Internal Curing of Concrete

Transcription

1 Internal Curing f Cncrete Ezekiel Enterprises, LLC Internal Curing f Cncrete One (1) Cntinuing Educatin Hurs Curse #CV111 Apprved Cntinuing Educatin fr Licensed Prfessinal Engineers EZ-pdh.cm Ezekiel Enterprises, LLC 301 Missin Dr. Unit 571 New Smyrna Beach, FL helpdesk@ezpdh.cm

2 Internal Curing f Cncrete Ezekiel Enterprises, LLC Curse Descriptin: The Internal Curing f Cncrete curse satisfies ne (1) hur f prfessinal develpment. The curse is designed as a distance learning curse that enables the practicing prfessinal engineer t understand internal curing in cncrete in varius applicatins. Objectives: The primary bjective f this curse is enable the student t understand the cncepts and benefits f internal curing fr cncrete pavements including mixture design, quality cntrl, and studies f practical applicatins ver the past tw decades. Grading: Students must achieve a minimum scre f 70% n the nline quiz t pass this curse. The quiz may be taken as many times as necessary t successful pass and cmplete the curse. A cpy f the quiz questins are attached t last pages f this dcument.

3 Internal Curing f Cncrete Ezekiel Enterprises, LLC Table f Cntents Internal Curing f Cncrete Intrductin... 1 Pavement Applicatins... 2 Mixture Design fr Internal Curing... 3 Cnstructin Aspects... 4 Prperties... 5 Practical Applicatins... 5 Summary... 6 References... 7 Quiz Questins...8

4 Internal Curing fr Cncrete Pavements This Curse prvides infrmatin n internal curing fr cncrete pavements by describing the primary cncepts behind internal curing as well as describing aspects f practical applicatins, mixture design, cnstructin, and quality cntrl. INTRODUCTION Internal curing prvides a mdern twist n gd curing practice by prviding water t the cementitius matrix after setting. Internal curing imprves the perfrmance f cncrete by increasing the reactin f the cementitius materials. Hwever, unlike cnventinal curing that supplies water frm the surface f cncrete, internal curing prvides curing water frm the aggregates within the cncrete (Figure 1). This is very beneficial since the depth that external water can penetrate is limited fr any cncrete, while internal curing water is dispersed thrughut the depth f the cncrete. In Nrth America, this water-filled inclusin is typically an expanded lightweight aggregate, althugh superabsrbent plymers, cellulse fibers, r recycled cncrete have been used (1, 2, 3). The water that is absrbed in the lightweight aggregate des nt cntribute t the classic definitin f the water-tcement rati. The water-t-cement rati is a descriptr f structure f the matrix and pres that develp in the fluid cncrete system. Once the cncrete sets, the structure and pre netwrk have been established, and water can nly aid in hydratin. The water in the lightweight aggregate will desrb (leave) the pres f the lightweight aggregate as the negative pressure in the pre fluid develps with setting and increases thereafter. Figure 1. An illustratin f cnventinal curing as cmpared with internal curing.

5 2 Internal Curing fr Cncrete Pavements Internal curing als can reduce autgenus shrinkage, since water frm the lightweight aggregate will wrk t fill pres that therwise can lead t autgenus shrinkage. The water frm the pres may eventually be lst t the envirnment; hwever, there are benefits in reducing the rate f shrinkage (4). Internal curing als can reduce transprt prperties (permeability, diffusin, and srptin) thrugh increased hydratin f the interfacial transitin zne arund the lightweight aggregate (2). Internal curing is especially beneficial fr mixtures cntaining high vlumes f supplementary cementitius materials that may require lnger times t hydrate (2). Internal curing can als make cncrete less susceptible t thermal cracking, as the built-in stress caused by autgenus shrinkage is substantially reduced (2). Many additinal benefits are being investigated, such as benefits in terms f reduced curling (5). PAVEMENT APPLICATIONS Internally cured cncrete has mstly been used in bridge deck applicatins t date; hwever, researchers have investigated extending the use f internal curing t pavements (5). The fllwing sectins describe recent activity in cntinuusly reinfrced cncrete pavements, jinted plain cncrete pavements, cncrete patches, and cncrete verlays. Cntinuusly Reinfrced Cncrete Pavement It has been speculated that the use f internal curing in mainline paving is likely mst beneficial t cntinuusly reinfrced cncrete pavement (CRCP) due t the reductin in shrinkage and curling, which may result in increased mechanical perfrmance and fatigue capacity (6). Observatins f CRCPs have indicated that early shrinkage is significantly reduced as cmpared with cnventinal cncrete, resulting in early crack spacing patterns f apprximately three times thse develped in cnventinal cncrete pavement sectins (6). Lnger term mnitring has shwn that this difference in crack spacing decreases ver time until the spacing is n the rder f 20 t 30 percent lnger than that in cnventinal cncrete. Hwever, cracks that ccur at later ages are smaller than thse that develp at earlier ages. This phenmenn has been bserved in in-service pavements in which the cracks in internally cured cncrete remain tighter than thse in cnventinal cncrete (6). In sme cases, the use f internally cured mixtures fr CRCPs has resulted in a reductin in cncrete elastic mdulus, which results in a reductin in stresses which, cmbined with lwer cefficient f thermal expansin and imprved strength, prvides a reductin in fatigue damage. This is smewhat affected by which aggregate sizes are intrduced in the internally cured cncrete mdificatin and whether the additinal aggregate is used t develp an ptimized mixture. In such instances, the resistance f the cncrete t flexure cracking is increased, which imprves pavement perfrmance life (6). Pavement perfrmance predictin using the Pavement ME Design sftware als has indicated that the incrpratin f internal curing in CRCP will result in slightly lnger crack spacing and tighter cracks. These changes are predicted t significantly increase the perfrmance life f the pavement as cmpared with cnventinal cncrete used in the same design (6). It has als been reprted that internal curing will result in a reductin in misture-related curling/warping (7). Research has als fund that internal curing reduces curling, cmpared t an equivalent mixture withut fine lightweight aggregate. The tendency tward a unifrm misture gradient may reduce slab curling and warping due t verall temperature and misture gradients; hwever, a reductin in curling frm misture gradient alne wuld be very beneficial in pavements. The benefits f internal curing in CRCPs are being investigated in a study fr the Illinis Tllway. Neutrn radigraphy is being used t examine misture gradients, and full test sectins are planned t be cast during the summer f 2016 (8). Jinted Plain Cncrete Pavement Internal curing in jinted plain cncrete pavements appears t have psitive effects n issues knwn t influence structural lngevity and durability. Small reductins in unit weight, elastic mdulus, and cefficient f expansin tgether with a small increase in strength prmte imprved structural integrity. The cmbined effect f these small imprvements results in a significant psitive impact n slab fatigue damage and assciated slab cracking in jinted cncrete pavements (6). The extended misture supply prvided by internal curing imprves cncrete pavement durability by reducing net misture lss and imprving hydratin.

6 Internal Curing fr Cncrete Pavements 3 Figure 2. Cnstructin-related permanent built-in temperature and shrinkage gradients. Internal curing reduces early age shrinkage and assciated plastic shrinkage cracking, as evidenced in bth labratry and field mnitring f cncrete placements. Anther ptential benefit t jinted pavements is a reductin in upward slab curling resulting frm internal slab misture gradients and stresses lcked in at the time f set resulting frm temperature gradients during curing. Stresses resulting frm curling, when cmbined with ladinduced stresses, can accelerate damage t pavement slabs. This reductin in curling stresses can imprve pavement perfrmance by reducing the ttal stress level, thereby limiting damage t pavement slabs (Figure 2). Patching and Full-Depth Panel Repair While the vast majrity f field trials have fcused n the use f internal curing fr bridge decks (5), field trials perfrmed in Indiana (9) in 2014 used internal curing with expanded slag aggregate in high early-strength, full-depth cncrete pavement patches (Figure 3). The applicatin f internal curing in the high early-strength patches prvided a cncrete with tw distinct benefits when cmpared with cnventinal cncrete: 1) reduced built-in stress caused by the restraint f shrinkage, which resulted in reduced cracking, and 2) increased water curing (frm inside the cncrete) after the patches are cvered with curing cmpund and pened t traffic. Figure 3. Internally cured mixtures being used fr panel replacement. MIXTURE DESIGN FOR INTERNAL CURING Except fr the lightweight aggregate, internally cured cncrete mixture design generally is identical t that f cnventinal cncrete with similar air cntent, water cntent, and carse aggregate cntent. Currently, internal curing in Nrth America is typically achieved by replacing a prtin f the cnventinal fine aggregate (i.e., sand) with a prewetted lightweight fine aggregate. This practice ccurs mainly due t lightweight aggregate availability, ecnmics, and the fact that the fine lightweight aggregates already appear n the apprved materials lists f many state highway agencies. Fine lightweight aggregate is generally preferable t carse lightweight aggregate because:

7 4 Internal Curing fr Cncrete Pavements The fine lightweight aggregate can prvide sufficient distributin f the curing water acrss the crss sectin t enable the internal curing water t reach the matrix. The use f cnventinal carse aggregate prvides cnventinal stiffness and fracture prperties fr the cncrete. The fine lightweight aggregate is generally less expensive than the carse lightweight aggregate. The fine lightweight aggregate can be evaluated using ASTM C1761 (10), althugh expanded clay, shale, slate, and slag have all been used effectively fr internal curing. mixture design fr the wide range f expanded lightweight aggregates in Nrth America (cnsidering the prperties f the lightweight aggregate such as specific gravity, 24-hur absrptin, desrptin) is shwn in Figure 5. There is a wide range f tls that can be used t prprtin a specific mixture; hwever, a simple spreadsheet is available t cnvert a cnventinal mixture design t an internally cured mixture design in a simple step using the prperties f the lightweight aggregates (5). This spreadsheet, alng with the majrity f internal curing cnversin tls, has been designed with a vlume f internal curing water that is equal t the vlume f chemical shrinkage t negate the effects f self-desiccatin. It is pssible t design at differing levels f internal curing water, which may be dne t accunt fr the majrity f internal curing benefits fr a lwer-cst mixture r t cmpensate fr drying shrinkage r ther surces f water lss. Figure 4. Typical mixture prprtin vlumes. Figure 4 prvides an example f the vlume prprtins f cnventinal and internally cured cncrete. The figure illustrates the similarities and differences between the design f a cnventinal six-bag mixture (water-t-cement rati f 0.36 and 6 percent air) and an internally cured mixture using the same vlume prprtins f the cnstituents. The example assumes 15 percent absrptin f the fine lightweight aggregate and a vlume f internal curing water that is equivalent t the vlume f chemical shrinkage f a typical cement (7 lb f water fr every 100 lb f cementititus materials). The vlume f sand that is replaced with fine lightweight aggregate will vary depending n the prperties (prsity and desrptin) f the fine lightweight aggregate and the design assumptin (prviding internal curing water t equal chemical shrinkage r prviding sme ther amunt f internal curing water). Using the mixture design frm Figure 4 and assuming that the internal curing water is designed t replace the chemical shrinkage water, an example t illustrate the Figure 5. Relatinship between absrptin and the vlume f fine aggregate replaced with fine lightweight aggregate. CONCRETE PAVEMENT CONSTRUCTION ASPECTS OF INTERNALLY CURED CONCRETE The prductin f internally cured cncrete is similar t that f cnventinal cncrete, but understanding and cntrlling the aggregate misture is mre critical in internally cured cncrete in rder t have a cnsistent mixture due t the higher absrptin values in the fine lightweight aggregate. The lightweight aggregate is pre-wetted in a stckpile prir t batching with a sprinkler r saker hse fr 48 hurs befre allwing a draindwn perid f at least 12 hurs, which enables the reductin f surface misture and greater unifrmity thrughut the stckpile.

8 Internal Curing fr Cncrete Pavements 5 The prcess fr batching internally cured cncrete is similar t that fr cnventinal cncrete. An extensive study nted that batching was perfrmed easily, with n prblems being reprted due t the additin f the lightweight aggregate ther than the need fr an additinal bin fr the pre-wetted lightweight aggregate (9). The absrbed misture and surface misture in the lightweight aggregate shuld be carefully measured and cntrlled t reduce the amunt f variability frm batch t batch. Tw methds are cmmnly used t determine the aggregate absrptin and surface misture: the paper twel methd and the centrifuge methd. The centrifuge has been shwn t be faster and mre accurate than the paper twel methd (9). Mst cnventinal quality cntrl tls used fr cnventinal cncrete apply t internally cured cncrete as well. It has als been nted that entrained air cntent can be measured using either ASTM C173 (the pressure methd) r ASTM C231 (the vlumetric methd) when fine lightweight aggregate is used (11). This is an imprtant finding, as it indicates that the mre rapid, cnventinal pressure methd can be used even thugh fine lightweight aggregate is being used, presumably due t the lightweight aggregate being well saturated befre testing. PROPERTIES OF INTERNALLY CURED CONCRETE The direct benefits f internally cured cncrete include a reductin in autgenus shrinkage, a reduced rate f drying shrinkage, reduced elastic mdulus, and imprved hydratin. Ra and Darter indicated that, in additin t these benefits, the cmbined effects f unit weight reductin, strength increase, and cefficient f thermal expansin reductin add up t a significant psitive impact n slab fatigue and assciated cracking initiating bth frm the tp f the slab and the bttm f the slab (6). There als exists a ptential fr reducing the pening f shrinkage cracks (including sawed jints), which imprves the shear transferability f the cracks and jints, leading t higher lad transfer efficiency and lnger service life. Ra and Darter als explred the practicality f incrprating internally cured cncrete prperties in the perfrmance predictins prduced by the Pavement ME Design sftware (6). They als cnsidered the impact f internally cured cncrete n cncrete pavement life-cycle cst analysis results and presented a radmap fr cntinued prgress in the applicatin f internally cured cncrete in cncrete pavements. PRACTICAL APPLICATIONS Internally cured cncrete has been used fr many cncrete facilities, the majrity f which have been bridge structures. Hwever, as nted belw, there have been a number f pavement applicatins, primarily in the Dallas-Frt Wrth area (12). The pavements in the Dallas area used a relatively small substitutin f intermediate aggregate sizes with lightweight aggregate. Sme examples f the prjects in Texas include: A 1,400-ft sectin f CRCP n the uter lane and shulder f State Highway 121 nrth f Dallas in The remainder f this prject was cnstructed with cnventinal cncrete t enable a perfrmance cmparisn. Fllw-up surveys indicated that the initial crack pattern that develped in the internally cured sectin exhibited much lnger spacing than is nrmal fr CRCP, and the cracks are als much tighter. After several years, the crack spacing became similar t that f the cnventinal sectins; hwever, the cracks remained much tighter. A high early-strength mixture in a sectin f an ff ramp n I-635 fr early pening t traffic in The use f internal curing had n adverse effect n early strength and was able t reducing shrinkage cracking. A 360-acre Unin Pacific intermdal terminal lcated 12 miles frm dwntwn Dallas, within the city limits f Hutchins and Wilmer. Minr jint spalls and limited cracking have been bserved. Perfrmance has been similar t the cnventinal sectins, with bth in excellent cnditin. A residential subdivisin in suth Frt Wrth, Windsr Park, cnstructed in A survey after 8 years in service identified n significant lngitudinal r transverse cracking, plastic shrinking cracking, spalling, r ther defects. In general, the pavement was in excellent cnditin. A residential subdivisin in nrth Frt Wrth, Alexandria Meadws Nrth, cnstructed in This prject cntained streets bth with and withut internally cured cncrete. A field survey revealed bth the internally cured cncrete and cnventinal pavement sectins

9 6 Internal Curing fr Cncrete Pavements Table 1. Example mixture designs shwing the replacement f nrmal weight aggregates with expanded shale lightweight aggregates (shwn in bld fnt). Mixture Cmpnent Cntrl Internally Cured Carse Lightweight Aggregate Mix Water (lb/yd 3 ) Cement (lb/yd 3 ) SSD nrmal weight carse aggregate (lb/yd 3 ) WSD shale lightweight carse aggregate (lb/yd 3 ) SSD nrmal weight fine aggregate (lb/yd 3 ) WSD shale lightweight fine aggregate (lb/yd 3 ) Water-reducing admixture (z/yd 3 ) High-range water-reducing admixture (z/yd 3 ) Rhelgy-cntrlling admixture (z/yd 3 ) Air-entraining admixture (z/yd 3 ) Target ttal air cntent (%) Water-t-cement rati All Lightweight Aggregate Mix were in excellent cnditin, with very limited cracking. N slab curl was identified. The mixtures used in the afrementined studies used intermediate size lightweight aggregate. Table 1 prvides an example f ne such mixture design that was used n the Unin Pacific intermdal facility in Dallas-Frt Wrth. This table prvides the mixture prprtins fr bth cnventinal and internally cured cncrete mixtures. The tw key benefits determined fr internally cured cncrete in cncrete pavement are structural lngevity and durability. Structural lngevity is imprved with internally cured cncrete due t its small reductin in unit weight, elastic mdulus, and cefficient f expansin, and a small increase in strength. These small effects, when cmbined, amunt t a significant psitive impact n slab fatigue damage and assciated slab cracking in jinted cncrete pavements. Likewise, internally cured cncrete leads t tighter crack penings and reduced punchut failures fr CRCP. Several case studies were analyzed using the Pavement ME Design prcedure, and the results indicated imprved perfrmance and lnger lives fr internally cured cncrete sectins. Life-cycle cst analyses fr these prjects shwed generally lwer csts fr internally cured cncrete as cmpared t cnventinal cncrete (6). SUMMARY Internal curing f cncrete has been the subject f many labratry investigatins ver the last tw decades. Internally cured cncrete has been used successfully in full-scale bridge decks and in cncrete fr CRCP, jinted plain pavement, and pavement patching prjects. Additinal field trials are underway t examine the ptential use f internally cured cncrete in a variety f paving applicatins. Field experience t date has shwn that, in general, internally cured cncrete has similar wrkability, similar strength and mechanical prperty develpment, reduced stress develpment and cracking, and similar r imprved durability when cmpared with cnventinal cncrete. Specific imprvements identified include reduced shrinkage, resulting in fewer and tighter cracks, imprved fatigue resistance, and reduced slab curling/warping. Perfrmance mdels, generated using the Pavement ME Design prgram, indicate that the perfrmance f pavements made with internal curing shuld be superir t cnventinal cncrete pavements, resulting in imprved lifecycle cst.

10 Internal Curing fr Cncrete Pavements 7 REFERENCES 1. Internal Curing f Cncrete, RILEM Reprt 41, Eds. K. Kvler and O.M. Jensen, RILEM Publicatins S.A.R.L., Bentz, D.P. and W.J. Weiss, Internal Curing: A 2010 State-f-the-Art Review, NISTIR 7765, U.S. Department f Cmmerce, February ACI Reprt n Early Age Cracking: Causes, Measurement and Mitigatin, ISBN Radlinska, A., F. Rajabipur, B. Bucher, R. Henkensiefken, G. Sant, and W.J. Weiss, Shrinkage Mitigatin Strategies in Cementitius Systems: a Clser Lk at Sealed and Unsealed Behavir, Transprtatin Research Recrd, Vlume 2070, pp , Ra, C. and M.I. Darter, Evaluatin f Internally Cured Cncrete fr Paving Applicatins, Reprt, Wei, Y. and W. Hansen, Characterizatin f Misture Transprt and Its Effect n Defrmatins in Jinted Plain Cncrete Pavements, Transprtatin Research Bard, Resler, J., D. Zllinger, and J. Weiss, Innvative Structural and Material Design f Cntinuusly Reinfrced Cncrete Pavement (CRCP) Illinis Tllway, Barrett, T. J., A.E. Miller, and W.J. Weiss, Dcumentatin f the INDOT experience and cnstructin f the bridge decks cntaining internal curing in 2013 (Jint Transprtatin Research Prgram Publicatin N. FHWA/IN/JTRP-2015/10). West Lafayette, IN: Purdue University ASTM Standard C1761/C1761M, 2012, Standard Specificatin fr Lightweight Aggregate fr Internal Curing f Cncrete, ASTM Internatinal, West Cnshhcken, PA, Jnes, W.A. and W.J. Weiss, Freezing and Thawing Behavir f Internally Cured Cncrete, ASTM - Advances in Civil Engineering Materials, 05/2015; 4(1): DOI: /ACEM , Friggle, T. and D. Reeves, Internal Curing f Cncrete Paving Labratry and Field Experiences, ACI SP 256, Eds. D. Bentz and B. Mhr, American Cncrete Institute pp CD ROM, 2008.

11 Internal Curing f Cncrete Internal Curing f Cncrete Quiz Ezekiel Enterprises, LLC 1. Hw des cnventinal curing differ frm internal curing f cncrete? Prvides water frm the surface f the cncrete Prvides water frm the aggregates within the cncrete Prvides water frm the bttm f the cncrete Nne f the abve 2. Internal curing can reduce: Transprt prperties Curling/warping/shrinkage All f the abve Autgenus shrinkage 3. T date, where has internally cured cncrete mstly been used? Clred cded applicatins Pavement applicatins Residential driveway applicatins Bridge deck applicatins 4. True r False: Internal curing makes cncrete less susceptible t thermal cracking True False 5. Internal curing in jinted plain cncrete pavements appears t have psitive effects n issues knwn t influence: Structural lngevity Durability Nne f the abve Bth a & b

12 Internal Curing f Cncrete Internal Curing f Cncrete Quiz Ezekiel Enterprises, LLC 6. Which tw benefits were fund when using internal curing in full depth cncrete pavement patches? Reduced cracking & shrinkage restraint Reduced built in stress & decreased water curing Reduced cracking & decreased water curing Reduced cracking & increased water curing 7. Why is fine lightweight aggregate preferable t carse lightweight aggregate? It can prvide sufficient distributin t enable the water t reach the matrix Prvides fr less stiffness and fracture prperties in the cncrete All f the abve Less expensive 8. Except fr, internally cured cncrete mixture design generally is identical t that f cnventinal cncrete. Air cntent Water cntent Lightweight aggregate Curse aggregate 9. The life-cycle cst analysis f internal curing prjects t be cnventinal cncrete. Higher than The same as Lwer than Nne f the abve 10. True r False: When using internal curing, the water that is absrbed in the lightweight aggregate des nt cntribute t the water-t-cement rati. True False