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7-14-04; 17:40 ;Lester Bulldln� ;3203955376 # 22/ 28 Article Request Page Page 20 of 26 tanks 1 and 2,water was brought near the surface of the tanks via capiIlary action.Also,even thmugh tanks 3 and 4 were drained,capillary action maintained water soil moisture content pmfiles in the�nks similar to those in tanks 1 and 2.Third,the rate of capillary rise in tanks 1 and 2 was relatively rapid. Data ahows that water reached the bottom edge of the top ECHO probe in tank 2 only 3.5 days after the test had commenc�. In other words,while top footings in tanks 3 and 4 received water on the first day of the test,it was not long afterwards that water was introduced into top footings in tanks 1 and 2. The rate of capillary rise in the air-dried sand and maximum capillary fringe depth in the sand were determined with an apparatus specially cons�tructed to measure these phenomena.Results from these tests showed that capillary rise in the air-dried sand could be expressed as a function of time as follows: Capillary Rise,mm=28 mm+ 109 mm x log(t/minute) [3] Where capillary riae is the dist�nce between the water table and the upward moving water front in the air-dried sand and t is elapsed minutes since the dried sand was brought into contact with the water.This equation is accurate for time periods,t,greater than 1 minute and less 80,600 minutes (8 weeks).Note that the base of the top ECIiO pmbe in Tank 2 was located approximately 420 mm (16.5 in.)above the water table level in Tank 2.For the amount of time it took for the water to reach this probe(i.e.,5040 minutes)equation 3 predicts a capillary rise of 431 mm. Overall,soil moisture levels in tanks 5 and 6 due to water treabment C were higher t1�an those in the other four tanks.In retrospect, soil moisture levels due to all water treatments should have been virhially identical.The primary reason for differences was attributed to the relatively low permeability of the geotextile fabric placed in each tank's water supply/drain line to keep sand from exiting the tanks.Without fabric or with a less permeable fabric,water added every 14 days to tarilcs 3,4, 5 and 6 would have been moved more rapidly thmugh the sand by gravitational forces. Without the restricting fabric,soil moisture profilea in all tanks should have been nearly identical within a couple hours of water application to tanks 3,4,5 and 6. Because of the lugh capillary rise,we were not able to assess the degree of hydration above the capillary fringe zone in a soil.For this reason,we would use coarser and more poorly graded sand if we were to reivn this experiment.In addition,we would allow for fi�cer movement of water in and out of tas�cs,and we would set up one tank with bottom drainage so that a gmund water table was not present in the tank. Specimen Moisture Content Regardless of water treatment,bottom footings had a significantly lower compressive strength than footings located at top and middle of the tanks(see Table 3 and figure 14).While there is an obvious tendency to attribute this to the hydration process,it is likely due to relatively high moisture content oFthe bottom footings at the time of removal,coring and testing. Several sludies have shown the compressive strength of a conerete specimen is decreased if its moisture content is uniformly increased throughout its volume.A good review of this research is provided by Bartlett and MacGregor(1993). One theory for this effect is that water absorbed into the pores of the hydrating cement particles(a.k.a.gel pores)has no place to go when the specimen is loaded.This results in a build-up of hydrostatic pressure that literally helps blow the specimen apart.Another theory is that excess water in the gel forces gel surfaces further apart,thus reducing Van Der WalIs forces between gel parkicles.These adhesive forces are proportional to the specific surface energy, and thus the critical stress required for cracking. Another possble reason for wetter cores having lower compreasive strengths is that wet cores are http://asae.frymulti.com/request2.asp?JI1�5&AID=14082&CID=1nv2003&v=&i=&T=1 6/4/2004