136 5 Materials for Lightweight Structures,Civil Infrastructure,Joining and Repair Longitudinal Pavement 日ase Subgrade Pavement Subgrade Figure 5.5.A pavement with base(gravel layer)undemeath and subgrade(soil layer)undemeath the base.The base layer allows water to flow through it in order to avoid the local collection of water.The longitudinal direction refers to the direction of traffic precipitate-known as ductile iron-is used instead.Ductile iron is akin to but different from cast iron,which has flake graphite precipitate.Due to the spheroidal graphite precipitate,ductile iron is stronger and more ductile than cast iron.This provides an example of a layered composite involving the use of metal as one layer and a cement-based material as the other layer. A ductile iron pipe has a cement mortar lining inside the pipe for the purpose of corrosion protection.Corrosion is particularly serious when the water is acidic.It causes tuberculation,which refers to the formation of small mounds of corrosion products on the inside surface of a pipe.Due to the centrifugal casting used to manufacture iron pipes,there is variation in the wall thickness along the length of a pipe.As a result,the durability of a pipe varies along its length. Both unreinforced concrete and steel-reinforced concrete are used for pipes. The unreinforced concrete has no steel reinforcement,but may have asbestos re- inforcement.Asbestos fibers are an effective reinforcement,but their carcinogenic character results in health concerns.Due to the water in the pipe and in the soil surrounding the pipe,corrosion is an issue.As a result,the corrosion of the steel rebars and the consequent degradation of the steel-concrete bond are an important consideration for steel-reinforced concrete pipes. 5.3 Materials for Joining Joining is at the heart of composite fabrication,since the creation of a composite involves the joining of various components,such as the joining of fiber and matrix. The bonding between the reinforcement and the matrix is critical to the mechanical integrity of a composite.136 5 Materials for Lightweight Structures, Civil Infrastructure, Joining and Repair Pavement Base Subgrade Pavement Subgrade Base Longitudinal Figure 5.5. A pavement with base (gravel layer) underneath and subgrade (soil layer) underneath the base. The base layer allows water to flow through it in order to avoid the local collection of water. The longitudinal direction refers to the direction of traffic precipitate – known as ductile iron – is used instead. Ductile iron is akin to but different from cast iron, which has flake graphite precipitate. Due to the spheroidal graphite precipitate, ductile iron is stronger and more ductile than cast iron. This provides an example of a layered composite involving the use of metal as one layer and a cement-based material as the other layer. A ductile iron pipe has a cement mortar lining inside the pipe for the purpose of corrosion protection. Corrosion is particularly serious when the water is acidic. It causes tuberculation, which refers to the formation of small mounds of corrosion products on the inside surface of a pipe. Due to the centrifugal casting used to manufacture iron pipes, there is variation in the wall thickness along the length of a pipe. As a result, the durability of a pipe varies along its length. Both unreinforced concrete and steel-reinforced concrete are used for pipes. The unreinforced concrete has no steel reinforcement, but may have asbestos re￾inforcement. Asbestos fibers are an effective reinforcement, but their carcinogenic character results in health concerns. Due to the water in the pipe and in the soil surrounding the pipe, corrosion is an issue. As a result, the corrosion of the steel rebars and the consequent degradation of the steel-concrete bond are an important consideration for steel-reinforced concrete pipes. 5.3 Materials for Joining Joining is at the heart of composite fabrication, since the creation of a composite involves the joining of various components, such as the joining of fiber and matrix. The bonding between the reinforcement and the matrix is critical to the mechanical integrity of a composite