Horizontal/Subsurface Infrastructure & Transportation

Maintaining joints in our concrete pavements is expected, but we question why we need to maintain so many linear feet. Seems reasonable that slabs could be bigger in plan dimension (currently 20ft x 20ft and/or 25ft x 25ft) to minimize the amount of joints we need to maintain. FAA set the standard on joint spacing, but couldn't there be some technical justification to go to bigger spacings? Many of today's wide body aircraft have gear loading that fall along the longitudinal joint line, perhaps that is justification to increase slab dimensions? Your thoughts? - #SpacedOut

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Manuel Bejarano P.E., M.ASCE
Engineer IV
Atkins
Orlando FL

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Airfield pavements with smaller slabs I.e. (225  to 278 sf) have lower warping stresses and are less prone to mid-slab cracking.    Google Earth the PHX airport for a visual example.  
The old Terminal 1 apron (no building just pavement slabs in a radial configuration at the west end of the terminal area was built in 1952 using then Navy criteria, 12.5 x 15 foot slabs, 3000 lb concrete, 9-inch with thickened edges.

Compare the pavement condition and mid-slab fractures with that of Terminal 2 apron, built in 1960 using then FAA design criteria, 20 x 25 foot slabs, 650 flex concrete, 11-inches thick.

Both areas have the same soil type, and 50 plus years of primarly Group III aircraft.

Current FAA design (cumulatiive stress) methods are based on aircraft loadings and ignore the  daily warping stresses that created by environmental variations.

My preference is to select slab size (ie joint spacings) so that the warping stresses never exceed 50% of the design flexural strength.

In my opinion, the reason that some organizations have such a problem with joint maintenance  is becaiuse of poor joint design (no chamfered joints), insufficient curing, and permitting any construction traffic on the pavement prior to the time that the concrete has cured to at least 75% of the design strength.  See the following paper "Improved Methods for Specifying and Placemnet of Hot Weather Pavement in Desert Environments, Transportation Systems Workshop, Austin, 2012 for detail based on experience of over 4.2 million square yards of pavement at Ft. Bliss TX.

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Jon Girand P.E., L.S., F.ASCE
Vice Pres
Arvada CO
(303) 658-0777
------------------------------

I agree with the desire to increase slab size. There is no doubt that joints are the weak spot in rigid pavements; the less the better. In addition, edge loading leads to the largest stress and corner loading leads to the largest deflection; so we would prefer the widebody tires to load on the center of the slab rather than on the longitudinal joints.

But remember that joints are a necessary feature for rigid pavement because concrete shrinks during curing and, more importantly, concrete curls due to temperature gradient and warps due to moisture gradient. The question is what is the limit.

• Stress due to friction: On page 62 of textbook Pavement Engineering: Principles and Practice (ISBN-13: 978-1439870358), the stress due to friction is σ_c =γ_c f_a L/2. For normal concrete, the induced tensile stress from friction would be lower than 100 psi, which is way below the flexural strength. "Therefore, in general, it does not appear that the joint spacing in plain concrete pavements is governed by stress due to friction alone."

• Stress due to drying shrinkage: I am curious about what the stress due to drying shrinkage is, but I have not found an equation for this yet. The textbook (page 63) estimates joint spacing by control the joint opening (ΔL) which has a relationship with the coefficient of thermal expansion α and the drying shrinkage coefficient ε of concrete. ΔL = C L (α Δt + ε ). An example calculation shows that 18 ft is the maximum slab length if the max. joint opening is 1/8''.

	Joint opening (in.)	CTE (*10-6)	delta T (F)	drying shrinkage (*10-6)	friction adjustment	L (ft)
1/8''	0.125	5.5	60	400	0.8	18

• Stress due to temperature curling: Edge stress can be estimated by σ_t = C E α Δt /2. An example shows that the curling stress in daytime could be up to 340 psi. When this stress is overlapped with stress due to traffic loading, it could easily reach the flexural strength of concrete and cause cracking. 

Therefore, the stress from temperature curling controls the size of slab. In places where the temperature gradient Δt is small, larger slab size could be possible. This agrees with my personal experience. Louisiana uses 20 ft slabs for years and did not see many mid-slab cracking, while the max. slab length is only 15 ft in Wisconsin. Can we push the existing "limit" to 25*25 or 30*30 is worth of investigating.

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Danny X. Xiao, Ph.D., P.E., M.ASCE
University of Wisconsin Platteville
Platteville WI
(608)342-7249
------------------------------

Original Message:
Sent: 02-27-2017 21:05
From: Jon Girand
Subject: Maintaining joints in concrete pavements

Airfield pavements with smaller slabs I.e. (225  to 278 sf) have lower warping stresses and are less prone to mid-slab cracking.    Google Earth the PHX airport for a visual example.  
The old Terminal 1 apron (no building just pavement slabs in a radial configuration at the west end of the terminal area was built in 1952 using then Navy criteria, 12.5 x 15 foot slabs, 3000 lb concrete, 9-inch with thickened edges.

Compare the pavement condition and mid-slab fractures with that of Terminal 2 apron, built in 1960 using then FAA design criteria, 20 x 25 foot slabs, 650 flex concrete, 11-inches thick.

Both areas have the same soil type, and 50 plus years of primarly Group III aircraft.

Current FAA design (cumulatiive stress) methods are based on aircraft loadings and ignore the  daily warping stresses that created by environmental variations.

My preference is to select slab size (ie joint spacings) so that the warping stresses never exceed 50% of the design flexural strength.

In my opinion, the reason that some organizations have such a problem with joint maintenance  is becaiuse of poor joint design (no chamfered joints), insufficient curing, and permitting any construction traffic on the pavement prior to the time that the concrete has cured to at least 75% of the design strength.  See the following paper "Improved Methods for Specifying and Placemnet of Hot Weather Pavement in Desert Environments, Transportation Systems Workshop, Austin, 2012 for detail based on experience of over 4.2 million square yards of pavement at Ft. Bliss TX.

------------------------------
Jon Girand P.E., L.S., F.ASCE
Vice Pres
Arvada CO
(303) 658-0777
------------------------------

Jon mentioned "Current FAA design (cumulatiive stress) methods are based on aircraft loadings and ignore the  daily warping stresses that created by environmental variations".

FAA currently has a project to study the magnitude of stresses developed in airport pavement concrete slabs due to warping and curling. We are instrumenting concrete slabs at airports in different geographical locations. Our current field installations include Atlanta's Hartsfield Jackson International Airport, and John F. Kennedy International Airport. In Spring this year, we will be installing sensors at Philadelphia International Airport. If the research shows that the stresses developed due to curling and warping are significant enough to be considered in pavement design, appropriate changes would be made in FAARFIELD.

 

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[Navneet] [Garg] [Ph.D., M.ASCE]
[Program Manager, National Airport Pavement & Materials Research Center ]
[Federal Aviation Administration]
[Atlantic City] [NJ]
[609-485-4483]
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Original Message:
Sent: 02-27-2017 21:05
From: Jon Girand
Subject: Maintaining joints in concrete pavements

Airfield pavements with smaller slabs I.e. (225  to 278 sf) have lower warping stresses and are less prone to mid-slab cracking.    Google Earth the PHX airport for a visual example.  
The old Terminal 1 apron (no building just pavement slabs in a radial configuration at the west end of the terminal area was built in 1952 using then Navy criteria, 12.5 x 15 foot slabs, 3000 lb concrete, 9-inch with thickened edges.

Compare the pavement condition and mid-slab fractures with that of Terminal 2 apron, built in 1960 using then FAA design criteria, 20 x 25 foot slabs, 650 flex concrete, 11-inches thick.

Both areas have the same soil type, and 50 plus years of primarly Group III aircraft.

Current FAA design (cumulatiive stress) methods are based on aircraft loadings and ignore the  daily warping stresses that created by environmental variations.

My preference is to select slab size (ie joint spacings) so that the warping stresses never exceed 50% of the design flexural strength.

In my opinion, the reason that some organizations have such a problem with joint maintenance  is becaiuse of poor joint design (no chamfered joints), insufficient curing, and permitting any construction traffic on the pavement prior to the time that the concrete has cured to at least 75% of the design strength.  See the following paper "Improved Methods for Specifying and Placemnet of Hot Weather Pavement in Desert Environments, Transportation Systems Workshop, Austin, 2012 for detail based on experience of over 4.2 million square yards of pavement at Ft. Bliss TX.

------------------------------
Jon Girand P.E., L.S., F.ASCE
Vice Pres
Arvada CO
(303) 658-0777
------------------------------

^{i agree with Jon that the reason to use short joints is to reduce the environmenal stresses due to curling and warping. Environmental stresses can easily be as much as stresses due loading and the easiest way to reduce them is to use shorter joint spacing to decrease the moment arm and uplift of the slab edges. The shorter spacing also means less movement at each joint opening so it stays tighter and causes less strain on the sealant, meaning it should last longer.} 

However, regarding the joint itself, i prefer the single saw cut (no widening) and the joints filled. It has been my experiences that it is not the water that causes issues but it is the incompressibles that get into the joint and eventually cause spalling. Keeping the joints narrow means bigger incompressibles can not get in. When one widens a joint, it essentially creates a funnel that channels water and incompressibles into the joint. Though i have less experience chamfered joints, i would expect the same. Use a 1/8 in (3 to 4 mm) wide cut, 1/3 the slab depth to make the joint then fill the joint up with sealant to keep out incompressibles. 

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James Mack P.E., M.ASCE
Director, Market Development
CEMEX
Houston TX
(713)598-6669
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Concrete will expand and shrink with changing in temperature and moisture.  This process will generate the cracks. According to ACI 330 - "Specification for Unreinforced Concrete"  Table 3.13 states that the spacing should not exceed 15 ft when the thickness is 6 “ and above. ACPA and FHWA recommends as well the limit of 15 ft.  You may design longer spacing limit, but it is more risk that going to be.
Thanks, 

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Hosam Salman P.E., F.ASCE
Supervising Engineer
Parsons Brinckerhoff Inc
Dallas TX
(214)583-3443
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We have been using 25'x25' slabs at Memphis International Airport for close to 20 years. Our pavements are in excellent condition. Despite the great history and experience, the FAA requires us to submit a Modification of Standards request for each project because their maximum allowable spacing is 20 feet. While each environment is unique, we attribute our success to thick slabs (typically at least 16 inches), the use of a flexible asphalt base layer, excellent subgrade preparation, and good maintenance. 

There is no one size fits all approach, but I agree that engineers should have fewer restrictions in pavement design criteria so that they can design properly for each owner and site. 

------------------------------
Nathan Luce P.E., M.ASCE
Manager of Engineering & Construction
MEMPHIS SHELBY COUNTY AIRPORT AUTHORITY
Memphis TN
(901) 922-8030
------------------------------

Original Message:
Sent: 02-28-2017 09:19
From: Hosam Salman
Subject: Maintaining joints in concrete pavements

Concrete will expand and shrink with changing in temperature and moisture.  This process will generate the cracks. According to ACI 330 - "Specification for Unreinforced Concrete"  Table 3.13 states that the spacing should not exceed 15 ft when the thickness is 6 “ and above. ACPA and FHWA recommends as well the limit of 15 ft.  You may design longer spacing limit, but it is more risk that going to be.
Thanks, 

------------------------------
Hosam Salman P.E., F.ASCE
Supervising Engineer
Parsons Brinckerhoff Inc
Dallas TX
(214)583-3443
------------------------------

I agree with Nathan Luce's statement below:

"There is no one size fits all approach, but I agree that engineers should have fewer restrictions in pavement design criteria so that they can design properly for each owner and site." 

While the design standards and text books offer guidance on stress, thickness, and joint spacing; specific designs and detailed analysis needs to be carried out to determine the appropriate design solution for each site and project.      

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Nathan Kebede P.E., M.ASCE
Pavement Engineer
Cartegraph
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Original Message:
Sent: 03-01-2017 08:38
From: Nathan Luce
Subject: Maintaining joints in concrete pavements

We have been using 25'x25' slabs at Memphis International Airport for close to 20 years. Our pavements are in excellent condition. Despite the great history and experience, the FAA requires us to submit a Modification of Standards request for each project because their maximum allowable spacing is 20 feet. While each environment is unique, we attribute our success to thick slabs (typically at least 16 inches), the use of a flexible asphalt base layer, excellent subgrade preparation, and good maintenance. 

There is no one size fits all approach, but I agree that engineers should have fewer restrictions in pavement design criteria so that they can design properly for each owner and site. 

------------------------------
Nathan Luce P.E., M.ASCE
Manager of Engineering & Construction
MEMPHIS SHELBY COUNTY AIRPORT AUTHORITY
Memphis TN
(901) 922-8030
------------------------------

Thanks Nathan for offering the successful experience at Memphis Airport. I would like offer a lesson at Green Bay Austin Straubel Airport. You can see this on Google Map. The taxiway between the two runways has slab size of 40 ft by 25 ft. Clearly most of the slabs cracked in the middle. Lessons learned: 40 ft joint spacing is too long.

I think the length of slab is limited by the combined stress from temperature curling and traffic loading. Since curling stress depends on temperature gradient Δt, we may be able to increase the joint spacing in locations where Δt is not significant. In addition, as Memphis' experience, thick slab reduces the stress due to loading; hence reduces the combined stress and enables the 25 ft to perform well. 

Looking forward to the result from FAA's project. The textbook on this topic can be improved to provide a better support on why 20 ft is the maximum.

------------------------------
Danny X. Xiao Ph.D., P.E., M.ASCE
University of Wisconsin Platteville
Platteville WI
(608)342-7249
------------------------------

Original Message:
Sent: 03-01-2017 08:38
From: Nathan Luce
Subject: Maintaining joints in concrete pavements

We have been using 25'x25' slabs at Memphis International Airport for close to 20 years. Our pavements are in excellent condition. Despite the great history and experience, the FAA requires us to submit a Modification of Standards request for each project because their maximum allowable spacing is 20 feet. While each environment is unique, we attribute our success to thick slabs (typically at least 16 inches), the use of a flexible asphalt base layer, excellent subgrade preparation, and good maintenance. 

There is no one size fits all approach, but I agree that engineers should have fewer restrictions in pavement design criteria so that they can design properly for each owner and site. 

------------------------------
Nathan Luce P.E., M.ASCE
Manager of Engineering & Construction
MEMPHIS SHELBY COUNTY AIRPORT AUTHORITY
Memphis TN
(901) 922-8030
------------------------------

Do you think that the cracking indicates that 40' is too long or that the 8:5 ratio (40:25) for the span is too far out of 'square'? I agree that 40' is a long free-span for SOG, even with adequate thickness, initial curing, subgrade and climate conditions, etc. However, I bet that a 16x10 panel might crack in many scenarios where a 16x16 wouldn't. I think that the proportions and size are both contributing to the cracking in this scenario.

Doug Provins, PE
Resident Engineer | Construction Manager
Kleinfelder
619.818.2427

------------------------------
Doug Provins, PE
Resident Engineer | Construction Manager
Kleinfelder
619.818.2427
------------------------------

Original Message:
Sent: 03-02-2017 10:38
From: Danny X. Xiao
Subject: Maintaining joints in concrete pavements

Thanks Nathan for offering the successful experience at Memphis Airport. I would like offer a lesson at Green Bay Austin Straubel Airport. You can see this on Google Map. The taxiway between the two runways has slab size of 40 ft by 25 ft. Clearly most of the slabs cracked in the middle. Lessons learned: 40 ft joint spacing is too long.

I think the length of slab is limited by the combined stress from temperature curling and traffic loading. Since curling stress depends on temperature gradient Δt, we may be able to increase the joint spacing in locations where Δt is not significant. In addition, as Memphis' experience, thick slab reduces the stress due to loading; hence reduces the combined stress and enables the 25 ft to perform well. 

Looking forward to the result from FAA's project. The textbook on this topic can be improved to provide a better support on why 20 ft is the maximum.

------------------------------
Danny X. Xiao Ph.D., P.E., M.ASCE
University of Wisconsin Platteville
Platteville WI
(608)342-7249
------------------------------


Original Message:
Sent: 03-01-2017 08:38
From: Nathan Luce
Subject: Maintaining joints in concrete pavements

We have been using 25'x25' slabs at Memphis International Airport for close to 20 years. Our pavements are in excellent condition. Despite the great history and experience, the FAA requires us to submit a Modification of Standards request for each project because their maximum allowable spacing is 20 feet. While each environment is unique, we attribute our success to thick slabs (typically at least 16 inches), the use of a flexible asphalt base layer, excellent subgrade preparation, and good maintenance. 

There is no one size fits all approach, but I agree that engineers should have fewer restrictions in pavement design criteria so that they can design properly for each owner and site. 

------------------------------
Nathan Luce P.E., M.ASCE
Manager of Engineering & Construction
MEMPHIS SHELBY COUNTY AIRPORT AUTHORITY
Memphis TN
(901) 922-8030
------------------------------

The length to width ratio of concrete pavements is extremely important.  Consider that control joints in sidewalks are typically spaced to match the width of the sidewalks resulting in square panels.  Also, there cannot be one standard joint spacing to fit all climates.  In extremely cold climates the surface temperature of the pavement may be 70% of the air temperature during the winter and 150% of the air temperature during the summer.  In Fairbanks, AK this could result in temperature differences between summer and winter of 200 oF.

Darrel Behrent, P.E. MASCE
Master of Science, Arctic Engineering
Cold Regions Engineering Instructor
Montana State University

------------------------------
Darrel Behrent P.E., MASCE
M.S. Arctic Engineering
Bozeman MT
(406)619-2910
------------------------------

Original Message:
Sent: 03-02-2017 11:50
From: Robert Provins
Subject: Maintaining joints in concrete pavements

Do you think that the cracking indicates that 40' is too long or that the 8:5 ratio (40:25) for the span is too far out of 'square'? I agree that 40' is a long free-span for SOG, even with adequate thickness, initial curing, subgrade and climate conditions, etc. However, I bet that a 16x10 panel might crack in many scenarios where a 16x16 wouldn't. I think that the proportions and size are both contributing to the cracking in this scenario.

Doug Provins, PE
Resident Engineer | Construction Manager
Kleinfelder
619.818.2427

------------------------------
Doug Provins, PE
Resident Engineer | Construction Manager
Kleinfelder
619.818.2427
------------------------------


Original Message:
Sent: 03-02-2017 10:38
From: Danny X. Xiao
Subject: Maintaining joints in concrete pavements

Thanks Nathan for offering the successful experience at Memphis Airport. I would like offer a lesson at Green Bay Austin Straubel Airport. You can see this on Google Map. The taxiway between the two runways has slab size of 40 ft by 25 ft. Clearly most of the slabs cracked in the middle. Lessons learned: 40 ft joint spacing is too long.

I think the length of slab is limited by the combined stress from temperature curling and traffic loading. Since curling stress depends on temperature gradient Δt, we may be able to increase the joint spacing in locations where Δt is not significant. In addition, as Memphis' experience, thick slab reduces the stress due to loading; hence reduces the combined stress and enables the 25 ft to perform well. 

Looking forward to the result from FAA's project. The textbook on this topic can be improved to provide a better support on why 20 ft is the maximum.

------------------------------
Danny X. Xiao Ph.D., P.E., M.ASCE
University of Wisconsin Platteville
Platteville WI
(608)342-7249
------------------------------

------Original Message------

The length to width ratio of concrete pavements is extremely important.  Consider that control joints in sidewalks are typically spaced to match the width of the sidewalks resulting in square panels.  Also, there cannot be one standard joint spacing to fit all climates.  In extremely cold climates the surface temperature of the pavement may be 70% of the air temperature during the winter and 150% of the air temperature during the summer.  In Fairbanks, AK this could result in temperature differences between summer and winter of 200 oF.

Darrel Behrent, P.E. MASCE
Master of Science, Arctic Engineering
Cold Regions Engineering Instructor
Montana State University

------------------------------
Darrel Behrent P.E., MASCE
M.S. Arctic Engineering
Bozeman MT
(406)619-2910
------------------------------