Concrete Slab Design
A number of issues can arise without the proper design of the concrete slab thickness and control joints. This section provides a few design recommendations. Generally, it is recommended that the subgrade material for concrete driveways is a granular “A” material to a minimum thickness of 100 mm on stable grade. Concrete thicknesses are normally recommended based on intended use:
Cars and pick-ups: 100 mm or 4"
Light trucks: 125 mm or 5"
Heavy truck usage: 150 mm or 6"
Steel reinforcements can be added. These reinforcements will not prevent cracks, but will help hold them together if they occur. Reinforcements can be either wire mesh or steel rebar placed in a grid pattern centered within the concrete. As concrete ages, initially it shrinks due to water evaporation
and cooling of the concrete, which can cause cracking. Proper placement of control joints can help control the location and the extent of cracking.
Control joints should be:
• Placed with a maximum joint spacing not exceeding 24 to 36 times the thickness of the slab.
• Cut as soon as possible without pulling the aggregate out, usually within 24-48 hours after the concrete has been placed. Newer early entry (green cutting) concrete saws allow for earlier cutting.
• Cut to a minimum depth of one quarter the thickness of the slab to ensure that the concrete cracks at the saw-cut location.
Subgrade and Site Preparation
Properly preparing the subgrade and site for the concrete driveway is important to prevent a number of deficiencies. If the subgrade is not properly prepared and compacted, uncontrolled shrinkage cracks can occur.
To prepare the subgrade, topsoil needs to be excavated to the native soil before placing the granular material to a minimum 100 mm (4 in) thickness. The granular material needs to be compacted to achieve a uniform top surface. There should be no frozen material in the natural soil and granulars. The grade must be a minimum of 2% sloped away from the building to allow for water drainage beneath the surface. Ensuring proper drainage of the aggregate base material is a critical component of the design. After the subgrade is prepared, the following preparations are needed for concrete placement:
• Isolation joints need to be installed against any existing concrete surfaces (house walls, garage floors, etc.)using asphalt-impregnated boards.
• Driveway should be shaped using stiff wood forms kept in position with stakes spaced not more than 1 m (3 ft) apart. The top of the stakes should be flush with, or slightly below the top of the form at finished grade.
• The exact location of control joints need to be planned before starting the project and their locations marked on the formwork prior to concrete placement.Type your paragraph here.
Materials and Specifications
Use of improper materials may cause numerous deficiencies, from scaling due to improper concrete selection to uncontrolled shrinkage cracks if too much water is added to the mix at the site. Chemical admixtures such as calcium chloride added to the concrete can cause discolouration and crazing. Pop-outs can occur if the aggregate contains soft porous materials.
Concrete selection should follow guidelines outlined in CAN/CSA A23.1 “Concrete Materials and methods of Concrete Construction/Methods of Test for Concrete.” The concrete supplied should be a Class C-2, 32 MPa, 0.45 water/cement ratio, with 5-8% air entrainment (assuming 20 mm coarse aggregate is used in the concrete mix design).
For coloured concrete, it is recommended that the colour be integrally mixed at the time of batching. Air entrainment should be measured after the colour has been mixed in. It is recommended that the contractor prepare a sample colour batch to ensure the customer’s expectations for colour are met.
Concrete as a material can be produced using by-products of other industries, such as fly ash and slag (referred to as supplemental cementing materials or SCMs), to provide a more sustainable approach to building construction. These materials, when used in the appropriate proportions, can increase both the strength and durability of the concrete. SCM options can be discussed with the concrete supplier.
Concrete Placement and Finishing
Proper concrete placement and finishing are important in preventing deficiencies. These can include:
• Discolouration if the flatwork finisher improperly estimates the timing of the finishing operations, resulting in a hard-troweled surface.
• Scaling when finishing operations are completed while the bleed water is still on the concrete surface.
• Blisters if the concrete surface is prematurely sealed due to improper finishing procedures or tools, a dry shake is prematurely applied to the concrete surface, or insufficient or excessive vibrations are used during concrete placement.
• Uncontrolled dry shrinkage cracks when improper finishing procedures are implemented or the installation of contraction joints is not completed in a timely fashion.
One of the critical elements in proper placement and finishing is to ensure there is sufficient labour on site when the concrete is delivered. Three or more people are generally required, depending on the size of the driveway, the weather, particular design aspects, etc. In addition, a number of steps should be followed during placement and finishing:
• The subgrade should be dampened without leaving freestanding water so the water is not drawn prematurely from the concrete.
• The proper amount of form release should be applied to the forms before placing concrete.
• A maximum 100 mm slump is usually adequate for placement.
Adding water to increase the slump or workability has a very detrimental effect on both the strength and durability of the concrete. Admixtures are available from the concrete producer to provide additional workability and slump without affecting the concrete, should a higher slump be desired.
• The concrete should be placed using wheelbarrows (or directly from concrete mixer if possible) and moved with shovels to rough grade, avoiding segregation.
• The concrete can be placed to finish grade using a straight board on edge between forms in a “sawing motion.” The edges of the forms should be tapped with a hammer to consolidate the concrete along the edges.
• The surface should be smoothed using a long-handled bull float (use an edging tool to finish edges). Steel or fresno trowels should not be used at any time, as they work the air out of the concrete surface, leaving a weakened surface. Air entrained concrete should never be power troweled.
• The final non-slip finish can be applied using a concrete broom, burlap drag, magnesium float or impressed mat finish.
• Overworking the fresh concrete causes scaling. This extra finishing is not required and will reduce the durability of the slab.
Rapid concrete surface drying can occur if there is low outdoor relative humidity or if it’s very sunny, windy and/or hot outside. Without proper protection, this can cause deficiencies including: crazing, plastic shrinkage cracks and flaking. Placing the concrete in cold conditions can also lead to pop-outs, scaling, blistering and weakened concrete strength. if placing concrete during a hot, dry or windy day, the following special precautions should be taken:
• Protect fresh concrete from rapid moisture loss by covering it with plastic sheets after screeding, or use wind screens, fog nozzles, vapour retardants, chemical curing compounds etc.
• Be prepared for faster setting times during warm weather.
• Protect hardening concrete by curing the concrete immediately after final finish.
Exterior concrete should not be placed when the air temperature is less than or equal to 5°C unless extra precautions are taken. If concrete freezes, then the durability and strength can be reduced by up to 50%. If placing concrete in cold weather:
• Allow a longer time before final finishing.
• Do not perform final finishing before concrete stops bleeding.
• Insulating concrete blankets may be necessary to prevent the concrete from freezing in its plastic state.
• Ensure that plastic sheeting used for curing purposes does not come in direct contact with the concrete. Plastic sheeting tends to leave colour streaks on the concrete surface where it is in direct contact with the concrete.
• Never place concrete on frozen ground. Curing and Sealing
Deficiencies often occur when concrete is insufficiently cured, when curing is started too late or early, or when concrete is improperly cured. These deficiencies may include discolouration, crazing, scaling, pop-outs, uncontrolled dry shrinkage cracking, and mortar flaking. For proper curing to occur, it is essential to maintain the required moisture condition and concrete temperature. Curing must start immediately after the final finish and can be accomplished by the following methods:
• Roll or spray on curing membranes/compounds applied as soon as the finishing operations are complete following manufacturer’s application instructions, or
• Wet burlap, soaking hoses, waterproof paper, or polyethylene can be used to keep the surface constantly wet for seven days.
Hoses should not be placed in direct contact with the surface until the concrete has obtained sufficient strength to prevent surface damage. If concrete is placed when temperatures can fall below 5°C, polyethylene sheets, insulated blankets or other cold weather curing methods should be used. Under normal conditions, the concrete may be sealed with a penetrating sealer according to the recommendations of the manufacturer:
• After 30 days if a curing membrane is used, or
• After a period of air drying if water has cured.
Deicers (e.g. salt) of any kind should not be applied until the concrete has gone through its first winter. If surface finishing and curing are done properly, the slab’s resistance to deicers will be improved over the long term. Ammonium-based deicers or products that contain nitrates or magnesium chlorides should not be used at any time. Damage from salts, deicers, including melt-off slush from vehicles is typically not covered under warranty. So it may be best to simply avoid the use of deicers. Sand, kitty litter or other environmentally friendly granular products that do not include nitrates or chlorides can be used instead of deicers for anti-slip/traction purposes. Snow and deicing salts should be removed from the concrete driveway so that they do not sit on the slab for an extended period of time. The driveway should be power washed or hosed down in spring to rinse off winter residues (salt and debris). Concrete sealers should be applied to prevent the ingress of chlorides from roads and sidewalks. Sealers should be reapplied as required (generally every two years for acrylic sealers and every five years for penetrating sealers). Ensure the use of compatible sealers over time: the use of an acrylic sealer initially is not compatible with future use of a penetrating sealer.
The industry standard for repair or replacement is greater than a 3/16 of an inch displacement (either height or gap). We may warranty cracks that exceed 3/16 of an inch in height or gap.
Popped or Flaked Concrete
We will repair or replace a section or sections of your flatwork if it can be defined within our warranty. Part of the decision to execute warranty repair or replacement will be determined if the minimum 20%, Industry Standard, of total area is affected. We may repair or replace our work if a minimum of 20% of the total work area is popped or flaking, except for the driveway approach.
Customer Care of Concrete
Do not drive on the “new” concrete for at least 14 days.
Do not allow water to drain beneath the slab…..settlement cracks may develop.
Do not apply deicing chemicals (salt) for snow and ice removal the first winter. As an alternative, sand can be used for traction.
WARNING: Never use deicers containing ammonium sulfate or ammonium nitrate (i.e. fertilizers). Such products are known to aggressively attack concrete.
Apply a good quality sealer. Contact your contractor, local Ready Mix producer or building supply store to purchase a concrete sealer. Follow the manufacturer’s instructions for sealer application. Re-application of the sealer is generally recommended every year. For stain removal, do not use harsh acids. Use a product specifically designed for the stain in question and for use on concrete.