Based on the existing experimental data for compressive strength values of different concrete mixes, a statistical analysis for the gathered data was conducted. The analysis revealed a model for predicting the compressive strength of concrete mixes at any age with the help of two constants (A) and (B) that are considered as a characteristic property for a concrete mix. The constant (A) is introduced as a rate of strength gain constant whereas, (B) is introduced as grade of strength constant.Once the values of constants (A) and (B) are defined for a concrete mix, the compressive strength at any age could be simply predicted without collecting data at that age. The values of (A) and (B) could be determined by one of two methods. Solving two simultaneous equations at two different ages while performing either design or trial concrete mix is a method that could be used to define the two constants. Other method is based on concrete strength at 28-day age. The proposed model was studied for different concrete mixes. The study covered some parameters including the influence of, mineral admixtures as a partial replacement of cement, metakaolin, nano silica fume, curing in water or lime and the effect of curing temperature.The analysis reveals that mixes containing no admixtures, mixes containing silica fume and cured at normal temperature, mixes containing nano silica and cured in water are following with high accuracy the proposed model. ª 2013 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center.
This study is planned to investigate the properties of Portland cement mixtures containing silica fume and mixed with saturated lime water. The conducted Portland cement mixes included three groups; cement pastes, cement mortars and cement concrete mixes. The main parameters were; type of mixing solution (water or lime-water) as well as the percentage of Portland cement replaced by silica fume. Consistency level, times of initial and final settings, compressive strength development, existence and intensity of CH crystals with age, pozzolanic activity as well as efficiency of the investigated matrices to delay the corrosion of embedded steel bars were the investigated properties. Test results show that using lime-water in mixing enhances consistency degree compared to the corresponding control mixes. Furthermore, it delays both initial and final setting times compared with traditional water due to the common ion effect principles. Moreover, combined use of limewater and silica fume enhances the pozzolanic reaction that was identified by the strength development at both early and later ages. The existence of CH crystals for higher percentages of silica fume (up to 30%) for further reaction at later ages was observed by XRD results. Moreover, combined use of silica fume and lime-water ensures a high alkaline media around steel bars from the moment of ingredients mixing as long as later ages despite of pozzolanic reaction that was identified from results of chloride attack.
This study is directed to evaluate the ability of using cone penetration test as a simple method to investigate the consistency level of fresh concrete. A cone of 30°apex angle attached with different load values was used. Eighteen concrete mixes divided into three groups were conducted. Three types of coarse aggregate were tried. Crushed dolomite, round gravel, and crushed basalt all of 20 mm maximum grain size were investigated. For each type of coarse aggregate, six levels of concrete consistency calibrated by standard slump test were tried. For the investigated mixes and at a specified consistency level, the displaced volume values were directly proportional to the applied load. The inclination of this relation is termed as the displaced volume rate (D.V). The results of cone penetration were analyzed and compared to the corresponding slump test values. The displaced volume per unit mass, bearing strength, as well as shear yield strength were the evaluated properties. The results introduce the cone penetration test as a simple instrument that could be adopted either at a laboratory or at site to evaluate fresh concrete workability. Moreover, it is being more sensitive compared to the well known slump test. It can simply and clearly distinguish between stiff mixes as well as floppy ones. Very useful numerical limits for the evaluated properties controlling the workability levels of very low, low, medium, high and very high were proposed. ª 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center.
An experimental testing program was undertaken to evaluate the applicability of using the principles of Archimedes' law for buoyancy to assess the fluidity of self compacting concrete (SCC) as well as fiber reinforced self compacting concrete mixes (FR-SCC). A cone instrument with different apex angle values (20, 30, 40 and 45°) was implemented. One type of steel fibers (SF), three types of polypropylene fibers (PP1, PP2 and PP3) as well as one type of glass fibers (GF) with different fiber volume fractions and aspect ratios were conducted. Fourteen FR-SCC mixes in addition to a control mix were examined. A new terminology called fluidity index (FI) was proposed and evaluated based on the displaced volume rate according to the buoyancy law through performing cone penetration test. Parallel with the penetration test, traditional flow ability and segregation resistance tests were performed.The results indicated that the proposed method is effective in evaluating the fluidity of FR-SCC mixes in terms of FI. FI values between 0.8 and 1.0 indicate SCC mixes whereas, FI lower than 0.8 indicate fresh concrete with fluidity not satisfying requirements of SCC. The proposed method can be performed either at laboratory or at field with and without sampling. Moreover, it can be directly adapted to the cast concrete in structural members provided that there is no obstruction for penetration. The direct proportion between FI and the slump flow test results indicated its viability to evaluate the fluidity of both FR-SCC and SCC mixtures. Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ncnd/4.0/).
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