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Figure 7.75: Typical Truck Mixer |
Figure 7.76: Large Truck Mixer |
| Major Topics on this Page | ||
| 8.1 | Truck Types | |
| 8.2 | Operational Considerations | |
| 8.3 | Summary | |
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 two basic truck types used for mix transport:
Standard transport truck requirements are contained in:
Truck mixers are the most common mode of PCC transport. They consist of a truck-mounted drum that rotates on an inclined axis. A typical mixing truck (see Figures 7.75 and 7.76) uses a 6.9 - 9.2 m3 (9 - 12 yd3) mixing drum, the size being limited due to gross vehicle weight of the loaded truck. When used to transport truck mixed PCC, drums can be filled to a maximum of 63 percent of their total volume. When used to transport central mixed PCC, drums can be filled to a maximum of 80 percent of their total volume (AASHTO, 2000). Generally, ready mixed concrete producers, load their trucks with a quantity at or near the rated mixer capacity (NRMCA, 2002). Mixing drums contain helical blades on their inside walls that are designed to push PCC to the bottom of the drum when rotated in the "mixing" direction and out to the discharge point when rotated in the opposite, or "discharge" direction. Most truck mixers discharge to the rear, however, front discharging truck mixers are gaining in popularity because the driver can drive directly onto a site and mechanically control the positioning of the discharge chute without the help of contractor personnel (NRMCA, 2002). Discharge is typically via a 3 - 6 m (10 - 20 ft.) chute. Truck mixers use revolution counters to keep track of total drum revolutions and may also be equipped with slump meters (usually accurate to about 12.5 mm (0.5 inches)) and digital water meters to monitor water usage (ACPA, 1995).
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Figure 7.75: Typical Truck Mixer |
Figure 7.76: Large Truck Mixer |
Drum rotation (see Video 7.12) is used for two purposes: mixing and agitation. High speed rotation (on the order of 12 - 15 rpm) is used to mix PCC ingredients into a homogenous material. This type of mixing typically takes between 50 and 100 revolutions depending upon PCC characteristics and environmental factors. After this period of mixing, the PCC is usually required to meet at least 5 of the 6 homogeneity specifications listed in Table 7.9. Samples for these specifications should be taken from widely separated portions but should also come from the middle 15 - 85 percent of the load so as not to be influenced by beginning and end of load abnormalities.
Table 7.9: Ready-Mix Concrete Homogeneity Test Requirements from AASHTO M 157 and ASTM C 94
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Parameter |
Maximum Permissible Difference in Results of Tests Taken from Two Locations in the PCC Batch | |
| Metric | English | |
| Weight per unit volume calculated to an air-free basis | 16 kg/m3 | 1 lb/ft3 |
| Air content | 1.0 % | 1.0 % |
| Slump | ||
| If average slump < 102 mm (4 inches) | 25 mm | 1.0 inch |
| If average slump is 102 - 152 mm (4 - 6 inches) | 38 mm | 1.5 inches |
| Coarse aggregate content (percent by weight retained on the 4.75 mm (No. 4) sieve) |
6.0 % | 6.0 % |
| Unit weight of air-free mortar (based on an average of all comparative samples tested) |
1.6 % | 1.6 % |
| Average 7-day compressive strength for each sample (based on an average of all comparative test specimens) | 7.5 % | 7.5 % |
Low speed rotation (about 2 rpm) is used to agitate the PCC to (1) maintain its homogeneity and (2) prevent slump loss while in transit. Truck mixers are equipped with a revolution counter to help maintain tight control over the total number of drum revolutions. Mixing, which is generally short in duration, is usually planned for a specific time or place. If not mixing, truck mixers usually operate in the low speed agitation mode. Mixing is typically done using one of the following three methods (NRMCA, 2002):
Video 7.12: Concrete Mixing Truck Drum Rotation Speeds
In general, short times between mixing and placement can better avoid the problems of premature hardening and slump loss that result from potential delays in transit. Regardless of the mixing mode, PCC is a perishable construction material. First, if it begins to set before being placed and consolidated it is of little use. Second, if it is mixed and agitated excessively it can loose its air entrainment or the effects of certain admixtures can diminish. Therefore, a typical specification will require that ready mixed PCC delivered to the paving site meet the following criteria:
Non-agitating trucks (see Figure 7.77 and 7.78) can be of any form but are typically end, bottom and side dump trucks. These trucks are not specifically designed to transport PCC but often work well for central mixed PCC when haul distances are short and mixing requirements are simple. Their chief advantages are:
Because they are not designed to transport PCC, dump trucks also have some severe disadvantages, which limit their use:
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Figure 7.77: End Dump Truck Discharging PCC in Front of a Paver |
Figure 7.78: End Dump Trucks Discharging PCC into Placer/Spreaders |
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 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 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 7.79.
Figure 7.79: Washout
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).
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:
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:
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:
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.
Mix transport can have a large impact on rigid pavement construction quality and efficiency. Mix characteristics such as homogeneity, workability, water content and air content are all affected by mix transport practices. In general, there are two types of mix transport trucks: the truck mixer and the non-agitated truck. The most common one, the truck mixer, hauls the mix in a large rotating drum giving it the capability to actually perform PCC mixing as well as the ability to agitate the PCC while in transport. Non-agitating trucks, which are typically dump trucks, are much simpler in design and are often used to transport central mixed PCC over short haul distances. Key considerations in mix transport are proper mixing and batch at the production facility, truck cleanliness, water management, unloading timeliness and location, and operation synchronization. If properly managed, mix transport can successfully move PCC over long distances with no impact to final pavement quality.
