Title no. 97-M58 Effect of Moisture Content on Water Transport in Silica Fume Concrete by James M. Aldred, S. Swaddiwudhipong, S. L Lee, and T. H. Wee Silica fume reduced water transport under all test regimes and regardless of initial moisture condition. Ten percent silica fume replacement in concrete with a water-binder ratio (w/b) of 0.6 was found, in general, to have water transport properties similar to oridnap portland cement (OPC) concrete with a w/b of 0.4. Unlike OPC concrete, however; the rate of wick action and mois- tureflow in silica fume concrete was relatively unaffected by initial moisture content. This suggests that its lower rate of desorption may limit the rate of water transport through saturated silica&me concrete. While water transport properties of silica fume concrete were generally detrimentally affected by the inclusion of a drying cycle and any resultant microcracking, any detrimental influence was similar to or less than that observed with the reference OPC concrete. Accordingly, silica fume concrete did not appear particu- larly sensitive to microcracking, although it was affected by limited early curing. Keywords: binder; curing; microcracks; silica fume. INTRODUCTION As a result of its pozzolanic reaction and filler effect, silica fume renders the pore structure of the concrete matrix more homogeneous by decreasing the number of larger pores, while the total porosity remains relatively unaffected.’ Ac- cordingly, silica fume has been found to increase strength and reduce water permeability; the latter is considered to be silica fume’s greatest asset. AC1 Committee 234 states that: “the low permeability characteristics of silica fume concrete and the corresponding improvements in long-term durability will provide the single most significant improvement to the concrete construction industry.“* Because of the strong and brittle matrix, however, there has been some concern regard- ing the potential microcracking in concrete containing silica fume, particularly during drying, and the subsequent detri- mental effect on concrete properties. Several studies have shown some retrogression in the long-term strength of silica fume concrete.3-6 De Larrard and Boistvironnois4 ascribed this strength reduction to the stresses developed during dry- ing. While there has been considerable research on the effect of drying on mechanical properties of silica fume concrete, little information is available on the water transport proper- ties. Therefore, the degree of the detrimental effect of result- ant microcracking is largely unknown. Gjerv’ found that a 10% silica fume addition to low-cement- content concrete reduced permeability by approximately three orders of magnitude. Hooton* showed more modest re- ductions in permeability at water-binder ratios (w/b) of 0.25; the degree of the reduction in permeability was found to re- duce with prolonged curing. Long et a1.9 also showed a greater reduction in permeability in concrete with a w/b of 0.7 compared with one with a w/b of 0.4. Their results also ACI Materials Journal/July-August 2000 showed that the relative benefit of silica fume replacement was less in specimens subjected to longer curing. Data on sorptivity with silica fume addition indicate that silica fume does reduce sorptivity. Ramakrishnan and Srinivasan” reported a reduction in the water absorption co- efficient in fiber-reinforced concrete containin when compared with the control. Sharafi et al.’ F silica fume showed that 10% silica fume reduces 30 min absorption from 2.9 to 1.7%. Morgan’* also found lower absorption in silica fume shot- Crete. Roy et a1.13 showed no effect of silica fume replace- ment on initial absorption, while tests by Lehtonen14 demonstrated a lower sorptivity rate in silica fume concrete, even though the final degree of saturation was the same as for the reference concrete. Other transport properties are also beneficial1 7 modified by the use of silica fume. During drying, Atlassi 5 recorded significant increases in the time to reach equilibrium, espe- cially in relative humidities (RHs) below 80%. Helsing- Atlassi16 showed that silica fume addition reduced desorp- tion at RHs greater than 30%. Sicard and Pans” reported that dessication shrinkage was reduced by more than 50% when using silica fume. There appears to be little published information on water vapor diffusion through silica fume concrete, but Rodhe” found that silica fume and fly ash addi- tion reduced the rate of moisture flow to approximately l/3. RESEARCH SIGNIFICANCE Research has shown some detrimental effect of microc- racking during drying on the mechanical properties of silica fume concrete. Little information, however, is available on the influence of drying on water transport properties. This re- search examines the effect of drying on a range of transport properties in concrete with and without silica fume. Parameters involved in water movement under conditions of one-sided drying, as would be found in basements and tunnels, are also included. Silica fume is widely used in such applications, but these data are limited. EXPERIMENTAL PROGRAM Concrete materials and mixture proportions The details of the concrete mixtures are given in Table 1. The ordinary portland cement (OPC) had a fineness of 3 17 m’/kg, a specific gravity of 3.15, and a C3A content of 8.64%. The fine aggregate consisted of natural sand with a fineness modulus of 2.60 and specific gravity of 2.62. The coarse aggregate was AC1 Mavriols Journal. V. 97, No. 4, July-August 2ooO. MS No. 99-219 received October 25, 1999, and reviewed under Institute publicauon policies. Copyright 0 2oo0, Amencan Concrete Institute. All tights reserved, includ- ing the making of copies unless permission is obtained from the copyright proprietors. Pertinent discusston will be published in the May-June 2001 ACIMaterids Joumd if received by February 1,200 1. 493