Precast concrete pavement03/03/2015
1. Why Precast?
What do we mean by faster construction? We’re not necessarily talking about how fast the pavement can be constructed, but rather how fast it can be opened to traffic. Conventional cast-in-place pavement requires several days of additional curing time after the concrete is placed before it is strong enough to withstand traffic loading. While “fast-setting” concrete mixtures have been developed for this purpose, these can be cost-prohibitive for large-scale pavement construction.
Reduced User Delay Costs
What are user delay costs? These are costs to the drivers of the roadway that are directly attributable to congestion caused by construction activities. Increased fuel consumption, lost work time, increased vehicle wear and tear, and increased air pollution are just a few of these costs. The savings in user delay costs realized through limiting construction to only off-peak travel times (at night or over a weekend) can be substantial. This is where the primary economic benefit of precast pavement will be realized.
Improved Durability and Performance
Precast concrete has a proven track record as a durable high-performance product for bridge and commercial building construction. This is the result of a high degree of quality control that can be achieved at a precast fabrication plant. High strength, low permeability concrete mixtures with a low water-cement ratio and uniform aggregate gradation are used routinely by precast fabrication plants. At most plants concrete batching and quality control is done on-site and the concrete is transported only a short distance from the batch plant to the forms, minimizing changes in concrete properties between the mixing and placing operations. What’s more, precast fabrication plants offer tremendous flexibility over the curing operation. Precast concrete elements can be fabricated indoors, they can be wet-mat cured, steam cured, and curing can be maintained as long as necessary after casting. Problems that can plague cast-in-place pavement construction such as surface strength loss, “built-in” curling, and inadequate air entrainment, can all be eliminated with precast concrete.
2. Why Prestressed Precast Pavement?
Prestressing has a proven track record for enhancing the performance and durability of concrete structures. And though it has seen very limited use in pavements, there are clearly benefits of prestressed concrete pavement, such as reduced cracking, reduced slab thickness, and bridging capability.
While conventional pavements are “designed” to crack at specific locations (at sawcut joints for JCP) or at regular intervals (CRCP), in general cracking is not desirable. Cracks can spall, they can permit water to penetrate the underlying base, they can fault, and they can eventually lead to severe pavement failures such as punchouts. Prestressing helps to minimize or even eliminate cracking. By putting a pavement in compression there is less likelihood of cracking due to tensile stresses. What’s more, the so-called “elasto-plastic” behavior of prestressed concrete will help keep any cracks that do form tightly closed.
Reduced Slab Thickness
While the underlying pavement structure is also a factor, the primary controlling factor in pavement thickness design is the magnitude and number of wheel load repetitions on the pavement over its expected design life. For a given pavement support structure and a given wheel load, tensile stresses in a thinner pavement will be higher than those in a thicker pavement. These higher stresses wear out or fatigue a concrete pavement faster. Prestressing can be used to reduce the tensile stresses in a thinner pavement slab to those of a much thicker pavement slab, increasing the design life of the pavement.
Why is this important? First is the savings in concrete material. Constructing an 8-inch thick pavement slab instead of a 12-inch-thick pavement slab will save more than 780 cubic yards of concrete per lane-mile. Secondly, for removal and replacement it is generally necessary to match the existing slab thickness. Most existing pavements that are in need of replacement are on the order of 8-10 inches thick. Prestressing permits in-kind replacement of the existing pavement with a pavement slab that will have a design life of a much thicker slab. Finally, slab thickness can often times be governed by overhead clearance constraints. When replacing a pavement under a bridge overpass, for example, it is often not possible to construct a thicker pavement than what was in place already without having to excavate base material.
Prestressing gives the pavement a certain “bridging” capability that permits the pavement slab to span small voids and “soft” base materials beneath the pavement. This is critical for pavement removal and replacement operations that are limited to short (overnight) construction windows when it is often not possible to recondition or replace the underlying base material.
Written by Lamcauham