Concrete - 100 years ago a new building material and today regarded as indispensable - camejust in time to be used from the very beginning of the hydropower era in the World, aroundthe turn of the century. It soon became the preferred material for construction of dams andpower-houses, although the performance with regard to durability proved to be poor in many cases, giving rise for concern. Most prone to damage were dams at mountain sites, exposed toharsh climatic conditions. The problem was obviously one of lack of know-how in the field of concrete technology. An important factor in mix design is the ratio between water and binder.As long as only binder is cement, the term w/c ratio cannot be misunder-stood. With theincreasing use of pozzolana as binder in addition to the cement, the term w/b ration is now widely used. The term w/c ration as used in literature shall be interpreted to include pozzolana

Alban Kuriqi

Gerti Kocileri

Epoka University

International Students’ Conference of Civil Engineering, ISCCE 2012, 10-11 May 2012, EpokaUniversity, Tirana, AlbaniaPlacement of Concrete in Hydropower Structures According To ClimaticConditions in AlbaniaGerti Kocileri1, Alban Kuriqi11Department of Civil Engineering, EPOKA University, AlbaniaAbstractConcrete – 100 years ago a new building material and today regarded as indispensable – camejust in time to be used from the very beginning of the hydropower era in the World, aroundthe turn of the century. It soon became the preferred material for construction of dams andpower-houses, although the performance with regard to durability proved to be poor in manycases, giving rise for concern. Most prone to damage were dams at mountain sites, exposed toharsh climatic conditions. The problem was obviously one of lack of know-how in the field ofconcrete technology. An important factor in mix design is the ratio between water and binder.As long as only binder is cement, the term w/c ratio cannot be misunder-stood. With theincreasing use of pozzolana as binder in addition to the cement, the term w/b ration is nowwidely used. The term w/c ration as used in literature shall be interpreted to includepozzolana.Keywords: Concrete,pozzolana,Durability,permeability,water tightnessINTRODUCTIONThe type of structure may influence mix design. Large massive structures pose otherrequirements than slender ones; mixes for heavily reinforced structures will be different fromsparsely reinforced ones. Examples are:Massive StructuresToo high temperature gradients and shrinkage, both resulting in cracks, are problemsassociated with massive structures. Internal temperature rises due to the heat developed duringhydration of the cement. As a rule of thumb, temperature gradients larger than 20°C over adistance of 300mm or less may result in cracks. The maximum temperature should also bekept below a certain limit, often set to around 65°C.Measures to reduce these problems are:- keeping contents of cement paste as low as possible. Result – stricter requirements forproper composition of the aggregate.- Using a low heat cement or replacing part of the cement with pozzolanic materials,normally pulverized fly ash. Silica fume is less efficient as temperature reducer, sincethe associated chemical reactions start early and develop heat. If a certain amount ofcement is replaced by a smaller amount of silica, however, the total result is reducedheat development.International Students’ Conference of Civil Engineering, ISCCE 2012, 10-11 May 2012, EpokaUniversity, Tirana, Albania- Cooling the mixing water with ise. This operation requires proper equipment andprocedures to ensure that all ice is converted to evenly distributed water beforeconcreting.For structure where temperature development must be controlled, test measurements shouldbe made. Massive structure will not be heavily reinforced, hence larger stone sizes and stiffconsistency may be used (see below).Slender, Heavily Reinforced Structures. The mix for slender structures with densereinforcement requires smaller stone sizes, and a plastic or flowing consistency to allowproper filling of the form.Early form removal is often requiring for such structural. This need requires rapid strengthdevelopment, and rapid hardening cement, reduced w/c ratio, or higher cement contents maybe considered.GENERAL REQUIREMENTS TO CONCRETE PROPERTIESConcrete dams are formed of cement-concrete placed in the dam body (Figure. 1.0). Concretedam section designed such that the loading produces compression stress only and no tensionare induced anywhere.The reinforcement is minimum mainly as temperature control. Concrete is placed in twoways: Reinforced concrete dam (RC dam) or Roller compacted concrete (RCC) dams. Thevariations of concrete dam include:1. Concrete gravity dam,2. Concrete arch dam and arch-gravity damFigure. 1.0 Type of Concrete DamInternational Students’ Conference of Civil Engineering, ISCCE 2012, 10-11 May 2012, EpokaUniversity, Tirana, Albania3. Multiple arch dam4. Double curvature or dome/cupola dam5. Buttress dam (head as diamond, roundhead, massive, decked etc)6. Hollow gravity dam7. Brick or rock masonry gravity damRubble/random/stone masonry to fill dam section. Concrete / mass concrete as bulk materialin dam section with steeper side slope. RCC section to take loadings, thus decrease section.Gravity dam: Stability due to its mass. Dam straight or slightly curved u/s in plan (no archaction). The u/s face is vertical or nearly vertical, d/s sloping.Consistency corresponding to the transport and casting methods used, and to the possibilitiesand methods of compacting. Concrete in dams can be placed by cranes – as opposed toconcrete in tunnels which is usually pumped and therefore requires high binder content forgood pump ability. For the slender slab and buttress dams, which are also heavily reinforced,consistency must be adjusted correspondingly.CRITERIA FOR SPECIFYING PROPERTIESConcrete strength class, as determined from requirements resulting from structural analysis,has been the predominant criterion for specifying concrete quality even when other properties,such as water tightness, are greater interest. This criterion may lead to disputes in practice,when the specified strength is incompatible with other requirements to the concrete.The situation has now changer, with growing awareness that long term durability shouldreceive greater attention. The first steps should be to establish what kind of deterioratingforces or mechanisms can be expected in the accrual environment for the structure. Suchphenomena may be deterioration of the concrete by aggressive agents, or deterioration of thestructure due to corrosion of the reinforcement. The investigation will translate into basicminimum requirements for the cement, for the w/c ratio and for the concrete, which will oftenrender a strength class which is higher than the one initially required or deemed necessaryfrom the structural analysis.Low permeability is a prerequisite for durability, w/c ratio; compaction and curing determinepermeability and also strength. Check of concrete strength can therefore continue to be a goodof concrete quality, once there is awareness of what really matters.CLASSIFICATION OF ENVIRONMENTThe Norwegian standards operate with four classes of environment based on expected degreeof aggressiveness and specify a maximum w/c ratio and recommended minimum cementcontent for each. The necessary and advisability of specifying minimum cement contents aredisputed. For ordinary structures, cement contents will as a rule is higher than therecommended minimum. For mass concrete such as gravity and arch dams, large foundationsetc…, lower cement contents are used in order to reduce temperature rise.The w/c ratio, however, is a direct indicator of the potential permeability of the cement pasteand of the concrete, and a key factor for durability.The four on environment are defined as follows:Mildly aggressive environment –LAIn practice, the LA class refers to indoor structures in a dry, non-aggressive atmosphere –offices, industry that emit no aggressive gases or aggressive liquid spill, dwellings etc.Moderately Aggressive Environment –NAInternational Students’ Conference of Civil Engineering, ISCCE 2012, 10-11 May 2012, EpokaUniversity, Tirana, AlbaniaThe NA class refers to ordinary, outdoor structures subject to normal levels of carbon dioxideand other gases in the atmosphere, normal acidic or contaminated precipitation, and structuresin fresh water non subject to freezing, indoor structures in a humid atmosphere.Severely Aggressive Environment – MAStructures subject to salts, chlorides, other aggressive agents, freezing thawing in fresh orsaline water, high levels of carbon dioxide or highly contaminated atmosphere.Especially Aggressive Environment –SAThis class refers to especially aggressive environment involving severe exposure to acids orother aggressive agents. Membranes to protect the concrete surface will be necessary.The requirements according to Norwegian standards are as shown in Table 1.0Table 1.0 Requirement according to Norwegian standardsClass Maximum ratiow/(c+Σk*p)1Recommended minimumBinder contentsC+Σk*p 1 kg/m3Mildly aggressive , LAModerately aggressive, NASeverely Aggressive, MaSpecially Aggressive, Sa0.9220.600.45Special225250300Treatmentp= weight of pozzolana if used. k= efficiency factor, with respect to the property in question.this figure is given in the NS 34201. It could probably have been set lower. In practice, atleast, the w/c ratio should never come close to this value structural concrete.As mentioned above, massive structure often contain substantially less cement per m3 in orderto reduce temperature rise. Binder contents as low as 170 kg/m3 are therefore acceptable formass concrete under certain conditions, e.g for the interior of mass structures, while theexposed surface concrete of the same structure may have higher binder content.The standards are aimed primarily at structural, reinforced concrete, and cannot be applied toorigidly to all hydropower structures. Hydropower structures will normally be in environmentsclassified as moderately aggressive, NA, or severely aggressive if subject to freezing whenwet. Concrete for buildings and structures exposed to no particular wear, without specialrequirements, might be specified with w/c ratio up to the maximum permissible for the NAclass, 0.60.According to the recommendations in the standards, all structures subject to freezing whenwet, e.g. all Norwegian concrete dams should be classified for Severely Aggressiveenvironment, with w/c ratio ≤ 0.45. This requirement has been modified in practice. Slender,reinforced dams, walls or watertight structures are classified as MA, while the w/c ratio formore massive structures may be in the range 0.55-0.45; this is accepted by the Norwegianpublic controlling body, the Safety and Emergency Planning Department of the NVE.1Civil engineering  Standard collection for concrete structureInternational Students’ Conference of Civil Engineering, ISCCE 2012, 10-11 May 2012, EpokaUniversity, Tirana, AlbaniaAMBIENT TEMPERATUREBoth high and low ambient temperature will affect concreting and design of the mix.When concreting is cold weather (below freezing) it must be ensured that:- The fresh concrete will not freeze- The internal temperature will be high enough to allow hydration to start- The strength development will be sufficiently rapid to make the concrete frost resistantbefore internal temperature falls below freezing.Part of measure will coincide with those necessary for early form removal – rapid hardeningcement, low w/c ratio and high cement content. Hot mixing water and heating of aggregatemay be required to get a sufficiently high temperature of the fresh concrete. In addition,insulation of the form to prevent freezing in the early stages may be required.When concreting in hot weather (tropical zones, or hot summer days in temperate zones), itmust be ensured that the fresh concrete temperature is so low that the maximum concretetemperature during hydration will not rise above 65°C. Cooling of the mixing water and of theaggregate may then be necessary. Aggregate storage piles should be shielded against sunlight,and also sprayed with water if there is risk o their completely drying out. The site ofconcreting must be shielded against direct sunlight. When casting massive structures forwhich temperature rise may be a problem, concreting should be limited to periods of theday/night when the temperature is below 20-30°C.Hot weather and high temperature of the fresh concrete will accelerate setting. This must beconsidered when determining consistency, use of admixtures and available time fortransporting, casting and compacting the concrete. The properties of fresh concrete with silicaare notably affected by the temperature. The effect of admixtures will also vary with thetemperature.LARGE GRAVITY DAMSWhile the majority of large gravity dams in Albania as built after 1950 have suffered from thelack of concrete technology know-how at the time and have performed poorly, there are someexceptions from the rule. Ulza dam as shown in figure (1.1) a 50 m high gravity structure onriver Mati, is perhaps the most conspicuous example of a professional concrete job from thisearly age. Another concrete gravity dam constructed on early age in Albania is also shkopetidam. Shkopeti Dam as shown in figure (1.2) was built from 1958 to 1963 on the Mati Riverdownstream of the Ulze project and about 25km from the mouth of the Mati at the AdriaticSea. It is a concrete gravity dam reaching a maximum height of 52m with a crest length of89m. The powerhouse is 250m downstream of the dam. It was rehabilitated during the 1990s.A project is underway to equip Shkopeti with a dam monitoring and alarm system whilepreparing an emergency action plan.International Students’ Conference of Civil Engineering, ISCCE 2012, 10-11 May 2012, EpokaUniversity, Tirana, AlbaniaFor today’s large gravity dam’s strong efforts are made to keep down the curing temperatureof the mass concrete through measures such as:- Use of a special low-heat, slow–curing cement type – the so-called” Dam cement”- Mixes that are lean in cement (150-200 kg/m3) and rich in silica fume (10-15 kg/m3).- Zoning of the dam cross section, with a core that is extra lean in cement between thesurface zones.- Use of cool aggregates and cold water, or ice, in the mix.Fig. 1.1 Ulza concrete gravity damFig. 1.2 Shkopeti concrete gravity damInternational Students’ Conference of Civil Engineering, ISCCE 2012, 10-11 May 2012, EpokaUniversity, Tirana, AlbaniaFor the all – important compaction process bulldozers with vibrator attachment areused. This procedure requires concrete of very stiff consistency.Normal height between concreting joints is 3 and the distance between vertical jointsapproximately 15 m. All horizontal joints are washed before setting – using air/waterjets to remove cement laitance.Design conditions and stability requirements, vertical joints etc, deviate very little from whatis used for small structures.CONLUSIONRelated to albania climate condictions also acording to long masurements of hydrologicalparameters like:humidity,wind speed,tension of evaporation and temperature that the mostimportant climate parameter for to define desing criteria for hydraulics structure ingeneraly,there are not such extremaly figure masure until nowadays.Minimum and maximutemperature achived are aproximatly -24 to 41,that means in albania and aspecialy in thosesite where are cunstructed or are planed to construct different hydraulic structures no specialcement and others aditive are required to use in concrete mix desing to improve or to increaseand decrease temperature of reaction between gradients of concrete when we place it onstructure.REFERENCES[1] High Strength Concrete in Norway. Utilization and Research. Utilization of HighStrength Concrete – Symposium in Lillehammer. Norway, June 20-23 1993.[2] Concrete in Hydropower Structure – Hydropower development, Published by NorwegianInstitute of Technology, Division of Hydraulic Engineering, N-7034 Trondheim, Dagfinna K.Lysne Professor.[3] Manual of Concrete Practice. Published annually by the American Concrete Institute(ACI)[4] Concrete Technology. 4th edition Nov. 1992. Pitman Publishing Limited. London.[5] Standart Specification for Blended Hydraulic Cements[6] Consultancy Service for the Dam Safety Survey for Hydropower Plants located on Drinand Mati River Cascades, Republic of Albania. DRAFT FINAL REPORT, Publishing byElectrowatt-Ekono AG, March 01, 2006.[7] Dam and Reservoir Engineering, book, Published by TARIQ. 2008

Placement of Concrete in Hydropower Structures According To Climatic Conditions in Albania

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Placement of Concrete in Hydropower Structures According To Climatic Conditions in Albania

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