CPV code 45231300 designates Construction work for water and sewage pipelines in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
45231300 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 45231000 (Construction work for pipelines, communication and power lines).