Mix transport involves all actions and equipment required to convey PCC from a batching facility to a paving site including truck loading, weighing and ticketing, hauling to the paving site, mixing (if required), agitation, placing, truck washing and truck return to the batching facility. The goal of mix transport is to delivery PCC to the paving job site that conforms to the specified mix design. Transport practices can affect such mix characteristics as (1) homogeneity, (2) workability, (3) water content and (4) air content. This section will discuss the types of trucks used for mix transport and the various considerations involved with mix transport.
There are several mix transport considerations or best practices that are essential to maintaining PCC characteristics between the production facility and the paving site. These considerations can generally be placed into four categories:
- Loading at the production facility
- Truck drum/bed and chute cleanliness
- Water management
- Unloading at the paving site
- Operation synchronization
Loading at the Production Facility
Loading at the production facility involves either batching ingredients into the mixing truck (for truck mixed PCC) or loading freshly mixed PCC into a transport truck (for central mixed or shrink mixed PCC). There are two potential issues with this transfer:
- Inadequate or improper material mixing. If the order of material batching is not carefully controlled, it is possible that they will be introduced in such an order that they do not adequately mix. Typically this happens when liquid admixtures are not mixed in with the water and fine aggregate, or when the portland cement and mixing water are introduced simultaneously. Also, certain admixtures should not come into contact with one another until they are in the mixing drum.
- Head packs. A head pack occurs when fine aggregate and portland cement become lodged in the drum entrance and are not mixed with the rest of the PCC. Head packs need to be detected during charging because they can frequently break away during discharge and end up in the finished pavement without being noticed.
- Cementitious balls. These are 60 – 75 mm (2.4 – 3 inch) balls of fine aggregate, portland cement and perhaps some coarse aggregate that do not get thoroughly mixed with the rest of the PCC. They are more prevalent in batches mixed for a small number of revolutions and can generally be avoided by a proper material loading sequence (one that starts loading water before the other ingredients and one that starts loading coarse aggregates before fine aggregates).
Truck Drum/Bed and Chute Cleanliness
Truck drums/beds and chutes should be kept clean to prevent the introduction of foreign substances into the PCC. Old PCC that remains in the drum/bed can reduce mixing efficiency and possibly break off during discharge and be incorporated into the pavement causing a potential weak spot (ACPA, 1995). Water is used to maintain cleanliness at three different stages of mix transport (ACPA, 1995):
- At the production facility after loading (called “wash off”). Applicable to mixing trucks, the driver should “wash off” the back of the mixer to prevent buildup of materials. The driver must take care to minimize water entry into the drum. Generally, a consistent amount of water (such as 20 liters or 5 gallons) is used to wash off the back hopper and fins because this water will run off into the drum. If the volume is known, it can be counted as part of the mixing water.
- After unloading at the paving site (called “wash down”). Applicable to mixing trucks, the chute and discharge hopper should be washed to prevent buildup of materials. Wash down locations need to be determined in advance because wash water should not be discharged into catch basins, road ditches or environmentally sensitive areas. PCC left in the drum after discharge can be either washed out or recycled.
- At the production facility at the end of the day (called “washout”). The drum/bed should be washed out at the end of the day to prevent material buildup. Prior to loading the next day, the drum should be run discharged or the bed dumped to eliminate any remaining water. See Figure 1.
Water is a relatively plentiful resource in the U.S. In rigid pavement construction it is used in PCC, for transport truck washing, dust control and PCC curing. Additionally, rain at the job site or along the PCC haul route can be a source of water. Finally, water can also be used to control PCC workability.
PCC performance is closely tied to its water-cement ratio and the amount of mixing water. Therefore, adding water to the mix between batching at the plant and final placement and consolidation must be a conscious decision that carefully considers all the potential consequences. All water inputs to the mix should be closely controlled, or managed. As an example, truck mixers are equipped with water tanks for water addition and cleaning, so it may be tempting to use this water to adjust PCC slump on the fly to account for a long haul time. If this type of addition is not in accordance with the water management plan (and it most often is not), avoid it – excess water can result in a multitude of untraceable problems such as excessive slump, mix segregation, poor finishing characteristics, bleeding and scaling. They key to water management is to use a consistent process and keep careful control of all water sources that go into the mix (ACPA, 1995).
Unloading at the Paving Site
PCC unloading involves removing the mix from the transport vehicle and placing it for use by the appropriate paving machinery. There are a couple of items to consider when unloading PCC:
- PCC should be unloaded quickly when it arrives at the paving site. This will minimize the time available for water evaporation and excessive agitation.
- PCC should be unloaded as close as possible to its final placement spot. This will prevent mix segregation that can occur if PCC is moved excessively. Paving machines are not designed to move PCC but rather to finish it.
Ideally, PCC plant production, truck transport and placement should be synchronized to the same rate to minimize accumulation of excess material in any one of these three segments. Realistically, however, this synchronization can be quite difficult because of varying laydown rates, unpredictable truck travel times and variable batching operations. Detailed information on operation synchronization can be found in:
American Concrete Pavement Association. (1995). Construction of Portland Cement Concrete Pavements. National Highway Institute Course No. 13133. AASHTO/FHWA/Industry joint training. Federal Highway Administration, Department of Transportation. Washington, D.C.
Ideally, all operations are designed to meet optimal PCC placement rates because a consistently moving PCC paver can, in general, produce a smoother pavement than one that must constantly start and stop while trying to match its paving rate to the material delivery rate. Truck transport should be planned such that the PCC transport rate (expressed in m3/hr or yd3/hr) closely matches plant production rate and paving rate. Some factors to consider are:
- Number of trucks to be used.
- Truck type.
- Average truck hauling capacity.
- Production facility output rate.
- Availability and condition of materials at the production facility.
- Time to wash off, down and out the transport truck.
- Waiting time at the production facility.
- Loading, weighing and ticketing time at the production facility.
- Distance between the production facility and the paving site.
- Average truck speed.
Traffic plays a large role in PCC delivery rates because it affects truck speed. Especially in congested urban areas, heavy and/or unpredictable traffic may substantially increase or at least vary truck travel time. As truck travel time increases more trucks are needed to provide a given PCC delivery rate. Additionally, PCC usually must be delivered within a specified amount of time. Therefore, as traffic gets worse, trucking costs increase. Additionally, the unpredictability of traffic may result in either long paver idle times as it waits for the next truckload of PCC or large truck backups as several trucks all reach the paving site or production facility at the same time.
In sum, synchronization should be the goal but it is often difficult to achieve (based on varying paving rates, haul time and traffic) and may result in paving inefficiency and degraded PCC quality.
Surveys-AASHTO State DOT’s Perspective Relative to the Use of Volumetric Concrete Mixers (Mobile Mixers) for State and Federally Funded Projects
- State DOT
- Does your state DOT permit the use of volumetric mixers (mobile mixers) in lieu of transit or central mixed concrete?
- If so, where do you permit and/or restrict their usage?
- Do you have any additional comments relative to usage or restriction of volumetric mixers (mobile mixers)?
- State Specifications, if provided
- Person Responding and Contact Information
AASHTO State DOT’s Perspective Relative to the Use of Volumetric Concrete Mixers by the Michigan DOT
- American Concrete Pavement Association (ACPA). (1995). Construction of Portland Cement Concrete Pavements. National Highway Institute Course No. 13133. AASHTO/FHWA/Industry joint training. Federal Highway Administration, Department of Transportation. Washington, D.C.↵