CONSTRUCTION EQUIPMENT AND PRACTICES FOR MANUFACTURE OF CONCRETE BATCHING AND MIXING Plant manufacturers standards.-For many years there has been a need for standard nomenclature and definitions for use in designating classes of batching plants. In 1960, the Concrete Plant Manufacturers Bureau issued the first edition of Concrete Plant Standards. In these standards five classes of batching systems are defined on the basis of three types of batchers. Thus for the first time there is standard nomenclature and definitions which may be used in specifying classes of batching plants. The corps is adopting the nomenclature and definitions of the concrete plant standards and these will be used in guide specifications for civil works concrete which are currently being revised for issue. Use of nomenclature and definitions which are recognized as standard by both manufacturers and contractors should largely eliminate misunderstandings with respect to plant requirements which have sometimes occurred in the past. Turbine mixer.-Prior to about 1957, in the United States essentially all concrete was mixed in drum-type mixers. These mixers rotate around a horizontal axis and the mixing action is generated by blades which are attached to the inside of the mixer drum and thus rotate with the drum. In contrast to the drum-type mixer, the turbine mixer is composed of a doughnut-shaped pan inside of which blades or plows rotate around a vertical axis to produce the mixing action (fig. 15). The mixed concrete is discharged through a door in the bottom of the pan. Tests by the corps on the turbine mixer indicated that it mixes concrete in less time than do the drum-type mixers. Tests indicated that it mixes concrete containing 11⁄2-inch maximum-size coarse aggregate very well, and concrete containing 3-inch maximum-size aggregate satisfactorily, but is not adapted to mixing of concrete containing 6-inch maximum size-aggregate. Tests were made on a mixer of 11⁄2 cubic yard capacity which was the largest available at that time. These mixers are now available in sizes up to 4 cubic yard capacity. Currently, use of turbine mixers is permitted on corps work for mixing concrete with up to 3-inch maximum-size aggregate. Until recently turbine mixers were available from only one U.S. manufacturer but it is understood that there are now two U.S. manufacturers and a Canadian manufacturer offering turbine mixers for sale. On a production basis, first cost of turbine mixers is said to be substantially less than first cost of drum-type mixers. Other cost factors including the cost of supporting structures for mixers and batchers are also said to favor turbine mixers. Rapid moisture indicators.-One of the essentials to control of quality of concrete is knowledge of the amount of "free water" (water on the surface of aggregates which becomes mixing water) in the aggregates, particularly the fine aggregate. This knowledge is essen tial in order to be able to batch the correct amount of additional water needed for mixing the concrete. Ordinary methods of determining free moisture in the fine aggregate (sand) which carries the greatest amounts and undergoes the widest variation in amount do provide data as rapidly or as frequently as desirable. Thus for a number of years work has been underway to develop means for making instantaneous determinations of amount of free moisture in fine aggregates. At least four devices using the electrical resistivity principle have been developed and are commercially available. Three of these were laboratory-tested by the corps and one of the three was fieldtested. All three were found to be capable of indicating changes in moisture content of fine aggregate but none could be depended on to continuously give accurate measures of moisture content. All three require frequent recalibration in order to maintain reasonable accuracy. In spite of their limitations, instruments based on the principle of resistivity are considered useful in controlling the water content of concrete but do not entirely fulfill the need for an accurate and continuously dependable instrument for this purpose. Price of these instruments range from about $200 to about $1,200. As far as is known, one instrument has been developed using the nuclear principle. One of these was laboratory-tested by the corps. The instrument showed considerable promise but there was a clear indication of need for further developmental work on the instrument and it is understood that such work is currently underway by the developer. As far as is known, no instruments using this principle are available commercially at this time. The corps has made limited use of instruments based on the principle of resistivity but has not found it prudent so far to require an instant moisture meter in each concrete plant. TRANSPORTING CONCRETE General. No new principles have been developed for transporting concrete in recent years; however, at least one new application of an old principle and two innovations of an old principle have been made. The new application of an old principle was in the use of compressed air (pneumatic) in transporting concrete. One of the innovations in an old principle was the development of a hydraulic bucket-dumping device which is self-contained and self-recharging; the other innovation was the development of a radio device for remote control of the dumping of a concrete bucket. Concrete bucket. The hydraulic principle has been utilized for a good many years for dumping large concrete buckets used in placing mass concrete. A source of compressed air in the forms has been a necessary part of this system until recently when a concrete equipment manufacturer developed a self-contained hydraulic system which recharges itself when the empty bucket is seated on a platform or other similar support for refilling. The dump of the bucket is activated by a pull on a short section of cord attached to the dumping mechanism and the rate of dump can be regulated by varying the pull on the cord. The same bucket can be equipped so that it can be dumped by radio control from a considerable distance. The remote dump has not been used so far because of the safety hazards involved. Air extrusion.-A device using air for transporting concrete for short distances through a pipeline has recently been developed. Transporting concrete by air in the past has been largely limited to the high-pressure method of transporting and placing concrete in tunnel linings known as the pneumatic method of tunnel lining. The new device utilizes low-pressure air and the movement of the concrete through the pipeline is described as air extrusion. The device has had limited use on corps work so far. PRECAST SLABS FOR FORMS Except for a few isolated cases the practice in the United States has been to use forms made of wood, steel, or other materials to contain and shape concrete as it is placed. In Russia in recent years precast slabs of concrete have been widely used as forms. The precast slabs are left in place and become a part of the structure. Cost comparisons made in the past have favored conventional forms of wood, steel or other materials over precast slabs. There have also been technical questions about use of precast slabs for forms which have not been satisfactorily answered and for this reason use of precast slabs as forms has not gained favor in the United States. In spite of past indications the question of use of precast slabs for forms is being reexamined to determine whether or not there is now any advantage to be gained in corps work by this practice. PREPACKED CONCRETE MIX A device consisting of a two-compartment rubber bag has been developed by a ready-mix-concrete producer in California for delivering "prepacked concrete mix" to a mixer at the construction site. Aggregates, water and cement thus may be batched at one location. and delivered to another location many miles away for mixing. Aggregate and water are placed in one compartment and cement in the other compartment sealed from the water. The bags which were manufactured by a rubber manufacturing company, are like an automobile tire in construction with rubber coating over nylon cord. The bags are estimated to have a life of about 1,000 reuses. As far as is known, this device has been used only by the developer up to now. For most civil works it would not be applicable but in unusual situations it might find usefulness and its use is permissible under the corps specifications. CONSTRUCTION EQUIPMENT AND PRACTICES-EARTH AND ROCK FILL DAMS; GROUTING; TUNNELING; DREDGING; REVETMENTS EARTH AND ROCKFILL DAMS Private industry has been encouraged to produce devices which will improve the construction of embankment fills. One item so developed is the heavy-rubber-tired roller which permits more economic compaction of earthfill. Such rollers are made by Bros, Inc., Southest Welding & Manufacturing Co., and Shovel Supply Co., among others. Another compaction item is the vibrating steel wheel roller for use in constructing roller granular fills for embankment. Vibration results in more uniform and greater density than is possible by pneumatic or tamping compaction. Principal suppliers of this equipment are Bros, Inc., and Shovel Supply Co. GROUT PUMPS Grout slurries or pastes for consolidation of subsurface materials may be injected by a variety of conventional pumps. To date the principal specifications which these machines have been required to meet have been stipulated as follows: "They shall be double-action, duplex-slush pumps of long-stroke type, having large valve openings and removable valve seats and removable liquid-end liners, and shall be of ample capacity and able to safely withstand the greatest pressure required." A variety of pumps have met these general requirements for grouting. The frequent tendency for thick portland cement grout to "choke" the pipeline through which it is pumped underground has been associated, in part, with the stroke pulse of conventional pumps. In an effort to remedy this pulse effect in the grout fluid line, a slush pump of the type described below has been developed and manufactured by Robbins & Myers, Inc., of Springfield, Ohio. Progressing cavity (Moyno type) pump. This recent development in the field of grout pumping equipment has received considerable attention by foundation engineers and construction contractors. Its principal application has been in connection with foundation consolidation by neat portland cement, or portland cement and sand, grouts. It has already been used with very satisfactory results on the Corps of Engineers Beaver and Greers Ferry Dams in Arkansas. The pump derives its designation, "Progressing Cavity," from its functional design. Stated simply, the device consists of a helical or screw-like rotor that turns within a double-helical or stator or cylinder. The screw-like movement of the rotor within the stator during operation of the pump, produces a cavity in the column of grout moving along the rotor. This discontinuity, however, is immediately rectified by the constant air pressure applied to the premixed grout entering the pump. The result is an even, pulsation-free movement of grout fluid from pump to ground. Observations of operations in the field indicate that control of grout pressures and grout consistencies at the |