An information series from the national authority on concrete masonry technology

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1 Povded by: Bown's Concete Poducts Lmted An nomaton sees om the natonal authoty on concete masony technology SEGMENTAL RETAINING WALL DESIGN TEK 15-5B Stuctual (2010) INTRODUCTION Segmental etanng walls (SRWs) uncton as gavty stuctues by elyng on sel-weght to esst the destablzng oces due to etaned sol (backll) and suchage loads. The sel-weght o the SRW system s ethe the weght o the SRW unts themselves ncludng aggegate coe ll used (n the case o conventonal SRWs) o the combned weght o the unts, aggegate coe ll used and the enoced sol mass (n the case o sol-enoced SRWs). Stablty s povded by a coheent mass wth sucent wdth to pevent both sldng at the base and ovetunng about the toe o the stuctue unde the acton o lateal eath oces. SRWs ae duable and long lastng etanng wall systems. The typcal sze o SRW unts, placed wthout mota (dy-stacked), pemts the constucton o walls n locatons wth dcult access and allows the constucton o tght cuves o othe complex achtectual layouts. Segmental etanng walls ae used n many applcatons, ncludng landscapng walls, stuctual walls o changes n gade, bdge abutments, steam channelzaton, wateont stuctues, tunnel access walls, wng walls and pakng aea suppot. Ths TEK povdes a geneal ovevew o desgn consdeatons and the nluences that heght, sol, loads and geomety have on stuctual stablty, based on Desgn Manual o Segmental Retanng Walls (e. 1). It s ecommended that uses o ths TEK consult local buldng codes to detemne addtonal SRW equements and the engneeng needs o the poject. Whee such specc equements do not exst, NCMA ecommends an engneeed desgn peomed by a egsteed poessonal on walls wth a total (desgn) heght, H, exceedng 4 t (1.21 m) (o uthe detal, ee to TEK 18-11A, Inspecton Gude o Segmental Retanng Walls (e. 3). TYPES OF SEGMENTAL RETAINING WALLS Conventonal (Gavty) Segmental Retanng Walls Conventonal (gavty) SRWs etan sols solely though the sel-weght o the SRW unts. They can be constucted wth ethe a sngle depth o unt o wth multple depths. The maxmum wall heght achevable usng a conventonal SRW s dectly popotonal to the unt s weght, wdth, ste geomety, suchage load and etaned sol type. Table 1 llustates the eect o nceasng the wall batte, unt wdth, unt's n-place densty (usng ethe a sold unt o unt wth aggegate coe ll), and bette qualty backll on the maxmum heght o a gavty wall. Sol-Renoced Segmental Retanng Walls Sol-enoced SRWs ae composte systems consstng o SRW unts n combnaton wth a mass o enoced sol. The sol s stablzed by hozontal layes o enocement, typcally a geosynthetc mateal. The enocement nceases the eectve wdth and weght o the gavty system. Geosynthetc enocement mateals ae hgh-tensle-stength polymec mateals. They may be geogds o geotextles, although cuent SRW constucton typcally uses geogds. Fgue 2 llustates a typcal solenoced segmental etanng wall and cuent desgn temnology. The geosynthetc enocement s placed between the unts and extended nto the sol to ceate a composte gavty mass stuctue. Ths mechancally stablzed wall system, compsed o the SRW unts and a enoced sol mass, s desgned to oe the equed esstance to extenal oces assocated wth talle walls, suchaged stuctues, Related TEK: 15-3A, 15-4B, 18-11A Keywods: geosynthetc enocement, etanng wall, segmental etanng wall, stuctual desgn NCMA TEK 15-5B 1

2 Unt wdth, n. (mm) Table 1 Gavty SRW Desgn Heghts o Vaous Unt, Sol and Wall Popetes (ee to cases on Fgue 1 and desgn paametes on next page) Level condtons w/50 ps suchage Cases 13 and 14 φ (deg) Retaned unt weght = 110 pc (1,762 kg/m 3 ) Retaned unt weght = (1,922 kg/m 3 ) Max. wall heght, t (m), o wall batte o: Max. wall heght, t (m), o wall batte o: 5 o 10 o 15 o 5 o 10 o 15 o 12 (305) (0.60) 2.0 (0.60) 2.6 (0.79) 1.3 (0.39) 2.0 (0.60) 2.0 (0.60) 24 (610) (1.40) 5.3 (1.61) 6.6 (2.01) 4.6 (1.40) 5.3 (1.61) 6.0 (1.82) 12 (305) (0.79) 3.3 (1.00) 4.0 (1.21) 2.6 (0.79) 3.3 (1.00) 4.0 (1.21) 24 (610) (1.82) 7.3 (2.22) 8.0 (2.43) 6.0 (1.82) 7.3 (2.22) 8.0 (2.43) Unt wdth, n. (mm) Slope 3:1 Cases 15 and 16 φ (deg) Retaned unt weght = 110 pc (1,762 kg/m 3 ) Retaned unt weght = (1,922 kg/m 3 ) Max. wall heght, t (m), o wall batte o: Max. wall heght, t (m), o wall batte o: 5 o 10 o 15 o 5 o 10 o 15 o 12 (305) (0.60) 2.0 (0.60) 2.6 (0.79) 2.0 (0.60) 2.0 (0.60) 2.0 (0.60) 24 (610) (1.21) 4.6 (1.40) 5.3 (1.61) 4.0 (1.21) 4.0 (1.21) 4.6 (1.40) 12 (305) (0.79) 3.3 (1.00) 4.0 (1.21) 2.6 (0.79) 3.3 (1.00) 3.3 (1.00) 24 (610) (1.82) 7.3 (2.22) 8.0 (2.43) 5.3 (1.61) 6.6 (2.01) 7.3 (2.22) q = ps 250 ps 250 ps = 120 pc ö = = ö = 130 pc = ö = 130 pc Case 1 Case 2 Case 3 Case 4 Case = 120 = ö = pc ö = = ö = pc 130 pc Case 6 Case 7 Case 8 Case 9 Case ps 50 ps pc = = 120 ö = ö = = 8 = ö = Case 11 Case 12 Case 13 Case 14 Case 15 Case 16 Cases 1 though 12 epesent sol-enoced SRW desgn cases. Cases 13 though 16 epesent conventonal SRW desgn cases, whee wall batte, ω, vaes: 5 o, 10 o o 15 o Fgue 1 Desgn Cases Coespondng to Table 1 and Fgues 3 though 5 2 NCMA TEK 15-5B

3 Desgn Paametes o Table 1: Mnmum acto o saety o base sldng, ovetunng and ntenal shea, 1.5 Toe slope 0 Mnmum masony cton educton acto, µ b, between SRW unt and aggegate levelng pad, 0.7 Mnmum shea capacty between SRW unts, 400 lb/t (5.8 kn/m) Angle o cton between SRW unts, 30 o Lve suchage s ntated behnd the ace o the wall Requed mnmum embedment at toe, H emb, 6 n. (152 mm) SRW unt weght, (1,922 kg/m 3 ), ncludes aggegate coe ll when used See Reeence 1 o typcal values o φ o vaous sol types o moe dcult sol condtons. Sol-enoced SRWs may also be eeed to as mechancally stablzed eath (MSE) walls, the genec tem used to descbe all oms o enoced sol stuctues. H H' Toe slope H emb H u W u Foundaton sol,ö q - Unom suchage Gavel ll Subdan system L E (n) = elevaton o geosynthetc enocement above top o levelng pad, t (m) H = total (desgn) heght o wall, t (m) H' = exposed heght o wall, t (m) H emb = wall embedment depth, t (m) H u = heght o segmental etanng wall unt, t (m) L = mnmum length o geosynthetc enocement, ncludng acng connecton, t (m) W u = wdth o segmental etanng wall unt, t (m) β = backslope angle om hozontal, degees γ = sol unt weght, pc (kg/m 3 ) φ = cton angle o sol, degees ω = wall batte, degees Fgue 2 Sol Renoced Segmental Retanng Wall Components 3 Renoced (nll) sol 1 Backll slope E (1) E (2) E (3) Compacted aggegate levelng pad,ö Geosynthetc enocement Retaned sol,ö DESIGN CONSIDERATIONS Geosynthetc Length and Spacng Fo sol-enoced segmental etanng walls, geosynthetc enocement nceases the mass o the composte SRW stuctue, and theeoe nceases ts esstance to destablzng oces. Geosynthetc length (L) s typcally contolled by extenal stablty o ntenal pullout capacty calculatons. Inceasng the length o the geosynthetc layes nceases the SRW's esstance to ovetunng, base sldng, beang alue and geosynthetc pullout. In some cases, the length o the uppemost laye(s) s locally extended to povde adequate anchoage (pullout capacty) o the geosynthetc layes. The stength o the geosynthetc and the ctonal nteacton wth the suoundng sol may also aect the geosynthetc length necessay to povde adequate pullout capacty. In addton, the equed length to acheve mnmum pullout capacty s aected by sol shea stength, backslope geomety and suchage load (dead o lve). The mnmum geosynthetc length equed to satsy extenal stablty ctea s also a uncton o the sol shea stength and stuctue geomety (ncludng wall batte, backslope, toe slope and suchage). As the extenal dvng oce nceases (as occus wth an ncease n backslope angle, educton n sol shea stength, o ncease n extenal suchage load (dead o lve)), the length o the geosynthetc nceases to satsy mnmum extenal stablty equements. Fgues 3 though 5 llustate the eect o backslope geomety, suchage, sol unt weght and sol shea stength on the mnmum equed geosynthetc length to satsy base sldng (FS = 1.5), ovetunng (FS = 1.5) and pullout (FS = 1.5). Regadless o the esults o extenal stablty analyses o sldng and ovetunng, the geogd length (L) should not be less than 0.6H. The pupose o ths empcal constant s to pevent the constucton o unusually naow enoced etanng walls. In NCMA TEK 15-5B 3

4 Wall Heght, H (t) BOTTOM GRID LENGTH Cases 1, 2, 3 Case Case 1 TOP GRID LENGTH Case 2 Case 3 Case Fgue 3 Flat Slope Cases, Vayng φ, γ and q Cases 1, 2, 3 and 4 Wall Heght, H (t) BOTTOM GRID LENGTH Cases 6, 7 Case 5 Case Case 7 TOP GRID LENGTH Case 6 Case 8 Case Note: The ICS analyss esults suggested possble global stablty poblems due to the consdeable top slope: the desgne s encouaged to vey wth the poject s geotechncal engnee all potental global nstablty poblems. Fgue 4 3:1 Top Slope Cases, Vayng φ and γ Cases 5, 6, 7 and 8 addton, t s ecommended that the absolute mnmum value o L be 4 t (1.2 m). A sucent numbe and stength o geosynthetc layes must be used to satsy hozontal equlbum wth sol oces behnd the wall and to mantan ntenal stablty. In addton, the tenson oces n the geosynthetc layes must be less than the desgn stength o the geosynthetc and wthn the allowable connecton stength between the geosynthetc and the SRW unt. The optmum spacng o these layes s typcally detemned teatvely, usually wth the ad o a compute pogam. Typcally, the vetcal spacng deceases wth depth below the top o the wall because eath pessues ncease lnealy wth depth. Vetcal spacng between geosynthetc layes should be lmted to pevent bulgng o the wall ace between geosynthetc connecton ponts, to pevent exceedng the shea capacty between SRW unts, to decease the load n the sol enocement and at the geosynthetc-srw unt connecton nteace. Fgue 6 shows that smalle vetcal enocement spacngs educe the geosynthetc enocement tensle load. Even when all ntenal and acal stablty alue modes can be satsed wth lage 4 NCMA TEK 15-5B

5 BOTTOM GRID LENGTH TOP GRID LENGTH Wall Heght, H (t) Case 11 Case 10 Case Case 9 Case 11 Case 10 Case 12 Case Note: The ICS analyss esults suggested possble global stablty poblems due to the consdeable top slope: the desgne s encouaged to vey wth the poject s geotechncal engnee all potental global nstablty poblems. The ICS acto o saety cannot be satsed o ths wall heght due to the combnaton o slope and sol condtons. The alue s caused by nstablty o the slope above the wall that may eque uthe nvestgaton n coodnaton wth the poject's geotechncal engnee. Fgue 5 2:1 Top Slope Cases, Vayng φ and ω Cases 9, 10, 11 and 12 Renocement Elevaton Above Levelng Pad, t Renocement Tensle Load, lb/t 24 n. spacng 16 n. spacng 8 n. spacng Fgue 6 Inluence o Renocement Vetcal Spacng on Calculated Renocement Tensle Load Wdth o SRW unt, W u, 12 n. (305 mm) SRW unt weght, (1,922 kg/m 3 ), ncludes aggegate coe ll when used Wall batte, ω, 3 o 8, as desgnated; toe slope 0 Angle o cton between SRW unts and geosynthetc, 40 Dect sldng coecent, C ds, 0.95 (mn.) Inteacton coecent, C, 0.7 (mn.) Mnmum shea capacty between SRW unts, 400 lb/t (5.8 kn/m) Desgn Paametes o Fgues 3 though 6: Angle o cton between SRW unts, Sol popetes as desgnated. When deent sol unt weghts (γ) ae consdeed, γ ees to the unted weght o the etaned sol Lve suchage s ntated behnd the ace o the wall Requed mnmum embedment at toe, H emb, 6 n. (152 mm) See Reeence 1 o typcal values o φ o vaous sol types NCMA TEK 15-5B 5

6 spacngs, howeve, a maxmum vetcal spacng between enocement layes o 24 n. (609 mm) s suggested to educe constucton stablty ssues. Note that some popetay systems may be capable o suppotng lage spacngs: a 32 n. (813 mm) maxmum spacng s suggested o these systems. Ths maxmum spacng lmts constucton ssues and also ensues that the enoced sol mass behaves as a composte mateal, as ntended by ths desgn methodology. Fo SRW unts less than o equal to 10 n. (254 mm) n depth, t s ecommended that the maxmum vetcal spacng o the enocement layes be no moe than twce the depth o the unt. Fo example, the maxmum vetcal spacng o a 9 n. (229 mm) deep modula block would be 18 n. (457 mm). Wthn these lmts, the wall desgne should choose an appopate maxmum enocement spacng o the popetay system used. Regadless o the enocement spacng, compacton o the enoced ll zone s geneally lmted to 6 to 8 n. (152 to 203 mm) (compacted heght) n ode to acheve the necessay densty and constucton qualty contol. Compacton lt thckness n the etaned zone s typcally lmted to the same heght; howeve, thcke lts can be accomplshed the speced densty can be acheved thoughout the ente lt thckness and t can be demonstated that thee ae no advese aects to the wall system peomance o aesthetcs. Regadless o the compacton method o equpment, the speced denstes should be met and any vaaton om the appoved speccatons must be authozed by the SRW desgn engnee o the poject. Gavel Fll and Danage Mateals Wheneve possble, wate should be dected away om SRWs. Howeve, when wate does each an SRW, pope danage components should be povded to avod eoson, mgaton o nes, and hydostatc pessue on the wall. Danage eatues o the SRW wll depend on ste-specc goundwate condtons. The wall desgne should povde adequate danage eatues to collect and evacuate wate that may potentally seep at the wall. The cvl ste engnee s typcally esponsble o the desgn o suace danage stuctues above, below and behnd the wall and the geotechncal engnee s typcally esponsble o oundaton pepaaton and subsuace danage beneath a wall. Reeence 1 addesses n detal the danage eatues and mateals equed o vaous gound wate condtons on SRWs. The gavel ll (omely known as the danage aggegate) and dan ppe shown on Fgue 2 should only be eled on to emove ncdental wate they ae not meant to be the pmay danage path o the system. The gavel ll acts manly as a compacton ad to educe hozontal compacton stesses on the back o the SRW unts dung constucton. It also pevents etaned sols om washng though the ace o the wall when desgned as a sol lte, and acltates danage o ncdental wate, theeby elevng hydostatc pessue o seepage oces. The dan ppe collects and evacuates any wate n the system though weep holes (maxmum 50 t (15.2 m) o.c. spacng) o dectly to a danage collecton system. The elevaton and damete o the dan ppe should be detemned by the wall desgne dependng on the specc ste condtons. The gavel ll should consst o at least 12 n. (305 mm) o a ee-danng aggegate nstalled behnd o the SRW unts, and the dan ppe have a mnmum damete o 3 n. (75 mm). Wall Batte Segmental etanng walls ae geneally nstalled wth a small hozontal setback between unts, ceatng a wall batte nto the etaned sol (ω n Fgue 2). The wall batte compensates o any slght lateal movement o the SRW ace due to eath pessue and complements the aesthetc attbutes o the SRW system. Fo conventonal (gavty) SRWs, nceasng the wall batte nceases the wall system stablty. Unt Sze and Shea Capacty All SRW unts povde a means o tanseng lateal oces om one couse to the next. Shea capacty povdes lateal stablty o the motaless SRW system. SRW unts can develop shea capacty by shea keys, leadng lps, talng lps, clps, pns o compacted columns o aggegate n open coes. In conventonal (gavty) SRWs, the stablty o the system depends pmaly on the mass and shea capacty o the SRW unts: nceasng the SRW unt wdth o weght povdes geate stablty, lage ctonal esstance, and lage esstng moments. In sol-enoced SRWs, heave and wde unts may pemt a geate vetcal spacng between layes o geosynthetc, mnmze the potental o bulgng o the wall ace. Fo desgn puposes, the unt weght o the SRW unts ncludes the gavel ll n the coes t s used. Wall Embedment Wall embedment s the depth o the wall ace below gade (H emb n Fgue 2). The pmay benet o wall embedment s to ensue the SRW s not undemned by sol eoson n ont o the wall. Inceasng the depth o embedment also povdes geate stablty when ste condtons nclude weak beang capacty o undelyng sols, steep slopes nea the toe o the wall, potental scou at the toe (patculaly n wateont o submeged applcatons), seasonal sol volume changes o sesmc loads. The embedment depth s detemned based on the wall 6 NCMA TEK 15-5B

7 heght and toe slope condtons (see Table 2), although the absolute mnmum suggested H emb s 6 n. (152 mm). Suchage Loadngs Oten, vetcal suchage loadngs (q n Fgue 2) ae mposed behnd the top o the wall n addton to load due to the etaned eath. These suchages add to the lateal pessue on the SRW stuctue and ae classed as dead o lve load suchages. Lve load suchages ae consdeed to be tansent loadngs that may change n magntude and may not be contnuously pesent ove the sevce le o the stuctue. In ths desgn methodology, lve load suchages ae consdeed to contbute to destablzng oces only, wth no contbuton to stablzng the stuctue aganst extenal o ntenal alue modes. Examples o lve load suchages ae vehcula tac and bulk mateal stoage acltes. Dead load suchages, on the othe hand, ae consdeed to contbute to both destablzng and stablzng oces snce they ae usually o constant magntude and ae pesent o the le o the stuctue. The weght o a buldng o anothe etanng wall (above and set back om the top o the wall) ae examples o dead load suchages. DESIGN RELATIONSHIPS Table 1 summazes the nluence o nceasng the wall batte, nceasng the unt wdth, nceasng the unt's n-place densty, and usng bette qualty backll on the maxmum constuctble heght o a gavty SRW to satsy sldng and ovetunng. Fgues 3 though 5 summaze the nluences wall geomety, backslope and sol shea stength have on the mnmum equed enocement length to satsy base sldng, ovetunng and pullout o a enoced SRW. These desgn elatonshps wee geneated usng consevatve, genec popetes o SRW unts. They ae not a substtute o poject-specc desgn, snce deences between popetes assumed n the tables and pojectspecc paametes can esult n lage deences n nal desgn dmensons o actos o saety. Although wall heghts up to 8 t (2.44 m) o conventonal (gavty) walls Table 2 Mnmum Wall Embedment Depth Slope n ont o wall Hozontal (walls) Hozontal (abutments) 3H : 1V 2H : 1V Mnmum H emb, to top o levelng pad A H'/20, 6 n. (152 mm) mn. H'/10, 6 n. (152 mm) mn. H'/10, 6 n. (152 mm) mn. H'/7, 6 n. (152 mm) mn. A H' s the exposed heght o the SRW, see Fgue 2 and 14 t (4.28 m) o sol-enoced walls ae pesented, popely engneeed walls can exceed these heghts. Fo a detaled dscusson o desgn and analyss paametes, the Desgn Manual o Segmental Retanng Walls (e. 1) should be consulted. Desgn cases 1 though 16 ae llustated n Fgue 1. All esults shown wee calculated usng the sotwae SRWall 4.0 (e. 2) povdng the appopate geosynthetc lengths to satsy sldng, ovetunng, and pullout (enoced walls only) saety actos; o the maxmum gavty wall heght to satsy sldng, ovetunng and ntenal shea. The nal numbe, dstbuton and stength o the geogds can only be detemned by a desgne o each specc SRW untgeogd combnaton to guaantee the appopate saety actos o ntenal, acal stablty and Intenal Compound Stablty (ICS) ae met (o moe detaled nomaton, see Reeence 1). The ICS can be met by educng the geogd spacng o nceasng the gd length o stength: the examples pesented hee wee calculated by educng the geogd spacng and mantanng the maxmum and mnmum geogd lengths o convenence. See TEK 15-4B, Segmental Retanng Wall Global Stablty, (e. 4) o moe detaled nomaton. Lage o commecal SRWs mght also eque oundaton sol competency, settlement, and global stablty analyses o a nal desgn n coodnaton wth othe poessonals n the poject that ae not addessed hee (o moe detals on oles and esponsbltes see TEK 15-3A, Roles and Responsbltes on Segmental Retanng Wall Pojects (e. 5)). I the oundaton and global analyses ultmately eque a modcaton to the wall desgn, ths must be done n coodnaton wth the SRW desgne. EXAMPLE A enoced SRW s speced o a poject that has the ollowng chaactestcs: H= 10 t (3.0 m) Backslope 3:1 Lve suchage= 0 ps All sols φ= and γ = (1,922 kg/m 3 ) Detemne the appoxmate geogd lengths (L) at the bottom and top o the etanng wall. Soluton Detemne the case that apples to ths poblem usng Fgue 1: Case 5 o ths example. Usng Fgue 4 (3:1 backslope), nd L/H o the gven sol condtons and o the desgn heght o 10 t (3.0 m). Bottom geogd: L/H= 0.71; L bottom = 0.71 x 10 t = 7.1 t (2.2 m) Top geogd: L/H= 0.92; L top = 0.92 x 10 t = 9.2 t (2.8 m) NCMA TEK 15-5B 7

8 Fo estmatng puposes, the volume o excavaton and enoced ll could be detemned om the obtaned data. The numbe, stength and dstbuton o the geogds can only be detemned by a desgne o the specc SRW unt-geogd combnaton to comply wth the appopate saety actos o ntenal, acal stablty and ICS. The ICS s dependent on the spacng, length and stength o the geogds: the desgne s encouaged to peom the appopate calculatons to vey the dstbuton o the geosynthetcs. NOTATIONS: C ds = dect sldng coecent C = nteacton coecent E (n) = elevaton o geosynthetc enocement above top o levelng pad, t (m) FS = acto o saety H = total (desgn) heght o wall, t (m) H' = exposed heght o wall, t (m) H emb = wall embedment depth, t (m) H u = heght o segmental etanng wall unt, t (m) L = mnmum length o geosynthetc enocement, ncludng acng connecton, t (m) q = vetcal unom suchage load, lb/t W u = wdth o segmental etanng wall unt, t (m) β = backslope angle om hozontal, degees γ = sol unt weght, pc (kg/m 3 ) γ = weght o oundaton sol, pc (kg/m 3 ) γ = weght o nll sol, pc (kg/m 3 ) γ = weght o etaned sol, pc (kg/m 3 ) µ b = mnmum masony cton educton acto φ = cton angle o sol, degees φ = cton angle o oundaton sol, degees φ = cton angle o nll sol, degees φ = cton angle o etaned sol, degees ω = wall batte, degees REFERENCES 1. Desgn Manual o Segmental Retanng Walls, 3d edton. Natonal Concete Masony Assocaton, Desgn Sotwae o Segmental Retanng Walls, SRWall 4.0. Natonal Concete Masony Assocaton, Inspecton Gude o Segmental Retanng Walls, TEK 18-11A. Natonal Concete Masony Assocaton, Segmental Retanng Wall Global Stablty, TEK 15-4B. Natonal Concete Masony Assocaton, Roles and Responsbltes on Segmental Retanng Wall Pojects, TEK 15-3A. Natonal Concete Masony Assocaton, NCMA and the companes dssemnatng ths techncal nomaton dsclam any and all esponsblty and lablty o the accuacy and the applcaton o the nomaton contaned n ths publcaton. NATIONAL CONCRETE MASONRY ASSOCIATION Sunse Valley Dve, Hendon, Vgna To ode a complete TEK Manual o TEK Index, contact NCMA Publcatons (703) Povded by: Bown's Concete Poducts Lmted 8 NCMA TEK 15-5B