Early-age cracking can occur in concrete if free shrinkage is prevented by the surrounding structure. This paper highlights recent findings to illustrate that shrinkage cracking is influenced by the geometry of the structure. A series of experimental results are presented from three different ring specimen geometries to illustrate that although these specimens had the same residual strain level (and similar residual stress), the age of cracking varied with specimen geometry. A second series of experiments was performed to illustrate that a geometry dependence also exists in specimens with moisture gradients. This paper describes how fracture mechanics concepts can explain this geometry dependent behavior under a uniform moisture distribution. Residual stress levels are computed, non-linear fracture mechanics failure criterion is applied to develop the time and geometry dependent tensile stress resistance curves, and the age of cracking is predicted. The theoretical simulations were found to compare reasonably with the experimental observations. A discussion is provided to illustrate how these considerations may be extended to the specimens with moisture gradients.

The testing proccdurc for the uniasial tension of concrctc in or&r to monitor tension softening bchwior is not cstablishcd )zt, bccausc thcrc arc scvcral difficulties for performing the tat. Various test conditions v,,hich wcrc somctimcs on the contrary have been proposed in the pzst 30 years. This paper suggests a rccommcndablc testing proccdurc for the uniwxial tension with fundamcnt:rl csplanations. In the prcscnt proccdurc, prismatic spccimcns having primary and guide notches on the laterals v,‘crc cmploycd. In addition, an original gear system w;is adopted for complctc elimination of secondary flcxurc or for maint;rining an artificial flcsurc at a conskrnt Icvcl. It was shown that the proposccl proccdurcs cnzblc to monitor tension softening curves :rnd to provide rcliablc test results.

Crack propagation in cement-based matrices reinforced with micro-fibers of steel and carbon was studied using contoured double cantilever beam specimens. Influence of fibers, sand and silica fume was quantified using crack growth resistance curves. It was demonstrated that these fibers enhance the resistance to both nucleation and growth of cracks, and that such fundamental fracture tests are very useful in developing micro-fiber composites with a high performance. The influence of number of variables which would otherwise have remained obscured in normal tests for engineering properties become apparent in the fracture tests. The paper points out the desired durability characteristics of these composites and discusses their current and future applications.

Much research has been performed on measuring the fracture toughness of concrete, but inconsistent toughness values tn the literature leave some questions yet unanswered. This paper provides results ol a broad-based experimental program designed to determine (/certain tests produce an accurmte measure of h-actute toughness for concrete. The results of this study can he used to help make rational dccistons when selecting a combination of specimen size. geometry and data reduction method to measure the fracture toughness of concrete. To be accurate, the fracture toughness value must he the same as would he obtained from an infinitely large test specimen. To show that a value of fracture toughness is accumte requires consistent values from tests using different size and geometry specimens and different data reduction methods. Therefore. this investigation uses three sizes of single edge, SE. and round double heam. RDB, specimens. More than one data reduction method was appltcd to the results of each sire and geometry combination. Four different data reduction methods wet-e used: Itnear elastic fractut-e mechanics. the two-parameter method. the size-effect method, and the Barker method. Results are presented from three hatches of concrete, which represent two dtstinctively different mixes. The fracture toughness values ohtatned were not consistent withtn each batch; therefore, the most aceurate value could not he shown conclustvely. Howevjer, several significant conclusions were formed. The most common laboratory specimen size. no more than 310 mm deep/tall, is UOI sufficiently large to ohtarn an accurate measure of fracture toughness for concrete using either specimen geometry. Even the lat-gest specimens. 1240 mm-tall RDB. experienced significant nonltnear fracture mechanics conditions for all of the concrete mixes. Combtntng the experimental results wtth numerical simulations could provide sufficient informatton to judge uhich of the fracture toughness values, if any, are close to the value that would be obtained from an tnftnitely large spectmcn.

Behavior of polyester polymer concrete (PC) with and without notch and graphite fiber was investigated using nondestructive and destructive testing techniques. The flexural strength of polyester PC was 14 MPa (2,000 psi). The effect of up to 6% chopped graphite fibers on the elastic modulus, shear modulus. Poisson’s ratio, flexural strength and fracture parameters were investigated. Nondestructive methods such as impact resonance and pulse velocity were used to determine the effect of notch depth on the mechanical and damping properties of PC. Fracture parameters, critical stress intensity factor Ktc and critical Jtc-integral were determined using single edge notched beam loaded in four-point bending by varying the initial notch-to-depth ratio from 0.2 to 0.7. By measuring the crack mouth openin g displacement (CMOD) during loading, the crack extension in the test specimen was determined. The critical stress intensity factor and critical J-integral for the polyester PC were 1.3 MN/m’ and 0.27 kN/m respectively. The addition of 6 mm long 6% chopped graphite fibers to the polyester polymer concrete improved the tlexural strength by 20% and Ktc and Jtc by over 25% and 125% respectively. Impact resonance test results were sensitive to the notch-to-depth ratio in the test specimen.