Rapid Slab Repair and Replacement of Airfield Concrete Pavement (2021)

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38 As defined in Chapter 1, FDR refers to full-depth removal and replacement of a portion of a slab or an entire slab by using either cast-in-place concrete or precast concrete. This chapter discusses the appropriate distresses, materials, design, and construction for an FDR project and presents an assessment tool to help users evaluate the considerations necessary to complete a successful FDR. Introduction to Full-Depth Repair FAA AC 150/5380-6C (FAA 2014b) identifies the following FDR types, which are based on how much of the slab is being repaired: • Partial-width, partial-slab repair (e.g., a corner break), • Full-width, partial-slab replacement (e.g., a deteriorated joint), and • Full-slab replacement (e.g., high-severity midslab cracking or shattered slab condition). For discussion, partial- and full-slab FDRs are grouped together. FDRs are typically either cast-in-place concrete or precast concrete slabs with deterioration extending through more than one-half the thickness of the existing concrete pavement. Precast slabs are generally used for full-slab replacements on airfields, either as an intermediate step when cast-in-place repairs are being made or as the final pavement. Figure 18 provides examples of installations of cast-in-place and precast FDRs. Cast-in-place FDR is the most common method reported by the airports that participated in this study. Table 13 lists advantages and disadvantages of this technique. Precast slab FDR is an alternative repair method that was used as early as the 1930s on high- ways and airfields for partial- or full-slab replacement, although widespread implementation of this method at that time was limited (Priddy et al. 2013). Interest in the use of precast concrete has increased at airports, especially in areas where very short closures are necessary (e.g., runway intersections). According to Tayabji et al. (2009), the main justifications for the use of precast concrete slabs are time savings or reduced closure times; otherwise, precast concrete pavement is typically not economically competitive. Table 14 lists advantages and disadvantages of precast slabs. Following are key considerations for determining the feasibility of precast slabs as an FDR option: • Extent and location of the damaged pavement; • Temporary or permanent repair; • Expected performance and life of the repair; C H A P T E R 4 Full-Depth Repair

Full-Depth Repair 39   Source: (a) Nichols Consulting Engineers, Chtd., and (b) Shiraz Tayabji. Figure 18. Examples of cast-in-place and precast full-depth repairs: (a) cast-in-place and (b) precast. Advantages Disadvantages • Familiarity of contractors with the equipment and process. • Proven technique for a range of distresses. • Difficulty in placement during certain adverse weather conditions. • Durability issues with some materials. • Curing time. Source: Williams et al. (2012), Priddy et al. (2013). Table 13. Advantages and disadvantages of using cast-in-place concrete for full-depth repair. Advantages Disadvantages • No on-site concrete curing time. • Conventional concrete can be used. • Quality of concrete is higher. • Can be prefabricated and stored. • Fabricated under controlled conditions. • Weather window for installation is broad. • Cost is much higher. • Moving and placing large panels may be difficult. • Slab edges and dowel bars may get damaged. • Experienced crews are required. • Crane height may penetrate FAA Federal Aviation Regulation (FAR) Part 77 surfaces and require closure of adjacent runways. Source: Buch et al. (2003), Olidis et al. (2010), Ashtiani et al. (2010), Priddy et al. (2013), Tayabji et al. (2012), and Chao (2018). Table 14. Advantages and disadvantages of precast slab concrete for full-depth repair.

40 Rapid Slab Repair and Replacement of Airfield Concrete Pavement • Existing pavement structure; • Feasibility of transporting and placing large, heavy panels or ability to manufacture panels on or near site and place them; and • Local availability of experienced contractors and precast yards. A more detailed description of precast slab repair methods is found elsewhere (Olidis et al. 2010, Ashtiani et al. 2010, Priddy et al. 2013, Smith and Snyder 2019). Given the complexity of these systems, they require independent engineering analysis and design for each project. Table 15 presents a few examples of precast installations that have been used for airfield pavements. Candidate Distresses and Conditions FDR is warranted when other methods (e.g., PDR, diamond grinding) can no longer be used to repair distresses effectively. In most cases, the distresses have severely diminished the structural integrity (i.e., load-carrying capacity) of the slab. The following pavement distresses are considered candidates for FDR [American Concrete Pavement Association (ACPA) 1995, Smith et al. 2014, FAA 2014b, and U.S. DOD 2018]: • Corner breaks (low, medium, and high severity); • Blowups; Panel Type Description Imagea Bottom dowel slots Precast slabs with bottom slots in the transverse joint faces of the slab to give room for the load-transfer dowel bars. Screws are used to adjust the final slab elevation, and grout is injected underneath the slab. This method can be used for single or multiple connected panel repairs. Top dowel slots Precast slabs with top slots in the transverse joint faces of the slab to give room for the load-transfer dowel bars. Screw-type leveling pads are used to adjust final slab elevation, and grout is injected underneath the slab. This method can be used for single or multiple connected panel repairs. ERDCb standard panel Consists of a standard panel with dowel bars at the middepth along both transverse edges and a second, or “terminal,” panel that has dowels in only one of the edges, while the other transverse edge has dowel receptacles for connecting it to a standard panel. The system uses a flowable fill or foam injection as a leveling course and to provide uniform support. a Row 1: Nichols Consulting Engineers, Chtd.; Row 2: Shiraz Tayabji; Row 3: Ashtiani et al. 2010. bPriddy et al. (2013). Table 15. Examples of precast slab installations for airfields.

Full-Depth Repair 41   • Transverse and longitudinal cracking (medium to high severity) or shattered slabs; • High-severity spalling or exposure of dowel bars; • Severe joint deterioration that is not a candidate for PDR; • Deterioration of existing repairs or adjacent to existing repairs (medium to high severity); • Deteriorated utility cuts or can lights; and • Materials-related distresses, such as alkali–silica reactivity, or medium- and high-severity D-cracking (as a stopgap measure). Figure 19 illustrates some of the common distresses that are candidates for FDR. Note that FDR may only provide a temporary solution when severe materials-related distresses (e.g., alkali–silica reactivity) or freeze–thaw distresses (e.g., D-cracking) are widespread. In these cases, FDR can be used to replace slab areas that pose a safety or operational hazard to aircraft in anticipation of a future, larger rehabilitation project. Concrete pavements exhibiting severe structural deterioration over an entire project or facility may be better suited for structural overlay or reconstruction. Material Selection Material selection requirements vary somewhat between cast-in-place and precast FDRs, as described below. Cast-in-Place Repairs Following are some of the factors to consider for cast-in-place materials: • Material strength gain required to complete work during the closure window after completion of other required FDR work items (e.g., demolition and removal of concrete, grade preparation, placing concrete). • Material costs, which generally increase with increased rate of strength gain. • Effect on unit repair costs of FDR repair size and project size (number of slabs to be replaced), larger repairs and larger projects typically being more cost-effective. Repair size can also affect the time required to complete each repair, in turn affecting productivity during short work windows. • Effect of the temperature and weather conditions anticipated during placement and curing on the selection of the repair material. General climate conditions, such as freeze–thaw cycling, can rule out the use of some aggregates and other materials. • Trade-offs between physical and mechanical properties of different repair materials (e.g., increases in early strength may correspond with increased shrinkage and reduced durability). • Experience working with the material. • Material performance (historical or as reported by other users). FAA AC 150/5380-6C (FAA 2014b) discusses the use of P-501 cement concrete pavement or state DOT materials for FDR. The P-501 specification states that state DOT materials are allowable for use on pavements for aircraft weighting less than 60,000 pounds. The U.S. DOD O&M Manual (2018) also indicates conventional concrete (ASTM C150 Portland Cement Type I–based mixes) are typically used but also discusses the use of ASTM C150 Portland Cement Type III–based mixes and proprietary materials for early opening to traffic requirements. The allowable closure time is often a driving factor in the selection of materials for cast- in-place repairs. In general, it is better to use the least exotic material that meets opening-to- traffic requirements. A conventional ASTM C150 Type I cement mix could take several days to

Source: Nichols Consulting Engineers, Chtd. Figure 19. Candidate distresses for full-depth repair: (a) corner breaks, (b) cracking (transverse or longitudinal), (c) shattered slab, (d) damage from alkali–silica reactivity (stopgap repair), and (e) widespread D-cracking (stopgap repair).

Full-Depth Repair 43   achieve adequate strength for opening to traffic. Concrete featuring either ASTM C150 Portland Cement Type I or Portland Cement Type III is commonly used with other mix design modifica- tions and can produce mixtures that are suitable for opening to traffic in as little as 6 to 8 hours. For earlier opening times, ESC mixtures using calcium aluminate or CSA cements can be used to meet strength requirements in as little as 2 to 4 hours (Priddy 2015). To achieve early strength gain in conventional concrete mixtures, the use of chemical admix- tures is common. These include accelerators and normal-range and high-range water-reducing admixtures. Note that calcium chloride or admixtures containing calcium chloride, as well as high-range water-reducing admixtures, are not permitted under Item P-501 “Cement Concrete Pavement,” in FAA AC 150/5370-10H, Standard Specifications for Construction of Airports (FAA 2018). Examples of the range in mixture proportions for different time-to-opening requirements are summarized in Table 16. Because ESC mixes typically contain higher cement contents and multiple admixtures, it is not uncommon for them to experience increased shrinkage, altered microstructure, and unexpected interactions (Van Dam et al. 2005). As a result, the long-term durability of these mixtures is potentially at risk. Several proprietary VHES cements (ASTM C1600) are commercially available. These specialty cements (e.g., CSA) can provide VHES to meet short time-to-opening time frames (<4 hours). Rapid-strength cements are typically more costly and can pose a challenge in handling and placing. These materials can be very sensitive to the weather conditions during installation and curing. For a new project—even if the construction crews have experience working with these materials—personnel should demonstrate their ability to properly install the materials off-site or should start work on the least-critical areas (e.g., aprons) before moving to critical areas (e.g., runways). Precast Slabs The most common material for precast slabs is conventional or accelerated portland cement concrete. Usually, the design strength requirements are the same as those for the existing concrete pavement, although early strength is often accelerated to facilitate stripping of forms. Conventional concrete is typically used because the slabs are produced before being hauled to the installation site. Precast slabs are also typically reinforced with deformed bars to control cracking that may occur during transport and handling and can be prestressed to reduce load-related stresses and associated cracking. Mixture Parameter Proportion, by Time to Opening 4–6 h 6–8 h 20–24 h Cement type I or III I or III I or III Cement content (lb/yd3) 650–895 715–885 675–800 w/cm ratio 0.38–0.40 0.36–0.40 0.40–0.43 Accelerator Yes Yes Yes or No Note: w/cm = water to cement. Source: Smith et al. (2014). Table 16. Typical constituent materials proportions for portland cement–based early-strength concrete mixtures.

44 Rapid Slab Repair and Replacement of Airfield Concrete Pavement Design The design process for FDR can vary in terms of the extent and nature of the repair (i.e., one slab versus multiple slabs, emergency versus nonemergency use). In some cases, standard details may be sufficient (e.g., emergency replacement of one slab) whereas specific plans and speci- fications may be necessary for larger projects. With FDR, alternative designs should also be considered and evaluated with respect to the need for accelerated construction (i.e., repairs to an apron pavement may be performed over a longer closure than repairs to a taxiway or runway). It is important to understand the existing pavement conditions as well as the extent of repairs to appropriately plan FDR. Cast-in-Place Repairs Design considerations, particularly for larger cast-in-place FDR projects, include the following: • Material requirements: Conventional materials can be used [FAA AC 150/5380-6C (FAA 2014b) references P-501 cement concrete pavement or state DOT materials], with the time allowed for strength gain being the major consideration. High-cement-content accelerated conventional mixes with a low w/cm ratio can achieve strengths relatively quickly but may exhibit shrinkage and early-age cracking. Proprietary materials can achieve strengths very quickly with use based on the manufacturer’s recommendations. Contractor experience is very important when unconventional materials are being used. • Provisions for incentives/disincentives when repairs have an impact on critical airport operations: Provide monetary incentives for early completion. Penalties for not returning pavement to service are generally much more severe for runways (e.g., $500/minute) but can be applied to other areas if they are critical to airport operations. • QC/QA: QC/QA for FDR procedures often follows standard specifications [such as P-501 cement concrete pavement (FAA 2018)] because quantities are greater (and, therefore, more controllable) than for typical PDR projects. Onsite inspectors and material testing technicians must be trained or have experience preparing beam specimens with VHES, if used. Additional test specimens or alternative strength measurement tests (such as maturity meters) need to be considered if short closure time frames are involved. • Opening-to-traffic requirements: Opening to traffic should be based on strength gain. The time allowed to achieve the required strength will depend on the allowable closure (e.g., strength needs to be achieved in less than 4 hours or in 24 hours or more). Some materials may set so quickly that time can be an indicator, but strength should ultimately be veri- fied. Environmental conditions (e.g., temperature, humidity) may affect or alter the rate of strength gain. • Provision for a preconstruction conference: It is advisable to hold a preconstruction meeting prior to closing pavement areas and allowing construction work to proceed. Establishing work methods and personnel responsibilities is significant to the success of FDR projects, and just-in-time training is extremely useful, especially when proprietary materials are being used. Construction of a repair demonstration (or test strip) on a noncritical pavement area (or off-site) is also suggested, particularly when new concrete materials or ESC are being used. Contractor experience (beyond experience with the materials) is also important when trying to coordinate and carry out rapid FDR projects in an area of aircraft operations. • Slab size requirements: For corner breaks and partial-slab repairs, FAA AC 150/5380-6C (FAA 2014b) indicates saw cuts be made at least 2 feet beyond the observed limits of damage. The U.S. DOD O&M Manual (2018) indicates that saw cuts should be a minimum of 3 feet from a joint. If the slab width is less than 20 feet, or if there are full-depth cracks within the interior area of the slab, full-width slab repair is required. The manual also indicates 10 feet

Full-Depth Repair 45   as the minimum repair dimension for airfield applications, to avoid rocking and pumping of the repair. • Joint considerations (doweling and tying): Load transfer needs to be provided at joints within large FDR areas and to reestablish joints with adjacent pavement. Load-transfer type (typically dowel or tie bar), size, and location need to be included. Details for installation— particularly drilling, alignment, and grouting—also need to be included. The planning and design process needs to consider what-ifs and develop appropriate contin- gencies. While not an exhaustive list, the following things should be considered: • Backup equipment and alternative sources of material production: All airports inter- viewed indicated the need for backup equipment and materials so that production facilities can avoid potential delays (or even not finishing the work within the required closure) due to equipment breakdown. Having backup supplies (e.g., dowel bars, PDR material) is also useful in the case of damage to adjacent concrete slabs or if the need to address unknown conditions (i.e., extend repair into an adjacent slab) arises. • Weather monitoring and mitigation: Ideally, FDR work should be scheduled during periods with historically acceptable weather for concrete placement. One agency interviewed had sufficient canopies to erect over the repair so work could continue during rain, if necessary. This is less of an issue for precast FDRs. • Alternate methods of reestablishing pavement surface: Some airports use precast slabs as temporary (or emergency) pavement if repair cannot be completed during a single shift. Alternate materials (such as asphalt) may also be considered. In addition: • Determine how utilities will be addressed: One survey respondent kept its in-pavement lights by coring around them. The concrete was then chipped off the cans and new concrete placed around the existing cans. Backup supplies can also be useful in case of damage to utilities or unknown conditions. • Plan site access and movement across the airfield well in advance: Haul time can become a significant factor, especially with ESC. Precast Slabs Many of the design items discussed for cast-in-place FDR also apply to precast slab FDR. Following are some of the differences: • Precast slab FDR is mainly used for full-slab replacement on airfields, but joint replacements are possible as well. Slabs are typically fabricated to produce approximately half-inch gaps around the entire panel to facilitate installation. Slab thickness is typically 0.5 to 1 inch less than the pavement being replaced; this allows for minor variations in slab thickness and base elevation while providing a gap for installing fine aggregate, grout, or urethane bedding material. Before fabrication and installation, the existing pavement thickness should be verified by coring or nondestructive means [e.g., ground-penetrating radar (GPR)] to avoid construction surprises. • Typically, specially proportioned mixtures that use ordinary portland cement are used for precast slabs. Early strength requirements are driven by the need to remove and reuse forms for the next cast, typically in less than 24 hours. Precast concrete strength at installa- tion is typically 5,000–6,000 pounds per square inch (psi)—far above typical design strength requirements. • Load-transfer is typically provided by dowels and tie bars, as required for the specific installa- tion. For connections with existing adjacent panels, the dowels and tie bars are typically drilled

46 Rapid Slab Repair and Replacement of Airfield Concrete Pavement and anchored in the existing slabs, and the precast slabs (fabricated with bottom slots or full- depth slots) are “dropped in” over the bars. Connections between multiple new precast slabs can involve slots in one panel and embedded dowels in the other. Alternatively, plain panels can be installed and dowels can be retrofit into slots cut across the joints with adjacent panels. Smith and Snyder (2019) provide details concerning precast dowel load-transfer systems. Panel reinforcement and embedment (e.g., lift-pins) needs to be designed appropriately. • Precast slab reinforcement and embedment (e.g., lift-pins, embedded jacks, grout ports, slot formers, etc.) needs to be designed appropriately. • Addressing in-pavement lighting (or other utilities) requires additional planning to ensure proper lighting alignment after installation. Note that lighting alignment will depend on both the installation of the can in the panel and on the proper elevation and rotation of the panel at installation. One project included in this study used two-piece cans and developed a solution for connecting the two pieces after panel placement. Another used adjustable light cans. • Establishment of haul routes and site access must consider the ability to transport slabs from the fabrication area to the repair site, including the maximum width permitted along the transport route. Full-sized airfield panels typically cannot be transported over roads, so other transport and/or fabrication options must be considered. • Backup equipment should include lifting cranes (or other equipment) needed to place the precast slabs. The impact of crane height to adjacent runway operation must be considered. Construction The generalized FDR construction procedure for cast-in-place includes (Hajek et al. 2011, Smith et al. 2014, FAA 2014b, U.S. DOD 2018): 1. Select repair location and mark boundaries. 2. Saw the repair boundaries and remove the damaged concrete without damaging the adjacent slabs to remain. Typically, 2 closely spaced parallel saw cuts are used at each boundary to minimize the chance of damaging the remaining concrete. 3. Restore the base, subgrade, and subdrains. 4. Restore the load-transfer system across the joints. 5. Replace any reinforcement. 6. Restore any expansion joints. 7. Place the new concrete. 8. Finish and texture to match the existing concrete. 9. Cure the concrete using the appropriate method. 10. Optional: perform diamond grinding. 11. Seal joints. 12. Open to traffic after proper curing. Preparation for precast FDR is quite similar through the removal steps. The following are general guidelines for precast concrete slab construction (Tayabji et al. 2009, Smith and Snyder 2019): 1. Determine the dimensions of the repaired area; this dictates the necessary panel size. This might not be possible for some larger repair areas (such as 25-foot by 25-foot slabs) due to limitations in transporting and placing such large, heavy precast slabs. Use of large cranes to place large precast slabs may also have limitations due to FAR Part 77 requirements. 2. Verify the existing concrete pavement thickness. It is suggested that the precast slab be 0.5 to 1.0 inch thinner than existing concrete to allow for variations in the thickness of the existing pavement/elevation of the base and to allow room for leveling.

Full-Depth Repair 47   3. Install saw cuts parallel and perpendicular to the center line; care must be taken to ensure panel fit. Typically, 2 closely spaced parallel cuts are made per boundary. 4. Place bedding materials according to precast system requirements. 5. Install load-transfer mechanisms as designed for the project. 6. Provide an expansion cap at one end of each dowel. This step is not done if slab gaps are filled with structural grout as is done in many applications. 7. Control dowel alignment with proper installation and cages. The use of narrow slots makes panel installation more difficult. 8. If possible, multitask during the installation process to reduce construction time. 9. If necessary, after installation, grind the surface to ensure a smooth ride. Seal transverse and longitudinal joints. Boundary Selection and Marking Selection of repair boundaries for airfield FDR typically includes the following considerations (Hajek et al. 2011, Smith et al. 2014, U.S. DOD 2018): • Full-width slab replacement is required if the original slab width is less than 20 feet. • Boundaries should encompass all deteriorated concrete and deteriorated subsurface layers. • It is not unusual for deterioration at the bottom of the slab to extend beyond what is visible on the surface, especially near the joints (Figure 20). The limits of deterioration can be determined by representative coring or nondestructive testing (e.g., ground-penetrating radar). Repair boundaries should be marked in advance in highly visible marking paint, as shown in Figure 21. Concrete Demolition and Removal Removal of the existing pavement can be very time-consuming. There are basically two methods for removal: the lift-out method and breakup-and-clean-out method. • Lift-out method: The lift-out method is usually faster and less damaging to the subbase, and is advisable whenever possible. Slabs are cut into manageably sized pieces to facilitate removal. Holes are drilled into the slab pieces and lift-pins inserted. Then the slab pieces are lifted out and placed on a truck by means of a crane or other construction equipment with steel chains connected to the lift-pins. An alternate lifting method is to use a vacuum-based slab lifter. To minimize damage to the adjacent slabs during lift-out operations, additional saw cuts can be made a few inches from and parallel to the repair boundaries and the wedges Source: Smith et al. (2014). Figure 20. Visual pavement deterioration at top and potential damage to the bottom.

48 Rapid Slab Repair and Replacement of Airfield Concrete Pavement of concrete removed to provide additional clearance during slab removal. This method is suggested when damage to adjacent slabs and subbase must be avoided. Wood shims can be wedged into saw cuts to minimize any rocking and potential spalling of adjacent pavement during removal. • Breakup-and-clean-out method: The breakup-and-clean-out method uses concrete breaking equipment (typically jackhammers) to completely break the concrete into smaller, manage- able pieces. This method should be avoided for slab replacements because it can damage adjacent slabs and results in disturbance to the base. However, this method must often be used when joints or slabs are so severely deteriorated that the lift-out method cannot be used. Furthermore, this method is used when damage to the base is not a critical concern or if only a few slabs are being repaired. Most of the airports contacted for this study have used the lift-out method with established sawing procedures (Figure 22). Sawing is often performed by using a set pattern that provides a narrower band around the perimeter to minimize the risk of damaging the adjacent pave- ment. Some agencies specify the use of angled interior cuts to facilitate removal. Most feel that overruns during sawing should also be avoided. One agency requires that a piece of steel plate be placed along the joint to prevent oversawing. Concrete chainsaws are a possible option in corners to avoid overruns (Figure 22). To expedite construction, many airports allow concrete sawing to be performed during a separate closure ahead of the closure for the pavement removal and replacement. The saw cut pattern is adjusted to avoid saw cuts near cracks, so as to avoid the creation of potential generators of FOD. An important step in allowing saw cutting to occur during an earlier closure is to have the pavement cleaned of sawing slurry prior to reopening. One airport turns to both in-house maintenance and ARFF personnel to help with saw slurry cleanup when needed, as they can quickly mobilize equipment when there is an urgent need. Source: Nichols Consulting Engineers, Chtd. Figure 21. Example of marked full-depth repair boundaries.

Full-Depth Repair 49   With the lift-out method, stabilized base material may be bound to the bottom of the pavement slab. One airport found that applying a dynamic load (not sufficient to break the pavement) could break the bond during removal. All demolition debris needs to be removed prior to continuation of grade preparation. Site Preparation Cast-in-Place Repairs If removal of the distressed pavement damages the base, it may be necessary to add new material that must be graded and compacted (Van Dam et al. 2005). As determined by the project engineer, if the repair area is too wet, it should be properly dried (ACPA 1995, Smith at al. 2014, U.S. DOD 2018). In some cases, extremely weak subgrade may need to be reme- diated. Loose base material must be removed prior to placement of the repair material, as long-term performance is dependent on the soundness or stability of the existing base or subgrade material. An alternative to using conventional backfill material is the use of flowable fill material (Smith et al. 2014). Flowable fill materials are easily placed; can be readily removed later, if needed; do not need to be compacted; and have sufficient compressive strength to provide acceptable support to prevent settlement. Flowable fill material is typically composed of portland cement, fly ash, 0.5-inch coarse aggregate, fine aggregate, and water. Means to trim the base (or subgrade) need to be available in the event the base is found to be too high (>1 inch). Different equipment, including a small milling machine or excavator, can be utilized, depending on the geometry of the repair and type of base material. Layer thickness tolerances in the FAA P-501 specification (FAA 2018) should be considered for assessing the need to trim the base prior to placing concrete. In the case of drainable bases, minor damage (e.g., <10% of the area) could be repaired with nondrainable material. If there is significant damage to a drainable base, similar material should be used for the repair. Source: (a) Nichols Consulting Engineers, Chtd., and (b) Applied Pavement Technology, Inc. Figure 22. Concrete demolition and removal: (a) lift-out method (wood shims are installed at slab edges to minimize damage to adjacent slabs) and (b) concrete chainsaw used to minimize saw cut overrun.

50 Rapid Slab Repair and Replacement of Airfield Concrete Pavement Precast Slabs For precast FDR, base preparation is generally dependent on the system or supplier being used. The same care for base repair and trimming necessary for cast-in-place repairs should be applied to the base for precast slabs. The thickness of the precast slab is often designed to be thinner than that of the existing slab (unless the slab will be ground flush following installation), and the gap between the base and the slab bottom must be filled with bedding material. Common bedding materials used to fill the gap between the leveled base and flat slab bottom include a thin layer of sand, flowable fill, grout, or polyurethane foam. Base preparation for precast slab FDR is much more critical than for cast-in-place FDR, because the base will determine the resulting grade of the slab surface. In some cases, extremely weak subgrade may need to be remediated. If the base is found to be too high, part of it can be removed. A small milling machine or excavator can be utilized for this purpose, depending on the geometry of the repair and the type of base material. Load-Transfer Restoration Cast-in-Place Repairs FDR typically requires restoration of load transfer to avoid differential movement that can cause spalling, rocking, pumping, faulting, and breakup of the FDR or adjacent slabs. Following are some key suggestions: • Use smooth dowel bars along all edges with existing pavement (full-slab replacement). • An exception to the previous suggestion is given in FAA AC 150/5380-6C (2014b) and in the U.S. DOD O&M Manual (2018). These guidelines allow use of tie bars at appropriate locations, such as nonworking joints (e.g., an inner panel joint for a partial-slab replacement) and crack locations. On aprons, dowels are often used at all joints because aircraft loadings may occur in multiple directions. • Dowels and tie bars are installed into the existing pavement as follows: – Verify slab faces are vertical and in sound condition. – Drill holes (avoiding existing embedded steel) with gang-mounted pneumatic drills, maintaining proper horizontal and vertical alignment. – Clean out drilled holes and ensure proper anchoring of the dowels (Figure 23) or tie bars. Either cement grout or two-component epoxy material can be used to anchor the dowels, and it is important that the material be effectively distributed around the circumference of the dowel. Source: Smith et al. (2014). Figure 23. Schematic of properly anchored dowel.

Full-Depth Repair 51   Figure 24 provides an example of load-transfer restoration using smooth, anchored dowel bars and tie bars. Precast Slabs Load-transfer restoration for precast FDR is based on the system being used. Load-transfer design and installation needs to be coordinated with the supplier or producer but will typically involve either cutting slots or drilling and anchoring dowels in the adjacent pavement for the load-transfer system. Placement and Finishing Cast-in-Place Repairs Preparation of conventional concrete for use in FDR is similar to that used for conventional paving operations. However, if VHES or HES concrete is used, the batching and placement time must be significantly shorter and may require the use of automated volumetric mixers that batch and mix the concrete on-site. Proprietary materials should be produced and placed according to the manufacturer’s recommendations. For VHES and HES concrete, the producer’s and contractor’s experience with the material is critical to successful placement. Placement of FDR materials often uses the edges of the existing pavement as forms, and the concrete is placed, consolidated, and finished by hand. The workability of the mixture needs to be appropriate for manual operations (i.e., more workable than a machine-placed mixture). The contractor should ensure the concrete is well-consolidated around the edges, load-transfer devices, and light cans by using spud vibrators without overvibration or overfinishing (Smith et al. 2014). Source: (a) Nichols Consulting Engineers, Chtd., and (b) C&S Engineers, Inc. Figure 24. Load-transfer restoration: (a) full-slab replacement using smooth, anchored dowel bars at all joint faces and (b) partial-slab replacement using smooth dowel bars at existing joints and tie bars at the inner panel joint interface.

52 Rapid Slab Repair and Replacement of Airfield Concrete Pavement It is important to finish concrete to a smooth, textured surface free of unevenness from the paving process. A general guide for finishing includes the following guidance (U.S. DOD 2018): • Use a straight edge to strike off repairs less than 10 feet (perpendicular to the pavement center- line). Use a vibratory screed to strike off repairs longer than 10 feet (longitudinal direction). • To avoid surface scaling and other durability problems, do not overfinish the concrete. • Match the surface texture with that of the adjacent slabs. Figure 25 shows a schematic of typical finishing techniques for repairs shorter and longer than 10 feet. Precast Slabs Precast slabs for airport applications are often fabricated at an on-site location in advance of construction to ensure that an adequate inventory of slabs exists before work proceeds. Full three-dimensional surveys are often obtained to accurately establish panel dimensions and surface geometry/warping before fabrication begins. Placement and finishing are performed in casting beds, usually with conventional placement techniques. Fabrication of the precast slabs needs to consider reinforcing steel; load-transfer devices; lifting lugs; embedded jacks; grout injection ports; grout distribution channels (if any); slot formers; and in-pavement light- ing, if included. The contractor will need to ensure that an adequate number of precast slabs is backlogged before construction begins. Repair area dimensions must be accurately addressed during the fabrication of precast slabs. Curing As with PDR, curing is important to FDR to ensure concrete strength gain. Following are key suggestions for proper curing practices (Smith et al. 2014, U.S. DOD 2018): • The approved curing procedure should be started as soon as the bleed water has dissipated from the surface of the concrete. • Curing methods to retain water include impervious paper, pigmented curing membrane, wet burlap, or polyethylene sheeting. The use of a white-pigmented curing compound (ASTM C309 Type 2) is most common, with typical application rates between 100 and Source: Nichols Consulting Engineers, Chtd. Figure 25. Finishing schematics for patches less than and greater than 10 feet long.

Full-Depth Repair 53   200 square feet per gallon to retain moisture. PAMS curing compounds (ASTM C309 Type 2, Class B) are typically more expensive than wax- and water-based compounds but are more effective at retaining water and should be considered. • For concrete with an early opening to traffic and concrete placed under low temperatures, insulation blankets can be used to keep the internal temperature of the concrete high. This accelerates the rate of hydration, which results in a rapid strength gain. For conventional concrete, insulation blankets can be used when the ambient temperature is low (<48°F), the winds are high, or both (American Concrete Institute 2016). Follow the manufacturer’s recommendations for proprietary products if curing methods differ from traditional methods. After curing, joints are prepared and sealed. Joint sealing operations can typically be performed during a subsequent closure following the sealant manufacturer’s recommendations if minimum curing requirements exist. Conventional curing methods are generally used for precast FDR at the place of fabrication. In some cases, external heat is applied to accelerate strength gain. Pavement Grinding and Grooving The finished concrete surface should be level with the surface profile of adjacent slabs. However, these repairs may result in increased roughness, and diamond grinding is often performed to restore rideability and create a smooth pavement (Smith et al. 2014). The FAA’s P-501 cement concrete pavement specification indicates that variances greater than 0.25 inch in 12 feet should be diamond ground, as this difference may allow water to pond and thereby increase the risk of aircraft hydroplaning (FAA 2018). For existing grooved surfaces (primarily runways), grooving should be reestablished in accordance with FAA AC 150/5320-12C, Measurement, Construction, and Maintenance of Skid- Resistant Airport Pavement Surfaces (FAA 1997). Care should be taken to match adjacent grooves and to not extend new grooving into adjacent slabs. Grooving can typically be performed during a separate closure after completion of the primary FDR work. Proper cleanup after grooving is an important step before reopening to traffic. Joint Resealing and Marking Following grooving, joints should be sawed to the proper width and cleaned, and a backer rod should be inserted to the correct depth to create the sealant reservoir. Joint sealant is placed in accordance with specifications and the manufacturer’s recommendations. FDR of large pavement areas can create the need to replace pavement markings and/or provide temporary pavement markings. FAA AC 150/5370-10H (FAA 2018) provides compre- hensive guidance on temporary and permanent pavement markings. Opening to Traffic Cast-in-Place Repairs There are two methods for determining when FDRs can be opened to traffic: • Specified minimum strength (typically 550 psi flexural strength for aircraft loading) or • Specified minimum time after completion of the repair (varies depending on material type). The logistics of measuring strength is difficult, especially for VHES materials. An alterna- tive method of assessing strength gain, such as ASTM C1074, Standard Practice for Estimating Strength by the Maturity Method, could be considered.

54 Rapid Slab Repair and Replacement of Airfield Concrete Pavement FAA AC 150/5370-16 (FAA 2007) states that a thorough inspection [by the (airport’s) project manager] should be done prior to reopening pavement to aircraft operations. The project manager must ensure all items meet the following requirements: • Construction materials have been secured. • Concrete has met the required opening strength. • All pavement surfaces have been cleaned and construction debris removed. • All surfaces have been marked for safe aircraft operation. Good communications must be established by airport operations to maintain good coordi- nation for closures and reopening. Also refer to FAA AC 150/5370-2G, Operational Safety on Airports During Construction (FAA 2017). Figure 26 shows examples of completed cast-in-place FDRs at airfield facilities. Source: (a–c) Nichols Consulting Engineers, Chtd., and (d) Rummel Construction. Figure 26. Examples of completed cast-in-place full-depth repairs: (a) full slabs, (b) partial slabs, (c) full slabs, and (d) replacement of in-pavement light.

Full-Depth Repair 55   Precast Slabs Precast FDRs do not have the concern of concrete curing. While the grout for the load-transfer system needs to achieve the required strength, it is typically a VHES material. Otherwise, the general cleanup items are as noted above. Full-Depth Repair Assessment Tool Figure 27 and the associated tables (Tables 17–19) present a framework for assessing some of the key variables considered during planning for and completing FDRs. This tool provides users with practical guidance on feasible techniques for repair of concrete pavement for a variety of facilities and closure times. Figure 27 presents a decision tree for identifying the location of the planned FDR. 1. Select the facility on which the FDR will be completed (i.e., runway, taxiway, or apron). 2. Identify the relative operational priority of the repair area (e.g., runway intersection, runway safety area). 3. Determine how much time is available to do the work. Closure time is divided into three groups: – Less than 8 hours (equivalent to an overnight closure), – 8 to 24 hours (equivalent to a 1-day closure), and – 24 to 60 hours (equivalent to a weekend closure). In many instances, work can be scaled to fit into the available closure time. That is, if a closure of less than 8 hours is the only available option for a given material and preparation method, the number of repairs may be limited in a given closure window to ensure all repair areas have achieved the required strength at the time of opening. Similarly, with longer closure time, a more conventional repair material can be used. Longer closure times also permit a higher production rate, as more time is available to work under a single mobilization. Each set of decisions leads to a table that summarizes the planning and construction considera- tions associated with the resulting box in Figure 27 (see Table 17 for FDR 1, Table 18 for FDR 2, and Table 19 for FDR 3).

Note: See Table 17, Table 18, and Table 19 for FDR 1, FDR 2, and FDR 3, respectively. 1Some taxiway connectors could remain closed to allow for longer curing times as long as work is completed within the allowable closure window. Source: Nichols Consulting Engineers, Chtd. Figure 27. Full-depth repair decision framework.

Full-Depth Repair 57   FDR 1 (<8 hours) Preparation Repair types and location • Partial slab: Encompass all deteriorated concrete and underlying layers (deterioration at the bottom of the slab can extend as much as 3 feet from visible surface distress). • Full slab: Extend repair to slab edges or include adjacent slabs if sound concrete is not present. • Consider closure time and placement rate in selecting quantity of repairs and ensure the final repair placed during the closure will be able to achieve the required strength at the time of opening. Material selection • Allowable closure time can dictate whether VHES materials are needed. Use the most conventional material that meets opening-to-traffic requirements. • Consider climatic conditions and temperature during placement and curing. • Precast slabs are a candidate. • Planning and coordination with precast supplier (panel sizes, locations, characteristics) is required. • Preproject fabrication and stockpiling of panels are required. Repair boundaries • Partial-slab repairs allowed. • Minimum width of 3 feet from joint. • Minimum length of 10 feet. • If repair extends over half the length of the panel, full-slab replacement is advised. Demolition/Removal Removal method • Before removal, saw cut boundaries. • Saw cut in advance to allow earlier closure (but no more than 1 or 2 days in advance). • Avoid saw cut overruns. • Consider shims to minimize slab rocking between closures, if needed. • Adjust saw cut pattern for existing cracks. • An additional saw cut at a specified offset (e.g., 4 inches) into the slab is done to avoid damaging adjacent slabs during removal. • Angle interior saw cuts to ease removal of center piece. • Lift-out method usually minimizes risk of damage to adjacent pavement. Sublayer repair • Compact with a plate compactor. • If excess moisture is present, remove or dry it before grading and compaction. • Option to replace some or all disturbed material with rapid strength flowable fill or lean concrete (with bond breaker). • Use geogrid if poor-quality layers are encountered. • Have means to trim stabilized materials, if needed. Load-transfer restoration and reinforcement • Dowel bar or tie bar size and spacing vary. • Dowel bars or tie bars along joints and edges tying into the existing pavement (except when an isolation joint is warranted). • Maintain proper horizontal and vertical alignment of dowels. • Ensure proper anchoring of dowels. • Provide reinforcement for odd-shaped slabs. Mixing and placement • • • • May require the use of automated volumetric mixers that batch and mix concrete on-site. If conventional concrete mixes are being used, follow standard mixing and placement practice, including consolidation with internal spud vibration. Make sure the concrete is well-consolidated around the edges without over-finishing. Follow the manufacturer’s recommendations for proprietary materials. Material Mixing and Placement Table 17. Summary of FDR 1 decisions. (continued on next page)

58 Rapid Slab Repair and Replacement of Airfield Concrete Pavement FDR 1 (<8 hours) Curing • • • Joint sealing • • Opening to Traffic Compressive strength • Flexural strength • Other • • Start curing as soon as the bleed water has dissipated from the surface of the concrete. Use the approved curing method, such as a white-pigmented curing compound. When warranted, use insulation blankets to maintain strength gain and protect concrete during cold weather. Saw the transverse and longitudinal joint sealant reservoirs of the repair area (do not form reservoirs with insert). Seal transverse and longitudinal joints around the perimeter of the patched area. Note: Joint sealing performed during separate closure. 3,500 psi for aircraft loading. 550 psi for aircraft loading. Maturity testing as alternative for opening criteria. Consider whether direct traffic loadings will be likely (i.e., only require strength for an emergency loading as opposed to continuous traffic). Finishing • • • • Use a vibratory screed if replacement slab is more than 10 feet long. Use a straight edge if replacement slab is less than 10 feet long. Do not over-finish concrete. Match texture with adjacent slabs. Table 17. (Continued). FDR 2 (8–24 hours) Preparation Repair types and location • Partial slab: Encompass all deteriorated concrete and underlying layers (deterioration at the bottom of the slab can extend as much as 3 feet from visible surface distress). • Full slab: extend repair to slab edges or include adjacent slabs if sound concrete is not present. • Consider closure time and placement rate in selecting quantity of repairs and ensure that final repair placed during the closure will be able to achieve the required strength at the time of opening. Material selection • Allowable closure time, high-early or moderate-early materials may be needed. Use the most conventional material that meets opening-to-traffic requirements; accelerated conventional concrete is most widely used material. • Consider climatic conditions and temperature during placement and curing. Repair boundaries • Partial-slab repairs allowed. • Minimum width of 3 feet from joint. • Minimum length of 10 feet. • If repair extends over half length of the panel, full-slab replacement is advised. Table 18. Summary of FDR 2 decisions.

Full-Depth Repair 59   FDR 2 (8–24 hours) Curing • Start curing as soon as the bleed water has dissipated from the surface of the concrete. • Use the approved curing method, such as a white-pigmented curing compound. • When warranted, use insulation blankets to maintain strength gain and protect concrete during cold weather. Joint sealing • Saw the transverse and longitudinal joint sealant reservoirs of the repair area (do not form reservoirs with insert). • Seal transverse and longitudinal joints around the perimeter of the patched area. Note: Joint sealing performed during separate closure. Opening to Traffic Compressive strength • 3,500 psi for aircraft loading. Flexural strength • 550 psi for aircraft loading. Other • Maturity testing as alternative for opening criteria. • Consider whether direct traffic loadings will be likely (i.e., only require strength for an emergency loading opposed to continuous traffic). Demolition/Removal Removal method • Before removal, saw cut boundaries. • Saw cut in advance to allow earlier closure (but no more than 1 or 2 days in advance). • Avoid saw cut overruns. • Consider shims to minimize slab rocking between closures, if needed. • Adjust saw cut pattern for existing cracks. • Consider an additional saw cut at a specified offset (e.g., 4 inches) into the slab to avoid damaging adjacent slabs during removal. • Angle interior saw cuts to ease center piece removal. • Lift-out method usually minimizes risk of damage to adjacent pavement. Sublayer repair • Compact with a plate compactor. • If excess moisture is present, remove or dry it before grading and compaction. • Option to replace some or all disturbed material with rapid strength flowable fill or lean concrete (with bond breaker). • Use geogrid if poor-quality layers are encountered. • Have means to trim stabilized materials, if needed. Load-transfer restoration and reinforcement • Dowel bar or tie bar size and spacing varies. • Dowel bars or tie bars along joints and edges tying into the existing pavement (except when an isolation joint is warranted). • Maintain proper horizontal and vertical alignment of dowels. • Ensure proper anchoring of dowels. • Provide reinforcement for odd-shaped slabs. Material Mixing and Placement Mix and placement • Follow standard mixing and placement practice for conventional concrete mixes, including consolidation with internal spud vibration. • Make sure the concrete is well-consolidated around the edges without over-finishing. • Follow the manufacturer’s recommendations for proprietary materials. Finishing • Use a vibratory screed if replacement slab is more than 10 feet long. • Use a straight edge if replacement slab is less than 10 feet long. • Do not over-finish concrete. • Match texture with adjacent slabs. Table 18. (Continued).

60 Rapid Slab Repair and Replacement of Airfield Concrete Pavement FDR 3 (24–60 hours) Preparation Repair types and location • Partial slab: encompass all deteriorated concrete and underlying layers (deterioration at the bottom of the slab can extend as much as 3 feet from visible surface distress). • Full slab: extend repair to slab edges or include adjacent slabs if sound concrete is not present. • Consider closure time and placement rate in selecting quantity of repairs and ensure that final repair placed during the closure will be able to achieve the required strength at the time of opening. Material selection • For allowable closure time, slightly accelerated conventional concrete mix will likely meet the opening requirements. • Consider climatic conditions and temperature during placement and curing. Repair boundaries • Partial-slab repairs allowed. • Minimum width of 3 feet from joint. • Minimum length of 10 feet. • If repair extends over half length of the panel, full-slab replacement is advised. Demolition/Removal Removal method • Before removal, saw cut boundaries. • Saw cut in advance to allow earlier closure (but no more than 1 or 2 days in advance). • Avoid saw cut overruns. • Consider shims to minimize slab rocking between closures, if needed. • Adjust saw cut pattern for existing cracks. • Consider an additional saw cut at a specified offset (e.g., 4 inches) into the slab to avoid damaging adjacent slabs during removal. • Angle interior saw cuts to ease removal of center piece. • Lift-out method usually minimizes risk of damage to adjacent pavement. Sublayer repair • Compact with a plate compactor. • If excess moisture is present, remove or dry it before grading and compaction. • Option to replace some or all disturbed material with rapid strength flowable fill or lean concrete (with bond breaker). • Use geogrid if poor-quality layers are encountered. • Have means to trim stabilized materials, if needed. Load-transfer restoration and reinforcement • Dowel bar or tie bar size and spacing vary. • Dowel bars or tie bars along joints and edges tying into the existing pavement (except when an isolation joint is warranted). • Maintain proper horizontal and vertical alignment of dowels. • Ensure proper anchoring of dowels. • Provide reinforcement for odd-shaped slabs. Material Mixing and Placement Mix and placement • If conventional concrete mixes are being used, follow standard mixing and placement practice, including consolidation. • Make sure the concrete is well consolidated around the edges without over-finishing. • Follow the manufacturer’s recommendations for proprietary materials. Finishing • Use a vibratory screed if replacement slab is more than 10 feet long. • Use a straight edge if replacement slab is less than 10 feet long. • Do not over-finish concrete. • Match texture with adjacent slabs. Table 19. Summary of FDR 3 decisions.

Full-Depth Repair 61   FDR 3 (24–60 hours) Curing • Start curing as soon as the bleed water has dissipated from the surface of the concrete. • Use the approved curing method, such as a white-pigmented curing compound. • When warranted, use insulation blankets to maintain strength gain and protect concrete during cold weather. Joint sealing • Saw the transverse and longitudinal joint sealant reservoirs of the repair area (do not form reservoirs with insert). • Seal transverse and longitudinal joints around the perimeter of the patched area. Note: Joint sealing performed during separate closure. Opening to Traffic Compressive strength • 3,500 psi for aircraft loading. Flexural strength • 550 psi for aircraft loading. Other • Maturity monitoring as alternative for opening criteria. Table 19. (Continued).