Effect of conditioning temperature on the strength and permeability of normal- and high-strength concrete

In order to evaluate the effect of the conditioning temperature on strength and permeability properties of concrete a series of compressive, indirect tensile and permeability tests were performed on concretes (designed to have 28-day compressive strengths of 40 and 100 N/mm²) conditioned at temperatures of 85 and 105 °C. The results show that, for both the normal- (NSC) and the high-strength concrete (HSC), comparable 28-day test results were obtained from strength tests performed on concrete conditioned at 85 and 105 °C. The permeability results were also somewhat similar for the two conditioning temperatures, although greater differences than previously reported were observed. Conditioning at both 85 and 105 °C was identified as adequate, with the preferred temperature of conditioning being 105 °C.     

Effect of the cement chemistry and the sample size on ASR expansion of concrete exposed to salt

Mortar bars and concrete prisms made with a very alkali-silica reactive limestone were stored at 38 °C in 1 M NaOH and NaCl solutions. A high-alkali (HA) cement and a low-alkali (LA) cement were used in order to evaluate the cement chemical composition on the expansion and on the chemistry of the pore water. The mortar bars immersed in 1 M NaOH presented much more expansion than mortar bars stored at 100% RH or in 1 M NaCl. The behaviour of the concrete prisms was completely different. Low expansion was obtained for concrete prisms made with the LA cement immersed for more than 5 years in 1 M NaCl solution, while the expansion was over 0.45% for concrete prisms made with the HA cement. Chemical equilibrium between the pore waters and the immersion solution was much longer to obtain for the concrete prisms (near 3 years) than for the mortar bars (less than 3 months). The results obtained in this study show that the type of sample used (mortar bars or concrete prisms) and the cement composition strongly influence the harmful effects of ASR in concrete exposed to salt.     

Properties of hot aluminosilicate concretes with various binders

Conclusions On the basis of results of a study of aluminosilicate concretes in the heated state, the following recommendations can be made for the use of these concretes, depending on the form of the binder and on the concentration of Al₂O₃ in the filler.Semiacid concrete in APB has fairly good strength and deformation properties, a high thermal-shock resistance, and can be used up to temperatures of 1350°C.Chamotte concrete in HAC is clearly not economical to manufacture. For performance, it can be totally replaced by chamotte concrete in AC or APB. The chamotte concrete in APB has better characteristics in the heated state than the concretes in other binders. It can be successfully used under conditions where there are sudden variations in the temperature and mechanical action.Aluminosilicate concretes in WG can reasonably be used in conditions of fairly intense abrasive action up to temperatures of 800°C. In places where the concrete is not subject to the action of molten metal or slag, it would not be reasonable to use concretes in WG containing >60% Al₂O₃.The high-alumina concretes in HAC have greater strength at high temperatures and therefore they must be used in those conditions; analogous concretes in APB can reasonably be used where there are sudden variations in temperature.The properties considered here of the concretes in HAC and APB are improved by increasing the concentration of Al₂O₃ and therefore the form of the filler for the concretes must be chosen in relation to the actual conditions of use.Translated from Ogneupory, No. 7, pp. 52–60, July, 1980.     

Carbon resistance of corundum concretes

Conclusions The resistance of the corundum concretes to carbon during direct interaction under the conditions of high temperature isothermal holding is quite high. After holding for a period of 120 h at 1580°C, the weight loss of the concrete specimens is insignificant (0.2–0.4%) without exception.Modifying the structure of the concretes using chromium oxide and a mixture of lignosulfonates makes it possible to reduce the weight loss in the carbon-bearing medium.Translated from Ogneupory, No. 4, pp. 11–15, April, 1988.     

The behavior of silicocarbonatite aggregates from the Montreal area

In a previous paper, it was concluded that silicocarbonatite aggregates from the Francon quarry, Montreal contributed to durability problems in Portland cement concrete. Results show that, at 2 days after casting, concrete made with silicocarbonatite aggregates contained over 1.5% more Na₂O than similar bars made with Exshaw limestone aggregates. A reaction involving the rare mineral dawsonite in the silicocarbonatite is thought responsible for the higher Na₂O content. In turn, this caused increased expansion of concrete bars made with alkali expansive aggregates. Also, concrete made with alkali-carbonate reactive Pittsburg aggregate showed more expansion when cured at 80 °C than bars cured at 23 °C. Concrete bars made with Exshaw limestone aggregates cured for 4 h at 85 °C showed late-stage expansion, which is attributed to delayed ettringite formation (DEF). However, no expansion was shown by heat-cured concrete prisms or mortar bars made with silicocarbonatite aggregates. Release of alkalis, aluminates and carbonates by the dawsonite reaction may have inhibited DEF. Concrete bars made with nonreactive Nelson dolostone and 10% silicocarbonatite cured at 80 °C for 4 h showed up to 0.15% expansion after several years at 23 °C and 100% relative humidity (R.H.), indicating that a deleterious reaction did occur.     

Effect of cooling methods on residual compressive strength and cracking behavior of fly ash concretes exposed at elevated temperatures

This paper presents the effects of cooling methods on residual compressive strength and cracking behavior of concretes containing four different class F fly ash contents of 10%, 20%, 30% and 40% as partial replacement of cement at various elevated temperatures. The residual compressive strength of the aforementioned fly ash concretes is measured after being exposed to 200, 400, 600 and 800 °C temperatures and two different cooling methods, for example, slow cooling and rapid water cooling. Results show that the residual compressive strengths of all fly ash concretes decrease with increase in temperatures irrespective of cooling regimes, which is similar to that of ordinary concrete. Generally, control ordinary concrete and all fly ash concretes exhibited between 10% and 35% more reduction in residual compressive strength because of rapid cooling than slow cooling except few cases. Cracks are observed over concrete specimens after being exposed to temperatures ranging from 400 to 800 °C. Samples that are slowly cooled developed smaller cracks than those rapidly cooled. At 800 °C, all fly ash concretes that are exposed to rapid cooling showed the most severe cracking. X‐ray diffraction analysis shows reduction of Ca(OH)₂ peak and formation of new calcium silicate peak in concretes containing 20% and 40% fly ash when subjected to 800 °C in both cooling methods. Thermo gravimetric analysis and differential thermal analysis results show increase in thermal stability of concrete with increase in fly ash contents. The existing Eurocode also predicted the compressive strength of fly ash concretes with reasonable accuracy when subjected to the aforementioned elevated temperatures and cooling methods. Copyright © 2014 John Wiley & Sons, Ltd.     

The experimental investigation of concrete carbonation depth

Phenolphthalein indicator has traditionally been used to determine the depth of carbonation in concrete. This investigation uses the thermalgravimetric analysis (TGA) method, which tests the concentration distribution of Ca(OH)₂ and CaCO₃, while the X-ray diffraction analysis (XRDA) tests the intensity distribution of Ca(OH)₂ and CaCO₃. The Fourier transformation infrared spectroscopy (FTIR) test method detects the presence of C-O in concrete samples as a basis for determining the presence of CaCO₃. Concrete specimens were prepared and subjected to accelerated carbonation under conditions of 23 °C temperature, 70% RH and 20% concentration of CO₂. The test results of TGA and XRDA indicate that there exist a sharp carbonation front. Three zones of carbonation were identified according to the degree of carbonation and pH in the pore solutions. The TGA, XRDA and FTIR results showed the depth of carbonation front is twice of that determined from phenolphthalein indicator.     

Influence of initial curing on sulphate resistance of blended cement concrete

The paper presents results of an investigation on the effect of initial curing conditions on the sulphate resistance of concrete made with ordinary portland cement and using pfa, silica fume and ground granulated blast furnace slag for partial replacement of cement. In addition, porosity and pore structure analysis of representative pastes was carried out to examine the relationship between these properties and sulphate resistance of concrete. The depth of carbonation in specimens of pastes was also determined.

Three different initial curing conditions immediately after casting of specimens were adopted, namely: WET/AIR CURED at 45°C, 25% RH; AIR CURED at 45°C, 25% RH; AIR CURED at 20°C, 55% RH. The results show that pore volume and pore structure of the paste bear no direct relationship with the sulphate resistance of concrete. The presence of a carbonated layer on the surface is generally accompanied by superior sulphate resistance—there are, however, important exceptions. Low humidity curing at high temperature (45°C) results in higher depths of carbonation but lower sulphate resistance than similar curing at 20°C.

The sulphate resistance of concrete increases with the replacement of cement with 22% pfa, 9% silica fume and 80% ggb slag. The sulphate resistance also increases due to drying out of concrete during early curing at low relative humidity and due to carbonation. The possible common factor which leads to this improved sulphate resistance is the reduced Ca(OH)₂ content which leads to smaller volume of the expansive reaction products with sulphate ions. The effect of initial curing at high temperature (45°C) is significantly harmful to the sulphate resistance of plain concrete but much less so to the blended cement concretes.     

Chloride resistance of high-performance concretes subjected to accelerated curing

The strengths and chloride penetration resistance of a series of high-performance concretes were measured after curing either at 23 °C or accelerated by heating to 65 °C. The results confirm that concretes containing silica fume (SF) or ternary blends of SF and ground granulated blast-furnace slag (GGBFS) exhibit improved chloride penetration resistance compared to those of plain Portland cement concretes. In addition, chloride penetration resistance of Portland cement concrete is adversely affected by accelerated curing. With the use of the ternary ordinary Portland cement (OPC)-SF-GGBFS binders, accelerated curing did not have detrimental effects on chloride penetration resistance and provided 18-h strengths in excess of 40 MPa.     

Aluminosilicate concretes bonded with water glass

Summary A composition was developed for aluminosilicate concrete with a water-glass bond with a density of 1.25–1.30 g/cm³ and with a setting accelerator-Portland cement.By reducing the density of the glass and eliminating the addition of sodium silicofluoride, the RUL is increased by 300° C compared with ordinary concrete used at present. The concrete possesses a high strength over the entire temperature range, a high abrasion resistance and excellent spalling resistance.The technical properties of the fireclay concrete suggest that this material can be used at up to 1300° C, and the aluminous at up to 1450° C in place of piece aluminosilicate goods.To solve the problem of the reliable and mass use of the recommended concretes in heat exchangers and in other parts of rotary cement furnaces it is necessary to carry out extra tests with the concretes on a bigger scale.     

Alkali mass balance during the accelerated concrete prism test for alkali-aggregate reactivity

The alkali mass balance was calculated in concrete specimens submitted to the storage conditions of the Canadian standard CSA A23.2-14A concrete prism test for expansion due to alkali-aggregate reaction (AAR). The alkali concentration of both the concrete pore solution expressed under high pressure and the water below specimens in storage pails (bottom water) was measured. Measurements were conducted over a 1-year period, which corresponds to the length of the above test. Two reactive aggregates were tested [Potsdam sandstone (PO) and Spratt limestone (SP)]. Each aggregate was incorporated in two concrete mixtures (mass concrete and structural concrete), for a total of four batches. Significant alkali leaching occurred at 38 °C while performing tests in high moisture storage conditions even though prisms were covered with plastic sleeves. After 52 weeks, the alkali loss ranged from 12% to 25% of the original Na₂O_e content of the concrete, depending on the mixture proportioning and the aggregate type. After estimation of the proportion of alkalis fixed in cement hydrates, it appears that about 23% to 39% of the original alkalis released by the cement are quickly sorbed on aggregate surfaces or have rapidly migrated inside aggregate particles, which may have been incorporated with time in the AAR product. After 52 weeks at 38 °C, the pore solution alkalinity expressed from mass concrete made with PO was 250 mmol/l, whereas the alkalinity was 270 mmol/l in mass concrete incorporating SP. Since prisms of both mixtures were still expanding at 1 year, these alkalinity values are above the thresholds required for sustaining AAR in these concrete mixtures.     

Synthesis, proton conductivity and methanol permeability of a novel sulfonated polyimide from 3-(2′,4′-diaminophenoxy)propane sulfonic acid

A novel sulfonated diamine monomer, 3-(2′,4′-diaminophenoxy)propane sulfonic acid (DAPPS), was successfully synthesized and the sulfonated polyimide (SPI) was prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and DAPPS. The resulting SPI, NTDA-DAPPS, was soluble in common organic solvents. The SPI membrane displayed proton conductivity σ values of 0.12-0.35 S/cm at temperatures ranging from 35 to 90 °C in liquid water, which were similar to or higher than those of Nafion 117 and sulfonated hydrocarbon polymers. The σ of the SPI membrane decreased significantly with decreasing relative humidity (RH) and became much lower than that of Nafion 117 at 30% RH. The SPI membrane displayed good water stability at 80 °C and was thermally stable up to 240 °C. It showed reasonable mechanical strength of a modulus of 1.3 GPa at 90 °C and 90% RH. Its methanol permeability P_M was 0.57×10⁻⁶ cm²/s at 30 °C and 8.6 wt% methanol in feed, which was a fourth of that of Nafion 117. As a result, its ratio of σ/P_M was 21×10⁴ S cm⁻³ s, which was about 4 times larger than that of Nafion 117, suggesting potential application of the SPI membrane for direct methanol fuel cell.     

Stress and strain state of concrete during freezing and thawing cycles

The objective of this work is to calculate the pressures, stresses, and strains induced into moist concrete during freezing and thawing. The applied theory is based on thermodynamics and the linear theory of elasticity. If no additional salts are dissolved in the pore water the inputs needed in the theory are relative humidity and temperature measured in the sample chamber and inside concrete and evaporable water amount in the pore structure. Theoretical results were compared with the test results made with two concretes cured under water or at 96% relative humidity. One of the concretes was air entrained and in the comparison concrete no air-entraining agents were used. In the test cylinders cured under water the largest tensional stresses in freezing occurred on the surface of the test cylinders both in the axial and tangential direction. The largest tensional stress was 2.2 MPa, both in air-entrained and in non air-entrained concretes. The largest tensional stresses in the warming phase took place at the end of the thawing period when the chamber temperature was around +5 °C. Then the maximum tension occurred in the middle of the concrete cylinder in the axial direction of the cylinder. This maximum tensional stress was over 2.5 MPa in the air-entrained concrete cured in the relative humidity of 96%. The thermodynamic pumping effect at the end of the thawing phase in every cycle can increase the pore water amount remarkably if free water or moisture is available on the surface of the structure or in the environment vapor. The thermodynamic pumping effect seems to be remarkably greater and more dangerous in air-entrained concretes.     

Effects of seawater on AAR expansion of concrete

Recently, AAR was identified in submerged piles of some bridges in tidal waters. Microstructural examination detected chloroaluminate salts in some cracks. To clarify whether seawater had influenced the deterioration an experimental program was planned to examine the effects of sodium chloride on AAR under various curing conditions.Concrete prisms containing either of highly-reactive, slowly-reactive or nonreactive aggregate, and either low or high alkali contents, were stored in saltwater (representing seawater) or at 100% RH, at temperatures of 38, 60 and 80 °C, for expansion measurement over 600 days, after which the temperature for those stored in saltwater was lowered to 23 °C, to check its effect on further expansion, which could be attributed to precipitation of ettringite and/or Ca-chloroaluminate.The results indicate that the type of aggregate and concrete alkali content had the greatest effect on AAR expansion. Exposure to saltwater did not have any significant effect on the AAR expansion.     

Strength properties of chamotte concretes based on phosphate binders

Conclusions The authors have investigated the strength properties of chamotte concretes based on phosphate binders over a wide range.They have shown the dependence of the compressive strength of the concretes on the type of phosphate binder used, the heat treatment temperature of the concrete, and its test temperature. Maximal compressive strength is exhibited by concretes roasted at 1450°C.They have established a relation between the physicochemical transitions taking place in a phosphate binder during heating and the strength characteristics of the concrete.Simultaneous introduction of phosphate binder and clay to the extent of 10–20% into chamotte concretes increases their strength at 20°C and during heating.Translated from Ogneupory, No. 6, pp. 32–36, June, 1979.     

Properties of pumice aggregate concretes at elevated temperatures and comparison with ANN models

The mechanical properties and thermal conductivity of concretes including pumice aggregate (PA) exposed to elevated temperature were analyzed by thermal conductivity, compressive strength, flexure strength, dynamic elasticity modulus (DEM) and dry unit weight tests. PA concrete specimens were cast by replacing a varying part of the normal aggregate (0–2 mm) with the PA. All concrete samples were prepared and cured at 23 ± 10C lime saturated water for 28 days. Compressive strength of concretes including PA decreased that reductions were 14, 19, 25 and 34% for 25, 50, 75 and 100% PA, respectively. The maximum thermal conductivity of 1.9382 W/mK was observed with the control samples containing normal aggregate. The tests were carried out by subjecting the samples to a temperature of 0, 100, 200, 300, 400 500, 600 and 700 °C for 3 h, then cooling by air cooling or in water method. The results indicated that all concretes exposed to a temperature of 500 and 700 °C occurred a significant decrease in thermal conductivity, compressive strength, flexure strength and DEM. An artificial neural network (ANN) approach was used to model the thermal and mechanical properties of PA concretes. The predicted values of the ANN were in accordance with the experimental data. The results indicate that the model can predict the concrete properties after elevated temperatures with adequate accuracy. Copyright © 2016 John Wiley & Sons, Ltd.     

Carbonation rates of concretes containing high volume of pozzolanic materials

The project studies the influence of fly ash and slag replacement on the carbonation rate of the concrete. The experimental work includes samples of pure Portland cement concrete (CEM I 42,5 R), blast-furnace slag concrete (CEM III-B), and fly ash blended concrete. To reveal the effect of curing on carbonation rate, the concretes were exposed to various submerged curing periods during their early ages. After that, the samples were subsequently exposed in the climate room controlling 20 °C and 50% RH until the testing date when the samples had an age of 5 months. Then, the accelerated carbonation test controlling the carbon dioxide concentration of 3% by volume, with 65% relative humidity were started to perform. The depth of carbonation can be observed by spraying a phenolphthalein solution on the fresh broken concrete surface. Finally, according to Fick's law of diffusion theoretical equations are proposed as a guild for estimating the carbonation rate of fly ash and blast-furnace slag concretes exposed under natural conditions from the results from accelerated carbonation tests.     

Effect of high temperatures on high performance steel fibre reinforced concrete

After being subjected to different elevated heating temperatures, ranging between 105 °C and 1200 °C, the compressive strength, flexural strength, elastic modulus and porosity of concrete reinforced with 1% steel fibre (SFRC) and changes of colour to the heated concrete have been investigated.The results show a loss of concrete strength with increased maximum heating temperature and with increased initial saturation percentage before firing. For maximum exposure temperatures below 400 °C, the loss in compressive strength was relatively small. Significant further reductions in compressive strength are observed, as maximum temperature increases, for all concretes heated to temperatures exceeding 400 °C. High performance concretes (HPC) start to suffer a greater compressive strength loss than normal strength concrete (NSC) at maximum exposure temperatures of 600 °C. It is suggested that HPC suffers both chemical decomposition and pore-structure coarsening of the hardened cement paste when C-S-H starts to decompose at this high temperature. Strengths for all mixes reached minimum values at 1000 or 1100 °C. No evidence of spalling was encountered. When steel fibres are incorporated, at 1%, an improvement of fire resistance and crack [F.M. Lea, Cement research: retrospect and prospect. Proc. 4th Int. Symp. On the Chemistry of Cement, pp. 5-8 (Washington, DC, 1960).] resistance as characterized by the residual strengths were observed. Mechanical strength results indicated that SFRC performs better than non-SFRC for maximum exposure temperatures below 1000 °C, even though the residual strength was very low for all mixes at this high temperature. The variations with colour, which occured, are associated with maximum temperatures of exposure.     

Elastic properties of refractory silica concretes

Conclusions The authors have designed an ultrasonic experimental apparatus and developed a procedure for using it to investigate the elastic properties of refractory concretes at 20–1500°C.They have determined the elastic moduli and investigated their pattern of change during heating and cooling of silica concretes based on water glass and aluminophosphate binder. They have established that the dynamic elastic moduli of these concretes vary within a wide range. During heating, the elastic modulus of the concretes sharply decreases within the range 50–500°C, then increases up to 1000°C, decreasing again on further rise in temperature.During cooling, the E_d exhibits the reverse pattern of change, but its values at the corresponding temperature are higher than during heating.The results obtained can be used to perfect the procedure for manufacturing refractory concretes and to determine their optimal conditions of service.Translated from Ogneupory, No. 4, pp. 38–43, April, 1979.     

Normalized age applied to AAR occurring in concretes with or without mineral admixtures

This paper presents a method for assessing the normalized age factors, which allow accelerated alkali-aggregate reaction (AAR) tests performed at various temperatures (20, 40 and 60 °C) to be related to the conditions encountered in situ in concrete structures. The evaluation of normalized age factors is based on the comparison of many experimental results taken from the literature concerning laboratory tests and in situ measurements. The use of these factors permits us to evaluate, from the results of an accelerated test performed at 60 °C, the protection time against AAR that could be expected for in situ concretes containing mineral admixtures (silica fume and fly ashes). The results show that, in addition to the inhibitory action of mineral admixtures leading to a strong decrease in the final AAR-swelling, the protection against abnormal expansion caused by AAR increases significantly when mineral admixtures are used. Abnormal expansion is expected at 2-4 years for plain concrete compared to 7-50 years for concrete with mineral admixtures.