R.N. Swamy

Thaumasite formation in Portland-limestone cement pastes

Small cylinders (10-mm diameter × 10-mm height) made at a water:solids ratio of 0.5 from Portland cement with 0, 5, 15, and 30% limestone additions were cured in water at room temperature for 28 days. They were subsequently stored in various solutions at 5°C for periods of up to 420 days. The pastes were inspected visually and examined by X-ray diffraction every 28 days. Selected samples were also examined by thermal analysis and scanning electron microscopy. Pastes containing fine limestone additions were susceptible to formation of thaumasite after only a few months of exposure to sulfate solutions. The extent of thaumasite formation was greater with increasing limestone additions and when magnesium sulfate was present in the solution. Thaumasite formation was then accompanied by formation of brucite and secondary gypsum. Calcium hydroxide was a reactant rather than a reaction product and C-S-H gel was also consumed.

ULTIMATE PUNCHING SHEAR STRENGTH ANALYSIS OF SLAB-COLUMN CONNECTIONS

Abstract A simple analytical model is presented to predict the ultimate punching shear strength of slab–column connections. The model is based on the physical behavior of the connections under load, and is therefore applicable to both lightweight and normal weight concrete. The model assumes that punching is a form of combined shearing and splitting, occurring without concrete crushing, but under complex three dimensional stresses. Failure is then assumed to occur when the tensile splitting strength of the concrete is exceeded. The theory is applied to predict the ultimate punching shear strength of 60 slab–column connections reported recently in literature, and designed to fail in shear, involving a large number of variables, such as type of concrete, concrete strength, tension steel ratio, compression reinforcement and loaded area. The results show very good agreement between the predicted and experimental values. The uniqueness of the model is that it incorporates many physical characteristics of the slabs and their failure behavior, and this is reflected by its ability to predict extremely well the results of tests conducted by researchers other than the authors.

Long term durability of Portland-limestone cement mortars exposed to magnesium sulfate attack

Mortar prisms made with Portland-limestone cement have been stored in air and in 1.8% magnesium sulfate solution at 5 °C and have been examined over a period of 5 years. This paper is primarily concerned with the results obtained at the end of this period. The limestone content in the samples varied from 0% to 35%, but the water to cement plus limestone powder ratio was kept constant. The status of the samples after storage for 5 years is reported based on visual examination and a thorough characterisation using X-ray diffraction, infra-red spectroscopy and scanning electron microscopy. The prisms stored in magnesium sulfate solution were all showing clear signs of deterioration, increasing in intensity with limestone content. The mortar prism with 5% limestone replacement was, however, seriously degraded in comparison with the ordinary Portland cement control prism, and it is shown that this was due to the thaumasite form of sulfate attack.

The thaumasite form of sulfate attack in Portland-limestone cement mortars stored in magnesium sulfate solution

Abstract Mortar prisms (40×40×160 mm 3 ) made at a water:solids ratio of 0.5 and a cement:aggregate ratio of 1:2.5 from Portland cement with 0%, 5%, 15% and 35% limestone additions were cured in water at 20 °C for 28 days. They were subsequently stored in air and submerged in 1.8% magnesium sulfate solution at 5 and 20 °C for a year. The prisms were inspected visually every 28 days, the solution was changed every 84 days, and selected samples were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) after specified intervals. The thaumasite form of sulfate attack was readily identified in the ordinary Portland cement (OPC) – 35% limestone mortar after 126 days storage in magnesium sulfate solution. The surface layer of the prism had spalled and was mushy, while the core was still solid and sound. Gypsum, thaumasite and brucite were identified in the surface layer. Secondary electron images of polished sections of deteriorated Portland-limestone cement mortars revealed the microstructure of the cement to be suffering from the thaumasite form of sulfate attack. The extent of this attack was greater at 5 °C than at 20 °C, although some thaumasite was formed even at the higher temperature.

A theory for the flexural strength of steel fiber reinforced concrete

Abstract A theory is presented to predict the flexural tensile strength of concrete reinforced with short, discontinuous steel fibers randomly oriented and uniformly dispersed in a cement-based matrix. The theory is based on a dual criterion of crack control and composite mechanics. The first crack in the fibrous composite occurs due to bond slip. The fracture process consists of progressive debonding of fibers during which slow crack propagation occurs. Final failure occurs due to unstable crack propagation when fibers pull out and the interfacial shear stress reaches the ultimate bond strength. The theory is supported by test data on fiber reinforced concrete, mortar and paste.

Use of mineral admixtures to prevent thaumasite formation in limestone cement mortar

Abstract Concrete 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 (ggbs) 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 varying part of the limestone cement with the above minerals. Siliceous and calcareous sand was used in order to study the effect of the sand type on the thaumasite formation. The specimens were immersed in a 1.8% MgSO4 solution and cured at 5 and 25 °C. The formation of thaumasite was checked and confirmed by visual inspection, strength tests, ultrasonic pulse velocity measurements, XRD and TGA. It is concluded that the use of specific minerals, as partial replacement of cement, inhibits the thaumasite formation in limestone cement mortar.

Influence of fiber geometry on the properties of steel fiber reinforced concrete

Abstract The influence of fiber diameter, length and volume fraction on the properties of steel fiber reinforced concrete in the fresh and hardened states is reported. The compactibility of fresh fibrous concrete decreases linearly with fiber aspect ratio. There is no unique relationship between fiber aspect ratio and ultimate flexural strength or compressive strength. The dynamic modulus of elasticity of fiber reinforced concrete is little different from that of plain concrete. The fibers, however, show substantial improvements in damping when the concrete is wet. It is shown that the ultimate flexural strength can be predicted by a composite mechanics equation. A unique relationship is also shown to exist between ultimate flexural strength and an “effective spacing” concept.

Fracture mechanism in concrete systems under uniaxial loading

Abstract The various factors influencing and the principal features of the deformational behavior and fracture of concrete and similar materials are discussed. The paper presents a unified theory that correlates the essential characteristics of concrete behavior under load with the phenomena of size effects on strength, stiffness and ductility. The model is based on a two-phase material one phase being microcracks. The model predicts satisfactorily the non-linear stress-strain behavior, size-dependent strength and fracture mode transition of concrete-like materials. The model also predicts a new phenomenon, the size effect on stiffness and ductility of such materials.

An evaluation of controlled permeability formwork for long-term durability of structural concrete elements

Abstract The long-term performance of a concrete slab (CPF slab) exposed to chloride ingress and atmospheric carbonation from the surface generated by controlled permeability formwork (CPF) is investigated. The results are compared with a similar slab exposed to long-term chloride ingress and atmospheric carbonation from the cast face (Control slab). Techniques such as X-ray diffraction (XRD) and differential thermal analyses (DTA) were employed to determine the resistance against carbonation while, mercury porosimetry was used for investigating the pore size distribution at the surface of the slabs. Amount of acid soluble chlorides was determined by using Volhards method. The CPF employed at the bottom of the mould was not fully effective in its intended purpose of generating a permanent and dense impermeable concrete layer adjacent to it when the design water-cement (w/c) ratio of the concrete mix was 0.60. This resulted in an almost similar extent of carbonation at the surface for both CPF and control slabs as shown by XRD and DTA studies. Similarly, there were no significant differences in the amount of chlorides and their depths of penetration for both CPF and control slabs, although the former was marginally superior in chloride penetration resistance at the surface.

The interfacial bond stress in steel fiber cement composites

Abstract In fiber cement composites most fibers are in a state of partial bond due to internal stresses arising from moisture migration during fabrication and subsequent volume changes in the matrix. A wide variation in the computed interfacial bond strength therefore occurs depending upon the type of test or when derived from phenomena such as crack spacing. In practice debonding of the fibers occurs in flexural tension in the presence of a strain gradient. This paper presents further data on steel fiber mortar and concrete to confirm the validity of the composite mechanics approach to predict the composite flexural strength. It is shown that the composition of the matrix and its strength properties influence the fiber-matrix interfacial bond stress and the relative contributions of the matrix and the fibers to the composite flexural strength.