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7-�4-04; 17:40 ;Lester Bullding ;3203955376 # 19/ 28 Article Request Page Page 17 of 26 Figure 16.Bulk density as a function of water/cement ratio for normally-hydrated concrete specimens. Discussion Overview In-situ hydrated concrete footings removed at 4, 12 and 24 weeks had mean compressive streng�khs of approximately 2570,3250 and 34701bf/in2,respectively.The same concrete mix,when normally hydrated and cured under ideal conditions for 28 days,had compressive strengths that averaged&om SOO lbf/in2 for a W/C ratio of 0.26,to 37001bf/in2 for a W/C ratio of 0.58. Based on a comparison of Wese numbers,one can conclude that in-situ hydration can produce concrete with significant strength, and that norrnally hydrated concrete strength is highly dependent on W/C ratio. A closer examination of the numbers behind these numbers reveals that in-situ hydration is likely a fuaction of dry mix gradation,initial consolidation,confinement pressure,unifomrity of dry mix after placement, as well as conditions related to water movement into the confined mix. Post Placement Hydratlon In-situ cement hydration falls under the broader category of what the authors have coined"post- placement hydration."Post-placement hydra.tion of cement or a dry concrete mix refers to any concrete manufacturing process in which cement is mixed with the aggregatss and positioned inside forms and molds,or placed in the gmund before the addition of water. In other worda,no mixing of the ingredients takes place after the addition of water. Aggregate Spacing Portland cement and other hydraulic cements set and harden by reacting chemically with water. This reaction is called hydradon.During hydration, each cement particle fomas a type of growth on its surface as calcium silicates are hydrated to form calcium hydroxide and calcium silicate hydrate (CSI�.This hydration continues as long as moisture and temperature conditions are favorable,and non-hydrated calcium silicates still exis�The rate of hydration depends upon the composition and specific surface area(i.e.,fineness)of the cement,the mixture proportions,and temperature. The main factor affecting concrete strength is the pmximity of hydrating cement particles to suaounding cement particles and aggregatea.The closer all solid particles are prior to hydration, . the greater We contact/bonding area will be between aggregate and fully hydrated cement particles as shown in 17.It follows that the greater the bonding area,the greater the force required to separate the aggregates,and the gceater the overall strength of the conglomeration. http://asae.frymulti.com/request2.asp?JID=S&AID=14082&CID=1nv2003&v=&i=&T=1 6/4/2004