Santee Cooper ShotcreteRepair in Moncks Corner

Santee Cooper, also known asThe South Carolina PublicService Authority, is a ma-jor electrical supplier with

corporate headquarters located inMoncks Corner, just north ofCharleston, South Carolina. Theco-operative entered into a contractin June 1987 to construct a threestory annex to their main corporateheadquarters. The structure wastypical for this part of South Caro-lina in that the foundation rested onpiling and the main structure was aconcrete frame. It was unusual be-cause all of the structural concretewas exposed.

During the fall of 1988, construc-tion on the annex was halted due toapparent deviation from the origi-nal architectural and possibly struc-tural design intent. Several optionsfor remedial action were discussed

by the owner, the architect, theconstruction manager, and the var-ious contractors involved. Theyconsidered several solutions, in-cluding complete demolition, handpatching, covering unsightly con-crete with aluminum panels, shee-trock, paint, and shotcrete.

Demolition was ruled out due toseveral factors including cost andtime constraints (the owner neededthe building). Hand patching ofconcrete was ruled out because ofthe massive extent of the problemand because of technical deficien-cies that made this repair impracti-cal. Covering the areas up was ruledout as this would completely alterthe architectural intent of the struc-ture. (This structure is an annex toa similar existing structure.) Shot-crete was finally settled on.

When the various options were

I~;';; l51· \.

Interior atrium during construction.

being discussed, a full service con-crete repair contractor was con-sulted for repair recommendations.In January 1989, Palmetto GuniteConstruction Co., Inc. entered intoan agreement with Santee Cooper toperform the concrete repairs andthe retrofit required to render thestructure useable.

The building was designed to bean architectural concrete structure- beams, columns, slabs, shearwalls and supporting infrastructurewere to be left exposed in the fin-ished building to show the worldhow functional and estheticallypleasing reinforced structural con-crete really is.

The original construction beganas most buildings are in this region,by driving the foundation piling.Following construction of the foun-dations the building started to rise

.....Surface area prepared for shotcrete placement.

28 Concrete InUernational

Interior forms for reveals.

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-.;......,.._ ~ .,...D~

Elevation drawing showing deviations from plane.

out of the ground, the concrete be-ing placed into conventionalwooden forms using pumps and insome cases a crane and bucket. Ap-parently, all was progressing asplanned until problems were en-countered somewhere between thestripping of forms and the initialefforts at installing the windows.Since I was not present during theoriginal construction, speculation asto what caused the problems wouldbe inappropriate.

The problems were varied. Theyincluded:

.. Slightly honeycombed concrete.• Voids in the concrete at inappro-priate places.• Misplaced steel with insufficientcover.• Wood, sawdust, and other trashleft in forms.• Severe honeycombed concrete... Concrete members that appearedto have moved or deflected after theconcrete was placed.e Window openings out of squaresignificantly more than the win-dows meant to go in the openings.• Architectural reglets and mem-bers out of intended alignment.It Walls not plumb... Walls, columns, and slabs withvariations of as much as severalinches from the described plane.

The problems listed should be de-scriptive enough to show that cor-rective action was warranted. Shot-crete was chosen as the repairmethod after determining that it

was the only method capable ofmeeting all of the criteria:.. Aesthetics similar to the originaldesign... Speed of application over a largearea.• Structural capabilities as good asthe originally designed convention-ally placed concrete... Bonding capabilities superior toany other construction methodknown.

The methods used were simpleand standard for this type of re-pair. The most troublesome diffi-culties occurred while determiningthe extent of the deficiencies andhow far the repairs had to go tosatisfy the original design intent.The actual repair began in Jan.1989 and was made adhering to thefollowing procedures:

Step 1 - Surveying andestablishing gradesThe extent of the variance fromgrade was difficult to determine dueto the massive extent of the prob-lem and the complexity and inter-dependence of various architecturalaspects (columns must line up verti-cally and horizontally, windowsmust be reasonably square, regletsand other "lines-of-site" must beconsistent throughout the structure,etc.) A licensed surveyor was re-tained to establish exactly what wasthere and to aid the contractor byproviding control points for estab-lishing final grades. Final grades

were established after consideringthe most economical combinationof removing what was there or add-ing more material, all the whilemeeting the original design intentand use of the member in question.

Grades were set using a combina-tion of forms and line wires. Cor-ners, reglets, slabs, windows andsuch were set using custom madeforms. Large flat work was shot tograde using line wires and thenhand finished. (Line wires are pi-ano wire set at a grade corner orplane in line with desired planes orexisting forms).

Step 2 - Busti·hammeringAll surfaces were chipped if neededand bush-hammered using hand-held pneumatic chipping hammers(15 lb [6.8 kg]) fitted with 2 in. (51mm) bush-hammer bits. Bush-ham-mering was performed to provide asurface conducive to chemical andmechanical bonding of the newshotcrete to the existing concrete.Although this process was relativelycostly, it was deemed essential inthis application to achieve the re-quired results.

Step 3 ~ SandblastingThe surfaces were sandblasted toremove dust and laitance left afterthe chipping operation. Laitance isa film formed on the surface whenconcrete initially sets up that willprevent shotcrete from bonding tothe existing concrete. Sandblasting

November 1991 29

Underside of beam with sawdust before repair began. Honeycombed coulmn to beam connection before repair.

was performed using standardequipment and medium coarsegrade sandblast sand.

Step 4 - WashingThe surfaces were then washed withwater under high pressure (1500 psi[10,300 kPaD to remove all sandand dust from sandblasting opera-tions. Again, this was necessary toprovide a clean surface to insureproper and complete bonding be-tween the old and new materials.

Step 5 - Attaching fabric2 x 2 in. (51 x 51 mm) welded wirefabric was attached and anchored toall surfaces. This was done to aidbonding and to reduce surfacecracking due to various causes suchas temperature and premature dry-ing under the extreme conditions ofthis project. The mesh aids bondingbecause of the mechanical fasteningof the anchors to the existing con-crete and the wire mesh, which willsubsequently be an integral part ofthe repair material.

Step 6 ~ SaturatingAll surfaces were saturated at leastovernight with soaker hoses beforeapplication of shotcrete. This stepwas essential for two reasons:.. South Carolina can be very hotduring the summer months and asthe temperature fluctuates betweenthe mid 70s at night to the upper90s in the day, tremendous amountsof energy can be stored in a con-crete structure such as this. By con-tinually wetting the surface, thetemperature in the immediate vicin-ity of the repair can be stabilizedand somewhat lowered. This is es-pecially true at the surface interface

of the old and new materials, whichis essential for a proper bond... Water saturation provides a me-dium (water) for the cement parti-cles to flow through into the parentmaterial (existing concrete). This isthe essence of the chemical andphysical bonding procedure. Theprocess happens because the shot-crete is applied at zero slump.

Only enough potable water isadded to facilitate hydration of thecement in the shotcrete. Cementparticles will seek out excess waterin an effort to further the hydrationprocess. If the excess water is lo-cated in the parent material, the ce-ment will migrate into the materialand create a fringe of cement phys-ically within the existing concrete.At the completion of the hydrationprocess, this hardened fringe willrepresent the meshing of the oldand new materials. This is a some-what simplistic but intuitive expla-nation of the bonding process. Anunderstanding of the mechanics ofa bond will make it obvious to thereader that the saturation water isimportant in the successful repairprocedure.

Step 7 - ShotcretingAlthough the shotcreting stage wasobviously important it was almostanti-climatic compared to the im-mense preparation required. Forexample, of the work force of up to87 people at the height of construc-tion, only about 20 were used forthis stage of the repair.

The care, experience, and atten-tion to detail of the nozzelmen andfinishers can not be understated.Equally, if not more important, wasthe same need for care, experience,

Completed column with reveal.

and attention to detail of each ofthe preparatory stages. The mottoof "Preparation, Preparation,Preparation" followed by the re-pair contractor was at times tediousto all involved from the owner tothe laborers and everyone in be-tween. This same tedious adherenceto the basics was also essential to asuccessful repair.

Shotcrete was applied using thedry-mix method. C-33 concretesand and Type I portland cementwere mixed using volumetric batch-ing equipment. Sand was main-tained at 4 to 6 percent moistureand was checked daily.

After mixing, the dry materialwas fed into rotary delivery ma-chines, which in turn fed the mate-rial into 1 Yz in. (38 mm) diameterhoses. The material was then pro-pelled through the hoses pneumati-cally to the nozzle for dischargeonto the walls and other receivingsurfaces. Water was not introducedto the mix until the mix reached thenozzle immediately before beingapplied to the final position. Thewater added had to be enough toensure full hydration of the mix andhad to be constant and homogene-ous throughout the matrix but theamount was limited by the need to

30 Concrete International

keep the mix at zero slump.A thorough understanding of this

statement will begin to give thereader a real appreciation of theskills the nozzleman must demon-strate in practicing his craft. In ad-dition to his responsibilities in con-trolling the mixing water, the noz-zIeman must ensure that allreinforcement is thoroughly en-cased, that no rebound material istrapped or encased in the freshlyplaced material, and that the mate-rial is placed thick enough but notso thick that sloughing occurs.

Step 8 - CuringThe final stage of the repair was thesame as for all freshly placed con-crete - curing. Wet curing waschosen for this project for manyobvious reasons, not the least ofwhich was the unknown of whetheror not to apply a coating such aspaint to the final surfaces. Wet cur-ing was carried out for a minimumof seven days over all repaired sur-faces using the same soaker hosesused in Step 6. The repairs werecompleted in Dec. 1989.

The final structure can be seentoday as an example of the success-

Completed Santee Cooper headquarter's annex.

ful repair of a major structure us-ing shotcrete. This structure hadbeen determined to be totally unac-ceptable for its intended use with-out the repairs.

The main conclusion to be drawnfrom this project is the acceptabil-ity and desirability of the shotcretemethod of concrete repair on majorstructures. No longer labeled asonly a swimming pool product, thisapplication is being demonstratedfor what it is: a truly viable andversatile repair and construction

placement method.Selected for reader interest by the editors.

ACI Member W.L.Snow, Sr. is Presi-dent and CEO ofPalmetto GuniteConstruction Co.,Inc. located in Rav-enel, South Caro-lina. He is a CivilEngineering gradu-ate of Auburn University. He is amember of ACI Committee 506, Shot-creting.