Evaluation of compressive strength of hardening silica fume blended concrete

Silica fume (SF) is a byproduct of induction arc furnaces and has long been used as a mineral admixture to produce high-strength and high-performance concrete. Owing to the pozzolanic reaction between calcium hydroxide and SF, compared with Portland cement, the hydration of concrete containing SF is much more complex. In this paper, by considering the production of calcium hydroxide in cement hydration and its consumption in the pozzolanic reaction, a numerical model is proposed to simulate the hydration of concrete containing SF. The degree of hydration of cement and degree of reaction of SF are obtained as accompanied results from the proposed hydration model. Furthermore, on the basis of the volume stoichiometries, mixing proportions and the degree of reactions of cement and SF, the gel–space ratio of hydrating blended concrete is calculated. Finally, the development of compressive strength of SF blended concrete is evaluated through Powers’ strength theory considering the contributions of cement hydration and SF reaction. The proposed model is verified through experimental data on concrete with different water-to-cement ratios and SF substitution ratios.     

Critical parameters for the compressive strength of high-strength concrete

This study investigates the influence of several material properties underlying the failure mechanism of high-strength concrete (HSC) under uniaxial compression. An experimental-numerical characterization of a single inclusion block (SIB) – an idealized composite comprising of a granite cylindrical core embedded within a high-strength mortar (HSM) matrix – is first carried out. Parametric studies are next conducted with the calibrated SIB model, to identify the critical parameters governing the failure of the idealized composite. The qualitative understanding obtained from the SIB is then utilized to design a series of experiments, exploring the extent of influence of the identified critical parameters on the compressive strength of HSC. Complementary experimental data in literature are also examined. For the range of specimens considered, it is found that the lateral strain capacity of mortar matrix has the most influence on the compressive strength of HSC.     

Two analogous methods for estimating the compressive strength of fibrous composites

In this paper, an analogy is made between the solution of micro-buckling of fibrous composites using a three-dimensional model and that of biaxial bending of reinforced concrete short columns. Two approaches are used; the first one uses a reciprocal formula and the second one uses a bilinear approximation to the non-dimensional stresses interaction equation, to estimate the compressive stress in a fibrous composite. The initial misalignment angles of composite fibers, which are the main parameters in the determination of the compressive strength of fibrous composites, are analogous to load eccentricities in concrete columns. The initial misalignment angles in both directions perpendicular to the axis of the fibers are defined by sinusoidal curves. The compressive strength of different fibrous composites, which also depends on the nonlinear shear stress–strain relationship of the matrix material, is estimated using the present approaches. The results obtained in this study agree well with experimental and analytical results available in the literature.     

Prediction of compressive strength of concrete

The paper, which summarises a survey and an analysis of empirical strength laws, was undertaken with a view to formulating a strength law interrelating the water-cement ratio-which determines the porosity of the hardened cement-paste-and Powers's Gel/Space ratio, giving the relative strength of the cement stone. Accordingly, a graphical procedure is given for predicting the strength of ordinary concretes for a given water-cement ratio and lean air-entrained concretes for a given “equivalent” water-cement ratio. Compressive strength may also be predicted by means of numerical formulae.     

On the compressive strength prediction for concrete masonry prisms

The results of a combined experimental program and numerical modeling program to evaluate the behavior of ungrouted hollow concrete blocks prisms under uniaxial compression are addressed. In the numerical program, three distinct approaches have been considered using a continuum model with a smeared approach, namely plane-stress, plane-strain and three-dimensional conditions. The response of the numerical simulations is compared with experimental data of masonry prisms using concrete blocks specifically designed for this purpose. The elastic and inelastic parameters were acquired from laboratory tests on concrete and mortar samples that constitute the blocks and the bed joint of the prisms. The results from the numerical simulations are discussed with respect to the ability to reproduce the global response of the experimental tests, and with respect to the failure behavior obtained. Good agreement between experimental and numerical results was found for the peak load and for the failure mode using the three-dimensional model, on four different sets of block/mortar types. Less good agreement was found for plain stress and plain strain models.     

On compressive strength variation in concrete

The interrelationship of the standard deviation, the coefficient of variation, and the compressive strength of concrete was studied on 32 building sites and 14 ready-mix plants, with more than 30,000 observations analysed. While the standard deviation was found to be practically strength-independent when overall values were considered, the coefficient of variation showed such independence in its daily values. It was further concluded that formal compliance with a set of nominal requirements does not necessarily secure the expected strength variation, in which respect the actual level of supervision is the dominant factor. The average overal standard deviation was of the order of 60 kg. per sq. cm. and the daily coefficient of variation—of the order of 10%, irrespective of nominal conditions of concrete production.     

Evaluation of the pull-out test to determine strength of in-situ concrete

A degree of correlation exists between compressive strength of 6×12-in (15×30-cm) concrete cylinders cured under standard conditions and pull-out strength of concrete cured under field conditions. The ratio pull-out strength: compressive strength varies directly with the compressive strength of concrete. At 3 days, this ratio varies from 18% for 4 795 (32.9 MN/m²) concrete to 46% for 1 145 psi (7.9 MN/m²). However, for any strength level the ratio does not change significantly with age. For the same concrete mix, the pull-out strength increased with increasing age, indicating the possible usefulness of these tests for comparative studies. The 28-day standard deviation and coefficient of variation of strength from pull-out test results varied from 15 to 45 psi (0.10 to 0.31 MN/m²) and from 2.3 to 5.0% respectively. The corresponding values from compressive strength test results were 4 to 120 psi (0.03 to 0.82 MN/m²) and 0.2 to 3.0% respectively, except for one mix for which the above values were 680 psi (4.7 MN/m²) and 11.4%.     

The compressive strength of a new ureaformaldehyde-based polymer concrete

Standard test specimens of ureaformaldehyde-based polymer concrete (PC) prepared with various amounts of ureaformaldehyde (UF) resin and cured at temperatures in the range 90 to 150 ° C for periods up to 21 d were tested in compression. The PC having a resin content of 8% and cured at 110 ° C for about 7 d, developed an ultimate compressive strength of 37 M Pa. The strength values of PC specimens are compared with those of Portland cement concrete (PCC) specimens prepared with different water/cement ratios and mix proportions. For certain mixes the compressive strength of PCC is surpassed by that of PC having a similar binder content and comparable workability.     

An experimental study on optimum usage of GGBS for the compressive strength of concrete

This paper presents a laboratory investigation on optimum level of ground granulated blast-furnace slag (GGBS) on the compressive strength of concrete. GGBS was added according to the partial replacement method in all mixtures. A total of 32 mixtures were prepared in four groups according to their binder content. Eight mixes were prepared as control mixtures with 175, 210, 245 and 280 kg/m³ cement content in order to calculate the Bolomey and Féret coefficients (K_B, K_F). For each group 175, 210, 245 and 280 kg/m³ dosages were determined as initial dosages, which were obtained by removing 30 percent of the cement content of control concretes with 250, 300, 350, and 400 kg/m³ dosages. Test concretes were obtained by adding GGBS to concretes in an amount equivalent to approximately 0%, 15%, 30%, 50%, 70%, 90% and 110% of cement contents of control concretes with 250, 300, 350 and 400 kg/m³ dosages. All specimens were moist cured for 7, 14, 28, 63, 119, 180 and 365 days before compressive strength testing.The test results proved that the compressive strength of concrete mixtures containing GGBS increases as the amount of GGBS increase. After an optimum point, at around 55% of the total binder content, the addition of GGBS does not improve the compressive strength. This can be explained by the presence of unreacted GGBS, acting as a filler material in the paste.     

Application of size effect to compressive strength of concrete members

It is important to consider the effect of size when estimating the ultimate strength of a concrete member under various loading conditions. Well known as the size effect, the strength of a member tends to decrease when its size increases. Therefore, in view of recent increased interest in the size effect of concrete this research focuses on the size effect of two main classes of compressive strength of concrete: pure axial compressive strength and flexural compressive strength. First, fracture mechanics type size effect on the compressive strength of cylindrical concrete specimens was studied, with the diameter, and the height/diameter ratio considered as the main parameters. Theoretical and statistical analyses were conducted, and a size effect equation was proposed to predict the compressive strength specimens. The proposed equation showed good agreement with the existing test results for concrete cylinders. Second, the size, length, and depth variations of a flexural compressive member have been studied experimentally. A series of C-shaped specimens subjected to axial compressive load and bending moment were tested. The shape of specimens and the test procedures used were similar to those by Hognestad and others. The test results are curve-fitted using Levenberg-Marquardt’s least squares method (LSM) to obtain parameters for the modified size effect law (MSEL) by Kim and co workers. The results of the analysis show that the effect of specimen size, length, and depth on ultimate strength is significant. Finally, more general parameters for MSEL are suggested.     

Effects of CuO nanoparticles on compressive strength of self-compacting concrete

In the present study, the compressive strength, thermal properties and microstructure of self-compacting concrete with different amounts of CuO nanoparticles have been investigated. CuO nanoparticles with an average particle size of 15 nm were added to self-compacting concrete and various properties of the specimens were measured. The results indicate that CuO nanoparticles are able to improve the compressive strength of self-compacting concrete and reverse the negative effects of superplasticizer on compressive strength of the specimens. CuO nanoparticles as a partial replacement of cement up to 4 wt.% could accelerate C–S–H gel formation as a result of the increased crystalline Ca(OH)₂ amount at the early ages of hydration. Increasing CuO nanoparticle content to more than 4 wt.%, causes reduced compressive strength because of unsuitable dispersion of nanoparticles in the concrete matrix. Accelerated peak appearance in conduction calorimetry tests, more weight loss in thermogravimetric analysis and more rapid appearance of peaks related to hydrated products in X-ray diffraction results, all indicate that CuO nanoparticles up to 4 wt.% could improve the mechanical and physical properties of the specimens. Finally, CuO nanoparticles improved the pore structure of concrete and caused shifting of the distributed pores from harmless to low harm.     

Investigation of the possibility of estimating concrete strength by porosity measurements

The possibility of estimating concrete or mortar strength by measuring the total water porosity by a very simple method is investigated. Cube strengths of mortar and concrete specimens are correlated with water porosity measured from ‘intact’ pieces taken from the failure cones of the crushed cubes. It is shown that it is not possible to acquire a satisfactory correlation of strength and the measured total water porosity of the mortar or concrete mix. However, a very promising correlation (correlation coefficient R>93%) is obtained if the ‘calculated average paste porosity’ P _p is substituted for the measured total water porosity of the mix P _m . P _p can be determined from the measured porosity P _m of the mix, the porosity of the aggregates and the volumetric proportions of the constituents. For the range of strengths investigated, it can be inferred that the strength class of a cement (OPC 35 or OPC 45) does not significantly influence the correlation, but a slightly different correlation seems to exist for pozzolanic cements.     

Evaluation of early concrete strength

With a view to verifying the feasibility of evaluating early concrete strength, an extensive experimental program was developed for testing the strength of steam-cured 8 cm concrete cubes after 17 hours (R _a ) and comparing the results with the strength of 15 cm cubes at 28 days (R _s ). The relationship of R _a to R _s depends on the steam curing cycle, on the maximum aggregate size and its influence in relation to sample size, and on the rate of loading during the test. The R _a -R _s regression line was calculated for different concrete mixes using various types of cement (to Italian standards), i.e., Portland 325 and 425, pozzolana 325 with fly ash and Portland 325 with a superplasticizing admixture. A linear relationship generally proved reliable as R _s =mR _a +c, where m and c are constants depending on the type of cement, the curing cycle and the maximum aggregate size. The coefficient of variation was about 4–5%, reaching 6% when all concrete mixes tested were considered.     

Effect of carbonation on the rebound number and compressive strength of concrete

Experimental research was performed to clarify the influence of carbonation on the rebound number and the strength evolution of concrete for three strength levels. The results reveal that the strength level dependent influence of carbonation is a source of errors in the existing equations for the strength reduction coefficient; these equations are used to compensate for the influence of surface carbonation in the rebound number method. A new equation for the strength reduction coefficient that can consider the influence of strength level was developed based on field test data extracted from technical reports of the Korea Research Institute of Standards and Science and of four universities. Over a wide range of strength levels, the equation shows good agreement with strength reduction coefficients established experimentally.     

Evaluation of compound compliance criteria for concrete strength

In the compliance control of concrete, different types of criterion are in use. In particular, compound criteria are often used since this type of criterion appears in the CEB-FIP Model Code for Concrete Structures. In this paper, the dependency between the minimumvalue and the mean-value criteria is shown not to be negligible. The usefulness of a minimum-value criterion is questioned, as generally it unnecessarily decreases the probability of acceptance for low defective fractions. The preliminary application of an outlier criterion is suggested as a more consistent approach. Finally, criteria of the type are proposed, where the numerical values of λ are fixed in such a way that the resulting OC lines are tangent to a continuous boundary line for the unsafe region.     

Prediction of FRP-confined compressive strength of concrete using artificial neural networks

Strengthening and retrofitting of concrete columns by wrapping and bonding FRP sheets has become an efficient technique in recent years. Considerable investigations have been carried out in the field of FRP-confined concrete and there are many proposed models that predict the compressive strength which are developed empirically by either doing regression analysis using existing test data or by a development based on the theory of plasticity. In the present study, a new approach is developed to obtain the FRP-confined compressive strength of concrete using a large number of experimental data by applying artificial neural networks. Having parameters used as input nodes in ANN modeling such as characteristics of concrete and FRP, the output node was FRP-confined compressive strength of concrete. The idealized neural network was employed to generate empirical charts and equations for use in design. The comparison of the new approach with existing empirical and experimental data shows good precision and accuracy of the developed ANN-based model in predicting the FRP-confined compressive strength of concrete.     

A microscopic approach to rate effect on compressive strength of concrete

The enhancement of concrete strength under uniaxial dynamic compression is investigated in this paper. The influence of the microcrack density to the concrete compressive strength is also discussed. Based on the sliding crack model, the compressive strength is obtained by considering the interaction between microcracks using the Kachanov method. Both free water viscosity and inertia effect are included by considering their influences on the dynamic stress intensity factor of crack tip under linearly increasing load. The relationship between the dynamic strength increase factor and the strain rate is obtained. The comparison between the results by the model proposed in this paper and those by available experiments indicates a favorable agreement.