Fracture toughness of macro-defect-free cement using small crack techniques
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I. INTRODUCTION
Cement is an inexpensive, readily available material with many desirable properties. Nevertheless, it is rather weak in structural applications compared with traditional ceramics such as alumina and zirconia. The fracture toughness (KIc) of conventional hydrated cement paste varies from 0.2-0.5 MPa m1/2,1-3 which is rather poor compared to other ceramics; e.g., alumina has K,c values which range from 2-6 MPa m1/2. The low toughness and large flaws lead to very low flexural strength, typically between 3 and 10 MPa.4 It has been demonstrated that porosity is by far the dominant controlling factor limiting the strength of hydrated cement paste.5"7 The removal of those large voids, i.e., macrodefects, by using polymers and plasticizers for rheological control and efficient mixing has generated a new class of materials: MDF (macro-defect-free) cements. Investigators have shown that the flexural strength of MDF cements then increases to 70 MPa and fracture toughness can reach 3.0 MPa m1/2.8 The measurement of fracture toughness of cement paste has been conducted mostly using the single-edgenotch-beam (SENB) method on ordinary portland cement2'3'7'9 and alumina cement.10 Some researchers used the double-cantilever-beam (DCB) technique on cement paste, mortars,11 and concrete,12 and double torsion (DT) to study the crack growth in hardened cement paste.13 There are many papers discussing the measurement of toughness of MDF cement. Eden and Bailey used the SENB technique and work of fracture to calculate KIc and proposed a fibrillar pull-out model for polymer modified portland cement.14 Mai et a/.15 attempted to use indentation cracks and large cracks in double cantilever beam (DCB) specimens to measure a)
Current address: University of Florida, Department of Materials Science and Engineering, Gainesville, Florida 32611.
1774
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 8, Aug 1990
Downloaded: 17 Mar 2015
crack resistance but succeeded only with the DCB technique. None of them employed the indentation-strength technique.16 This technique is attractive because it involves crack sizes observed in most advanced materials applications. Since there appears to be a crack size effect on fracture resistance, it is important to know the behavior, i.e., strength and toughness, for small cracks as well as large cracks. Typically, small cracks control the strength of the material. Fracture surface analysis (FSA), which has been proven very powerful in glass and ceramics, is used to determine the fracture toughness of MDF cements. It is our goal in this paper to investigate the applicability of the indentation method and FSA to MDF cements. II. EXPERIMENTAL
MDF cement was prepared by mixing 83 parts (by weight) of ordinary portland cement (a calcium silicate based material), 14 parts of de-ionized water, and 3 parts of polymer (polyacrylamide). For better processing, the polymer was first dissolved in water, then mixed with cement particles. After shear mixing, the paste was extruded into a plate and
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