3. Summary of Significant Accounting Policies (continued) Development costs of oil sand properties Construction in progress Impairment of non-current assets Repairs and maintenance The costs of acquiring, exploring, evaluating and developing oil sands properties are capitalized when it is expected that  these costs will be recovered through future exploitation or sale of the property. Capitalized development costs of oil  sand properties are considered to be tangible assets. These assets are not depreciated as they are not currently available  for use. When proven and probable reserves are determined and development is approved, capitalized development  costs for oil sand properties are reclassified to mineral properties and leases within property, plant and equipment. Assets in the course of construction are capitalized as construction in progress. On completion, the cost of construction  is transferred to the appropriate category of property, plant and equipment, and depreciation commences when the  asset is available for its intended use. The carrying amounts of non-current assets are reviewed for impairment whenever facts and circumstances suggest  that the carrying amounts may not be recoverable. If there are indicators of impairment, the recoverable amount of the  asset is estimated in order to determine the extent of any impairment. Where the asset does not generate cash flows  that are independent from other assets, the recoverable amount of the cash generating unit to which the asset belongs  is determined. The recoverable amount of an asset or cash generating unit is determined as the higher of its fair value  less costs to sell and its value in use. An impairment loss exists if the asset’s or cash generating unit’s carrying amount  exceeds the recoverable amount and is recorded as an expense immediately. Value in use is determined as the present value of the future cash flows expected to be derived from continuing use of  an asset or cash generating unit in its present form. These estimated future cash flows are discounted to their present  value using a pre-tax discount rate that reflects current market assessments of the time value of money and the risks  specific to the asset or cash generating unit for which estimates of future cash flows have not been adjusted. Fair value  less costs to sell is the amount obtainable from the sale of an asset or cash generating unit in an arm’s length transaction  between knowledgeable, willing parties, less the costs of disposal. For mining assets, when a binding sale agreement  is not readily available, fair value less costs to sell is estimated using a discounted cash flow approach. Estimated future  cash flows are calculated using estimated future prices, mineral reserves and resources, operating and capital costs.  All assumptions used are those that an independent market participant would consider appropriate. Indicators of impairment and impairment of exploration and evaluation assets or oil sands development costs are  assessed on a project-by-project basis or as part of the existing operation they relate to. Tangible assets that have been impaired in prior periods are tested for possible reversal of impairment whenever  events or changes in circumstances indicate that the impairment has reversed. If the impairment has reversed, the  carrying amount of the asset is increased to its recoverable amount but not beyond the carrying amount that would  have been determined had no impairment loss been recognized for the asset in the prior periods. A reversal of an  impairment loss is recognized into profit immediately. Repairs and maintenance costs, including shutdown maintenance costs, are charged to expense as incurred, except  when these repairs significantly extend the life of an asset or result in an operating improvement. In these instances,  the portion of these repairs relating to the betterment is capitalized as part of plant and equipment. 86 Teck 2012 Annual Report Notes to Consolidated Financial Statements  Years ended December 31, 2012 and 2011