of low volume change is required. As preplaced aggregate concrete construc- 

 tion is of a specialized nature it is advisable that the work be undertaken 

 by qualified personnel experienced in this method of construction. The 

 physical properties of preplaced aggregate concrete are similar to those of 

 conventional concrete; therefore, the same allowable working stresses used 

 for conventional concrete structural design may be used (U.S. Army Engineer 

 Waterways Experiment Station, 1954). 



(2) Tremie Concrete . Tremie placement is a method frequently used 

 to place concrete underwater. By the tremie method the concrete is deposited 

 under the surface of fresh concrete previously placed. Placement is usually 

 by gravity feed from above the water surface through a vertical pipe con- 

 nected to a funnel-shaped hopper at the top (Fig. 22). Tremie concrete 

 flows outward from the bottom of the pipe pushing the existing surface of 

 the concrete outward and upward. As long as flow is smooth so that the 

 concrete surface adjacent to the water is not physically agitated, high- 

 quality concrete will result. Placement can also be carried out through 

 other liquids lighter than concrete such as bentonite slurry to suit special 

 conditions. Tremie concrete is used primarily for cofferdam or caisson 

 seal, underwater structural sections such as bridge piers, drydock walls, 

 floors, etc., and as a seal for precast tunnel sections. 



The concrete mix proportions for tremie placement differ from ordinary 

 structural mixes because of the need to have the mix flow into place 

 slowly by gravity without vibration or mechanical help. The mix should be 

 proportioned for a slump 15 to 23 centimeters (6 to 9 inches) . It is 

 generally preferable to use a natural round gravel rather than crushed rock 

 because of flow requirements. The maximum size aggregate is usually 38.1 

 millimeters. However, a nominal size of 19.0 millimeters or 9.51 millimeters 

 (3/4 or 3/8 inch) can be used for complex sections and critical flow 

 conditions. The proportion of fine aggregate (sand) is usually in the 

 range of 40 to 50 percent of the total weight of aggregate. Water-reducing 

 retarding admixtures conforming to ASTM Standard C494 have been found to be 

 an aid in placement of the concrete, and the retarding effect slows the 

 rate of heat development and provides flatter slopes with less laitance 

 (Williams, 1959). Air-entraining admixtures and pozzolans are also benefi- 

 cial to flow characteristics. The concrete temperatures should be kept as 

 low as practical, usually below 21.1 Celsius to improve placement and 

 structural qualities. The recommended maximum water-cement ratio for 

 concrete deposited by tremie under water is 0.44 by weight. 



The compressive strength of rich, high-slump tremie concrete mixes will 

 often be approximately 28 to 56 megapascals per square meter (4 000 to 8 000 

 pounds per square inch) at 28 days. Curing conditions are excellent and 

 shrinkage is low. The surfaces that will be in contact with the concrete 

 should be free of mud, marine growth, sewage, and other matter. Bond to 

 clean surfaces of steel, rock, and timber is generally excellent. The heat 

 of hydration developed in rich mixes produces high early strength even when 

 water temperature is as low as 4.4 Celsius (40 Fahrenheit). When large 

 masses of tremie concrete are placed, volume change due to heat development 

 may warrant special consideration. The use of suitable instrumentation may 

 be required to monitor the temperature rise in these structures. 



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