Modeling mechanical performance of lightweight concrete containing silica fume exposed to high temperature using genetic programming

In this study, the mechanical performance of lightweight concrete exposed to high temperature has been modeled using genetic programming. The mixes incorporating 0%, 10%, 20% and 30% silica fumes were prepared. Two different cement contents (400 and 500 kg/m³) were used in this study. After being heated to temperatures of 20 °C, 200 °C, 400 °C and 800 °C, respectively, the compressive and splitting tensile strength of lightweight concrete was tested. Empirical genetic programming based equations for compressive and splitting tensile strength were obtained in terms of temperature (T), cement content (C), silica fume content (SF), pumice aggregate content (A), water/cement ratio (W/C) and super plasticizer content (SP). Proposed genetic programming based equations are observed to be quite accurate as compared to experimental results.     

Effects of curing regimes and cement fineness on the compressive strength of ordinary Portland cement mortars

Curing techniques and curing duration have crucial effects on the strength and other mechanical properties of mortars. Proper curing can protect against moisture loss from fresh mixes. The objective of this experimental work is to examine the compressive strength of ordinary Portland cement mortars (OMs) under various curing regimes and cement fineness. Six different curing methods including water, air, water heated, oven heated, air–water, and water–air were applied to the specimens and also six groups of mortars were used. The results showed that the highest and lowest compressive strengths are attributed to the specimens of OPC mortar water cured using grounded OPC for duration of 6 h (OM–G6–wc) and OPC mortar air cured under room temperature with oven heated after demoulding of the specimens at 60 °C for duration of 20 h (OM–OH–ac), respectively. The maximum levels obtained of compressive strengths at 7, 28, and 90 days are 57.5, 70.3, and 76.0 MPa, respectively.     

The effect of activator concentration on the residual strength of alkali-activated fly ash pastes subjected to thermal load

This investigation reports on a comparative study of the residual compressive strength at different temperatures of alkali-activated fly ash (AAFA) pastes chemically activated using sodium silicate with three different concentrations named 20, 30 and 40 (wt.%). The behaviour of different mixtures in conditions of rapid temperature changes was studied. Water quench test was applied to determine thermal shock resistance. The traditional Portland cement pastes were used as a reference. The temperatures ranging from 200 °C to 1000 °C with an increment of 200 °C has been examined. Pore solution pH and compressive strengths before and after exposure to elevated temperatures were determined. The various decomposition phases formed were identified using X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential thermal analysis (DTG) and scanning electron microscopy (SEM). The results indicated that fly ash activated by sodium silicate is more able to resist degradation caused by exposure to elevated temperature than Portland cement system as its relative strengths are superior. In the hardened AAFA pastes, as activator concentration dosage increased as the relative compressive strengths and thermal shock resistance decreased. The AAFA system is able to maintain a sufficient high pH to retain the passive protective layer on the surface of any reinforcing steel present.     

Influence of class F fly ash on the abrasion–erosion resistance of high-strength concrete

This study investigates the abrasion–erosion resistance of high-strength concrete (HSC) mixtures in which cement was partially replaced by four kinds of replacements (15%, 20%, 25% and 30%) of class F fly ash. The mixtures containing ordinary Portland cement were designed to have 28 days compressive strength of approximately 40–80 MPa. Specimens were subjected to abrasion–erosion testing in accordance with ASTM C1138. Experimental results show that the abrasion–erosion resistances of fly ash concrete mixtures were improved by increasing compressive strength and decreasing the ratio of water-to-cementitious materials. The abrasion–erosion resistance of concrete with cement replacement up to 15% was comparable to that of control concrete without fly ash. Beyond 15% cement replacement, fly ash concrete showed lower resistance to abrasion–erosion compared to non-fly ash concrete. Equations were established based on effective compressive strengths and effective water-to-cementitious materials ratios, which were modified by cement replacement and developed to predict the 28- and 91-day abrasion–erosion resistance of concretes with compressive strengths ranging from approximately 30–100 MPa. The calculation results are compared favorably with the experimental results.     

Influence of soaking and polymerization conditions on the properties of polymer concrete

An experimental investigation was conducted to study the effect of soaking time and polymerization temperature on the mechanical and physical properties of polymer-impregnated concrete. Soaking time was controlled in 4, 8, 12, 16, 20 and 24 h, polymerization temperature was set at 70, 80 and 90 °C for 0.5, 1, 2, 4, 6, 12 and 24 h in impregnation process, respectively. Cylindrical concrete specimens with water/cement ratios of 0.45 and 0.65 were impregnated with methyl methacrylate (MMA) and benzoyl peroxide (BPO) mixtures. The polymer loading increases as immersion time increases until 12 h. Based on compressive strength and surface absorption, optimum polymerization temperature is 70 °C for Mix A (high cement content) and 80 °C for Mix B (low cement content). Polymer impregnation not only increases concrete strength and resistivity but also greatly decreases surface absorption comparing with normal concrete. SEM and MIP observations indicate that the micro-pores and meso-pores of PIC specimens are filled with PMMA and the total pore volume and maximum pore size are reduced significantly.     

Post-fire residual mechanical properties of concrete made with recycled rubber aggregate

This study investigates the effects of elevated temperatures on the residual mechanical performance of concrete produced with recycled rubber aggregate (RRA). Four different concrete compositions were prepared: a reference concrete (RC) made with natural coarse aggregate and three concrete mixes with replacement rates of 5%, 10% and 15% of natural fine and coarse aggregate by RRA from used tyres. Specimens were exposed for a period of 1 h to temperatures of 400 °C, 600 °C and 800 °C, after being heated in accordance with ISO 834 time–temperature curve. After cooling down to ambient temperature, the compressive strength and the splitting tensile strength were evaluated and compared with reference values obtained prior to fire exposure. For the replacement rates used in the present experiments, the obtained results show that concrete made with recycled rubber aggregate (CRRA) present a thermal response that is roughly similar to that of RC; in addition, although residual mechanical properties of CRRA are noticeably more affected than those of RC, particularly for higher exposure temperatures, the relative reduction should not prevent it from being used in structural applications.     

Effect of elevated temperature on the mechanical properties of concrete produced with finely ground pumice and silica fume

This study investigated the effect of elevated temperature on the mechanical and physical properties of concrete specimens obtained by substituting cement with finely ground pumice (FGP) at proportions of 5%, 10%, 15% and 20% by weight. To determine the effect of silica fume (SF) additive on the mechanical and physical properties of concrete containing FGP, SF has been added to all series except for the control specimen, which contained 10% cement by weight instead. The specimens were heated in an electric furnace up to 400, 600 and 800 °C and kept at these temperatures for one hour. After the specimens were cooled in the furnace, ultrasonic pulse velocity (UPV), compressive strength and weight loss values were determined. The results demonstrated that adding the mineral admixtures to concrete decreased both unit weight and compressive strength. Additionally, elevating the temperature above 600 °C affected the compressive strength such that the weight loss of concrete was more pronounced for concrete mixtures containing both FGP and SF. These results were also supported by scanning electron microscope (SEM) studies.     

The effect of high temperature curing on the strength and carbonation of pozzolanic structural lightweight concretes

An experimental study on the compressive strength and carbonation depth of lightweight concrete mixes that contain pulverized fuel ash (PFA) and silica fume (SF) as cement replacements is presented in this paper. Mixes that had a relatively high replacement level of PFA at 25, 40, and 55% and of SF at 5, 10, and 15% by weight were compared. The results indicated that accelerated curing at 60 °C for 3 days improved the 28-day compressive strength of the PFA- and SF-incorporated mixes but resulted in higher carbonation of the mixes compared with that under normal temperature curing. Mixes that had 25% PFA or 5–10% SF as partial cement replacements had slightly higher strength under accelerated curing and slightly lower strength under normal curing than the control mix. At higher replacement levels of PFA and SF, further lower strength and higher carbonation was observed.     

Mechanical properties of normal and high strength concretes subjected to high temperatures and using image analysis to detect bond deteriorations

The effect of high temperatures, up to 250 °C, on mechanical properties of normal and high strength concretes with and without silica fume was investigated, and image analysis was performed on split concrete surfaces to see the change in bond strength between aggregate and mortar. Specimens were heated up to elevated temperatures (50, 100, 150, 200, 250 °C) without loading and then the residual compressive and splitting tensile strength, as well as the static modulus of elasticity of the specimens were determined. For normal strength concrete residual mechanical properties started to decrease at 100 °C, while using silica fume reduced the losses at high temperatures. In terms of percent residual properties, high strength concrete specimens performed better than normal strength concrete specimens for all heating cycles. Image analysis studies on the split surfaces have been utilized to investigate the effect of high temperatures on the bond strength between aggregate and mortar. Image analysis results showed that reduced water–cement ratio and the use of silica fume improved the bond strength at room temperature, and created more stable bonding at elevated temperatures up to 250 °C.     

Strengths and flexural strain of CRC specimens at low temperature

Crumb rubber concrete (CRC) is made by adding rubber crumbs into conventional concrete. This study undertakes an experimental study on the cubic compressive strength, axial compressive strength, flexural strength and splitting tensile strength of CRC specimens at both ambient temperature 20 °C and low temperature ?25 °C. The flexural stress–strain responses were also recorded. The averaged size of rubber crumbs used in the study is about 1.5 mm. Four levels of rubber contents are investigated, which are 0%, 5%, 10% and 15% by volume, respectively. The mix design aimed at 40 MPa of compressive strength and 100 mm of slump for all the CRC specimens. The results show that CRC increases its magnitude in strengths when temperature decreases, which is similar to the case of conventional concrete, but still exhibits ductility in low temperature. The conclusion from this study is that CRC may be more beneficial in its application in low temperature environments than in ambient temperature environments.     

Performance of thermally damaged fibre reinforced concretes

Mechanical characteristics of Fibre Reinforced High Performance Concrete (FR-HPC) subjected to high temperatures were experimentally investigated in this paper. Three different concretes were prepared: a normal strength concrete (NSC) and two High Performance Concretes (HPC1 and HPC2). Fibre reinforced concretes were produced by addition of steel or polypropylene fibres in the above mixtures at dosages of 40 kg/m³ and 5 kg/m³, respectively. A total of nine concrete mixtures were produced and fibres were added in six of them. At the age of 120 days specimens were heated to maximum temperatures of 100, 300, 500 and 700 °C. Specimens were then allowed to cool in the furnace and tested for compressive strength, splitting tensile strength, modulus of elasticity and ultrasonic pulse velocity. Reference tests were also performed at air temperature (20 °C). Residual strength of NSC and HPC1 was reduced almost linearly up to 700 °C and 500 °C, respectively whereas the residual strength of HPC2 was sharply reduced up to 300 °C. Explosive spalling was observed on both HPC. Addition of steel fibres increased the residual strength up to 300 °C, but spalling still occurred in HPC1 and HPC2. Such an explosive behavior was not observed when polypropylene fibres were added in the mixtures; however, in this case the residual mechanical characteristics of all concretes were significantly reduced.     

Influence of calcined kaolin on mortar properties

The use of calcined clay, in the form of metakaolin (MK), as a pozzolanic material for mortar and concrete has received considerable attention in recent years. The present paper describes the results of a research project initiated to study the calcination of local kaolin at various temperatures (650–950 °C) and durations (2, 3 and 4 h) to produce MK with a high pozzolanic activity. The pozzolanic activity was assessed by 28-days compressive strength and hydration heat methods. The maximum identified activity was obtained at 850 °C for 3 h duration. An increase of both hydration heat and compressive strength was obtained when ordinary Portland cement was replaced by 10% MK. The use of ternary blended cement improves the early age and the long-term compressive strength. The durability was also enhanced as better acidic resistance was observed.     

Formulation of blended cement: Effect of process variables on clay pozzolanic activity

Chemical, physical and mineralogical properties of crude and calcined local kaolinitic clay were studied in detail in order to use it as an artificial pozzolan. The aim of this study was to investigate and optimize the properties of mortars in which calcined clay is employed as a pozzolan.A three variable (calcination time: X1, calcination temperature: X2 and % of calcined clay in the blended cement: X3) rotatable orthogonal composite design was set up. It was concluded that the compressive strengths were governed by the calcination temperature and the percentage of the calcined clay in the blended cement. It was proven that the strengths could be improved by increasing simultaneously the percentage of incorporation and the calcination temperature of the clay. It was also demonstrated that at temperatures lower than 700 °C, the increase of the calcination time, improved the compressive strength, while above 700 °C, the opposite effect was observed. Finally, a blended cement composition has been formulated and optimized using the desirability functions. The optimized blended cement contains 25% of calcined clay, heated for 3 h at a temperature of 750 °C.     

Beam test on bond behavior between high-grade rebar and high-strength concrete after elevated temperatures

In this paper, post-heating bond behavior between high-grade rebar and C80 high-strength concrete (hereafter, HSC) is studied. The high-grade rebar is HRBF500 fine grained steel with a yield strength of 500 MPa and the concrete grade C80 denotes compressive strength not lower than 80 MPa. First, the residual mechanical behavior of both high-grade rebar and HSC were tested after fire exposure. Second, the beam bond test was carried out to study the bond behavior between high-grade rebar and HSC after exposed heating at 200 °C, 400 °C, 500 °C and 600 °C, respectively. During the bond test, the influence of temperature, bond length, and some construction measurements on the bond–slip behavior were compared and evaluated. The investigation demonstrates that (1) the bond strength between high-grade rebar and HSC decreases while the peak slip increases with the elevated temperature, especially when the temperature exceeds 400 °C and (2) the confinement effect of steel wire mesh can help to improve rebar׳s bond behavior. Third, the bond–slip model between high-grade rebar and HSC for post-heating is proposed.     

Multiaxial tensile–compressive strengths and failure criterion of plain high-performance concrete before and after high temperatures

Multiaxial tensile–compressive tests were performed on 100 mm × 100 mm × 100 mm cubic specimens of plain high-performance concrete (HPC) at all kinds of stress ratios after exposure to normal and high temperatures of 20, 200, 300, 400, 500, and 600 °C, using a large static–dynamic true triaxial machine. Friction-reducing pads were three layers of plastic membrane with glycerine in-between for the compressive loading plane; the tensile loading planes of concrete samples were processed by attrition machine, and then the samples were glued-up with the loading plate with structural glue. The failure mode characteristic of specimens and the direction of the crack were observed and described, respectively. The three principally static strengths in the corresponding stress state were measured. The influence of the temperatures, stress ratios, and stress states on the triaxial strengths of HPC after exposure to high temperatures were also analyzed respectively. The experimental results showed that the uniaxial compressive strength of plain HPC after exposure to high temperatures does not decrease completely with the increase in temperature, the ratios of the triaxial to its uniaxial compressive strength depend on brittleness–stiffness of HPC after different high temperatures besides the stress states and stress ratios. On this basis, the formula of a new failure criterion with the temperature parameters under multiaxial tensile–compressive stress states for plain HPC is proposed. This study is helpful to reveal the multiaxial mechanical properties of HPC structure enduring high temperatures, and provides the experimental and theory foundations (testing data and correlated formula) for fire-resistant structural design, and for structural safety assessment and maintenance after fire.     

Thaumasite form of sulfate attack in limestone cement mortars: A study on long term efficiency of mineral admixtures

Concrete and mortar made from limestone cement may exhibit a lack of durability due to the formation of thaumasite. The addition of minerals that improve the concrete durability is expected to slow down the formation of thaumasite. In this work the effect of natural pozzolana, fly ash, ground granulated blastfurnace slag and metakaolin on the thaumasite formation in limestone cement mortar is examined. A limestone cement, containing 15% w/w limestone, was used. Mortar specimens were prepared by replacing a part of limestone cement with the above minerals. The specimens were immersed in a 1.8% MgSO₄ solution and cured at 5 and 25 °C. The status of the samples after a storage period of 5 years was reported based on visual inspection, compressive strength, mass measurements, ultrasonic pulse velocity measurements and analytical techniques. It is concluded that the use of specific minerals, as partial replacement of cement, inhibits thaumasite formation in limestone cement mortar.     

Effects of calcination temperature of kaolinite clays on the properties of geopolymer cements

The aim of this work is to determine the most convenient calcination temperature of kaolinite clays in view of producing geopolymer cements. In this light, the clay fractions of three kaolin minerals were used. The clay fractions were characterized (chemical and thermal analyses and X-ray diffraction) and then calcined in the temperature range of 450 and 800 °C. The obtained amorphous materials were dissolved in a strongly alkaline solution in order to produce geopolymer cements whose pastes were characterized by determining their setting time, linear shrinkage and compressive strength. Hardened geopolymer cement paste samples were also submitted to X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy analyses. The setting time of geopolymer cement pastes produced from the clay fractions calcined at 450 °C was very long (test samples could be handled easily only after 21 days at the ambient atmosphere of the laboratory). For the clay fractions calcined between 500 and 700 °C, the setting time of geopolymer cement pastes reduced with increasing temperature and varied between 130 and 40 min. Above 700 °C, the setting time began to increase. The linear shrinkage of the hardened geopolymer cement paste samples aged between 21 and 28 days attained its lowest value around 700 °C. Above 700 °C, the linear shrinkage began to increase. The compressive strength of the hardened geopolymer cement paste samples was between 11.9 and 36.4 MPa: it increased with samples from the clay fractions calcined between 500 and 700 °C but dropped above 700 °C.It can be concluded that the most convenient temperature for the calcination of kaolinite clays in view of producing geopolymer cements is around 700 °C.     

Effects of thermal cycles on mechanical properties of an optimized polymer concrete

The aim of this study is the design, fabrication and experimentally characterization of an optimized polymer concrete (PC). To this end, three factors, namely: the aggregate size, epoxy resin weight percentage, and chopped glass fiber percentage; are considered as the influencing factors on the compressive strength, bending strengths and interfacial shear strength between the PC and steel. The number of tests which are necessary to simultaneously optimize three above strengths of the PC are reduced based on the design of experiment using the orthogonal array technique or so-called Taguchi method. Comparison of the predicted strengths based on the Taguchi approach with the measured experimental results shows a good correlation between them. Afterward, the effect of three freeze/thaw thermal cycles; 25 °C to ?30 °C (cycle-A), 25 °C to 70 °C (cycle-B) and ?30 °C to 70 °C (cycle-C) for 7 days; on the strengths of the optimized PC is experimentally investigated. Comparison of the experimental results for the mechanical strengths measured at room temperature (RT) and above thermal cycles shows that the compressive strength of the optimally designed PC is not affected by heating and cooling cycles. On the other hand, the bending strength is more affected by exposing PC to the thermal cycle-B. The interfacial shear strength becomes affected by exposing the PC to cycles-A and -B, whereas no changes are observed on this strength by exposing to the thermal cycle-C. In general, among the three thermal cycles, cycle-B exerted the most deteriorating effect on the strengths.     

Influence of fly ash on strength and sorption characteristics of cold-bonded fly ash aggregate concrete

Cold-bonded fly ash aggregate concrete with fly ash as part of binder or fine aggregate facilitates high volume utilization of fly ash in concrete with minimum energy consumption. This paper investigates the influence of fly ash on strength and sorption behaviour of cold-bonded fly ash aggregate concrete due to partial replacement of cement and also as replacement material for sand. While cement replacement must be restricted based on the compressive strength requirement at desired age, replacement of sand with fly ash appears to be advantageous from early days onwards with higher enhancement in strength and higher utilization of fly ash in mixes of lower cement content. Microstructure of concrete was examined under BSEI mode. Replacement of sand with fly ash is effective in reducing water absorption and sorptivity attributable to the densification of both matrix and matrix–aggregate interfacial bond. Cold-bonded fly ash aggregate concrete with a cement content of 250 kg/m³, results in compressive strength of about 45 MPa, with a total inclusion of around 0.6 m³ of fly ash in unit volume of concrete.     

Production of low cost self compacting concrete using bagasse ash

Self compacting concrete (SCC) is a development of conventional concrete, in which the use of vibrator for compaction is no more required. This property of self compacting concrete has made its use more attractive all over the world. But its initial higher supply cost over conventional concrete, has hindered its application to general construction. Therefore, for producing low cost SCC, it is prudent to look at the alternates to help reducing the SSC cost. This research is aimed at evaluating the usage of bagasse ash as viscosity modifying agent in SCC, and to study the relative costs of the materials used in SCC.In this research, the main variables are the proportion of bagasse ash, dosage of superplasticizer for flowability and water/binder ratio. The parameters kept constant are the amount of cement and water content.Test results substantiate the feasibility to develop low cost self compacting concrete using bagasse ash. In the fresh state of concrete, the different mixes of concrete have slump flow in the range of 333 mm to 815 mm, L-box ratio ranging from 0 to 1 and flow time ranging from 1.8 s to no flow (stucked). Out of twenty five different mixes, five mixes were found to satisfy the requirements suggested by European federation of national trade associations representing producers and applicators of specialist building products (EFNARC) guide for making self compacting concrete. The compressive strengths developed by the self compacting concrete mixes with bagasse ash at 28 days were comparable to the control concrete. Cost analysis showed that the cost of ingredients of specific self compacting concrete mix is 35.63% less than that of control concrete, both having compressive strength above 34 MPa.