碾压混凝土施工外语.doc

8 RCC as a New Construction Method（RCC作为一种新的施工方法）8.1 General Construction Considerations (一般情况下施工过程需考虑的因素)There are considerable differences between vertical, block-by-block construction （垂直块状浇筑）of dams with conventional mass concrete and horizontal, lift-by-lift construction（水平通仓浇筑）of dams with RCC. The construction plant for RCC is usually, less costly, and utilized at a higher capacity factor （利用率）than that required for conventional concrete gravity dams. But the fast pace of placement required to achieve the economies possible with RCC imposes a greater discipline (要求)on the planning, scheduling(进度), and coordination(协调) in the field between designer(设计) and contractor(承包商).The RCC design approaches described in Chaps. 4, 5, and 6 diverge(不同点在于.) in some construction procedures. In all cases, however, the mixing(拌合), transporting(运输), spreading(摊铺), and compacting (碾压)of relatively large volumes of material are concentrated into a short time interval. In the United States, the time limit from mixing plant to final compaction is about 45 min. It is about twice that in Japan. In addition, the Japanese RCD method requires that lifts be cured (养护)for as long as 36 h before placement (浇筑)of the next layer of RCC. In the United states and elsewhere, the goal has nearly always been to come as close as possible to continuous placement of RCC. 8.1.1 SchedulingConstruction scheduling for RCC is less forgiving than for mass concrete(大体积混凝土). Although some mix designs can have greater tolerances than conventional concrete, RCC is still a sensitive concrete and the critical path (关键路线)for placing RCC is focused on completing one lift at a time. There are no alternate monolithic (整块的、独块的)blocks to form or place while problems are being analyzed.To avoid costly delays over disputes on specification（规范）compliance（执行）, the authority to resolve engineering problems must be vested in a well trained field organization. Lines of communication between the project engineer（项目工程师）and contractor must be clearly established and used frequently. Clarity(廉洁), candor（坦诚）, and cooperation are very important in achieving the speed of placement possible with RCC.Thorough field and classroom training of construction crews（施工队伍）and inspectors（检查员） in the handling and placement of RCC is essential. Lean, dry RCC mixes resemble damp（潮湿的） gravel（砂砾石）and are often placed by road-building or earth-dam crews using earthmoving equipment. However, all the desired characteristics of the product are those of concrete. Meeting the specifications requires educating workers to the specific requirements of RCC as a new construction material requiring specific construction methods.Because so much depends on fast, efficient placement of the RCC, all related activities such as foundation cleanup, access, assembly of embedded components(预埋件), and stockpiling （贮存、堆存）of materials must be meticulously planned and scheduled well before construction starts. Construction of forms and assembly of embedded items should be planned and scheduled so that as much of the work as possible is done off the dam, or if necessary, from the top of a lift （升程）during shift（工作班）changes.Lift scheduling is often complicated by concerns over heat buildup. Because the production rate of RCC can be very high, controls to limit thermal cracking（温度裂缝）may restrict the season or time of day that placement is allowed as well as the rate of placement.At Upper Stillwater Dam, all of the RCC was placed during a five-month weather window each year from May to October and only during two 8-h shifts starting at 8 p.m. This was necessary in order to keep placement temperatures below the 50oF (10oC) specification limit. At Willow Creek and Galesville dams, placement was limited to no more than three and four lifts, respectively, each 24 h.Conversely, at Elk Creek and Stagcoach dams, the contractor was encouraged to place RCC as fast as possible. At peak placement, six 1-ft (0.3-m) lifts were placed in a day at Stagecoach. The 150-ft-high (46-m) dam required 44,500 yd3 (34,000 m3) of RCC which was placed during 37 days in the summer of 1988. To keep placement temperatures low at Elk Creek, the specifications required that the most massive sections be constructed during the late winter and early spring.8.2 Aggregate Production and Plant Layout (砂石料生产和混凝土工厂布置)Maintaining adequate supplies of acceptable aggregate is particularly important to RCC scheduling. More than half of the aggregate required for an entire construction season may need to be stockpiled well ahead of the start of construction in order to keep up with extraordinarily high demand during the placement season. This can also provide some easing(缓解)of cash flow（现金流）and scheduling requirements during the placement season. Large stockpiles also allow for more economical sizing of production facilities and for blending of material that may be out of specification. If precooling of aggregate is desired to keep placement temperatures low, stockpiling in the winter provides that opportunity.Continuous raw feed to stockpiles is necessary throughout the construction season to keep up with RCC production. At Upper Stillwater Dam, the largest in the United States, the contractor had to produce 9000 tons/day (8160 metric tons) of 3/4- and 2-in. (19- to 50-mm) aggregate from soft sandstone to keep up with its RCC placement schedule of 7200 yd3/day (5500 m3/day) during the final construction season in 1987.The location, size, and shape of aggregate piles must be coordinated with the concrete plant location and method of feed. If conveyors (传送带、皮带机)are not used, several large loaders may be required to feed the mixers on large RCC projects. To achieve RCC production of 900 yd3/h (690 m3/h), for instance, four 12-yd3 (9 m3) front-end loaders could be needed to allow for a reasonable interval between loadings. The haul distances（运距）, dumping procedures（卸料过程）, and turnarounds need to be planned carefully to operate efficiently and safely.At Elk Creek Dam, the aggregate was produced from an andesite-basalt(安山玄武岩) outcrop（露头） about 3000 ft (910 m) upstream and to the right of the dam. Rock was hauled to the primary jaw crusher, located in the quarry(采石场), using 85-ton 77-metric-ton end dump trucks（后卸式卡车）. Rock was crushed to 9-in (230-mm) maximum sizes. The aggregate was transported from the quarry down a 12 percent slope by a 56-in-wide, 2085-ft-long (1.4 x 635 m) conveyor belt rated at 1000 tons/h (907t/h) and stored in a surge pile using a radial stacker(栈式料仓). A series of cone crushers and screens then produced 3-in (75-mm), 1-in (38(mm), and 3/4-in (19-mm) aggregate sizes. The aggregate was transferred from these stockpiles by three 7-yd3 (5.4 m3) front-end loaders into movable hoppers discharging onto five conveyors which fed the batch plant surge piles. From there, the material was loaded from reclaim tunnels onto 36-in-wide (910-mm) conveyor belts and transported to the RCC and conventional concrete batch plant aggregate bins(贮料仓). The Elk Creek layout is shown in Fig. 8.3。Three concrete mixing plants about 300 ft (90 m) upstream of the dam were used to produce the bedding grout, conventonal concrete, and RCC for Elk Creek. Conveyors were used to move the mix from the plants to the dam and deposit it on either side of a regulating outlet structure.The concrete plant layout and location should be selected to minimize energy requirements whether the RCC is transported to the dam by conveyor or haul vehicles. The intent should be to minimize haul distances, vertical lift, and exposure of the RCC to sun, wind, or rain. If trucks or other vehicles are used for hauling, the plant should be located on a raised, free-draining area. This avoids making mud and helps to prevent tracking of foreign material onto the dam.Fueling and field maintenance of equipment should not be done on the RCC surface because of the likelihood of contamination(污染) and consequent problems in establishing a secure bond between lifts at that point. Fuel spills were a problem at Monksville Dam because the contractor was allowed to refuel on the dam. That potential problem was addressed at Elk Creek Dam by specifying that the contractor use a movable refueling pad （垫）on the surface of the dam to collect and contain fuel spillage. The contractor chose instead to refuel equipment and perform maintenance off the surface of the dam.8.3 Mixing RCCIn most cases, the methods used for transporting, spreading, and compacting RCC will not affect production as much as the speed and efficiency of mixing. Thus, the mixing plant capacity should exceed the laydown capacity(设计容量).Achieving the desired product consistency and quality at continuous, high production rates requires good concrete plant design and rigorous maintenance. RCC mixes are relatively harsh and sticky, and the lean, dry material has no fluid properties. The design of misers, transfers, and hoppers must take these characteristics into consideration in order to avoid caking（结块）and loss of capacity.The mixing method must produce a homogeneous mixture of the ingredients. This factor determines the mixing time and, to a large extent, the production rate. Tests should be done to determine retention times（拌合时间）required for each mix. The variable-speed pugmill mixers （叶片式搅拌机）used during 1987 at Elk Creek Dam required 38s of mixing for 6-yd3 (4.5-m3) batches. At Willow Creek Dam, the retention time in the 9-yd3 (6.8 m3) drum misers was 75 s.The trend in design is to use one RCC mix for the body of the dam. For large projects that require a variety of mixes, the plant chosen for the job must be able to change mix designs quickly and little or no mechanical or manual manipulation to plant components. On most projects, different plants are used to produce conventional concrete and RCC.For major projects the weights of all ingredients should be digitally recorded as a function of time, date, and mix design. Because of the sensitivity of RCC to excess water, the plant should be equipped with instrumentation to determine the fine aggregate moisture content. In Japan, automatic water-batching systems are adjusted according to results of continuous monitoring of the moisture content of sand in the sand storage piles. This System has been used on most Japanese RCD projects and has resulted in precise control of moistllre colltent. In the United States most contractors feel continuous visual inspection of the RCC placement or a Vebe test is a more reliable control of water content than moisture meters in the aggregate piles.Proper blending or ribboning of the aggregates and cementitious material on the charging belt as they are fed into the miser will help to speed mixing time and avoid buildup of the sticky material. Achieving the proper timing and angle of introduction of water into batehed mixtures is also important. The proper sandwiching of material to achieve best results with RCC often is different from conventional mass concrete mixes. Each plant and mix combination has its own quirks（特点）, so the exact method of ribboning mix constituents（要素） can only be determined by trial and error(反复试验).High-speed batching and mixing can best be achieved with individual weigh systems for aggregate. Accumulative weigh systems make it more difficult to fine-tune the ribboning of constituents on the belt.8.3.1 Mixing plantsVarious combinations of batch and continuous concrete plants using drum and pugmill mixers have been used to produce RCC. Continuous-mix plants can provide higher output capability than batch plants, and the most sophisticated continuous mixers can produce the same degree of control as batch plants. Land use and labor requirements are generally less for continuous plants.Drum mixers have the advantage of using less energy than pugmills and provide good control of batch proportions. Pugmills are faster and are generally more portable than drum mixers. Pugmills have been used on all RCC dams in Japan. Pugmills need to be carefully designed to avoid maintenance problems and excessive wear on paddles and plates, particularly for mixes with large aggregates. In drum mixes, excessive buildup of the sticky material can occur, reducing capacity and mixing efficiency. Redesign of fins（鳍状物） has helped to avoid this problem in some cases. Considerable cleanup of the drums on a daily basis is often required.For high-volume production, larger-than-average-capacity drum mixers may be necessary to cope with problems encountered when mixing no-slump RCC. Blending a zero-slump (塌落度)mixture with a high proportion of fines often causes long mix and discharge times. At Willow Creek, a four-bin Noble 600 plant with two 9-yd3(6.8-m3) Erie Strayer drums was used. The plant had been proven on an earlier highway job at 600 to 750 yd3(460 to 570m3) times were 30 s longer than for the fluid highway paving mix, and the drums had to be derated（降级为）to between 7 l/2 and 8 yd3 (5.7 and 6 m3) because the diameter of the discharge end was increased to speed dumping. On average, the plant produced 400 yd3/h (310 m3/h) of RCC and peaked during one shift at 438 yd3/h (335 m3/h).Four 250-yd3/h (190 m3/h) IHI-Hydam pugmills made by Ishikawajima Construction Machinery were used by the Japanese contractor for mixing batched RCC at Elk Creek Dam. The double-shaft(双轴) pugmills were hydraulically driven and allowed for variable rotation speeds during the charging(进料), mixing, and discharging processes .The coarse aggregate was crushed basalt sized from 3/4 to 3 in (19 to 75mm) maximum. Batching was computerized using Erie-Strayer controls. The maximum output of the four 6-yd3 (4.6-m3) mixers was 1014yd3/h (775 m3/h). That was achieved on October 29, 1988.Two different batch-type mixing plants were used to produce RCC at Upper Stillwater. The first plant built at the site was a Noble 600 with two 8-yd3 (6-m3) tilting drum mixers. A second plant, a Noble 600 modified to accept two 4-yd3(3 m3) Nikko high-intensity, spiral-flow pugmill mixers, was added later. Each plant achieved peak production rates of about 420 yd3/h (320 m3/h) in 1986. Abrasive quartzite sandstone aggregate with a silica content of about 80 percent created extraordinary maintenance problems. Problems were greater with the Nikko bottom-discharge pugmill mixers, which suffered from high wear rates on the paddles and breakage of the liner plates(衬砌板).For the drum mixers, rubberized plastic liners were installed and these worked well. The high-wear steel liner plates on the Nikko mixers had to be replaced five times during the 1986 season, however. That was due mainly to breakage caused by the shock of loading and mixing RCC. Initially, the contractor switched to milder steel liner plates. They broke at about the same rate as the high-wear plates but they cost less and were more readily available.For the 1987 season, a belt-drive system was installed on the Nikko mixers to allow a 10 percent reduction in paddle（搅拌） speed. Retention times were kept the same so that wear was reduced some-what. In addition, a 3/8-in-thick (9.5-mm) layer of 11,000-lb/in2 (76MPa) epoxy-silica（环氧-硅石） grout was troweled onto the backs of the liner plates before the sixth set of plates was bolted to the mixer wall. The grout is commonly used as a coating for nuclear reactor containments and has the beneficial characteristics of being durable but relatively soft. Placing it between the plates and the mixer wall solved the breakage problem.RCC production peaked in 1987 during the first week in June at an average of 548 yd3/h (419 m3/h) during two production shifts per day. Production from both plants during the entire month averaged 515 yd3/h (394 m3/h) over 59 shifts. The RCC batching and delivery system produced 266,000 yd3 (203,000 m3) in June 1987. The dam was topped out (达到坝顶高程)on August 8, 1987.Two ARAN ASR-200 continuous-mix pugmill plants were used successfully to produce 183,000 yd3 (140,000 m3) of RCC placed over 17 weeks at Copperfield Dam in Australia. Crushed alluvial gravel with a maximum size of 2.5 in (63 mm) was used as aggregate. Each plant produced a different mix, one with cement and fly ash for interior concrete and one with only cement for the exterior faces. Slow progress in jump-forming the 330-ft-long (100-m), central overflow spillway controlled RCC production rates on much of the dam. Hence, neither of the self-contained, mobile mixing plants was used to capacity although both were needed in order to produce the different mixes simultaneously. The average hourly output was 290 yd3 (220 m3) for the first plant and 234 yd3 (179 m3) for the other.A contractor-designed continuous-mix plant produced consistent, high-quality RCC at Monksville Dam. Mixer proficiency test results are shown in Fig. 8.4. The plant consisted of three 8-yd3 (6-m3) feed bins with adjustable-speed belts straddling a 36-in-wide (914-mm) main feed belt; a 75-ton (68-t) cement silo(筒仓) with aerators and one vane feeder（螺旋进料器）; a Davis pugmill with a capacity of 500 yd3/h(382 m3/h); a stacker belt; and an 18-yd3 (14 m3) two-stage surge bin.The plant could be operated manually or by a computer programmed to initiate the production sequence starting at the surge hopper(涌浪式加料斗). Sensing devices included a feed-belt scale（带称）; tilt gages on each feed bin; an electronic water flow meter; and a revolution counter（旋转式称量台） for metering cement through the ital display was available for real-time measurement of aggregate feed. For aggregate, cement, and water, the system provided cumulative data every 5 min.8.4 Transporting RCC Transport of RCC can be by scraper(铲土机) conveyor, bottom-and rear-dump trucks, large front-end loaders or a combination of these. Continuous, high-speed conveyors appear to be the most desirable method for large RCC jobs. Scrapers have worked well on most medium-size project