Designing Roller compacted concrete (RCC) dams

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DEGREE PROJECT IN

CIVIL ENGINEERING AND URBAN MANAGEMENT

STOCKHOLM, SWEDEN 2018

Designing Roller compacted

concrete (RCC) dams

SHAYMA AL BAGHDADY

LINNEA KHAN

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Designing Roller compacted

concrete (RCC) dams

S

HAYMA

A

L

B

AGHDADY

L

INNEA

K

HAN

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TRITA-ABE-MBT-18366, 2018 ISBN 978-91-7729-869-4

KTH School of ABE SE-100 44 Stockholm SWEDEN © Shayma Al Baghdady & Linnea Khan 2018

Royal Institute of Technology (KTH)

Department of Civil and Architectural Engineering Division of Concrete Structures

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Abstract

Concrete is the most common building material in the world and it consists of aggregates, cement and water that harden over time, it is also known as a composite material. The use of concrete is very versatile due to its resistance to wind and water and its ability to withstand high temperature. These qualities make concrete a suitable building material for large structures such as dams.

A dam is a huge construction that needs massive amount of concrete to build it with and that leads to high cost, so alternative methods should be considered to minimize the cost of constructing the dams. One method is building the dams with Roller Compacted Concrete (RCC), which by definition is a composite construction material with no-slump consistency in its unhardened state and it has achieved its name from the construction method. The definition for a no-slump consistency is a freshly mixed concrete with a slump less than 6 mm. The RCC is placed with the help of paving equipment and then it is compacted by vibrating roller equipment. The RCC ingredients are the same as for the conventional concrete but it has different ratios in the materials that are blended to produce the concrete. It differs when it comes to aggregates because both similar aggregates used in conventional concrete or aggregates that do not fulfill the normal standards can be used in the RCC mixtures. This means, for example that aggregates found on the construction site can be used for the RCC. Compared to when constructing a conventional concrete dam, which is usually built in large blocks, the RCC dam are usually built in thin, horizontal lifts, which allows rapid construction. This reduces the amount of formwork, but also the demand for man-hours are less due to the usage of machines for spreading and compacting, ultimately making it a cheaper method. Building with RCC has become very popular around the world because of its advantages and new methods have been developed over the past two decades, adapted to the experience gained after each project. All RCC dams that has been built, usually faces challenges both during and after construction, and it includes everything from temperature variations, cracks to leakage.

The main purpose of this master thesis is to create a guideline for how to design and construct dams with RCC and the idea is to be able to use it as a basis for future dams. The requirements of Eurocode 2 and RIDAS are the basis of the criteria that the dam must fulfill and information of what is expected of the RCC is presented in this thesis. Furthermore an example for design of an existing embankment dam to an RCC dam has been presented in this thesis. The embankment dam needs to be rebuilt in order to increase the safety of the dam and the goal of the case study was to determine the dimensions of the new RCC dam.

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Sammanfattning

Betong är det vanligaste byggmaterialet i världen och det är ett material som består av ballast, cement och vatten som härdas över tiden, även känt som ett komposit material. Användningen av betong är mycket mångsidig tack vare dess motståndskraft mot vind och vatten och dess förmåga att motstå höga temperaturer. Dessa egenskaper gör betong ett lämpligt byggmaterial för stora strukturer som dammar.

En dam är en enorm konstruktion som kräver massiva mängder av betong för att bygga den med och det leder till höga kostnader, därför bör alternativa metoder övervägas för att minimera dessa. Ett förslag till en metod är att bygga dammar med Roller Compacted Concrete (RCC), som per definition är ett komposit material med ett sättmått på mindre än 6 mm i sitt ohärdade tillstånd. RCC har erhållit sitt namn från sin byggmetod, då den sprids med hjälp av utrustning för att lägga vägar och sedan kompakteras den med en traktordriven vibratorvält. Ingredienserna för RCC är samma som för konventionell betong, men den stora skillnaden utgörs av att det är olika mängd-förhållanden av de material som blandas för att producera denna betong. Det skiljer sig också när det gäller ballasten, eftersom både liknande ballast som används i konventionell betong eller ballast som inte uppfyller de normala standarder kan användas för RCC. Det betyder att exempelvis, ballast som man erhåller på byggarbetsplatsen kan användas för att producera RCC.

I jämförelse med när man bygger en traditionell betongdamm, som vanligen byggs i stora block, så bygger man oftast en RCC damm i horisontella lager vilket ger möjligheten för snabbt byggande. Detta reducerar behovet av att använda gjutformar, men även antalet mantimmar på grund av användningen av maskiner för spridning och kompaktion. De här faktorerna gör det till en billigare metod. RCC dammar har blivit populärt att bygga runt om i världen på grund av dess fördelar och nya metoder har utvecklats under de senaste 20 åren anpassade efter erfarenheten man har erhållit efter varje projekt. Alla RCC dammar som byggts stöter ofta på utmaningar både under och efter byggandet och det har med, allt från temperatur variationer, sprickor, och läckage, att göra.

Huvudsyftet med det här examensarbetet är att skapa en guide för hur man designar och bygger en RCC damm och tanken är att man ska kunna använda den som en grund för framtida dammbyggen. Kraven från Eurokod 2 och RIDAS är grunden för kriterierna som dammen ska uppfylla och information om vad som förväntas av RCC är presenterat. En fallstudie har gjorts, där ett exempel på en design för en RCC damm som ska ersätta en befintlig fyllningsdamm i Hylte är presenterad. Fyllningsdammen är i behov av ombyggnation för att höja säkerheten av dammen och målet med fallstudien är att avgöra dimensionerna för den nya RCC dammen som ska placeras där.

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Preface

This is a Master of Science thesis written at the Division of Concrete Structures, Department of Civil and Architectural Engineering at KTH Royal Institute of Technology during the period of January-June 2018. Dr. Richard Malm, KTH, supervised the thesis subject.

We would like to thank Dr. Richard Malm for the support and guidance he has given us during this period. We would also like to express our gratitude to adj. Erik Nordström for providing us with guidance and material for our case study.

Stockholm, June 2018

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1 Introduction

1.1 Background

Concrete is the most common building material in the world and it is a composite material. It consists of a mixture of coarse aggregates, water and cement that hardens over time. The use of concrete is versatile due to its resistance to wind and water and its ability to withstand high temperature (PCA, 2018). These qualities make concrete a suitable building material for large structures such as dams and hydropower plants.

The main reasons for building dams in today’s society are water supply, irrigation, flood control, navigation, sedimentation control and hydropower. Most of the dams that have been constructed are single-purposed, where irrigation and hydropower are the most common ones. There are also multipurpose dams, which are increasing in number especially in developing countries. The importance of these projects for the developing countries is a great deal, due to a single investment contributes to economic and domestic benefits for the population. (ICOLD, 2017)

There are different types of dams, and in Sweden the most common ones are embankment-and concrete dams, see Figure 1.1. Embankment dams are typically referred as “rock fill” or “earth fill” dams depending on the material used. Commonly used materials are natural rock, soil or waste materials which are obtained from mining operations. Concrete dams are categorized in three common types: arch, gravity and buttress. See Figure 1.2, 1.3 and 1.4. The main difference is the way they are designed. The arch dam is somewhat thin in the cross-section and it resembles a part of an ellipse or a circle. The gravity dam is built up of vertical concrete blocks that are joined together by seals in the joints. The buttress dam is also vertical concrete blocks but with reduced concrete mass. (ASDSO, 2017)

Figure 1.1 Example of an embankment dam, consisting of (1) clay, (2) a drainage layer, (3) gravel, (4) (5) stone, and (6) a drainage well. (Vattenkraft.info, 2009)

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Figure 1.2 Gordon Dam, Tasmania, an arch dam. (Wikiwand, 2018)

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Figure 1.4 A buttress dam in the village Rätan. (Vattenkraft.info, 2009)

A concrete dam is a large construction that needs massive amount of concrete, resulting in the use of a large amount of cement. This leads to both a high cost and a negative impact on the environment due to CO2 emissions from cement production. Therefore alternative methods

should be considered to minimize the cost of constructing the dams and to minimize the use of cement. One method is building the dams with Roller Compacted Concrete (RCC), which is a concrete that is compacted by vibrating roller equipment. RCC ingredients are the same as the conventional concrete but it has different ratios in the materials that are mixed to produce the concrete. It is known for its rapid construction method. An example of an RCC dam is shown in Figure 1.5.

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Figure 1.5'RZQVWUHDPIDFHRI%H\GD÷ dam

1.2 Purpose and limitations

The purpose of this Master thesis is to develop a guideline for how to design and construct a roller compacted concrete dam in Sweden, which may be of use in the future. This thesis will bring all the basic knowledge needed about RCC and bring up critical parts for when designing a dam. The research questions that have governed this thesis are:

What are the differences in design of conventional concrete dams and roller compacted dams?

Would it be possible to build a roller compacted dam in the Sweden, taken into account the cold Swedish climate?

Which design standards are used for RCC dams internationally?

This thesis will not include a life-cycle analysis; however, it will be discussed how CO2

emissions from RCC affects the environment. A life-cycle cost will not either be considered in this thesis. The case study, which is performed in this thesis, will not include a cross-section analysis. It will only consider stability calculations.

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1.3 Scope of the thesis

This thesis begins with a theory chapter, Chapter 2, which provides what materials are chosen and used; along with what properties are expected for the roller compacted concrete.

Chapter 3 brings up the typical challenges with this type of dam during and after construction, giving example of dams that had similar challenges.

Chapter 4 constitutes the design of the roller compacted concrete, with criteria’s of the cross-section and stability presented from Eurocode 2 and RIDAS, which is the Swedish guideline for dam safety.

Chapter 5 constitutes the case study, where a dam is designed, with the help of stability calculations, on a given topography.

Chapter 6 the discussion is presented, bringing up the possibility of constructing RCC dams in Sweden.

Chapter 7 presents the proposed future study.

Appendix A provides the full calculations that were made for the case study in Chapter 5 and the inputs in CADAM.

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2 Roller compacted concrete (RCC)

and dams

2.1 Definition of RCC

Roller compacted concrete, also known as RCC, is a composite construction material with no-slump consistency in its unhardened state. It has achieved its name from the construction method where the RCC is placed with the help of standard or high-density paving equipment and then it is compacted or consolidated with rollers. The definition of a no-slump concrete is a freshly mixed concrete with a slump less than 6 mm, where the slump is the difference between the height of the mould and the highest point of the specimen, see Figure 2.1 (Maxi, 2017).

Figure 2.1 The consistency of the concrete is tested with the help of the slump test (Maxi, 2017)

This consistency allows that the following lifts can be placed directly after a previous lift has been compacted. Compared to conventional concrete, the materials that are used for the RCC are usually of a wider range. When mixing the RCC the philosophy is to use adequate paste volume to fill the aggregate voids, without using more water than is needed for a decent workability. (USACE, 2000)

The hardened RCC and conventional concrete have similar properties, when it comes to durability, and RCC can therefore be used for building dams. Constructing RCC dams has become immensely popular throughout the world due to the advantages it comes with. The main advantages are the rapid construction process, reduced costs and smaller environmental impact due to less cement. (ACI, 2011)

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2.2 Material

2.2.1 Selection of material

2.2.1.1 Cementitious materials

Cement is one of the key components in concrete production and the most used type is the Portland cement. Cement consists of a mixture of limestone and clay which is then heated, producing a substance called “clinker” that contains calcium, silicate, alumina and iron oxide (PCA, 2018). There are different types of cement depending on the concretes application and these are listed in Table 2.1.

Table 2.1 Types of Portland cement and their general features (Jennings H et al., 2010)

Portland cement type Description Applications

Type I Normal General construction (most

buildings, bridges,

pavements, pre-cast units etc.) Type II Moderate sulfate resistance Structures exposed to soil or

water containing sulfate ions. Type III High early strength Rapid construction, cold

weather concreting. Type IV Low heat hydration (slow

reacting)

Massive structures such as dams. (Nowdays rare.) Type V High sulfate resistance Structures exposed to high

levels of sulfate ions.

When selecting type of cement for RCC and conventional concrete, some factors must be taken into account such as; the quality and the type of the cement, how it reacts with pozzolan, the manufacturer’s capability to deliver sufficient quantities and the delivery cost to site.

The most commonly used cement in the RCC mixture is the type II due to the low heat generation at early ages and the longer set times, which may lead to control or reduction of thermal cracking. Generally for cements with low heat generation, the development of strength is slower than e.g. for cements of type I and type III. (USACE, 2000)

Cement in RCC mixtures can be partially replaced with pozzolan for the following reasons (ACI, 2011):

1. To reduce heat generation; 2. To reduce costs;

3. To be used as supplemental fines in the mixture; 4. Reduce CO2emissions.

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Natural pozzolans and fly-ash are example of different types of pozzolans. According to SS EN 206, the amount of pozzolan in a RCC mixture can vary from none to up to 25% by volume depending on the exposure class. There are different ways to add the fly ash, either during the production of cement or it can be added directly to the concrete mixture. There are two methods the fly ash may be added, if it is added directly to the concrete mixture: the k-value concept or EPCC.

According to ASTM C618 (2018), pozzolans are classified into three categories shown and described in Table 2.2.

Table 2.2 Classes of pozzolan, according to ASTM C618 (2018)

Class Description

N Raw or calcined natural pozzolan

F A low-calcium fly ash

C A high-calcium fly ash

Class F fly ash is the used pozzolan in both RCC and conventional concrete, and it gives the concrete enhanced properties such as decreased permeability and thereby higher seepage control. It has also the ability to control the heat gain effectively as well as it provides resistance against sulfates and sulfides. (Headwaters Resources, 2017)

2.2.1.2 Aggregates

Aggregates can be obtained from excavations for the dam or from rock quarries. The quality and the grading of the aggregates have a great effect on the properties of the RCC. The grading influences the workability of the mixture, the total void ratio and the capability to efficiently consolidate or compact the RCC. (USBR, 2017)

The common nominal maximum size, also known as NMSA, of coarse aggregate particles can vary from project to project, but usually 25 mm has been used to prevent segregation during transportation, spreading and compacting. However, there are projects that has used up to 75 mm sized coarse aggregate. On the other hand, for fine aggregates the preferable size is commonly 75 μm for RCC with low cementitious material content, (USACE, 2000). Typical grading curve that may be used for aggregates in RCC is shown in Figure 2.2.

Too great amount of fine aggregates may cause a reduction of the workability, demand for more water, followed by a loss of strength. If plastic fines are used in the mixture, it is of great importance that an analysis of durability is made. This will determine, from a practical point of view, how to meet the structural design requirements. (ICOLD, 2003)

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Figure 2.2 Suggested combined aggregate grading with coarse and fine aggregate gradation bands

In Sweden, the aggregate standard is governed by the Swedish Standards Institute and the aggregates are with high quality which results in concrete with good strength properties.

2.2.1.3 Chemical admixtures

There are two types of admixtures that are commonly used in RCC mixtures: water-reducing admixtures and air-entraining admixtures.

According to ASTM C494 (2018), the chemical admixture is categorized in to eight types, all described in Table 2.3, depending on the desired properties of the RCC mixture.

Table 2.3 Types of admixtures, according to ASTM C494 (2018)

Type Purpose

A Water-reducing admixture

B Retarding admixture

C Accelerating admixture

D Water-reducing and retarding admixture

E Water-reducing and accelerating admixture

F Water-reducing, high range admixture

G Water-reducing, high range and retarding admixture

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The most commonly used admixture is type D, which gives increased workability of RCC and longer setting times. (ACI, 2011)

The air-entraining admixture is added to create small bubbles of air uniformly through the RCC, as well as in conventional concrete. The benefit of this admixture is that the concrete gains damage resistance when it is exposed to repeated cycles of freezing and thawing when saturated. It is on the other hand not commonly used in RCC, because it is hard to create the voids of the proper size and distribute it evenly due to the no-slump consistency. However, if this type of admixture is to be used, a Vebe time (see Section 2.2.2.3, Figure 2.5) less than 20 seconds is typically required. (ACI, 2011)

2.2.2 Mixture proportions

2.2.2.1 RCC mixing

The mixture proportioning procedure for RCC and conventional concrete are about the same, except that for RCC, some differences due to no-slump consistency and the relatively low water content can appear. The consistency RCC has to be sufficiently stable to tolerate the vibratory roller’s weight and other heavy machines, but at the same time, it must have sufficient workability to fill up the voids between the aggregate particles with mortar or paste during the compaction. (ACI, 2011)

The main difference between the RCC and conventional concrete is the ratios of the components which are represented in Figure 2.3 and 2.4 below. It also differs when it comes to aggregates because RCC mixtures can use similar aggregates used in conventional concrete or aggregates that do not fulfill the normal standards. (USBR, 2017)

Figure 2.3 Typical mixture proportions for conventional concrete (PCA, 2018)

11 16 6 26 41

Conventional

concrete

Cementitious material, 11% Water, 16% Air, 6% Fine aggregate, 26% Coarse aggregate, 41%

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Figure 2.4 Typical mixture proportions for RCC (PCA, 2018)

As can be seen in Figure 2.4, the RCC has a low proportion of air voids, which means that air-entraining admixtures are required to increase the air voids and thereby obtain a frost resistant concrete. When the air voids are developed artificially, they are not filled with water and therefore remain filled with air. Ice can form in these empty voids without applying pressure on the pore walls and by that capillary saturation can be achieved without exceeding the critical limit. (Rosenqvist, 2016)

In Table 2.4, some examples are shown of RCC mixture proportions from different dam projects that have been constructed over the years. All the quantities for each component are given.

Table 2.4 Examples of mixture proportions of RCC dams (ACI, 2011)

Dam Water [kg/m3_] Cement, [kg/m3_] Pozzolan, [kg/m3_] w/c Fine aggregate, [kg/m3_] Coarse aggregate, [kg/m3_] Air-entrained admixture, [cm3_/m3_] Air [%] Water-reducing admixture [cm3_/m3_] Al Wehdah 25 70 60 0.4 910 1365 13 2 -Camp Dyer 90 82 82 1.1 750 1344 4 3.6 2 Santa Cruz 101 76 75 1.3 728 1365 4 2.3 2 Upper Stillwater 94 79 173 1.2 729 1292 - 1.5 7 Willow Creek 110 104 47 1.1 645 1625 - 1.2 -10 13 1.5 35 40.5

Roller compacted concrete

Cementitious material, 10% Water, 13% Air, 1.5% Fine aggregate, 35% Coarse aggregate, 40.5%

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The water-cement ratio law, which is exemplified in Table 2.4, is generally used for fully consolidated mixtures, where the compressive strength of RCC is a function of the water-cementitious material ratio for mixtures that are fully compacted. This general relationship is represented in Figure 2.5.

Figure 2.5 General relationship between the compressive strength and the w/c (ACI, 2011)

For mixtures with dry consistency, where the voids are not fully filled with paste, the compressive strength is determined by the moisture-density relationship.

In general, poorly compacted mixtures consist of less-than-optimum moisture, which leads to a loss in strength and density. This can be counteracted by adding water to the mixture and thereby increasing the paste volume to fill the voids. A mixture that is fully consolidated and exceeds the optimum moisture usually creates a higher compressive strength. The tensile and shear strength, along the lift surfaces, typically determines the design strength while the compressive strength is more of an indicator of the quality of the concrete. (ACI, 2011)

2.2.2.2 Classification of RCC mixtures

The RCC mixture may vary when it comes to the amount of cement and this leads to the classification of RCC mixtures according to Table 2.5.

Table 2.5 Classification of RCC mixture (Chryso, 2018)

Classification Cement [kg/m3_]

Lean paste RCC <100

Medium paste RCC 100-150

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The lean paste RCC is a mixture containing low amounts of cementitious material and these mixes are drier in consistency, leading to a less workable mixture with a Vebe time (See 2.2.2.3, Figure 2.6) less than 30 seconds. The minimized use of the cement or/and pozzolan leads to cost saving. An advantage with lean mixtures is that the concrete, which is created, has low internal temperature during the hydration and low elastic modulus but the concrete tend to have high creep rates which are important to consider.

For a lean mixture, compared to a high paste mixture, the quality of the bond is reduced between the lifts of the RCC is reduced. Lean mixtures provide adequate strength for sliding stability, but seepage is expected and measures needs to be taken to control it and this is described in section 3.2 After construction.

When constructing with high paste RCC the goal is that it performs just as well as a conventional concrete dam. This type of mixture contains a greater amount of pozzolan and cement, more than 150 kg/m3, to be able to obtain a density near the theoretical air-free density. (Hansen K.D et al.,1991)

2.2.2.3 Considerations

Workability

The workability is a way of determining the RCC capacity to be situated and compacted without damaging segregation. The definition of segregation is the separation of the concrete ingredients from each other and thereby resulting in a non-uniform mix. It is affected by cement, water, fly ash and fine aggregates. To check if the mixture is workable, a Vebe apparatus is typically used to measure the mixture consistency. (USACE, 2000)

The Vebe apparatus is connected to a vibrating table and filled with fresh concrete, which is compacted into a conical mould, see Figure 2.6. The mould is then removed and a clear plastic disc is placed on the top of the concrete. The vibrating table is started and the time it takes for the whole disc to fully come in contact with the concrete is the Vebe time.

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The Vebe time, which is achieved for the RCC mixture, is used very similarly to the slump test for conventional concrete mixtures. According to SS-EN 12350-3:2009, to obtain an adequate workability, a Vebe consistency of 5-30 seconds is desired which will contribute to a uniform density through the whole lift, good bonding between the lifts, support of compaction equipment and easy compaction.

Durability

The durability of RCC will depend on the quality of the materials that are used, the exposure conditions and the expected performance level of the structure. Freezing-thawing and erosion caused by e.g. aggressive waters are processes that affect the durability and can be avoided by protecting the external sides, exposed to water, with conventional concrete. This can be done on both the upstream- and downstream face, all depending on if the faces are exposed of deterioration due to water or chemicals. (USACE, 2000)

Segregation

In order to reduce the risk of segregation of RCC during placing, spreading and transporting, it is of great importance to produce a cohesive mixture. Segregated material lead to a loss of properties of the RCC and this occurs in low-cementitious content mixtures due to its grainy consistency. By adding fine aggregates and controlling the moisture content this can be prevented. Mixtures with high paste-content are generally less likely to segregate due to being more cohesive. (ACI, 2011)

Heat generation

The heat generation during hydration of cementitious materials needs to be considered when designing massive RCC structures. Figure 2.7 shows an example of how the hydration heat of cement governs the rise of the temperature. The goal is to minimize the heat that is developed during the hydration to avoid the risk of thermal cracking but at the same time achieve sufficient strength growth by creating a suitable combination of pozzolan and cement. To achieve the optimal combination, tests on different percentages of pozzolan and cement mixtures are typically conducted. (ACI, 2011)

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Construction-conditions

The use of equipment and the requirements for construction is important to consider, due to the possibility of damaging the compacted RCC when it is placed. When rollers and hauling trucks are exposed to a RCC mixture with high workability it tends to leave wheel-tracks, which is also known as rut. For that reason, there should be a restriction from operating on a compacted surface until it reaches final set. (ACI, 2011)

2.2.2.4 Proportioning approaches

Two different proportioning techniques may be used when designing a new RCC dam and two main approaches has been developed. The first approach is the concrete approach and it is appropriate for high paste RCC mixtures, while the second approach is the soil approach and it suits RCC mixtures that are lean. These approaches are further explained below.

In the concrete approach RCC is considered to be a true concrete and that it is composed by clean and well-graded aggregates. When the RCC mixture is fully consolidated, the strength will be inversely proportional to its water-cement ratio. The consistency of the mixture using this approach is usually more viscous and the concept of having an adequate amount of paste to fill all the voids in the aggregate is applied. This is because fully compaction needs to be achieved with a no-slump consistency. However, it is important that a measurable slump does not appear which could occur if the mixture contains more paste than necessary. According to ICOLD (2003), the concrete approach is used in several methods that all has minor differences but still follow the general process described in the following steps:

1. Increase the coarse and fine aggregates gradation to achieve minimum voids in the mixture.

2. Fill the voids in the fine aggregate with paste by choosing a suitable paste/mortar ratio. The material used should pass 75 μm sieve to be acceptable.

3. Adjust the proportion of the concrete components (cement, water, fly ash and admixtures) to obtain the appropriate mean strength.

4. Use the Vebe apparatus to achieve coarse aggregate volume that will lead to an acceptable workability.

5. Investigate if there is enough cementitious material and added fines to obtain the desired permeability.

6. Investigate that the fine/coarse aggregate ratio is as optimized as possible by comparing it to gradation curves.

7. Investigate that the generated heat during hydration does not exceed the expected limits.

8. Make any modifications that are needed and re-check the design.

The soil approach is based on the moisture-density relationship. Compared to the concrete approach, the RCC is considered as a processed soil which is enriched with cement. To find optimum moisture content for a specific amount of cement and aggregates, the mixture has to be able to carry a compaction effort corresponding to the vibratory rollers. Thereby a

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maximum dry density can be obtained. It is common that all the voids in the aggregate are not filled with paste after compaction when using the soil approach. (USBR, 2017) When using the soil compaction method, the following steps are usually used (Harrington D et al., 2010):

1. Select well-graded aggregates

2. Choose a mid-range cementitious content

3. Develop plots of the moisture density relationship and verify that the moisture ratio is acceptable

4. Cast samples to measure the compressive strength

5. Test the specimens and choose required cementitious content 6. Calculate the mixture proportions

For conventional concrete, the mix design is similar to the RCC procedure based on the concrete approach, where the strength and water-cement ratio are the main aspects. The difference when testing the consistency of the conventional concrete is that slump is allowed. Some examples of slump values, suitable for different kinds of concrete, are presented in Table 2.6. (ACI, 2017)

Table 2.6 Recommended slumps for different types of concrete applications, according to ACI (2017)

Conventional concrete constructions

Slump [mm]

Maximum Minimum

Reinforced foundation and walls and footings

75 25

Plain footings, caissons and substructure walls

75 25

Beams and reinforced walls 100 25

Building columns 100 25

Pavements and slabs 75 25

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2.2.3 Properties of hardened concrete

2.2.3.1 Durability

Concrete and RCC dams are in general exposed to deterioration effects from abrasion/erosion, freezing/thawing and chemical attacks, which lead to degradation of the concrete. The interior of RCC can experience internal frost damage, due to ice-crystallization in the pore system. However, this can generally be counteracted with the help of air-entraining admixtures, which create larger volume of air voids for the water to spread out on. An alternative could be to have a higher quality concrete on the exterior than the interior concrete to avoid cracking, spalling and loss of material from the surface, thereby achieving a high durability.

Abrasion/erosion damage may occur in RCC dams because it is subjected to factors such as ice and waterborne sediments, see Figure 2.8. The quality of aggregates and the compressive strength determines the RCC’s resistance against abrasion/erosion. (ACI, 2011)

Figure 2.8 A spillway that has been subjected to hydro-abrasion (Eriksson D, 2018)

Generally, RCC has a poor resistance to freezing/thawing but by using conventional concrete or air-entrained admixture the risk of frost damages can be reduced (USBR, 2017). Figure 2.9 shows typical frost damages that can appear on hydraulic structures.

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2.2.3.2 Strength

Similarly to conventional concrete, the mixture proportions of the components and the degree of compaction influence the strength of the RCC. The voids between aggregates in the mixture can be completely filled with paste or not, depending on the classification the mixture according to Table 2.5, and this determines the basic strength relationship for the RCC. Table 2.7 gives a general view of the compressive and tensile strength for the different classification of RCC mixtures. (USACE, 2000)

The compressive strength is an indicator for the total strength of RCC and it increases with time. It is comparable to that of conventional concrete, generally ranging from 28 to 41 MPa. (Roller Compacted Concrete, 2013) During the design phase, tests of the compressive strength are performed on cores that are drilled out from the built structure. The cores compressive strengths are compared to the compressive strengths of cylindrical specimens of trial mixtures see Table 2.8. These specimens are prepared typically 7, 28, 90, 180 days, and 1 year to be able to follow the mixtures strength gain. The modulus of elasticity and Poisson’s ratio can also be determined with the help of these specimens. (ACI, 2011)

The factors that influence the compressive strength of the RCC are water/cementitious material content, aggregates and the degree of compaction. For fully compacted RCC, the reduction in water content leads to increase in compressive strength. Although, water content below the optimum will cause voids in the mixture and that gives a weaker compressive strength. To obtain a compressive strength that is equal to conventional concrete, high-quality aggregates have to be used. (USBR, 2017)

Table 2.7 Strength for RCC (ICOLD, 2003)

Lean paste RCC Medium paste RCC High paste RCC

Compressive strength [MPa]

Mean 11.6 15.2 20.7

Direct tensile strength [MPa]

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Table 2.8Comparison of compressive strength of RCC: construction control cylinders vs. cores (ACI, 2011)

Core strength [MPa]

Strength 17 0 18 13 36 Age, days 730 0 90 365 365 Strength 9 ₁₄ ₁₄ ₁₄ ₃₄ Age, days 90 42 28 180 180 Cylind er st reng th [ MPa ] 365 _days 16 - - 9 74 90 days 9 ₁₁ ₂₁ - ₁₈ 28 days 3 9 ₁₈ 2 ₁₃ NM SA [m m] ₇₅ ₇₅ 37.5 50 50 w/ c 1.00 1.43 0.82 0.93 0.39 Pozzol an [kg/ m 3 ] 33 0 62 77 ₁₇₃ Ce m en t [kg/ m 3 ] 70 66 ₁₂₅ 71 79 Da m Elk Cre ek Middle For k S tacey Spillwa y S

tagecoach Upper _Stillwa

te

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The tensile strength is the main consideration for the loading design and, compared to the compressive strength, it is mainly governed by the bond of the aggregates. In a RCC structure, the lift joints are the weakest zones and, as for a conventional concrete structure, the tensile strength in these points is an important property. (USACE, 2000)

The most suitable way to test the tensile strength between the lifts is with a direct tensile strength test. The result from this type of test depends on the lift joints maturity, the preparation of joint surface etc. If the tensile strength of the parent (unjointed) RCC is of interest, then a splitting tensile strength test of horizontal cores is generally used. This test is, compared to the direct tensile strength test, easier to carry out and is not as sensitive to micro-cracking from drying and thereby gives more consistent results. The tensile strength in the RCC joints is always lower than in the parent (unjointed) concrete and therefore, the tensile strength in the lifts will determine the design. In Table 2.9, the tensile strength and percentage of bonded joint strength are represented from some projects. (USACE, 2000)

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Table 2.9Tensile strength of drilled cores of RCC dams (ACI, 2011) Test type DT DT % bond ed jo in s 95 90 Te nsile strength [MP a] 0.9 1.6 Compr es sive strength [MP a] 14 _31.9 Age, days 90 5000 Joint type NB P NM SA [m m] ₅₀ ₅₀ w/ c 0.63 0.35 Pozzol an [kg/ m 3 ] 121 195 Ce m en t [kg/ m 3 ] 74 89 Dam Olive nha in Upper Stilllwa te r

Notes: Joint type: NB = no bedding; B = bedding mortar or concrete; P = parent concrete. Test type: DT = direct tensile test; ST = splitting tensile test.

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One of the most important properties of RCC is shear strength of lift joints, which is the sum of the cohesion and the internal friction for bonded, intact lift joints. It can generally, be described by using Coulomb’s equation, shown below. (USACE, 2000)

ݏ = ܿ + ݌ tan ߶ (eq. 2-1)

Where:

s = unit shear stress c = unit cohesion p = unit normal stress

߶ = the angle of internal friction

Parent shear strength test and lift joint shear strength test are two types of test that are typically performed when determining the shear strength of the RCC and examples of values from different projects are shown in Table 2.10. The parent shear strength test can be developed from cylinder specimens made in the laboratory or from cores drilled from a finished RCC dam. The RCC shear strength is usually similar to those for conventional concrete. The cohesion in this test varies with the amount of cement, paste and age, while the friction angle is affected by the aggregate type. (USACE, 2000)

The other test that is performed is the lift joint shear strength and it generally determines the critical shear strength for design. Compared to the shear strength of conventional concrete dam, the RCC shear strength for lift joints can be lower. The cohesion varies, as in the previous test, with the amount of cement paste but also the preparation of the lift joint and the exposure. Conditions improving the cohesion, and thereby strengthen the bond between the lifts, can be done by placing the RCC lifts rapidly over a fresh joint surface, applying an additional bonding mixture concrete or bedding mortar between lifts or increasing the cement content of the mixture. (USACE, 2000)

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Table 2.10Examples of shear strength of drilled cores in RCC dams (ACI, 2011) R es idual sh ea r ߶ ,deg 40 42 44 40 R es idual sh ea r cohes ion [kP a] 552 207 69 69 Shear ߶,deg 67 55 69 56 Peak cohes ion [kP a] 758 ₂₀₆₈ ₁₅₈₆ 586 Co re compres s iv e st rengt h [MP a] 14 27 18 10 Ag e, days 415 120 365 345 Jo in t ty pe NB P B NB NMSA [mm] 75 50 50 63 w/c _1.09 _0.37 _0.80 _1.50 Pozzolan [kg/ m 3 ] 51 ₁₇₃ 66 0 Ce m en t [kg/ m 3 ] 51 79 67 74 Da m G alesv il le Upper S ti llw at er Vi ct or ia Zi nt el Canyon

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2.2.3.3 Permeability

Permeability of RCC can be controlled by the degree of compaction, placement method, mixture proportioning and the use of bedding mortar on the lift surfaces. Due to the construction method of RCC dams, seepage can occur between horizontal lifts and through vertical contraction joints or cracks which leads to a reduction in tensile and shear strength. (USACE, 2000)

The main concern for permeability in concrete dams is the seepage in lift joints. To produce watertightness, high cementitious content mixtures are needed to provide sufficient bond with a newly placed lift. Lower cementitious content mixture does usually not provide adequate watertightness and needs to be treated with bedding mortar between the lifts. According to ACI (2011), RCC mixtures containing a paste and fine aggregate volume of 18-22% will contribute generally to a sufficient level of impermeability, which is similar to conventional concrete.

2.2.3.4 Density

The definition of density is the mass per unit volume and the RCC density relies on the degree of compaction and the density of the aggregate. Many RCC mixtures have low water content and insufficient entrained air that leads to a slightly higher density compared to conventional concrete. The conventional concrete has a density of approximately 2400 kg/m3and can vary, whereas the density for RCC can be 1-3% greater and land on a density of 2424-2472 kg/m3. (USACE, 2000)

2.2.3.5 Elastic properties

The modulus of elasticity and Poisson’s ratio are the elastic properties of RCC, as well as for conventional concrete, and they are affected by factors such as strength, age, paste volume and aggregate type. (ICOLD, 2003)

The modulus of elasticity is the ratio between the normal stress and the corresponding strain and is generally, for a given aggregate type, a function of strength. It is usually assumed that the modulus of elasticity in compression is the same as in tension. When designing dams, it is desirable to have a low modulus of elasticity to decrease the possibility of cracking at a particular stress level, but this leads to greater deformation. To determine an acceptable low modulus of elasticity is important and can be done with laboratory tests on specimens. The stresses and strains, obtained from the strength tests, are plotted and the ratio between stress and strain will give the proper modulus of elasticity.

The poisson’s ratio is the ratio between the transverse strain and the axial strain due to a uniformly distributed axial stress. Conventional concrete and RCC has generally similar values of Poisson’s ratio and it varies typically between 0.17-0.22. (ACI, 2011)

Table 2.11 shows typical modulus of elasticity and Poisson’s ratio for some cases of RCC mixtures.

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Table 2.11 Examples of elastic properties for different completed dams, according to ACI (2011) Poi ss ion ’s r ati o 365 _days - 0.21 0.14 - -90 _days - _0.19 _0.14 _0.18 _0.21 26 _days _0.17 _0.14 _0.13 _0.19 -7 days - 0.13 - - -Modu lus o f e las ti ci ty [GP a] 365 days 22.82 22.34 11.79 - 17.72 90 _days 13.17 15.58 9.10 19.17 14.82 26 _days _7.58 12.41 7.10 18.41 10.62 7 days - 9.38 - 15.17 4.69 Compr ess ive S tr eng th [ MP a] 365 days 11.7 21.0 36.0 26.1 10.7 90 _days 8.6 _15.0 _24.2 _18.3 7.5 26 _days _6.8 _8.9 _14.7 _12.7 _4.3 7 days 4.4 4.4 9.4 6.9 19 w/ c 1.03 0.88 0.47 1.06 2.0 NM SA [m m] ₇₅ ₅₀ ₅₀ ₇₅ ₇₅ Da m

Concep cion Sant

a Cr uz Upper _Stillw at er Wi llo w C reek Zi nt el Cnayon

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2.2.3.6 Creep

Creep is a time-dependent deformation and under sustained long term loading, the creep will continue at a decreasing rate and increase in strain. High modulus of elasticity and high strength in the RCC will cause low creep, while low strength and modulus of elasticity leads to larger creep values. The desired creep values are typically high to gradually relieve stress and strain development caused by foundation restraint and exterior and thermal loading. According to Eurocode 2, the creep can be represented by the formula:

ߝ௖௖(λ, ݐ଴) = ߮(λ, ݐ଴) ή ቀఙ_ா೎

೎ቁ (eq 2-2)

where,

ߝ = creep deformation

߮(λ, ݐ଴) = final value of creep coefficient

ߪ௖= compressive strength, MPa

ܧ = static modulus of elasticity, GPa ݐ = time after loading, days

The first part of the above equation represents the long-term effect of creep after loading, while the second part represents initial elastic strain from the load. Some examples of creep properties are given in Table 2.12.

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Table 2.12 Strain and creep properties of some laboratory RCC mixtures, according to ACI (2011) Modu lus of elas ti ci ty [G P a] - ₁₀ ₁₄ ₁₀ ₁₂ ₁₃ 8 ₁₁ ₁₃ Com pre ss iv e str engt h [MP a] 4 7 9 ₁₄ ₂₉ ₃₅ 3 8 ₁₂ Creep coef fi ci ent s f(K ) 0.12 0.08 0.03 0.04 0.01 0.02 0.20 0.11 -[10 -6 /K P a] 0.20 0.11 0.07 0.10 0.08 0.08 0.29 0.16 0.08 Loadi ng age, da ys 7 ₂₈ ₉₀ ₂₈ ₁₈₀ ₃₆₅ 7 ₂₈ ₉₀ w/ c 1.20 1.20 1.20 0.43 0.43 0.43 1.61 1.61 1.61 Pozzol an [kg/ m 3 ] 0 0 0 ₁₇₀ ₁₇₀ ₁₇₀ ₁₉ ₁₉ ₁₉ Ce m en t [kg/ m 3 ] 90 90 90 77 77 77 47 47 47 Da m Concepc ion Upper S til lw at er Wi llow C reek

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2.2.3.7 Volume change – shrinkage

It is important, for all massive concrete structures, to minimize uncontrolled cracking due to volume changes. These volume changes are divided into different types such as, drying shrinkage, autogenous shrinkage and thermal contraction.

Drying shrinkage is defined as the volume reduction that the concrete exhibit due to the moisture migration when it is exposed to lower relative humidity environment. It depends on the water content and the features of the aggregate that are used. Due to the lower water content in RCC, the drying shrinkage is either similar or lower than for conventional concrete. Compared to autogenous shrinkage, drying shrinkage takes place over a longer period of time. (USACE, 2000)

Autogenous shrinkage is the deformation that occurs during constant temperature with no exchange of moisture with its environment. The chemical shrinkage is the driving force and when hydration develops a volume change in the interior of the concrete occurs, without losing or gaining moisture in the concrete. Autogenous shrinkage is dependent on the properties of the used materials and the mixture proportions. Generally, higher strength properties may lead to higher autogenous shrinkage. (Barcelo L et al., 2005)

The thermal contraction takes place when the hydration process causes the concrete to reach temperature higher than the ambient temperature. When the hot concrete starts to cool down to the surrounding temperature, it contracts and reduces in volume. (BASF, 2014)

2.2.3.8 Thermal properties

The thermal properties, which include specific heat, coefficient of thermal expansion, conductivity and adiabatic temperature rise, are of great importance for both RCC and conventional concrete. In Table 2.13, examples of thermal properties for various projects are presented. It is recommended to test the mixture because the thermal properties vary significantly depending on aggregate and cementitious type and content. (ACI, 2011)

The adiabatic temperature change depends on the total cementitious content and percentage of pozzolan in the mixture. Compared to conventional concrete, RCC with low cementitious material content has lower temperature rise (ICOLD, 2003).

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Table 2.13 Thermal properties of some laboratory RCC mixtures, according ACI (2011) A diaba tic t emp er atu re ris e Change in ϶C 28 _days _13.9 _11.1 _18.3 _26.7 _12.2 7 days 13.3 8.9 16.1 20.2 0 3 days 11.7 7.2 13.9 13.3 7.2 In it ia l ϶C _19.4 6.7 _16.1 _12.2 _11.7 C oefficient of expans ion [m illio nt hs / ϶C ] 11 7 _5.4 - 7 Conduct iv it y [W /m ϶ K ] 1.9 1.7 2.9 - 1.8 Di ff us iv it y [m 2 /h] 0.003 0.003 0.004 - 0.003 Speci fi c heat [J/k g ϶C ] 1047 754 1089 - 921 A ggrega te type Igni mb ri te Sands tone A llu vi al gr an ite Qua rts Ba sa lt Pozzol an [kg/ m 3 ] 0 ₂₃ ₆₆ ₂₀₄ ₁₉ Ce m en t [kg/ m 3 ] 90 56 66 93 47 Da m Concepc ion Elk Cre ek Sata C ruz Upper S til lw at er Wi llo w C reek

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2.2.3.9 Tensile strain capacity

Cracks occur when a restrained volume change causes strain which exceeds the tensile strain capacity. There are several factors that affect the tensile strain capacity such as rate of loading, cementitious content, type of aggregate, age and strength of the concrete and aggregate shape (natural round vs. angular, which is produced by crushing). Development of tensile strains in the concrete occurs due to volume changes and external loads applied to the structure. (ACI, 2011)

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2.3 Construction

2.3.1 Foundation considerations

For RCC dams, the foundation considerations are the same as for the conventional concrete dams. When designing, several aspects must be taken into consideration such as the loads from the dam and the stresses that are distributed on the foundation, the suitableness of the rock foundation, and the required quantity of surface treatment and excavation to achieve a suitable foundation.

The friction angle of the joint surfaces, the orientation and dip angles of key joint sets and the loads that are transmitted to the foundation must be taken into account when evaluating the foundation stability. Cohesion between the foundation and the dam is important to control sliding resistance of the contact surface. (USBR, 2017)

Conventional concrete is generally used to build a platform between the foundation and the RCC dam because this is the most critical point of the structure. After this, the RCC is placed in layers on a leveled surface. However, there are projects that directly have started with RCC on the foundation.

The goal when designing a new RCC dams is to reduce the amount of leveling concrete due to the use of conventional concrete is generally more expensive than RCC and it may have different properties. To avoid the use of leveling concrete, a thin layer of high-slump bedding concrete can be placed onto the rock and then spread over the RCC and compact it while the bedding concrete is fresh. This way the two materials merge into one after compaction and the mortar and grout of the bedding concrete creates sufficient bond. (ACI, 2011)

2.3.2 Construction method

When designing a new RCC dam it is important to consider the basic purpose of the dam and assure that the set requirements, e.g. cost, watertightness, appearance etc, are fulfilled. These aspects will determine for instance the mixture proportions of the RCC and the shape of the dam.

Aggregates that are used for RCC mixtures can be found on the construction site or it can be transported from an aggregate producer. Stockpiling aggregates is necessary for an RCC dam construction and before starting the RCC work. Huge stockpiles are needed because the aggregate production capability might be exceeded by the usage rate of the aggregate during the placement of the RCC. (ICOLD, 2003)

When producing aggregates it is critical to stockpile it cold or warm, depending on the time of the year that the production is done. There is a specified maximum temperature for the aggregates during placement and if warm weather is anticipated, then pre-cooling of the aggregates may be required which can be done by sprinkling water on the stockpiles to create evaporative cooling. It is however, important to consider the moisture of the aggregates when it is time to mix the RCC mixture. In general, if low placement temperatures are desired then stockpiling during the winter is the best option. (USBR, 2017)

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The most crucial part of construction is that the RCC is transported, placed, spread and compacted as rapidly as possible. The time from the start of mixing to after it has been compacted should not exceed the initial set time of the RCC mixture. For mixtures containing no or little pozzolan the placing, spreading, and compacting should be executed within 30-45 minutes of mixing and this general rule is suitable for mixtures containing retarding admixtures. However, this time can be adjusted depending on the weather conditions. For warmer weather the time should be reduced, while for cooler weather the time can be extended. (ICOLD, 2003)

Compared to when constructing a conventional concrete dam, RCC dam differs in the aspects of the layout, planning and equipment. Rather than building in blocks the RCC dam are usually built in thin, horizontal lifts which are advanced from one abutment to the other. When constructing RCC dams, the demand for man-hours are less compared to conventional concrete due to the usage of machines for compacting and spreading the concrete (see Figure 2.10), reduced amount of formwork and decreased joint preparation. RCC dams can be built with curved or straight axes, with inclined or vertical upstream face and with a downstream face with a vertical or an inclined slope depending on what is suitable for the given site. With time there has been a development of placing methods and the goal has been to place multiple RCC lifts in a short amount of time, before the initial set of the concrete is reached. This leads to an improved bond between each lift and the need for placing bonding mortar can be avoided.

Figure 2.10 RCC compaction (Shaw, 2010)

The sloped layer placing method (SLM) is one of the newer methods, where 10 small-volume lifts are rapidly placed on slopes from 1:10 to 1:40 that are later built up to a single layer which may have a thickness up to 3 m, see Figure 2.11. (ACI, 2011)

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Figure 2.11 Sloped layer placing method, according to ACI (2011)

Bulldozers are the general equipment that is used for spreading RCC and it is typically placed in a 300-350 mm thick layers. An uncompacted lift gives the dozers the ability to work on the surface without damaging it. When it comes to the thickness after compaction, the most common lift thickness is 300 mm because it is suitable to work with in the field but it can be up to 500 mm thick. Factors such as maximum approved exposure time of a lift before placing the following one, affect the selection of the RCC layer thickness. Another factor is to use the maximum allowable lift thickness of a RCC mixture and to obtain the specified minimum density after spreading and compaction. For minimum potential weaknesses in the dam, thicker lifts are chosen which leads to longer exposure times but fewer joints between the lifts. If instead improved bond is required, then thinner lifts are chosen which consequently means more joints but these can be covered a lot sooner. It has to be taken into consideration that each project is unique and different lift thicknesses may be optimal. (ICOLD,2003)

Upholding the bond between lifts is important for both RCC and conventional concrete dams in order to fulfill the necessary factors of safety for usual, unusual and extreme loading conditions. The following requirements are essential to obtain an adequate bonding between the lifts (USBR, 2017):

1. Having a RCC mixture with adequate workability and sufficient amount of paste and mortar.

2. To control segregation when placing the RCC.

3. Achieving an adequate compaction with the vibrating roller. 4. Thorough cleanup of the lift surfaces.

5. Using a bonding layer of concrete or mortar between lifts.

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As mentioned earlier, the bond between the lifts are important especially for hydraulic structures. The quality of the bond relies on the type of joint treatment that is required. The joint treatment is dependent on the time between placements of the lifts and the time relies upon the RCC mixture and the surrounding temperature at the site. There are three types of joints, which are explained further. (USBR, 2017)

x Hot joint (fresh joint) appears when placing a new RCC lift over the earlier placed lift that has not reached its initial set (between 6-12 hours from placement). The common cleanup treatment (Type 1) includes the removal of loose materials and free water and then cleaning the lift surface with vacuum equipment. If the mixture contains no pozzolan or if the placing occurs during warm temperature conditions, then the time period 6-12 hours is reduced down to 4 hours.

x Cold joint appears between the initial and final set (6-24 hours after placement). The cleanup treatment (Type 2) includes cleaning with air- or air water jetting to remove concrete that is defected then vacuuming the surface to remove remaining loose materials or water. Depending on the design conditions bonding mortar may be needed.

x Construction joint appears after the final set of the concrete (24-48 hours after placement). The cleanup treatment (Type 3) is important and it includes high-pressure water jetting or wet sand blasting to remove loose materials and water, then a mechanical broom and vacuum is followed. Bonding mortar is commonly needed. To control cracking caused by thermal volume change, contraction joints are used and is an important part of the design. Contraction joints formed by a crack inducing plate is done by firstly spreading the RCC to the contraction joint alignment and then prepare a vertical form plate for the joint with some external support to keep the plate vertical. On both sides of the vertical plate, RCC is spread with manual labor. It is common to use plastic sheets around the vertical plate which is later on removed, leaving the plastic to act as a bond breaker. See Figure 2.12. Another method to create a contraction joint is with a vertical plate attached to an excavating machine called backhoe, which has a bucket attached to its arm. The galvanized steel plate is installed by vibrating it into the compacted lifts into pre-made joint location and these acts like bond breakers due to they are left in the RCC. The joints placing and spacing is determined by the temperature change and the time period it develops, the foundation restraints, the creep relaxation, the tensile strain capacity of the concrete, the applied loads and the coefficient of thermal expansion (USBR, 2017). According to Cotoi (2015), a general rule could be to place the contraction joints every 15 m throughout the dam.

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The construction of contraction joints can vary from a superficial crack and control of seepage, to detailed joints with drain holes, tubes for grouting and water stops. If the contraction joint consists of the superficial crack and seepage control construction, then a wood strip of 40x45 mm is installed as a crack inducer and treated or sealed with a joint sealer and a foam. A contraction joint that consists of a water stop and a drain, commonly places the water stop in conventional concrete at a certain distance from the dams upstream face and joint filler is placed on both downstream and upstream of the water stop. Contraction joints with drain holes are formed during the time when the RCC is placed and the drain hole is connected to the drainage gallery through an outlet pipe. (ACI, 2011)

Galleries are important to have in dams over 30 m in height in order to create places that are used for inspection and observing the behavior of the dam, drilling drain and grout holes into the foundation and seepage draining. It is important to place the galleries so that they do not interfere with the construction of the dam. There are many construction methods for designing galleries e.g. conventional forming method with or without the use of conventional concrete or excavation of gravel in-fill from the gallery area. (ACI, 2017)

The design of spillways that is used for conventional concrete dams is applicable for RCC dams. There are four kinds of spillways, when overtopping is desired, for RCC dams and they are: naturally sloped RCC spillways, stepped RCC spillways, stepped conventional concrete spillways and sloped conventional concrete spillways. It is common to use conventional concrete steps for the spillways when constructing RCC dams and it can be constructed either after the RCC is complete, similar to how it is done to most smooth spillway facings, or it can be done lift by lift with the RCC. If the spillway is not strengthened with conventional concrete, then the RCC can be used if the water flow velocity is less than 8 m/sec. (USACE, 2000)

When the RCC has been placed and compacted, continuous curing is important just as for conventional concrete. The RCC is a drier mixture and the surface tends to dry faster during warm weather. Therefore, it should be maintained in a moist condition with water for 14 days or until the next lift is placed. The RCC must be protected from extreme temperature changes until it gains adequate strength and the construction should cease if the rain exceeds 2-3 mm/h. If vehicles are allowed on the surface during rain, the tires may damage the surface and turn the material soft. This situation may need waiting until the RCC has hardened and cleanup can be done or removal of the entire lift is done. (USBR, 2017)

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3 Challenges during construction

3.1 During construction

The main reasons for designing new RCC dams are the economy, the speed of constructing it and the positive environmental affect due to less cement use. As for every other type of dam, the RCC dam may face challenges during construction. There are typical conditions which need to be taken into consideration, and those are topography, foundation, geology, access conditions, climate conditions, available materials and characteristics of the river flow. The handling of these conditions can be found from earlier experience from previous projects. However, there is no standard solution when designing a RCC dam, or any other type of dam, due to characteristics of the specific site, which makes every project unique in design. (Griggs T et al., 2012)

Nowadays, RCC dams are being constructed in all types of climate all over the world. This means that the circumstances for construction can be exposed to extreme cold or warm climate conditions, which both can lead to cracking in the structure. During construction, when the placing of the lifts takes place, the initial hydration can affect the maximum temperature to increase or decrease depending on the ambient condition and exposure time. The major concerns when cracking in the RCC occurs are leakage control, durability and appearance. The most difficult factor to control after construction is the leakage and often results in an unwanted loss of water and is problematic to maintain. (ACI, 2011)

When constructing a RCC dam in cold climate, it is important to consider possible difficulties due to rapid cooling of the massive RCC construction and how this could be handled can be seen in the project of Xingjiang Shimenzi Reservoir. The region has temperatures below zero around 1/3 of the year and temperature can reach -36°C, making this a very harsh climate to build in. Important measures were taken, where the first step was to create a mixture with good freezing and thawing resistance with the help of air-entraining admixture. When the RCC mixture was transported to site it was insulated and the placement area was fully insulated with insulating layers. Actions were taken to protect the existing RCC against frost by using weaving cloth, gravel and grass cushions to enclose the surface of the RCC during the time when there was low activity in the construction phase. (Berga L et al., 2003)

Constructing in warm climate can be challenging as well. When placing the RCC lifts in warm conditions, the surface will absorb the direct heat from the sun which will cause an increase of the mixtures temperature and it will generate the hydration more rapidly. If the surface of the lift is exposed during a long time, it will have the chance of absorbing so much solar energy that it will force the internal temperature to increase. (ACI, 2011) That is why rapid placement is suitable in this case and an example of a dam that was built during warm conditions is the Al Wehdah dam on the border between Jordan and Syria. This project started May 2003 and during the summer months the temperature could reach up to 40ͼC. This was taken into account by incorporating chilled water into the mixture and thereby reducing the placement temperature of the RCC. Also, the coarse aggregates were kept chilled with the help of a moving wet belt system, which in other words is a cooling tunnel and it used water that was chilled down to 3ͼC. (Water Power, 2009)

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3.2 After construction

The design and construction develops with time and each completed project and this provides experience and knowledge, which can be used in future RCC dam designing. Therefore, it is important to achieve reports or records with accurate results of the performance and to learn from earlier mistakes that has lead to unsatisfactory performance. After the construction is completed core samples are drilled out and they often contain the joints between the lifts which can be examined and tested for strength.

A new RCC dam will experience thermal stresses that are directly related to volume changes of the concrete, due to the hydration process and the ambient temperature variations leading to cracks. The dam may be exposed to temperature differences up to 60-70ͼC, within a summer to winter period, resulting in opening and closing of cracks and joints in the RCC dam. (USBR, 2017) These temperature variations are commonly considered when designing the dam and can be anticipated with the help of FE- analysis. An example of the temperature distribution is shown in Figure 3.1.

Figure 3.1 Example of temperature distribution in a concrete dam according to USBR (2006).

Cracking can be expected in any type of concrete dam and develops because of tensile strain, caused by the cooling of the concrete from the peak temperature. A common thing to do to prevent cracks, is to install crack or joint inducers in the bigger dam constructions to help control the cracking and they are placed every 15 m, see Section 2.3.2, with drain holes and upstream water stops. The cracks that are of concern, when the dam is in use, are the ones that are below the waterline and are wide enough to let water pass through. These cracks can be repaired or sealed with different method and materials and one way is to use a sealant consisting of polysulfide and polyurethane or cement grouting. The repairing could be done when the water reservoir is lowered or underwater crack sealing can be implemented with e.g. quick-set cement. (ACI, 2011)

Seepage can be one of the most challenging parts after construction. If it is not handled properly, it can lead to internal failure modes where the lifts will slide, see Figure 3.2, due to water flowing through the cracks causing a hydrostatic pressure. Due to the water, leaching of the concrete will occur, which will increase the deterioration.