74
3220.033
Palikij, G.D.; K.W. Howell
1955
1955 and After
The contact zone is the transition layer of material connecting the coarse aggregate particle with the enveloping continuous mortar matrix. Analysis of this linkage layer requires consideration of more than the adhesion developed at the interface and should include the transitional layer that forms between the two phases. Collapse of the structural integrity of a conglomerate may come from the failure of one of the two phases or from a breakdown in the contact zone causing a separation of the still intact phases. The various mechanisms that act to maintain continuity or that cause separation have not received the same attention as has the air void system necessary to protect the paste. Aggregates are frequently dismissed as being “inert” fillers and as a result they and the associated transition zone have received very modest attention.In order that concrete perform satisfactorily in severe exposure conditions it is essential that a good bond develop and be maintained between the aggregate and the enveloping continuous mortar matrix.
Based on studies of mature concrete, the following were identified:
1. Expanded aggregates are well bonded to the mortar matrix.
2. Silicon content is higher at the interface between the expanded aggregate and the cement paste than in the cement paste generally.
3. Cracking and local areas of high calcium content were observed at the aggregate-cement paste interface in normal weight concrete.
The first two factors would account, in part, for the long term good behavior of lightweight concrete in severe exposure.