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- Title: Lesson6
- Type: Notes
- School: Washington
- Course: SPAN 595
- Term: Winter
6: Lesson Portland Cement Concrete CEE 595 Construction Materials Winter Quarter 2008 Lesson 6: Portland Cement Concrete Topics Traditional Portland Cement Concrete 6.1 Introduction (Chapter 1 and Powers et al) 6.2 Hydraulic Cements (Chapter 2 and USGS references) 6.3 Fly Ash, Silica Fume, Other Pozzolans (Chapter 3) 6.4 Mixing Water for Concrete (Chapter 4) 6.5 Aggregates for Concrete (Chapter 5) 6.6 Admixtures for Concrete (Chapter 6) 6.7 Proportioning Normal Concrete Lesson 6: Portland Cement Concrete Topics (continued) Traditional Portland Cement Concrete Lesson 6a: PCC Case Study New Tacoma Narrows Bridge Lesson 6b: PCC Case Study US 395 Test Sections Lesson 6c: PCC Case Study WSDOT PCC Intersections Lesson 6d: PCC Case Study WSDOT PCC Pontoons for Hood Canal Floating Bridge Lesson 6: Portland Cement Concrete This lesson contains a modest amount of material. It is difficult to cover traditional PCC in one week however we will do what we can. The primary purpose of the lesson is to refresh the knowledge you already have about PCC or learn something about this important construction material if you have limited exposure to it. The basic reference is excellent Design and Control of Concrete Mixtures. The version on the CD contains the latest information. Lesson 6: Portland Cement Concrete You are encouraged to pick and choose information from the PCA publication. This PowerPoint along with the case studies will be helpful but, undoubtedly, incomplete. There are so many factors that make for a well-designed and constructed concrete project. These notes focus on basic material issues but job site specific conditions are always critical such as the weather conditions, transport of the fresh mix, placing and finishing, etc. Lesson 6.1 Introduction Major Topics Introduction Freshly Mixed Concrete Hardened Concrete Durability 6.1 Introduction For those that have studied or worked with PCC, this portion of Lesson 6 will be a straightforward review. For those that have not, the basics associated with PCC is important to most working in construction and specifically heavy construction. 6.1 Introduction Concrete: Basically formed by two components aggregate and paste. Paste is a mixture of portland cement and water. Paste chemically reacts with water to form cementing products (hydration) that bind aggregate into a rocklike mass. 6.1 Introduction Paste has two types of air: entrapped and entrained. Entrapped air occurs in all PCC mixtures usually small amounts. Entrained air is deliberately designed into the PCC mixture to aid the durability of the hardened PCC. Volumes Paste: 25 to 40% of total volume Aggregates: 60 to 75% of total volume Refer to PCA, Chapter 1, Figure 1-2 for additional details. 6.1 Introduction Aggregate described by two sizes: fine and coarse aggregate. Fine aggregate: 100% passes a 9.5 mm (3/8 inch) sieve. Coarse aggregate: Generally maximum aggregate size is about 37.5 mm (1.5 inches) but this can be much larger. Ranges down to 9.5 to 2.36 mm to (3/8 inch to No. 8). Aggregates will be covered in more detail in 6.5 of these notes (PCA, Chapter 5). 6.1 Introduction The less water in the paste (and hence the total PCC mixture) in general the better for the hardened material. Less water in the PCC mixture offers the following benefits: Higher compressive and flexural strength Lower permeability and improved water tightness Increased resistance to weathering Improved bond between PCC and reinforcing steel Reduced drying shrinkage and cracking (which can also occur due to construction placing and curing operations) Reduced volume changes due to wetting and drying. 6.1 Introduction Quote: The less water used, the better the quality of the concrete provided the mixture can be consolidated . The course instructors could not agree more. Refer to PCA, Chapter 1, Figure 1-4. This figure shows the relative proportions of water. Designated by water to cement ratios (weight of water divided by the weight of portland cement). The w/c ratios in the figure range from 0.25 (very low ratio) to 0.70 (a very high ratio). 6.1 Freshly Mixed Concrete Subtopics include Mixing Workability Bleeding and settlement Consolidation Hydration, Setting Time, and Hardening 6.1 Freshly Mixed Concrete Mixing Ensures that the separate PCC components are mixed properly. This will be covered in more detail in Lesson 4.9 (which covers PCA, Chapter 10). Workability Quote from PCA, Chapter 1: The ease of placing, consolidating, and finishing freshly mixed concrete and the degree to which it resists segregation is called workability. Workability is a critical element in concrete construction. 6.1 Freshly Mixed Concrete Workability Factors that influence the workability of concrete: 1. The method and duration of transport. 2. Quantity and characteristics of cementitious materials. 3. Concrete consistency which is defined by the measurement of slump. 4. Grading, shape, and surface texture of fine and coarse aggregates. 5. Entrained air 6. Water content 7. Concrete and ambient air temperatures 8. Admixtures 6.1 Freshly Mixed Concrete Workability Figure 1-6 (PCA, Chapter 1) illustrates the influence of temperature on the slump of two mixes each with a different portland cement. Bleeding and Settlement Bleeding: The development of a layer of water on the PCC surface (most often noted on PCC slabs). Bleeding cause: Settlement of the cement and aggregate particles. 6.1 Freshly Mixed Concrete Bleeding and Settlement Bleeding results in a higher w/c ratio at the top of the PCC which results in lower strength and durability. Consolidation An introduction to the positive features associated with vibration of PCC to obtain its final shape or form. 6.1 Freshly Mixed Concrete Hydration, Setting Time, and Hardening Hydration: Chemical reaction between cement and water. Unhydrated portland cement a combination of many compounds. Four compounds make up 90% or 75% of portland cement more of a typical cement: Tricalcium silicate Dicalcium silicate Tricalcium aluminate Tetracalcium aluminoferrite 6.1 Freshly Mixed Concrete Abbreviations Tricalcium silicate: 3CaO SiO2=C3S Dicalcium silicate: 2CaO SiO2=C2S Tricalcium aluminate: 3CaO Al2O3=C3A Tetracalcium aluminoferrite: 4CaO Al2O3 Fe2O3=C4AF 6.1 Freshly Mixed Concrete Tricalcium silicate (C3S): Hydrates and hardens rapidly and is largely responsible for initial set and early strength. Dicalcium silicate (C2S): Hydrates and hardens slowly and contributes largely to strength increase at ages beyond one week. 6.1 Freshly Mixed Concrete Tricalcium aluminate (C3A): Liberates a large amount of heat during the first few days of hydration. Gypsum added to the cement during final grinding slows the hydration rate of C3A. Tetracalcium aluminoferrite (C4AF): Used to assist in manufacturing of cement. Hydrates rapidly but contributes little to strength. Most PCC color effects due to C4AF and its hydrates. 6.1 Freshly Mixed Concrete Hydration, Setting Time, and Hardening All types of portland cement contain the same four compounds just in different amounts. Calcium silicates + water form: Calcium hydroxide about 25% by weight Calcium silicate hydrate (tobermorite gel) about 50% by weight Calcium silicate hydrate most important for PCC engineering properties. 6.1 Freshly Mixed Concrete Hydration, Setting Time, and Hardening Calcium silicate hydrate forms interlocking structure between other crystalline phases, remaining unhydrated cement grains, and aggregate particles. As PCC hardens, the overall volume remains essentially unchanged. 6.1 Freshly Mixed Concrete Hydration, Setting Time, and Hardening Hardened paste has pores containing water and air. Fewer pores results in higher strength. Goal is to use no more water than necessary to hydrate the portland cement. 6.1 Freshly Mixed Concrete Hydration, Setting Time, and Hardening Powers et al showed in a 1948 publication that 0.4 grams of water is required to completely hydrate 1.0 gram of portland cement however complete hydration of portland cement is rare. 6.1 Freshly Mixed Concrete Hydration, Setting Time, and Hardening Initial rate of hydration gypsum is added to the ground cement to control the initial setting of PCC. Other factors that control initial set time are: Fineness of portland cement Amount of water added Admixtures Temperature at the time of mixing. 6.1 Hardened Concrete Subtopics include Curing Drying Rate of Concrete Strength Density Permeability and Watertightness Abrasion Resistance Volume Stability and Crack Control 6.1 Hardened Concrete Curing Curing: Increase in strength with time. Curing continues if: Unhydrated cement is present. The concrete remains moist or has a relative humidity above 80%. The concrete temperature remains favorable. Space is available for hydration products to form. 6.1 Hardened Concrete Curing Curing stops if: Concrete temperature drops below freezing. Concrete relative humidity drops below about 80%. 6.1 Hardened Concrete Drying Rate of Concrete Quote from PCA, Chapter 1: Concrete does not harden or cure by drying. Freshly mixed concrete has adequate water for curing but this quickly changes. Insufficient moist curing for a floor slab can result in a weak surface which is subject to dusting under traffic. Drying concrete Shrinkage occurs due to drying Drying shrinkage a primary cause of PCC cracks 6.1 Hardened Concrete Drying Rate of Concrete Moisture content of PCC after several months typically 1 to 2% by total mass of PCC. 6.1 Hardened Concrete Strength Compressive strength of concrete is an often specified requirement. Typically, the compressive strength is reported as a function of a 28 day cure. However, many different cure periods are specified depending on project requirements. The basic relationship between w/c ratio and strength has been known for about 100 years. 6.1 Hardened Concrete Strength fc Specified compressive strength is designated Two types of compressive strength tests Mortar: 50 mm X 50 mm (2 in. X 2 in.) cubes PCC cylinders: 150 mm diameter X 300 mm high (6 in X 12 in). Sometimes smaller cylinders are used that are 100 mm diameter X 200 mm high (4 in X 8 in). Specified compressive strength ranges General use applications: 20 to 40 MPa (3,000 to 6,000 psi) Special bridge and high-rise building applications: 70 to 140 MPa (10,000 to ' 6.1 Hardened Concrete Strength Flexural strength (or modulus of rupture) sometimes used in the design of pavements and slabs. Approximate correlations with compressive strength f c' Flexural Strength = 0.7 to 0.8 Flexural Strength = 7.5 to 10 f c' (in MPa) (in psi) Direct tensile strength approximately 8 to 12% of compressive strength. 6.1 Hardened Concrete Strength Splitting tensile strength approximately 8 to 14% of compressive strength. Modulus of elasticity (E) ranges between 14,000 to 41,000 MPa (or 2 to 6 million psi) for normal weight concrete. Modulus of elasticity can also be approximated from compressive strength E = 5,000 f c' E = 57,000 f c' (in MPa) (in psi) 6.1 Hardened Concrete Density Typical concrete density 2200 to 2400 kg/m3 (137 to 150 lb/ft3) Density varies as a function of Aggregate Amount of air entrapped or entrained Water and cement contents Some mix water does evaporate from the concrete when exposed to ambient conditions this amounts to about 0.5 to 3% of concrete weight. 6.1 Hardened Concrete Density Specialty concrete density can range from as low as 240 kg/m3 (15 lb/ft3) to as high as 6000 kg/m3 (375 lb/ft3). 6.1 Hardened Concrete Permeability and Watertightness Watertightness: Ability of concrete to hold back water without visible leakage. Permeability: Amount of water transmitted through concrete when water under pressure. 6.1 Hardened Concrete Permeability and Watertightness Permeability of concrete a function of: Permeability of the paste Permeability and gradation of the aggregate Quality of paste and aggregate transition zone Relative proportion of paste to aggregate. 6.1 Hardened Concrete Abrasion Resistance Pavements, floors, and hydraulic structures should have a high abrasion resistance. Concrete abrasion resistance linked to: Compressive strength hence w/c ratio and curing conditions. Type of aggregate Surface finish or treatment Refer to Supplemental Lesson 6b for additional insight into a form of 6.1 Hardened Concrete Volume Stability and Crack Control Hardened concrete volume changes due to: Temperature Moisture Stress Thermal volume changes of hardened concrete about the same as for steel. 6.1 Hardened Concrete Volume Stability and Crack Control Two basic causes of cracks in concrete: Stress due to applied loads Stress due to drying shrinkage or temperature changes when concrete is restrained. Drying shrinkage is an inherent property of concrete but it can be minimized by mix design and curing and Reinforcing steel to keep cracks closed Joints (more information is available via 6.1 Hardened Concrete Volume Stability and Crack Control Thermal stresses induced by ambient temperature changes can cause cracking this can be a major issue for early age concrete. Thermal stresses a major factor to consider in designing PCC jointed pavements. Lesson 6.2 Major Topics Types of cements Production of cements Cement supply Location of Washington State cement plants 6.2 Types of Cements Type I: General purpose cement. Type II: Protects PCC against moderate sulfate attack. Generates less heat than Type I. Some cement manufacturers can meet both the Type I and II requirements with one cement. Type III: Provides high strength PCC with a shorter cure period. Similar to Type I but the clinker is ground finer thus allowing more rapid hydration. 6.2 Types of Cements Type IV: Produces less heat during hydration but slower strength gain. Sometimes used with mass concrete. Type V: Used for PCC exposed to severe sulfate action from soils or groundwater. Blended Cements: Refer to Chapter 2. Special Cements: A wide variety of cements are available for specific applications refer to Table 2-4, Chapter 2. World cement production for 2003 top 10 producing countries Country 1. China 2. India 3. US including Puerto Rico 4. Japan 5. Korea 6. Brazil 7. Italy 8. Russia 9. Spain 10. Thailand 6.2 Hydraulic Cements Production Production (millions metric tons) 750 110 93 72 56 40 40 40 40 35 6.2 Hydraulic Cements Production US production in 2003: 87 million metric tons of portland cement 4.5 million tons of masonry cement Produced at 118 plants in 37 states and Puerto Rico by 39 companies. Annual imports of hydraulic cement: 21 million tons Total cement use in US: 112 million tons/year (imports about 20% of consumption) 6.2 Hydraulic Cements Production Import sources: 19% Canada 18% Thailand 12% China 7% Venezuela 44% Others (32 other countries) 75% to ready-mixed concrete producers 13% to concrete product manufacturers 6% to contractors (mostly road paving) 6% others US cement applications: 6.2 Cement Shortages in the US--2004 US cement supply is currently short of demand why? Strong construction markets Long lead times needed to bring new cement plants online (permitting process) and lots of capital. Freight Limited availability of transport ships for importing more cement Shipping rates increased significantly during 2004 6.2 Cement Shortages in the US--2004 6.2 Local Cement Production In Seattle, portland cement is produced by Ash Grove Cement Lafarge Cement (formerly Holnam Cement and before that Idea Cement) 6.2 Local Cement Production--South Seattle Industrial Area 6.2 Location of Ash Grove and Lafarge Plants in Seattle West Seattle Bridge Ash Grove Plant Site Lafarge Plant Site 6.2 Lafarge Cement Plant Seattle Photo source: Rob Shogren, Lafarge 6.2 Lafarge Cement Plant British Columbia Photo source: Rob Shogren, Lafarge Lafarge 6.2 Kiln Production of Clinker Photo source: Rob Shogren, Lafarge 6.2 Lafarge Kiln Production of Clinker Photo source: Rob Shogren, Lafarge 6.2 Ash Grove and Lafarge Plants in Seattle Use of Scrap Tires as Fuel Both cement plants in Seattle use scrap tires as a portion of the fuel for their kilns. National-wide about 290 million scrap tires are generated each year with about 233 million being consumed (or about 80%). Cement plants are estimated to consume about 53 million tires per year (or 18% of total scrap tires generated). Benefits to cement manufacturers Reduces energy costs Less nitrogen oxide emissions compared to other fuels Tire-derived fuel (TDF) is becoming a standard practice. Refer to ASTM D6700. 6.3 Fly Ash, Silica Fume, Other Pozzolans These are broadly classed as supplementary cementitious materials and are used in about 60% of ready mixed PCC produced in the US. Definitions Pozzolan: A siliceous or aluminosiliceous material that, in finely divided form and in the presence of moisture, chemically reacts with the calcium hydroxide released by the hydration of portland cement to form calcium silicate hydrate and other 6.3 Fly Ash, Silica Fume, Other Pozzolans Definitions Fly Ash: The most widely used supplementary cementitious material in concrete, is a byproduct of the combustion of pulverized coal in electric power generating plants. Conforms to ASTM C618 Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. Definitions Silica Fume: Silica fume, also referred to as microsilica or condensed silica fume, is a byproduct material that is used as a pozzolan. This byproduct is a result of the reduction of high-purity quartz with coal in an electric arc furnace in the manufacture of silicon or ferrosilicon alloy. Condensed silica fume is essentially silicon dioxide (usually more than 85%) in noncrystalline (amorphorous) form. It has a spherical shape and is extremely fine with particles less than 1 m in diameter and with an average diameter of about 0.1 m, about 100 times 6.3 Fly Ash, Silica Fume, Other Pozzolans 6.3 Fly Ash Produced from coal burning power plants Three types of fly ash according to ASTM C618 Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete Class N: Raw or calcined natural pozzolans Class F: Fly ash normally produced from burning anthracite or bituminous coal. This class has pozzolanic properties. Class C: Fly ash normally produced from lignite or subbituminous coal. This class has pozzolanic properties and some cementitious properties. 6.3 Locally Produced Fly Ash In Washington State, the only fly ash producing power plant is located in Centralia, WA and is owned and operated by TransAlta Corp (based in Calgary). ISG Resources markets the Class F fly ash recovered from electrostatic precipitators. The power plant consumes about 5 million tons of coal per year with about 4 million tons being mined from a 14,000 acre facility near Centralia. The coal mined at Centralia is classified as bituminous or soft coal. Over 50% of the electricity produced in the US is via coal fired power plants. 6.3 Effects on Freshly Mixed PCC Effect Fly Ash Silica Fume More needed but can be offset with water reducer but can be Sticky offset with high range water reducer Less More air entraining needed Water Less needed Requiremen ts Workability Improved Bleeding Less and Segregation More air Air Content entraining needed 6.3 Effects on Freshly Mixed PCC Effect Fly Ash Silica Fume Could increase or decrease -Can be difficult to finish Improved -- Heat of Lower Hydration Setting Time Retards set time Finishability Pumpability Cure Time Improved Improved Longer 6.3 Effects on Hardened PCC Effect Strength Abrasion Resistance Drying Shrinkage and Creep Permeability AlkaliAggregate Reactivity Fly Ash Silica Fume Neutral or Increase increase Neutral Should increase but due to higher strength Neutral Reduces Reduces Neutral Reduces more than fly ash Reduces 6.4 Mixing Water for PCC Almost any natural water that is drinkable and has no pronounced taste or odor can be used as mixing water for making concrete. 6.5 Aggregates for PCC Chapter 5 in the PCA publication contains a substantial amount of detailed information about aggregates for PCC. To a limited extent, aggregates were introduced in Lesson 2. Further information will be provided on aggregates in the HMA Lessons. A few of the more significant aspects of PCC aggregates will be noted in the following slides. No attempt is made to cover all the details available in Chapter 5. 6.5 Aggregates for PCC Table 5.2: Characteristics and Tests of Aggregates is an excellent summary and suggests that this is a detailed topic! Note in Table 5.2 two ASTM standards ASTM C125: Standard Terminology Relating to Concrete and Concrete Aggregates ASTM C294: Standard Descriptive Nomenclature for Constituents of Concrete Aggregates These two standards should be reviewed since they cover basic terms and terminology sort of a language primer for concrete. Most of the terms will be familiar. 6.5 Aggregates for PCC Fineness Modulus (FM): This is an index of the fineness of an aggregate. The lower the FM, the finer the gradation. The FM is used in proportioning PCC mixes (PCA, Chapter 9). Particle shape and surface texture Mostly influences the properties of freshly mixed concrete but not hardened concrete (unlike hot mix asphalt which requires crushed aggregate to achieve good long-term performance). 6.5 Aggregates for PCC Absorption and surface moisture Review the following terms Oven dry Air dry Saturated surface dry Damp or wet Figure 5-12 is helpful in reviewing the above terms. 6.5 Aggregates for PCC Alkali-Aggregate Reactivity Review carefully the content in Chapter 5 on alkali-silica reactions (ASR). This is a very serious PCC topic since it is generally preventable if early measures are taken. ASR can cause major damage to PCC structures. The extent of ASR problems vary throughout the US since the basic problem lies with the aggregate used in the PCC and generally most PCC aggregate is locally produced. 6.5 Aggregates for PCC Recycled concrete aggregate This is another topic that requires some attention. There are a number of possible uses for recycled concrete including use as aggregate for new PCC however there are risks associated with that use. The use of recycled concrete pavement as aggregate has experienced severe performance problems on at least one project in Michigan. 6.6 Admixtures for Concrete Admixtures are those ingredients in concrete other than portland cement, water, and aggregates that are added to the mixture immediately before or during mixing. Types of admixtures: Air-entraining admixtures Water-reducing admixtures Plasticizers Accelerating admixtures Retarding admixtures Hydration-control admixtures Corrosion inhibitors Shrinkage reducers Alkali-silica reactivity inhibitors Coloring admixtures 6.6 Admixtures for PCC Table 6-1 provides an excellent overview of concrete admixtures by classification Some of the most commonly used admixtures include Air entraining admixtures Water reducers Water reducer high range Superplasticizers 6.6 Admixtures for PCC Air entrainment became a standard practice for most types of concrete in about 1945. The work that led up to the wide-spread use of air entrainment started much earlier than 1945. 6.6 Admixtures for PCC ASTM C494: Standard Specification for Chemical Admixtures for Concrete lists Types A through G (a number of which are covered in PCA, Chapter 6, Table 6-1) Type A: Water reducing admixtures Type B: Retarding admixtures Type C: Accelerating admixtures Type D: Water reducing and retarding admixtures Type E: Water reducing and accelerating admixtures Type F: Water reducing, high range admixtures Type G: Water reducing, high range, and 6.7 Proportioning Normal Concrete Mixtures Mix design: The process of determining required and specifiable characteristics of a concrete mixture. Mixture proportioning: Refers to the process of determining the quantities of concrete ingredients, using local materials, to achieve the specified characteristics of the concrete. A properly proportioned concrete mix should possess these qualities: Acceptable workability of the freshly mixed concrete Durability, strength, and uniform appearance of the hardened concrete 6.7 Proportioning Normal Concrete Mixtures So how do you decide what concrete durability or strength is needed? Determine the minimum strength needed via Building code Durability requirements Other design requirements An example of building code requirements is the International Building Code (IBC) typical requirements from the IBC follow. As you likely know, building code requirements tend to be detailed thus the criteria shown are only a small sample. 6.7 PCC Code Requirements IBC, Chapter 19: The IBC makes extensive use of ACI 318. Let us take a look at typical code requirements Section 1904: Durability Requirements these are based on three separate criteria which are: Water-cementitious ratio Freezing and thawing exposures Sulfate exposures Criteria for each of the three criteria will be shown in the following images. 6.7 PCC Code Requirements IBC, Chapter 19: Section 1904: Durability Requirements these are based on three separate criteria which are: Water-cementitious ratio Minimum specified compressive strengths (f c) can be as low as 2,500 psi for negligible exposure to 3,500 psi for severe exposure (exposures determined by project location and type of construction). Maximum water-cementitious ratios and minimum f c for concrete exposed to sulfate containing solutions Maximum w/c ratios range from 0.50 to 0.45 Minimum f c ranges from 4,000 to 4,500 psi (28 day cure) Freezing and thawing exposures 6.7 PCC Code Requirements IBC, Chapter 19: The IBC makes extensive use of ACI 318. Let us take a look at typical code requirements Section 1904: Durability Requirements these are based on three separate criteria which are: Water-cementitious ratio Freezing and thawing exposures Air entrainment requirements Maximum water-cementitious ratios and minimum f c for concrete conditions Maximum w/c ratios range from 0.50 to 0.40 Minimum f c ranges from 4,000 to 5,000 psi (28 day cure) Sulfate exposures 6.7 PCC Code Requirements IBC, Chapter 19: The IBC makes extensive use of ACI 318. Let us take a look at typical code requirements Section 1904: Durability Requirements these are based on three separate criteria which are: Water-cementitious ratio Freezing and thawing exposures Sulfate exposures Similar to w/c ratio criteria 6.7 Proportioning Steps Select required strength Tables 9-1 and 9-2 show minimum strength requirements for various exposure conditions. Table 9-3 shows typical compressive strengths for various water-cementitious ratios. Example: Compressive strength = 7,000 psi @ 28 day cure for a w/c ratio = 0.33 (non-air entrained). 6.7 Proportioning Steps Select aggregates Maximum aggregate size examples include Max aggregate size should not exceed 1/5 the narrowest dimension between sides of forms nor the clear space between reinforcing bars etc. Slab (unreinforced): Max size should not exceed 1/3 stab thickness. High strength PCC (greater than 10,000 psi): Max aggregate should be no more than inch. Bulk volume of coarse aggregate: Refer to Table 9-4. 6.7 Proportioning Steps Select air content and initial water content Depends on exposure conditions for the concrete. Refer to Figure 9-4 and Table 9-5. Table 9-5 is a function of slump, max aggregate size, and whether there is a need for air entrainment. The table provides an important mix proportion ingredient the approximate amount of mix water. The water contents shown in Table 9-5 are for crushed aggregate. If rounded gravel is being used (often the case), then it is recommended that water reductions be made in the estimate. 6.7 Proportioning Steps Select slump Refer to Table 9-6 for recommended slumps for various types of construction. Select cementing materials content and type The amount of cement should be minimized for economy but must be enough to ensure quality (hence performance) of the concrete. To minimize water and cement requirements include (1) the stiffest practical mixture, (2) the largest practical maximum size of aggregate, and (3) the optimum ratio of fine-to-coarse aggregate. Water/cementitious ratio is a primary factor that is used to determine cement 6.7 Proportioning Steps Select cementing materials content and type (cont.) Minimum cement contents are often specified for durability requirements. As examples: Severe freeze-thaw conditions: 564 lb/cu. yd. Placement of concrete underwater: 650 lb/cu. yd. Flatwork: Refer to Table 9-7. Limits on cementitious materials other than portland cement. For concrete exposed to deicers, typical limits are shown in Table 9-8. For example: Fly ash and natural pozzolans: limit 25% (by mass) Silica fume: limit 10% 6.7 Proportioning Steps Proportioning approaches Proportioning based on field data Proportioning based on trial mixes The absolute volume method (illustrated by Example 2 (US units), PCA, Chapter 9) is commonly used by laboratories. Elements of the absolute volume method include Conditions and specifications Cement required information must include: (1) cement type, and (2) relative density of the cement Coarse aggregate required information must include: (1) max aggregate size, (2) specific gravity, (3) absorption, (4) dry rodded bulk 6.7 Proportioning Steps Proportioning approaches Elements of the absolute volume method include Conditions and specifications (cont.) Fine aggregate required information includes: (1) specific gravity, (2) absorption, (3) actual (lab) moisture content, (4) Fineness Modulus (FM). Determine required strength Determine water-cement ratio Check clearances for coarse aggregate Select needed air content Select target slump 6.7 Proportioning Steps Proportioning approaches Elements of the absolute volume method include (cont.) Select initial water content Calculate cement content Determine coarse aggregate content Determine dosages for admixtures Determine fine aggregate content Make moisture corrections Prepare trial batch Make adjustments based on results from trial batch 6.7 Proportioning Steps Proportioning approaches Proportioning by trial batches (cont.) Example 5, PCA, Chapter 9 provides an overview for proportioning a concrete mix via the absolute volume method using multiple cementing materials and admixtures. This example may be of interest to some of you. Concrete mixes for small jobs: Advice is contained in Chapter 9 for this situation again this may be of interest. Discussion Forum Discuss the pros and cons associated with concrete mixes with low water-cement ratios (say w/c ratios of 0.40 or less). For the cons stated, how might they be mitigated. So we are all discussing the same application, make the concrete application a pavement project using fixed forms (not slip forming). Lesson 6: References Powers, T. and Brownyard, T. (1948), Studies of the Physical Properties of Hardened Portland Cement Paste, Bulletin 22, Portland Cement Association, reprint from the Journal of the American Concrete Institute, Detroit, Michigan, March 1948. Hosmatka, S., Kerkoff, B., and Panarese, W. (2003), Design and Control of Concrete Mixtures, 14th Edition, Portland Cement Association, Skokie, Illinois. USGS (2004), Mineral Commodity Summaries, US Geological Survey, January 2004. Lesson 6: References RMA (2003), US Scrap Tire Markets, 2003 Edition, Rubber Manufacturers Association, Washington, DC, July 2004. ICC (2000), International Building Code, International Code Council, Falls Church, Virginia, March 2000. ACI (2003), Mass Concrete, ACI 207.1R-96, ACI Manual of Concrete Practice Part 1 2003, American Concrete Institute, Farmington Hills, MI.
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Path: Washington >> STAT >> 583 Fall, 2008
Path: Washington >> STAT >> 583 Fall, 2008
Path: Washington >> STAT >> 583 Fall, 2008
Path: Washington >> STAT >> 592 Fall, 2008
Path: Washington >> STAT >> 592 Fall, 2008
Path: Washington >> STAT >> 593 Fall, 2008