Fibre-reinforced cemented soils compressive and tensile strength assessment as a function of filament length

This study aims to develop a dosage methodology based on tensile and compressive strength for artificially cemented fibre reinforced soils considering filament length. The controlling parameters evaluated were the fibre length (l), the cement content (C), the porosity (η) and the porosity/cement ratio (η/C_iv). A number of unconfined compression and split tensile tests were carried out in the present work. The results show that fibre insertion in the cemented soil, for the whole range of cement content studied, and the increase of reinforcement length improve unconfined compressive and split tensile strengths. It was shown that the porosity/cement ratio, in which volumetric cementitious material content is adjusted by an exponent (0.28 for all the fibre-reinforced and non-reinforced cemented soil mixtures) to end in unique correlations for each mixture, is a good parameter in the evaluation of the unconfined compressive and split tensile strength of the fibre-reinforced and non-reinforced cemented soil studied. Analysis of variance (ANOVA) performed on the results of a factorial experiment considering the effect of adjusted cement content, fibre length and porosity showed that all of these factors are significant in affecting the measured changes in split tensile and unconfined compressive strength values. Finally, unique dosage relationships could be achieved linking the unconfined compressive strength (q_u) and the split tensile strength (q_t) of the sandy soil studied with porosity/cement ratio (η/C_iv) and fibre length (l).     

Optimizing the evolution of strength for lime-stabilized rammed soil

In the present study,unconfined compressive strength(q_u)values of two lime-treated soils(soil 1 and 2)with curing times of 28 d,90 d and 360 d were optimized.The influence of void/lime ratio was represented by the porosity/volumetric lime content ratio(η/L_(iv))as the main parameter.η/L_(iv) represents the volume of void influenced by compaction effort and lime volume.The evolution of qu was analyzed for each soil using the coefficient of determination as the optimization parameter.Aiming at providing adjustments to the mechanical resistance values,the η/L_(iv) parameter was modified to η/L_(iv)~C using the adjustment exponent C(to make q_u-η/L_(iv) variation rates compatible).The results show that with the decrease of η/L_(iv)~C.qu increases potentially and the optimized values of C were 0.14-0.18.The mechanical resistance data show similar trends between q_u and η/L_(iv)~C for the studied silty soil-ground lime mixtures,which were cured at ambient temperature(23±2)℃ with different curing times of 28—360 d.Finally,optimized equations were presented using the normalized strengths and the proposed optimization model,which show 6% error and 95% acceptability on average.     

Effects of lime addition on geotechnical properties of sedimentary soil in Curitiba,Brazil

The soil of the Guabirotuba geological formation(Paraná Basin, Brazil) has physico-mechanical properties which are not suitable for its utilization in pavement construction, in protection of hillsides and slopes, or as shallow foundation support. Treatment of this soil by lime addition would improve its usability. The present context intends to determine the ratio between the splitting tensile strength(q_t)and the unconfined compressive strength(q_u) of clayey soil in the metropolitan region of Curitiba City,which has been treated with different lime contents and curing times. The control parameters evaluated include lime content(L), curing time(t), moisture content(w), and ratio of porosity to volumetric lime content(η/L_v). It was observed that the q_t/q_u ratio is between 0.17 and 0.2 in relation to the curing time,and an exponential relation exists between them. Meanwhile, the unconfined compressive strength of lime-treated soil was found to be approximately four times the initial value.     

Durability against wetting-drying cycles for cement-stabilized reclaimed asphalt pavement blended with crushed rock

Pavement rehabilitation and reconstruction generate large quantities of reclaimed asphalt pavement (RAP). The improvement of the engineering properties of this RAP is required in order to enable it for use as environmentally friendly alternative construction material in road pavements. The durability of RAP when blended with crushed rock (CR) and stabilized with Portland cement was investigated in this paper. The CR replacement was found to improve the compactibility and durability of the stabilized RAP/CR material. For a particular RAP:CR ratio, the compaction curves of cement-stabilized RAP/CR blends were found to be essentially the same for all cement contents, but different for unstabilized blends; i.e., the maximum dry unit weight of cement-stabilized RAP/CR blends is higher than that of unstabilized RAP-CR blends. The wetting-drying (w-d) cycles led to a loss in weight of the cement-stabilized RCA/CR blends and to a subsequent reduction in strength. The w-d cycle strengths (q_u(w-d)) for a state of compaction (dry side, wet side or optimum water content) at any w-d cycle could be approximated from the corresponding initial soaked strength (prior to w-d tests) (q_u0). The q_u0 of cement-stabilized RAP/CR blends increased with an increasing CR replacement and an increasing cement content. Assuming that the CR replacement also results in an increasing cement content, w/[C(1?+?kCR_c)] was proposed as a critical parameter for developing q_u0 and q_u(w-d) predictive equations where w is the water content at the optimum water content, C is the cement content, k is the replacement efficiency, and CR_c is the CR content. Based on the q_u(w-d) predictive equation developed here, a design procedure for the laboratory mixing of cement-stabilized RAP/CR blends was proposed, which would be valuable for an accurate determination of the ingredients (RAP:CR ratio and cement content) required to attain the necessary strength at the design service life.     

The effects of curing time and temperature on stiffness,strength and durability of sand-environment friendly binder blends

Agricultural-industrial wastes, like rice-husk ash (RHA) and carbide lime (CL), have great potential applications in such earthworks as the stabilization of slopes and pavement layers and the spread footings and bed of pipelines, particularly in the regions near where the waste is produced. Present research evaluates the potential use of RHA mixed with CL as a binder, improving strength, stiffness and durability properties of a uniform sand. Two different curing temperatures, 23 °C and 40 °C, and curing periods, 7 and 28 days, of compacted sand-RHA-CL blends (distinct dry unit weights and contents of RHA and CL) were evaluated to determine the importance of these changes on the reactions between the materials. The experimental program aims to assess the following parameters: initial shear modulus (G₀), unconfined compressive strength (q_u), and accumulated loss of mass (ALM). Studies have been carried out to quantify these parameters as a function of a novel index called porosity/volumetric binder content (η/B_iv). The results showed higher values of G₀ and q_u, as well as a small rate of ALM with reduction of porosity and with rise of the environment friendly binder content. The latter is achieved either by increasing eith the RHA or the CL content. The curing temperature acts as a catalyser, accelerating the pozzolanic reactions between RHA and CL. Longer curing periods also benefit reactions between materials by enhancing their geotechnical properties. An analysis of variance (ANOVA) was carried and the results showed the dry unit weight, RHA content and curing type are significantly effect the strength results. It was also possible to verify that curing for 28 days at 23 °C and for 7 days at 40 °C are statistically equivalent in terms of strength. The G₀ results after weathering cycles tended to decrease in specimens at a 40 °C curing temperature and increase in specimens at a 23 °C curing temperature.     

Modelling tensile/compressive strength ratio of artificially cemented clean sand

The present work proposes a new theoretical model for predicting both the splitting tensile strength (q_t) and the compressive strength (q_u) of artificially cemented sand and assesses their ratio for a given material. The proposed model is based on the concept of the superposition of the failure strength contributions of the sand and cement phases. The sand matrix obeys the concept of critical state soil mechanics, while the strength of the cemented phase can be described using the Drucker-Prager failure criterion. The analytical solutions are compared against the results of tests on three different types of cemented clean sand cured for different time periods. While the analytical relation fits the experimental data well, it also provides a theoretical basis for the explanation of some features related to the experimentally derived strength relationships for cemented clean sand. The value of the power relationship between the strength and the porosity/cement ratio index seems to be governed by the soil matrix properties, while the interdependency of the strength and the curing time can also be captured. For a given cemented sand, the model equally confirms the existence of a unique tensile/compressive strength ratio (q_t/q_u), independent of the curing time and primarily governed by the compressive to tensile strength ratio (or the friction properties) of the cement. It is also confirmed that the q_t/q_u ratio changes within a narrow range for different frictional properties of the cementing phase.     

An Improved Method for Estimating In-Situ Undrained Shear Strength of Natural Deposits

An equation for estimating in-situ undrained shear strength (q_u(I)) of natural deposits is derived as q_u(I)/2c_u(I)=1.0-0.285 ln p_m/S₀ through the unconfined compression test (UCT) and K₀ consolidated-undrained triaxial compression test (CK₀UC). The q_u(I) of natural clay deposits can be estimated from the q_u value multiplied by the reciprocal number of q_u(I)/2c_u(I) of the equation using the suction (S₀) and q_u obtained from UCT for a specimen, where c_u(I) is in-situ shear strength measured from CK₀UC and p_m is two times the effective overburden pressure divided by three. The q_u(I)/2c_u(I) values were unrelated to I_p, q_u and p_m/S₀ and the mean value of these ratios was 0.98 in the range of I_p=26~110 and q_u=12~178 kPa. The mean values of the ratios for q_u, q*_u(I) and q_u(I) to 2c_u(I) were 0.629, 0.998 and 0.977 and the standard deviation of those ratios were 0.14, 0.10 and 0.16, respectively. Therefore, it can be seen that the improved method is appropriate as well as Shogaki's basic method (q*_u(I)). The mean values of q_u(I)/2c_u(I) were 0.94, 0.99 and 0.91 for Iwai organic and soft clay plus Kahokugata clay, respectively. The coefficient of variations of the q_u and q_u(I) values were (13~14)% and unrelated to soils, q_u or q_u(I) values. Therefore, the applicability of the improved method newly developed in this study can be confirmed for Kahokugata and Iwai clays as well as Iwai organic soils. The proposed method is a simple and easy one for practical engineering usage.     

Hydration of coal–biomass fly ash cement

This paper presents possibilities of use of fly ashes from co-combustion bituminous coal and biomass in cement production process. Both fly ashes coming from co-combustion bituminous coal and biomass and the ones from bituminous coal combustion were analysed. The following properties of cement were tested: heat of hydration, Ca(OH)₂ content, unreacted C₃S content and microstructure. The results showed that fly ashes from co-combustion coal and biomass retard cement hydration. Cement samples containing coal-biomass fly ashes demonstrate adverse features like lower heat of hydration, higher Ca(OH)₂ content and lower rate of C₃S hydration in comparison to the ones containing fly ashes from bituminous coal. The incorporation of coal-biomass fly ashes in cement results in an increase of porosity of cement paste, leading to a microstructure of lower density.     

Strength Development in Cement Stabilized Low Plasticity and Coarse Grained Soils: Laboratory and Field Study

Laboratory and field strength development of cement stabilized coarse-grained soils are studied in this paper. A phenomenological model to assess the laboratory strength development is developed. The model is divided into the dry and the wet sides of optimum water content. At the optimum and on the wet side of optimum, the strength development in cement stabilized soils at a particular curing time is dependent only upon the soil-water/cement ratio, w/C, which can reflect the combined effects of water content and cement content. It is moreover premised that the relationship between strength and water content is symmetrical around the optimum water content (OWC) in the range of 0.8 to 1.2 times the OWC. The proposed model is useful for assessing the strength development wherein water content, cement content and compaction energy vary over a wide range. Only the test result of a single laboratory trial is needed. From the field study, it is found that the field roller-compacted strength, q_ufr is lower than the laboratory strength, q_ul under the same dry unit weight, soil-water/cement ratio and curing time due to several field factors. The ratio q_ufr/q_ul varies from 50 to 100%. Non-uniformity in mixing soil with cement is realized by the ratio of field hand-compacted strength to laboratory strength, q_ufh/q_ul ranging from 0.75 to 1.2. For most data, the field roller-compacted strength is 55 to 100% the field hand-compacted strength. This might be caused by the difference in compaction method and curing condition between laboratory and field stabilization. From this field observation and the proposed model, a practical procedure for repairing damaged roads using the pavement recycling technique is introduced. The procedure consists of the determination of cement content, the execution of the field stabilization and the examination of the field strength. It can save on sampling and laboratory testing and hence cost.     

Assessment of strength development in blended cement admixed Bangkok clay

Fly ash and biomass ash have been widely accepted as waste materials substituting Portland cement. In this paper, the role of these two ashes on the strength development of cement admixed low-swelling Bangkok clay is investigated via unconfined compressive (UC) test and thermal gravity (TG) analysis. Fly ash and biomass ash are dispersing materials, increasing the reactive surface of the cement grains. The pozzolanic reaction does not play any significant role on the strength development with time since the amount of Ca(OH)₂ is insufficient to react with the ashes. The contribution of the dispersing effect to the strength development is regarded akin as an addition of cement. Based on this premise, the clay–water/cement ratio hypothesis for blended cement admixed clay is proposed for analyzing and assessing the strength development. Even with the difference in water content, cement content and ash content, the blended cement admixed clay samples having the same clay–water/cement ratio, w_c/C possess practically the same stress–strain response and strength. The relationship among strength, clay–water/cement ratio, and curing time for the blended cement admixed Bangkok clay is finally developed and verified. It is useful to assess the strength at any curing time wherein water content, cement content, and ash content vary over a wide range by using the test result of a single laboratory trial. For the economic mix design (the most effective dispersing effect), an addition of 25% ash is recommended. It can save on the input of cement up to 15.8%.     

Life-cycle cost analysis of highway noise barriers designed with photocatalytic cement

This study examines the environmental and economic feasibility of concrete noise barriers containing photocatalytic cement using a life-cycle cost analysis (LCCA). Photocatalytic concrete contains titanium dioxide (TiO₂) which allows for the oxidation of air pollutants to occur on the surface of the building material. Design variables studied include the cementing material type (general use (GU) cement, ground granulated blast furnace slag (GGBFS) used as cement replacement, and photocatalytic (PCAT) cement), and the thickness of a photocatalytic concrete cover. The LCCA accounts for the CO₂ and NO_x generated during manufacturing and the NO_x (NO, NO₂) oxidised during the life of barriers containing photocatalytic concrete. A key outcome from this study revealed that at a 40-year service life, assuming a 6 mg/h/m² NO_x degradation rate, a barrier designed with 100%GU cement and a 25 mm photocatalytic concrete cover has an annual cost that is 7%, 30%, and 36% greater than the 100%GU, 35% and 50%GGBFS barriers without a photocatalytic cover, respectively. Results of this analysis also indicated that the application of a 25 mm photocatalytic concrete cover to concrete containing 35 and 50%GGBFS is more economically feasible than 100%GU concrete, irrespective of the service life and pollution degradation rate.     

Strength Development in Cement Admixed Bangkok Clay: Laboratory and Field Investigations

The in-situ deep mixing technique has been established as an effective means to effect columnar inclusions into soft Bangkok clay to enhance bearing capacity and reduce settlement. In this paper, an attempt is made to identify the critical factors governing the strength development in cement admixed Bangkok clay in both the laboratory and the field. It is found that clay-water/cement ratio, w_c/C is the prime parameter controlling the laboratory strength development when the liquidity index varies between 1 and 2. Based on this parameter and Abrams' law, the strength prediction equation for various curing times and combinations of clay water content and cement content is proposed and verified. This will help minimize the number of trials necessary to arrive at the quantity of cement to be admixed. Besides the w_c/C, the strength of deep mixing column is controlled by the execution and curing conditions. For low strength improvement (laboratory 28-day strength less than 1,500 kPa), the field strength of the deep mixing columns, q_uf, made up from both dry and wet mixing methods is higher than 0.6 times the laboratory strength, q_ul. The q_uf/q_ul ratios for the wet mixing columns are generally higher than those for the dry mixing columns. This higher strength ratio is due to the dissipation of the excess water in the column (consolidation) caused by the field stress. The water to cement ratio, W/C, of 1.0 is recommended for the wet mixing method of the soft Bangkok clay. A fast installation rate was shown to provide high quality for low strength columns. Suggestions are made for improving the deep mixing of soft Bangkok clay, which are very useful both from economic and engineering viewpoints.     

Embodied energy in cement stabilised rammed earth walls

Rammed earth walls are low carbon emission and energy efficient alternatives to load bearing walls. Large numbers of rammed earth buildings have been constructed in the recent past across the globe. This paper is focused on embodied energy in cement stabilised rammed earth (CSRE) walls. Influence of soil grading, density and cement content on compaction energy input has been monitored. A comparison between energy content of cement and energy in transportation of materials, with that of the actual energy input during rammed earth compaction in the actual field conditions and the laboratory has been made. Major conclusions of the investigations are (a) compaction energy increases with increase in clay fraction of the soil mix and it is sensitive to density of the CSRE wall, (b) compaction energy varies between 0.033 MJ/m³ and 0.36 MJ/m³ for the range of densities and cement contents attempted, (c) energy expenditure in the compaction process is negligible when compared to energy content of the cement and (d) total embodied energy in CSRE walls increases linearly with the increase in cement content and is in the range of 0.4-0.5 GJ/m³ for cement content in the rage of 6-8%.     

Modelling tensile/compressive strength ratio of fibre reinforced cemented soils

The present work proposes a theoretical model for predicting the splitting tensile strength (q_t) - unconfined compressive strength (q_u) ratio of artificially cemented fibre reinforced soils. The proposed developments are based on the concept of superposition of failure strength contributions of the soil, cement and fibres phases. The soil matrix obeys the critical state soil mechanics concept, while the strength of the cemented phase can be described using the Drucker-Prager failure criterion and fibres contribution to strength is related to the composite deformation. The proposed developments are challenged to simulate the experimental results for fibre reinforced cemented Botucatu residual soil, for 7 days of cure. While the proposed analytical relation fits well the experimental data for this material, it also provides a theoretical explanation for some features of the experimentally derived strength relationships for artificially fibre reinforced cemented clean sands. A parametric study to analyse the effect of adding different fibre contents and fibre properties is provided. The proposed modelling developments also confirm the existence of a rather constant q_t/q_u ratio with moulding density, cement and fibre contents .     

Effect of fiber-reinforcement on the strength of cemented soils

This study aims to verify the differences in the strength of an artificially cemented sandy soil with and without fiber reinforcement. The controlling parameters evaluated were the amount of cement, porosity, moisture content, and voids/cement ratio. A series of unconfined compression tests and suction measures were carried out. The results show that fiber insertion in the cemented soil, for the whole range of cement studied, causes an increase in unconfined compression strength. The UCS increased linearly with the amount of cement and reduced with the increase in porosity (η) for both the fiber-reinforced and unreinforced specimens. A power function fits well as the relation between unconfined compressive strength (UCS) and porosity (η). Finally, it was shown that the voids/cement ratio is a good parameter in the evaluation of the unconfined compressive strength of the fiber-reinforced and unreinforced cemented soil studied.     

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.     

Physicochemical and consolidation properties of compacted lateritic soil treated with cement

The consolidation of a fine-grained lateritic soil, treated with compound Portland cement (CEMII/BM 32.5 N) up to 9% by weight of the dry soil and prepared at three different molding water contents (ω_DRY, OMC, and ω_WET), was investigated by means of a one-dimensional consolidation test. The physicochemical and microstructural properties of the compacted lateritic soil-cement mixture were investigated using Raman spectroscopy, polarized light microscopy (PLM), scanning electron microscopy (SEM), and pH measurement. The results show that cement admixtures resulted in the formation of tobermorite, afwillite, ettringite, portlandite, and calcite. However, tobermorite and afwilite, which are calcium silicate hydrates (CSH) whose mechanisms of formation are the pozzolanic and alkali silica reactions, appear from 6% added cement. The fixing point of the pH (12.4) is also obtained from 6% added cement. It is the threshold value at which the material begins to develop an adequate mechanical performance. In general, as the content of cement in the soil is increased, the yield stress increases from 1 to 3 times in comparison to untreated soil. For effective vertical stresses smaller than the cement-induced yield stress, the primary consolidation process for specimens treated with cement is 2–7 times faster than that for specimens not treated with cement, while for effective vertical stresses higher than the cement-induced yield stress, the primary consolidation process for specimens treated with cement is about 0.5–1.5 times faster than that for specimens not treated with cement. Permeability and secondary compression are reduced 1–9 times and 2–11 times that of the untreated samples, respectively. These changes are attributed to the creation of chemical bonds and aggregation that accompany the addition of cement. The results also show that it would be desirable for soil samples to be prepared at the dry side of optimum (ω_DRY) when the optimum moisture content (OMC) is not reached at the site. These results indicate that significant and desirable changes in soil behavior can be achieved when the soil is admixed with CEM II/BM 32.5 N cement, thus providing the possibility of using the tested lateritic soil in road construction.     

Electrochemical disinfection of Escherichia coli in the presence and absence of primary sludge particulates

Electrochemical (EC) residual disinfection of Escherichia coli (E. coli) in the presence and absence of primary sludge particulates (PSPs) was studied. The kinetics followed a first-order rate law. When PSPs were absent, the EC residual disinfection rate coefficient (k) increased linearly with EC pretreatment energy (E_C, 0–0.63 kWh/m³). However, with 143 mg PSPs/L, k first increased linearly with E_C (0–0.28 kWh/m³) and then decreased linearly with E_C (0.28–0.42 kWh/m³). H₂O₂ was detected during EC pretreatment in PSPs-free samples and the H₂O₂ concentration (C_H) increased with E_C (0–0.83 kWh/m³) linearly. Chloride was detected in PSPs aqueous samples (143 mg PSPs/L) and its concentration (C_C) changed during EC pretreatment: initially, a decrease of C_C was observed when E_C increased from 0 to 0.28 kWh/m³, followed by an increase of C_C when E_C increased 0.28–0.42 kWh/m³. In both cases, k correlated to the initial post-EC chloride concentration (C_CI) in an inverse linear relationship. This two-stage change of C_C and k was caused by a combination of two reactions: anodic oxidation of chloride and the reaction of chloramines with excess chlorine. This paper explains the mechanisms underlying EC residual disinfection in the presence and absence of PSPs, and proposes a feasible strategy for EC disinfection when PSPs are present, an approach that could be useful in the treatment of combined sewage overflow (CSO).     

Coarse blast furnace slag as a cementitious material,comparative study as a partial replacement of Portland cement and as an alkali activated cement

The cementitious performance of a coarse granulated blast furnace slag, 2900 cm²/g, was investigated in concretes of 230, 280 and 330 kg binder/m³. First, the slag partially replaced 30%, 50% and 70% of Portland cement, the strength reduced as the amount of slag increased; however, for high binder contents, similar strengths were attained for lower Portland cement contents. Second, the slag was alkali activated with sodium silicate (moduli 1.7 and 2) at 4%, 6% and 8% %Na₂O, the strength increased with the amount of slag in the concrete and developed faster as %Na₂O increased. The microstructures of both type of concretes were dense; however, the strengths of activated slag were superior at similar binder loads, indicating that the hydration products of activated slag are of higher intrinsic strength.