Concrete Sidewalks

Concrete sidewalks are installed so pedestrians may have a strong, level, and consistent traveling path through the right of way and often through grassy sodden areas. A stone base is commonly specified (though not necessarily required) underneath the sidewalk to provide a firm level surface for concrete placement, to provide drainage, and to resist movement over time. If a stone base is not required by the engineer on project plans, it is often still used to provide a smooth and compactable placement surface.

The standard thickness required for concrete sidewalks is 4″ and this is the necessary thickness to resist shear loads from common foot traffic and differential movement between the slab and the subgrade. It is uncommon to find sidewalks any less thick than 4”. For sidewalks adjacent to a drive where vehicle traffic crosses, it is common for sidewalk thicknesses to be 6″ or 8″. The most common width for a sidewalk is 4-5′ though they may be as small as 2-3′ and as wide as 8-10′ or more.

There are safety standards governed by both the Americans with Disabilities Act and local municipalities which dictate the slope of concrete sidewalk to assure they don’t fall outside of certain global tolerances. The most commonly used maximum slopes for sidewalks are 1:60 (1 degree or 2%) per foot cross slope and 1:20 (3 degrees or 5%) longitudinal slope. This helps keep grade changes from being too drastic for wheelchairs, the elderly, children, the disabled, etc.

After the forms are set to the proper grade by way of string lines and levels, the prepared stone base grade may be either too high, too low, or just right. Depending on the installer, they may prefer the prepared base to be a little high so they can scrape the stone down to just the right grade, or vice versa, slightly low so some stone base can be added to get it up to grade. Final compaction is then done, often with a plate compactor.

Control joints are required transversely on sidewalks due to the nature of sidewalks to crack. These joints can be grooved with a jointer tool or sawcut and are commonly at +/- 5′ intervals. The width of the joint is typically 1/8″ and the depth should be at least 1/4 of the depth of the slab.

It is common for 1/4 – 1/2″ thick expansion joint material be installed every 30-45′ longitudinally which isolates each long piece in case of differential movement of an entire stretch of walk.

A 1/4″ radiused edger tool is typically used at the edges of the walk and at the contraction joints to provide a smooth, aesthetically positive look. Concrete walks typically receive a broom finish to reduce the danger of slipping and receive a sprayed on membrane curing compound to help with the curing process.

A curb ramp is a sidewalk transitional zone from an elevated area to a lower area, often an access ramp from an elevated sidewalk to the road (near the curb). A curb ramp follows many of the guidelines for sidewalks only the thickness, finish, and the slope are slightly different. The thickness of a curb ramp is commonly a minimum of 6″ thick. The slope is typically specified to not exceed 12:1 ( approximately 5 degrees or 8.33%) and the finish on a curb ramp is typically roughened more than sidewalks to resist slippage dangers from the increased slopes. It is extremely critical when installing curb ramps that ADA and other standards are being followed as any slopes beyond specification may warrant complete removal and replacement (due to safety concerns).

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MAJOR INDUSTRY PLAYERS

Manufacturing

Concrete manufacturing is a regional activity with costs being driven largely by both demand and proximity of dry materials; aggregate, sand, cement, etc. Concrete plants are often stationed directly with or nearby rock quarrys, either leasing the property or quarry owned. Ready mix plant proximity also plays into the costs of concrete by way of delivery and fuel costs, not to mention the fluctuating fuel costs themselves.

Top cement manufacturers in the US however from largest to smallest are: Oldcastle (Atlanta), Cemex (Houston), LaFarge (Herndon, VA), Heidelberg (Allentwon, PA), Holcim (Dundee), Vulcan Materials (Birmingham), Colas S.A. (Roseland, NJ), Martin Marietta (Raleigh), among others.

FOREMAN AND FIELD NOTES

Scheduling

During the hot summer months, sidewalk pours are commonly done as early as possible to assure cooler temps during the volatile hydration window.

Walks that occur behind concrete drives may be poured with a fast setting mix if it is desired that the area be opened up to vehicular traffic sooner than 7-14 days.

Walking on concrete sidewalk should be held off for a minimum of 24 hours but scuffs are possible until the 72 hour period.

Means and Methods

After excavation, subgrade achievement, rough stone base placement/compaction, form placement, and finish grading, concrete sidewalks are placed. See concrete flatwork ‘edge forming’ for more detailed description.

Concrete sidewalks are most commonly placed directly off the chute of the ready mix truck or buggied in on a powered ‘georgia’ buggy or bucket on a machine. For smaller projects, a wheelbarrow may be utilized for relaying concrete from the truck to the forms.

The concrete is rough placed with ‘come-alongs’, struck off or ‘screeded’, bled of water, broomed, tooled, then spray-cured.

Concrete bleeding times are dependent on thickness, porous or non porous sub-base, ambient temperature, water content in the mix, admixtures (water reducing), and amount of fines in the mix. But a standard bleed time for 4” thick concrete sidewalk is anywhere between 30-70 minutes. Windy days can pull the bleed water to the surface quicker.

After screeding and before floating, any bleed water must have risen to the top and evaporated. Bleed water can be reduced by tweaking the mix; i.e. minimize slump, use of air entrainment, no jobsite water addition, smaller cement ‘fine’ particles.

There is also an additional waiting period, often interlapping with the bleeding period, for the concrete to slightly harden, reduce in slump, and increase viscosity to a point where it is more easily finished without being easily scuffed or marred with tools and brooms.

After any bleed water has escaped, the walks are bull floated (often horinzontally to forms), darbied, edged, broomed, and tooled. After the walks are finished, they are sprayed with curing compound or water cured (burlap or visqueen) to seal in the moisture to encourage hydration.

Some walks may have sawcut contraction joints cut to ¼ depth of the overall slab depth. In this case sawcutting is most commonly done the next day while the walks are cured enough to resist saw blade damage, but soft enough to be cut easily. Chalk line is usually snapped at the cut intervals. Sawcutting can be done with a cut-off saw and a steady hand, or a walk behind power saw. Cuts will leave dust on the surface which will need to be cleaned after operation.

The standard longitudinal slope for sidewalks is usually 5% (+/- 5/8” per foot) maximum. The standard transverse slope on sidewalks is usually greater than 1% (+/- 1/8” per foot) but 2% (+/- ¼” per foot) maximum. Ramps for ADA access have significant and ever-changing guidelines, but are commonly not to exceed 8% (+/- 1” per foot)

After initial cure (often the next day, but sometimes sameday if pour is early enough), forms are stripped, edges are backfilled as necessary, and site is cleaned and restored.

Interaction with Other Trades

Standard, non-accelerated concrete will commonly need to cure for at least 3 days if not 7 before any asphalt paving or compaction activities (vibratory rollers) can be done nearby.

Concrete sidewalks should not be placed in areas where construction equipment traffic will pass over them frequently as replacements are costly and contentious affairs.

Installation Tips & Tricks

Common concrete truck chutes are assembled in +/- 48”pieces, max out at +/- 24’ length, and can often rotate up to +/- 85 degrees. Maximum chute height is 6’ with only one chute on and reduces approximately 1’ in height for every chute piece added. However, lower concrete slumps will not travel easily down the chute for longer distances so this should be considered.

Concrete ready mix trucks consume an area 10’ wide and 15’ high to move and maneuver and can get stuck in wet, muddy, or otherwise weak ground surfaces.

Ready mix trucks typically have a tank of water on them for jobsite mix addition, washing out the mixer, and for cleaning the truck and concrete tools. The driver will commonly want to use his water for his own purposes first, but will often allow the contractor to use the remaining if needed.

Traffic is a consideration for sidewalk concrete placement especially for the duration of the ready mix trucks access. Two lane roads will likely need to be flagged for routing traffic around the ready mix truck safely.

Someone on the concrete crew will commonly take the lead and provide visual guidance to the concrete truck driver with regards to chute placement and amount of concrete to be released from the chute.

If an entire project or intended area is intended to be completed in one sitting, it is important to order extra concrete (often ¼ to ½ cubic yard) to assure enough concrete is available. Otherwise more concrete will need to be ordered putting the pour at risk of a cold joint, and cpotentially causing a ‘short’ load charge (often placed on orders from 1-6 cubic yards, depending on the supplier.

Other additional charges are for: Saturday or Sunday delivery, night delivery, admixtures, fuel charges, hot water, ice, etc.

Concrete tools will need cleaned after every use in order to minimize hardening buildup and maximize life span.

EQUIPMENT AND TOOLBOX TALK

Essential Tools

Round point shovels, come-alongs, straight edge (aluminum, steel, or wood), bull float, darbys, edgers, jointers, brooms, water, chalk line, hammer, double headed-nails, lumber/steel/aluminum forms, level, ‘Chapin’ sprayer.

Recommended Tools

‘Georgia’ buggy, gloves, eye protection, ‘smart’ level, rubber boot leggings.

CAVEATS

What can go wrong

A fully loaded concrete truck weighs +/- 33 Ton. It will often crack 4-5” thick existing concrete, especially when a poor sub-base exists, and sometimes even 6-8” concrete depending. It can also crack and/or shove asphalt in thicknesses of 4” or less, especially over top a weak or thin sib base.

Floating before bleeding can cause dusting, scaling, and delamination as the water/cement ratio on the surface will be too high.

Concrete should not be inside the mixer truck for more than 60 minutes after being mixed with water at the plant. Some have reported it may be ok up to 90 minutes but this should be done at one’s own risk and depends on many outside factors.

Overhead wires should be at least 15.5’ off the ground per many local codes but be cognizant of any low hanging wires with dump and concrete trucks.

Insufficient manpower to place and finish concrete quick enough can lead to wasted concrete and concrete that can’t be properly finished.

ESTIMATING ASSISTANT

Productivity Conditions and Factors

Concrete placement is a large factor in how many square feet of sidewalk can be installed in a day. Concrete truck chute access improves productivity.

Wider walks (6-8’ wide or more) with less lineal feet of tooling, edging, and jointing per square foot leads to better productivity. However very wide walks or sidewalk pavement areas may require scaffolding/bridging for access, and considerable knee pad work which will reduce productivity.

‘Picture-framed’ tooling joints reduces productivity.

Reduced bleeding water time frame can increase productivity.

Waiting for proper finishing consistency can also reduce productivity if other work can not be done in the interim. Because cost-bourne mix accelerators may be used to speed up initial set, the contractor will have to weigh the possibility of this lost time with other productive items that can be done, I.e. cleanup, restoration, stripping forms from previous day, etc.

Productivity Range

Forming walks – 20-35 feet per man hour

Placing concrete – 2-6 cubic yard per man hour

Broom Finish – 45-75 square feet per man hour

Place curing compound – 500-700 SF per man hour

Manpower

2 finishers to 1 laborer is recommended as a minimum for smaller sidewalk pours.

Crews as large as 7-13 workers may be utilized for larger sidewalk pours.

Manpower Task Breakdown

Laborers are commonly responsible for material/tool relaying to finishers, edge forming, concrete placement, screeding, stripping forms, applying curing compound, sawcutting, site cleanup/restoration, cleaning tools, etc

Finishers are commonly responsible for darbying, floating, brooming, tooling, edging, etc.

For non commercial non-unionized projects, finishers will often help with laborers tasks as well. But with larger unionized projects, the union will often forbid their finishers to do certain labor related tasks.

Material Considerations

In residential type work, adding water at the site to retemper the concrete is fairly common, though on highly regulated projects this is often not allowed by engineers as it affects the mix design and water cement ratio.