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Reinforced concrete buildings with moment-resisting frames comprising beam–column joints (without joint shear reinforcement) designed prior to introduction of seismic codes are shear deficient when subjected to seismic loading, thereby mostly fail in shear at the core of the beam–column joint. However, those designed to the new seismic codes may fail by flexural hinging at the interface of the beam–column joint due to the yielding of the beam reinforcement at the location of highest stress demand (usually the beam–column joint interface). The shear failure has been precluded through the provision of transverse reinforcement at the joint in new design and the use of carbon fiber–reinforced polymer retrofitting in old buildings. Plastic hinge formation at the interface of the beam–column joint is critical because of its penetration into the joint and its effect on bond deterioration. In this study, eight corner-external beam–column joint specimens of 1/3 scale of a typical moment-resisting frame, made without transverse reinforcement, were tested for monotonic and reversed cyclic test under displacement-controlled regime. The control specimens failed by flexural hinging at the beam–column joint interface. The experimental results have been validated using the finite element model. The specimens were retrofitted with unidirectional carbon fiber–reinforced polymer of different layers and different length. After retrofitting, the plastic hinge was relocated to the curtailment end of the carbon fiber–reinforced polymer. The relocation of the plastic hinge resulted in higher load capacity and ductility.

This paper presents a study on the shear behavior of reinforced concrete (RC) beams strengthened by jacketing the surfaces of the beams using ultra-high performance fiber reinforced concrete (UHPC). The surfaces of the RC beams were prepared by sandblasting and UHPC was cast in situ over the surfaces of RC beams. The beams were strengthened using two different strengthening configurations; (i) two longitudinal sides strengthening (ii) three sides strengthening. The bond between normal concrete and UHPC was examined by conducting splitting tensile strength and slant shear strength tests on composite cylindrical specimens cast using normal concrete and UHPC. The control and strengthened beam specimens were tested using four-point loading arrangement maintaining different shear span-to-depth ratios. The results of tested beams showed the beneficial effects of strengthening the RC beams using UHPC, as evident from enhancement of the shear capacity and shifting of the failure mode from brittle to ductile with more stiff behavior. In addition, a non-linear finite element model (FEM) was developed to examine the sufficiency of the experimental results used to study the shear behavior of control and strengthened beams. The failure loads and the crack patterns determined experimentally matched well with those predicted using the proposed model with a reasonably good degree of accuracy.

A comparison of the thermal performance and energy efficiency of walls constructed using autoclaved aerated concrete (AAC) block with thermal insulating plaster and reflective coating and t...

Concrete water tanks have a stringent serviceability requirement in terms of limiting crack widths. Excessive cracks or cracks running across the full depth of these tanks could result in l...

The focus of this paper is to investigate the effect of column axial load levels on the performance of shear deficient reinforced concrete beam column joints (BCJs) under monotonic and cyclic loading. The problem of interaction between shear stress in BCJ and axial load on column has been addressed in this work by initially postulating a mechanistic model and substantiated by an experimental test program. This was achieved by conducting appropriate tests on seven BCJ sub-assemblies subjected to monotonic and reversed cyclic loading, with varying levels of the column axial load. Experimental results were further validated using a finite element model in an ABAQUS environment. The effect of variation of compressive strength of concrete was considered in a subsequent parametric study, in order to obtain sufficient data, and utilized to develop a new shear strength model for BCJs which includes influences of all the important parameters required to predict the shear strength of BCJs. The results showed that column axial load affects the seismic performance of BCJs significantly. Experimental results demonstrated that at initial stages of loading, increase in axial load enhances the shear capacity of the joint and reduces its ductility. However, when the column axial load/axial strength ratio increases to about 0.6–0.7, shear strength starts to decrease rapidly, leading to pure axial failure of the joint. The magnitude of axial load/axial capacity ratio also dictates the failure mode and development of crack patterns in BCJs. Results of reverse cyclic tests on BCJs showed that high value of axial load/axial capacity ratio increases the initial stiffness of BCJ but rate of stiffness degradation is accelerated after peak strength attenuation.

Development of alkali-activated binder (AAB) is fast becoming a viable alternative to ordinary portland cement (OPC). However, key engineering properties of such a binder depend strongly on the chemical composition and fineness of the source materials in addition to the concentration of alkaline activators. In order to improve the reaction kinetics and subsequently, the mechanical strength of alkali activated concrete (AAC), nano-SiO2 was proposed as a partial replacement to natural pozzolan (NP) in this study. As the composition of alkaline activators also plays a vital role on the properties of AAC, trial mixtures were prepared with varying sodium silicate to sodium hydroxide (SS/SH) ratio. Subsequently, NP was partially replaced with nano-SiO2 in the selected mix to enhance the properties further. Fresh properties, such as, setting time of alkali-activated pastes (AAP) and the workability in terms of flow of mortar were determined. Evolution of compressive strength of concrete cured at 60 °C was monitored. Scanning electron microscopy (SEM) and X-ray diffration (XRD) were utilized to determine the morphology and mineralogy of the AAP, respectively. In addition, FTIR was used to identify functional groups formed in the AAP. The study reveals that a SS/SH ratio of 2.5 was suitable in achieving superior strength. AAC prepared using NP and nano-SiO2 exhibited improved strength and microstructural characteristics. However, 5% nano-SiO2 showed significant enhancement in the compressive strength and microstructural characteristics as compared to those prepared with other replacement levels. Consequently, the results of this study provides important information on developing NP-based sustainable building material with remarkable enhancement in the properties by adding nano-SiO2.

In this paper, the lateral response of unreinforced masonry walls (URM) subjected to different levels of axial load is studied numerically and experimentally. The walls in general are noted to exhibit increasing levels of lateral strength when the magnitude of applied compressive axial force is increased. Beyond a certain threshold magnitude of axial compression, this trend in the URM interaction diagram is then noted to be reversed in which the wall exhibits a weaker lateral response as the axial force exerted on the wall increases. An experimental program was conducted to study the lateral load response of sandstone and limestone walls. The finite element simulation using a plastic-damage model in terms of parameters that account for hardening and softening of the wall components in an ABAQUS environment was conducted, and the relationship between lateral and axial force that encompasses the entire spectrum of failure modes was established. The model was tested against available experimental data, and the interaction between lateral and axial force has been expressed in a ready to use form. The results obtained from the plastic-damage-based FEM simulation have also been compared to existing mechanistic models.

Evolution of heat due to an exothermic reaction between cementitious materials and water plays a significant role in the temperature rise in mass concrete at an early age. The temperature gradient between the core of mass concrete and its surfaces leads to the development of thermal stresses and may cause cracking if these stresses exceed the evolving tensile strength of concrete. This paper presents the results of an experimental and finite-element investigation of heat generation and dissipation in full-scale mass concrete blocks 2 m × 2 m × 2 m in size. A concrete mix with 100% OPC and 70% cement replaced by GGBFS was used in these blocks. The temperature profile across the depth of the block was measured at the centre, side and corner of the block. A semi-adiabatic calorimeter was used to measure the heat of hydration of the two concrete mixes. Finite-element simulation of these blocks captured with good accuracy the measured evolution of temperature in the blocks. Numerical simulation was subsequently used to investigate the influence of the size of the blocks, placement temperature of concrete, ambient temperature, solar radiation and type of formwork on the temperature rise and the associated potential of cracking in the mass concrete blocks.

In deeper wells, high pressure and temperature post cementing operations put extreme stresses on the cement sheath and dramatically affect the cement integrity. So, the design of the cement slurry is very important to ensure the durability and long-term integrity of the cement sheath. The design should lead to a better zonal isolation and strengthen the bonding with the casing. Furthermore, it should help to shield the casing from corroding and unpredictable shock loads in deeper zones and sustain plugs to control the lost circulation in fractured and high permeability zones. Different additives were used to provide these functions such as; polymers, fibers, and bio-medicine in concrete history. Because of their large surface areas and small size, those particles are highly being used in the petroleum industry in the latest decades. In this study, the usage of polypropylene fibers (PPF) is evaluated for oil well cement at high pressure and high temperature (HPHT) condition. The polypropylene fibers were added to the base cement in a different amount to evaluate its influence on enhancing the performance of cement. The results obtained showed that adding polypropylene fiber to the base cement has a negligible effect on cement density, rheological properties, and free water. Using polypropylene fiber (0.75% base weight of cement (BWOC)) with the base cement reduced the thickening time by 75%; from 317 minutes to 78 minutes. The addition of Polypropylene fiber (0.5% BWOC) increased the compressive strength of class G cement by 17.8%. Polypropylene fiber can be used as an alternative material for silica floor in shallow wells. Polypropylene fiber has a significant effect on reducing the porosity and the permeability of the base cement.

Nanotechnology provides a wide variety of methods to resolve industrial issues, which could not be addressed previously using customary methods. It helps enable researchers to alter properties of bulk materials at the nanometer scale. Various nanomaterials have been successfully applied in many areas of petroleum engineering, particularly in drilling fluids, lost circulation, enhanced oil recovery (EOR), and cementing. This study examines the mechanical and microstructural properties of oil well cement with nanozeolite. During this research, API Class G cement was used with various concentrations of nanozeolite. Compressive strength development of Class G cement, with and without nanozeolite, was studied using an ultrasonic cement analyzer (UCA) for 24 hours under high-pressure and high-temperature (HP/HT) conditions. The porosity and permeability of set Class G cement admixed with nanozeolite was also analyzed in an automated permeameter/porosimeter after 24 hours of curing. Microstructural examination of cement samples was performed using scanning electron microscopy (SEM). Three important parameters during well cementing operations included time to achieve 50- and 500-psi compressive strength and time to achieve 2,000-psi compressive strength. These parameters were significantly altered by adding a small percentage of nanozeolite to the neat Class G cement. The addition of nanozeolite resulted in a decrease in transition time and accelerated achievement of 2,000-psi strength. Furthermore, the porosity and permeability of the set Class G cement specimens with nanozeolite decreased substantially, thus indicating a dense microstructure of the matrix. This was confirmed by microstructural investigations using SEM. Nanozeolite is nonhazardous, nontoxic and is compatible with API Class G cement. Nanozeolite can be an effective oil well cement additive because it enhances early strength, and the final compressive strength helps improve cement durability. The accelerated compressive strength development can help decrease wait-on-cement (WOC) time, thus lowering operation costs. Additionally, denser microstructure can help restrain the invasion of corrosive formation fluids.

In oil and gas well cementing, a robust cement sheath is required toensure long-term integrity of the wells. Successful completion of cementing jobhas become more complex, as drilling is being carried out in highly deviated andhigh pressure-high temperature (HPHT) wells. To gain desired compressive strength and rheological properties has become hard and complex task in HPHT wells. The use of nanomaterials inenhanced oil recovery, drilling fluid, oil well cementing and other applications isbeing investigated. Being smaller in size and having a high surface area, nanomaterials can provide solutions to challenges faced in HPHT cementing. This paper presents the results of an experimental studyconducted to investigate the effect of nanoclay as an additive, on themechanical and rheological properties of Type-G cement slurry under HPHTconditions. Nanoclay based cement mixes were prepared by replacing cementwith 1%, 2% and 3% BWOC nanoclay, and admixed with silica flour, and otherchemical admixtures. The cement slurries were prepared and cured at 290°F temperature and 3000 psi pressure condition as per API 10-B. Compressive strength and rheological properties were measured after conditioning the slurry at 194°F temperature and atmospheric pressure conditions as per API standard. It is observed that addition of nanoclay at1% BWOC in cement slurry, enhances the compressive strength as compared to base mix without nanoclay and other percentages of nanoclay (2% & 3%) BWOC. Further, it is investigated that nanoclay addition enhances the reological properties of cement slurry that could help in mud displacement depending on requirement.

To prevent cement sheath damage, a robust cement sheath is required to ensure long-term integrity and stability of the well from the imposed stresses. Successful completion of cementing job has become more complex, as drilling is being carried out in high pressure high temperature (HPHT) wells. Recently, application of nanomaterials in enhanced oil recovery, drilling fluid, oil well cementing and other applications are being explored. This paper presents the outcomes of an experimental study conducted to investigate the effect of nanoclay as an additive on the properties of Type G cement slurry under HPHT conditions. An organically modified nanoclay in which natural montmorillonite is modified with a quaternary ammonium salt is used in a cement mix, typically used in the Saudi Arabian oil field under HPHT conditions. Nanoclay based cement mixes were prepared with 1%, 2% and 3% BWOC nanoclay, along with silica flour, and other chemical admixtures. Development of compressive strength properties, microstructural studies using SEM and XRD techniques and permeability and porosity studies were conducted. It was observed that nanoclay at 1%BWOC reduced the permeability and porosity and enhanced the compressive strength with the accelerated strength development at early ages. A crystal product that is interweaved and joined together to build strong and well-proportioned structure is developed in the harden cement paste. The smaller size particles fill the capillaries of the cement paste that resulted in a dense structure of the matrix. In addition, this novel slurry formulation with low permeability and porosity and high compressive strength, is suitable in zones where the possibility of gas migration is very high.

Growing demand to drill high-pressure, high-temperature (HPHT) wells requires improved technology to overcome the HPHT challenges. Designing and testing for cost-effective cementing at simulated downhole conditions for HPHT gas and oil wells poses a challenge necessitating special consideration in the choice of cement slurry. A successful cementing job is dependent on how quickly and efficiently cement achieves its strength. Another critical aspect is to efficiently displace the mud out of the annulus by designing the cement slurry with desired rheology. Achieving desired mechanical and rheological properties of cement becomes harder and more complex at HPHT conditions. A wide variety of admixtures are commonly mixed with the oilwell cement slurries to accommodate the extensive range of pressure and temperature and achieve enhanced mechanical and rheological properties. Carbon nanotubes are one of the most recent admixture options. This paper describes a study carried out to examine compressive strength and rheological properties (plastic viscosity, yield stress and gel strength) of oilwell cement slurries integrating chemical additives and multi-walled carbon nanotubes (MWCNT) at HPHT conditions. The study determined that integrating MWCNT in oilwell cement led to substantial increase in the compressive strength values. The rheological properties of oilwell cement slurries are greatly reliant on temperature, water/cement ratio and the admixture used. The study indicated that using MWCNT in cement slurries improved the rheological properties of cement slurries and therefore their displacement efficiency in challenging conditions.

Growing demand to drill High Pressure High Temperature (HPHT) wells requires improved technology to overcome the HPHT challenges. Oil well cementing has a greater impact on the production potential of a well as compared to other operational actions. A successful cementing job is dependent on how quickly and efficiently cement achieves strength. Achieving desired mechanical properties of cement becomes harder and complex at HPHT conditions. A wide variety of admixtures are commonly mixed with the oil well cement slurries to deal with the extensive range of pressure and temperature to acquire enhanced mechanical properties depending on the combination of admixture used. Carbon Nanotubes (CNT), declared as "Miracle Material", have been one of the most recent admixtures being used to achieve the desired mechanical properties of cement slurries. This paper highlights the study carried out to examine the mechanical properties comprising compressive strength of oil well cement slurries integrating chemical additives and CNT under high pressure high temperature (HPHT) conditions. Three different percentages of functionalized multi-walled CNT's (MWCNT), at 0.1%, 0.25% and 0.5% bwoc, were mixed with Type-G oil well cement together with other additives used in HPHT applications. Compressive strength investigations (Destructive and UCA) were carried out on those slurries. SEM investigations were also conducted on selected MWCNT mixes. The study indicated that incorporation of MWCNT, at a very small percentage, in cement slurries resulted in a significant increase in the compressive strength values.

Microstructural composition of the hardened cement pastes are analyzed using nanoindentation and 29Si MAS NMR after curing time periods of 7 and 28 days. Two curing conditions, room condition (20°C with 0.1 MPa pressure) and an elevated condition (80°C with 10 MPa pressure) are prepared to hydrate cement pastes [water to cement (w/c) ratio of 0.45]. The degree of hydration of the cement paste quantified using nanoindentation was compared with that from 29Si MAS NMR. From nanoindentation of the hardened cement pastes, microstructural hydration products are characterized with respect to the corresponding modulus of elasticity. A hydration product, which has a relatively high modulus of elasticity over other known hydration products, was found in the hardened cement paste cured in elevated temperature and pressure. The effect of high pressure on the composition of the hydration product is discussed and it is hypothesized that the packing density of calcium-silicate-hydrate (C-S-H) might increase when a cement paste is hydrated under high temperature and pressure.