Design and Control of Concrete Mixtures CHAPTER 14

Design and Control of Concrete Mixtures CHAPTER 14
Durability Design and Control of Concrete Mixtures CHAPTER 14 Design and Control of Concrete Mixtures, 16th edition, Chapter 14 - Durability

Overview Deterioration mechanisms and mitigation Abrasion and Erosion
Freezing and Thawing Exposure to Deicers and Anti-Icers Alkali-Aggregate Reactivity Alkali-Silica Reaction Alkali-Carbonate Reaction Carbonation Corrosion Sulfate Attack Salt Crystallization or Physical Salt Attack Delayed Ettringite Formation Acid Attack Seawater Exposure This module will discuss the durability of concrete, addressing the factors that affect durability and the various deterioration mechanisms and the mitigation strategies for each.

Durability The durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties for the expected service life of the structure. To ensure durability of concrete, care in materials selection, design, and construction is most important, with increased care necessary in more severe environments.

Durability Although a durable structure is expected to serve without deterioration or major repair before expiration of its design life, it must not be presumed that provision of durability is a substitution for maintenance. Even structures designed and constructed to a high durability standard require regular inspection and routine maintenance.

Factors Affecting Durability
Concrete subjected to severe exposure conditions should be highly impermeable. Permeability refers to the ease of fluid migration through concrete when the fluid is under pressure or to the ability of concrete to resist penetration by water or other substances. Diffusivity refers to the ease with which dissolved ions move through concrete. Permeability and diffusivity are influenced by porosity, but are distinct from porosity. Porosity is the volume of voids as a percent (or fraction) of the total volume. Permeability and diffusivity are affected by the connectivity of the voids. This schematic shows two hypothetical porous materials with approximately the same porosity. However, in the material on the left the pores are discontinuous (as would be the case with entrained air bubbles), while in the other the pores are continuous. The material on the right would be much more permeable than the one on the left.

Permeability The image shows the relative sizes of the various pores and solids found in concrete. The capillary pores are primarily responsible for the transport properties.

Permeability Adapted from Powers 1958
As a rough guide, the permeability versus capillary porosity for cement paste is plotted in the figure. It can be seen that as the porosity increases above about 30%, the permeability increases dramatically. Decreasing the porosity below 30% reduces the permeability, but any additional benefits obtained are relatively minor. The pore system of cement paste becomes discontinuous at about 30% porosity. Adapted from Powers 1958

Permeability The time required for capillary pores to become discontinuous with increasing hydration of the cement is shown in the table to the right. It is notable that mixtures with a water-cement ratio greater than 0.7 will always have continuous pores. Whiting 1989

Approximate Age Required to Produce Maturity at Which Time Capillaries Become Discontinuous for Concrete Continuously Moist-Cured The table illustrates the relationship between water-cement ratio, permeability and curing. Observe the importance of using a low water-cement ratio. Typically, the water-to-cementitious materials ratio is limited to a maximum of 0.40 to 0.50 when concrete is being designed for durability, depending on the specific conditions of exposure. Powers and others 1959

Permeability Test Methods Used to Determine Various Permeability-Related Properties Various methods (both direct and indirect) exist to determine permeability of concrete. Several methods from ASTM, AASHTO, and the Army Corps of Engineers are outlined in this table.

ACI 318 Categories for Durability
This table shows the four main durability-related exposure categories covered in ACI 318. The specifier selects the relevant exposures for each component of the concrete structure, and determines the one that requires the greatest resistance in terms of the lowest water-to-cementing materials ratio and the highest minimum concrete strength. ACI 318 only addresses exposures to freezing and thawing, soluble sulfates in soil or water, conditions that need precautions to minimize corrosion of reinforcing steel, and conditions that will need low permeability for concrete members in contact with water. Additional durability concerns specific to a project need to be separately addressed by the specifier, such as ASR.

Cracking and Durability
Cracks allow ingress of moisture Adversely affects durability regardless of concrete quality Cracks can be controlled through jointing and reinforcement No matter how good the quality of concrete, if the concrete cracks extensively, moisture can enter the concrete and adversely affect its durability. Two basic causes of cracks in concrete are: (1) stress due to applied loads, and (2) stress due to volume changes when concrete is restrained. Random cracking may be avoided by the use of proper joint spacing to predetermine the location of the cracks, or by the use of properly sized and positioned reinforcing steel to reduce crack widths or increase joint spacing.

Abrasion and Erosion Defined by ACI 2010 Abrasion
“…wearing away of a surface by rubbing and friction.” Erosion “…progressive disintegration of a solid by the abrasive or cavitation action of gases, fluids, or solids in motion”. Concrete surfaces that are exposed to strong mechanical stress require high abrasion and erosion resistance.

Abrasion and Erosion Wear on concrete surfaces can occur in the following situations: Floors and slabs due to pedestrian and wheeled traffic Pavements and slabs subject to vehicular traffic Sliding bulk material Impact stress of heavy objects Erosion of hydraulic structures from the impact of objects transported by the fluid Cavitation in hydraulic structures Abrasion by ice in marine structures

Abrasion and Erosion Courtesy of R.D. Hooton
Shown here is a picture of aggregate exposed after decades of use on a concrete pavement. Courtesy of R.D. Hooton

Abrasion and Erosion Shown here is an example of erosion in the stilling basin of Kinzua Dam, Pennsylvania. Cavitation is the result of bubbles collapsing in a fast moving stream of water. Vapor bubbles are formed as the water moves over surface irregularities and later collapse explosively, causing damage to the concrete surface.

Abrasion and Erosion Testing
ASTM C779- Standard Test Method for Abrasion Resistance of Horizontal Concrete Surfaces Others include: ASTM C418- Standard Test Method for Abrasion Resistance of Concrete by Sandblasting ASTM C944- Standard Test Method for Abrasion Resistance of Concrete or Mortar Surfaces by the Rotating-Cutter Method ASTM C1138- Standard Test Method for Abrasion Resistance of Concrete (Underwater Method) Rotating drum Rotating wire brushes ASTM C779, Standard Test Method for Abrasion Resistance of Horizontal Concrete Surfaces, the most commonly referenced concrete abrasion test method, offers three loading regimes: revolving disks, dressing wheels, and ball bearings. There is little correlation between the different loading regimes in this test method, making it difficult to predict wear from one mechanism based on data from another test. Other available tests for abrasion and erosion include ASTM C418, Standard Test Method for Abrasion Resistance of Concrete by Sandblasting, which uses the depth of wear under sandblasting; ASTM C944, Standard Test Method for Abrasion Resistance of Concrete or Mortar Surfaces by the Rotating-Cutter Method, which uses rotating cutters; ASTM C1138, Standard Test Method for Abrasion Resistance of Concrete (Underwater Method), which simulates the effects of swirling water or cavitation; clamping concrete slabs inside a rotating drum filled with steel shot or aggregate; and rotating wire brushes.

Abrasion and Erosion Resistance
Test results indicate that abrasion resistance is closely related to the compressive strength of concrete. This graph shows results of abrasion tests on concretes of different compressive strengths and different aggregate types. Liu 1981

Abrasion and Erosion Resistance
It is critical that the surface of the concrete be as durable as possible, which requires careful selection of finishing techniques for interior and exterior applications. A steel-troweled surface resists abrasion better than a surface that had not been troweled. This graph illustrates the effect surface treatments, such as metallic or mineral aggregate surface hardeners, have on abrasion resistance of hard steel troweled surfaces. Brinkerhoff 1970

Freezing and Thawing Exposure
Freeze-thaw exposures range from moderate (category F1), such as a façade element exposed to occasional moisture, to very severe (category F3) For example, a pavement or bridge decks in continual contact with moisture and exposed to deicer chemicals or freezing seawater

Freezing and Thawing As the water in moist concrete freezes, it produces osmotic and hydraulic pressures in the capillaries and pores of the cement paste and aggregate. If the pressure exceeds the tensile strength of the surrounding paste or aggregate, the cavity will dilate and rupture. Deterioration is visible in the form of cracking, scaling, and disintegration, as shown.

Freezing and Thawing Concrete damage due to freezing and thawing cycles is the result of complex microscopic and macroscopic interactions closely related to the freezing behavior of the pore solution. Ice in capillary pores (or any ice in large voids or cracks) draws water from surrounding pores to advance its growth. Shown above is a sample of sawn and polished concrete damaged by freeze-thaw cycles.

Freezing and Thawing Pore Size Distribution Adapted from Setzer 1997
The pore solution of concrete contains a high quantity of dissolved ions which lower the freezing point. The freezing point of the pore solution is dependent on pore size; the smaller the pore, the lower the freezing point, as shown in the table. Adapted from Setzer 1997

Freezing and Thawing Water expands 9% upon freezing
Critical saturation of pores at 91.7% filled Osmotic pressures caused by differential alkali concentrations Osmotic pressures considered major in salt scaling Hydraulic pressures are caused by the 9% expansion of water upon freezing; in this process growing ice crystals displace unfrozen water. If a capillary is above critical saturation (86 to 91.7% filled with water), hydraulic pressures result as freezing progresses. Osmotic pressures develop from differential concentrations of alkali solutions in the paste. Osmotic pressures are considered a major factor in salt scaling.

Exposure to Deicers and Anti-icers
V-shaped joints are a common sign of the effects of freeze-thaw damage in concrete pavements. Some joints exhibit an inverted V-shaped deterioration as shown here. Courtesy of D. Harrington

Exposure to Deicers and Anti-icers
Scaling is more severe in poorly drained areas because more of the deicer solution remains on the concrete surface during freezing and thawing. The graph shows cumulative mass loss for mixtures with a water to cement ratio of 0.45 and on-time finishing. Pinto and Hover 2001

Deicers and Anti-icers
This table summarizes the most commonly used snow and ice control materials (NCHRP 2007). Chloride-based salts containing sodium, calcium, magnesium, and potassium (NaCl, CaCl2, MgCl2, and KCl) comprise the majority of deicers used to melt snow and ice. These chemicals work well because they lower the freezing point of the precipitation that falls on concrete pavements.

Deicers and Anti-icers
Deicers that contain ammonium nitrate or ammonium sulfate are not recommended for use on concrete because they severely attack concrete. All deicers can aggravate scaling of concrete that is not properly air entrained. Studies have shown that concentrated calcium chloride solutions can chemically attack concrete.

Freeze-Thaw and Deicer Scaling Testing
ASTM C666 Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing ASTM C672 Standard Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals The most commonly used tests for freezing and thawing and deicer scaling are ASTM C666, Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing, and ASTM C672, Standard Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals. These test methods normally subject samples of concrete to a number of freezing and thawing cycles in order to obtain an accelerated indication of the degree of deterioration associated with long-term exposure.

Durability Factor A durability factor is then calculated according to the expression below: Where: DF = durability factor of the specimen tested Pn = relative dynamic modulus at N cycles (%) N = number of cycles at which the test specimen achieves the minimum specified value of Pc for discounting the test or the specified number of cycles of the test, whichever is less M = specified number of cycles of the test (typically M = 300)

Freeze/Thaw and Deicer Scaling Testing
As an indication of the degree of freeze-thaw resistance, it is suggested that a concrete of poor frost resistance would have a durability factor below 20%, while concrete with good frost resistance would have a durability factor greater than 80%. (Non-Air Entrained Concrete) Pinto and Hover 2001

Freeze/Thaw and Deicer Scaling Mitigation
The resistance of hardened concrete to freezing and thawing and deicers in a moist condition is significantly improved by the use of intentionally entrained air, as shown in this graph. Bates and others 1952, and Lerch 1960

Effect of weathering on boxes and slabs on the ground at the Long-Time Study outdoor test plot; Skokie, Illinois. Specimens at top are air-entrained. Specimens at bottom exhibiting severe crumbling and scaling are non-entrained. All concretes were made with 335 kg (564 lb) of Type I portland cement per cubic meter (Cubic yard). Periodically, calcium chloride deicer was applied to the slabs. Specimens were 40 years old when photographed.

Methods to Control Freeze/Thaw
ASTM C457 Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete calculated spacing factor, L, of less than mm (0.008 in.) specific surface, α, of 24 mm 2 mm 3 (600 in 2 in 3 ) The spacing and size of air voids are important factors contributing to the effectiveness of air entrainment in concrete. ASTM C457 describes a method of evaluating the air-void system in hardened concrete. Most authorities consider the following air-void characteristics as representative of a system with adequate freeze-thaw resistance: calculated spacing factor, L, (an index related to the distance between bubbles but not the actual average spacing in the system) of less than mm (0.008 in.) and specific surface, α, (surface area of the air voids) of 24 square millimeters per cubic millimeter (600 sq in. per cubic inch) of air-void volume, or greater.

Freeze/Thaw and Deicer Scaling
This graph illustrates the relationship between spacing factor and total air content. Measurement of air volume alone does not permit full evaluation of the important characteristics of the air-void system; however, air-entrainment is generally considered effective for freeze-thaw resistance when the total volume of air in the mortar fraction of the concrete is about 9 ± 1% or about 18% by paste volume. Pinto and Hover 2001

Deicer Scaling Pinto and Hover 2001
A good air-void system with a low spacing factor (maximum of 200 micrometers) is perhaps more important to deicer environments than saturated frost environments without deicers. The relationship between spacing factor and deicer scaling is illustrated in this graph. Pinto and Hover 2001

Deicer Scaling Pinto and Hover 2001
This graph illustrates the overriding impact of air content over water-cement ratio in controlling scaling. Pinto and Hover 2001

Deicer Scaling Scaling resistance may decrease as the amount of certain SCMs increase. The ACI 318 building code limits the maximum dosage of the following SCMs, by mass of cementing materials, for deicer exposures. Fly ash-25%, Slag- 50% Silica fume- 10% Total SCM content should not exceed 50% by mass of the cementitious materials.

Freeze/Thaw and Deicer Scaling
Guidelines to ensure adequate concrete performance: Adequate air void system Low w/cm Minimum strength of 31 MPa (4500 psi) ACI 318 limits on SCM content Proper finishing Minimum 7 days moist curing Minimum 30-day drying period Adequate Drainage Breathable sealer When concrete in service will be exposed to cycles of freezing and thawing or deicing chemicals, consult local guidelines on allowable practices and use the following guidelines to ensure adequate concrete performance: An adequate air content (a minimum of 5% to 8% for 19 mm [3/4 in.] nominal size aggregate) with a satisfactory air void system (having a spacing factor ≤ mm (0.008 in.) and a specific surface area of 24 mm2/mm3 (600 in.2/in.3) or greater A low water-to-cementitious materials ratio (≤ 0.45) A minimum compressive strength of 31 MPa (4500 psi) for concrete exposed to freezing and thawing cycles that will be in continuous contact with moisture and exposure to deicing chemicals Fly ash, slag cement, and silica fume dosages not exceeding 25%, 50%, and 10%, respectively with combinations not exceeding 50%, by mass of cementing materials, for deicer exposures, unless otherwise demonstrated by local practice or testing Proper finishing after bleed water has evaporated from the surface A minimum of 7 days moist curing at or above 10°C (50°F) A minimum 30-day drying period after moist curing prior to exposure to freeze-thaw cycles and deicers when saturated Adequate drainage (1% minimum slope, 2% preferred) For additional protection, consider applying a breathable sealer after the initial drying period

Alkali-Aggregate Reactivity
Alkali-silica reactivity (ASR) More widespread Alkali-carbonate reactivity (ACR) Limited to small set of aggregates Aggregates containing certain constituents can react with alkali hydroxides in concrete. The reactivity is potentially harmful only when it produces significant expansion. Alkali-aggregate reactivity (AAR) has two forms---alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). ASR is of greater concern than ACR because the occurrence of aggregates containing reactive silica minerals is more widespread. Alkali-reactive carbonate aggregates have a specific composition that is not common.

Alkali-Silica Reactivity
Typical indicators of ASR might be any of the following: a network of cracks; cracks with straining or exuding gel; closed or spalled joints; relative displacements of different parts of a structure; or fragments breaking out of the surface of the concrete (popouts).

Alkali-Silica Reaction
This image shows popouts caused by ASR of sand-sized particles

Two step process: Alkali hydroxide + reactive silica gel  alkali-silica gel Alkali-silica gel + moisture  expansion For alkali-silica reaction to occur, three conditions must be present: reactive forms of silica in the aggregate, high-alkali (pH) pore solution, and sufficient moisture. The reaction can be visualized as a two-step process: 1. Alkali hydroxide + reactive silica gel = reaction product (alkali-silica gel); 2. Gel reaction product + moisture = expansion. The presence of gel does not always coincide with distress, and thus, gel presence does not necessarily indicate destructive ASR. Conversely, certain aggregates produce relatively little gel, yet can lead to significant and deleterious expansion.

Test Methods for Alkali-Silica Reactivity
This table summarizes the test methods for ASR. ASTM C1778 provides guidelines on these test methods.

Materials and Methods to Control ASR
ASTN C1778 Standard Guide for Reducing the Risk of Deleterious Alkali-Aggregate Reaction in Concrete Use of SCMs Low-alkali concrete Limit alkali content of cement Low-alkali cement <0.60% Na20e Lithium-based admixtures Current practices include the use of supplementary cementing materials or blended cement proven by testing to control ASR or by limiting the alkali content of the concrete. Low-alkali portland cement with an alkali content of not more than 0.60% (equivalent sodium oxide) has been successful for ASR resistance with slightly reactive to moderately reactive aggregates. However, it is the total concrete alkalis that are of greatest importance. Lithium-based admixtures are available to control ASR in fresh concrete.

When pozzolans, slag cements, or blended cements are used to control ASR, their effectiveness must be determined by tests such as ASTM C1567 or C1293. ASTM C1567 with a 14 day expansion limit is the most common method used to evaluate effectiveness of control measures. Where applicable, different amounts of pozzolan or slag cement should be tested to determine the optimum dosage. Expansion usually decreases as the dosage of the pozzolan or slag cement increases. Fournier 1997 ASTM C1567 Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method) ASTM C1293 Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction

This chart shows steps to control ASR. The solid lines show the preferred approach. However, some agencies may want to reduce the amount of testing and accept a higher level of risk and this can be achieved by following the direction of the hashed lines. It is important to distinguish between the reaction and the resulting damage from the reaction. In the diagnosis of concrete deterioration, it is most likely that a gel product will be identified. But, in some cases significant amounts of gel are formed without causing damage to concrete. (Adapted from ASTM C1778)

To pinpoint ASR as the cause of damage, the presence of deleterious ASR gel must be verified. A network of internal cracks connecting reacted aggregate particles is an almost certain indication that ASR is responsible for cracking. A petrographic examination (ASTM C856) is the most conclusive method available for identifying ASR gel in concrete.

Alkali-Carbonate Reaction
Observed with certain dolomitic rocks Rare due to general unsuitability of reactive rocks Mechanism includes dedolomitization: CaMgCO3 + (alkali)OH  Mg(OH)2 + CaCO3 + K2CO3 + (alkali)OH Crystallization of brucite is expansive Reactions observed with certain dolomitic rocks are associated with alkali-carbonate reaction (ACR). Reactive rocks usually contain large crystals of dolomite scattered in, and surrounded by, a fine-grained matrix of calcite and clay. ACR is relatively rare because aggregates susceptible to this reaction are usually unsuitable for use in concrete for other reasons, such as strength potential. There is still some debate about the mechanisms of ACR, but some attribute the expansion to dedolomitization, or the breaking down of dolomite. Dedolomitization proceeds according to the following equation: CaMgCO3 (dolomite) + alkali hydroxide solution --> Mg(OH)2 (brucite) + CaCO3 (calcium carbonate) + K2CO3 (potassium carbonate) + alkali hydroxide. The dedolomitization reaction and subsequent crystallization of brucite may cause considerable expansion.

Alkali-Carbonate Reaction
Best preventive measure: avoid reactive aggregates Selective quarrying Blend reactive aggregate with unreactive aggregate Limit aggregate size Low-alkali cement, SCMs, and lithium admixtures are not effective mitigators The best and most practical preventative measure has been to avoid the use of these aggregates. ACR-susceptible aggregate has a specific composition that is readily identified by petrographic testing. If a rock indicates ACR susceptibility, one of the following measures should be taken to reduce the likelihood of damage: selective quarrying to completely avoid reactive aggregate; blend aggregate according to Appendix in ASTM C1105; or limit aggregate size to the smallest practical. Low-alkali cement, SCMs, and lithium compounds are generally not found to be effective in controlling expansive ACR.

Carbonation Carbonation of concrete is a process by which carbon dioxide typically in from the atmosphere penetrates the concrete and reacts with the various hydration products, such as calcium hydroxide, to form carbonates. Depth of carbonation is typically measured by applying phenolphthalein solution to a freshly fractured surface of concrete. The characteristic bright pink color appears when the pH is above 9.5, as shown in the upper right of this image. Where the pH has been reduced to below 9.5 (whether by carbonation or other causes), there is no color change of the indicator.

Carbonation The depth of carbonation, dc, is approximately linear to the square root of the time of carbonation tc With: d0 = a parameter that depends on curing and early exposure. It becomes smaller with the later start of carbonation t0 a = a factor that contains parameters resulting from concrete composition, curing and exposure conditions The equation is conservative. Carbonation is slower than predicted by the 𝑎 𝑡 𝑐 relationship, if the concrete element is at least occasionally exposed to moisture.

Carbonation The amount of carbonation is significantly increased in concrete with: High water to cement ratio Low cement content Short curing period Low strength Highly permeable (porous) paste Ensuring that the concrete exhibits sufficiently low permeability best reduces the rate of carbonation. To reduce early carbonation, concrete needs to be protected from drying for as long as possible.

Sulfate Exposure Sulfate exposures range from moderate (category S1), to very severe (category S3) when determining sulfate content of the soil by ASTM C1580 and water by ASTM D516.

Corrosion Courtesy of R.D. Hooton
Concrete protects embedded steel from corrosion through its highly alkaline nature. The high pH environment in concrete (usually greater than 13.0) causes a passive and non-corroding protective oxide film to form around the steel. However, the presence of chloride ions from deicers or seawater can destroy or penetrate this film leading to corrosion. Corrosion of steel, is an expansive process---the byproduct of corrosion, rust, induces significant internal stresses and eventual spalling of the concrete over reinforcing steel. Courtesy of R.D. Hooton

Corrosion Source: Detwiler and Taylor 2005
The corrosion of steel reinforcement is an electrochemical process. For corrosion to take place, all elements of a corrosion cell must be present: an anode, a cathode, an electrolyte, and an electrical connection. Once the chloride corrosion threshold of concrete (about 0.15% water-soluble chloride by mass of cement) is reached, an electric cell is formed along the steel or between steel bars and the electrochemical process of corrosion begins. Dissolution of the iron takes place at the anode. The ferrous ions combine with hydroxyl ions, oxygen, and water to form various corrosion products. Source: Detwiler and Taylor 2005

Corrosion Adapted from Herholdt and others 1979
The iron and hydroxide ions form iron hydroxide. The iron hydroxide further oxidizes to form rust or other iron oxides as illustrated in this chart. The volume of the final product may be more than six times the volume of the original iron resulting in cracking and spalling of the concrete. The cross-sectional area of the steel can also be significantly reduced. The specific corrosion products formed depend on the availability of oxygen. Adapted from Herholdt and others 1979

Corrosion Detwiler and Taylor 2005
Chlorides---Chloride ions act as catalysts to the corrosion reaction. They break down the passive layer by a process illustrated in this schematic. The localized microcell corrosion that takes place under these circumstances is called pitting. Detwiler and Taylor 2005

Corrosion Various factors affect the rate of corrosion of steel. These include: Water Oxygen The pH of the concrete Chlorides Temperature Electrical resistivity of the concrete Permeability/Diffusivity of the concrete Cathode-to-anode area Various factors affect the rate of corrosion of steel.

Corrosion Adapted from Uhlig and Revie 1985
The pH affects the rate of corrosion. Below a pH of about 11, the passive layer of concrete breaks down allowing chloride ions to more rapidly penetrate the concrete. Below a pH of about 4, the protective film on the steel dissolves. Adapted from Uhlig and Revie 1985

Nonferrous Metals Zinc Aluminum Lead Alloys containing these metals
Nonferrous metals are also frequently used for construction in contact with portland cement concrete. Metals such as zinc, aluminum, and lead – and alloys containing these metals – may be subject to corrosion when embedded in, or in surface contact with, concrete.

Corrosion Testing ASTM C876 ASTM C1152 ASTM C1218 ASTM C1524 ASTM G109
Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete ASTM C1152 Test Method for Acid-Soluble Chloride in Mortar and Concrete ASTM C1218 Test Method for Water-Soluble Chloride in Mortar and Concrete ASTM C1524 Test Method for Water-Extractable Chloride in Aggregate (Soxhlet Method) ASTM G109 Standard Test Method for Determining Effects of Chemical Admixtures on Corrosion of Embedded Steel Reinforcement in Concrete Exposed to Chloride Environments ASTM C1202 Standard Test Method for Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration Deductions concerning corrosion activity of embedded steel can be made using the information obtained from ASTM C876, Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete. Acid-soluble chloride content of concrete is measured in accordance with ASTM C1152, Test Method for Acid-Soluble Chloride in Mortar and Concrete. Testing to determine water-soluble chloride ion content should be performed in accordance with ASTM C1218, Test Method for Water-Soluble Chloride in Mortar and Concrete. ASTM C1524, Test Method for Water-Extractable Chloride in Aggregate (Soxhlet Method), can be used to evaluate aggregates that contain a high amount of naturally occurring chloride. ASTM G109, Standard Test Method for Determining Effects of Chemical Admixtures on Corrosion of Embedded Steel Reinforcement in Concrete Exposed to Chloride Environments, can be used to determine the effects of admixtures on the corrosion of embedded steel reinforcement in concrete exposed to chloride environments. Where stray currents are expected, the electrical conductivity of the concrete should be explicitly addressed by limiting the allowable charge passed as measured by ASTM C1202.

Materials and Methods to Control Corrosion
Reduced permeability w/cm of 0.4 or less 7 days moist curing Use of SCMs with extended curing Alumina-bearing cements – bind chlorides Low chloride content materials Sufficient cover To maximize chloride (corrosion) resistance, reduce permeability by specifying a maximum water-cement ratio of 0.40 or less and at least seven days of moist curing. Judicious use of one or more SCM, combined with extended moist curing, can effectively reduce the permeability, diffusivity, and electrical conductivity of concrete. Cements with high C3A contents and/or slag cement or alumina-bearing pozzolans are also frequently used because of their effectiveness in binding chlorides. Admixtures, aggregate, and mixing water containing chlorides should be avoided. Increasing the concrete cover over the steel also helps slow down the migration of chlorides.

Epoxy-Coated Reinforcing Steel
Fusion-bonded epoxy-coated reinforcing steel, shown here, is very popular for the construction of marine structures and pavements, bridge decks, and parking garages exposed to deicer chemicals. The epoxy coating prevents chloride ions and other corrosive chemicals, moisture, and oxygen from reaching the steel. ASTM D3963

Other Types of Reinforcing Steel
Epoxy coated reinforcing steel Stainless steel Nickel-plated steel Galvanized steel Fiber-reinforced plastic (FRP) reinforcement Occasionally, selective use of stainless steel reinforcement in zones exposed to high chloride concentrations can ensure a long service life in that part of the structure, provided the concrete itself is made sufficiently resistant to avoid other types of deterioration. Nickel-plated steel will not corrode when embedded in chloride-free concrete. The nickel plate will provide protection to steel as long as no discontinuities or pinholes are present in the coating.

Corrosion Inhibitors and Cathodic Protection
Corrosion inhibitors such as calcium nitrate are used as an admixture to reduce corrosion Cathodic protection reverses the natural electron flow through concrete and reinforcing steel by inserting a nonstructural anode in the concrete Organic-based corrosion inhibitors, based on amine and fatty ester derivatives, are also available. A nonstructural anode forces the steel to act as the cathode by electrically charging the system. Since corrosion occurs where electrons leave the steel, the reinforcement cannot corrode, as it is receiving the electrons. Using more than one protection method simultaneously can result in significant savings in maintenance costs.

Sulfate Attack Sulfates (for example calcium sulfate, sodium sulfate, and magnesium sulfate) can attack concrete by reacting with hydrated compounds in the hardened cement paste. Sulfate ions attack calcium hydroxide and the hydration products of C3A, forming gypsum and ettringite in expansive reactions. Concrete beams after seven years of exposure to sulfate-rich wet soil in a Sacramento, California, test plot. The beams in a better condition have low water-cementitious materials ratios, and most have sulfate resistant cement. The inset shows two of the beams tipped on their side to expose decreasing levels of deterioration with depth and moisture level. Deterioration of concrete exposed to sulfates is often a combination of chemical sulfate attack and physical salt crystallization. Stark 2002

Salt Crystallization/Physical Salt Attack
Salts may or may not contain sulfates and they may or may not react with the hydrated compounds in concrete. Examples of salts known to cause weathering of exposed concrete include sodium carbonate and sodium sulfate. Groundwater enters the concrete by capillary action and diffusion.

Delayed Ettringite Formation (DEF)
Delayed ettringite formation (DEF) refers to the delayed formation of ettringite, in which the normal early formation of ettringite that occurs in concrete cured at ambient temperature is interrupted as a result of exposure to high-temperatures (between 70°C and 100°C [158°F to 212°F]) during placement or curing.

Delayed Ettringite Formation (DEF)
DEF is characterized by the development of rims around the aggregates, sometimes filled with ettringite. An abundance of water is necessary for the formation of ettringite. Because of the risk of delayed ettringite formation, as well as the deleterious effects of elevated temperature on durability, concrete temperatures above 70°C (158°F) should be avoided. The use of SCMs will help reduce the risk of DEF. Courtesy of Z. Zhang and J. Olek

Acid Attack Most acids disintegrate portland cement concrete
Protective treatment may be impossible Sacrificial calcareous aggregate – even surface wear and neutralizing effect Minimize paste content and permeability Most acidic solutions will disintegrate portland cement concrete. The rate of disintegration will be dependent on the type and concentration of acid. Acids attack concrete by dissolving both hydrated and unhydrated cement compounds as well as calcareous aggregate. Siliceous aggregates are resistant to most acids and other chemicals and are sometimes specified to improve the chemical resistance of concrete, especially with the use of chemically-resistant cement, however they are more prone to ASR. In certain acidic solutions, it may be impossible to apply an adequate protective treatment to the concrete.

Seawater Exposure PC3A <0.40 w/cm SCMs Slag Pozzolans
Cement used in concrete for a marine environment must balance the benefit of higher C3A content to bind chlorides with the need for sulfate resistance. Portland cements with tricalcium aluminate contents that range from 4% to 10% have been found to provide satisfactory protection against seawater sulfate attack, as well as protection against reinforcement corrosion by chlorides. Slag cement and alumina-bearing pozzolans are very effective in binding chlorides as well as providing sulfate resistance. For this reason, specially formulated marine cements generally contain high volumes (65% or more by mass) of slag cement.

Seawater Exposure Moderate sulfate exposure.
Abundant supply of oxygen and seawater Cycles of wetting and drying Cycles of freezing and thawing The action of waves, floating objects, and sand Splash zone most secure Seawater is considered a moderate sulfate exposure

Summary Abrasion Carbonation Corrosion De-icer Scaling DEF Erosion
Freeze/Thaw Sulfate Attack ASR Acid Attack Seawater Exposure

Design and Control of Concrete Mixtures CHAPTER 14

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Design and Control of Concrete Mixtures CHAPTER 14

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Presentation on theme: "Design and Control of Concrete Mixtures CHAPTER 14"— Presentation transcript:

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About project

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