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40 Curing PraCtiCes Plastic shrinkage cracks occur in the top surface of concrete when job conditions are so dry that moisture is removed from the surface at a faster rate than it is replaced by bleed water (ACI Com- mittee 224 2008). The rate of evaporation depends on the air and concrete temperatures, air relative humidity, and wind speed. If the rate of evaporation approaches 0.2 lb/ft2/h, precautions against plastic shrinkage cracking are needed. The potential for plastic shrinkage cracking can be minimized by not placing concrete when conditions are not favorable or taking precautionary methods such as fog spraying. The National Ready Mixed Concrete Association nomogram is often used to determine evaporation rates (ACI Committee 305 2014). ACI 224R-01 states that the most effective curing environment is to keep the concrete continu- ously wet with a wet cover in contact with the surface of the concrete for at least 7 days (ACI Com- mittee 224 2008). At the end of the curing period, the cover is to be left in place until it and the concrete surface appear dry. Shrinkage cracking in hardened concrete can occur shortly after the end of water curing if the concrete is allowed to dry too rapidly. ACI 224R-01 also states that rapid dry- ing can be prevented by using a curing compound after water curing. Wet curing of concrete bridge decks is illustrated in Figures 14 and 15. Based on the results from the survey for this synthesis, all responding agencies except one wet cure concrete bridge decks. However, the duration of wet curing varies from 3 to 14 days, as shown in Figure 16. Clearly, most states cure the deck concrete for 7 to 14 days. This is consistent with the length of curing periods reported in a 2012 survey (Russell 2013). Most agencies wet cure the decks using soaker hoses in combination with prewetted burlap and plastic sheeting, curing blankets, or plastic-coated burlap. In a survey for NCHRP Synthesis 333 published in 2004, agencies in the United States and Canada indicated a range of curing periods from 3 to 14 days, the most frequent time period being 7 days (Russell 2004). However, between 2004 and a survey in 2012, the percentage of agencies specifying 7 days or fewer decreased from 87% to 67%, and the percentage specifying 14 days had increased from 11% to 24%. In the 2012 survey, only two states reported fewer than 7 days of wet curing. Based on their experiences with cracking on the westbound Kernville viaduct, the Pennsylva- nia DOT (PennDOT) adopted the following construction considerations for the eastbound bridge (Spangler and Tikalsky 2006): ⢠Place positive moment regions on 1 day followed by the placement of negative moment regions no less than 3 days later. ⢠Apply moist curing immediately after concrete finishing and maintain continuously for 10 days with curing compound applied thereafter. ⢠Increase vigilance in quality control and quality assurance operations. Based on a research study for the Indiana DOT, Frosch et al. (2002) recommended a minimum 7-day wet curing process to reduce overall shrinkage strains and that the drying shrinkage of the concrete mix should be minimized through concrete mix design and materials selection. Concrete compressive chapter four effeCts of ConstruCtion PraCtiCes on CraCking
41 FIGURE 14 Application of wet burlap within minutes of strike off (Courtesy: Michigan DOT). FIGURE 15 Application of polyethylene sheeting to ensure wet curing (Courtesy: Texas DOT). FIGURE 16 Duration of wet curing for concrete bridge decks.
42 strengths higher than specified by structural design are not required and can exacerbate deck cracking. Higher concrete compressive strengths can require additional cementitious materials that produce con- cretes with higher shrinkage, higher tensile strength, and higher modulus of elasticity. The higher ten- sile strength is beneficial in reducing cracking, but its benefit is more than offset by the higher shrinkage and higher modulus of elasticity, which contribute to higher tensile stresses. Weather Conditions Krauss and Rogalla (1996) identified that weather conditions, such as high temperatures and low humidity, and inadequate curing are construction factors that affect deck cracking. They recommended that concrete placement cease or protective measures be taken during periods of high evaporation. Casting concrete decks at night significantly reduced deck cracking. They recommended wet curing during hot weather and for a period of at least 14 days. Kivisto (2003) reported two instances when construction practices led to cracking in bridge decks. The first instance happened in 1999, when the specifications required the contractor only to fog the deck to keep it wet below placing wet burlap. During the deck placement, the wind speed increased, and the manual fogging operation was not able to keep up with the rate of evapo- ration. As a result, several areas of map cracking were evident in the deck after completion of the curing period. As a result, the specifications were revised to require placement of wet burlap within 15 minutes of finishing and the burlap to be maintained in a wet condition for 7 days after placement of the deck. A second instance of deck cracking occurred in 2002, when the contractor did not have the work bridges set up behind the paving machine for immediate application of the wet burlap. The contractor tried to fog the deck from the ends and sides of the bridge. As the wind increased, the manual fogging was not able to keep up with the surface evaporation. Transverse cracks at 5-ft intervals occurred throughout the deck. PlaCement length and ConstruCtion sequenCe Construction sequence can influence transverse deck cracking in the negative moment regions of continuous structures. If the negative moment region is cast before the positive moment region, the casting of the positive moment region introduces tensile stresses in the top of the negative moment region. The practice of casting the positive moment region ahead of the negative moment region is aimed at alleviating this situation but may not always be practical. Although placement sequences are specified in the bridge plans, contractors often employ their own placing sequence (Schmitt and Darwin 1995). These new sequences should be approved by the engineer, but frequently they are not recorded. Given the limited data available on the subject, researchers have found varying levels of importance in placement sequence and its relationship to cracking. Perfetti et al. (1985) calculated the concrete stresses caused by the construction sequences of 16 continuous steel girder bridges built in North Carolina. They reported no consistent correlation between the incidence of transverse deck cracking and the maximum tensile stresses induced by only the dead load of the concrete. However, when the tensile stresses from live load were combined with the residual dead load stresses, a correlation existed between the calculated tensile stresses and the incidence of deck cracking. They recommended that alternate casting sequences be considered to minimize tensile stresses. Schmitt and Darwin (1995), in an investigation of 40 bridge decks in northeast Kansas, could not identify any relationship between cracking and placement length for monolithic bridge decks. How- ever, cracking clearly increased as placement length increased for bridge deck overlays. Krauss and Rogalla (1996) reported that placement sequence is important but that the sequence is not a primary cause of transverse deck cracking.
43 Babaie and Fouladgar (1997) pointed out that flexural transverse cracks over supports of continu- ous structures can be lessened by placing concrete in the center portion of adjacent spans before placing the concrete over the supports. Ramey et al. (1997) recommend a detailed placing procedure as follows: ⢠Place complete deck at one time when possible. ⢠Place simple span bridges one span per placement, or if span is long, divide the deck longitu- dinally and place each strip at one time. If this cannot be done, place the center of the span first and then place other portions. ⢠If multiple placements are made on continuous beams, place middle spans first and wait 72 hours between placements. Use a bonding agent to enhance bond at construction joints. According to Saadeghvaziri and Hadidi (2002), earlier studies (Cheng and Johnston, 1985; Per- fetti et al. 1985) reported that placement length and sequence do not appear to influence crack- ing. However, later studies suggested that placement length, sequence, and rate of placement may have some effects on deck cracking, and Saadeghvaziri and Hadidi (2002) recommended placement sequences similar to those suggested by Ramey et al. (1997) and that the placement sequence be specified. Kochanski et al. (1990) recommend placing concrete at a rate faster than 0.6 span lengths per hour. In an analytical study, Issa (1999) attributed cracking to sequence of placement and recom- mended placing concrete in positive moment regions first. Hopper et al. (2015) made the same recommendation. The Uphapee Creek Bridge in Alabama consists of seven equal simple spans of 114 ft (Mokarem et al. 2009). Almost 100% of the cracks in the bridge deck occurred in the transverse direction and were located in the quarter-span lengths at the end of each span. The middle half of each span was relatively free of cracks. The center portion of each span was cast first and the quarter lengths were cast several days later. The measured properties of the concrete used in the quarter and center lengths were similar. The casting of the quarter lengths after the center would induce compressive stresses in the deck of the center portion and may explain why the center portion was relatively crack free. other PraCtiCes Frosch et al. (2002) also suggested that alternatives to stay-in-place metal deck forms should be con- sidered because the pan shape causes stress concentrations and the pan prevents inspection of the underside of the deck. The use of a metal angle with a leg upturned into the deck should be discontinued because the leg produces a crack initiator along the edge of the girdersâ top flanges. summary of ConstruCtion PraCtiCes Construction practices that can reduce the likelihood of deck cracking include the following: ⢠Applying wet curing procedures immediately after concrete finishing and maintaining the surface wet for at least 7 days. ⢠Applying a curing compound after the wet curing period. ⢠Where practical, using a placement sequence minimizes tensile stresses in previously placed fresh concrete. ⢠In hot or low humidity conditions, placing concrete at night.
