High Strength Israel

High strength concrete-Stage I: Part 2-properties of concrete with local aggregates Assoc. Prof. Katz Amnon, Dr Dancygier Avraham, Prof. Bentur Arnon, Dr Leviathan Itai Abstract 1. Objectives: Investigation of the properties of high strength concrete prepared with typical Israeli's aggregates. Investigation of the mechanical properties of concretes made with these aggregates (compressive strength, tensile strength in flexure and splitting, modulus of elasticity). Defining the relationships between these properties with respect to the type of aggregates. 2. Findings: this part of the work tested high strength concrete prepared from typical local aggregates: limestone from Ein-Harod and Modiyim, dolomite from Hanaton and Wadi-Arah, and Basalt from Bteha. The concretes were prepared with low water to cement ratio that varied between 0.50 and 0.30, using cement CEM I 52.5N without additives such as fly ash or microsilica. The following conclusions were found: Compressive strength: the 28 days compressive strength has reach ~90 MPa and strength increased to ~100 MPa at 90 days. No significant difference was found between the various stones. Though, in the narrow range of the results (97-108 MPa), basalt aggregate was found in the upper range, and limestone, in the lower range. Rate of compressive strength development was higher as concrete strength increased, unlike the common equations to predict strength development according to various standards. These equations show reasonable correlation with concrete in the known range (up to ~60 MPa), but they underestimate the strength at early age. Alternatively, estimation of concrete strength at late age (28 days and thereafter) according to the results at early age may overestimate the strength. Flexural strength has slightly changed by the effect of stone type. The trend found here were opposing to those found for compressive strength, i.e. the flexural strength of concrete with basalt aggregate was slightly lower than that made with limestone aggregate. The development of flexural strength was very rapid. At age 2 days it was ~80% of the 28 days strength, much more than the observed values for compressive strength, or the values expected by the standards. Some reduction of the flexural strength was observed with concretes prepared with low water/cement ratio after age 7 days. It is possible that this observation results from self desiccation that leads to shrinkage of concretes with low water/cement ratio. This phenomenon should be re-evaluated for a longer period of time in the second part of the study, including its effect on the design of concrete elements (mainly pre-stressed concrete). As a result from the rapid development of flexural strength compared with compressive strength the conversion factors from compressive to flexural (or tensile) strength are higher at early age (0.13-0.15) than at 28 days (0.11-0.13). Modulus of Elasticity: concretes with limestone exhibited somewhat lower modulus that the dolomite aggregates. Good correlation with the values shown in

SI 466-1 in the range of strength equivalent to B- 60 (water/cement ratio of 0.50). at the high strength range the modulus of the limestone aggregate was 41-43 GPa, and higher values were obtained for dolomite: 50 GPa and 45 GPa for aggregates from Wadi-Arah and Hanaton, respectively. Interesting to know the average value observed for basalt aggregate (~44 GPa). 3. Recommendations: at the end of this stage of the study it seems to be too early to address guidelines for the design of concrete elements made with high strength concrete, without full completion of the whole study. The next stage should focus on two types of stone and examine water/cement ratio in the range of 0.40-027, including the use of microsilica. The phenomenon of reduction in flexural strength at late ages should be studied in addition.