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The structure of a flexible and rigid road pavement comprises several courses designed to transfer the trafficking loads to the natural subgrade at the bottom. Typically, the upper courses (i.e., surface and binder layers) are bound using bitumen or cement, while the other courses (i.e., base and subbase layers) are generally solely comprised of unbound granular materials. Unbound granular materials for road bases may exhibit insufficient properties (e.g. low bearing capacity, susceptibility for frost action), which then results in substantial pavement distress and reduction of the pavement life.

Due to continual increases in traffic loads and changes in environmental conditions, innovative technologies are sought to improve the mechanical properties of unbound course materials to provide enduring solutions. Some addition of a stabilizing agent such as cement, bitumen, lime or some non-traditional agents can improve the properties of unbound course materials. Among these different stabilized materials, cement-bound materials develop a quite high stiffness and strength, and exhibit good performance for pavement serviceability and high durability. Stabilized bases can provide cost-effective solutions to many common designs and construction situations.

What is Cement Treated Base?

Cement Treated Base (CTB) is a traditional method applied in road bases materials to improve its engineering properties. Cement-Treated Base is a type of Soil-Cement describing an intimate mixture of native soils and/or manufactured aggregates with measured amounts of portland or blended cement and water that hardens after compaction and curing to form a strong, durable, frost-resistant paving material. Other descriptions such as soil-cement base, cement-treated aggregate base, cement-stabilized roadbed, and cement-stabilized base are sometimes used. CTB is widely used as a pavement base for highways, roads, streets, parking areas, airports, and materials handling and storage areas. It is suitable as a base for asphalt pavements or subbase for concrete pavements.

The soil and aggregate materials for use in CTB may consist of (1) any combination of gravel, stone, sand, silt, and clay; (2) miscellaneous material such as caliche, scoria, slag, sandshell, cinders, and ash; (3) waste material from aggregate production plants; (4) high-quality crushed stone and gravel base course aggregates; or (5) old flexible pavements, including the pulverized bituminous surface and stone or gravel base course.

In CTB construction the objective is to obtain a thorough mixture of an aggregate/granular material with the correct quantity of portland or blended cement and enough water to permit maximum compaction. The completed CTB must be adequately cured to both let the cement hydrate and to harden the cement-aggregate mixture.

Advantages of Cement Treated Base

CTB possesses its own unique structural characteristics. CTB pavements are designed for both economy and long service life. CTB provides a stiffer and stronger base than an unbound granular base. A stiffer base reduces deflections due to traffic loads, which results in lower strains in the asphalt surface. This delays the onset of surface distress, such as fatigue cracking, and extends pavement life. CTB materials provide additional strength and support without increasing the total thickness of the pavement layers. Depending on project needs, CTB increases the construction speed, enhances the structural capacity of the pavement, or in some cases reduce the overall project time.

Base thickness of CTB is reduced because of high bearing strength compared to unbound granular base thicknesses. The thickness of CTB is less than that required for granular bases carrying the same traffic because CTB is a cemented, rigid material that distributes the load over a large area. The strong uniform support provided by CTB results in reduced stresses applied to the subgrade. A thinner cement stabilized section can reduce subgrade stresses more than a thicker layer of untreated aggregate base. Subgrade failures, potholes, and road roughness are thus reduced.

CTB’s slab-like characteristics and beam strength are unmatched by granular bases that can fail when interlock is lost. This happens when wet subgrade soil is forced up into the base by traffic loads. Hard, rigid CTB is practically impervious. It resists cyclic freezing, rain, and spring-weather damage. CTB continues to gain strength with age even under traffic. This reserve strength accounts in part for CTB's excellent performance.

Rutting is reduced in a CTB pavement. Loads from channelized traffic will displace unbound granular material beneath flexible surface pavements. Moisture intrusion can destroy unstabilized pavement bases, but not when cement is used to bind the base. CTB pavements form a moisture-resistant base that keeps water out and maintains higher levels of strength, even when saturated, thus reducing the potential for pumping of subgrade soils.

CTB when used as a subbase layer under concrete pavements, it prevents mud-pumping of fine-grained subgrade soils under wet conditions and heavy truck traffic. In addition to preventing mud-pumping, CTB provides a uniform, strong support for the pavement, provides a firm, stable working platform for construction equipment, prevents infiltration of subgrade into the subbase, prevents subbase consolidation under traffic, and provides increased load transfer at pavement joints.

CTB is versatile as it can be either mixed in place using transverse-shaft pulvermixers or traveling mixing machines and compacted after blending or mixed in a central plant, either be continuous-flow or batch-type pugmill mixers, where it is hauled to the placement area and spread on a prepared subgrade or subbase and compacted.

Mixing Plant for CTB Material

The continuous-flow-type pugmill plant is the most common for producing CTB material. The plant setup is typified by a hopper or bulkhead feeder system containing the soil/aggregate, a screening device to remove oversized material from the raw soil/aggregate feed prior to mixing, a cement silo, surge hopper and feeder, main feeder belt, and revolving-blade pugmill mixer. The plant is equipped with metering and feeding devices that will add the soil/aggregate, cement, and water into the mixer in the specified quantities.

Cement is usually metered onto the soil/aggregate main feeder belt just prior to entering the pugmill. Water is metered and added by means of spray bars mounted above the pugmill. The mixed material is discharged into a holding hopper and then into haul trucks. Soil/aggregate and cement are mixed sufficiently to prevent cement balls from forming when water is added. The mixing time shall be that which is required to secure an intimate, uniform mixture of the soil/aggregate, cement, and water.