Concrete sidewalk construction
Build to last
|For a structure to perform well, everything must be analyzed and thought out before its construction when drawing up the plans and specifications, and then things must be done according to the instructions given.It doesn’t take much for a structure to have to be redone after a few years of service; one simple deviation may mean failure. (see photo)|
To avoid premature aging because of cracking
The construction contractor, in addition to having good equipment and competent, experienced workers, must strictly comply with best construction practices and perform the work in accordance with the plans and specifications.
Construction of a sidewalk requires proper preparation of the site (see the type section). In addition, a careful choice of materials (aggregate, concrete, etc.) and equipment used is important. The concrete must also be made in the best conditions possible. General Building Specifications sums up the main steps to follow when doing work.
The main joints for a concrete sidewalk
Concrete is a rigid material. Sidewalks are exposed to many outside stresses such as: variations in temperature and humidity and foundation settlement that may make them crack. Proper jointing when constructing a sidewalk can control contraction cracks in fresh concrete and ensure long term freedom of movement for the structure. The role of each type of joint is described in the table below..
This concrete’s resistance to sulphate is extremely good and effective and municipalities and governments require it in their specifications.
Isolation joints are used to separate the sidewalk slab when it abuts a wall, a column, a footing, etc.
Construction joints are full depth and are used as a stop line at the end of a day’s work or when concrete placement is interrupted for more than 30 minutes.
The contraction joints are installed in the upper part of the sidewalk ( 25 % of the thickness) to create a point of weakness where the shrinkage cracks, caused by the drying can initiate. Theses joints are spaced from 24 to 30 times the thickness of the sidewalk (about 4,5 m).
Expansion joints, consisting of a compressible material, are placed at 60 linear meter intervals along the sidewalk. They allow thermal contraction and expansion.
Esthetic joints are made in the upper part of the sidewalk (depth of 20 mm) and are usually placed at 1.5 meter intervals; they have no significant impact on the structure.
General building specifications
|The subgrade should have a uniform bearing capacity, be level or uniformly sloped, and be free of sod, organic matter, large rocks, concrete residues, etc.Any filled sections should be compacted by successive layers that do not exceed 300 mm. They should also extend at least 300 mm outside the form line.|
Levelling the subbase
The subbase in contact with the concrete must be composed of a free draining material (20 mm clean, crushed rocks). This material must be well compacted during the final levelling to prevent any excessive movement of the foundation when the structure is in use. Levelling must be done to ensure uniformity in the concrete section of the sidewalk. The subbase section should extend beyond the sidewalk section on all sides.
A drain installed at the base of the sidewalk is necessary to evacuate the water coming from the foundation of the sidewalk and the street towards the well. This drainage is essential to minimize the ground’s movements during the freezing and thaw periods.
Forms should be straight, free from warping, and strong enough to ensure true, vertical lines. When using a slipform machine, test a specific section to verify the conformity of the forms used.
Expansion and isolation joints
Isolation and expansion joints should be planned, positioned, and ready to be made before concrete placement.
Ready-mix concrete in accordance with CAN/CSA-A23.1-M90, class C-2 should be used. It should have excellent resistance to scaling due to freeze/thaw cycles and de-icing products.
The subbase should be adequately sprayed before concrete placement. The concrete should be placed within a reasonable timeframe. It should be consolidated by hand or by mechanical methods. After placing, the concrete should be levelled to the desired grade, then floated using an aluminum or magnesium float to eliminate high and low spots.
Edging is required along the length of lateral forms and at isolation and construction joints. The concrete should be cut away from the forms using a pointed trowel before the edging tool is used. Edging should begin after the bleed water has evaporated. After the bleed water has evaporated and the edging has been done, the concrete should be floated with a wood, aluminum, or magnesium hand float to produce a smooth, even texture.
A slip-resistant surface can be produced by brooming the surface before the final set of the concrete. Esthetic joints should be done after this final finishing step.
After the finishing operations and when the bleed water has evaporated from the sidewalk surface, the concrete must be protected against excessive water evaporation. Satisfactory moisture content in the concrete must be maintained for at least 7 days to ensure proper hydration of the cement. Regardless of the curing method used, the concrete should be allowed to air dry for a period of one month following the curing period, before any de-icer salts are applied; this will give the concrete better resistance to scaling caused by the freeze/thaw cycle.
Saw cutting contraction joints
(in cases where joints have not been preformed)
Saw cut the contraction joints to a depth of 25% of the thickness of the concrete section when the concrete has sufficient resistance (6 to 18 hours after placing and before drying shrinkage cracks appear). Maximum joint spacing should be 4.5 meters.
Specifications: Cement concrete sidewalks
City sidewalks represent an important investment for a municipality. Many metres are constructed each year. In addition to these new structures, several metres must be replaced due to breakage, premature aging, or excessive scaling. When building these structures, it is important to ensure that everything is done to produce « HIGH-PERFORMANCE STRUCTURES. »
This document, which covers the construction of Portland cement concrete sidewalks, is based on the following standards:
|CAN3-A266.2||Chemical Admixtures for Concrete|
|CAN/CSA-A23.1-A23.2||Concrete Material and Methods of Concrete Construction/Method of Test for Concrete|
|NQ 2629-520||Concrete sidewalks and curbs mixed on site.|
Construction of high-performance sidewalks
Sidewalks are fairly simple to build. They are not reinforced. Their bending load is usually low.
However, they are exposed to harsh weather conditions including numerous freeze/thaw cycles, de-icing salts, frequent wet/dry cycles, etc.
In addition, other factors such as soil movement due to freezing, loss of carrying capacity caused by excessive settlement, or material transport due to erosion, can result in premature weathering of the structures.
For all these reasons, during the construction stages, it is important to ensure that all the technical references (plans and specifications) involve high-performance structures.
High-performance durable concrete
High performance concrete means that it will retain its structural integrity for many years. Our severe winters require the fabrication of highly durable concrete structures that can withstand freeze/thaw cycles.
Premature scaling of sidewalks can be the result of poor-quality aggregates or the use of concrete that does not conform to the minimum standard requirements.
The main factors, which may affect concrete resistance to scaling are listed in the table below.
Factors affecting concrete resistance to scaling when exposed to freeze/thaw cycles in the presence of de-icing chemicals
To ensure long-term service ability of the structure, the municipality should demand documentation from the concrete supplier attesting that the materials delivered to the job site meet the minimum required characteristics.
The municipality must also require assurance from the contractor responsible for the concreting work that the product used will perform well.
In return, the municipality must provide contractors with an adequate soil analysis. This can avoid additional costs due to unforeseen occurrences and allow contractors to be better able to guarantee the work done and to do it in the timeframes set out in the schedule.
If the municipality requires a delivery date for the work accompanied by a penalty clause for any possible delays, then it should also offer a bonus system to encourage early delivery.
Although the municipality must generally award work to the lowest bidder, insofar as possible, it should favour the contractor who offers the best guarantee for long term performance.
There are tests specifically designed to assess the quality of the concrete of existing concrete structures (freeze-thaw resistance, scaling and compressive strength resistance, etc.).
The following table sums up in a few lines the main requirements a municipality might formulate for high-performance concrete.
(plastic concrete of normal density)
|Minimum requirements for durable concrete exposed to de-icing chemicals 28 day compressive strength (f’c 28d): 32 MPa min.|
Recommended cementing material content:
Volume of large aggregates (5-20 mm):
|407 kg/m3 (130 L/m3)
90 mm max. *
|. Optimal characteristics of the air space network (CSA A23.2-17C)|
|Air space mean spacing factor, (L):
Air space volume percentage:
Air space volume surface:
|+- 150 um
+- 6 %
+- 30 mm2/mm3 min.
|. * This value must not exceed 40 mm when using slip formwork.|
|Scaling resistance of exposed concrete surface (ASTM C-672)|
0 à 400 g/m2
400 à 800 g/m2
800 à 1200 g/m2
|Resistance to rapid freeze-thaw cycles (ASTM C-666, procedure A, 300 cycles)|
|Value of Durability Factor
90 % à 100 %
80 % à 90 %
60 % à 80 %
|Freeze-Thaw Cycle Resistance
|* This value must not exceed 40 mm when using slip formwork.|
As shown in this table, certain requirements are intended for the manufacturer whereas others apply to the contractor responsible for the concreting. Municipalities may require a certain class of scaling and durability in regard to the degree of exposure of structures and desired performances.