McGraw-Hill Construction is a registered provider with The

Transcription

1

2 McGraw-Hill Construction is a registered provider with The American Institute of Architects Continuing Education System. Credit earned on completion of this program will be reported to CES records for AIA members. Certificates of completion for non-aia members are available on request. This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.

3 This presentation is protected by US and international copyright laws. Reproduction, distribution, display and use of the presentation without written permission of TXI is strictly prohibited.

4 After completing this course you will be able to: Explain how micro-cracks cracks develop in concrete even before the mix is set. Discuss how even very small cracks can cause big problems in concrete over time. Describe how the internal curing process works. List the key benefits that internal curing brings to concrete. Discuss how high-performance concrete contributes to sustainability.

5 The Course is organized into the following sections Section 1: Concrete Basics Section 2: Concrete Deterioration: Causes and Impacts Section 3: Internal Curing: Process and Benefits Section 3a: Projects already completed Section 4: High Performance Lightweight Concrete and Sustainability

6 SECTION 1 Concrete Basics

7 Concrete is a Stone-like Building Material Made by Mixing: Cement Sand (fine aggregate) Gravel (coarse aggregate) WaterW t Its fluid-like state prior to hardening allows for ease of workability and a vast variety of possible forms. Once hardened, concrete, if worked properly, is durable and extremely strong in compression. Quality control is essential not only in the selection and proportioning of its separate ingredients, but in its handling, placement, and curing.

8 Concrete: The Basic Recipe Ingredients: Portland Cement Fine Aggregate Coarse Aggregate Water Air (both entrapped and entrained) Process: Add ingredients together and mix thoroughly Place in container of desired shape and form Allow mixture to set Remove container Decorate as desired Allow mixture the time to reach requisite strength Concrete being pumped into insulating stay in place forms

9 Concrete Ingredients Portland Cement Manufactured from Lime, Silica, Iron Oxide and Alumina In its dry state, Portland Cement is a very fine powder. When mixed with water, a paste is formed. As the paste begins to set, the cement loses its plasticity. Initial Set Occurs Within a Few Hours Final set takes about 4-8 hours. Cement Continues to Hydrate and Gain Strength Portland cement paste binds the fine aggregate and coarse aggregate. It is the glue

10 Concrete Ingredients Aggregate Chemically inert ingredients in concrete held together by Portland Cement Affect the quality of the concrete, reduce shrinkage of the concrete, and for economic purposes, serve as filler Must be hard, durable, and clean Any material coating can negatively affect the concrete mix Proportion of fine to coarse is important Good proportions results in better workability, optimum density, and better strength and durability.

11 Concrete Ingredients Aggregates classified by size: Fine: 1/4 or Less in Diameter Sand or other suitable fine material Fills spaces between coarse aggregate Allows good workability and smooth surfaces Coarse: from 1/4 to 1 1/2 in Diameter Crushed stone or gravel Larger than fine, but small enough to fit comfortably between reinforcement bars More cement required for smaller coarse aggregates, so size of coarse aggregate has impact on concrete cost Larger aggregates are more difficult to work with and may be harder to handle Choice of aggregate impacts strength th and workability

12 Concrete Ingredients Lightweight aggregates Volcanic sources Byproducts from coal combustion Manufactured Expanded shale, clay and slate Weight reduction benefits allow Thinner fire resistant slabs Longer spans Expressive architectural features Additional floors Lower column and bearing wall loads Common applications Structural lightweight concrete Insulating concrete

13 Concrete Process Mixing On-Site Ready-mixed Pre-cast Placing Formwork Reinforcement Pouring/Depositing Consolidation Finishing

14 Concrete Process Not drying a chemical process Crystals form on surface of each cement particle Crystals grow and expand until they link up with other cement particles or adjacent aggregates Can take place under water continues long after concrete has reached final set As long as water and un-hydrated d cement are present Years

15 Concrete Process Curing Strongly influences properties of hardened d concrete Durability, strength, water-tightness, abrasion resistance, and resistance to freeze/thaw cycles and salts Begins after exposed surfaces have hardened enough to resist marring Ensures continued hydration of the cement and strength gain of the concrete Concrete must be kept moist Correct temperature must be maintained Benefits versus economics should dictate the length of time a structure is under cure

16 Concrete Process Curing methods: external Spraying Water Membrane-forming compounds (Create moisture barriers) Polyethylene sheets Curing blankets Curing methods: internal Internal curing New technology Ensures a continuous supply of moisture from within the concrete to allow continued cement hydration over time

17 SECTION 2: Concrete Deterioration: Causes and Impacts

18 Three Basic Mechanisms Shrinkage cracking ( Early Age cracking) Chemical shrinkage Drying shrinkage Plastic shrinkage Thermal shrinkage Permeability Porosity of concrete allows water and other agents, like deicing salts, to get in Freeze-thaw disintegration can result Reinforcing steel can corrode Chemical reactions Alkali-aggregate li reactions Other chemical attacks

19 Shrinkage Cracking All concrete shrinks Shrinkage by Itself: no problem Except when concrete is restrained By walls, columns, reinforcing steel, subgrade, etc. In Practice, concrete is almost always restrained Or when plastic shrinkage cracking occurs Which can happen even when concrete Isn t restrained

20 Shrinkage Cracking What happens when restrained concrete shrinks? It cracks. Why? Concrete is strong compression; not so strong in tension (roughly 10% of compressive strength) Most concrete cracks if moved more than 1-2 mils per foot But shrinkage can shorten concrete by 5-10 mils per foot Something has to compensate = Cracking Shrinkage is not the problem; shrinkage cracking is.

21 Shrinkage Cracking Key contributor to Early Age cracking Concrete undergoes significant volume change (shrinkage) early in the curing process The cement paste (not the aggregate) shrinks Cracks can begin invisible to the eye, and over time become an ongoing maintenance expense or render a structure unserviceable

22 Shrinkage Cracking Early Age cracking can be a significant problem Creates tensile stresses within the concrete Tensile strength of concrete = only 10% of compressive strength Cracks develop when tensile stress Exceeds tensile strength Especially problematic in high-strength concrete with low water to cement (w/cm) ratios More cement translates into greater shrinkage The greater the cement content the greater the heat generated resulting in more thermal stress

23 What Causes Shrinkage Cracking? Four basic types of shrinkage Chemical shrinkage Caused by hydration Drying shrinkage Evaporation of mix water over time Plastic shrinkage Evaporation of mix water shortly after placement While concrete is still plastic Thermal Shrinkage Change in temperature causes changes in volume Overall shrinkage is combined effect of all three

24 Chemical Shrinkage Not driven by evaporation Occurs internally The result of a chemical process = hydration Can occur in sealed or unsealed curing conditions Hydration causes a reduction in the absolute volume of solids and liquids in the hydrating paste Creates tensile stresses within the concrete Cracks develop when tensile stress exceeds tensile strength Especially problematic in high-strength g concrete Higher cement content increases likelihood of cracking

25 Chemical Shrinkage Includes autogenous shrinkage Autogenous = occurring independent of external influence When water for the hydration process runs out, self-desiccation (drying-out) of the paste occurs Results in autogenous (internally-driven) shrinkage

26 Chemical Shrinkage An ongoing process Continues to occur as Long as the cement hydrates

27 Drying Shrinkage Is driven by evaporation Occurs in unsealed conditions Occurs over a longer period of time than plastic Shrinkage Not just immediately following placement When the concrete is restrained, tensile stresses and cracks Develop in the hardened concrete Cracks form inside the concrete, Not just on the surface Restrained drying shrinkage = Most common cause of concrete cracking

28 Plastic Shrinkage Also driven by evaporation Also in unsealed conditions Occurs when water evaporates from the surface of freshly yplaced concrete Water evaporates faster than bleed water can replace it Bleed water = internal mix water that t rises to the surface of freshly placed concrete Results in capillary stress that can lead to cracking Occurs within the first few hours

29 Plastic Shrinkage Strongly influenced by weather Relative humidity Dryness Wind Tensile stresses develop in the surface concrete Shallow cracks result Can be fairly wide at the surface

30 Thermal Shrinkage Cement hydration is an exothermal process The combination of heat of hydration and heat/cold from the ambient conditions can result in sufficient temperature differences to cause stress in the concrete When the tensile stress exceeds the tensile strength of the concrete, cracking occurs

31 Permeability Porosity Concrete is porous Allows ingress of moisture and ions by: Flow, diffusion or sorption These processes generally referred to as permeability Porosity is negatively affected by poor design, quality control and/or field placement Two key attributes influence permeability Nature of the hardened cement paste Nature of the interfacial a transition t zone (ITZ) between ee the paste and the aggregate and/or any reinforcing steel or fibers in mix concrete

32 Permeability The hardened cement paste Strongly influenced by length and effectiveness of curing The Interfacial transition zones (ITZ) Different from the cement paste Typically more porous, richer in calcium hydroxide, weaker and more soluble than the cement paste Can be a path of least resistance for moisture and other harmful Substances like deicing salts Is therefore prone to microcracking

33 Permeability Problems Corrosion of reinforcing steel Concrete is alkaline protects reinforcing steel from rust Chloride (for example, from deicing salts) can destroy the passivating layer of alkalinity around the steel, allowing it to rust Rust expands against the concrete Puts it in tension Result: concrete cracks Freeze-thaw deterioration Water can enter concrete s pore structure If sufficient water is present and it freezes, the concrete cracks and flakes Good quality concrete with good quality paste will avoid these problems

34 Permeability Problems Corrosion and freeze-thaw Water and chlorides can migrate through concrete via cracks Both can also migrate through the pore structure itself, even in the absence of cracking It is for this reason that decreasing concrete permeability is important for ensuring overall durability

35 Chemical Reactions Alkali-aggregate g reaction Between alkali hydroxides in the concrete and minerals in certain aggregates Forms a gel around the aggregate particles If the gel gets wet, it expands and can destroy the concrete One solution: add certain supplementary cementitious materials (SCMs) Silica fume, fly Ash, and blast-furnace slag Other chemical attack agents Chlorides Acids

36 Cracking in Practice Multiple Processes Interacting Over Time

37 SECTION 3 Internal Curing: Process and Benefits

38 What is Internal Curing of Concrete? A process that ensures a continuous supply of moisture from within the concrete mix Itself for the development of cement hydration with age Enables a high h relative humidity within the pore structure t of the concrete Reduces internal drying ( Self-Desiccation ) Extends hydration process Results: increased strength and durability Especially appropriate for High Strength concrete Larger cement content, lower water-to-cementitious materials ratios (w/cm) Particularly l vulnerable to self-desiccation and early age cracking plus high supplemental cementitius mixes

39 What is Internal Curing of Concrete? Internal curing provides a set of water-filled reservoirs within the concrete that supply water on demand to the hydrating cement paste from the time of mixing (i.e., for reducing plastic shrinkage and maintaining workability) until the time when moisture equilibrium is achieved between the reservoirs and the surrounding cement paste (i.e., for reducing autogenous and drying shrinkage). from Internal Curing Improves Concrete Performance Throughout Its Life Concrete in Focus

40 What is Internal Curing of Concrete? A long history Original research dates to the 1950 s and focused on: Negating the impacts of self-desiccation in high strength concretes Reducing the chemical ca ( autogenous ) ous shrinkage that accompanies self- desiccation, and reducing the early age cracking that accompanies chemical shrinkage Studies continue to look at: Impacts on drying and plastic shrinkage Impacts on thermal cracking Impacts on concrete strength Impacts on permeability and durability Impacts on alkali-aggregate interaction Impacts on supplementary cemetitious materials Internal curing has positive impact in all areas

41 How does Internal Curing Work? Provides a set of water-filled reservoirs within the concrete Reservoirs supply water on demand to the hydrating cement From the time of mixing Until the time when moisture equilibrium is achieved between the reservoirs and the surrounding cement paste What provides the reservoirs? Lightweight aggregate batched at a high degree of absorbed water Can be substituted for normal weight aggregate to facilitate internal curing Especially appropriate for high cement content concretes which are vulnerable to self-desiccation and early age cracking Net Result? Reduction in shrinkage cracking and associated problems Better concrete

42 Lightweight Aggregate Three Basic Types Volcanic Byproducts of coal combustion Manufactured: expanded shale, clay and slate (ESCS) ESCS Ceramic materials produced d by expanding and vitrifying select shales, clays and slates in a rotary kiln Strong, physically stable, durable, environmentally Inert, light in weight, highly insulative Absorptive

43 Lightweight Aggregate: How Does It Work? Lightweight aggregate is porous Absorbs and holds more Water than normal weight aggregate Also similar in elastic properties to the surrounding cement matrix Lightweight aggregate is porous, cement paste is porous Creates a more effective transition zone between aggregate and cement

44 Lightweight Aggregate: How does It Work? Interfacial Transition Zone (ITZ) Historically Overlooked as a Major Factor in Concrete Durability Aggregate Typically Dismissed as Inert Filler Now Receiving Much More Attention As the Significance of Internal Curing Continues to Grow

45 Lightweight Aggregate: How Does It Work?

46 The Benefits of Internal Curing Reduces shrinkage and shrinkage cracking Chemical Drying Plastic Reduces permeability Creates a denser microstructure in the cement paste Through interactions with supplementary cementitious materials Improves the microstructure of the interfacial transition zones Net Result: improved performance and durability over time Can also improve workability and increase strength

47 Reduces Chemical Shrinkage Cracking Provides on-demand water to the hydration process Concrete mix does not self-desiccate because water needed for Hydration does not run out Can significantly reduce and/or eliminate autogenous Shrinkage Reduces or eliminates chemical shrinkage cracking Strengthens the concrete

48 Reduces Drying Shrinkage The lightweight g aggregate g provides more water than needed to counteract self-desiccation Additional water replenishes any water lost from the cement paste due to drying Works in unsealed conditions Where shrinkage occurs because of hydration (chemical) and evaporation (drying)

49 Reduces Plastic Shrinkage Similar to impact on drying shrinkage Additional curing water replenishes any water lost from the cement paste due to evaporation during and immediately following placement Reduces capillary stress in the system Resulting in less consolidation Dramatically lowers the potential for plastic shrinkage cracking

50 Reduces Thermal Cracking An Auburn University Study Reveals Compared to a normal weight aggregate bridge deck mix a mix with rotary kiln lightweight aggregate doubles the time to first cracking with just a nominal dosage The study compared the four mixes under identical curing conditions ( simulating both mild and hot weather conditions) Internal curing with rotary kiln lt wt aggregate reduces or eliminates early age cracking from autogenous and thermal shrinkage.

51

52 erature ( F) Temp CTRL 95 F IC 95 F SLW 95 F ALW 95 F Concrete Age (hrs)

53 Reduces Permeability By creating a denser paste microstructure Internal curing water provides moisture needed for chemical Reactions of supplementary cementitious materials (SCMs) Silica fume, fly ash, slag, etc. Produces a denser microstructure Fewer and smaller unhydrated cement particles Smaller capillary pores

54 Reduces Permeability By improving the interfacial transition zones (ITZ) that surround aggregate Eliminates the wall effect that surrounds normal weight aggregate Allows a stronger, more continuous bond between the lightweight aggregate and the hydrated cement paste Removes preferential paths for the ingress of fluids and deleterious chemicals

55 Summary of Benefits Internal curing can: Improve slump retention, workability and finishability of fresh concrete Reduce deformations and cracking due to chemical, drying and plastic shrinkage and thermal shrinkage Increase strength and decrease permeability By increased hydration By improved interfacial transition zone microstructures Produce a more forgiving concrete that is less sensitive to poor Field- curing conditions Significantly extend the service life of concrete

56 Potential Impacts: Service Life Benefits Case Study (National Research Council Canada) Compared 4 concrete bridge deck options NC: normal concrete with no supplementary cementitious materials (SCM) HPC: high-performance concrete with 25% SCM HPC-IC: high-performance concrete with 25% SCM and internal curing VHPC-IC: High-performance concrete with 25% SCM, internal curing, and a lower water-to-cementious materials ratio

57 Potential Impacts: Service Life Benefits Case Study results Estimated service life (time to initiate delamination or spalling) NC: 19 Years HPC: 38 Years HPC-IC: 46 Years VHPC-IC: 68 Years

58 Potential Impacts: Service Life Benefits Case Study conclusions The use of internal curing can increase the service lives of highperformance concrete bridge decks by almost 10 years Due to the absence of initial cracking The estimated extension may be conservative Reduction in permeability was not accounted for Service life can be extended by 50 years when internal curing is used with very high performance concrete vs. normal Water-to-cementitious to materials ratios (w/cm) of.30 vs..40 Internal curing can bring additional economic and environmental Benefits By delaying times at which repairs and rehabilitation is required

59 Examples of Service Life Benefits 50-year-old bridges and 80-year-old concrete ships Lightweight structural concrete used in both applications Core samples revealed very low levels of microcracking Result: high levels of resistance to weathering and corrosion The Roman Port of Cosa Built in 273 BC using lightweight concrete made from natural volcanic materials Recognized that expanded aggregates worked better in a marine Environment than the locally available beach sand and gravel Port piers still standing after 2000 years Only became obsolete because of siltation

60 Applications for Internal Curing in Concrete Bridge decks Paving High strength concrete Watertight structures Sewage treatment plants Hazardous waste containment Airports Parking structures Concrete in severe environments Probability of plastic shrinkage cracking Architectural concrete Concrete with supplemental l cementitious material

61 Projects

62 Union Pacific Intermodal Facility TxDoT SH 121 Mainline Paving TxDot Bridge IH- 30 Denver Water Authority Water Tank

63 Union Pacific Intermodal Facility January 2005, Hutchins, Texas 250,000 cubic yards of concrete with Internal curing 650 and 750 psi at 28 days 2 central mix batch plants on site

64

65

66 SH c.y. mainline paving, Dallas Texas Placed November 2006 Lt Wt fines used for internal curing and to optimize overall gradation Compressive strength exceeded design by 2000 psi Crack survey measured average cracking spacing 31 Width of cracks smaller than conventional paving

67

68

69 IH 30 bridge deck Dallas Tx (txiesc.com) Denver Water Board NY DOT 12 to 16 bridge decks Indiana DOT 2 bridge decks California recently approved lt wt fines for IC

70 Txiesc.com ESCSI.org DaleBentz (NIST) Contact Information

71

72 SECTION 4 High-performance Concrete & Sustainability

73 Sustainability Benefits Durability Extended service life Structural efficiency Construction efficiency Energy efficiency

74 Durability Internal curing with lightweight aggregate: Provides for better hydration of the cementitious material Increases time-dependent quality of concrete relative to normalweight aggregate g Produces a more forgiving concrete that is less sensitive to poor field-curing conditions Impact Reduced micro-cracking Reduced permeability Increased durability

75 Extended Service Life Internal curing with lightweight aggregate: Creates concrete with significantly longer service life than Comparable normal-weight concrete Impact Reduced replacement and repair costs Reduced replacement and repair impacts on the environment Improved cost/benefit ratio over the service life of the structure

76 Extended Service Life Example The Pantheon Completed in 27 BCE (rebuilt ADE) Incorporates concretes that vary in density from bottom to top of the dome Uses lightweight structural concrete to span the dome Diameter 142 ft (43.3 m) Not exceeded for more than 1900 years Impact In continuous use for over 1800 years The definition of Extended Service Life

77 Structural Efficiency Buildings with structural lightweight concrete Thinner fire-resistant i slabs Lightweight concrete more fire-resistant than normal-weight Longer spans Additional floors added to existing structures Additional floors possible on sites with poor soil conditions Impact Can reduce foundation loads Allowing smaller footings, fewer piles, smaller pile caps, less reinforcing Can reduce dead loads Allowing smaller supporting members (girders, beams, etc.) Reduces the amount of material used in a building Reduces cost and environmental impact

78 Structural Efficiency New Bridges with structural lightweight g concrete Wider bridge decks Additional lanes Longer spans Fewer piers Impact Can accomplish more with less Reduced environmental impact for equal or better overall performance

79 Structural Efficiency Precast Longer or larger precast members can be manufactured without increasing weight Fewer columns or piers Fewer joints More cost-effective to transport Impact o o o Can ship more members per load Can accomplish more with less Reduced environmental impact for Equal or better overall performance

80 Construction Efficiency - Materials and Resources Transportation o o Transportation costs and environmental impact are directly related to weight of concrete products Transportation cost savings = 7x additional cost of lightweight aggregate 25% more ready-mix lightweight concrete can be delivered to the job site per truck load than with conventional concrete Impact o o o Fewer truck trips needed Less transportation energy use Less pollution

81 Energy Efficiency Thermal performance Lowering concrete density increases thermal resistance R-Value of 90 lb/ft 3 lightweight concrete = 260% better than R-value of 135 lb/ft 3 concrete Impact Better energy performance vs. normal-weight 44% less heating gpeak load for exterior walls 51% less cooling peak load for exterior walls 12% less total building heating peak load 2% less total building cooling peak load 2.2% 2% less total t building energy consumption (Energy cost study of a big box retail building modeled at several location in Omaha, NE, by Henderson Engineering, Kansas City, MO.)

82 Lightweight Structural Concrete and LEED Key Credit Categories Energy and Atmosphere Materials and Resources Indoor Environmental Quality Innovation & Design Process

83 Lightweight Structural Concrete and LEED Energy and Atmosphere Low thermal conductivity Lightweight concrete building components increase Thermal resistance of the structural envelope Mass Wall Construction Reduces energy use by absorbing and re-releasing heat Thermal lag shifts peak heating/cooling loads to off peak times ReducesThermal Bridging Minimizes heat flow bridges in building assemblies

84 Lightweight Structural Concrete and LEED Materials and Resources Lightweight Concrete Building Components Can Be Used to Retrofit Structures with Minimal Impact on Foundations Local Production Producers of Lightweight Structural Concrete in Close Proximity to Jobsites o Virtually all of the US within 500 Miles of a Manufacturing Location

85 Lightweight Structural Concrete and LEED Indoor Environmental Quality Better Thermal Performance Will Contribute to a Thermally Comfortable Environment Better Indoor Air Quality Not a Food Source for Mold

86 Lightweight Structural Concrete and LEED Innovation & Design Process Lightweight Building Components Require Fewer Truckloads of Material for the Same Application Reduces Energy Use, Air Pollution and Congestion

87 Course Summary

88 This Course has Reviewed: 1. Concrete Basics The importance of effective curing and its impacts on durability The role of hydration in the curing process 2. Concrete Deterioration The types and causes of shrinkage cracking Chemical shrinkage Drying shrinkage Plastic shrinkage Thermal cracking The role of permeability in concrete performance The impacts of shrinkage cracking and high permeability Corrosion Freeze-thaw damage Premature deterioration Reduced service life

89 This Course has Reviewed: 3. Internal Curing o o o What it Is A process that provides on-demand moisture to hydrating cement paste How it Works Relies on lightweight aggregate batched at a high degree of absorbed water What its Benefits Are Reduces shrinkage and shrinkage cracking Reduces permeability Improves performance and durability of concrete Extends service life

90 This Course has Reviewed: 4. Sustainability o The Key Sustainability Benefits of Internal Curing and High Performance Concrete Durability Extended service life Structural efficiency Construction efficiency Energy efficiency

91 Images in the Presentation were Provided Courtesy of the Following Organizations and are Gratefully Acknowledged Portland Cement Association (7, 8, 9, 11, 13, 16, 26, 27, 29, 34, 35) Expanded Shale, Clay and Slate Institute (12, 32, 42, 44, 45, 59, 64, 66) National Research Council of Canada (22, 35, 36, 53, 54) Federal Highway Administration (19) Turner Fairbanks Highway Research Center (18) NIST - National Institute of Standards and Technology (39) Moon Won PE Ph.D. Texas Tech. University (21, 22)

92 Dale Bentz, NIST Jason Weiss, Purdue University Anton Schindler, Auburn University Benjamin Byard, Auburn University John Ries, ESCSI

93 Thank You. This concludes the American Institute of Architects Continuing Education System Program Please take the test to receive your AIA Credits Questions? Please contact TXI at Sponsored by: TXI