Property, plant, and equipment – We record property, plant, and equipment at cost. We provide for depreciation and amortization on plant and equipment generally using accelerated methods during the first half of the estimated useful lives of the assets, and the straight-line method thereafter. The estimated useful lives of our plant and equipment generally range from 10 to 40 years for buildings and five to 15 years for machinery and equipment. No depreciation expense is recorded on construction in progress until such assets are placed into operation. Depreciation expense related to plant and equipment was $714 million in 2013, $715 million in 2012, and $712 million in 2011.
We review the carrying amounts of long-lived assets for impairment if events or changes in the facts and circumstances indicate that their carrying amounts may not be recoverable. We assess impairment by comparing the estimated undiscounted future cash flows of the related asset grouping to its carrying amount. If an asset is determined to be impaired, we recognize an impairment charge in the current period for the difference between the fair value of the asset and its carrying amount.