As in the case with all other construction materials, there are some materials which will corrode if they are brought into contact with concrete. A great deal of the corrosion will stem from alkaline solution in the concrete. These alkaline solutions are inescapable because of the chemical composition of portland cement. It is this chemical composition that forces a reaction between concrete and certain metals or glass. Aluminum is one of the most chemically active metals. It is usually a good idea to provide a protective coating around any aluminum objects that will come in contact with concrete. Lead and zinc when exposed to air rapidly develop protective films, however, these films are absent in lead and zinc embedded in concrete. Usually bituminous felt or bituminous coatings protect lead and zinc from corrosion. Copper is only a problem if chlorides are present and then the corrosion is small. Corrosion can be reduced by not laying pipe directly on a sand, earth or vermiculite base layer and then casting the concrete over it. This can be avoided by supporting pipes on small concrete blocks. Wood blocks should not be used because they can cause corrosion.