Shear Cracking Phenomena / Solutions John M. Becker, P.E. ACPA Pennsylvania Chapter 55 th ANNUAL MEETING – Celebrating Concrete Victories! – NOV 27-29, 2018
The Definition of a Good Presentation A Go Good Beg Begin innin ing An And Ho Hopefully Cl Close se To Toge gether A Good ood E Ending ing
1 – First Placement In this example there are six 30-inch tie-bars per 15-foot long slab (tie-bars are equally spaced 30-inches apart) 15” 15” Typically placed in warmer months This was the tie-bar design standard in Pennsylvania (prior to circa 2008-10) #5 tie-bars, 30-inches long (either Grade 40 or Grade 60)
2 – Joints Don’t Open Uniformly Dominant Joint (typically every third to fifth joint opens up first) Mainline placed in warmer months Note: Many causes, including high-friction, high-strength bases, or perhaps very low-friction bases, also thick pavements.
3 – Tied Shoulder Placement Typically First joint to open up placed in shoulder does not in cooler necessarily line up months with mainline dominant joint Mainline Dominant joints placed in typically open up even warmer wider with cooler months temperatures Note: Ambient (seasonal) temperatures often different between mainline paving and shoulder paving.
4 – The Following Summer Joints Try to Close Up Note: Edge restrained from closing by tiebars, base friction, and/or mortar intrusion.
Shear Cracking –Driving Lane Sinusoidal Shaped Shear Crack Cracks Initiate over Dowels
Shear Cracking –Passing Lane
Shear Cracking Across the U.S. Alaska Hawaii None (that C/S was aware of) One project (mortar intrusion sometimes cited as root cause) Occasionally Source: Survey of ACPA Chapter/State Executives, October/November 2018
A Word About Corner Congestion 6-inches
Shear Cracking in Pennsylvania Five projects since 2004 150 slabs documented Only 1 project since 2010 (that project had a base design flaw) Topic discussed at Annual Tour/Meetings in 2004, 2008, 2016 With Spec Re-Write in 2018, Department wanted problem solved by October 31, 2018
PennDOT’s Proposed Solution for Preventing Shear Cracking -Expansion Joints Every 180+/- feet 2. Outside Shoulder 1. Driving Lane 1. Passing Lane 3. Inside Shoulder 2. Outside Shoulder 2. Driving Lane 1. Passing Lane 1. Inside Shoulder Concerns Adjacent joints opening up resulting in loss of aggregate interlock Long-term performance of expansion joint material Constructability; how to slip form with DBI Has never been tried in the U.S. for preventing shear cracking
Preventing Shear Cracking (2004 ACPA R&T Update) Prevent Mortar Intrusion Use tar or duct tape at transverse joints, both on the side and on the top When sawing joints for second placement, flush out saw slurry that gets into joints from first placement Reduce friction of the slab/treated base interface with coat of curing compound or a thin layer of sand across the base Sand bond-breaker (banned years ago by PennDOT) Curing compound (group questioned effectiveness) Try to pave adjacent lanes in similar environmental conditions Not always practical with project schedules and MPT schemes
Preventing Shear Cracking -Continued (2004 ACPA R&T Update) Eliminate one or two tie-bars near the dominant joint One Eliminated Per Standards in Circa 2008-2010 Have we adequately reduced the risk with this change? 30” 30”
Preventing Shear Cracking -Continued (2004 ACPA R&T Update) Eliminate one or two tie-bars near the dominant joint Eliminate (by cutting off or not drilling into slab) one more on each side of a dominant joint if paving in the fall? 30” 120” 30” 60”
Preventing Shear Cracking Other Options (Done on One Project in 2004) Full-Depth Sawing of Shoulder allows Room for Mainline to Expand Sawcut Full-Depth the Shoulder to Match the Width of the Dominant Joint in the Travel Lane Mainline dominant joint This option works ONLY for Un-Dowelled Shoulders
Preventing Shear Cracking (Other Options) Encourage first joints to open up in the second placement to match Dominant Joint in First Placement By Creating a Weakened Plane Absent intervention, these rarely line up When second placement is a shoulder, tooling shoulder (1” depth) in fall paving has been effective in preventing the development of shear cracks Tooling not suggested for travel lanes; use of an early entry saw could be considered for fall paving
Where We Appear to Have Landed Expansion joints will not be required Re-emphasis on sealing/smearing of the sides of transverse joints to prevent mortar intrusion Observations are that this is not consistently being done No more duct tape Specifications already requires that first transverse joints to open up (i.e., the dominant joint) shall be marked Observations are that this is rarely being done Specification language already suggests that when doing multiple placements, the first joints cut in the second placement should be parallel to dominant joints in the first placement Consensus is that we may have already solved the problem by going from six tie-bars per 15-foot slab to five tie-bars per slab BUT…..
Where We Appear to Have Landed -More We will eliminate one additional tie bar at dominant joints when placing concrete pavement adjacent to slab previously placed or any slab placed Criteria still TBD When temperature for second placement is below 50F OR when delta temperature for second placement is 35F below first placement OR simply between October 1 and March 1 Joints in second placement parallel to dominant joints may be saw-cut using an early entry saw or by other means to “encourage” the first cracks in the second placement to line-up with first placement dominant joints
Where We Are Heading Pennsylvania Tie-bar design will be re-evaluated as part of our Department /Industry Jointing Group for standard slabs /thicknesses Seven #4 bars? Four #6 bars? Placement of tie-bar any closer than 30” from transverse joints off the table Other materials (e.g., FRP?) Tie-bar spacings for non-standard slabs also to be evaluated 12x12 slabs 6x6x6 slabs Keeping it simple is an objective! ACPA R&T is valuable document but it merits updating
If Shear Cracks Develop R&R - My Recommendations Cut tie-bars to eliminate stress Cross-stitch crack Drill 2-inch diameter stress relief hole and fill with flexible material
Acknowledgements • Geoff Clarke, Swank Construction • Jenifer Albert, FHWA/Pennsylvania • Brian Eberhart, Callahan Paving • Terry Dreher, Turnpike • Lowell Jensen, New Enterprise • George Dunheimer, PennDOT, • Rich Jucha, ACPA/PA Philadelphia/King of Prussia • Dennis Morian, QES • Josh Freeman, PennDOT • Dave Scuillo, Golden Triangle Pavement Design • Mark Snyder, PERC • Dave Jarvis, PennDOT/QA • Clayton Stahl, Gulisek • Bill Kovach, PennDOT, Uniontown • ACPA & Chapter/State folks • Lydia Peddicord, PennDOT • Steve Waalkes (Author of ACPA R&T) Pavement Design • Brent Trivelpiece, PennDOT/QA 55 th ANNUAL MEETING – Celebrating Concrete Victories! – NOV 27-29, 2018
Thank You John M. Becker, P.E., President ACPA Pennsylvania Chapter 800 N. 3 rd Street, Suite 503 Harrisburg, PA 17102 Jbecker@pavement.com 717-215-0509 www.LongerLifePavement.com 55 th ANNUAL MEETING – Celebrating Concrete Victories! – NOV 27-29, 2018
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