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Construction Technology Update, 1998-12-01
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Surface preparation of the concrete substrate
Mailvaganam, N. P.; Pye, G. B.; Arnott, M. R.
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NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=76d98169-e665-418c-928a-1e065a48ae29 https://publications-cnrc.canada.ca/fra/voir/objet/?id=76d98169-e665-418c-928a-1e065a48ae29
b y N.P. Ma ilv a ga na m , G.B. Py e a nd M.R. A rnott
Surface preparation is a critical factor in the performance of coatings and
repair materials applied to concrete. This Update discusses the technical
requirements and the various methods for preparing a surface.
C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o . 2 4
Th e su rface p rep aration of con crete in read in ess for th e ap p lication of a coatin g or rep air m aterial in clu d es all th e step s taken after th e rem oval of th e d eteriorated con crete. Man y of th e sam e step s ap p ly w h en little or n o con crete is rem oved . Prop er p rep aration p rovid es a d ry, even an d level su rface free of d irt, d u st, oil an d grease. Rem oval of su rface con tam in an ts allow s p rim ers an d rep air m aterials to h ave d irect con tact w ith th e su bstrate, in creasin g th e su rface area an d rou gh n ess of th e su rface, an d p rovid in g in creased an ch orage of th e ap p lied m aterial.
Th e op tim al con d ition of th e con crete su rface, h ow ever, d ep en d s on th e typ e of rep air bein g u n d ertaken an d th e con d ition of th e su bstrate. Also, it is n ot alw ays p os-sible to d eterm in e w h ich m aterial m u st be
rem oved , becau se th e zon es of d am aged or d eteriorated con crete are som etim es n ot w ell d efin ed . Th u s, th e best ap p roach is to rem ove m aterial u n til aggregate p articles are bein g broken rath er th an sim p ly bein g p ried loose from th e m atrix.
Before ap p lyin g cem en t-based rep air m aterials, th e su bstrate sh ou ld be satu rated an d th e su rface th en d ried to p reven t th e rap id loss of w ater from th e rep air m aterial (to th e su bstrate) an d su bsequ en t sh rin kage an d crackin g. How ever, for resin -based m aterials th e con crete su rface m u st be d ry for m axim u m ad h esion to be ach ieved .
Prior to th e ap p lication of coatin gs, th e m oistu re con ten t of th e su bstrate sh ou ld be ch ecked by on e of th e follow in g m eth od s: 1) u sin g an electrod e-typ e m oistu re m eter. For th is m eth od , read in gs of <5% m ois-tu re con ten t by m ass are requ ired before th e coatin g can be ap p lied .2,3
2) p lacin g a sh eet of p lastic on th e con crete su rface for th e sam e len gth of tim e th e rep air m aterial w ou ld take to cu re (n or-m ally 24 h ou rs). If, after th is tior-m e, th ere is visible m oistu re, a p oor bon d m ay resu lt if “n on -breath able” m aterials are ap p lied .
Wh en ever d eteriorated con crete is rem oved u sin g im p act tools, th e su rface of th e rem ain in g con crete m ay be d am aged . If th is latter d am aged layer is n ot rem oved , th e rep air m aterial w ill d ebon d from th e su bstrate.1,3,7 Th e rem ain in g con crete
sh ou ld th erefore u n d ergo fu rth er p rep ara-tion u sin g w et san d blastin g or h igh -p res-su re w ater jettin g to rem ove th is d am aged
Surface Preparation of the
Concrete Substrate
In ord er to ch oose th e best con crete rem oval m eth od or com bin ation of m eth od s, th e follow in g safety, en viron m en tal an d job-related in form ation sh ou ld be obtain ed :1
• Is th e rep air job to take p lace in sid e a bu ild in g? • Are th ere an y restriction s w ith resp ect to n oise, d u st,
vibration , exh au st fu m es an d d isp osal of w astew ater? • If th e w ork is to be d on e on elevated d ecks, are th ere an y
w eigh t restriction s on th e p rop osed rep air equ ip m en t? • How th ick is th e con crete or coatin g th at h as to be
rem oved ?
• Wh at is th e con d ition of th e con crete? If it is sou n d , w h at is its estim ated com p ressive stren gth ?
• Is th e rein forcin g steel close to th e su rface?
• Wh at kin d of n ew su rface w ill be ap p lied an d h ow th ick w ill it be?
• Is th ere an existin g coatin g th at h as to be rem oved ? If so, w h at kin d is it?
2
su rface m aterial. Usu ally th e rem oval of lim ited areas of con crete to p erm it a rep air requ ires th e saw cu ttin g of th e p erim eter of th e areas to m in im ize feath er-ed gin g.
M e t hods of Surfa c e Pre pa ra t ion Chemical Cleaning
Con crete con tam in ated w ith oil, grease or d irt can be clean ed w ith d etergen t, trisod iu m p h osp h ate or variou s p rop rietary con crete clean ers. Th e u se of th ese m aterials sh ou ld be follow ed by vigorou s scru bbin g an d th orou gh rin sin g w ith w ater to rem ove all resid u e. Solven ts sh ou ld n ot be u sed to clean con crete sin ce th ey w ill d issolve th e con tam in ate an d carry it d eep er in to th e con crete. Mu riatic acid is relatively in ef-fective in rem ovin g oil an d grease.3,4
Mechanical Cleaning
Mech an ical clean in g d evices are of tw o typ es, rotary an d im p act. Rotary equ ip m en t in clu d es d iscs an d grin d ers u su ally u sed on low com p ressive stren gth con crete su bstrates th at d o n ot h ave a steel trow elled fin ish . Th ese d evices are n ot effective on h ard d en se con crete, w h ich th ey are likely to p olish rath er th an abrad e.5,6
Impact tools and scabblers. Devices su ch as bu sh h am m ers, scabblers an d n eed le gu n s w ill effectively rem ove several m illim etres of su rface. Scabblers u se com -p ressed air to h am m er -p iston -m ou n ted bits in to th e con crete su rface; th is rou gh en s th e su rface m ore th an eith er abrasive blastin g or sh otblastin g. Varyin g d egrees of su rface p rep aration m ay be ach ieved , d ep en d in g on w h ich h am m er h ead s are u sed . Scabblin g op eration s are d u sty an d n oisy an d p rod u ce som e vibration . Becau se im p act tools p u l-verize th e con crete an d can cau se fractu rin g of th e con crete su bstrate, it m ay be n eces-sary to u se w ater jettin g or w et san d blastin g for a fin al su rface clean in g.5
Scarifiers. Scarifyin g m ach in es ap p ly a rotatin g circu lar cu ttin g w h eel to th e con crete su rface. Dep th of cu t can be m ore p recisely con trolled th an w ith a scabbler. Differen t styles of in terch an geable cu tter assem blies can be u sed for clean in g, grin d in g an d ligh t or h eavy m illin g. Like scabblers, scarifiers are n oisy, p rod u ce vibration s an d gen erate a great d eal of d u st, alth ou gh th e latter can be con trolled by u sin g a d u st collector attach m en t. Th ese m ach in es are
effective on old floors, an d w ill su ccessfu lly rem ove old p ain t or cu rin g com p ou n d s, bu t are relatively exp en sive an d h eavy, an d requ ire skilled op erators.2
Blast Cleaning
Blast cleaning includes abrasive sandblasting, both w et an d d ry, sh otblastin g, an d w ater-jet clean in g.
Sandblasting. San d blastin g m ach in es u se com p ressed air to eject a h igh -sp eed stream of san d (p article size ran gin g from No. 8–10 m esh ) or som e oth er abrasive from a n ozzle. A fin er san d (No. 20 m esh ) is u sed to rem ove laitan ce, an d an an gu lar san d th at cu ts better th an rou n d ed san d is u sed to rem ove a coatin g from th e con crete. Th e air sou rce of a san d blastin g m ach in e m u st be equ ip p ed w ith an effective oil trap to avoid con tam i-n atioi-n of th e coi-n crete su rface d u rii-n g th e p rep aration p h ase.5
Th e h ard n ess of th e con crete is im p ortan t in d eterm in in g w h eth er san d blastin g is th e m ost econ om ical m eth od of clean in g for ap p lication s requ irin g m ore th an ligh t clean in g. San d blastin g can be u sed for fin al su rface p rep aration to rem ove laitan ce, d irt, oil an d oth er con tam in an ts. Wh en th e d ry m eth od of san d blastin g is u sed , d u st an d clean -u p are p roblem s. Th e large volu m es of abrad ed con crete an d san d are collected by an in d u strial vacu u m an d w orkers u su ally w ear air-fed h elm ets. Becau se of d u st an d associated h ealth p roblem s, d ry san d blastin g is n ow u sed sp arin gly.
Shotblasting. A m etallic abrasive (steel sh ot) is u sed in sh otblastin g m ach in es to scou r th e con crete su rface. Sh ot is p rop elled by a rotatin g w h eel, im p acts on th e con crete su rface an d rebou n d s in to a recovery u n it. Th is m eth od is typ ically u sed for clean in g or scarification of th e con crete to d ep th s of u p to 3 m m (1/ 8
"
). Wh en th e floor is to be coated w ith an ep oxy or u reth an e coatin g, fin e sh ot is u sed to p rod u ce a very ligh t etch in g (bru sh blast) of th e su rface. Th e p rofile for th is typ e of blast is typ ically 4 or 5 m ils (0.1–0.2 m m ) d eep . A bru sh blast breaks th e glazed su rface of a con crete floor an d p rovid es a rou gh textu re to im p rove ad h esion of th e coatin g.Wh en a top p in g or overlay is to be p laced , a m ore vigorou s blast is u sed to exp ose san d p articles. Th e m ore aggressive blast abrad es d eep er, rem ovin g th e m ortar
m atrix d ow n to th e coarse aggregate, som e-tim es to a d ep th th at leaves p rotru d in g coarse p articles. Th ere are th ree factors th at in flu en ce th e d ep th of blast:1
• size of th e abrasive (coarse sh ot etch es th e su rface m ore d eep ly);
• am ou n t of abrasive (an abrasive con trol valve allow s th e op erator to in crease th e flow of th e abrasive for a d eep er etch ); • sp eed of th e m ach in e (slow er sp eed is
n eed ed for a d eep er etch ).
Th ese factors, in ad d ition to th e clean in g p ath w id th , th e d esired rem oval d ep th , th e h ard n ess of th e con crete, an d th e p resen ce of p reviou s coatin gs, affect p rod u ction rates. For exam p le, a h eavy elastom eric coatin g on an old floor w ill cau se sh ots to bou n ce off th e su rface rath er th an to scou r it. Ad d ition ally, if th e existin g coatin g h as w orn off in sp ots, th e bare con crete w ill becom e m ore d eep ly etch ed , p rod u cin g an irregu lar su rface. Wh en a th ick top p in g is to be ap p lied , th e irregu lar su rface w ill n ot be a p roblem , bu t w h en a coatin g is to be u sed , a u n iform su rface is n eed ed .
If th e p reviou s coatin g is th icker th an 3 m m (1/ 8
"
), or h as w orn off in sp ots, it sh ou ld be rem oved w ith a scarifier or strip -p in g m ach in e before th e su rface is sh ot-blasted .1,4 Bein g able to con trol d u st is on eof th e m ajor ad van tages of sh otblasters. An d sin ce n o w ater is u sed , th e su rface is im m ed iately read y for th e ap p lication of coatin gs th at requ ire a d ry su rface.
Waterblasting. Th is m eth od con sists of d irectin g a h igh -velocity, h igh -p ressu re w ater jet to th e con crete su rface th rou gh a sp ecially d esign ed n ozzle that travels transversely alon g a boom , sw eep in g back an d forth across th e con crete su rface as th e equ ip m en t ad van ces in crem en tally. Th e equ ip m en t can be u sed in ap p lication s ran gin g from laitan ce rem oval to h yd rod e-m olition of con crete to d ep th s of u p to 30 m m (12
"
).Th e w ater p ressu re, th e speed of the nozzle as it moves alon g th e boom , an d th e sp eed of th e m ach in e — all of w h ich can be ad ju sted — con trol th e d ep th of rem oval.
Th e jet cu ts a series of grooves an d w ater p ressu re breaks u p th e con crete betw een th e grooves. Th is m eth od is very effective w h en u sed as a fin al step in su rface p rep a-ration . Its m ain lim itation is th e collection an d d isp osal of w astew ater. Waterblastin g d ebris m u st be rem oved d aily to p reven t it from h ard en in g. Frequ en tly th is m eth od is u sed on brid ge an d p arkin g garage d ecks to rem ove th e con crete su rface to a d ep th of u p to 50–75 m m (2–3
"
).Th e ad van tages of th is m eth od are: • Th ere is n o d u st, an d n oise is m in im al. • Th ere are n o m ech an ical vibration s th at
m igh t cau se stru ctu ral d am age.
• Th e m ach in e selectively rem oves d eteri-orated con crete an d leaves good con crete in tact.
• Th e rein forcin g steel is n ot d am aged as it cou ld be by scarifiers or scabblers. • Th e rem oval of d eteriorated con crete is
faster th an by con ven tion al m eth od s su ch as jackh am m ers. Rem oval rates can ran ge from 0.28–0.85 m3/ h (10–30 ft3/ h )
an d 46.45–74.32 m2/ h (500–800 ft2/ h )
w h en u sed as a scarifier to rem ove su r-face m aterial to a d ep th of 6 m m (1/ 4
"
).Acid Etching
Acid etch in g rem oves en ou gh cem en t p aste to p rovid e a rou gh en ed su rface, w h ich im p roves th e bon d betw een th e rep lacem en t m aterials an d th e su bstrate. Becau se of th e p oten tial for corrosion , ACI Com m ittee 515 recom m en d s th at acid etch in g on ly be u sed w h en n o altern ative m ean s of su rface p rep aration is accep table.2,7
Flame Cleaning
Flam e clean in g is gen erally u sed to clean con crete su rfaces th at are to receive coatin gs or resin ou s overlays. Th is m eth od is p articu larly u sefu l for oil-stain ed floors becau se it p erm its th e ap p lication of coatin gs to th e con crete im m ed iately after. A sp ecial m u lti-flam e oxy-acetylen e blow p ip e is p assed over th e con crete su rface at u n iform sp eed . Th e th ickn ess of th e con crete layer rem oved d ep en d s on th e sp eed at w h ich th e blow p ip e is m oved an d th e p rop erties of th e con crete. Th e m ost su itable blow p ip e sp eed lies betw een 0.02 m / s (0.066 ft/ s) an d 0.03 m / s (0.099 ft/ s). Con crete an d coatin g rem oval in volves both th e sp allin g an d m eltin g off of th e su rface. Th e laitan ce layer is u su ally rem oved to a d ep th of 1 or 2 m m (0.04
"
or 0.08"
) an d in a few3
Figure 1.Ultra high-pressure cavitation jet
in stan ces u p to 4 m m (0.16
"
). Th e m oistu re con ten t of th e con crete h as th e greatest effect on con crete rem oval — com p letely d ry slabs d o n ot p rod u ce m u ch sp allin g, w h ile slabs soaked in w ater p rior to flam e clean in g p rod u ce u n iform con crete rem oval.Eu rop ean exp erien ce7in d icates th at
flam e clean in g d oes n ot p rom ote th e m igra-tion of d eep -seated oil to th e su rface, d oes n ot rem ove th e alkalin ity of th e m atrix — th e su rface grad u ally attain s alkalin ity sim ilar to th at of n ew con crete — an d d oes n ot p rom ote th e d evelop m en t of an y visible cracks in th e su rface.
Th e m eth od h as p roven u sefu l for su ch ap p lication s as th e recoatin g of con crete floors or th e rem oval of d efective elas-tom eric w aterp roofin g m em bran es from p arkin g d ecks.
Sum m a ry
Th e lim itation s of each su rface p rep aration m eth od — th e d u st, n oise an d vibration gen erated ; th e p oten tial for corrosion ; an d th e p ossibility of d rivin g oil an d grease d eep er in to th e con crete — w ill d eterm in e th e ap p licability of a given m eth od to a p articu lar typ e of bu ild in g or occu p an cy (e.g., h osp ital, office bu ild in g) an d to th e state of d eterioration of th e stru ctu re. Also, th e ad van tages afford ed by a p articu lar m eth od to ach ieve a sp ecific resu lt (su ch as ligh t su rface abrasion ) w ill h elp clarify th e ap p rop riate op tion .
Re fe re nc e s
1. Equ ip m en t For Clean in g or Prep arin g Con crete Su rfaces For Rep air. Con crete Con stru ction , Ju ly 1984, p p . 456-459. 2. ACI 541, Gu id e to Rep air of Con crete –
Draft 1, Oct. 1987.
3. CSA Tech n ical Com m ittee on Rep air of Con crete Bu ild in gs – Draft 1, May 1989. 4. ACI-362R–85. State of th e Art Rep ort on
Parkin g Stru ctu res. ACI Jou rn al, Ju ly-Au gu st 1985, p p . 544-560.
5. Bits an d Blad es: Wh at Makes Th em Cu t Faster an d Last Lon ger? Con crete Con stru ction , Sep t. 1985, p p . 753-761. 6. Con crete Saw in g an d Drillin g. Con crete
In tern ation al, Sep t. 1980, p p . 46-48. 7. Mailvagan am , N.P., Rep air an d Protection
of Con crete Stru ctu res. Ch ap . 9, CRC Press, Boca Raton , FL, 1991.
N.P. Ma ilv a ga na mis a Prin cip al Research Officer in th e Bu ild in g En velop e an d S tru ctu re Program of th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction . G.B. Py ean d M.R. A rnott are sen ior tech n ical officers in th e sam e p rogram .
“Construction Te chnology Up d a te s” is a se rie s of te chnica l a rticle s conta ining p ra ctica l inform a tion d istille d from re ce nt construction re se a rch.
For more information, contact Institute for Research in Construction, National Research Council of Canada, Ottaw a K1A 0R6
Telephone: (613) 993-2607; Facsimile: (613) 952-7673; Internet: http://irc.nrc-cnrc.gc.ca
© 1998
Nation al Research Cou n cil of Can ad a Decem ber 1998
