For this project we designed a new retaining wall to replace an existing failed retaining wall. The house and deck has an overlooking view of the St. Lawrence River near Brockville, ON. Part of the design was to ensure the structure was aesthetically pleasing to compliment the views and atmosphere.
The new retaining wall features buttresses for support and a new poured concrete retaining wall. The wall will be clad in an architectural stone for a more natural look.
Our office is located in Brockville, Ontario. We can assist you with your upcoming renovation of your deck, patio or landscaping. Our designs come 3D modeled for better visualization and constructibility. From the 3D model we will create permit and construction drawings to get your project to the next step.
A retaining wall and deck design located on the St. Lawrence River near Brockville, ON.
Recently the Ontario Provincial Government announced they are making cuts to the Environmental Commissioner of Ontario’s office and has also produced a less than enthusiastic environmental policy. Silencing the environmental critic isn’t going to change the fact that we have to do something about Climate Change.
Since the Ontario government isn’t going to take responsibility for our environment it falls on our businesses to take up the challenge.
That got us thinking, what can engineers do to reduce green house gases, especially in the construction industry?
Our office at IN Engineering has adopted a substantive environmental policy. We offer engineering services that are completely paperless including drafting and plans, invoicing and reporting. We also use Google servers for all of our file hosting that means our data is stored in the most efficient way possible (compared to local office servers that require a lot of energy). However, as an engineer we have the ability to specify products and materials that lean towards reduced green houses gases.
For demonstrative purposes we designed a beam for a residential property. The beam is supporting a roof and is subject to snow, weight and a maintenance person load. The length of the beam is 10 feet and it supports a tributary width of 10 feet.
Concrete Beam
The first beam designed was a concrete beam. In order to support the loads the beam has to be 8″ deep x 6″ wide with 2 – 20M bars at the bottom for tension reinforcing. It also requires 10M stirrups at 12″ for shear reinforcing. This design is at 98% capacity in bending resistance. The CO2 emissions for cement was researched to be 410 kg per cubic metre [1]. The reinforcing steel in the concrete 0.762 kg of CO2 per kg of steel, 50% of the steel was assumed to be recycled.
The carbon footprint of the concrete beam was estimated to be about 343kg.
Concrete is responsible for about 5% of CO2 emissions worldwide, this is predominately due to the fact that it takes a lot of energy and burning to produce cement. Concrete also requires quarries to produce aggregates and sand which destroy the natural environment. Even worldwide availability of construction sand is becoming an issue – only beach sand is appropriate for concrete as river sand is too smooth.
Steel Beam
The second beam designed is a steel beam. To support the loads, a W5x16 beam was selected with a maximum capacity of 61% in deflection. The CO2 emissions for steel were estimated at 0.762 kg of CO2 per kg of steel and 50% recycled.
The carbon footprint of the steel beam was estimated to be about 28kg.
Wood Beam
The last beam designed was a built up wood beam made of 3-2×12’s and Spruce-Pine-Fir No.1/2. The beam is at 99% capacity in bending. Wood actually sequesters carbon dioxide at about 1.7 kg of CO2 per kg of wood [3]. This means the wood beam actually has a negative carbon footprint.
The carbon footprint of the wood beam was estimated to be about -3400kg (stored).
There are actually more environmental benefits for using sustainable wood products. Forests will sequester carbon as they grow and mature to harvesting. At the end of the life of a wood product it can go to a landfill where it will decompose and produce methane. If the methane is collected then the environmental efficiency of wood increases. Wood is also a light building material, meaning it takes less effort to ship to site and construct.
[1] Environmental Impact of Concrete, https://en.wikipedia.org/wiki/Environmental_impact_of_concrete
[2] Carbon Footpring os Steel, http://www.newsteelconstruction.com/wp/the-carbon-footprint-of-steel/
[3] Canadian Wood Council - Carbon Calculator, http://cwc.ca/carboncalculator/
When the air temperature is forecasted to fall below 5 degrees Celsius within 24 hours of placing concrete then special considerations apply for concrete construction. These clauses are in section 7.1.2 of CSA A23.1/A23.2, Canadian Standard for Concrete Materials and Methods of Construction.
If the forecasted temperature is suspected to drop below 5 degrees celsisus (41F) then protection is required. All snow and ice has to be removed from the forms and surface. De-icing salts cannot be used to de-ice the forms. Adequate protection has to be provided to keep the concrete at a minimum of 10 degrees celsius (50F) for the duration of curing, which is typically 3 days. Protection can be heated enclosures, coverings, insulation or any combination of these. Another consideration is that the granular base needs to be preheated before pouring concrete. These prevents such deficiencies as blisters during troweling and delaminations. Using a concrete curing accelerant can also prevent these deficiencies.
Corners, edges and thin sections of concrete are the most vulnerable locations in cold weather and need more protection than plane surfaces. Once the compressive strength reaches 7 MPa it will have sufficient strength to resist frost damage.
Protection should remain in place until the concrete has cooled to the right temperature. This will prevent cracking due to a sudden temperature change.
Heated Enclosures
The enclosures should be constructed to withstand snow and ice build up and being mostly air-tight. The enclosure should have enough space to allow air to circulate over the concrete. Heat can be provided by forced hot air, stationary heaters, hydronic heaters or other approved types. The concrete surface should be protected from any exhaust from the heaters. Carbon Dioxide from direct fire heaters can negatively effect the curing of the concrete.
Protective Covers and Insulation
The cover and insulation should be determined based on the expected temperature differential and wind chill factor. Other factors would include the size and shape of the structure and the amount of cement in the concrete mix.
CSA Standard
Additional information can be found in CSA A23.1/A23.2 and is available to be order on their website. Additional information on cold weather concrete can be found in the American Concrete Institute Standard ACI 306R.
Concrete reinforced with steel rebar is one of the most common construction materials around. Concrete is cheap, easy to install and plentiful. Concrete structures have been known to last a long time and has been used in construction for thousands of years.
Concrete does require periodic inspections and maintenance. When engineers will most commonly perform a visual inspection of concrete. The engineer is looking for several deficiencies that may reveal that the concrete structure requires a rehabilitation. The most common resource for visual inspections of concrete is the Ontario Structural Inspection Manual (OSIM). This document is produced by the the Ministry of Transportation of Ontario (MTO). The document is written for bridges but can be referenced for other concrete structures as well.
The OSIM recommends that all bridges and retaining walls be inspected every two years. An acceptable standard for residential and commercial concrete structures would be every 5 years. Retaining walls, balconies and foundations that are in poor repair should be inspected more frequently until they are repaired or replaced.
OSIM groups common concrete defects as lists criteria for the severity. The follow defects are from the OSIM.
Corrosion of Reinforcement
Steel reinforcing (rebar) inside of concrete can rust and corrode due to the process known as electrolysis. As water and chloride ions infiltrate the concrete the reinforcement will break down due to corrosion.
Light – Light rust stain on the concrete surface;
Medium – Exposed reinforcement with uniform light rust. Loss of reinforcing steel section less than 10%;
Severe – Exposed reinforcement with heavy rusting and localized pitting. Loss of reinforcing steel section between 10% and 20%;
Very Severe – Exposed reinforcement with very heavy rusting and pitting. Loss of reinforcing steel section over 20%.
Delamination
Concrete that is separated from the main body but not completely is delaminated. Most commonly, as corroded rebar expands it separated the outer portion of the concrete. Delamination is most commonly detected by using a hammer to lightly tap the concrete and to listen for a distinct hollow sound. Most of the time delaminated concrete is not able to be visually observed.
Light – Delaminated area measuring less than 150 mm in any direction.
Medium – Delaminated area measuring 150 mm to 300 mm in any direction.
Severe – Delaminated area measuring 300 mm to 600 mm in any direction.
Very Severe- Delaminated area measuring more than 600 mm in any direction.
Spalling
Spalling is when large concrete pieces break completely away from the structure. Spalling is the next step after delamination where corroded rebars expands and breaks off concrete. Spalling may also be caused by impacts and overloaded concrete structures.
Light – Spalled area measuring less than 150 mm in any direction or less than 25 mm in depth.
Medium – Spalled area measuring between 150 mm to 300 mm in any direction or between 25 mm and 50 mm in depth.
Severe – Spalled area measuring between 300 mm to 600 mm in any direction or between 50 mm and 100 mm in depth.
Very Severe – Spalled area measuring more than 600 mm in any direction or greater than 100 mm in depth.
Cracking
Observations of cracks is the most common deficiency in concrete structures. A crack is a fracture within the cement and aggregate of the structure and may be a surface crack or completely through the member. Cracking is caused by a number of processes. Most small cracks are caused by shrinkage, as the concrete cures it shrinks and cracks slightly. Concrete performs poorly in tension, when it is being pulled apart and is susceptible to cracks anytime it is subjected to tensile loads. Generally small cracks less than 0.3mm is not of a structural concern and is attributed to the shrinkage of the concrete.
Hairline cracks – less than 0.1 mm wide.
Narrow cracks – 0.1 mm to 0.3 mm wide.
Medium cracks – 0.3 mm to 1.0 mm wide.
Wide cracks – greater than 1.0 mm wide.
Scaling
Scaling is the local flaking and cracks due to the freeze-thaw deterioration of concrete. This defect is most common in poorly finished concrete.
Light – Loss of surface mortar to a depth of up to 5 mm without exposure of coarse aggregate;
Medium – Loss of surface mortar to a depth of 6 to 10 mm with exposure of some coarse aggregates;
Severe – Loss of surface mortar to a depth of 11 mm to 20 mm with aggregate particles standing out from the concrete and a few completely lost.
Very Severe – Loss of surface mortar and aggregate particles to a depth greater than 20 mm.
Disintegration
Disintegration of concrete is the physical deterioration of concrete into small fragments and particles. This will most likely be observed on the exterior side of the structure where it is exposed to weather.
Light – Loss of section up to 25 mm in depth with some loss of coarse aggregate;
Medium – Loss of section between 25 mm and 50 mm deep with considerable loss of coarse aggregate and exposure of reinforcement;
Severe – Loss of section between 50 mm and 100 mm deep with substantial loss of coarse aggregate and exposure of reinforcement over a large area.
Very Severe- Loss of section in excess of 100 mm deep and extending over a large area.
Erosion
Erosion is caused by sand and gravel particles suspended in water damaging submerged concrete. This will be common in concrete structures that are exposed to water and flow.
Light – Loss of section up to 25 mm in depth with some loss of coarse aggregate;
Medium – Loss of section between 25 mm and 50 mm deep with considerable loss of coarse aggregate and exposure of reinforcement;
Severe – Loss of section between 50 mm and 100 mm deep with substantial loss of coarse aggregate and exposure of reinforcement over a large area.
Very Severe – Loss of section is in excess of 100 mm deep and extending over a large area.
Further Inspections
Following the visual inspection the engineer may recommend that non-destructive testing techniques be applied. These tests can determine the strength of the concrete, degree of corrosion in the rebar or reveal other deficiencies.
Although this information comes from the Ontario Structural Inspection Manual and is predominately applied to bridges and culverts, this information can be used on any concrete structure. If you have a residential or commercial property with concrete foundations, retaining walls or structure and you notice some of these defects it is time to hire an engineer to do a review. It is important to properly maintain and inspection concrete before it gets too severe.
Reference: Ministry of Transportation Ontario, Ontario Structure Inspection Manual (OSIM). Government of Ontario, [PDF] April 2008
If there is something around your property that you are concerned about then it is time for a structural inspection from a qualified professional engineer. Whether it is wood, masonry, concrete or steel, we will inspect, report and make recommendations on repairs. We have performed many structural inspections on all types of construction and building size.
Foundation Inspections
One of the most important aspects of a structure is the foundation. Many problems can arise in a foundation including leaks, large cracks, and settlement. If you have cracks greater than 3mm then that could be a sign of a severe problem. Continuous and long cracks could be a sign of foundation settlement which will compromise the rest of the structure. If you suspect the crack goes through the footing (the strip of concrete under your foundation wall) then repairs may be immediately required. Most foundations are constructed from concrete, or stone, but if you have a wood foundation regular inspections should be considered.
Beam and Column Inspections
If you are doing a renovation and have exposed the beams and columns of the structure, this would be a good time to do a quick structural inspection to make sure everything is framed properly. These quick and inexpensive inspections will ensure that your primary structure is supporting the loads properly.
Balconies and Deck Inspections
Balconies and decks are exposed to weather and require regular maintenance more often. If you suspect there may be a problem with a balcony or a deck then it’s time for a structural inspection. You should also consider regular inspections every few years.
Retaining Wall Inspections
If you have a retaining wall that is leaning, sliding, or damaged then it is time to have it inspected. Retaining walls pose a serious risk to safety and need to be properly designed.
