In 1907, a bridge in Quebec collapsed, killing 75 workers on the spot. Earlier, Theodore Cooper, a famous New York bridge designer had noticed a ¼ misalignment on the steel girders but didn’t think it was serious enough to warrant a redesign. What resulted was a national catastrophe that could have been avoided.
Construction materials, especially those used in precast concrete buildings must be of the highest quality possible. After the 1907 accident, the first building standard was developed by the Canadian Standards Association (CSA). Today, the country has over 2600 standards in fifty-six different areas. Below are 7 different materials used in precast concrete buildings, each of which must meet the requisite standards and building codes.
This is a critical material in the construction of buildings. The Canadian Standards Association (CSA) standard A3001 recommends 6 cement types, each of which meets specific chemical and physical.
- The multipurpose, general use (GU) cement is recommended for all manner of applications that don’t require a special type of cement. This material can be used on masonry blocs, pavements, sidewalks, floors, pipes, and buildings.
- The MS Moderate Sulfate Resistant Cement protects concrete from minor sulphate attacks. It’s ideal for structures that are in contact with underground water or soils with a higher than normal sulphate concentration, but not too high.
- The MH Moderate Heat of Hydration Cement is unique in that it slowly generates minimal heat compared to general use cement. It can be used in large-scale construction projects such as columns, dams, bridge piers, or thick retaining walls, among other structures.
- Another type, the HE High Early Strength Cement, is somewhat similar to the GU cement, but produces high strength in significantly less time, mostly in seven days or less. Unlike general-use cement, this type of cement has finely-grounded particles. It is ideal for projects that need to be expedited since it takes fewer days to cure.
- To significantly reduce the heat resulting from the hydration process, you need LH Low Heat of Hydration Cement. The great thing about this cement is that it slowly builds resistance compared to other cement types. It is ideal for huge construction projects that require minimal heat hydration.
- For concrete that requires high sulphate exposure, you will need High Sulfate Resistance Cement. This is especially ideal in areas with high sulphate content in their groundwater or soil. The concrete gains strength slowly with sulphate resistance cement compared to the general hydraulic cement.
- The last type is the White Portland Cement, which is different from the regular cement that features a grey colour. It, however, meets the relevant CSA specifications. It is mostly used to construct architectural structures such as terrazzo, precast curtain walls, cement paint, cladding panels, decorative concrete, ceramic grout, or stucco or mortar. CSA recommends five types of portland cement to be used for various purposes. These include low heat hydration, moderate, normal, sulphate-resisting, and high early strength cement. While general-use cement is the most popular, sulphate-resisting and high early strength types of portland cement are also readily available whenever needed.
2. Structural Steel
Steel is critical in any structure that requires reinforcement, which is literally almost any type of building. To avoid accidents such as the one that happened in 1907, the steel one uses must meet the requisite standards and codes. It must meet the applicable CISC Code of Standard Practice in terms of design, fabrication, and erection. The code has been revised eight times, with the last revision in 2018. The key is to make sure the steel structure adheres to the most recent revision of the CISC Code of Standard Practice as it applies to structural steel.
Concrete is a mixture of water, coarse aggregates and cement. Upon mixing, it takes time to harden in a process known as curing. The ratio of cement and water used determines its properties, which include workability, strength, resistance to radiation or heat, and durability.
Water is another substance used in precast buildings. Ideally, it should be free of impurities that might impair the durability and strength of the concrete. Using recycled water may increase the number of alkalis and soluble salts in concrete. Always test to confirm everything meets the minimum standards. Also, make sure alkali segregates are not employed.
5. Waterproofing Admixture
When using two or more admixtures together, test them for compatibility with the cement. Consider undertaking admixture experiments with specific materials to determine the best dodge for the required performance.
For precast structures, a crystalline waterproofing admixture is an excellent choice. Carefully develop the integral crystalline admixture powder so it mixes with concrete capillary pore structures to produce a permanent waterproofing system.
Ideally, precast concrete aggregates are extremely durable. They do not decay or dissolve as a result of weather activity. Freezing, thawing, moisture content fluctuations, and temperature changes are all weathering activity factors to consider.
CSA A23.2:19 covers aggregate testing methods. Special Aggregates may be required to provide desired characteristics like colour and texture for exposed aggregate surfaces.
7. Reinforcement Bars
Carbon steel bars are often used as reinforcement material and must adhere to CSA G30.18:21. This standard covers two types of bars:
- Type R – used for general applications
- Type W – used when restrictions in mechanical properties and chemical composition are desired. Type W bars are more ductile and more weldable than other steels. Deformed bars have the W indication rolled into the surface.
The reinforcement should be clean and devoid of hazardous materials that may impede binding with concrete when inserted into the mould after fabrication. Such materials include retarders, grease, loose mill scale, and loose rust oil.
Since the 1907 bridge accident that killed 75 workers, the Canadian Standards Association came up with codes and standards to regulate the quality of construction materials to deter similar accidents in future. The codes are regularly revised to ensure that building materials used in the country are of the highest possible standards.