Volume 6 hydro power 6 14 – durability design of concrete hydropower structures

Volume 6 hydro power 6 14 – durability design of concrete hydropower structures Volume 6 hydro power 6 14 – durability design of concrete hydropower structures Volume 6 hydro power 6 14 – durability design of concrete hydropower structures Volume 6 hydro power 6 14 – durability design of concrete hydropower structures Volume 6 hydro power 6 14 – durability design of concrete hydropower structures

S Jianxia, Design and Research Institute, Yangzhou City, Jiangsu Province, China

© 2012 Elsevier Ltd All rights reserved

6.14.1 Introduction

6.14.1.1 General

6.14.1.1.1 Background of concrete hydropower structures

6.14.1.1.2 The inadequacy of concrete hydropower structures

6.14.2 Early Cracking

6.14.2.1 General

6.14.2.2 Definition of Early Cracking

6.14.2.3 The Causes of Early Cracking

6.14.2.4 Controlling the Cracking of Concrete Hydropower Structures

6.14.2.4.1 Different design concepts on crack width and monolith size

6.14.2.4.2 Experience of limiting crack width and monolith size

6.14.2.4.3 The influence of reinforcement on concrete cracking

6.14.2.5 State-of-the-Art Joints

6.14.2.5.1 General

6.14.2.5.2 Joint type

6.14.2.5.3 Joint filling materials

6.14.2.5.4 Present condition of joints

6.14.3 Durability Problems

6.14.3.1 General

6.14.3.2 How Concrete Hydropower Structures Become Damaged

6.14.3.3 Carbonation and Reinforcement Corrosion

6.14.3.5.2 Influential factors and prevention methods

6.14.3.6 Chloride and Sulfate Attack

6.14.3.9.2 Abrasion- and cavitation-induced deterioration in China

6.14.4.3 The Direction of Future Research

6.14.5 How to Maintain a Durable Concrete

6.14.6 Case Histories for Durable Concrete

6.14.6.1 Shi Lianghe Key Project in China

6.14.6.2 Fengman Hydropower Structure in China

6.14.6.3 Haikou Key Project in China

6.14.6.4 Butgenbach Dam in Belgium

6.14.6.5 Baoying Key Project in China

6.14.7 Conclusion

References

6.14.1 Introduction

6.14.1.1 General

6.14.1.1.1 Background of concrete hydropower structures

Concrete is one of the oldest and most durable building materials Its earliest known use was for a hut floor in the former Yugoslavia, dating from 5600 BC Later, more notable examples of the use of concrete include the Great Pyramid of Giza, Egypt, and the Pantheon in Rome, Italy (Figure 1) Portland cement came into use in 1854 and quickly became one of the most versatile and frequently applied construction materials in the world, owing to its availability, low cost, suitability for making simple shapes, and fire-resistant properties Concrete, however, is a brittle material with very low tensile stress, which limits its application To solve this problem, reinforced concrete started to be used in the mid-nineteenth century Nowadays, most concrete hydropower structures are applied in combination with steel reinforcement, which brings several advantages

Thermal effects Early cracking Cement shrinkage

Foundation stiffness and friction concrete hydraulic structures

Alkali−aggerate reaction (AAR) Seepage water scouring Abrasion and cavitation

Figure 1 The Pantheon, Rome, Italy [1]

● Steel can withstand a lot of tension but is not able to resist large compressive loads The opposite is true of concrete; therefore, the mass of concrete provides stability and protects against buckling

● Unprotected steel is subject to corrosion under normal atmospheric exposure, whereas concrete remains stable The alkalinity of concrete provides a passive environment where steel is less likely to corrode

● Steel and concrete possess similar coefficients of thermal expansion (CTEs) The CTE of steel is 1.2
10−5 and that of concrete is 1.0–1.5
10−5 When steel and concrete are combined, they do not exert undue strains on one another when subjected to wide temperature extremes

Although concrete can be reinforced or prestressed with steel, which can enhance its tensile and flexural strength, it is still a material that cracks easily Many concrete structures suffer cracking, spalling, loss of strength, or steel corrosion These faults are not only expensive to repair, but they also limit the life of structures

Most of the concrete hydropower structures in China have a life of only 50 years In Europe and the United States, the structures have a longer life but not as long as one might expect of such structures Research is needed to shed more light on the durability of concrete hydropower structures The aim is to at least double the life of concrete hydropower structures in the next century 6.14.1.1.2 The inadequacy of concrete hydropower structures

Generally, research on the inadequacy of concrete hydropower structures is divided into two branches, with one focusing on early cracking and the other on durability (see Figure 2)

The main causes of the early cracking of concrete include thermal effects, cement shrinkage, and foundation conditions These are the main factors that limit the monolithic dimension of concrete hydropower structures The main factors that influence the durability

of concrete hydropower structures include carbonation and reinforcement corrosion, freezing and thawing, concrete expansion and contraction, chloride and sulfate attacks, alkali–aggregate reaction (AAR), seepage water scouring, abrasion, and cavitation

The early cracking of concrete damages a whole structure’s integrity and makes it vulnerable to other faults due to an increase in permeability and a loss of mechanical resistance in the concrete Therefore, early cracking is actually a reason for concrete deterioration as well In this chapter, early cracking is treated separately from the durability problem because of the following:

Figure 2 Inadequate performance of concrete hydropower structures

● Controlling early cracking is very important for the correct performance of concrete hydropower structures during their lifetime

It is cheaper to repair early cracks than it is to fully restore a structure that has fallen into further disrepair

● The conditions that cause concrete to crack early generally happen over a short period in contrast to the conditions that cause long-term durability problems

This chapter will focus on concrete hydropower structures Concrete hydropower structures have all the characteristics of concrete structures, but since they are generally situated in water, they are more likely to experience the problems associated with water These problems include freezing and thawing, chloride and sulfate attacks, and AAR

To prevent concrete from cracking, transverse or longitudinal contraction joints are generally placed at the site where cracks are expected to occur However, each transverse and longitudinal joint induces a point of weakness where the joint filling material may age or be flushed away by seepage water or become permeable (when the joints are required to be impermeable) Therefore, making reliable stress estimations to determine the onset of cracking and the width of the cracks for young concrete is the basis for the dimension design of concrete hydropower structures

6.14.2.2 Definition of Early Cracking

According to the hydration degree of cement paste, concrete formation is generally divided into three stages: plastic stage, early stage, and matured stage [2]

1 Plastic stage

The plastic stage is the time from when the concrete is cast to when the concrete turns from a plastic to a solid state The end of this stage is called set time For plain concrete, the set time is reached about 6–12 h after concrete casting At the plastic stage, concrete is in a plastic state and under no stress Cement hydration is quite strong and the physical and chemical properties are very unstable The volume changes tremendously

2 Early stage

The early stage refers to the period lasting between 12 h and 90 days after concrete casting It is generally divided into two parts The first part is from 12 to 72 h after concrete placement At this stage, the cement hydration process is half complete and the microstructure of concrete is basically formed The stress and elastic modulus develop very fast The second part refers to the 72 h

to 90 days after concrete placement Cement hydration is almost finished at the end of this stage, and the development of the stress and elastic modulus slows down The concrete is nearly matured

3 Matured stage

The matured stage refers to the 90 days after concrete casting Slight hydration is still taking place inside the concrete The physical and chemical characteristics of the concrete are still developing but very slowly Research on the early cracking of concrete tends to be focusing on this stage presently

In this chapter, the term ‘early cracking’ refers to cracks that happen during the 12-h to 90-day period after concrete casting The main factors that cause early cracking will now be examined

6.14.2.3 The Causes of Early Cracking

When a concrete structure is relatively thick, excessive heat from cement hydration occurs The heat at the surface of the concrete

is easily released at the ambient temperature, while the center of the concrete remains at a high temperature This results in a large temperature difference in mass concrete, especially when the ambient temperature changes abruptly This then causes the center of the structure to expand at a different rate than the sides, and when the shrinkage or expansion is limited by the structure’s foundations (external restraint) or the structure itself (internal restraint), cracks may happen The main factors that influence the occurrence of thermal stress are:

● the cement content of the concrete – the more cement present in the concrete, the greater the chance of cracking occurring;

● ambient conditions – these include the air and subbase temperatures, and the friction of the subbase; and

● the size of the monolith – the greater the size of the monolith, the more difficult it will be to transmit hydration heat to an ambient environment and the greater the limitation of the subbase To prevent the influence of those effects, a hydropower structure is generally divided into several parts with joints

There are many ways to reduce concrete thermal stress The generally accepted strategies are to decrease the cement content or use low-heat cement to reduce the amount of heat produced, to apply low thermal expansion and crushed aggregate, to slow down the hydration process through the use of various admixtures, to reinforce the concrete with steel or fibers, to precool the concrete constituent materials, to decrease the internal temperature, to protect the exposed surfaces and formwork from environmental extremes, to section off or split concrete for a single structure with temporary or permanent joints, and to add concrete in several lifts

or pours

The casting and curing for mass concrete is very important The general thermal controlling process for a hydropower structure during construction is as follows: temperature anticipation → thermal stress controlling scheme → concrete casting and curing → temperature monitoring → scheme adjusting

6.14.2.3.2 Autogenous shrinkage

Autogenous shrinkage is the uniform reduction of internal moisture due to cement hydration, which is typical of high-strength concrete Autogenous shrinkage contributes significantly to concrete cracking when the water–cement (w/c) ratio is less than 0.4 [3] The use of concrete with a somewhat higher w/c ratio can mitigate this problem However, the strength and impermeability of concrete will be decreased if the w/c ratio is increased

The main factors that influence the autogenous shrinkage of concrete are w/c ratio and concrete maturity Concrete maturity is mainly influenced by ambient temperature and the type of cement There is no effective way to mitigate the autogenous shrinkage for high-performance concrete (HPC)

6.14.2.3.3 Drying shrinkage

Drying shrinkage is caused by nonuniform drying of concrete after curing and the removal of forms It increases with a greater w/c ratio [4] Cracks caused by cement drying shrinkage generally happen 1 week after concrete casting or shortly after concrete finishes curing

Ambient conditions such as air temperature, wind, relative humidity, or sunlight may draw moisture from exposed concrete structures, which will cause water content to gradient from the inner core to the surface of the concrete As the moisture is lost in the small pores of the surface concrete, the surface tension of the remaining water tends to pull the pores together, which results in a loss

of volume over time However, this shrinkage will be limited by ambient conditions such as the inner concrete or the foundations

If the tensile stress produced surpasses the tensile strength of the surface concrete, cracks will appear

Hydropower structures such as stilling basins, flashboards, and working bridges, whose surface areas are large relative to their volume, are more likely to have cracks caused by drying shrinkage

Two factors influence concrete drying shrinkage: the concrete characteristics and the ambient conditions Concrete characteristics include w/c ratio, cement type, dosage of cement, aggregate characteristics and dosage, admixtures, and so on Ambient conditions include air temperature, relative humility, and so on, which may draw moisture from the concrete

Techniques for reducing concrete drying shrinkage are as follows: to apply low shrinkage cement or cement with low hydration heat, to decrease the quantity of cement (as long as the concrete satisfies the strength requirements and can obtain adequate placement), to decrease the w/c ratio, to maintain at a minimum the quantity of fine aggregate that will just produce adequate workability and finishing characteristics, to prolong the moisture curing time of concrete, to set contraction joints (temporary or permanent), and to apply admixtures such as a water reducer or a shrinkage reducing admixture

6.14.2.3.4 Uneven settlement of the foundations

There are two possible reasons for the uneven settlement of the foundations: one is the unevenness of the foundation soil and the other is the unevenness of the stress works on the foundations Uneven settlement of the foundations can cause often deep or even transverse cracks to occur The direction of the cracks is influenced by the condition of uneven settlement Cracks are generally perpendicular or 30–45° to the vertical direction When settlement is finished, the width and length of the cracks will remain stable

In China, national design codes for hydraulic structures dictate that the mean settlement for a monolith is not allowed to be larger than 10 cm, and the uneven settlement for a single structure is not allowed to be larger than 5 cm

6.14.2.3.5 Foundation friction

Besides foundation settlement, foundation friction is another factor that may induce uncontrolled cracking because of the high friction and, in some cases bonding, between the foundations and the concrete slab The friction or bonding restrains the concrete’s volume change (shrinkage or contraction), inducing higher stresses than might occur in concrete that directly comes into contact with the foundations The higher the foundation friction, the more likely that cracks will occur For concrete sat on rock foundations, the problem caused by foundation friction is more prominent

Friction-initiated cracks are likely to happen from the bottom of the slab and travel toward the slab surface Cracks from high friction can be erratic in orientation but follow zones of restraint between the concrete and the subbase

To decrease the friction between the concrete and the subbase, a thin layer (generally 10–20 cm) of cementitious material (such

as cement, asphalt, sand, or clay) is often applied at the interception However, this method can only be applied when the horizontal force working on the structures is relatively small and the structures are stable under the decreased friction force 6.14.2.4 Controlling the Cracking of Concrete Hydropower Structures

6.14.2.4.1 Different design concepts on crack width and monolith size

The process of avoiding the onset and further development of cracks in concrete is very complicated, and accurate solutions are not available At present, countries fall into one of two groups in the way they deal with the cracking of concrete:

● For countries in the first group, there are no definite rules on the maximum crack width or the maximum size of the monolith In national design codes for hydraulic structures, calculations concerning cracking are only for reference and engineers design structures based on their own experience Many hydropower structures have no contraction/expansion or settlement joints

If cracks that do occur cannot be blocked, other complementary repair methods will be undertaken, such as drainage Japan, the United Kingdom, and the United States apply this design concept

● For countries in the second group, there are formulae in national design codes that must be obeyed in estimating cracks caused due to loading There are no specific methods for estimating cracks caused by deformation However, there are limitations on the maximum monolith size It is supposed that if the size of the monolith is limited, the problems of cracking will not exist The Soviet Union, some European countries, and China adhere to this design concept

6.14.2.4.2 Experience of limiting crack width and monolith size

Cracks that can be seen with the naked eye are generally wider than 0.02–0.05 mm Cracks with a width less than 0.05 mm are not harmful to the structure No-cracking concrete refers to concrete whose crack width is less than 0.05 mm [5]

The criteria applied by engineers all over the world to limit the width of cracks are more or less the same, as follows:

● If no harmful ions are present in the environment and there are no seepage control requirements, the maximum allowable crack width is 0.3–0.4 mm

● If the environment contains slightly harmful ions and there are no seepage control requirements, the allowable crack width is 0.2–0.3 mm

● If the environment contains very harmful ions and the structure has seepage control requirements, the allowable crack width is 0.1

6.14.2.4.3 The influence of reinforcement on concrete cracking

Thanks to research, the potential for cracks to appear due to thermal stress, foundation condition, and structural load can be estimated accurately with software on the basis of finite element methods However, how the cracks will develop, especially under the influence of reinforcement, and the exact interrelationship between concrete drying shrinkage and steel reinforcement are topics for future research

It is widely agreed that steel reinforcement can decrease the width and depth of a crack once a crack emerges Cracks in reinforced concrete will be denser and smaller compared with cracks in non-reinforced concrete

6.14.2.5 State-of-the-Art Joints

6.14.2.5.1 General

In order to prevent concrete cracking caused by uneven settlement, thermal effects, autonomous shrinkage, drying shrinkage, or other effects, different parts of hydropower structures are often connected with transverse joints These joints are generally set at the site

where the internal force is low or the elevation of the base changes abruptly The normal width for the joints is about 1.0–2.5 cm The joints permit minor differential movements between adjacent blocks, and in their absence major transverse cracks may develop 6.14.2.5.2 Joint type

Nearly every concrete hydropower structure has joints that must be sealed to ensure the integrity and durability of the structure Joints are divided into two categories: temporary joints and permanent joints

Temporary joints: Temporary joints refer to construction joints – the actual surfaces where two successive placements of concrete meet To assist in the construction and the placement of concrete, temporary joints are designed and positioned at certain locations during the placement of mass concrete The concrete at the point of stoppage becomes a construction joint when concrete placement continues Size of placement and time are contributory factors in the use of construction joints In monolithic placements, temporary joints are required to be fully bonded across the construction joint for structural integrity If the concrete construction is not too large, temporary joints may be designed to correspond with permanent joints

Permanent joints: Permanent joints are generally set for concrete expansion, contraction, or uneven settlement Permanent joints can

be made to be cross-sectional or just shallow grooves on the concrete’s surface Shallow grooves are generally formed by a sawing machine They are applied only as an auxiliary method to facilitate concrete expansion or contraction Cross-sectional joints divide a structure into two or more monoliths To avoid the abutting concrete elements from touching each other, they are often filled with soft and elastic materials

Joints are further divided into two types according to their direction: vertical joints and horizontal joints Vertical joints are normally flexible, allowing for uneven settlement between different parts (Figure 3) Horizontal joints (Figure 4) are generally rigid, allowing for only slight horizontal movements

6.14.2.5.3 Joint filling materials

For hydropower structures, joints are often required to endure both minor structural movements and high hydrostatic pressures Therefore, the joints need to be robust, flexible, and sometimes waterproof To meet those requirements, the space of the joint needs

to be filled with some premolded, nonabsorbent, nonreactive, nonextruding materials that are either rigid or flexible Rigid materials are metallic materials: steel, copper, and, occasionally, lead Flexible materials include polyvinyl chloride (PVC) mats, rubber, asphalt-impregnated fiber sheets (or rolls), or compressible foam strips Figure 5 illustrates the application of compressible foam mats in a concrete structure

Figure 3 Vertical joints

Asphalt felt

2 cm in thickness Horizontal copper waterstop

Filled with asphalt

Vertical copper waterstop

Asphalt felt

Asphalt well

2 cm in thickness 2 mm Steel shell

Figure 4 Horizontal joints

Figure 5 Foam mats

Joint filling materials must be resistant to harsh weather conditions, alkalis, fungi, musts, oils, greases, and other agents, as well

as silicone or polyurethane-based mastics They are installed across the joints One end of the joint filling material is often embedded in concrete, while the other end is free in the adjoining monolith to permit slight movement of the concrete without any restriction

Copper is an expensive joint filling material, yet it is very durable and waterproof (Figures 3 and 4) Copper joints are generally applied where water stoppage is very critical For very important projects, two lines of copper waterstop are often required The general thickness for copper waterstop is 1.2–1.4 mm

The joint filling materials such as PVC or rubber are more affordable than copper, but they age quickly, especially in sunlight Nevertheless, they are common types of waterstop In China, PVC or rubber is mainly used as the joint-filled material for small hydropower structures or as the remedy of the main waterstop

6.14.2.5.4 Present condition of joints

The selection of joint-filling material differs from country to country, often depending on a country’s economic conditions and the availability of materials In China, copper joints are widely used as joint-filling material for important hydropower structures However, copper is expensive and working with it is labor intensive, which limits its application in developed countries In Europe, rubber joints are more frequently used, but their quality is yet to match that of copper joints

Following the expansion of the rubber and plastics industry, some new shapes of rubber or PVC joints have started to be used in hydropower structures in China (Figures 6 and 7) Figure 8 is rolls of rubber waterstop applied in a hydropower structure However, these types of joints are still at an experimental stage since it is not known how long their life will be

Figure 6 PVC waterstop

Figure 7 Rubber waterstop

Figure 8 Rolls of rubber waterstop

Although joints are being widely used and presenting operational results, the repair or recovery of the joints is very difficult should an accident or localized rupture in the joints occur after they have been installed Because of the durability problem and the difficulty of repairing joints, the joints are often sited at the weakest point of a hydropower structure Therefore, it is preferable to construct a concrete hydropower structure without joints if cracks are to be prevented

In the quest to build environmentally sustainable concrete hydropower structures, it is clear that durability is a more important consideration than strength [6] Durable concrete can be defined as concrete that is designed, constructed, and maintained to perform satisfactorily in the expected environment for the specified designed life It is the ability of the concrete to resist any process

of deterioration

In the early part of this century, research into the durability of concrete is focused on long-term performance standards, materials, optimization techniques, the science of chemical admixtures, and advancing the understanding of aggregate–paste transition zones [7] The latter part of the century could offer opportunities to research affordable defect-free concrete that is able

to stand the test of time in any predicable environment

6.14.3.2 How Concrete Hydropower Structures Become Damaged

The main cause of concrete damage is concrete permeability resulting from the environment [8] If concrete is constructed correctly, the microcracks and voids inside it will not form interconnection pass ways to the surface of the concrete; the concrete is basically waterproof After a certain period of time, however, the voids and microcracks in the concrete will enlarge due to environmental factors such as freezing–thawing cycles, loading, and chemical attacks Once an unconnected pass way becomes interconnected, reactive ions or water can enter the concrete, causing the pass way to enlarge further and the concrete to gradually fall into a state of disrepair due to inflation, cracking, weight loss, and decreased strength

The factors that influence the durability of concrete hydropower structures include carbonation and reinforcement corrosion, freezing and thawing, expansion and contraction, chemical attacks, AAR, seepage water scouring, abrasion, and cavitation Other aspects such as structural design, construction defects, biological process, and fire damage can also influence the durability of concrete hydropower structures but to a lesser extent

6.14.3.3 Carbonation and Reinforcement Corrosion

6.14.3.3.1 General

In fresh concrete, the presence of abundant amounts of calcium hydroxide gives concrete a very high alkalinity with a pH of 12–14 However, when carbon dioxide, which is present in the air, dissolves in the pore solution of the surface concrete and reacts with calcium hydroxide according to the following equation, the pH value will drop:

CaðOHÞ2 þ CO2 → CaCO3 þ H2O This is called carbonation The pH value of a fully carbonated concrete is about 7 (neutral) During carbonation, the outer part of concrete is affected first, but with the passage of time, the inner mass also becomes affected as carbon dioxide diffuses inward from the surface If all influencing factors remain constant, the depth of penetration of the carbonation front is thought to be proportional to the square root of the time of exposure Concrete carbonation and reinforcement corrosion are major factors that affect the durability of concrete hydropower structures in North America, Europe, the Middle East, and other parts of the world The problem is more serious in places where the temperature and levels of humidity are relatively high

6.14.3.3.2 Working mechanism

Normal carbonation results in a decrease of the porosity of concrete, making the carbonated paste stronger Carbonation is, therefore, an advantage in non-reinforced concrete but a disadvantage in reinforced concrete When concrete is at an early age, the high alkalinity results in the formation of a passive film on the surface of the embedded steel in reinforced concrete This layer is durable and self-repairing, and as long as the film is undisturbed, it will protect the steel from corrosion When carbonation happens, the alkalinity of concrete reduces If the pH value of the carbonated concrete drops below 10.5, the passive layer will decay, exposing the steel to moisture and oxygen and making it susceptible to corrosion The solid corrosion product (rust) occupies a larger volume (2–2.5 times of the previous volume) than that of the steel destroyed and exerts a pressure on surrounding concrete, causing cracking and spalling

For a hydropower structure that is subject to concrete carbonation and reinforcement corrosion, the concrete appears to be sound with relatively little macroscopic cracking initially Over time the macroscopic cracks enlarge and the concrete surface is stained by reddish corrosion products Furthermore, spalling of the concrete covering the reinforcing steel is visible due to the formation of voluminous corrosion products Eventually, severe spalling of the concrete covering the reinforcing steel is evident, leaving the reinforcing steel bars directly exposed to the atmosphere Besides carbon dioxide (CO2), hydrochloric acid (HCl), sulfur dioxide (SO2), and chlorine (Cl2) in the environment can also decrease the high alkalinity of concrete and break the passive film over steel However, this is not very common generally

6.14.3.3.3 Influential factors

The speed of carbonation is influenced by two factors: the quality of the concrete and the environmental conditions

Quality of the concrete: For good quality concrete that is properly strengthened and has no cracks and little porosity, the expected rate of carbonation is very low The quality of concrete is influenced by the w/c ratio, cement content, type and dosage of admixture, cement grade, type of curing aid, the water curing period, and the temperature when the concrete is initially cast and exposed A decrease in the w/c ratio and an increase in cement content and the water curing period will result in a decrease in concrete carbonation

Generally, the exposed surfaces of concrete should be kept wet for at least 7 days from the date of placing However, longer curing periods – up to 28 days – are recommended for blended cement Concrete that is initially cast and exposed in the winter exhibits lower carbonation than concrete that is cast in the summer Compared with plain concrete, concrete that incorporates mineral admixtures such as fly ash generally shows higher resistance to carbonation

Environmental conditions: The speed of carbonation is also affected by environmental conditions such as relative humidity, carbon dioxide density, and ambient temperature Carbonation occurs fastest when the relative humidity reaches 55% [9] Above this value, with an increase in the relative humidity, the speed of carbonation decreases Although the carbonation speed slows down for concretes when the relative humidity is 90–95%, the rate of corrosion is at its highest Concrete carbonation increases with

rising carbon dioxide levels, and this fact goes some way to explaining why some concrete structures located close to highways or factories show little resistance to carbonation The ambient temperature also influences the rate of chemical reactions Carbonation speed will increase with an increase in temperature With an increase in temperature of 10 °C, the rate of reaction

is approximately doubled

6.14.3.3.4 Prevention methods

Having examined the factors that affect the speed of carbonation (the quality of the concrete and the environmental conditions), it is possible to deduce two ways to prevent concrete carbonation from occurring One way is to make concrete stronger, for example, by decreasing the w/c ratio while increasing the amount of concrete covering the steel, thus preventing concrete from cracking However, this method can only be used for newly built concrete structures A second way is suitable for old concrete structures It involves the application of specialized surface coatings if the depth of carbonation is less than the depth of the concrete covering the steel 6.14.3.4 Freezing and Thawing

6.14.3.4.1 General

Hardened cement paste, like sand and stone aggregates, is a porous solid and will absorb water If the temperature drops to freezing point, the water in the porous concrete will freeze The transition of water from a liquid state to a solid state involves an increase in volume by about 9%, which can damage the cement paste by pushing the capillary walls and generating hydraulic pressures Concrete subjected to continuous or frequent wetting is susceptible to damage by freezing and thawing cycles Several cycles of freezing and thawing of water may result in the spalling of concrete

The freezing and thawing of plastic or green concrete is serious and usually results in permanent damage, even with a single cycle The volume change that accompanies the freezing tends to increase the space between particles of cement and aggregate so that later bridging by hydration products can only be partly achieved Such concretes, when hardened, have sharply reduced strengths and much higher porosity

6.14.3.4.2 Influential factors

Concrete’s resistance to freezing and thawing depends upon several parameters, such as pore size, the distribution of pores and capillaries, the age of the concrete, aggregate type, the degree of saturation, and freezing–thawing cycles Deterioration of concrete caused by freezing and thawing may occur when approximately 91% of the pores are filled with water The freezing–thawing cycle will start from the first freezing and thawing and will continue throughout successive winters, resulting in repeated loss of concrete surface Concrete with a high water content and a high w/c ratio is less resistant to freezing and thawing than concrete with a low water content Because concrete is workable, the level of water in the mix is normally much greater than that needed for hydration Excess water in the mix is undesirable because spaces filled with water in the original mix become voids in the concrete when the water not used in hydration evaporates

Following the curing period, normal drying of dense concrete is vital in aiding resistance to damage caused by freezing and thawing because once the original water in the concrete has been largely used by hydration or lost by evaporation, the process of reabsorption through normal rewetting is very slow In dry concrete, it is difficult to achieve a near-saturation state with exposure to periodic wetting Therefore, adequate drainage to provide rapid water runoff is an important way to prevent damage caused by freezing and thawing

Improving the quality of concrete: Concrete that is dense is difficult to permeate and has a low w/c ratio is relatively resistant to the effects of freezing and thawing In actual fact, high grade concrete (grade C50 or above) is generally more resistant to freezing and thawing because the higher cement content needed for producing the higher strength concrete effectively lowers the w/c ratio

6.14.3.5 Concrete Expansion and Contraction

6.14.3.5.1 General

Temperature-related expansion and contraction of surface moisture exerts a mechanical action and results in the gradual wearing of concrete’s surface Concrete expansion and contraction happens in those areas where the humidity and temperature changes periodically

As already mentioned, hardened concrete is a porous solid that can absorb water If the ambient conditions such as air temperature, wind, relative humidity, and sunlight change, the moisture (or water content) or temperature in concrete will change and, hence, cause structures to expand or contract If those volume changes are limited by aggregates, foundations, or neighboring structures, cracks may occur

Thin hydropower structures, whose surface areas are large relative to their volume, are subject to the strains of concrete expansion and contraction Cracks caused by concrete expansion and contraction can occur on upper structures such as flashboards or bridges – the splash zone of the piers, the stilling basin, and the apron being frequently exposed to air

6.14.3.5.2 Influential factors and prevention methods

The deterioration of concrete resulting from expansion and contraction is influenced by two main factors: the geometric condition

of the structure and the ambient conditions For hydropower structures exposed to the air or in the splash or tidal zone, irregular cracks are very likely to occur However, the ambient conditions are generally very difficult to change The only way to relieve the deterioration of concrete through expansion and contraction is to decrease the limitation of neighboring structures or to make the structures more crack resistant The use of aggregates with low coefficient of expansion may also prevent cracks from happening but

in a less effective way

6.14.3.6 Chloride and Sulfate Attack

Once chloride ions have reached the reinforcement in sufficient quantities, they will depassivate the embedded steel by breaking down the protective oxide layer normally maintained by the alkaline environment, and hence cause the reinforcement to corrode The rust formed has a larger volume than that of the steel consumed, which will cause concrete to crack

The speed of chloride diffusion is influenced by two main factors: the quality of the concrete and the ambient conditions For dense concrete, the diffusion speed of the chloride ions will be slow, and hence more time will be taken for chloride ions to reach the steel surface The environment to which the reinforced concrete structure is exposed affects the extent of a chloride attack as well The speed of chloride penetration is greatly influenced by the degree of saturation

There are generally two ways to prevent a chloride attack The first method involves enhancing the quality of concrete (e.g., by choosing proper materials, having adequate cover over reinforcements, paying attention to the environmental changes during construction) and concrete coating The second method can be very effective for newly built structures or for old structures whose chloride quantity at the depth of the reinforcement is below the chloride threshold

6.14.3.6.3 Sulfate attack

A sulfate attack occurs in hardened concrete when sulfates, found in sea water, in some soils or in wastewater, react with the tricalcium aluminates (C3A) in Portland cement paste The reaction causes a material called ettringite to form The ettringite produced occupies a greater volume within the concrete than the calcium aluminate hydrates, which results in concrete expansion and irregular cracking The cracking of concrete provides further access to penetrating substances and to progressive deterioration The higher the C3A content of the cement, the more chance there will be of a sulfate attack

Gypsum (calcium sulfate) is present in some of the clay soils in the south of England and also occurs in desert soils at locations where the water table is close to the surface The more soluble sulfate salts – sodium sulfate (Glauber’s salts) and magnesium sulfate (Epsom salts) – may be present in some rock formations in the United Kingdom but are more extensive in the alkali soils of North America In northeast China, a lot of hydropower structures are experiencing sulfate-induced deterioration

In addition to the two methods that can help to prevent a chloride attack, concrete can also be protected against a sulfate attack

by limiting the aluminates to a level between 3% and 8% According to research, blended cements perform better than ordinary Portland cement when subjected to a sulfate attack, and pozzolanic materials such as fly ash, silica fume, and rice husk ash provide moderate resistance

alkali þ reactive silica þ water → ASR → expansive gel The gel produced by the reaction has a very strong affinity for water, and thus has a tendency to swell Once the gel is formed, it may migrate through the porous structure of the concrete, following preexisting voids and fractures If it meets with water, the gel may absorb the water and expand, causing microfracturing of the material and, ultimately, a characteristic map-cracking pattern on exposed surfaces

Besides ASR, carbonate minerals may also cause deterioration of concrete due to an alkali attack but in an uncommon way (alkali–carbonate reaction (ACR)) Until recently, ACR was found only in North America in some concretes containing dolomite aggregates The vast majority of sources of dolomitic limestone aggregates have not been found to be deleteriously reactive, and have been used successfully for the production of concrete [10]

The reaction time of AAR ranges from 2–3 to 40–50 years depending on the different aggregates in the concrete For ACR, cracks will happen only after 2–3 years, while for ASR, the reaction time is generally around 10–20 years For some ASR, the reaction time can be very long, with cracks happening after 40–50 years However, once cracks appear, the compressive strength and the modulus

of elasticity of the concrete will decrease tremendously, which is very difficult to rectify

Often, the first external signs of alkali reaction in a structure are short fine cracks on the surface radiating from a point The cracks occur adjacent to fragments of reacting aggregates and are caused by the outward swelling pressure As time passes, the cracks propagate and eventually join up to form maplike patterns (see Figure 9) The macrocracks, which appear on the surface, are generally found to be 25–50 mm deep and to occur roughly at right angles to the surface

6.14.3.7.2 Sodium equivalent

In order to make simple comparisons between different cements, the alkali content is usually expressed as total alkali content or

‘sodium equivalent’ To obtain the equivalent sodium oxide content, the potassium oxide content is factored by the ratio of the molecular weights of sodium oxide and potassium oxide, and added to the sodium oxide content [10]

Total alkali ðas equivalent Na2OÞ ¼ Na2O content þ 0:658
ðK2O contentÞ The alkali content of cement depends on the materials from which it is manufactured and also to a certain extent on the manufacturing process, but it is usually in the range of 0.4–1.6%

In 1941, the United States declared that, in order to prevent AAR, the aggregates used in concrete construction must be tested and the alkali content of cement must be less than 0.6% More than 20 countries, including China, have accepted and added this criterion to their national standards In many countries such as New Zealand, England, and Japan, cement is produced with the alkali content lower than 0.6%

6.14.3.7.3 AAR in the world

Damage due to AAR in concrete was first recognized in the United States in 1940 and has been observed in many countries since then The maintenance costs of concrete structures suffering from AAR in Western Europe accounts for approximately 10% of the

Figure 9 ASR attack, Ontario, USA