accelerated degradation which may be due to hydraulic fracturing 

 of the concrete by entrapped water. 



(2) Placement of mass underwater concrete is being tested. 

 Tremie concrete mixes and placement procedures are being investigated 

 at the University of California and mass concrete placement in the 

 deep ocean is being tested by U.S. Naval Civil Engineering Laboratory 

 at Hueneme. Also, investigation of thixotropic admixtures to prevent 

 segregation of concrete when flowing through water is being con- 

 sidered. 



(31 Failure of dolos units and other armor units indicates 

 the need for additional investigation to find more stable shapes, 

 or the possible need for reinforcement to tie the member internally 

 and improve its flexural strength. Repairs to precast concrete 

 units of this kind, as well as other concrete structures have been 

 carried out using epoxy injection. Continued tests on repaired units 

 are needed to determine fatigue and ultimate strength as well as 

 to gain a better understanding of the impact on other properties 

 of concrete. 



d. Other Types of Concrete and Grout . 



(1) Bituminous Concrete . Asphalt is made from crude oil and re- 

 fineries in recent years are using crude from many different sources, making 

 the characteristics of presently produced asphalts different from those 

 previously produced. This results in problems in the handling and placing of 

 asphalt cement and in its performance. Asphalt-related problems generally 

 divide into two categories: workability problems and performance problems. 

 Workability problems, which make asphalt more difficult to mix and place, 

 seem to be common. They result from mixing the different crude sources. 



Some evidence indicates that equipment changes, such as using drum plant 

 mixing in which the aggregate and asphalt are added to a drum simultaneously, 

 can result in a softer asphalt with a higher moisture content. The intro- 

 duction of vibratory compactors, which densify by dynamic energy, requires a 

 different compaction process than steel tandem rollers. 



There are many variables that can affect an asphalt: cement, fines, 

 aggregate, temperature of mix and roll, etc. More research is needed to 

 identify and clarify the role of variables. More investigation of the 

 compaction process is necessary. 



(2) Preplaced Aggregate Concrete and Portland Cement Grout . This is 

 essentially a special application of Portland cement concrete; therefore, the 

 investigations relating to Portland cement can also result in better use of 

 this type of concrete. By investigating the performance of past, present, 

 and future projects using preplaced aggregate and Portland cement grout, 

 improved techniques and other applications should be found. 



(3) Soil Cement . Soil cement is also a special use of Portland 

 cement and additional investigation can create an understanding of how to use 

 local soils, especially the very fine and clay type soils, successfully. 



388 



