ACI 325 13R:2006pdf free download.Concrete Overlays for Pavement Rehabilitation.
break and seat—technique similar to crack and seat. except conducted on jointed reinforced concrete pavements and using higher impact energy; uses more impact energy to rupture the steel or break its bond with the concrete to ensure independent movement, and seating with a heavy roller.
crack and seat—technique involving fracturing the existing jointed plain concrete pavements into pieces I to 4 ft (0.3 to 1.2 m) on a side by inducing full-depth cracks using a modified pile driver, guillotine hammer, whip hammer, or other equipment, and seating with a heavy roller.
curling—concrete distortion, usually in a slab, resulting from differential temperatures.
drainage, subsurface—inclusion of specific drainage elements in a pavement structure intended to remove excess surface infiltration water from a pavement.
equivalent single-axle loads (ESALs)—summation of 18 kip (80 kN) single-axle load applications used to combine mixed traffic to design traffic during the analysis period.
falling weight deflectometer—device in which electronic sensors measure the deflection of the pavement as a result of an impact load of known magnitude; results can be used to estimate the elastic moduli of subgrade and pavement layers and the load transfer across joints and cracks.
faulting—difference of elevation across a joint.
fracturing, slab—tcchnique in which an existing portland-cement concrete pavement is cracked or broken into smaller pieces to reduce the likelihood of reflection cracking.
hot-mix asphalt (HMA)—an asphalt cement-aggregate mixture that is mixed, spread. and compacted at an elevated temperature: also commonly referred to as “asphalt concrete” or “asphalt.”
joint orientation—alignment of transverse joints in a concrete pavement with respect to the centerline of the pavement.
layer, separator—layer of hot-mix asphalt. bituminous material, or other stress-relieving material used at the interface between an unbonded concrete overlay and the existing concrete pavement to ensure independent behavior.
leveling course—thin layer of hot-mix asphalt or other bituminous material to produce a uniform surface for paving.
load transfer—means through which wheel loads are transferred or transmitted across a joint from one slab to the next.
life-cycle cost analysis (LCCA)—economic assessment of competing pavement design alternatives in which all significant costs over the life of each alternative are considered. LCCA is used to evaluate a design solution. Life-cycle costs may be measured for different designs to determine which design will meet the economic and performance goals.
mill—process using drum-mounted carbide steel cutting bits to remove material from a pavement and provide texture to promote bonding with an overlay.
overlay. bonded concrete—hydraulic cement concrete overlay bonded directly to an existing concrete pavement to form a monolithic structure.
2.2.1.2 Jointed reinforced concrete pa’einent—JRCP is a hydraulic cement concrete pavement system containing dowels, characterized by long joint spacings and distributed reinforcing steel in the slab to control crack widths. Slab lengths are generally more than 20 ft (6 m), and may be as much as 60 ft (18 in). Current pavement practice is away from JRCP designs. and construction is rare because the longer joint spacing results in more joint movement. When midsiab cracks occur, the light reinforcing may not be enough to hold the cracks tightly. If JRCP is used, the recommended maximum joint spacing is 30 ft (9 m) (FHWA 1990). Deformed bars or deformed welded wire reinforcement are recommended at a minimum steel content of 0.19% (Darter cc at. 1997).
2.2.1.3 (‘ontinuouslv reinforced concrete pavement— CRCP, also known as continuous concrete pavement, is a pavement with uninterrupted longitudinal steel reinforcement and no intermediate transverse expansion or contraction joints. Reinforcement design for CRCP overlays is similar to that for new design. The recommended minimum steel content is 0.60%, and the use of deformed bars is strongly recommended (Darter et al. 1997). The depth of reinforcing steel has a significant effect on crack opening, and steel placement closer to the top surface may provide tighter cracks and better long-term performance (Dhamrait and Taylor 1979; Roman and Darter 1988). A minimum concrete cover of 2.5 in. (65 mm) is recommended for protection of the reinforcing steel against corrosion.
2.2.2 concrete overlays of existing concrete pavements— Concrete overlays of existing concrete pavements arc generally classified according to the proposed bonding condition between the new overlay and the existing pavement. They may be placed in a bonded, partially bonded, or unbonded condition, the selection of which depends largely upon the condition of the existing pavement and Ofl the future traffic levels. Table 2.2 summarizes some key characteristics of each of these concrete overlay types (Hoerner et al. 2001).
2.2.2.1 Bonded concrete overlays—A bonded concrete overlay consists of a thin layer of concrete (typically 3 to 4 in. 175 to 100 mml thick) that is bonded to the existing pavement (Fig. 2.2). These are used to increase the structural capacity of an existing pavement orto improve its overall ride quality,
2.2.3.2 Ultra -thin and this, whit eiopping—UTW and thin whitetopping overlays are both designed assuming that the overlay bonds to the existing asphalt. Most overlays in this category are UTW, although there have been a few thin whitetopping projects constructed: this is an area of increasing research interest. Thin whitetopping represents an extension of the UTW to thicker overlays for pavements carrying heavier traffic.
UTW is a process in which a thin layer of concrete (between 2 and 4 in. 150 and 100 mmj thick) is placed over a rutted or cracked asphalt pavement (ACPA 1998). In a UTW project, the existing hot-mix asphalt surface is cold milled to enhance the bond between the concrete overlay and the pavement to create a monolithic structure. Milling also removes surface irregularities and provides a more uniform surface for overlay placement.
UTW was originally intended for parking lots, residential streets, low-volume roads, general aviation airports, and hot-mix asphalt intersections where rutting is a problem but no other significant structural deterioration is present (ACPA 1998). Since the mid-l990s. its use has been extended to highway applications in Alabama. Kansas. Missouri. and other states.
UTW overlays employ short slabs, typically square and with joint spacing between 2 and 6 ft (0.6 and 1.8 m). This is to help reduce bending and thermal curling stresses. Figure 2.5 shows a schematic of a UTW overlay (Grogg et al. 2(X) 1).
The use of UTW grew rapidly during the I 990s. with over
200 projects in 35 states since 1992 (ACPA 2000a). As of
2003. Tennessee had constructed the most (JTW projects.
followed closely by Kentucky and Kansas. All UTW
projects have been jointed, plain concrete overlay designs.
hut some have used fiber-reinforced concrete (FRC).
Thin whitetopping overlays are between 4 and 8 in. (100 and 200 mm) thick concrete slabs with joint spacing between 6 and 12 ft (1.8 and 3.7 m) that are placed on a milled asphalt pavement. As with the UTW design. milling of the asphalt surface is intended to promote bonding between the overlay and the existing pavement. This bonding is accounted for in design. and improves the performance of the overlay.
The maximum coarse aggregate size permitted in concrete mixtures is a function of the pavement thickness or the amount of reinforcing steel (if used) (ACI 211.1: PCA 2002). The largest and most practical maximum coarse aggregate size should be used to minimize paste content, reduce shrinkage, minimize costs, and improve mechanical interlock properties at joints and cracks (Van Dam ci al. 2002). Although maximum coarse aggregate sizes of 0.75 to I in. (19.0 to 25.0 mm) have been common, some agencies are examining the use of larger maximum coarse-aggregate sizes (1.5 w 2 in. 137.5 to 50 mmj) for conventional concrete paving. For thinner overlays (such as bonded concrete or UTW), however, smaller maximum coarse-aggregate sizes are required. For unreinforced pavement structures, the Portland Cement Association (PCA) recommends a maximum aggregate size of one-third of the slab thickness (PCA 2002). For more information on aggregates. refer to ACI 22lR and 221.1k.
ACI 211.1 and PCA (2002) provide guidance on the selection of the appropriate waler-cementitious material ratio (w/cm). A maximum w/cm of 0.45 is common for pavements in a moist environment and subjected to cycles of freezing and thawing (PCA 2002). Lower wiem values are used on thinner concrete overlays (bonded overlays and UTW) to accelerate strength gain and to minimize drying shrinkage (McGhee 1994: ACPA 1998). Low water and paste content, however. are more important than the w/cm in minimizing shrinkage.
Various admixtures (ACI 2 12.3R) are commonly introduced into concrete mixtures:
Air entrainment protects the hardened concrete from freezing-and-thawing deterioration and deicer scaling and also helps increase the workability of fresh concrete. significantly reducing segregation and bleeding (PCA 2002). Typical entrained air contents of concrete pavement are in the range of 4 to 6%:
Accelerators increase the rate of concrete strength development. In pavement, they are commonly used in full-depth repairs or on fast-track paving projects in which early opening times are required. Calcium chloride is commonly used as a set accelerator. Nonchloride accelerators should be used if steel reinforcement and dowels are present: and
Water reducers are added to concrete mixtures to reduce the amount of water required to produce concrete of a given consistency. This allows for a lowering of the w/crn while maintaining a desired slump. and thus has the beneficial effect of increasing strength and reducing permeability (Van Dam et at. 2002).
Supplementary cementitious materials, such as fly ash, slag cement, and silica fume may be used as additions to concrete mixtur. These materials may be placed in addition to the portland cement or as a partial substitution for a percentage of the portland cement. Of these, fly ash is the most commonly used. Fly ash is a by-product of coal-fired power plants, and may be classified as either Class C.
Phase 3: Select preferred solutions—This step considers both monetary and nonmonetary factors to select the alternative deemed most appropriate for design conditions and constraints.
In general. the pavemcnt-rclaed aspects of this process are well defined, and the guidelines provided in numerous references can be used to identify technically feasible and preferable rehabilitation alternatives.
The selection process is much simpler if only concrete overlay alternatives are considered, but there is no practical value in limiting rehabilitation choices in that way. The choice among concrete overlays is often clearly defined by the engineering criteria. Where more than one type of concrete overlay is feasible, the selection may be based on life-cycle costing, because the impact of construction. which affects most of the nonmonetary factors, is similar for all concrete overlays. When other rehabilitation alternatives, such as hot-mix asphalt overlay alternatives, concrete pavement restoration alternatives, and reconstruction, are also considered, user costs and non monetary factors become more relevant. The decision matrix shown in Table 3.1 can be a useful tool for identifying the alternative that best satisfies multiple selection criteria. One limitation of this approach is that it is difficult to rate different alternatives so that the relative merits of each alternative are properly represented in the rating for many of the factors.
Although most state highway agencies regard user cost and nonmonetary factors as very important decision factors in rehabilitation selection, there are no generally accepted means of combining these factors. For the most part, an informal process is used, although systematic procedures for rehabilitation selection are currently being developed under ongoing research projects.
CHAPTER 4—BONDED CONCRETE OVERLAYS 4.1 —Introduction
A bonded overlay increases the overall structural capacity of the pavement, but that structural benefit only occurs when the overlay and the underlying concrete behave monolithically. Thus, effective bond between the concrete overlay and the existing pavement is critical to the performance of these overlays. When a bonded overlay is properly constructed and the application is appropriate, its expected advantages are that it lasts longer than conventional hot-mix asphalt overlays, and it provides a higher level of serviceability over its service life.
Bonded concrete overlays have been used as a pavement rehabilitation technique for almost 90 years (Hutchinson 1982; Delatte and Laird 1999). A number of highway agencies have substantial experience with bonded overlays, both in the U.S. and internationally. Among some of the agencies in the U.S. that have constructed bonded concrete overlays are Texas. Iowa, Pennsylvania. Louisiana. Virginia, Illinois, and California. These experiences cover a wide range of overlay.
7.3.4 Curing—Effective curing of the new UTW overlay promotes continued cement hydration and strength gain by controlling the rate of moisture loss in the concrete slabs. Although curing is important to all concrete pavements, it is even more critical to UTW overlays because their high surface area-to-volume ratio make them more susceptible to rapid moisture loss. Curing is most often accomplished through the application of a curing compound immediately after the final texturing of the concrete surface, as discussed in Section 2.6.5.
7.3.5 Joini sawing and sealing—Timely joint sawing is required to establish the contraction joints in the concrete pavement and prevent random cracking. Because of the great amount of joint sawing required on UTW overlays, joint sawing should commence as soon as the concrete has developed sufficient strength such that the joints can be cut without significant raveling or chipping. This will typically be within approximately 3 to 6 hours after concrete placement. The contractor should ensure that there are sufficient saw-cutting crews available for the work and that all crews are familiar with the prescribed joint-sawing patterns. Because of the need to get on the pavement as soon as possible. the use of lightweight early-entry saws is particularly advantageous for UTW overlay construction.
Criteria for saw-cut depths on UTW overlays have not been established, hut a minimum 1 in. (25 mm) deep cut appears to perform satisfactorily for both transverse and longitudinal joints (ACPA 1998). The joints arc typically sawed to a width of 0.12 in. (3 mm). Generally, the joints in UTW projects arc not scaled, although a few agencies have constructed experimental UTW sections comparing sealed and nonsealed joints.
7.4—Performance
Because this is a new and evolving technology, very little long-term performance data are available for UTW and thin whitetopping overlays. The available data suggest that UTW overlays are a viable pavement rehabilitation alternative for low-volume roadways. Performance of specific projects is discussed in Smith Ct al. (2(X)2).
7.4.1 ACPA UTW performance ei’aluations—In 1995 and
1996. the ACPA conducted detailed condition surveys on nine UTW projects: six in Tennessee and three in Georgia. The purpose of these surveys was to examine the early performance of UTW overlays, and the nine projects were selected primarily because they were some of the older LJTW overlays (Cole 1997).
Based on the results of the condition surveys, which represent approximately 4 to 5 years of performance data, the following conclusions were drawn (Cole 1997):
Nine of the 10 sections are rated in excellent condition.