1、45.1CHAPTER 45CONCRETE DAMS AND SUBSURFACE SOILSCONCRETE DAMS . 45.1Methods of Temperature Control. 45.1System Selection Parameters. 45.3CONTROL OF SUBSURFACE WATER FLOW 45.3SOIL STABILIZATION. 45.4Thermal Design 45.4Passive Cooling 45.4Active Systems 45.5EFRIGERATION is one of the more important to
2、ols of theRheavy construction industry, particularly in the temperaturecontrol of large concrete dams. It is also used to stabilize both water-bearing and permanently frozen soil. This chapter briefly describessomeofthecoolingpracticesthathavebeenusedforthesepurposes.CONCRETE DAMSWithout the applica
3、tion of mechanical refrigeration during con-struction of massive concrete dams, much smaller constructionblocks or monoliths would have to be used, which would slow con-struction. By removing unwanted heat, refrigeration can speed con-struction, improve the quality of the concrete, and lower the ove
4、rallcost.METHODS OF TEMPERATURE CONTROLTemperature control of massive concrete structures can beachieved by (1) selecting the type of cement, (2) replacing part ofthe cement with pozzolanic materials, (3) using embedded coolingcoils, or (4) precooling the materials. The measures used depend onthe si
5、ze and type of structure and on the time permitted for its con-struction.Cement Selection and Pozzolanic AdmixturesThe temperature rise that occurs after concrete is placed is dueprincipally to the cementing materials heat of hydration. This tem-perature rise varies directly with cement content per
6、unit volumeand, more significantly, with the type of cement. Ordinary Portlandcement (Type I) releases about 180 Btu/lb, half of which is typicallygenerated in the first day after the concrete is placed. Depending onspecifications, Type II cement may generate slightly less heat. TypeIV is a low-heat
7、 cement that generates less heat at a slower rate.Pozzolanic admixtures, which include fly ash, calcined clays andshales, diatomaceous earths, and volcanic tuffs and pumicites, maybe used in lieu of part of the cement. Heat-generating characteristicsof pozzolans vary, but are generally about one-hal
8、f that of cement.When determining system refrigeration load, heat release datafor the cement being used should be obtained from the manufac-turer.Cooling with Embedded CoilsIn the early to mid-1900s, the heat of curing on large concretestructures was removed by embedded cooling coils for glycol orwa
9、ter recirculation. These coils also lowered the structures temper-ature to its final state during construction. This is desirable wherevolumetric shrinkage of a large mass is necessary during construc-tion (e.g., to allow the contraction joint grouting of intermediateabutting structures to be comple
10、ted).In an embedded-coil system, thin-wall tubing is placed as a grid-like coil on top of each 5 or 7.5 ft lift of concrete in the monoliths.Chilled water is then pumped through the tubing, using a closed-loop system to remove the heat. A typical system uses 1 in. OD tub-ing with a flow of about 4 g
11、pm through each embedded coil.Although the number of coils in operation at any time varies withthe size of the structure, 150 coils is not uncommon in larger dams.Initially, the temperature rise in each coil can be as much as 8 to10F, but it later becomes 3 to 4F. An average temperature rise of6F is
12、 normal. When sizing refrigeration equipment, the heat gain ofall circuits is added to the heat gain through the headers and con-necting piping.For a typical system with 150 coils based on a design tempera-ture rise of 6F in the embedded coils and a total heat loss of 6Felsewhere, the size of the re
13、frigeration plant is about 300 tons. Fig-ure 1 shows a flow diagram of a typical embedded-coil system.Cooling with Chilled Water and IceThe actual temperature of the mix at the time of placement hasa greater effect on the overall temperature changes and subsequentcontraction of the concrete than any
14、 change caused solely by vary-ing the heat-generating characteristics of the cementing materials.Further, placing the concrete at a lower temperature normally re-sults in a smaller overall temperature change than that obtainedwith embedded-coilcooling.Becauseof theseinherentadvantages,precooling mea
15、sures have been applied to most concrete dams.The preparation of this chapter is assigned to TC 10.1, Custom-EngineeredRefrigeration Systems.Fig. 1 Flow Diagram of Typical Embedded Coil SystemFig. 1 Flow Diagram of Typical Embedded-Coil System45.2 2010 ASHRAE HandbookRefrigerationGlen Canyon Dam ill
16、ustrates the installation required. The con-crete was placed at a maximum placing temperature of 50F duringsummer, when the aggregate temperature was about 87F, cementtemperature was as high as 150F, and the river water temperaturewas about 85F. Maximum air temperatures averaged over 100Fduring the
17、summer. The selected system included cooling aggre-gates with 35F water jets on the way to the storage bins, addingrefrigerated mix water at 35F, and adding flaked ice for part of thecold-water mix. Subsequent cooling of the concrete to temperaturesvarying from 40F at the base of the dam to 55F at t
18、he top was alsorequired. The total connected brake power of the ammonia com-pressors in the plant was 6200 hp, with a refrigeration capacityequal to making 6000 tons of ice per day.The maximum amount of chilled water that may be added to theconcrete mix is determined by subtracting the amount of sur
19、facewater from the total mix water, which is free water. Frequently, if achemical admixture is specified, some water (usually about 20% ofthe total free water) must be added to dissolve the admixture. Thislimits the amount of ice that can be added to the remaining 80% offree water available. After t
20、he amount of ice needed for cooling isdetermined, the size of the ice-making equipment can be fixed.When determining equipment capacity, allowances should also bemade for cleaning, service time, and ice storage during nonproduc-tive times.When calculating heat removal, consider ice to be 32F whenint
21、roduced into the mixer. Chilled water is assumed to be 40Fentering the mixer, even though it may be supplied at a lower tem-perature.Cooling by InundationThe temperatures specified today cannot be achieved solely byadding ice to the mix. In fact, it is not possible on heavy constructionof this type
22、(in view of low cement content and low water/cementratio specified) to put enough ice in the mix to obtain the specifiedtemperatures. As a result, inundation (deluging or overflowing) ofaggregates in refrigerated water was developed and was one of thefirst uses of refrigeration in dams.When aggregat
23、es are cooled by inundation with water, generallythe three largest sizes are placed in large cylindrical tanks. Nor-mally, two tanks are used for each of the three aggregate sizes toprovide back-up capacity and a constant flow of materials. Coolingtanks, loaders, unloaders, chutes, screens, and conv
24、eyor systemsfrom the tanks into the concrete plant should be enclosed and cooledfrom 45 to 40F by refrigeration units, with blowers placed atappropriate points in the housing around the tanks and conveyors.Peugh and Tyler (House 1949) determined the inundation (orsoaking) time required by calculatio
25、ns and actual tests. Pilot testscorroborated their computations of aggregates cooling times asindicated in Table 1.This study indicated that immersion for about 40 min brings theaggregate down to an average temperature of 40F. Theoretically,smaller sizes can be brought to the desired temperature in
26、less time.Considerations such as the rate at which cooling water can bepumped make it unlikely that a cooling period less than 30 minshould be considered. Any excess cooling provides the neededsafetyfactor.However,thelimitingfactorontheoverallcycleisthecooling time for the largest aggregate, which i
27、s nearly 45 min, plusabout 15 min for loading and unloading. Back-up capacity shouldbe considered for maintaining a constant flow of materials.Air-Blast CoolingAir-blast cooling, a more recent development than inundation,does not require particular changes to material handling or addi-tional tanks f
28、or inundation; instead, cold air is blown through theaggregate in batching bins above the concrete mixers. Also, the aircycle used in cooling can be used in heating aggregates duringcold weather. The aggregate is cooled during the final stage ofhandling; this does not increase the moisture content.T
29、he compartmented bins where air-blast cooling usually occursare generally sized so that if any supply breakdowns occur, the mix-ing plant will not have to shut down before a particular pour can becompleted. On this basis, the average concrete octagonal bin abovethe mixing plant on a large job holds
30、at least 600 yd3, which is usu-ally more than adequate to allow time for air cooling. However, cer-tain minimum requirements must be considered. If possible, basedon a 1 h loading and cooling schedule, at least 2 h of storage volumeshould be provided for each size aggregate that air will cool. Themi
31、nimum volume should be 1.5 h plus cycling time, based on thetables shown for cooling 6 in. aggregate.The bin compartment analysis shown in Table 2 may be used asa starting point in determining the air refrigeration loads and staticpressures. This type of analysis should give approximately equalstora
32、ge periods for each aggregate size used. In practice, after air-cycle cooling is calculated, more air volume is needed in the smalleraggregate compartments, and less in the sand and aggregate, toobtain maximum cooling. This is because of the higher air resis-tance in the smaller aggregate sections a
33、nd the fact that air does notcool the sand compartment as effectively.ComputingAir-BlastCoolingLoads. To calculate the requiredcooling, several assumptions must be made:Assumption 1. Normally, the lowest temperature of air leavingthe cooling coils is 38 to 40F. Lower air temperatures may beachieved,
34、 but should not be trusted: a temperature lower than 35Fusually causes rapid frosting of the coils.Assumption 2. Heat transfer between the aggregate and air is only80to90%effective.Toallowforairtemperatureriseintheducts,heatleakage, pressure drop, etc., an empirical factor of 85% may be used.Table1
35、TemperatureofVariousSizeAggregatesCooled by InundationTime,min.Aggregate Size, in.6 3 1.5 0.75 0.3751 85F 69F 49F 39F 38F2 77 59 41 38 (42)5 66 46 (45) 38 (42)10 56 40 (42)15 50 3820 4625 4430 4240 4050 39Source: Peugh, V.L. and I. Tyler. 1934. “Mathematical theory of cooling concreteaggregates.” In
36、 House (1949). Numbers in parentheses are from tests by R. McShea.Note: Temperatures listed are at cobble center with assumed thermal diffusivity of0.07 ft2/h. Aggregate initial temperature is 90F and cooling water temperature is35F.Table 2 Bin Compartment Analysis for DeterminingRefrigeration Loads
37、 and Static PressuresMaterialSize, in. lb/yd3%ofTotalNo. of BinCompartmentsBin%Stones6 to 3 800 21.33 4 25.003 to 1.5 700 18.67 3 18.751.5 to 0.75 650 17.33 2 12.500.75 to 0.25 600 16.00 3 18.75Sand 1000 26.67 4 25.00Total 3750 100.00 16 100.00Note: In practice, relative sizes vary; amounts shown ar
38、e for an assumed design mix ofprincipal classes of concrete. On any given job, several design mixes requiring differ-ent amounts for each size are needed.Concrete Dams and Subsurface Soils 45.3Thus, with 80F aggregate and 40F cooling air, the effective tem-perature differential would be 0.85(80 40)
39、= 34F.Assumption 3. The rise in temperature of air passing through theaggregate compartment normally will not exceed 80% of the differ-ence between the entering aggregate and the entering air tempera-tures.Thuswith80Faggregateand40Fair,thetemperatureriseofthe air is 0.80(80 40) = 32F. The maximum te
40、mperature of thereturn air is 40 + 32 = 72F.Assumption 4. An allowance should be included for heat leakageinto air ducts on the aggregate bin sides, normally about 2% of atotal air-blast cooling load.Another important consideration is the static pressure against airflowing through a body of aggregat
41、es. The resistance to air flowingthrough a bin varies as the square of the air volume or velocity; thisis summarized in Table 3. The resistance pressure listed is for a unitheight of aggregate. To use values in Table 3, the cross-sectionalarea of the aggregate compartment and the height of the aggre
42、gatecolumn must be known. Manufacturers of concrete mixing bins andmixing equipment can supply this information.Other Cooling MethodsAs specifications require lower and lower placing temperatures,direct methods of cooling sand and cement have been attempted toobtain or lower the heat removal require
43、d. No method has beenproven to cool cement. Sand-cooling methods that have been triedand found to be unsuccessful areWater inundation. The increase in free moisture content ofbatched sand makes correctly proportioning the mix difficult.Moving sand through screw conveyors with hollow flights.Chilled
44、water was pumped through the flights, but because com-ponents were cooled below the dew point, resulting in condensa-tion, serious handling problems occurred.Vacuum systems, which evaporate surface moisture to reduceaggregate temperature. The unreliability of the equipment and thebatch nature of the
45、 process precluded success.An alternative method of cooling sand, air cooling, is being triedon some projects, but it is too early for final results of this method.For small pours, liquid nitrogen is sometimes used to reducethe temperature of the mixture to the final pour temperature.Nitrogen is use
46、d because the initial capital costs are considerablylower than a mechanical system, but because of the high cost ofmanufacturing nitrogen, the operation cost is much greater than amechanical system. Cost comparison must be done on a case-by-case basis.SYSTEM SELECTION PARAMETERSFor larger installati
47、ons, plant selection depends on a number offactors, including the following:Normal pouring rate, yd3/h (contractor or contract specified).Maximum pouring rate, yd3/h (contractor or contract specified).Total allowable mixing water, lb/yd3(usually contract specified).Required concrete placement temper
48、ature (usually contract spec-ified).Concrete temperature when coming from the mixer (to be deter-mined considering materials handling to placement site, time intransit, and storage at placement site).Average ambient temperature and aggregate temperature duringperiod of maximum placement. The average
49、 ambient temperatureof the aggregates is assumed to be the mean ambient temperature(including night and day) during the period of storage, which isdetermined by the amount of storage capacity provided by thecontractorandtherateofconcreteplacement.Iftheminimumlivestorage is, for example, 100,000 tons of aggregates and the pour-ing rate is 2000 yd3/day/day, consumption is approximately3800 tons of aggregates per day. If the job is working a 6 dayweek, this provides 26 days of storage. Unless weather conditionsare unusual, the temperature of rock delivere
