Stabilizing, Repairing & Strengthening Cracked …canadianmasonrycontractors.com/designers/wp-content/uploads/sites/...Stabilizing, Repairing & Strengthening Cracked Traditional Masonry-3-A

From an article published in the Jan/Feb’07 issue of Construction Canada magazine.1

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Stabilizing, Repairing & Strengthening CrackedTraditional Masonry ~ Case Studies

Paul Jeffs ~ PJ Materials Consultants Limited

Case Study One - Hart Hall1

Completed in two stages during 1910 and 1920, Mount Allison University’s Hart Hall is one of several historicbuildings located on its Sackville, N.B campus.In keeping with most of the campus’ structures, the masonry was

constructed using red and buff sandstone units predominantly fabricated from a locally sourced quarried stone.Over time, it became obvious Hart Hall’s masonry was having difficulty accommodating its imposed loads dueto natural movement of the building. As a result, considerable cracking became progressively evident. Prior tothe author’s involvement, several attempts had been made at restoration, but damage continued to occur.

Following several inspectionsthat culminated in a 2004investigative study, thechallenge was to create astrategy that repaired thedamage, improved themasonry’s durability, andbetter accommodated thebuilding’s movement. Thesegoals needed to be achievedwithout dramatically changingthe established dynamics ofthe more than 90-year-oldbuilding (See Photograph 1).To this end, it was consideredunlikely masonry restorationcould be successful unless thecauses - rather than just theresults - of the damage werecorrected.

Photo. 1. Hart Hall - prior to extensive restoration.

Stabilizing, Repairing & Strengthening Cracked Traditional Masonry

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Investigation Process

The investigation process began with thorough inspections of theexterior masonry, as well as the condition of the interior walldecorations. A 25-m (82-ft) boom man-lift was used to facilitate closeviews of the upper levels of the exterior masonry. The locations andpatterns of cracks were noted during the inspections, particularlywhere they reflected through masonry units and building trim ratherthan ‘stepped’ through mortared joints (See Photograph 2).

The original construction drawings were unavailable, but on-siteinspections confirmed the foundation walls had been built usingtraditional mass masonry construction techniques. With thesemethods and masonry style, the above-grade exterior wythes ofmasonry were typically fabricated to specific dimensions, dressed toa ‘rock-face’ finish, and laid in a random modular pattern ashlarcoursing style. (Interior wythes were constructed using random rubblestone.) Below-grade, the exterior wythes were also constructed usingrandom rubble stone.

As construction of the interior and exterior wythes proceeded, the core of the masonry assembly would havebeen progressively formed by filling the space in between with off-cuts and rubble laid in a lime/sand mortar.It was estimated the total thickness of the constructed foundation walls was approximately 610 mm (24 in).During the investigation, it was ascertained the below-grade foundation walls had not been waterproofed,indicating water had most likely infiltrated the core rubble through deteriorated mortar joints, causing the rubblemortar to disintegrate.

Voids would have been almost certainly formed over the years as progressively more lime passed into thesolution and percolated the mortar, leaving behind a granular debris. On drying, some of this material wouldhave filtered downward, following the path formed by the lime solution. The effect of this hidden deteriorationmechanism would have been a redistribution of the imposed load away from the interior to the exterior wythesof the masonry assembly. This resulted in increased differential movement of the supported masonry above.Such excessive movement during the extremes of Canadian weather, combined with the increased loadsupported by the exterior masonry wythes, would almost certainly have resulted in progressive cracking.

During the investigation, ground-penetrating radar (GPR) non-destructive testing (NDT) equipment was usedat random, accessible locations to confirm the presence of voids and to provide an indication of their extent. Aninterpretation of test data confirmed the high probability of the existence of considerable voids. However, thesewere predominantly limited to the foundation wall inner-core rubble.

Photo. 2. Example of extensive cracking


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A study of crack locations, extent, and patterns indicatedthe original design of the window lintel and sill units didnot extend them sufficiently beyond the jamb stones(Photograph 3). This deficiency manifested in aconcentrated distribution of loads immediately adjacent towindow openings, particularly at jamb locations. The resultwas considered to be a major contributor to the degree ofcracking that had occurred in masonry units, almostcertainly exacerbated by the destabilized foundation walls.

A report following the investigation detailed the likelycauses of the building’s damage and recommended arestoration strategy that not only addressed the masonry’spoor condition, but also corrected the deficienciesconsidered to be its cause.

Specification and Bid Development

It was considered essential that specifications established not only the required materials and techniques, butalso the reasoning behind their selection. For this reason, the contract documents included the restorationphilosophy, as well as the strategy that had been carefully formulated from the post-investigation studies. Thelatter information was incorporated into the specifications as the scope of work.

Contract drawings were prepared to detail the extent of the work and included specific information for thevarious techniques required for the restoration. A pre-qualification process preceded the invitations to tender,resulting in masonry companies with the required minimum level of expertise in restoring heritage structuresbidding for the project. Pre-selected specialist subcontracting companies were pre-qualified within thespecifications for the critical grouting of the foundation walls. The company required to perform specialisttesting during the grouting operations was also pre-qualified and named within the specifications.

In view of the overall specialist nature of the required restoration work, specifications were developed using aprescriptive, rather than performance-based format. Therefore, each stage of the operation was described indetail within the ‘execution’ portion of each section.

Restoration Strategy and Project

The project began with excavation around the building perimeter to facilitate the installation of below-gradedrainage, foundation wall waterproofing, and inner-core rubble grouting.

Installation of Masonry Ties

450-mm (17.7-in.) long stainless steel helical masonry ties were installed across the thickness of the masonryassembly on a 600 x 400-mm (24 x 16-in.) grid pattern. The installed ties were concentrated at the locationswhere the masonry was to be subsequently grouted. In addition to their positive influence on improving themasonry assembly’s stability, the ties were required to resist the pressures generated by the grouting operations.

Photo. 3. The lintels do not project sufficiently beyond the

lintels.


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Testing of ties was carried out in advance of the installation process and this confirmed satisfactory anchoragescould be achieved when axially loaded up to 3116 N (700 lbf). The masonry ties were then installed using a dry-set method, whereby 10-mm (0.375-in.) diameter ties were spun through 8-mm (0.3125-in.) diameter holes thatwere first drilled through mortar joints upward at 45-degree angles. This installation method ensured the tiespenetrated and terminated in stone, rather than mortar.

Cellular Foamed Cement Grouting

To facilitate grouting, 12-mm (0.5-in.) diameter holes were drilled through mortar joints within the exposedbelow-grade masonry to terminate within the inner-core rubble. Injection tubes were then installed into the holesand the masonry ‘rough-parged’ to facilitate containment of the fluid grout during the subsequent corestabilization work (See Photos 4 & 5). Injection tubes were also installed through mortar joint intersections inthe above-grade portion of the masonry up to approximately the locations of the first window lintels.

A cellular foam grout was then mixed and pumped through the injection tubes to fill any voids within the inner-core rubble of the masonry. This part of the restoration strategy was designed to further stabilize the foundationwalls and provide for a more uniform load distribution across the cross-section of the masonry assembly. Thecellular grout was selected for its beneficial load absorbing characteristics, as well as its low-strength andlightweight properties. Benefits were also derived from the very low level of mixing water required within agiven volume of grout, thereby generating correspondingly low residual moisture within the masonry corefollowing cement hydration.

The liquid cellular grout, which was formulated to have a wet density between 720 and 800 Kg/m (45 and 503

pcf), was produced by blending a pre-determined weight of foam into a 0.45 water/cement (w/c) ratio grout (SeePhotograph 6). A methyl cellulose-type admixture was combined with the cement/water grout to providethixotropic properties to the plastic grout, minimizing leakage to the interior through cracks and gaps. Togenerate the foam, a proprietary surfactant-type admixture was diluted in water and forced through a nozzlewhen fed from a pressurized steel cylindrical tank (See Photograph 7). Following wet density assurance testingfor each batch, the foamed cement grout mixture was injected into the core using a grout pump fitted with 50-mm (2-in.) diameter lines.

Photo. 4. Injection tubes were installed through joints Photo. 5. The below-grade masonry was then rough-parged.


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The nozzle end was fitted with a pressure gauge, shut-off valve and a return line, which allowed control of theflow of grout and pumping pressures at the grout injection point. Approximately 10 m (350 cf) of grout was3

injected into the inner-core rubble voids, which represented about 7.5 per cent of the estimated volume ofgrouted masonry. Compressive strength of the hardened grout was routinely tested to be typically between 2 and5 MPa (300 to 750 psi) at 28 days.

Below-Grade Waterproofing and Drainage

After the grouting operation, theinjection tubes were removed and asand/cement mortar was parged over therough masonry surfaces of the below-grade portion of the foundation walls toprovide a smooth substrate. A cold-applied, waterproofing sheet membranesystem was then installed. (Photograph8) A relatively new (at that time) primingsystem was used, which considerablyreduced the waiting period otherwiserequired to facilitate sufficient drying ofthe parged surfaces. Once thewaterproofing operations werecompleted, new below-grade drainagepipes were installed and connected toexisting catch-basins. The excavatedareas were then back-filled andscaffolding was erected to facilitate theremaining above-grade work.

Photo. 7. Generating foam.Photo. 6. Adding foam to the grout mixer.

Photo. 8. Applying the waterproof membrane system..


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Masonry Joint Reinforcing

6-mm (0.25-in) diameter helical stainless steelmasonry reinforcing rods were embedded withinslots cut predominantly within horizontal joints andencapsulated within a high-strength mortar. (SeePhotograph 9) The rods were set back sufficientlywithin the joints to facilitate subsequent face-pointing and hide their location. The purpose of therods was to transfer gravity loads away from thewindow jamb units into the adjoining massmasonry.

Repointing

The mortar within all the above-grade masonry joints was cut back a minimum depth of 25 mm (1 in.) to soundmortar. Hand tools were primarily used for the cutting-out operations. The prepared joints were then repointedusing a proprietary pre-packaged hydraulic lime/sand mortar with a 1:3 ratio and containing a custom-grade dry,clean, sharp sand. The material was selected to provide compatibility with the dimension stone masonry units,particularly with regard to moisture vapour transmission properties. This is considered to be important in viewof the desirability for entrapped moisture to readily ‘breathe’ to the exterior where it could evaporate. It wasrecognized that conventional cement-based mortars could potentially inhibit these properties and would mostlikely be less compatible with regard to the accommodation of natural movement during temperature change.The specified sand grading and mortar joint finishing techniques were both selected to assist in producing amoderately weathered appearance and to optimize moisture vapour transmission properties.

Although the original mortar within the frontelevation masonry joints had been pink, thedecision was made not to pigment the repointingmaterial. This was largely due to pre-projectinspections, which confirmed the original mortarhad not in fact been pigmented. The apparent pinkcolour of the original pointing and some of theolder repointing mortar was actually runoff fromthe sandstone masonry units which, over time, hadstained the joints (See Photograph 10). The jointson the north elevation of the building wereunstained, since they had been sheltered from therain by overhanging trees.

Photo. 9. Installing helical reinforcing rods

Photo. 10. The original white lime based mortar revealed under poorly

repointed pigmented mortar


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Previous unsatisfactory attempts to match the discolouration - which created large unattractive variations inappearance - justified the decision to not pigment the repointing mortar. It was also taken into account that itwould be extremely difficult to continually achieve a uniform appearance when using pigments in small batchesof mortar to repoint the entire masonry joints. Additionally, there would be no assurance that uniformweathering of the pigmented joints would occur over time. Although the selected repointing mortar resulted ina change to the pre-project appearance of the building, the result more closely resembled the original appearanceof the masonry.

Masonry Repair

Cracked masonry units - particularlywindow opening lintels - were ‘stitched’using stainless steel helical masonry ties(See Photograph 11). After routing theopenings, cracks through masonry unitswere repaired using proprietary pre-packaged repair mortars, speciallyformulated for compatibility withsandstone (Photograph 12). In accordancewith established heritage restorationpractices, the repair mortars were designedto blend with either the original colour ofthe red sandstone masonry units or theoriginal colour of the buff sandstonebuilding trim units. No attempts weremade to match or blend with themasonry’s weathered appearance.

Proprietary repair mortars were used torestore damaged stone masonry units andareas where brackets for a redundant fireescape previously existed. Also, fivewindow sill units, which previously did not protrude sufficiently beyond the masonry, were extended using newsandstone units, pinned and bonded to ground sill faces with epoxy resin adhesive.

Post-Project Considerations

In accordance with established industry practices, the client was provided with a comprehensive post-projectreport. This contained all contract documents and records, including as-built data, minutes of meetings, siteinstructions, change orders, and product data. Additionally, it provided recommendations for future maintenance,including close-up inspections on an annual basis for three years.

The Restored Building

After experiencing its ninth winter the results are encouraging. Regular inspections have confirmed there areno signs of failure of the repaired areas and no new damage has occurred.

Photo. 11 (Left). Stitching a cracked lintel. Photo. 12 (Right) Cracks were

repaired using pigmented proprietary repair mortars.


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Photo 13: The Black House situated on Mount Allison University’s

campus in Sackville, New Brunswick

Case Study Two ~ Black House

Built in 1896, the Black House (also known as JohnHammond House) is also situated on Mount AllisonUniversity’s campus. (See Photograph 13) ThisNational Historic Site heritage building wasconstructed on natural stone foundations with theground and first floor masonry predominantly shingleand shake construction.

Unfortunately, approximately 15-years or more ago,cracks began to form in the masonry. The cracksextended through the primary stone masonry units aswell as through sills. (See Photographs 14 through 17.)

An examinationof the interiorbasement wallsrevealed thatc o n s i d e r a b l el e a k a g e h a doccurred overmany years and itwas obvious thatwater had causeddeterioration ofthe inner corer u b b l e . T h ec r a c k s w e r etherefore almostcer t a in ly ther e s u l t o fdestabilization ofthe foundationwalls and anoverload of theexterior wythe ofmasonry.

It was determined that - before below grade waterproofing, crack repair and repointing could be carried out -the inner core rubble should be grouted. This work was carried out during the Fall of 2009.

Photos 14 through 17. Cracks through masonry units had occurred at several locations.


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Photo 18: (Left) Over-sized holes were first drilled out and a smaller pilot hole drilled within the hole

to terminate with at least 50% embedment length. Helical ties were then installed and proof load tested.

Photo 19: (Right) Proof-loading the ties.

Photo. 20: Injection tubes under installation - the upper rows have yet

to be installed in the photograph.

During project start-up,tests were carried out toensure adequate capacityof the helical masonryties. Oversized holeswere drilled through theinter ior wythe ofm a s o n r y ( S e ePhotograph 18) and tiesinstalled at five locationsinto the exterior portionof the masonry innercore.

Direct tensile loads wereapplied to each of thefive ties until a provingload of at least 700-psiwas achieved. (SeePhotograph 19) The testswere considered acceptable since more than adequate safety factors were thereby assured. The ties were theninstalled throughout the interior foundation walls at approx 600mm x 400mm spacings, the objective being toprovide restraint to any adverse effect from the potential development of pressure within the core rubble - andto provide a better composite action throughout the thickness of the masonry assembly.

The next stage of the work involved theinstallation of injection tubes in holes that hadbeen drilled through the interior stone or jointsto intersect with inner core rubble. (SeePhotograph 20) The spacings of the tubes waspre-determined and approved.


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Photo 21: (Left) Foam generator pump pressures were adjusted until an acceptable foam

yield was achieved. Photo 22: (Right) The volume of foam added to the cement grout

was adjusted during the mock-up trials to achieve the specified yield weight.

Photo 23: (Left) Mixing the cement grout in a colloidal mixer. Photo 24: (Right) Generating the foam.

Prior to commencement of thegrouting operations, mock-up trialswere carried out to ensurecompliance of both the generatedfoam and the foamed cement withthe specified yield weights. (SeePhotographs 21 & 22) Aprequalified inspection companywas used for the testing.Adjustments were made until theyields were acceptable.

Once the formulations werefinalised, the grouting workcommenced by mixing batches ofgrout within a colloidal (high shearaction) grout mixer. (SeePhotograph 23) The grout was thentransferred to a conventionalhorizontal spiral blade mortar mixer. Foam was then generated using a pre-determined concentration of asurfactant admixture in water forced by pressure through a nozzle. (See Photograph 24) The pre-determinedvolume of foam was then added to the mortar mixer and folded into the grout.


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Photo 25: Test cylinders were made from the cellular foamed

grout.

Photo 26: The grout was injected through the ports under a

controlled low pressure.

Test cylinders were made from a batch of mixedcellular foam grout for subsequent testing by theprequalified testing laboratory. (See Photograph 25)

The cellular foamed grout was then injected under lowpressure into the voids within the inner core rubble.(See Photograph 26) A gelling agent was included inthe grout mixture to produce a thixotropic property thatprevents the grout continuing to flow once pumpingceases.

The volume of grout injected through each (referenced)port was recorded. During project review visits, goodpractices were observed to ensure good flow of groutwhile maintaining optimum injection requirements. Onsome occasions, contact was established considerabledistances from the point of injection - indicatingpositive connection of a large number of voids withinthe inner core rubble. It was estimated thatapproximately 5 cubic metres of grout was injected -the volume representing about 15% voids within themasonry assembly; a high figure. An interestingobservation was that the areas where the highestvolumes of injected grout were recorded aligned withthe exterior location of the most severe damage that hadoccurred to the masonry.

Cellular Foamed Cement Grout

Cellular foamed cement grout was selected because ofthe author’s previous experiences gained originallywithin the mining industry and principally because ofits lightweight, low strength properties - as well as itsinherent ability to absorb stress. These properties,together with the grout’s ability to deform considerablyunder load at a constant yield stress and redistributedamaging point load stresses. (These unique properties

also present opportunities for seismic retrofitapplications for historical structures.) The density of thehardened cellular foam grout can be varied byadjustment of the quantity of foam introduced into thebase grout, as well as by incorporating a filler, such assand. Generally, densities less than 400-Kg/m (25-pcf) produce unstable hardened material, while the benefit3

of foaming becomes diminished once densities exceed about 1600-Kg/m (100-pcf). For neat cement grouts this3

range of densities will generally produce 28-day compressive strengths between 1.5-MPa and 14-MPa (200 and2000-psi) with a relatively reasonable linear relationship.


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AcknowledgementThe author would like to acknowledge and thank Mount Allison University forpermission to use the information gained during their on-going campus restorationand maintenance programme.

Conclusions

Plans to restore and stabilize badly cracked traditional stone masonry buildings should typically be based on aclear understanding of all factors that may potentially have contributed to the damage. As such, a thoroughinvestigation of hidden, as well as visible, conditions can be essential. Non-destructive testing may often providevaluable assistance in this regard. Once a strategy has been developed, any changes in the building’s dynamicsafter restoration should also be evaluated, particularly how these alterations will potentially be accommodatedby the restored masonry. The author’s experience indicates the preferred strategies are typically thoseaccommodating the way in which masonry reacts to changes in temperature, wind-loading, etc., rather than thosebased on restricting the building’s natural ability to ‘move.’

ABOUT THE AUTHOR

Paul Jeffs has a career spanning over 40 years within the construction industry and hasexperience from around the world. Before the establishment of PJ Materials Consultants in1989, Paul was employed for almost 20 years by a UK-based multi-national group. In 1976he transferred from England to the Middle East, living for three years in Bahrain and Iran.During this time he was involved in many construction projects throughout the ArabianPeninsula, including Bahrain, Saudi Arabia, Kuwait, United Arab Emirates and Qatar.

In 1979 he moved to Japan and established a regional base from where he became involvedin projects throughout South East Asia and the Far East, including Japan, the Philippines, theRepublic of Korea, Hong Kong, Taiwan, Indonesia, Singapore and Malaysia. Prior toemigrating to Canada in 1983, he was involved in construction projects in South Africa andIndia.

Paul provides professional development technical training in Ontario through PJ Materials Consultants and across Canadathrough the Continuing Technical College of Dalhousie University. Those who have attended include Engineers,Architects, Authorities, Contractors, Materials Suppliers, etc. He has also been an instructor for the ProfessionalDevelopment Centre of the University of Toronto providing course modules and special event courses as part of theirBuilding Science Certificate Program. Paul has also presented for many organizations, such as the Capital Projects &Design ~ Precinct Properties Branch of the Legislative Assembly of Ontario, the National Capital Commission, theCanadian Society for Civil Engineering, the Canadian Dam Association and the Ontario Building Envelope Council(Toronto & Ottawa Chapters).

Paul has been a guest lecturer at several Canadian universities, he has authored and presented papers at many nationaland international conferences and has been a regular presenter of technical training courses in the Middle East.

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