State of the Art on Fiber Reinforced Concrete

Transcription

1 State of the Art on Fiber Reinforced Concrete Recent Advances on the use of FRC for Pavement Design Presented to: 015 Concrete Pavements Plus Seminar Michael Mahoney, P.Eng. Director of Admixture and Fiber Marketing Past President, Fiber Reinforced Concrete Association The Euclid Chemical Company Cleveland, OH November 015 Presentation Topics Current Industry use of Fibers Technical Design Aspects More than just fibers. Case Studies 1

2 Why do we use Fibers? Short answer: To control cracks from forming in concrete during both the plastic and hardened state Concrete is strong in compression but weak in tension. Like the placement of steel rebar, fibers are placed in concrete to transfer stress, modify the cracking behavior and possibly increase strengths and long term performance. Quality Fiber Materials types of fibers shapes of fibers quantity of fibers The Lingo - Terms and Notes FRC SnFRC SFRC Microfibers Macrofibers Aspect Ratio Fiber Reinforced Concrete Synthetic Fiber Reinforced Concrete Steel Fiber Reinforced Concrete (replace the C with S for shotcrete ie: SnFRS) smaller fibers for plastic shrinkage protection - used as secondary reinforcement used to describe structural fibers ; larger coarse fibers like steel and macro-monofilaments ratio of length to equivalent diameter (L/d); - usually only used for macro-fibers Dosage Rates: 1 lb/yd 3 = 0.6 kg/m 3 1 kg/m 3 = 1.7 lbs/yd 3

3 How to Differentiate the Fiber Types micros & macros In general, the industry has accepted that steel macro-fibers and older microsynthetic fibers (fibrillated, monofils, etc) are not used under the same conditions. Micro-synthetics secondary reinforcement; plastic shrinkage only Steel fibers industrial floor design; replacement of heavier reinforcing configurations Synthetic macro-fibers can be thought of like steel fibers, but simply not made of steel. The physical characteristics of these fibers (length, tensile strength, diameter, etc.) are all different, when compared to traditional micro-synthetics. dosages of macro-fibers should be calculated by engineering requirements. Basic Concept The main subject matter is not Fibers; It is Fiber Reinforced Concrete Benefits to contractors, engineers, owners and RM producers: Significant time savings and reduced overall construction costs For floors, no steel and wire to layout and move for laser screed operations Ability to work with RM and fiber supplier to guarantee fiber requirements to Engineer Access to new design tools and CODE Approvals Successful Track Record 3

4 Designing with Fibers How do we know that the right fiber type and dosage has been specified in my concrete? Fibers for Pavements and Floors Concrete floors and pavements must resist dynamic wheel loads, static rack loads and uniformly distributed loads. They must also withstand the damaging effect of fork truck traffic and impact from falling loads or equipment Fiber reinforced concrete, which is designed as a homogeneous material, combines easy processing and high reliability Applications Factories & Warehouses Hangers Concrete overlays & pavements Bridge Decks Pavements and Parking areas Rehabilitation projects 4

5 Specification Support Not all fibers are created equal! Calculated fiber dosages force manufacturers to provide test data and documentation that a specific fiber type is suited for the application This will protect all parties involved fiber supplier, GC, RM Supplier & owner. Fiber specifications should not call out specific dosage rates but rather desired performance and required compliances (ASTM, etc.) - fiber alternate shall be macro-fiber (steel or synthetic) complying with ASTM C1116 and provide equivalent tensile and/or bending resistance to # 4 rebar (Grade 60) placed from top of a 6 slab or mid-depth in a 8 wall.. Standard Test Methods for FRC: ASTM C1609 ASTM C 1609 can be used to find the performance of FRC in terms of residual strength. Closed-loop four-point bending test is required for proper data collection in post peak. T (toughness) Flexural strength (psi) Residual Strength Ratio (%) f R r L P. bd e,3 or R T, T f bd r 100% Example: 600 psi Example: 35% psi Implies what % of the load can be carried by the fibers after cracking 5

6 Design Concept Temperature and Shrinkage Steel Subgrade friction equations have traditionally been used for the determination of the amount of distributed steel needed to control cracks in slabs/pavements. We need to calculate the fiber dosage to provide the same level of reinforcement ratio. Area of steel: Working stress of steel: Stress from temperature gradient: fa. L. w As f s s. As. F fs b. h C. E.. T T y Set the two equal (with safety factor) Residual strength of FRC: frc ARS or R f e, 3 r /100 Design Example Temperature and Shrinkage Steel Design an 8 slab (f c=4,000 psi with WxW - 6 x6 mesh at.5 depth. How much fiber do we need for the same performance (plastic shrinkage and temperature crack control) A s 0.04 in / ft ( 0.04%) s. As. Fy ,000 fs 3 b. h 18 psi Also check for stress from temperature gradient: 6 6 C. E.. T 0.8(410 ) (5.510 ) 10 T 88 With safety factors, we will need low volume macro-synthetics to match this performance psi ARS 88 psi or R R e,3 e,3 f r 88 psi 88 / % 6

7 Design Concept Bending Moments ACI 318 method for calculating moment capacity (in a cracked beam): If fibers are substituted for steel rebar, the flexural stress is carried by fibers, called residual stress. The residual strength capacity of fiber-reinforced concrete may be obtained using: a Bending moment: M. As. Fy. d M. y Bending (flexural) strength: b I Residual strength of FRC: frc ARS or R f e, 3 r /100 Set the two equal (with safety factor) Design Example Bending Moments Design a 6 slab (f c=4,000 psi with #4 o.c. at 3 depth. How much fiber do we need for the same flexural performance? A s in / ft ( 0.1%) As. f y a 0." ' 0.85 f. b c a 0. M. As. Fy. d ,000(3 ), 940 in lb M. y 6M b 318 psi I b. h 1 6 ARS 318 psi or R f 318 psi R e,3 e,3 r 318 / % Find appropriate fiber dosage for specific manufacturer to meet this performance 7

8 Design Potentials Potential reduction in slab thickness when macro fibers are used at higher dosages. (eliminates the minimum required cover) The thinner the slab, the higher the stress, so we will need more fibers to provide higher residual strength. This can be calculated using the beam equation or the drag/temperature equations. For unreinforced concrete, we can use micro fibers at lb/yd 3 for better performance in crack control and durability. Joints The most common question that comes up on floors and pavements designed with fiber reinforced concrete is the spacing of joints. While it is possible to increase joint spacings, it is still recommended to utilize current joint spacing and cutting practices as outlined with ACI and other technical documents for normal floors. R&D for fibers is happening now with some proprietary mixtures being used in the market. There are many other factors that control joint spacing recommendations: subgrade characteristics concrete quality (gradation, w/cm ratio, supplemental materials) site conditions (interior / exterior, curing, finishing) 8

9 Concrete Shrinkage must be considered Concrete shrinkage can affect (or be affected) by various characteristics of a slab on ground cracking - caused by subgrade restraint, curing, mix design, others. curling - long term serviceability issue load transfer - design, performance and maintenance joint spacing - mitigate cracking, design implications Theories of Shrinkage ACI 3 graphic curves dependent on many factors no influence from fibers importance of curing illustrated mix design can influence testing diligence is very important 9

10 Typical Pavement Failures new pavement, conventional sawcut w/wo dowel baskets tight crack with aggregate interlock shrinkage = wider joint opening; loss of aggregate interlock and load transfer more moisture accessibility to base additional cracking at joints, loss of ability to transfer load early stages of failure concrete loss and spall accelerated failure typical pavement deterioration Fiber Reinforced Advantage fibers provide crack control, improved durability and load re-distribution if shrinkage occurs, load transfer can still be maintained; 3D reinforcing is maintained throughout matrix If edge loading causes cracks along joints, concrete integrity is maintained preventing spalling and loss 10

11 Set Expectations define the role of fibers and mix design implications slump, etc. provide education on cracking mechanisms cracks may still occur! document problems types of cracks, widths, location, timing FRC cracks should maintain integrity much longer than unreinforced cracks FRC works! faster, cheaper, cleaner, greener Watch for evolving specifications the market is changing; get educated! Projects and Success Stories 11

12 Whitetopping RTA Park n Ride Medina, OH replacement of 4 x 1 W8.5 road mesh with 5.5 pcy of macro-fiber

13 Additional Case Study Cleveland area Parking Lot evaluate shrinkage and concrete properties for different combinations of admixtures and fibers plain concrete high performance macro-fiber macro-fiber and SRA macro-fiber and SCA macro-fiber, SRA and SCA evaluate possibility of extended joints on exterior concrete have a showcase project for R&D and promotional efforts educate the ready-mix and contracting community on successful FRC design Start to Finish An FRC Parking Lot 13

14 Parking Lot Research Project more photos. 14

15 Real world measurements each slab will have joint widths monitored over the next year with correlating temperatures measurements will be compared to actual shrinkage values as measured by ASTM C157 modified tests in lab anticipated that combination fiber, SRA, SCA slab will achieve best performance Thank you for your attention Questions and Comments? 15