Good Practices for Concrete Road Pavement Construction

Transcription

1 Good Practices for Concrete Road Pavement Construction Mark B. Snyder, Ph.D., P.E. President, International Society for Concrete Pavements Vice-President, ACPA Pennsylvania Chapter Critical Factors for Smooth Pavement A good grade and track line for paving String line management A good concrete mixture Consistent concrete workability Continuous supply of concrete to paver Well-maintained paving equipment Proper operation of paving equipment Good consolidation and finishing of concrete A skilled and dedicated crew 1

2 Overview Foundation Preparation Pre-Paving Setup Paving Operations Joint Layout It Starts from the Ground Up Roadbed (subgrade and subbase) design and construction are key to: Long-term performance Smoothness (initial and long-term) 2

3 What is Good Support? Uniformity in material and grading (most important!) Resistant to erosion Engineered to control subgrade soil expansion/frost heave Foundation Preparation Subgrades 3

4 Subgrades Obtain uniform support by controlling: Expansive soils Frost-susceptible soils (frost heave) Pumping Wet Soils Unstabilized Subgrades 1. Grade to match roadway plans 2. Cross haul to avoid abrupt changes 3. Compact at optimum moisture content 4. Identify soft spots and fix 5. Protect from rain (if necessary) by tight blading and finishing with smooth drum roller 6. Fine grade to plan elevations within tolerances 4

5 Stabilized Subgrades 1. Trim to match roadway plans but finish the grade below the final grade elevation 2. Spread stabilized agent as evenly as possible 3. Mix, add water and compact 4. Finish grade 5. Cure the subgrade Foundation Preparation Subbases 5

6 Place Base to Specified Tolerances Minimize loss of concrete Minimize/eliminate thickness penalties Enhance smoothness Enhance pavement performance General Notes on Subbases Thick subbases (greater than 150mm) are typically not beneficial, and therefore are not recommended The width of the subbase should accommodate the paving equipment by extending at least 1 m beyond the width of the pavement on each side. Recommended minimum thickness: Unstabilized: 100mm CTB or LCB: 100mm ATB: 50mm Recycled materials are commonly used 6

7 Unstabilized Subbases 1. Mix a uniformly moist, homogeneous material 2. Place using preferred method 3. Compact to required density w/min effort 4. Trim to plan elevation and tolerances 5. Moisture content key for during construction of subbase AND immediately before paving Stabilized Bases Includes soil-cement, cement-treated base, asphalt-treated base, permeable asphalt-treated base Strength issue for stabilized base Specify min/max values 7-day compressive strength: 5.0/8.0 MPa psi common Base stiffness affects pavement performance Potential for random cracking high for very stiff bases 7

8 Pre-Paving Setup Safety Ride quality is pointless if someone is hurt Plan out truck routes Use of highway patrol Daily safety meetings Teach employees to watch traffic and each other 8

9 Safety Pay attention to broom handles, saws and protrusions in traffic Watch for backing trucks and make sure alarms are functional Be aware of surroundings Pre-Paving Setup SLIPFORM PAVING 9

10 String lines String line management very important to final smoothness of pavement String lines control the steering of the paver String lines control the elevation and slab thickness String line Alignment Sensing Wand Elevation Sensing Wand Set Stringline Can be wire, cable, woven nylon, polyethylene rope, or another similar material Clean and tight splices Rigid stakes watch for staking errors Maximum stake spacing of 7.6m No perceptible sagging Adjust stake spacing to fit conditions See Staking Interval Calculator at apps.acpa.org for recommendations on vertical curves 10

11 Set String Line (cont.) Continually check tension Place winches at 300m String lines on both sides of paving? Some situations require cantilever or trusses for sensors to reach string line Setting the String Line 1. Set reference hubs at proper interval place a string line support stake outside of each hub 2. Set stake arm to the proper elevation 3. Install string line 4. Tighten string line 5. Check installation 11

12 Once Set, the Paver Does the Rest Stringless Paving: Example Leica s Direct Connect 3D Control System Software communicates directly with networked Microprocessor Control System 12

13 Pre-Paving Setup FIXED FORM PAVING Application Advantages Fixed Form Paving Tight tolerances - side clearance Multiple changes of width Blockouts Intersections - quadrants open to traffic 13

14 Application Advantages Fixed Form Paving Maintain quality during short concrete supply Critical staging for traffic control Small inexpensive equipment can place concrete An adaptable system Forms Typical form - steel section Wide, flat base for stability Rail to carry paving equipment Pin (stake) holes to anchor to base Connections for adjacent forms Normally, height same as pavement thickness 14

15 Setting Forms Forms Before ordering or using check Specifications for requirements Approved for use on project Paving equipment needs Paving process needs Adequacy of stockpiled forms Order forms timely 15

16 Forms Additional requirements Tightly lock to ends of adjacent forms Stake with a minimum of two pins for each 3 m section Stable under weight or vibration of equipment Clean and oil before use Recycle or discard any bent, twisted or broken forms Setting Forms Form setting is critical construction step Uniform support Proper maintenance Proper alignment and elevations contribute to smooth pavement 16

17 Setting Forms Set forms to proper grade - alignment Eyeball horizontal and vertical alignment Check depth between forms Check width between forms Check joints between forms Pre-Paving Setup INSTALLATION OF DOWELS 17

18 Transverse Contraction Joints mm dia. Smooth Dowel Doweled D/2 D D/3 Reservoir 3-9 mm (typ.) Undoweled Sealant Deflection Load Transfer 0% Load Transfer 100% Load Transfer Wheel Load Approach Slab Wheel Load Direction of Traffic Leave Slab Unloaded LT = Loaded Direction of Traffic Approach Slab Leave Slab 18

19 Effects of Joint Load Transfer on Pavement Behavior Deflections in Concrete Pavement Outside Pavement Edge 3.6m Lanes 5 D i 3 D i 3 D i 2 D i D i D i Longitudinal Centerline (acts as tied PCC Shoulder) Undoweled Transverse Joint Doweled Transverse Joint Typical Dowel Bars Typical length = 450 mm Typical diameter Roads: mm Airports: 38 50mm Epoxy or other coating typically used in harsh climates for corrosion protection 19

20 Dowel Bar Stresses Shear, bending and bearing (dowel on concrete) Stresses increase with increased loading Stresses decrease with increased foundation support Shear and bending are rarely critical, rarely analyzed. Bearing stresses can be critical to performance! 20

21 Optimized Dowel Spacing Trend toward reducing standard dowel installations from 12 dowels per 3.6-m lane to 11 increase distance from lane edge to outside dowels to reduce incidence of paver-induced misalignment Concentrated dowels in wheel paths Common in dowel bar retrofit applications Some trends for new construction Evaluate bearing stresses for alternate spacings using DowelCAD software 21

22 Optimized Dowel Designs Reduce bearing stress while holding cross-sectional area constant (or reducing it) Examples: Elliptical Dowels Plate Dowels 22

23 Dowel Guide and Tech Brief Available! National Concrete Pavement Technology Center at Iowa State University /index.cfm 23

24 Placing Dowels Dowel Bar Placement Issues Baskets Basket rigidity (wires/design) Basket stability cut shipping wires (?), anchor type/length, base stability Dowel debonding agent Basket length Mechanical implantation Consolidation around dowel bars Dowel debonding agent Dowel alignment 24

25 Placing and Staking Dowel Baskets See cptechcenter.org for NCC Guide to Dowel Load Transfer Dowel Location Identification 25

26 Dowel Misalignment Examples 51 Placing Tiebars 26

27 Pre-Paving Setup PAVING EQUIPMENT SETUP Paving Equipment Setup 27

28 Final Grade and Cross Slope Check Vibrator Setup 28

29 Effect of Paver Speed on Consolidation Slipform Paver Setup 29

30 Edge Slump Edge slump can arise from a number of factors Materials Equipment Early detection and corrective action are required Most important with stage construction Slipform Mold (Pan) Setup Preliminary leveling of paving machine s frame and then slip-form mold Check joints in the pan Adjust center to account for cross-slope Check alignment Adjust edges for edge slump 30

31 Typical Paving Clearance Zone The minimum clearance zone needed for a standard concrete paver operation is ~1.2 m per machine side: ~0.9 m for the paver track and workers ~0.3 m for paver control string line Modified Paver for Zero Clearance Notice edge of paver riding next to curb 31

32 PAVING OPERATIONS Concrete Placement Deposit concrete as close to paver as possible Avoid stop-and-go operation Maintain uniform speed and head No end loaders or backhoes to distribute concrete 64 32

33 Concrete Placement Issues Do not add water to concrete in front of (or behind!) paver Reduced strength Reduced durability Proper vibration effort Control consolidation across paving width Provide just enough fines at surface for a tight finish This is also affected by mix design Consolidation The internal vibrators on the paver fluidize the concrete for extrusion Adequate consolidation Required around dowels and tie bars Throughout the slab 33

34 Poor Consolidation Lower in-place strength, honeycombing Over-Consolidation Over vibration can cause settlement, loss of air void system, less durable concrete IMCP Manual 34

35 Headers: Beginning and End of Day Construction What are some of the concerns? Consolidation Dowel alignment Finishing (over-finishing) Source: Shiraz Tayabji, Fugro Consultants, Inc Finishing Operations Minimal hand finishing do not over-finish Surface does not have to be super-smooth Longer straight edges produce smoother surface Do not add water to facilitate finishing if used, it should be fogged, not sprayed Finishers have final say on PCCP smoothness & surface durability Source: Shiraz Tayabji, Fugro Consultants, Inc High Spalling Potential 35

36 Use 5-m float for smoothness Need for finishing is minimized by Selecting a workable mix Properly operating the paving 71 equipment CONCRETE PAVEMENT TEXTURE Goals: Safe, Smooth and Quiet 36

37 Texturing and Smoothness Provide friction and skid resistance Texturing options: Drag textures Longitudinal tining Transverse tining Diamond grinding Innovative techniques Texture Types Transverse Tine Conventional Diamond Grinding Traffic Longitudinal Tining Exposed Aggregate 37

38 Source: Iowa State University,

39 Noise vs. Friction Average OBSI Level (dba) Diamond Grinding Drag Longitudinal Tining Transverse Tining Other Average DFT/CTM-Estimated SN40S (ASTM E 274 Skid Trailer, Bald Tire) Next Generation Concrete Surface (NGCS) vs. Conventional Diamond Grinding (CDG) NGCS CDG 39

40 Equipment Head Differences NGCS Head Conventional Diamond Grinding Head NGCS Texture MicroTexture Grooves for Macro Texture 40

41 Concrete Curing Need to maintain adequate moisture & temperature regimes Inadequate curing leads to Excessive moisture loss at surface => plastic shrinkage cracking Weak surface => durability problems Excessive slab warping (built-in warp) Must assure timely curing behind paver Source: Shiraz Tayabji, Fugro Consultants, Inc. Curing Methods Ponding/continuous sprinkling Burlap/cotton mats Plastic sheeting Curing compounds Source: Shiraz Tayabji, Fugro Consultants, Inc. 41

42 Curing Compound Curing Compounds Coating placed on concrete surface to prevent moisture loss Resin or wax based Clear or white-pigmented Advantages: Easy to use - not labor intensive Economical ASTM C 309 & AASHTO M148 (Type 2, Class B) All curing compounds are not created equal even if they meet specs Water retention is a key to successful curing 42

43 Curing Compound Application Time of application Apply as soon as surface sheen has disappeared Use automated equipment for uniform coverage Cover all exposed surfaces (incl. sides) Re-apply at joints after sawcutting Typical application rate: 4 to 5 m 2 /l ( ft 2 /gal) Curing time: Typically 72 to 96 hours Check nozzles regularly for uniform spray (clogging) Source: Shiraz Tayabji, Fugro Consultants, Inc

44 Nozzle Height 200 mm PCCP 250 mm PCCP Timing of Curing Final finishing follows concrete placement There is some wait to ensure surface is ready for texturing/tining Once bleeding stops & texturing is done, curing can start Too early curing - bleed water accumulates under and damages curing membrane Too late curing - damages surface & results in plastic shrinkage cracking If grinding is used for texture, curing can begin as soon as bleeding stops Note: Slipform paving concretes do not bleed much!! Source: Shiraz Tayabji, Fugro Consultants, Inc. 44

45 Joint Sawing Why saw? Saw timing Sawing window Maturity testing HIPERPAV Early-entry saws Joint width Mixture effects Why Joint Concrete Pavement? Primary Reason: control natural cracking ft ft 45

46 Joint Sawing Factors that Shorten Window Sudden temperature drop High wind, low humidity High friction base Bonding between base & slab Porous base (PATB) Retarded set (delayed and shortened) Delay in curing application 46

47 Crack Prediction with HIPERPAV Hot-Weather Concreting Applicable when air temp. > 25 C for 3 days Potential problems include Rapid loss of slump; Reduced air content Premature stiffening; Plastic shrinkage cracking Thermal cracking Use of less cement & more supplementary cementing materials (slag, Class F FA, etc) Reduce heat of hydration Class C FA not recommended Adjust admixture dosage Source: Shiraz Tayabji, Fugro Consultants, Inc. 47

48 Temperature of Slabs Placed at Different Times Surface Temperature, ºC :00 12:00 4:00 8:00 12:00 4:00 am pm am Time Paved at 7:30 am Paved at 9:00 am Paved at 11:15 am Paved at 1:15 pm Paved at 3:30 pm Surface Temperature, ºF Source: Shiraz Tayabji, Fugro Consultants, Inc Mitigating Hot-Weather Effects Consider setting up on-site miniweather station Monitor evaporation rates Monitor sawcutting window Avoid use of hot cement or FA directly from supplier Use chilled water Use waterborne monomolecular evaporation retarder Consider night-time placement Delay paving operations Source: Shiraz Tayabji, Fugro Consultants, Inc. 48

49 Joint Layout Why Joint Concrete Pavement? Other reasons we joint concrete pavements: Divide pavement into construction lanes or increments. Accommodate slab movements. Provide load transfer via placed dowels. Provide uniform sealant reservoir. 49

50 Why Joint Concrete Pavement? Not for lane delineation! Influence of Slab Geometry on Stresses 50

51 Joint Layout Critical to crack control Typically decided by engineer and included in project plans No knowledge of contractor, equipment, processes Hard to precisely place things like utilities Contractor may be allowed to develop plans but, even if not, field adjustments can and should be made Field Adjustments are Necessary Adjust joints that are within 1.5m of a utility asset! Must isolate utilities as shown on plans Contractor must also consider impact of moving joints!!! 51

52 If Joints Aren t Properly Adjusted What If I Have to Dead-end a Joint? 52

53 What If I Have to Dead-end a Joint? Joint Layout Design EB237 Concrete Pavement Field Reference: Pre-Paving IS006, Intersection Joint Layout IS061 Design and Construction of Joints for Concrete Streets R&T Update 6.03 Concrete Roundabouts TB010 Design and Construction of Joints for Concrete Highways TB017 Airfield Joints, Jointing Arrangements and Steel TB019 Concrete Intersections: A Guide for Design and Construction 53

54 Looking for More Details? Order ACPA s Field References and search: EB237: Pre-Paving EB238: Paving EB239: CPP PDF or Printed Member: $13 - $15 Non-Member: $50 - $60 54

55 FREE Proper Procedure Checklists! Acknowledgments American Association of State Highway and Transportation Officials (AASHTO) American Concrete Pavement Association National Concrete Pavement Technology Center (Iowa State University) Shiraz Tayabji, Fugro Consultants, Inc. U.S. Federal Highway Administration Pavement Technology Program U.S. National Highway Institute 55

56 Questions? 56