Design and Control of Concrete Mixtures CHAPTER 2 Paving Design and Control of Concrete Mixtures CHAPTER 2 Design and Control of Concrete Mixtures, 16th edition, Chapter 2 - Paving

Overview History Paving Types Jointed Plain Continuously Reinforced Concrete Overlays Pervious Precast Roller-Compacted Concrete Cement Modified Soil Cement Treated Base Full-Depth Reclamation This module covers the history of concrete pavement and the various paving types that have been developed. Both traditional and innovative approaches will be discussed.

History Concrete pavements have been a mainstay of the U.S. infrastructure for over 100 years. Early concrete pavements improved ride quality.

Pavement Types Jointed plain concrete pavements Continuously reinforced concrete pavements Concrete overlays Pervious concrete Precast pavements Roller-compacted concrete Traditionally, concrete pavements have been built using conventional type mixtures either in fixed forms or slipformed. However, a number of different approaches are finding acceptance among agencies and contractors. A description of each pavement type follows:

Jointed Plain Concrete Pavements Jointed plain concrete pavements (JPCP) have regularly spaced transverse sawn contraction joints that allow the concrete to move horizontally with changes in temperature and moisture. Vertical movement between the transverse joints is controlled by smooth steel dowels, which assist with transferring load between slabs, reducing slab thickness. The fundamental aim is to force cracks to form in controlled locations rather than in random locations.

Desired Properties of JPCP and Approaches Desired properties and how they may be specified are shown in the table.

Construction of JPCP Central batch plant showing stationary mixer loading into dump trucks. Concrete is typically transported to the point of placement using this method. For smaller projects, it may be delivered in truck mixers ensuring that concrete is uniform from batch to batch while taking allowance for haul times.

Construction of JPCP Fixed-form paving is generally used in small or irregular sections where slipform paving is not practical.

Construction of JPCP Slipform paving operation. This is generally for placements that require high production rates, such as mainline paving pictured above.

Construction of JPCP Longitudinal tining the surface of concrete pavement has been shown to reduce noise. The purpose of texturing is to increase friction in wet weather.

Construction of JPCP Dowels can be placed in prefabricated dowel baskets and secured in place prior to pacing, or they can be inserted into the fresh concrete during placement using a dowel-bar inserter (pictured above).

Continuously Reinforced Concrete Pavements Continuously reinforced concrete pavements (CRCP) contain continuous longitudinal and transverse reinforcement through the entire pavement. The reinforcement is designed to control transverse crack widths and hold cracks tightly together. It is not designed to help carry traffic loads. As a result, CRCP systems do not include transverse contraction joints, but do include transverse and longitudinal construction joints.

Materials and Mixture of CRCP Mixtures for CRCP systems are the same as those used for JPCP. Vibrators on slipform pavers will have to run above the top steel yet still consolidate the full thickness of the slab (pictured above).

Construction of CRCP CRCP design involves selecting longitudinal and transverse reinforcement in order to provide desired performance for the design life. Factors affecting steel details include: crack spacing, crack width, steel stress, and bond development length.

Concrete Overlays Concrete overlays are a means of extending or improving the life of a pavement, while making most use of the system already in place. Concrete overlays may be classified as bonded and unbonded.

Structural Design Design process (pictured above). The choice between using a bonded or unbonded overlay is strongly dependent on the condition of the existing pavement.

Bonded Overlays A bonded concrete overlay makes the overlay and existing pavement one monolithic structure.

Pervious Concrete Pavements Pervious concrete allows rainwater to pass through the surface and percolate into the underlying layer.

Pervious Concrete Pavements Pervious concrete pavements reduce stormwater runoff, flash flooding, standing water, and may eliminate the need for on-site holding ponds or buried stormwater retention structures.

Material Properties of Pervious Concrete Typical material properties for pervious concrete are shown in the table.

Construction Pervious concrete is usually placed and then struck off with a vibratory screed.

Construction Compaction and finishing is generally accomplished by a weighted roller-screed that spins as it is pulled across the fresh concrete.

Construction Joints should be made soon after consolidation using a rolling joint tool.

Precast Concrete Pavements Precast pavements are constructed using prefabricated concrete slabs installed over a prepared subbase or existing pavement.

Precast Concrete Pavements Examples of precast applications Precast panels can be used for isolated full-depth repairs, such as joint replacements, for single or multiple consecutive slab replacements, or for total reconstruction of an entire section. (Tayabji 2013)

Precast Concrete Pavements Placing a precast panel.

Roller-Compacted Concrete Roller compacted concrete is placed with roller pavers for compaction. The largest difference between RCC mixtures and conventional concrete mixtures is that RCC has less paste content.

Strength development of RCC (Harrington 2010)

Roller-Compacted Concrete Typical mixture proportions. Compared to conventional concrete, RCC pavements generally are not air entrained, have a lower water and paste content, and requires a larger fine aggregate content in order to produce a combined aggregate that is well graded and stable under the action of a vibratory roller. (Harrington 2010)

Recommended RCC gradation envelopes Recommended RCC gradation envelopes. Aggregate selection can have a strong influence on performance. (Harrington 2010)

Cement Modified Soil Subgrade A cement-modified soil (CMS) is a soil material that has been treated with a relatively small proportion of portland cement in an amount that is less than that required to produce hardened soil-cement. Typical cement modified soil cross section.

Cement Modified Soil Subgrade The improvements in engineering properties of a soil due to the addition of small quantities of cement can include: Reduction in moisture content of in-situ soil Reduction in plasticity characteristics as measured by Plasticity Index (PI) Reduction in the amount of silt and clay size particles Increase in the California Bearing Ratio (CBR) Increase in shearing strength Decrease in volume-change properties

Construction A sheepsfoot roller is used to compact CMS.

Cement Treated Base Typical CTB section. If the materials available for a base are insufficient to support the loads from the pavement, then they can be enhanced by the addition of a cementitious material.

Cement Treated Base Constituents Native Soils Gravels or Aggregates Portland Cement Water A cement treated base typically includes the constituents listed. The objective is to obtain maximum compaction by mixing an aggregate/granular material with the correct quantity of portland cement and water.

Cement Treated Base Benefits A stiffer base Reduction in subgrade failures, pumping, rutting, joint faulting, and road roughness Base thickness is reduced Marginal aggregates can be used

Cement Treated Base Engineering Properties of Cement Treated Base are shown in the table.

CTB Construction Cement is placed dry onto the surface of the in-place aggregate using mechanical spreader equipment

Full-Depth Reclamation Re-compaction Hot mix asphalt Granular base Portland cement Full-depth reclamation may be considered when the existing pavement or base and/or subgrade is damaged and inadequate for the current or future traffic.

Construction Full-depth reclamation requires a reclaimer mixer, grader, cement spreader, water truck, and roller. A reclaimer machine typically makes an initial pass over the existing flexible pavement, pulverizing the HMA surface and blending it with the base and/or subgrade material. Water may be added during this mixing stage to bring the material up to optimum moisture content. The material is then graded accordingly. Next, cement is spread either dry or in slurry form in a controlled manner onto the surface. The reclaimer then mixes the cement into the pulverized material until the material is thorougly mixed. This is followed by compaction, final grading, curing, and surfacing. Smooth-wheeled rollers are used for compaction. The FDR should be cured for from three to seven days.

Summary Jointed Plain Concrete Pavements Precast Concrete Pavements Continuously Reinforced Concrete Pavements Roller-Compacted Concrete Concrete Overlays Subgrades and Bases Bonded Overlays Cement Modified Soil Subgrade Unbonded Overlays Pervious Concrete Pavements Cement Treated Base Full Depth Reclamation

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