In the 80 years since the shotc

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1 Developments in shotcrete for repairs and rehabilitation BY DUDLEY R. MORGAN In the 80 years since the shotc rete process was deve l o p e d, s h o t c rete has played a va l u a b l e role in repair and re h a b i l i t a t i o n projects. One of its major attributes is excellent bond to the substra t e, usually superior to the bond a c h i e ved with cast-in-place conc re t e. This has made shotcrete particularly well-suited for repair of vertical and overhead surfaces. Many developments in shotcre t e technology during the 1980s have enhanced shotcreting capabilities. These include advances in shotcre t- ing materials technology and imp roved methods for batching, mixing, supply, and application. The d e velopments have stemmed larg e- ly from the desire of engineers and c o n t ractors to improve the quality and durability of inplace shotcre t e, i n c rease shotcreting pro d u c t i v i t y and economy, and expand the ra n g e of shotcrete applications. Fo l l ow i n g a re some highlights of these advances as they pertain to shotcre t e for remedial work. ADVANCES IN SHOTCRETE MA- TERIALS Be f o re the 1980s, most shotcre t e used for repair and rehabilitation in No rth America was made of conventional portland cement and sand mixtures applied by the drymix shotcrete pro c e s s. Some polym e r-modified shotcrete was used for remedial work in aggre s s i ve exp o s u re conditions. There also was limited use of wet-mix shotcre t e, p ri m a rily for larg e - volume pro j e c t s. To d a y, both dry- and we t - m i x s h o t c retes often contain supplem e n t a ry cementing materi a l s, such Using wet-mix shotcrete reinforced with high volumes of polypropylene fibers, worker stabilizes a ravelling highway rock cut. The shotcrete also contains silica fume. as fly ash and silica fume, as additions or partial cement re p l a c e- m e n t s. These materials improve s h o t c rete workability and perf o r- m a n c e. Fly ash Fly ash is used in shotcrete for many of the same reasons it s used in concre t e. It : Im p roves the workability and pumpability of wet-mix shotc rete made with harsh aggreg a t e s Reduces the heat of hyd ra t i o n and, consequently, the potential for thermal cracking in thick s e c t i o n s Im p roves sulfate re s i s t a n c e Co n t rols alkali-aggregate re a c- t i o n s Produces a more economical m i x t u re A recent development is high-vo l- ume fly ash, wet-mix shotcre t e. The m i x t u re contains 40% to 60% fly ash by mass along with water re d u c e r s and high dosages of superplasticize r s to produce a low water-cement ra t i o. The resulting shotcrete has good vo l- ume stability (low drying shri n k a g e capacity), good fre eze-thaw dura b i l i- t y, and ve ry low chloride perm e a b i l i- ty compared with conventional we t - mix, portland-cement shotcre t e. These are desirable chara c t e ri s t i c s for repair and rehabilitation applicat i o n s, but further re s e a rch is needed to assess high-volume fly ash shotc rete perf o rm a n c e. To date, the shotc rete has mainly been used for capping and sealing exposed rock slopes and bedro c k. Silica fume Scandinavian countries pion e e red the use of silica fume in we t - mix shotcrete in the late 1970s and early 1980s. Re s e a rchers found that substituting between 7% to 15% by mass of cement with silica fume off e red seve ral benefits: Im p roved adhesion and cohesion of plastic shotcre t e Ability to build up greater thicknesses of shotcrete with little or no accelerator added Im p roved pro p e rties in hardened shotcre t e, part i c u l a r l y c o m p re s s i ve and flexura l

2 Figure 1. An extreme example of shotcrete improperly applied to mesh reinforcement shows build-up of shotcrete on the face of the mesh and shadowing and voids behind. s t re n g t h s When using silica fume in we t - mix shotcre t e, it s necessary to add both a water- reducer and superp l a s t i c i zer to control the water demand of the mix. Ot h e rw i s e, the use of silica fume will re q u i re excess wat e r, which can be detrimental to ove rall shotcrete quality and lead to e xc e s s i ve shrinkage cra c k i n g. In 1983, re s e a rchers in British Columbia, Canada, conducted extens i ve lab and field investigations into the potential use of silica fume in d ry-mix shotcre t e. They found the benefits to be even more pronounced than for wet-mix, silicafume shotcre t e. These include: Substantial improvement in adhesion and cohesion of plastic s h o t c re t e Ability to build up ove rhead layers as thick as 16 inches in a single pass with little or no accelera t o r Im p roved resistance to washout w h e re freshly applied shotcre t e is subject to running water or tidal cyc l i n g Im p roved economy through reductions in rebound and inc reases in productivity ra t e s Im p rovements in the pro p e rt i e s of hardened shotcre t e, including c o m p re s s i ve and flexura l s t rengths and fre eze-thaw re s i s- t a n c e Im p roved resistance to chemical attack and penetration of deicing salts Since these inve s t i g a t i o n s, tens of thousands of tons of dry- m i x, silica-fume shotc rete have been used in va ri o u s i n f ra s t ru c t u re repair projects in No rth Ameri c a. Accelerators Sh o t c rete acc e l e rators allow build-up of thicker layers of shotc rete in a single p a s s, enhancing p ro d u c t i v i t y. They also reduce the time of initial set and increase early (8 to 24 hours) s t rength development. De s p i t e these benefits, accelerators can adversely affect shotcrete perf o r- mance and should not be used unless considered essential to the job. Ac c e l e rators usually reduce shotc rete strength at later ages. The extent of strength reduction depends on va rious factors, including the type of accelerator used and its dosage ra t e. Ge n e ra l l y, later- a g e s t rength reductions are most pronounced with accelerator types and dosage rates that promote flash sett i n g. Most accelerators also significantly increase the drying shri n k a g e of portland-cement concre t e. Fi e l d o b s e rvations have ve rified this effect in shotcre t e. In an examination of 60 shotcre t e - re p a i red bridges in Canada, the widest shrinkage cra c k s we re found in shotcrete repairs that used accelera t o r s. Fo rt u n a t e l y, with the advent of silica fume, it s no longer necessary to rely only on accelerators for achieving thick layers of shotcre t e build-up in a single pass. Si l i c a f u m e, howe ve r, does not substantially reduce initial and final set times of portland cement shotcre t e. T h e re f o re, it may be necessary to use some accelerator in projects req u i ring rapid set and high-early (8- hour) compre s s i ve stre n g t h s. High-early-strength cements An altern a t i ve to using shotcre t e a c c e l e rators is to use re g u l a t e d - s e t and high-early- s t rength cements. Tests show that shotcretes containing these cements develop higher e a r l y- and later-age strengths than chemically accelerated shotcre t e s. They also have good durability under fre eze-thaw conditions. Va rious cements are ava i l a b l e having different setting times. The most rapid-setting cements are m o re suitable for use in sitebatched, wet-mix shotcre t e. They cannot be used in central-mix or t ransit-mix shotcretes with long retention times before discharg e. REINFORCEMENT Be f o re the 1980s, most shotcre t e was re i n f o rced with conve n t i o n a l c o n c rete re i n f o rcing steel or we l d- e d - w i re mesh. Although these re i n- f o rcement materials are still the most widely used in repair shotc re t e, steel and polypro pylene fibers a re now increasing in use because they offer some perf o rmance adva n t a g e s. Steel fibers In the early 1970s, a major advance in shotcrete technology was the development of steel-fiber- re i n- f o rced shotcrete (SFRS). SFRS is particularly useful for remedial applications in aggre s s i ve chemical or m a rine environments because it resists corrosion better than shotcre t e with conventional steel re i n f o rc e- ment. As long as the shotcrete mat rix retains its inherent alkalinity and remains uncracked, deteri o ration of SFRS is unlikely. Co r ro s i o n of the discreet steel fibers occurs only to the depth of surface carbonation in the shotcre t e. If corrosion of the surface fibers is aesthetically obj e c t i o n a b l e, a flash coat of plain, unre i n f o rced shotcrete can be applied. SFRS has another advantage: It s m o re user friendly and less prone to p roblems caused by inadequate w o rk m a n s h i p. For example, it eliminates the shadowing and vo i d i n g p roblems sometimes encountere d in conventionally re i n f o rced shot-

3 c rete repairs (Fi g u re 1). Such defects usually occur as a result of one or m o re of the following factors: Air pre s s u re is too low at the n oz z l e. Nozzle is too far away from the receiving surf a c e. Sh o t c rete is applied at an incorrect angle to the receiving surf a c e. Fresh or hardened ove r s p ray or rebound isn t properly cont rolled and dealt with. Steel-fiber re i n f o rcement addition rates va ry from about 60 to 140 pounds per cubic yard, depending on job re q u i rements and fiber type and size. Ge n e ra l l y, higher fiber addition rates are used in stru c t u re s subject to seve re stresses and stra i n s imposed by: Impact or explosive forc e s He a v y, repeated, dynamic cyc l i c l o a d i n g Large exposed surf a c e s, which a re more susceptible to shri n k a g e c ra c k i n g Polypropylene fibers In i t i a l l y, polypro pylene fibers (both monofilament and collated, f i b rillated) we re added to shotcre t e at the same dosage rates as used for ready mixed concrete about 1 1/2 pounds per cubic yard. At this ra t e, the benefits of polypro pylene fibers a re mainly limited to enhancing resistance to plastic shrinkage cra c k- ing and making freshly placed shotc rete less susceptible to sags and tears during finishing. Re c e n t l y, extensive re s e a rch has been conducted on high-vo l u m e p o l y p ro pylene fiber dosage rates in wet-mix shotcrete for use as a capping and sealing material. Fiber addition rates we re 6 3/4 to 11 3/4 pounds per cubic yard, producing a pseudoductile material with load versus deformation chara c t e ri s t i c s equal to some mesh and steel-fiberre i n f o rced shotcre t e s. The extra fiber re i n f o rcement also re d u c e d c racking potential. Hi g h - vo l u m e p o l y p ro py l e n e - re i n f o rced shotcre t e has been used in British Co l u m b i a to rehabilitate ra velling highway rock cuts and repair eroded bri d g e a b u t m e n t s. BATCHING, MIXING, AND SUP- PLYING Dry-mix shotcrete Dry-mix shotcrete for re m e d i a l applications can be batched, mixed, and supplied by: Ce n t ral or transit batching with transit-mix supply Site batching, using vo l u m e t ri c or mass batching techniques Dry, bagged premix supply Site batching is the pre d o m i n a n t means of dry-mix shotcrete supply in the United St a t e s. Co m p a red with t ransit-mix-supply shotcre t e, sitebatched shotcrete has a shorter period between first contact of water with the cementing system to application. This gives the plastic shotc rete better adhesive and cohesive c h a ra c t e ri s t i c s. A disadvantage of SHOTCRETE REPAIR WITHSTANDS SEVERE MARINE ENVIRONMENT Concrete berth faces in the Port of Saint John, New Brunswick, Canada, are subject to one of the most severe environments in the world. With a tidal range of 28 feet and winter temperatures as low as -22_ F, the intertidal regions of the berth faces usually are subject to more than 200 freeze-thaw cycles a year. To make matters worse, the structures were built long before the development of air entrainment and much of the original aggregate used in the concrete is alkali reactive. The result has been severe deterioration and loss of concrete from the berth faces. In some older sections, Standing on a barge that lowers with the tide, workers repair berth faces be applying a wet-mix, silica-fume shotcrete reinforced with steel fibers. built in 1913, daylight could be seen through concrete almost 20 feet thick. Extensive testing was undertaken to satisfy the Port Authority that the selected repair material would be durable in this aggressive environment. The repair alternatives investigated included cast-in-place concrete, preplaced aggregate concrete, and shotcrete. Shotcrete proved to be the most economic. The wet-mix, silica-fume shotcrete used was central-mix batched and transit-mix supplied, with steelfiber reinforcement added to the transit mixer at the jobsite. Accelerator was added only to the shotcrete used for filling deep holes in the berth faces before application of the structural facing shotcrete. To apply the shotcrete, workers stood on a barge that served as a moving scaffold. As the tide fell, the barge dropped at a rate of about 4 feet per hour. Because of the severe deterioration of the existing concrete, the shotcrete was anchored with 3 1/4-footlong grouted dowels placed vertically and horizontally at about 4 1/2-foot centers. Shear reinforcement connected anchor heads. First applied in 1986, the shotcrete repair has now undergone more than 1,000 freeze-thaw cycles without showing any freeze-thaw distress. Some drying shrinkage cracking has occurred, but the cracks are less than inch wide. The Port Authority continues to use the same remedial system for berth face repairs every construction season.

4 site batching can be greater va ri a b i l- ity in mix pro p o rtioning. But this depends on the batching equipment used. Mobile vo l u m e t ri c batching equipment, when pro p e r- ly calibrated, can consistently produce shotcrete of uniform quality. The pre f e r red system in Canada is bagged premix. Its advantages include precision mass batching, unif o rm i t y, simplicity of use, and ability to be tra n s p o rted to re m o t e j o b s i t e s. All ingredients are premixed in the bag, including steelfiber re i n f o rcement and, if re q u i re d, p owd e red accelera t o r s. When using transit-mix supply, d o n t allow dry-mix shotcrete to age e xc e s s i vely before discharg e. Ge n e r- a l l y, discharge all the shotcrete within 45 minutes of batching. In hot we a t h e r, even shorter times may be re q u i red. Also, silica fume is less eff e c t i ve in increasing application thicknesses and reducing re b o u n d in transit-mix-supplied, dry- m i x s h o t c rete than in bagged pre m i x. T h a t s because the bagged pre m i x a l l ows less time between first contact of water with cement and silica fume to shotcrete application. Wet-mix shotcrete Any of the batching, mixing, and supplying pro c e d u res used for conventional concrete production can be used for wet-mix shotcre t e. Ing redients such as silica fume and steel-fiber re i n f o rcement can be added at the batch plant or ons i t e. Ac c e l e ra t o r s, if used, are added to the shotcrete at the nozzle duri n g d i s c h a rg e. Silica-fume adm i x t u res are a vailable in liquid s l u r ry and dry p owder form s. Most of the adm i x t u res are n e u t ra l. That means that the Figure 2. Nozzleman s helper removes overspray and rebound using an air-water lance during top-down shotcrete repair of a dam face. s h o t c retes to which they are added retain the same slumps and air contents. Dry, powd e red, silica-fume adm i x t u re added onsite is a part i c u l a r- ly efficient way to produce we t - m i x, silica-fume shotcre t e. The contra c- tor simply orders a standard - s i ze d load of conventional plain shotcre t e f rom a ready mix supplier. The silica-fume admixture is then added d i rectly to the transit mixer onsite f rom bags in a pre d e t e rmined quant i t y. Steel fibers also can be added to the transit mixer onsite. Wet-mix shotcrete to be exposed to fre eze-thaw conditions must be adequately air- e n t rained. Be c a u s e a p p roximately half the air in the m i x t u re is lost during pumping, shooting, and pneumatic consolidation, start with high air contents (about 9% to 12%) in the shotcre t e supplied to the p u m p. Figure 3. An artificial rockscape made with wet-mix silica fume shotcrete creates rustic surroundings for a whale pool at an aquarium in Vancouver, British Columbia, Canada. SHOTCRETE APPLICATION AND FINISHING Application techniques Recent equipment adva n c e s that have simplified shotcrete application include: Robotic shotc rete placement equipment, particularly useful for tunnel re p a i r s Im p rovements in we t - m i x pumps and dry-mix guns Im p rovements in ancillary shotc rete equipment, such as shotcre t e n ozzles and predampening units for use with dry, premixed shotcre t e s Special dispensing units for adding silica-fume slurry at the nozz l e, with or without accelera t o r s, in the dry-mix shotcrete pro c e s s Despite these adva n c e s, the quality of the final in-place shotcrete still l a rgely depends on the skills of the n ozzleman. It s important when applying either dry- or wet-mix shotc rete to hold the nozzle at the correct distance from the re c e i v i n g s u rf a c e. If the distance is too gre a t, s h o t c rete can build up on the face of any re i n f o rcing steel, resulting in s h a d owing and voiding behind the re i n f o rc i n g. Si m i l a r l y, holding the nozzle at an i n c o r rect angle to the receiving surface can result in voids of incomplete consolidation and in more rebound. With a few exc e p t i o n s, such as encasing large-diameter re b a r, the shotcrete stream should hit the receiving surface at right angles. In the dry-mix shotcrete pro c e s s, c o m p ressed air conveys the damp s h o t c rete materials from the gun, d own a hose, to the noz z l e, where the remainder of the mix water is added. Sometimes other materi a l s, such as accelerators or silica-fume

5 s l u r ry, are added at the noz z l e. The n ozzleman must control the shotc reting technique and the amount of water and materials added at the n oz z l e. The nozzleman also tells the gun opera t o r, usually by hand sign a l s, to either increase or decre a s e the volume of air and materials conve yed through the hose. See Re f e r- ences l and 2 for guidance to drymix shotcrete nozzling techniques. The wet-mix shotcrete pro c e s s differs from the dry-mix process in that all shotcrete ingre d i e n t s, exc e p t a c c e l e ra t o r s, are thoroughly mixed b e f o re discharge into the delive ry equipment. Co m p ressed air then c o n veys the premixed shotcre t e d own the hose to the noz z l e, where h i g h - p re s s u re air is added to pro j e c t the shotcrete onto the receiving surf a c e. Liquid accelera t o r, if re q u i re d, is added through a separate line at the noz z l e. Applying wet-mix shotcrete is simpler than applying dry-mix shotc re t e. The nozzleman only has to c o n t rol the amount of air added at the nozzle and the shotcreting techn i q u e. The nozzleman, howe ve r, lacks control over the consistency of the wet-mix shotcrete delive re d. This must be correct before disc h a rge into the shotcrete pump. Re f e rence 1 discusses wet-mix shotc rete nozzling techniques. Reducing rebound and overspray Rebound is shotcrete materi a l leaner than the original mixture that ricochets off the receiving surf a c e. It usually consists of the coarser agg regate particles in the shotcre t e mix with some adhering mort a r. Studies show that rebound not only has a lower cementing-materi a l s content than the original shotcre t e, but also is highly va ri a b l e. For these re a s o n s, never incorporate re b o u n d into the applied shotcre t e. It can result in localized failures in shotcre t e re p a i r s. It s best to re m ove rebound continuously during shotcrete application using air wands, water jets, b rooming, or other means (Fi g u re 2). Once hardened, rebound is much harder to re m ove. Wet-mix shotcre t e, properly designed and applied, has ve ry little rebound. Dry-mix shotcrete adhere s best and has the least re b o u n d when shot at the wettest stable cons i s t e n c y. Adding too little water can p roduce excess dust and re b o u n d. Using silica fume in dry-mix shotc rete also helps minimize re b o u n d. O ve r s p ray is shotcrete materi a l deposited away from the intended receiving surf a c e. It s usually carri e d by nozzle air curre n t s. Unlike rebound, fresh ove r s p ray can be inc o r p o rated into freshly applied s h o t c rete without any adverse eff e c t s. Ha rdened ove r s p ra y, on the other hand, should always be rem oved. It can create porous zo n e s b e t ween successive layers of shotc re t e, which can cause delaminat i o n s. Re m oval methods include chipping, sandblasting, or highp re s s u re waterblasting. Finishing A va riety of surface finishes can be achieved with shotcre t e, from a plain, sack-rubbed finish to a detailed, sculpted finish simulating n a t u ral rock (Fi g u re 3). In repair applications where aesthetics is not a c o n c e rn, shotcrete left in its natura l gun finish usually is adequate. Bu t some repairs must conform to a specific finished surface texture. This can be achieved with appro p riate finishing pro c e d u res discussed in Re f e rences 1 and 2. Wet-mix shotcrete is genera l l y easier to finish than dry-mix shotc rete because it has a more workable consistency. Excess material is t rimmed, sliced, or scraped to gra d e using a rod or trowel. This gives the s h o t c rete a natural rod finish sufficient for many remedial applicat i o n s. Brooming, floating, or trowe l- ing the surface can produce other t e x t u re s. Another approach to finishing is applying a thin flash or finish coat using a sanded mixture. Apply the m i x t u re at a plaster-like consistency, then screed, float, or trowel to a c h i e ve the re q u i red surface texture. Dudley R. Morgan is chief materials engineer for Hardy BBT Ltd., Burn a- b y, BC, Canada. He also is a member of American Concrete Institute (ACI) Committee 506, Shotcre t e, and the Canadian representative for the International Tunnelling Association Shotcrete Working Group. Ac k n ow l e d g m e n t This article is condensed from a paper Mo rgan pre p a red for pre s e n- tation at the CANMET In t e rn a t i o n a l Symposium on Ad vances in Co n- c rete Technology in At h e n s, Gre e c e, May References 1. ACI 506R-90, Guide to Shotcrete, 1990, American Concrete Institute, Box 19150, Detroit, MI T. R. Crom, Dry-mix Shotcrete Nozzling, 1978, The Crom Corp., Gainesville, FL. Publication # C Co py right 1991, The Ab e rd e e n Gro u p. All rights re s e rve d