Treated lumber is used
to preserve the wood and should be used where the wood touches the
concrete as in the picture above.
Chromated copper arsenate (CCA) is a wood preservative used for timber treatment, in use since the mid-1930's. It is a mix of copper, chromium, and arsenic formulated as oxides or salts. It preserves the wood from decay fungi, wood attacking insects, including termites, and marine borers.
It also improves the weather-resistance of treated timber and may assist paint adherence in the long term.
Timber or lumber that is treated with a preservative generally have it applied through vacuum and/or pressure treatment. The preservatives used to pressure-treat timber are classified as pesticides. Treating timber provides long-term resistance to organisms that cause deterioration. If it is applied correctly, it extends the productive life of timber by five to ten times. If left untreated, wood that is exposed to moisture or soil for sustained periods of time will become weakened by various types of fungi, bacteria or insects.
CCA is known by many trade names, including the world-wide brand Tanalith. The chromium acts as a chemical fixing agent and has little or no preserving properties; it helps the other chemicals to fix in the timber, binding them through chemical complexes to the wood's cellulose and lignin. The copper acts primarily to protect the wood against decay fungi and bacteria, while the arsenic is the main insecticidal component of CCA.