Vivek Bindiganavile

Thermal characterisation of a lightweight mortar containing expanded perlite for underground insulation

This paper aims to investigate the use of expanded perlite in mortar, for further application of shotcrete to thermal insulation of underground mines. Mixes were designed according to the typical proportions of underground shotcrete, with the sand volumetrically substituted by expanded perlite. Tests of samples were conducted at four ages. Transient plane source technique was utilised to measure the thermal properties. The results showed reduced weight, decreased thermal conductivity, deteriorated thermal diffusivity, and sacrificed mechanical strength with perlite addition. Experimental data analysis and explanation in this paper would establish useful fundamentals for further application of expanded perlite to underground shotcrete.

Drop-Weight Impact Response of Fibre Reinforced Cement Based Foams

This paper describes the strength and toughness of fibre reinforced cement based foams subjected to bending under variable strain rates. Drop-weight impact tests were conducted on notched beams with cast density between 475 kg/m3 to 1200 kg/m3. The foams were reinforced with 0–0.2% polypropylene microfibre by volume and their role in improving the dynamic properties is illustrated. The structure of the cell wall within the foams was engineered through a suitable choice of foaming agent, such that foams with a predominantly closed cell structure were produced. Companion tests were conducted as per ASTM standards in compression and flexure under quasi-static loading. The results were compared to other brittle cellular solids from the literature, and the mechanical properties are described in terms of the relative density, defined as the ratio of the density of the cellular composite relative to that of Portland cement paste in the cell wall. The study shows that under quasi-static loading, the compressive strength, elastic modulus and modulus of rupture of plain mixes scale with the square of the relative density. On the other hand, the flexural toughness factor scaled linearly with it. Fibres were seen to increase the flexural strength at all rates of loading, regardless of cast density. However, the Poissons ratio remained unaffected by the density of the mix. Further, cement based foams were seen to be strain rate sensitive but, the existing CEB-FIP model for cementitious materials vastly overestimates the dynamic impact factor at lower cast densities.

Thermal Conductivity of Hydrated Paste in Cement-Based Foam Microstructure

This article presents the thermal constants of hydrated cement paste, which constitute the air-void wall of cement-based foam microstructure, and examines its influence on moisture, age, pozzolanic admixture type, and content. Along with the reference mix, containing portland cement only, six other mixes were prepared by replacing cement with fly ash, silica fume, and metakaolin, up to 20 % by mass in the binder. The Transient Plane Source, a thermal analyzer that conforms to ISO test standards, was employed to evaluate the thermal constants. Here, the measurements were made at 60, 120, 210, and 300 days. The results revealed that the drop in thermal conductivity for the cement paste was significant in the earlier age but progressively became insignificant as the hydration advanced. It was also found that adding fly ash and silica fume in higher dosages resulted in reducing the conductivity, while metakaolin exhibits a reverse trend. Furthermore, the reduction in the moisture content due to pozzolanic admixture was also noticed. Finally, an empirical formulation was developed to predict the thermal conductivity of the hydrated cement paste, which was also validated against the results of past research.

Specimen Size Effects and Dynamic Fracture Toughness of Cement-Based Foams

AbstractThis paper presents the effect of geometrically similar specimen size on the mechanical properties of cement-based foams. The experimental study also investigates fracture-mechanical parameters under flexural impact. A preformed foam was employed to prepare the specimens using a synthetic foaming agent that was known to achieve a stable bubble structure. The cement-based foams were prepared to a cast density of 475  kg/m3 and were examined under compression and flexure. Along with a plain unreinforced mix, polypropylene fiber at a volumetric fraction of 0.2% was used to study the effect of microfiber reinforcement. The tested specimens scaled between one to four times in dimension. Parameters associated with their dynamic-fracture mechanics were evaluated under flexure with the help of a drop-weight impact tester equipped with a high speed imaging system. Bažant’s size effect model and the multifractal scaling model were employed to describe the size effect. It was seen that when subjected to comp...

Cylindrical models of heat flow and thermo-elastic stresses in underground tunnels

Purpose The trapped geothermal heat in the infinite rock mass through which mine tunnels are excavated is a great threat to the safety of personnel and mine operating equipment in deep underground hot mines. In order to lessen the temperature inside the tunnel a considerable amount of energy is being spent by the way of using ventilation and cooling systems to dissipate the heat. However, operational costs of the system rise quite considerably, especially as the mines get deeper. Shotcrete is used both as a structural lining and as an effective insulation to reduce the heat load on the ventilation and cooling system within such tunnels. The paper aims to discuss these issues. Design/methodology/approach In order to analyse this problem of heat flow and thermal stresses and their time dependent pattern, several cylindrical models, in both analytical and numerical forms, are discussed and compared in this paper. Findings This study shows the validation of ABAQUS® software to predict the time dependent temperature and the thermal stresses in mine tunnels through the comparisons with the available analytical models. Further, thermal insulation effects of shotcrete are also evaluated with these theoretical models and it is found that all the models gave results in close agreements with one another. Originality/value Therefore, this study provides the theoretical proof for advantages in applying shotcrete as the thermal insulation layer in underground mines.

Effect of Pozzolanic Admixtures on the Fresh Properties of Cement-Based Foam

This experimental study was conducted to investigate the influence of pozzolanic admixtures on the fresh properties of cement-based foams. The cement-based foam mixes were tested at three different cast densities namely, 800, 600, and 400 kg/m3. Along with a reference mix, other series were prepared in which fly ash, silica fume, and metakaolin were added to the binder at up to 10 % and 20 % replacement by cement mass. The Marsh cone test and the flow cone test techniques were employed to measure the flowability and spreadability for 21 cement-based foam mixes. The results show that the addition of pozzolanic admixtures increases the flow time and the longest time recorded with metakaolin. A linear relationship of spread with the density of the mixes was found in this study. It was also found that with the addition of fly ash and silica fume, there was an increase in the demand for foam content. On the other hand, adding metakaolin reduced this demand. Based on the experimental results, an equation to predict the spreadability of the mixes with pozzolanic admixtures has been suggested.

Process dependence of shotcrete for repairs

In the study presented here, the authors illustrate that the properties of steel fibre reinforced shotcrete are a function of the manufacturing process. The paper examines the causes for the higher penetration resistance in dry-mix shotcrete, while highlighting the differences in composition and rebound during the fresh state. A strong influence of the spray process is witnessed on the hardened properties under both compression and flexure. For an identical fibre content, the toughness of shotcrete is noticeably different from that of the corresponding cast concrete. It is seen that the optimal post-crack performance is achieved via the wet process technique.

Evaluating Sulphate Resistance of Cement-Based Systems by Sulphate Content Determination after Exposure

This paper describes approaches to evaluating the resistance of cement-based composites to sulphate attack. The conventional approach of evaluation by means of measuring expansion is discussed in comparison with the sulphate diffusion, which was quantified as a function of depth. Besides CSA Types GU and HS, a 30:70 blend of fly ash and cement Type GU was also examined. The specimens so produced were immersed in a sulphate solution as per ASTM C1012 and retrieved variously after 7, 14, 28, 56 and 84 days of exposure. As expected, Type HS cement performed best with minimum expansion and sulphate ingress. On the other hand, the Type GU cement showed lower expansion and sulphate ingress in comparison to the fly ash blended binder. Although bearing identical porosity, the blended binder had the smallest median pore size. Therefore, the sulphate ingress and consequent ettringite production likely cracks the blended system more than the other two. Significantly, after longer durations of sulphate exposure, the blended system showed higher tensile strength which implies a healing of cracks through ettringite formation.

Crack Growth Resistance in Fibre Reinforced Geopolymer Concrete Exposed to Sustained Extreme Temperatures

Portland cement concrete (PCC) is now second only to potable water in per capita consumption. And notwithstanding its numerous benefits, Portland cement itself is responsible for between 4 to 5% of the world’s manmade greenhouse gas emissions. In this context, geopolymer concrete is a promising alternative, wherein the Portland cement binder is replaced entirely by supplementary cementitious materials triggered by alkaline activators. Relatively little is known on the fracture response of this system, especially when exposed to extreme temperatures. The study reported here focused on the crack growth response of such a system prepared with Class F fly ash and reinforced with steel and polymeric fibres up to 1% volume fraction. The geopolymerization was effected with a blend of sodium hydroxide and sodium silicate to achieve a compressive strength of 30 MPa at 28 days. The resulting geopolymer concrete was subjected to temperatures between-30 oC to 300 oC, sustained for 2 hours. A fibre blend of steel to polypropylene in the mass ratio of 4:1 was incorporated. Based on the results, four different stages for fracture behaviour were identified with superior fibre efficiency seen at sub-zero temperatures.

Enhancing the Impact Resistance of Historical Stone Masonry Units with Fibre-Reinforced Hydraulic Lime Mortars

This paper describes the dynamic response of sandstone masonry units bound with fibre-reinforced mortars comparing a Portland cement-lime system with hydraulic lime. A drop-weight impact machine was used to generate stress rates up to 107 kPa/s. The dynamic impact factor and stress rate sensitivity were evaluated for the flexural strength of the sandstone and mortar, and for the bond strength of the unit, and further, the pattern of failure was noted in the units for each mortar mix and loading rate. Polypropylene microfibres were incorporated at 0%, 0.25% and 0.5% volume fraction into the mortar. Results show that the flexural bond strength was more sensitive to stress rate than the flexural strength of the mortar, at similar rates of loading. Further, the stress rate sensitivity of the bond strength decreased with an increase in the fibre content. Also, whereas the flexural toughness factors for the stone-mortar bond fell with fibre reinforcement in the stronger Portland cement-lime system, the bond improved with fibre addition when employing hydraulic lime mortar.