introduCtion Water Water added to cement produces a chemical reaction termed hydration. The physical characteristic of this reaction is the formation of a gel when the cement is exposed to water. The gel is formed by the penetration of water into the cement particles causing softening and establishing a colloidal suspension. The taking up of water by the clusters of cement particles is the actual hydration. A small amount of water is required for hydration but additional water is required to produce a workable mix. There is, however, a relation between the amount of water used and the strength of the resulting concrete. The amount of water must be limited to that which will produce concrete of the quality required. This is seldom a factor in concrete used in precast concrete pipe because the manufacturing processes utilize relatively dry mixes. Water used for mixing concrete should be free of acids, alkalies and oil, unless tests or experience indicate that water being considered for use and containing any of these materials is satisfactory. Particularly to be avoided is water containing organic matter which may interfere with the hydration of the cement. Most specifications require that the mixing water be suitable for drinking. Aggregates Aggregates are granular material of mineral composition, such as sand, gravel or crushed stone, combined with a cementing medium to form concrete. Aggregates should have sufficient strength to develop the full strength of the cementing matrix and be of such character that the binding material will adhere to the surface. Aggregates are classified by the general terms fine and coarse aggregate. Fine aggregate consists of material ranging from a size passing a 9.5 mm (3/8 inch) down to material just passing the number 100-sieve. Coarse aggregate ranges from the maximum size for sand to a varying upper limit determined by the pipe wall thickness and production considerations. The maximum size ordinarily used in pipe manufacture is 19 mm to 25 mm (3/4 to 1-inch). Aggregates for concrete pipe meet the requirements of ASTM Standard C 33, except for gradation requirements. This specification limits the amount of deleterious substances and also covers requirements as to grading, strength and soundness. stormwater treatment teChnologies box seCtions manhole and CatCh basin material ConCrete pipe LAFARGE PIPE eMANUAL PAGE 15