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41 Introduction As part of the project, short telephone interviews were held with several airports known to have experienced ASR on their airside pavement facilities. The primary purpose of these interviews was to document the experiences of the airports in addressing ASR, particularly in terms of the types of treatments that were used and the level of success. The airports were selected based on their geographical diversity and on their range of experience in dealing with ASR on airside pavement facilities. Phoenix Sky Harbor International Airport Phoenix Sky Harbor International Airport (PHX) has a history of issues with ASR on several of its pavement facilities. In general, concrete pavements that were built in the 1950s and 1960s did not exhibit any ASR due to the use of a non-susceptible aggregate source; however, starting with pavements built in the 1980s, ASR began to emerge in several areas of the airfield due to a change in aggregate source. The most severe cases of ASR were found on the Terminal 4 apron (north and south concourse) pavements, which were built around 1985. These pavements were typically 18 inches (460 mm) thick with 20 ft by 20 ft (6.1 m by 6.1 m) slabs and were placed on an aggregate base. The ASR started to appear in the early 2000s and exhibited the following symptoms: â¢ A âwetâ map-cracking appearance, even during the hot, dry summer months, as shown in Figure 19. â¢ Closing of pavement expansion joints. â¢ Pavement growth and expansion, moving the caissons that supported the jet bridges, which in turn affected the operation of some doors and damaged some walls and floor tiles in the terminal building. â¢ Heaving of adjacent sidewalks (which were tied to the apron concrete). â¢ Movement of utility manhole fixtures. â¢ Shifted alignment of joints across adjacent slabs. Interestingly, the ASR did not cause a significant spalling problem in the Terminal 4 apron pavements. An independent firm took cores from several locations in the apron area and performed petrographic evaluation that confirmed that ASR was present in the concrete. This represented one of the first airports in a hot and dry climate to document the presence of ASR. C H A P T E R 4 Case Examples
42 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports In response to the ASR distress manifestations that were observed on Terminal 4, the airport undertook several different remedial actions: â¢ Creation of a 2-ft (0.6-m) wide, asphalt concrete-filled trench adjacent to the terminal building to alleviate the pressure damage. This was effective in reducing the damage to the building, but the asphalt concrete âhumpedâ up as expansion continued and required continuous maintenance in the form of milling. â¢ Creation of a 2- to 2.5-inch (51- to 63-mm) pressure relief joint in the concrete between the adjacent concourses (see Figure 20). This was done full-depth using a diamond-bladed saw, but the joint was observed to close up within a few years. â¢ A lithium nitrate solution was applied on the surface of the pavement, which was performed over 3 consecutive nights. The pavement area was then monitored for an extended period of time. There were no visible signs of the lithium treatment slowing or mitigating the ASR distress or expansion, so this treatment was considered ineffective. â¢ Although spalling was not that widespread, some partial-depth repairs have been placed (see Figure 21). A proprietary two-part polyurethane-based material (FlexSet Concrete Repair by Roklin Systems, Inc.) is commonly used as a partial-depth repair material and, based on interviews, has been used with success. FlexSet can be opened to traffic in as little as 30 minutes. Figure 19. Map cracking at PHX (courtesy of Applied Pavement Technology, Inc.). Figure 20. Sawed relief joint between adjacent concourses at PHX (courtesy of Jeff Stempihar, NCE).
Case Examples 43 The airport seals the joints to help keep incompressible materials out of the joints. Preformed compression sealants are the preferred sealing material for new concrete construction, but in resealing operations, silicone sealant is used. The airport has not used waterproofing or penetrating surface sealers, but did consider the use of an asphalt concrete overlay; however, concern was expressed that the overlay might trap moisture and actually accelerate the ASR distress, so they did not consider it further. As a result of the ASR distress, the airport recently completed a reconstruction of the Terminal 4 south apron and is conducting a gate-by-gate reconstruction of the Terminal 4 north apron to remove the affected pavement. This is not only because of the pressure damage issues, but also because of a general concern that the pavement deterioration could accelerate at any time, resulting in an unacceptable risk of FOD. In addition to Terminal 4, the airport has also observed ASR in the Terminal 3 apron and in Taxiway B7, both of which date to the early 1980s. The ASR in Terminal 3 is more localized and is not as widespread as Terminal 4, although isolated areas are currently under- going reconstruction as part of the Terminal 3 Modernization Project. Taxiway B7 looks very similar to the cracking in Terminal 4, exhibiting ASR over perhaps 50 to 60% of the area. The Taxiway B7 pavement has exhibited significant edge spalling where it abuts the adjacent runway. The airport has performed a regular program of partial-depth patching to help maintain their ASR-affected pavements. When working to remove deteriorated concrete as part of the partial-depth repair, there have been instances where the concrete continues to break away as workers try to get down to sound concrete, requiring careful removal operations. For example, a backhoe with a hammer was observed to easily break through the concrete when doing repairs and thus great care must be exercised during the repair operations to avoid damaging concrete that is slated to remain in place. For new concrete construction, the current FAA P-501 specification is followed, which calls for aggregate testing and the addition of either fly ash or lithium nitrate admixture to mitigate ASR. An ASTM C618 Class F fly ash is more commonly used at typical volumes of 25 to 30% by mass of total cementitious materials. Figure 21. Partial-depth repair (and map cracking) at PHX (courtesy of Applied Pavement Technology, Inc.).
44 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports Hartsfield-Jackson Atlanta International Airport Hartsfield-Jackson Atlanta International Airport (ATL) first documented ASR in 1984 on Runway 8R, the â40-day wonderâ runway pavement that was built in 1969. This was a 16-inch (406-mm) thick jointed reinforced concrete pavement (JRCP) placed on a 6-inch (150-mm) unbound aggregate base layer and a 6-inch (150-mm) soil-cement layer on top of a prepared subgrade. The pavement featured 25-ft (7.6-m) by 75-ft (22.8-m) panels. The initial investiga- tion found pattern cracking along the joints that was first identified as D-cracking. However, petrographic analysis of pavement cores revealed the presence of âreaction rimsâ around the aggregate associated with ASR. The cracking was also observed to extend vertically to the depth of the reinforcement (about 8 inches [203 mm]) and turned horizontal; this resulted in significant delamination and large pieces of concrete breaking off, causing significant FOD. The runway was ultimately reconstructed in 2006. ASR was later observed on several other runways of similar age and design, as well as on other facilities. Runway 9L, built in 1974 with 16-inch (406-mm) slabs and 25-ft (7.6-m) by 75-ft (23-m) panels, was confirmed to have ASR in 1990. Runway 9R, also of the same construc- tion era but with 25-ft (7.6-m) by 50-ft (15.2-m) panels, was also confirmed with ASR in 1990. Runway 9L remains in service today, but Runway 9R was reconstructed in 1999 due to the effects of ASR. Figure 22 is a micrograph of a fractured surface treated with sodium cobaltinitrite that stains sodium-rich ASR gel yellow. This is from a core retrieved from Runway 9L and shows deposits around the darkened coarse aggregate perimeter stained yellow, indicating ASR gel (see the darker arrow/the red arrow if viewing the PDF). The lighter arrow (the blue arrow if viewing the PDF) points to secondary ettringite that infilled a crack at the aggregate-paste interface, which remained white after the staining. Secondary ettringite is a common feature observed in ASR-affected concrete, a result of the ASR and not a cause of the damage. The primary symptoms of the ASR on these facilities were map cracking and progressive spalling. The isolation joints at fixed structures were noted to close up, and bulging of adjacent asphalt shoulders was also observed. Figure 23 shows a photo of typical ASR cracking on a ramp facility at ATL. Figure 22. Micrograph of fractured surface from Runway 9L (yellow stain indicates ASR gel) (courtesy of Tara Cavalline).
Case Examples 45 ATL has instituted an aggressive patching program to maintain the serviceability of their pavements that are exhibiting ASR. The airport currently uses CTS Rapid Set for partial-depth repairs and requires minimum 6-inch (150-mm) depths to help ensure good performance. In some cases, a polymer concrete may be used. A unique aspect of the partial-depth repairs used at ATL is the addition of a horizontal reinforcing steel mat that is anchored to the adjacent sound concrete with tiebars (see Figure 24). This type of partial-depth repair has been used for more than 20 years at ATL as an effective means of addressing slab spalling (Greer et al. 2014). When slab deterioration is excessive or accompanied by structural cracking, full-depth repairs will be performed. Many of these repairs are placed under short, 10-hour construction windows, but, when feasible, there has been a recent effort to allow longer 3-day construction windows that permit the use of more conventional repair materials. The airport uses silicone joint seals to keep moisture and incompressible materials out of the joint. The airport has not used waterproofing or penetrating surface sealers, but has experimented with the use of topical applications of lithium nitrate on several facilities, including Taxiway Figure 23. ASR cracking pattern at ATL (courtesy of Tara Cavalline). Figure 24. PDR with reinforcing steel at ATL (Greer et al. 2014).
46 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports Bravo and Runway 9L. Those applications, applied to a short series of slabs, did not appear to offer any benefits or changes in performance. The Runway 9L application was the subject of the IPRF study and included two test sections on the western end of the runway, along with a control section (Whitmore 2009). An initial application of lithium was applied in January 2006 in two coats of approximately 444 ft2/gal (10.8 m2/L), with follow-up applications in August and December 2006. The multiple lithium treatment applications increased the depth of penetration from about 0.25 inches (6 mm) to about 0.70 inches (18 mm). After 4 years of monitoring, no changes in performance or expansive movements were observed. The airport does not consider asphalt concrete overlays as a rehabilitation option and instead has programmed the ASR-affected pavements for reconstruction. Even with ASR, the airport has obtained 35 to 40 years of service from their runway pavements. Although low alkali cement was not required for concrete at ATL in the 1970s through the 1990s, a review of mill test reports for cement used in the reconstruction of Runway 9L (1974) indicated that the cement would meet the requirements for low alkali cement even though it was not specified (Greer et al. 2013). As a result, this is believed to be the reason the ASR has remained in the low- to moderate-severity category on Runway 9L, enabling a long service life. For new concrete, the airport specifies low alkali cement and the addition of an ASTM C618 Class F fly ash to mitigate ASR. ASR has not been observed on any of the pavements constructed under these specifications. Bangor International Airport Bangor International Airport (BGR) is a joint civilâmilitary public airport on the west side of Bangor, ME. In the mid-1990s, the airport documented ASR in several of its concrete airfield facilities, including its only runway (Runway 15â33), and several taxiways and aprons. The distress first showed up in the form of map cracking, but significant expansive forces were also present in the pavement that contributed to blowups, joint buckling, pushing of fixed structures such as trench drains, joint misalignment, and shoving of adjacent asphalt pavement and shoulders. Figure 25 illustrates some of the conditions observed at BGR. The most critical case of ASR occurred on Runway 15â33, the airportâs only runway. The runway pavement is a JRCP design that ranges in thickness from 17 to 19 inches (430 to 480 mm) at the ends to 15 inches (380 mm) in the central portion of the runway. The pavement rests on an aggregate base and the slab dimensions are nominally 25 by 50 ft (7.6 by 15.2 m), although shorter panels are found in the outer portions of the runway. Cores at selected locations in the pavement showed that the map cracking typically extended vertically from the surface to a depth of about 2 to 4 inches (50 to 100 mm) and also often exhibited horizontal cracking at that depth and below. Petrographic examinations performed in 1995 confirmed the presence of ASR, with additional examinations performed in 2001 to evaluate the level of progression. The problem was identified in both the coarse and fine aggregates, and the petrographic examination showed the presence of clear to milky-white alkali-reaction product in portions of the observed cracks, particularly those around the periphery of affected coarse and fine aggregate particles. Throughout the 1990s and into the early 2000s, BGR performed full- and partial-depth repairs to maintain the serviceability of the runway pavement. Particularly during the summer months, an extensive patching program was performed to address the spalling and deterioration caused by ASR. Various rapid-setting materials were used for these partial-depth repairs. In the late 2000s, the airport began using Crafco TechCrete and DS Brown Delpatch. Both mitigations were successfully employed. A minimum repair depth of 3 inches (76 mm) is specified.
Case Examples 47 Expansion joints were frequently cut into the pavement to accommodate movements, and these ranged in width from 6 inches (150 mm) to 8 to 10 ft (2.4 to 3.0 m) or more. Saw binding was common when making the initial saw cut because of the built-up expansive forces that existed in the pavement. The wider systems essentially removed a significant portion of a concrete slab and filled it with an aggregate base (Item P209) and an asphalt concrete surface (Item P401) and were very effective at accommodating the significant amounts of movement encountered. While these were susceptible to humping, the airport would regularly mill these off as the conditions dictated. After performing an extensive evaluation of the runway pavement, the airport placed a 5-inch (125-mm) HMA overlay on the ASR-afflicted concrete pavement in 2004. As part of this work, the airport sawed and sealed âjointsâ in the HMA overlay at locations corresponding with the longitudinal and transverse joints in the underlying reinforced concrete pavement. This was done to more easily maintain any subsequent reflection cracking in the HMA overlay and to help (c) (a) (b) Figure 25. ASR manifestations at BGR: a) map cracking (courtesy of Applied Pavement Technology, Inc.), b) misaligned joints caused by slab movement (courtesy of Applied Pavement Technology, Inc.), and c) shoving of adjacent asphalt pavement (courtesy of Jason Homiak).
48 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports improve overall performance. The saw cut joints in the HMA overlay were nominally 0.5 inches (13-mm) wide by 0.5-inches (13-mm) deep and are periodically resealed by the airportâs main- tenance forces, typically using a Crafco rubberized sealant meeting the Item P605 specification. The airport believes the sawing and sealing of the HMA overlay has been a strong contributor to its performance. After more than 13 years of service, the overlaid pavement is performing well and there have been no apparent detrimental effects of the HMA overlay on the progression of ASR in the underlying pavement. The reinforcement in the underlying concrete pavement is likely contributing to the effectiveness of the overlay by controlling mid-panel crack deterioration. Other concrete pavement structures within the airport are also afflicted by ASR. For example, Taxiway A exhibited ASR and received an HMA overlay that featured several different test sections evaluating different pre- and post-overlay treatments, including surficial milling of the concrete, slab fracturing (break/seat and rubblization), installation of a stress-absorbing membrane interlayer, and sawing and sealing joints in the HMA overlay. Of the various alter- natives, the section that showed the best overall performance was the mill and overlay option that included sawed and sealed joints. In addition, several apron areas exhibited some of the most severe ASR conditions at the airport, possibly exacerbated by the non-reinforced panels and the restraint forces produced by their confinement. In the late 1990s, the airport began specifying the addition of an ASTM C618 Class F fly ash to mitigate ASR in new mixtures. Because of the high level of aggregate reactivity, the fly ash is typically added at levels of between 30 and 40% of the total cementitious content by mass. Since the adoption of this specification, no ASR issues have been observed in their new concrete construction. Colorado Springs Airport The City of Colorado Springs Municipal Airport (COS) is a city-owned, public civilâmilitary airport located southeast of Colorado Springs. The airport recognized ASR on several of its facilities in about 2004, including on Runway 17L-35R as well as on several taxiways (most prominently Taxiway E) and several terminal ramps. These concrete pavements were all built in 1996 and generally featured 14-inch (355-mm) slabs with either 20-ft by 20-ft (6.1-m by 6.1-m) or 18.75-ft by 18.75-ft (5.7-m by 5.7-m) panels. During this period, the airport also made extensive use of potassium acetate deicers, and there appeared to be greater surface damage on those ASR-affected concrete pavements that received more frequent applications of the deicer. As described in Chapter 2, an IPRF study (Rangaraju and Olek 2011) indicated that potassium acetate does not achieve adequate penetration in the field to accelerate ASR, but can make the crack pattern more visible. After about 8 to 10 years of service, these 1996-era pavements began to exhibit all of the signs of ASR distress, including staining of joints and cracks, pattern cracking around joints (see Figure 26), map cracking, spalling, closing of expansion joints, blowups, and damage to light cans (see Figure 27). Petrographic analysis of cores taken from the facilities confirmed ASR. COS instituted a major maintenance program consisting of both partial-depth and full- depth repairs shortly after the signs of ASR were first observed. Given the more localized areas associated with the ASR distress, partial-depth repairs were widely used. Experience indicated that sound concrete would be exposed at depths of 2 to 4 inches (50 to 100 mm) into the slab, so the typical depth of the partial-depth repairs was not more than 5-inches (125-mm) deep. Both conventional concrete and proprietary (TechCrete) materials were used; the airport reported more success with the TechCrete (performance over more than 5 years) because of its ability to accommodate slab movement. Figure 28 shows a photo of a partial-depth repair using TechCrete, while Figure 29 shows photos of concrete partial-depth repairs.
Case Examples 49 Figure 26. ASR pattern cracking around joints (courtesy of Applied Pavement Technology, Inc.). Figure 27. Shoving and spalling of taxiway lights at COS (courtesy of Applied Pavement Technology, Inc.).
50 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports Figure 28. Partial-depth repair (TechCrete) at COS (courtesy of Applied Pavement Technology, Inc.). Figure 29. Variation in performance of concrete partial-depth repairs at COS (courtesy of Applied Pavement Technology, Inc.). A topical application of lithium nitrate was used on a series of six slabs on Taxiway E in 2004. Two separate applications were placed about 12 months apart; the treatment was found to have no effect on the progression of the ASR or the development of associated distress or expansion. In fact, one of the treated panels deteriorated to the point that it had to be replaced. Given the continuing progression of ASR and the significant maintenance investments, the airport began a program of reconstruction in 2006. Although other solutions, such as asphalt
Case Examples 51 or concrete overlays, were considered, it was believed that total reconstruction was the best option. Runway 17L-35R and Taxiway E were the first to be reconstructed, and several other taxiways were reconstructed short time later. Currently, about 80% of the 1990s era ASR-affected concrete pavement has been replaced. The airport undertook a major initiative to formulate new concrete mix designs to avoid the development of ASR in new concrete paving, including the incorporation of non-ASR- susceptible aggregate. ASTM C1260 is used to evaluate aggregate materials, and ASTM C1567 is used to evaluate the aggregate with ASTM C618 Class F fly ash, which is added at quantities of about 15%. Underdrains are also included on new pavement construction to remove excess water. To date, no ASR issues have been observed on any of the new concrete pavements. Key Takeaways from Case Examples A few key takeaways from the various case examples are highlighted below: â¢ ASR was typically observed after about 10 to 15 years of service. â¢ Pattern cracking around the joints and general map cracking throughout the slab were common initial signs of ASR, followed thereafter by spalling and pressure-related damage (blowups, shoving of fixed structures and adjacent pavements). â¢ Various severity levels of ASR are present, with less aggressive forms still allowing for 25 or more years of performance (ATL) whereas more aggressive forms can significantly shorten pavement life (COS). Cement alkalinity and overall exposure conditions also contribute to the aggressiveness of the reaction. â¢ Partial-depth repairs are the primary method used to maintain facilities in serviceable conditions and to control FOD. A range of materials are used, including several different types of proprietary materials that offer early opening times and can accommodate expansive movements. â¢ Full-depth repairs are used when slabs become severely deteriorated. â¢ The use of PDR, FDR, and PRJ are commonly used by airports as interim treatments as a way of maintaining the serviceability of the pavement until more substantial rehabilitation can be programmed. â¢ One airport applied an HMA overlay to rehabilitate an ASR-affected runway pavement and has seen good performance after 13 years of service even though the underlying concrete continues to move. The HMA overlay was sawed and sealed after construction, and this is apparently contributing to its good performance. The reinforcement in the underlying concrete pavement is likely contributing to the effectiveness of the overlay by controlling mid-panel crack deterioration. â¢ A few airports have tried topical application of lithium compounds on their ASR-affected pavements, but these have not demonstrated any effect on slowing or preventing the progres- sion of ASR. â¢ All airports have adopted new specifications to prevent ASR in new concrete pavement construction, and these, to date, have been taken to be very successful. Key aspects of these specifications are the use of ASTM C1260/C1567 to screen aggregates and the use of ASTM C618 Class F fly ash to mitigate ASR if the aggregates are deemed to be susceptible.