International Concrete Abstracts Portal

Top-down premature mid-slab transverse cracking was investigated for a jointed plain concrete pavement project with joint spacing of 4.88 m and located on I-96 in southeastern Michigan. The environmental (curling/warping) stresses were evaluated using conventional linear temperature gradient analysis (1) and a recent developed method for non-linear gradient analysis (2). Slab deflection profiles and temperature gradients for different times of day demonstrated that a built in upward slab curling was present, equivalent to a linear negative temperature gradient of 0.03 C/mm or greater. This condition increases curling stresses at mid-slab and outer edge during morning hour temperature conditions as the built in curling condition provides added negative thermal gradients. In addition, increased joint and corner uplift occurs, a condition, which favors loss of slab base support. For these conditions, finite element analysis for truck tandem axle loading at the edge of transverse joints predicts substantial increased slab deflection and top tensile stresses. Further, loss of contact moves the maximum tensile stress towards the mid slab region along the outer edge, where also curling stresses are highest. The combined tensile stresses were found to be significant and can initiate top down transverse cracking. Once surface cracks are initiated they tend to propagate inward and downward from repeated truck loading.

Field engineers have observed that Jointed Plain Concrete Pavements (JPCP) exhibit irregular joint cracking patterns. Upon inspection, it was seen that most of the joints remain uncracked at early ages, while many of those joints that do crack experience excessively large crack widths. This results in a quicker localized deterioration of these joints, nd ultimately a shorter life span of the highway. This phenomenon, as well as many of the early-age mechanical properties of concrete, was investigated in this study. This paper describes the study of the early-age JPCP joint cracking. On-site highways. The pavements were monitored for slab temperature profile histories, ambient temperature histories, transverse joint crack developemnt and overall behaviors. The concrete temperature histories were obtained at selected locations for each investigation using embedded thermocouples and a contact infrared thermometer. Crack growth histories were obtained for each site by measuring the crack widths at each joint. A time and location dependent analysis was developed which gives an acceptable representation of the observed cracking pattern. The analysis is based on such factors as frictional forces between the JPCP and its underlying layers. The results of this study can be used to help control the locations of joint cracking and crack widths of early-age concrete pavements.

An experimental study was conducted to evaluate the mechanical properties of mortar containing flyash subjected to both thermal and chemical activation. Mortar specimens containing class F. flyash and various activators were prepared. Up to 50% by weight of cement was replaced with flyasha dn the results were compared with the control mixture. IN order to activate the gydration reafctions, additives such as sodium hydroxide (NaOH) and potassium hydroxide (KOH) were used at a rate of 2.5 % of total binder weight. Thermal activation was achieved using an autoclave curing process. Both the strengthening and toughening mechanisms were studied in compression, flexure. The fracture results are analyzed using the fracture energy method, the two -parameter facture properties remain virtually at the same level. R-Curves provide a more descriptive measure of fracture response. Autoclave curing of high flyash mortar samples results in a mrked increase in the strength but a marginal reduction in ductility.

A current research project on hydration kinetics, mechanical properties and early age stress behavior of blended cement conducted at the University of Michigan is reviewd in this paper. A number of experiments including calorimetry and differential thermal analysis were performed to investigate hydration kinetics. The mechanical properties investigated included the compressive strength, splitting tensile strength, Young's modulus, creep compliance, relaxation modulus, and coefficient of thermal dilation. The early age stress behavior was studied by measuring the stress developed in a uniaxially restrained concrete member. In addition, the deformation due to autogeneous shrinkage was also measured experimentally. The experimental data could be used to quantify degree of hydration,, and temperature effects on hydration, and could be used as imputs for predicting the early age stress development in concrete.

The bond stress versus slip behavior of steel fibers cut from twisted wire strands (which are made from at least two round steel wires wrapped helically around each other) is investigated and compared to the behavior of a single triangular steel fiber (Torex) twisted along its axis. Single fiber pullout tests simulating fiber pullout in a cracked tensile specimen were used. Parameters investigated are: 1) the embedded length of the fiber in the matrix; 2) the number of individual wires making a strand fiber; 3) the pitch distance of the fgiber; 4) the compressive strength of the mortar matrix (44 and 84 Mpa); and 5) the tensile strength of the fiver. It is observed that, depending on the combination of parameters (embedded length, matrix, strand type and pitch distance) the bond stress-slip response can be elasto-plastic in shape or slip softening after the peak. However, the twisted wire strand fibers are less efficient than the single Torex fiber, in terms of peak pullout load and pullout energy.