All inanimate objects tend to disintegrate. In the case of concrete, the inclusion of reactive ingredients or embedded materials in the mix helps speed up this natural function. This is true of magnesia, sugars, certain aggregates used in concrete, salts added to the concrete, aluminum conduits, and the use of a lignin base. For example, the expansive action of magnesia, even if incorporated in the cement will always cause trouble. In two very large shopping centers of identical design in the Washington area, one ceiling showed large areas of failure near the ventilation grilles after 18 months of use. The cause was plaster with an abnormal percentage of magnesia. Lignites are also a problem. In the late 1920's, a replacement for cinder concrete lead to the use of sawdust concrete. The concrete was strong and lightweight but the lignites were reactive and could catch on fire. Similarly, the aggregate can cause a problem. In 1923, the concrete encasing wooden piles in Los Angeles harbor were found to be worm eaten. Investigation showed that the holes were formed by the growth of small borers, native to the local rock used in the concrete mix.