. In addition, reliable informationconcerning the adhesion to concrete and steel is still to beexplored. Composite materials such as fibre reinforced plastic(FRP) can also be used to retrofit the existing structures [18].However, it is important to note that the use of FRP is oftendictated by strain limitations [19]. The large differences in strengthand coefficients of thermal expansion can result in bond deteriora-tion and splitting of concrete [20].

Table 1Physical properties of aggregates used.

Aggregates Relative density (SSD condition) Water absorption (%)

Fine aggregate 2.63 1.20Coarse aggregate 2.60 0.99

92 S. Ahmad et al. / Construction and Building Materials 27 (2012) 91–96

Epoxy injection has also been used as repair material [6]. This isan economical method of repairing non-moving cracks in concretestructure and is capable of restoring the concrete to its pre-crackedstrength but the epoxy bonded systems also exhibit some disad-vantages. For example the diffusion closeness, poor thermal com-patibility with base concrete, sensitive to moisture on theadherents at time for bonding, hazardous working environmentfor the manual worker and the problem of minimum temperatureof assemble in cold climates. It is, therefore, of interest to replacethe epoxy adhesive with a mineral based bonding agent. Manycementitious mortars contain cellulose ethers as an additive to im-prove water retention and workability. However, after setting anddrying they will adhere poorly. In addition, cementitious mortarsare very hard, brittle and inflexible materials, whereas for manyapplications flexible and deformable cementitious materials areessential. Un-modified mortar made with Portland cement is nor-mally used as repaired material in some field applications. How-ever, these kinds of mortars have some disadvantages due toinorganic nature of the material [21]. These disadvantages include(1) low tensile, flexural, shear, impact and bond strengths, (2) largedrying shrinkage, (3) tendency to crack with changes in tempera-ture and moisture, (4) relatively high moisture absorption and per-meation, (5) low resistance to corrosive agents, and (6) rapid loss ofgauging water in thin surfacing by evaporation and substrateabsorption. Afridi et al. [22] examined that abundance of cracksand pores were present in electron micrograph of a hardenedunmodified cement mortar.

Compatibility of the repair materials with the existing substrateand durability under various conditions in service are of muchgreater importance. Apart from bond and compressive strengththere are some other properties such as dimensional stability(shrinkage), coefficient of thermal expansion, modulus of elasticityand permeability which should be taken care during selection of arepair material. The other factors like permeability, chemical andelectrochemical compatibility should also be considered [23].

Polymer modified mortars (PMM) are considered to overcomethese limitations [24]. Also from the viewpoint of sustainabledevelopment in the construction industry, environment-consciousPMM has been developed with a great interest in recent years.The PMM with high performance, multi-functionality and sustain-ability are expected to become the promising construction mate-rials in Pakistan in the 21st century. Polymer modified mortars(PPMs) are related to setting shrinkage control, thermal proper-ties and temperature dependence; lightweight or porous nature[24].

PPMs are environmentally friendly and can be classified as con-struction materials that support the sustainability aspects of con-struction. These types of cementitious materials address concernsabout saving natural resources, provide longevity to infrastruc-tures, and protect the environment. Not only it strengthens theRCC structural members but also makes a highly durable repairalong with excellent adhesion characteristic even to difficult sub-strates [25]. The polymer films are responsible for improvedmechanical and durability characteristics of such systems that re-sults in enhanced properties compared with conventional cementmortar mixes [24]. As a consequence for almost all applicationsin modern construction, the modification of cementitious mortarswith polymers is a must. Availability of polymers as redispersiblepolymer powders has helped to develop prepackaged polymermodified cementitious mixes requiring only water to be added atconstruction site prior to application. Prepackaged polymer modi-fied cementitious are highly helpful in improving the handling pro-cedures and to avoid mixing error. Even then the people inconstruction industry are reluctantly using this product in as suchform because only very little research data is available and argu-ments are on cost factor.

The usage of PMM as strengthening material for pre-cracked RCmembers is not well documented; no sufficient research data isavailable on use of pre-packaged PPM except some successful casehistories of repair [26]. Such current case histories and results onsome of its other properties [22,27] encouraged to investigate per-formance behaviour of the RC beams using PMM. The research re-ported in this paper was aimed to develop a new retrofittingtechnique, especially for a developing country like Pakistan, result-ing more effective and cost benefiting repair and strengthening forrestoration of pre-cracked RC structures. The proposed techniqueconsists of application of PMM in cracked beams to increase itsload carrying capacity.

2. Experimental programme

2.1. Materials

2.1.1. Constituents of concreteThe cement used was Ordinary Portland cement complying with ASTM type 1.

Fine and coarse aggregates used were from the local sources in Pakistan. The coarseaggregates consist of crushed stones having a maximum size of 10 mm. The phys-ical properties of aggregates used are summarised in Table 1. Tap water was used inmanufacturing the concrete. Steel form-work (moulds) was used for casting beams.Deformed steel Grade 40 for # 3 bars and Grade 60 for # 4 bars were used in the testbeams.

2.1.2. Materials for repairPolymer modified mortar (PMM); a commercial prepackaged formulation

developed locally in Pakistan was used as repair material. This is a proportionatemix of cement, sand, an organic film-forming re-dispersive polymer powder andother related ingredients in appropriate ratios. A comparison of properties of PPMwith those of normal (unmodified) mortar properties are shown in Table 2. ThePMM into the crack was injected by following the procedure as laid down by themanufacturer [25].

2.2. Mix proportions

The proportions of the concrete mixes for preparing RC beams were 1:2.4 bymass of cement, sand and gravel respectively. The mix was designed to produce aconcrete with 28 days cylindrical strength of about 30 MPa. The water–cement ratio(w/c) was kept 0.50. The workability of concrete was measured in terms of slump asper ASTM C 143 [28] and a slump of 35 mm was noted. An average cylindrical com-pressive strength of 30.5 MPa was achieved.

2.3. Preparation and curing of beams

Beams with a constant cross-section of 152.4 mm � 304.8 mm and a length of3353 mm were prepared in two categories, namely Category HB and Category GB.There were three beams in each category, resulting altogether six full-scale beams.Category HB had flexural reinforcement only, and these beams were designated asHB-1, HB-2 and HB-3. Category GB had the flexural as well as shear reinforcement,and these beams were designated as GB-1, GB-2 and GB-3. The cross-sections of thebeams for Category HB and Category GB are shown in Fig. 1a and b respectively.Two days after casting, the forms were removed and the beams were wrapped inwet hessian cloth. Curing was continued by keeping the hessian cloth wet untilthe testing at the age of 28 days.

2.4. Testing procedure

RC beams were simply supported and loaded in flexure under a two points load-ing conditions. The position of the loads and the set up of the machine are shown inFig. 2. The load was applied in increments till the cracks appeared to have a width of1 mm which is a value well beyond the allowable limits defined by ACI 318-05 [29].The crack width was determined using a non-destructive crack inspection device(crack comparator). The load at crack width equal to 1 mm, mid-span deflectionsand failure loads were recorded.

Table 2Comparison of PMM with ordinary mortars [21].

Property PMM Ordinarymortar

Unit weight (kg/l) 1.9 2.0Air content (%) 8.2 6.1Water retention (%) 96.6 70Total pore volume (�10�2 cm3/g) 10.3366 11.253128 day compressive strength (MPa) 320 234Maximum deflection (�10�1 mm) 1.0 0.42Maximum extreme tensile fibre strain (�10�6) 1231 385Maximum tensile strain (�10�6) 380 82Flexural modulus of elasticity (�104 MPa) 0.631 0.736Tensile modulus of elasticity (�105 MPa) 0.227 0.263Crack coefficient (�10�2 cm2/kg) 0.020 0.037Adhesion in tension (MPa) 2.2 5.0Water absorption (%) 9.3 12.2Water permeation (g) 6 66Freeze thaw durability factor 72 1091 day carbonation depth (mm) 10 2191 day chloride ion penetration depth (mm) 10.5 22.5Apparent chloride ion diffusion coefficient

(�10�9 cm2/s)0.2 13.2

(a) Beam of HB Category (b) Beam of GB Category

152 mm

304 mm

4#4

2#3

4#4

152 mm

304 mm

Fig. 1. Cross-sections of beams.

3048 mm

1015 mm 1015 mm 1015 mm

Proving Ring

Hydraulic Jack

Load Distributor Strain Data Logger

Deflection Gauge Strain Gauge

Fig. 2. Test specimens and loading arrangement.

S. Ahmad et al. / Construction and Building Materials 27 (2012) 91–96 93

Prior to injecting the PMM, repair area was prepared and reinforcement wasexposed by blast cleaning. It was insured that surface was free of oil, dirt, andloose material. The repair area was saturated with water to develop a saturatedsurface-dry condition. Standing water was removed with cotton cloth. PMM wasmixed with water to obtain a mix of standard consistency and then injectedinto the crack according to the manufacturer’s instructions. Cotton mats wereapplied on the exposed layer of mortar within 10 min after finishing, and thenwet curing was started immediately. Curing was continued for a minimum of3 days.

3. Results and discussion

Non-repaired beams tested till the cracks appeared to have awidth of 1 mm are termed as controlled beam. After these damages(in terms of cracks) occur, beams were repaired using PMM. Thesesbeams tested after repairing are known as repaired beams. The fail-ure patterns of beams are shown in Fig. 3. The first crack in all thebeams formed approximately 10–50 mm from the centre line atthe region of maximum moment. This implies that yielding of steelreinforcement started almost at the same region. Although eachcategory had three RC beams with same cross-section and rein-forcement details, average values were used in discussing the re-sults. The slight variation displayed in the results of samecategory beams appears to be due to the variation in the concrete’smodulus of rupture as well as in the repair materials. The lowervalues in the same category may be attributed to the poor mixingand inadequate curing. As the first crack is normally very suddenand may remain invisible for a certain period of time, the values re-corded might not exactly be the same with the actual first crackloads. The increase in load and deflection at 1 mm crack widthand beyond this are summarised in Table 3. The discussion on re-sults obtained is made in the following sections.

3.1. Effects of repairing material on beams of HB Category

Experimental results regarding average enhancement in thegained strength (in terms of load carrying capacity) and averagedeflection of repaired beams of HB Category as compared with con-trol beams is reported in Table 3. The deflections in control and re-paired beams for Category HB (with flexural reinforcement only)are shown in Figs. 4a–4c. For HB Category, an average increase inload by 22% over the control beams was observed to produce acrack width of 1 mm. This enhancement in load carrying capacitywas increased by 20% as compared with control beam beyondthe development of crack of 1 mm width. In this Category the aver-age deflection was 39% and 11% more than the control beams be-fore and after development of crack of 1 mm width respectively.Therefore, the capacity enhancement due to PMM applicationwas gained. However, the beams could not utilise their full mo-ment strength and failed due to the widening of shear cracks. Itcan be observed that the relationship between the applied loadand the corresponding deflection is approximately linear up tothe cracking load. After cracking, the increase in deflection wasnoted; hence, causing deviation from linearity up to the failure.

3.2. Effects of repairing material on beams of GB Category

Once the existing cracks were structurally repaired by PMMapplication, the RC beams of Category GB also behaved with im-proved load carrying capacity. Experimental results regardingaverage enhancement in the gained strength (in terms of load car-rying capacity) and average deflection as compared with controlbeams is reported in Table 3. The deflections in control and re-paired beams for Category GB (with both shear and flexural rein-forcement) are shown in Figs. 5a–5c. For this category, there wasan average increase (36%) in the load to produce a crack width of1 mm as compared to the control beams and this enhancement(in comparison) was observed up to 53% after the developmentof crack width of 1 mm. The average deflection at 1 mm crackwidth was 43% more than the control beams and this rate inenhancement continued unchanged up to beam failure. This meansthat the beams have utilised full or maximum flexural strength andfailure occurred due to both flexural and shear cracks. However,the shear crack observed was thicker in size as compared to thoseexhibited in the repaired beams of HB Category. It can be observed

Fig. 3. Various beams after failure.

Table 3Comparison of repaired beams with control beams.

Beam ID Increase at 1 mm crackwidth (%)

Increase beyond crack width1 mm(%)

Load Deflection Failure load Deflection

HB 22 39 20 11GB 36 43 53 43

0

3

6

9

12

0 20 40 60 80 100

Def

lect

ion

(mm

)

Load (kN)

Control beamRepaired beam

Fig. 4a. Deflections of control and repaired HB-1 beam.

0 20 40 60 80 100

Load (kN)

0

3

6

9

12

Def

lect

ion

(mm

)Control beamRepaired beam

Fig. 4b. Deflections of control and repaired HB-2 beam.

0

3

6

9

12

Def

lect

ion

(mm

)

Load (kN)

Control beamRepaired beam

0 20 40 60 80 100

Fig. 4c. Deflections of control and repaired HB-3 beam.

94 S. Ahmad et al. / Construction and Building Materials 27 (2012) 91–96

that similar to the Category HB, there exists a straight line relation-ship between the applied load and the corresponding deflection upto the cracking load. However, after cracking, the deflections werefound to increase rapidly and hence, considerably deviation fromlinearity until the beam failed.

3.3. Efficiency of reinforcement pattern on capacity enhancement

A comparison of experimental values has been reported in Table3 and these values are presented in Figs. 4a–5c. Experimental re-sults show that a considerable increase in both the load carryingcapacity and the deflection was observed for repaired beams ofGB Category. The load carrying capacity beyond 1 mm crack at fail-ure for GB Category (Figs. 4a–4c) was found to be more than 2.5times than that of the GB Category (Figs. 5a–5c). However, thisenhancement was 64% before the development of 1 mm crack.

The increase in deflection was noted both before and after thedevelopment of 1 mm crack in repaired beams of GB Category.However, the increment before the development of 1 mm crack(10%) is marginal as compared to that (291%) after the crack. Thiscould be due to absence of shear reinforcement and reinforcementin the compressive zone. This is also in agreement to the failuremode observed for GB Category where most of the repaired beamsfailed after exhausting their moment capacity having reserved po-tential for the shear capacity.

0

3

6

9

12

Load (kN)0 20 40 60 80 100

Def

lect

ion

(mm

)

Control beamRepaired beam

Fig. 5a. Deflections of control and repaired GB-3 beam.

0

3

6

9

12

Def

lect

ion

(mm

)

Control beam

Repaired beam

Load (kN)0 20 40 60 80 100

Fig. 5b. Deflections of control and repaired GB-2 beam.

0

3

6

9

12

Def

lect

ion

(mm

)

Load (kN)

Control beamRepaired beam

0 20 40 60 80 100

Fig. 5c. Deflections of control and repaired GB-3 beam.

S. Ahmad et al. / Construction and Building Materials 27 (2012) 91–96 95

It was noted that For HB-1, the repair technique was effectivefor shear cracks. Shear failure was observed by a quite new devel-oped shear crack for the HB-1 repaired beam rather than the re-paired cracks. Shear failure was also observed for GB-3, eventhough they had the shear reinforcement. It shows the effective-ness of modified mortar technique for flexural cracks.

RC beams of Category GB with PMM application exhibited thesame or slightly higher initial stiffness than their counterpart con-trol beams. This proved that the applications of PMM helped to ‘‘re-store’’ their initial pre-cracked stiffness. However, only repairedHB-1 beam of Category HB showed a considerable stiffness after50% of the load caused a crack width of 1 mm in the controlledbeams. Also no improvement in stiffness was observed by usingthis retrofitting technique. Additional tests on RC beams areneeded to verify the results reported in this paper and to identifyother parameters which might affect the PMM application.

4. Conclusions

On the basis of the results obtained from this study, the follow-ing conclusions can be drawn:

(i) RC beams can be strengthened by repairing the existing flex-ural and shear cracks with PMM application and this canlead to a considerable (36%) increase in the load carryingcapacity.

(ii) Most of the beams have failed in shear, which representsthat PMM application injection is more effective for flexuralcontrol of cracks. One of the beams has failed due to openingof repaired cracked which strongly suggests that applicationof PMM is also efficient in the repair of cracked concretestructures.

(iii) Repaired RC beams of Category GB (with flexural and shearreinforcement) showed more stiffness than the beams ofCategory HB with flexural reinforcement only.

(iv) Application of PMM exhibited no improvement in the stiff-ness of repaired beams for both the categories except at ini-tial stage.

It is worthwhile to mention here that in this study when loadfrom the beams was removed the crack width decreased to a cer-tain value (less than 1 mm) due to which less PMM was applied tocrack, but in a practical field when the members will remain underpermanent loading condition, more PMM may be required to injectin the cracks and more efficient results may be obtained.

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