Tucked away in the pristine wilderness of Pasco County in western Florida is the newest of five ready mixed concrete plants of Keys Concrete Industries. The plant stands just south of the intersection of State Road 52 and Ehren Cutoff Road, some five miles west of Interstate 75 leading south into the greater Tampa area. The plant operates as a testimony to the possibilities of managing the convergent needs of nature, agriculture, and emerging residential and commercial growth (separately, note this month's NRMCA Environmental Excellence Awards, page 16). To those in the industry, perhaps most noteworthy is the batching speed of the plant.

The Ehren Cutoff Plant is capable of weighing up a 10-yd. load and putting it into the truck in under 1 min. 15 sec., resulting in a truck-to-truck time of well under two minutes. In a worst-case scenario, the plant is capable of batching 300 yards per hour under normal operating conditions. The question most often asked about the plant is: Why the need for that type of speed, particularly when the market volume in the area is comparatively small?

Batching speeds in the ready mixed concrete industry are increasing rapidly. Four factors that have a significant impact on plant decisions must be considered in view of changing market conditions and profitability goals:

1. The need to satisfy current market volume demands
2. The need to satisfy current market speed demands
3. The need to improve truck and operator efficiency
4. The need to satisfy future market speed and volume demands

In a discussion of the above, the following terms are relevant:

• Batch time: Time from batch start to the completion or finish of truck loading.
• Cycle time: Time from one truck finished loading to the following truck finished loading.
• Rated plant capacity: Theoretical capacity of plant, assuming no time elapses between end of one load and start of a second load.
• Operational plant capacity: Actual plant capacity given that some time will elapse between the end of one batch and the start of another under normal operations. (This article assumes 30 seconds for this change from one truck to the next and 10-yd. load sizes.)

Satisfying current market volume demands

One common method used to determine required plant capacity is to balance such volume and mixer truck requirements. The calculations generally use the following logic, balancing round trip times with plant capacity:

Assumptions

Expected market volume/day	800 yd./day
Expected average volume/truck/day	32 yd.
Expected average round trip time	100 min.

Calculations

Truck requirement	25 trucks (Note: 800/32)
Cycle time required (truck-to-truck)	4 min. (Note: 100 min. rtt/25 trucks)
Theoretical batch time required	3.5 min.
Rated plant capacity	180 yd./hour

Using this method to plan for capacity, the plant is in balance with fleet size. With a cycle time of four minutes, truck-to-truck, an operational capacity of approximately 150 yd. per hour can be estimated. This, in turn, requires a batch time of 3½ minutes and, therefore, a plant with a rated capacity of approximately 180 yd. per hour. Problems associated with using this method of planning for plant capacity include:

• Queuing times can significantly reduce truck and driver efficiency when a number of trucks return to the plant within a short period of time. (Leaving a plant at four minutes per load does not guarantee evenly spaced returns to the plant.)

• This method does not address the problem of satisfying customers with faster off-load times while maintaining an existing account base.

Satisfying current market speed demands

While the volume of a particular market may not have changed significantly over the last few years, it is probable that speed requirements have changed. Much of the demand for increased speed may be attributed to technological changes in concrete placement, which enable crews to handle loads much faster than just a few years ago. Such innovations include laser screeds, high volume pumps, and better survey control. Today, it is not unusual for even a small crew to be able to place concrete at rates in excess of 120 yd. per hour.

These accelerated placing rates have had a dramatic impact on ready mixed concrete production that is often not addressed in plant planning and evaluation. Increasingly, we see the batching rate of a plant to be a primary factor in scheduling instead of mixer truck fleet size, the historical determinant. But what impact do high-volume placing techniques have on batching speed? Some considerations of speed planning are worth examining.

Taking the example from the preceding section, let us assume that we have a plant with an operational capacity of 150 yd. per hour. Further, let us suppose that Contractor A, one of the company's better customers, typically uses a boom pump and places concrete at a rate of 120 yd. per hour, or one load every five minutes. In order to provide concrete to this customer while maintaining the remainder of the company's base, how fast a plant is needed?

To provide perfect service to Contractor A alone — an ideal situation, indeed — and to give him a load every five minutes, we must first have a plant capable of batching 120 yd. per hour. Again, in a perfect world, in order to both provide for this customer and accommodate even one additional order, a plant capable of batching 240 yd. per hour would be required. At anything less than 240 yd. per hour, batching a load for this customer every five minutes would not be possible. For example, given a plant that could batch every three minutes, or 200 yards an hour, we would have a six-minute gap between loads if any other load were batched between loads for Contractor A.

In the real world, then, it is evident that the 150-yd.-per-hour plant is strained with a customer who often pours 120 yd. per hour, even if that plant meets the average volume requirement of 800 yd. per day. The plant that can produce only 150 yd. per hour is limited to 30 yd. per hour in orders for other customers on those days when Customer A is pouring. Even then, the excess of 30 yd. per hour is available only at the cost of providing less than optimum service to Contractor A.

A market may be defined, therefore, not only by the number of anticipated yards per day, but also by peak hourly demands of its customers. This factor becomes increasingly important as placement technology continues to improve. Today, even in moderate-sized markets, it is common for at least one contractor to place concrete at rates approaching or exceeding 120 yd. per hour, for numerous contractors to be placing at 60-80 yd. per hour, and for a large part of the remaining contractors to be placing at 30-50 yd. per hour. Clearly, plants with batching capabilities less than 150 yd. per hour are becoming increasingly irrelevant, and soon plants under 180 yd. per hour will be marginal.

Increasing truck and driver efficiency

A common statement heard when discussing the virtues of fast plants is, “Why should I have a two-minute batch plant when my drivers spend six minutes on the slump rack?” Simply, with a two-minute batch plant, trucks can get on the way to the job every two minutes regardless of the time spent on the slump rack (within reason, of course). If it is known that the drivers will spend four minutes on the slump rack, building two slump racks (time on the rack divided by batch time) is much cheaper than buying more trucks or building another plant. If they spend six minutes at the rack, one might try building three racks. Experience also indicates that when trucks are coming to the slump rack quickly, the preceding drivers will move faster to get out of the way, particularly if that is company policy.

It is assumed that batch time has little impact on round-trip time and truck or driver efficiency during the round trip. If average round-trip time is 100 minutes, a four-minute batch time will add only two minutes to the round-trip time including a batch time of two minutes. However, a huge difference exists in the number of trucks that each of the two plants can get out of the gate: a two-minute batch plant can move 30 trucks out in one hour; a four-minute plant can get only 15 trucks out in the same length of time (regardless of time spent at the slump rack).

Second, the queuing effect of faster plants is critical. With a fleet size of 25 or more, six trucks returning to the yard within a short period of time is a common occurrence. In a plant with a cycle time of four minutes, the last truck will have been in the yard for 20 minutes of nonproductive time before being batched. Total lost time for the five waiting trucks is 60 minutes. A two-minute batch time will result in a 12-minute time loss for the last truck and a combined loss for the five trucks of 30 minutes. Though the individual losses seem negligible, as a typical occurrence happening many times throughout each day, the cumulative effect is significant. The queuing effect accounts not only for added cost; more significantly, perhaps, it results in diminished quality of service to customers.

A final effect is that more orders can be taken for the plant that batches in two minutes as compared to the four-minute plant. Why? Trucks are able to get out of the yard faster. This greater volume can lead to another significant, though somewhat unpredictable, productivity increase — even without an increase in the number of trucks.

The combined results of reduced round-trip time, decreased queuing effect, and greater volume are not entirely quantifiable. Yet, on one plant upgrade, from which were drawn some of the observations presented here, the change from a 150-yd.-per-hour plant to a 300-yd.-per-hour plant resulted in a productivity increase of 18 percent, with market and fleet size variables remaining constant. Subsequently, the upgrade led to a large gain in market share due to increased capacity and better service.

Satisfying future market requirements

Meeting future demand is such an obvious justification for increased batching speeds that it hardly warrants mentioning. However, though market volume requirements historically have been recognized in plant planning, future technological advances that may impact plant requirements have not been fully considered. On the production side, the industry has seen advances in mixer technology including boosters, front discharge mixers, and mixer designs that have increased load capacity and load height while decreasing load times. On the contractor side, the changes cited above have increased the contractors' demand per hour, thereby stepping up production requirements at the plant. Improved computerized scheduling and dispatching techniques have extended the range of tracking and control to facilitate dispatching trucks from a single plant and, thereby, increasing the number of trucks that can be handled at the plant. Finally, improved computer batching controls allow faster weigh ups and discharges and, consequently, faster batch times.

The end result is that speed is less expensive than ever before. Purchasing and constructing a 300-yd.-per-hour plant may cost $100,000 more than erecting a 150-yd.-per-hour plant, but the view down the road sweetens the deal.

1. The 300-yd.-per-hour plant can support up to 50 trucks, while the 150-yd.-per-hour plant can support only 25 (average volume method, 100 minute RTT).
2. The 300-yd.-per-hour plant allows support of a 120-yd.-per-hour contractor in addition to the maintenance of a sizable customer base (or two 120-yd.-per-hour customers, plus others), while the 150-yd.-per-hour plant can accommodate one 120-yd.-per-hour contractor and little else.
3. Assuming a (conservative) 10 percent productivity gain with the 300-yd.-per-hour plant, the outcome is roughly $1.50-per-yard improvement. Basically, the faster plant's cost is covered in 80,000 yards (ignoring time-value of money which is low at current interest rates).
4. Service is significantly improved with the 300-yd.-per-hour plant.
5. Future technological and market advances are generally not a problem with the faster plant.

Since rarely do two companies share the same methods of generating rate-of-return calculations, look at one set of very simple numbers: If the 300-yd.-per-hour plant costs $100,000 more than the 150 -yd.-per-hour facility, and if these plants will be in use for 20 years, the cost of the faster plant is approximately $400 per month (not counting the time-value of money). In view of life-cycle economics, the investment is relatively small for obvious returns.

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Jerry Kyckelhahn heads Applied Technology Consultants, Palm City, Fla. He holds a B.S. in Civil Engineering and an M.S. degree in Mineral Economics from The Citadel and Colorado School of Mines, respectively. He oversaw plant and site design and construction of Keys' Ehren Cutoff operation.

Ehren Cutoff Plant: Fast and furious

The plant at Ehren Cutoff Road was designed and manufactured by Merts, Inc. according to the requirements of Keys Concrete President Jimmy Keys. After analyses of other company plants had been completed, a goal was established for the Ehren Cutoff property to balance the weigh-up and discharge times in order to achieve plant-loading efficiency. As an example, Keys Concrete, which uses three cementitious materials in most of its loads, i.e., cement, fly ash, and slag, optimized the time involved in weighing up these materials by employing two cement weigh hoppers instead of one. Such measures to accelerate cement weigh-up were necessary in order to meet the other “material ready” times of aggregate and water. The end result was balanced times and, hence, faster batches with minimal added cost. Other elements of the plant's design, cited in the following description, contribute as well to its smooth operation and higher productivity.

Starting at the rear of the facility, the plant is fed aggregates through a 10-ft. ∞ 10-ft. front-end loader hopper. To minimize ramping, the loader hopper has “wings” on three sides to maximize its effective volume and eliminate spillage. The hopper is plastic-lined to facilitate the flow of material on to the 30-in. belt of the radial stacker feeding the overhead aggregate storage bins. In order to meet batch times, the loader has to maintain a cycle time of 34, achieved by means of an efficient material storage design to minimize travel distances and by minimal ramping (note photo, preceding page).

The overhead aggregate storage is rated at 225 tons. The overhead bins feed the aggregate weigh hopper through eight 18-in. ∞ 18-in. double-acting clamshell gates driven by 4-in.-diameter, 10-in. stroke cylinders. The 472-cu.-ft. weigh batcher is equipped with two 18-in. ∞ 45-in. gates tapered to 10-in. ∞ 45-in. with four-15,000-lb. load cells. Average aggregate weigh-up time for 10-yd. loads is approximately 22 seconds.

Cementitious materials are stored in a split silo with a total capacity of 998 barrels and a single 622 barrel-capacity silo. Cement is fed from the single silo into one weigh batcher while fly ash and slag feed into a second weigh batcher. All cementitious materials are vertically fed, eliminating screws and air slides. Constant material flow is optimized by using a Rotron high-volume, low-pressure air blower for aeration.

Both fresh water and reclaim water are stored in separate holding hoppers, which feed a single water weigh hopper equipped with a 6-in. butterfly valve discharging through dual 4-in. pipes.

Critical to fast batch times in a single-stage plant is getting the material into the truck quickly: it is here that the plant differs from conventional, low-profile facilities. The plant uses a 42-in. charging conveyor that carries the material at a rate of up to 800 lb. per second. Strict inching controls are imposed through features of the Command Alkon Spectrum batch computer, and specialized sequences are used to improve loading efficiency. The drop from the aggregate charge conveyor to the truck is approximately 13 ft., allowing the material to accelerate and to effectively spread out for a quick charge. At the end of the fall is a ceramic-lined hopper through which aggregates, cements, waters, and admixtures enter the truck.

To avoid problems caused by materials “backing up” waiting to enter the truck, a 5-yd. surge hopper is located between the charge conveyor and the ceramic-lined charging hopper. The surge hopper comes into play when a truck does not accept charging at the expected rate. The surge hopper can then fill with aggregates and allow slower feeding into the truck. The unique design of the charging hopper then permits the tail water to assist in flowing the backed up aggregate into the truck. This design feature has eliminated the problem of materials backing up and flowing out onto the loading platform when trucks are not properly aligned under the plant. With a 24-in. drop from loading ramp to slab, trucks are placed at a sufficient angle to facilitate material feeding and reduce spillage.

The site fully supports the loading speed of the plant through both automation and design. Traffic flow was planned to support both front and rear discharge mixers, as were the slump racks. The layout minimizes loader cycle time to keep the aggregate overheads charged. The water system for the plant uses touch-screen/PLC controls to allow full control of all waters used in the plant and all sediment pits from the batch office. Staffing requirements consist of a batch man and a loader operator, regardless of the volume requirements placed on the plant.

Allowing its speed to support all levels of anticipated operations while minimizing operational costs and concerns, the Ehren Cutoff plant epitomizes the successful application of technology within the concrete industry.
— J.Kyckelhahn