EARLY AGE COMPRESSIVE AND TENSILE STRENGTH DEVELOPMENT

1. VMA High Paste Graded Aggregate Mineral Fillers Mechanical Performance of Self Consolidating Concrete M. D. D'Ambrosia, D. A. Lange , A. J. Brinks University of Illinois at Urbana-Champaign OBJECTIVE Determine how SCC strategies… • high paste content • VMA (thickeners) • smaller aggregate & controlled gradation • HRWR, SP (CAE) • Mineral fillers & additives …change hardened properties… • segregation • shrinkage and creep • entrained air system • permeability • strength …and affect performance. • early age cracking • long term durability • surface scaling • freeze-thaw resistance • abrasion resistance EXPERIMENTS • Slump flow of SCC mixtures was measured for quality control • A separate rheological study was conducted (L. Shen, L. Struble, J. Hidalgo) • Drying began at concrete age of 1 day • Environment was 50% RH and 23°C • An LVDT extensometer was used to measure deformation • Sealed barrier of aluminum foil applied to impose symmetric drying • Feedback controlled closed loop system applies a restraining force with servo hydraulic actuator • Instron Controller operated by a Restraint Simulation Program (RSP) (LabView) • Restraint Simulation Program keeps strain virtually constant over time (to within 0.005mm ) • Summation of restrained deformation allows for creep calculation • Autogenous shrinkage of sealed specimens was measured from time of casting • Internal relative humidity in sealed prism was also measured to asses the driving force for autogenous shrinkage UIUC SCC DATABASE • SCC proportions differ from ordinary concrete • Selected control mixtures from literature explore various strategies of SCC mixture proportioning EARLY AGE COMPRESSIVE AND TENSILE STRENGTH DEVELOPMENT AUTOGENOUS SHRINKAGE AND TOTAL SHRINKAGE DURING DRYING • Average w/cm = 0.41, w/p = 0.35 • 33% contain limestone powder (LSP) • 52% contain fly ash, 37% contain slag • 30% contain slag and fly ash • 0% contain both LSP and pozzolans • 45% contain VMA • 1% contain both VMA and LSP • Only 3% contain 3 different size aggregates • SCC mixtures tend to have low w/c ratio and high paste%, and thus higher strength than most ordinary concrete. When compared to OPC with same w/c and paste%, strength is similar, indicating that SCC admixtures had little effect of strength • Low w/c drives autogenous shrinkage, which can then be a major contributor to total shrinkage at early age. High early age shrinkage leads to tensile stress and cracking when concrete is restrained RESTRAINED STRESS DEVELOPMENT, STRESS STRENGTH RATIO AND RELAXATION BY TENSILE CREEP SUMMARY • An investigation of the EA mechanical behavior of SCC has revealed a potentially high risk for cracking in some mixtures • Mechanical properties can be influenced by higher paste content and low w/c, BUT it is best not to treat SCC as a group of materials with similar mechanical behavior. • Autogenous shrinkage may cause significant stress at early age • SCC Mix Design should minimize cement paste content and use a w/cm that avoids significant autogenous shrinkage while achieving necessary flow characteristics • Providing external water during curing in field applications will delay shrinkage stress development at early age and reduce the overall magnitude of shrinkage and cracking risk. • Stress measurements indicate that to reduce the risk of early age cracking in SCC, using a w/cm ratio of 0.40 to 0.42 can prevent autogenous shrinkage from causing significant stress, while at the same time minimizing drying shrinkage. • Stress-strength ratios demonstrate that microcracking and damage may be occurring as early as one or two days after drying at early age. • Creep capacity is directly proportional to paste content and is inversely proportional to w/cm ratio. The high stress-strength ratio of SCC1 induced microcracking damage  High cracking risk • In SCC4 the stress develops rapidly due to the lack of relaxation by creep and damage occurs rapidly right before failure Also high cracking risk