P. H. Morris

A worldwide correlation for exponential bed particle size variation in subaerial aqueous flows

The particle size of the bed sediments in or on many natural streams, alluvial fans, laboratory flumes, irrigation canals and mine waste deltas varies exponentially with distance along the stream. A plot of the available worldwide exponential bed particle size diminution coefficient data against stream length is presented which shows that all the data lie within a single narrow band extending over virtually the whole range of stream lengths and bed sediment particle sizes found on Earth. This correlation applies to both natural and artificial flows with both sand and gravel beds, irrespective of either the solids concentration or whether normal or reverse sorting occurs. This strongly suggests that there are common mechanisms underlying the exponential diminution of bed particles in subaerial aqueous flows of all kinds. Thus existing models of sorting and abrasion applicable to some such flows may be applicable to others. A comparison of exponential laboratory abrasion and field diminution coefficients suggests that abrasion is unlikely to be significant in gravel and sand bed streams shorter than about 10 km to 100 km, and about 500 km, respectively. Copyright (C) 1999 John Wiley & Sons, Ltd.

Exponential longitudinal profiles of streams

A theoretical model is presented which shows that streams with low solids concentrations and low lateral inflows, whose bed load sediments undergo either comminution or hydraulic sorting under steady or quasi-steady conditions, have exponential profiles. Similar streams in which both communition and sorting are significant have exponential profiles only if they are short Sediment threshold and flow depth estimates based on the theoretical model are consistent with field and laboratory data from the literature. A comparison of the model and communition and sorting data from the literature strongly suggests that hydraulic sorting and comminution dominate in short and long natural streams, respectively. No examples of natural streams of intermediate length with exponential longitudinal profiles were found, suggesting that neither sorting nor comminution is dominant in such streams.

Tensile Properties of Early-Age Concrete

In this paper, the tensile properties of concrete at very early ages and their measurements are reviewed, and the need for further study is clearly highlighted, A newly developed apparatus and procedures for uniaxial direct tensile testing of concrete specimens at ages of 1.5 hours or more after mixing, together with the experimental results obtained, are reported. An improved knowledge of various important early-age properties is presented. The tensile strength, Youngs modulus, and fracture energy of early-age concrete are all found to increase very slowly during the first 3 hours or so, but significantly increase thereafter. Strong correlations are shown to exist between these three parameters, especially between tensile strength and fracture energy. The high values of fracture energy obtained strongly suggest that early-age cracking of concrete involves a significant zone of plastic straining or microcracking in the vicinity of the crack tip. Early-age concrete is also shown to be more ductile than mature concrete.

Correlations for mine tailings consolidation parameters

ABSTRACT Because of the excessive time required for the determination of the compressibility and permeability of fine mine tailings, estimates of the volume required for the storage of such tailings are often based on very limited data. Correlations based on the index properties of mine tailings are presented that enable reliable estimates of the required parameters to be obtained rapidly. The index properties themselves may be determined within a week using small quantities of tailings. The correlations are based on data for disparate types of tailings and a variety of test methods. They are thus applicable to most mine tailings, but must be applied with caution to tailings in which chemical bonding may curtail consolidation or that contain significant quantities of relatively coarse particles or clays with fibrous particles.

Worldwide correlations for subaerial aqueous flows with exponential longitudinal profiles

It is shown that very strong, worldwide correlations exist between the bed concavity coefficients of a wide range of subaerial aqueous flows with exponential longitudinal profiles and both the corresponding stream segment lengths and exponential bed particle size diminution coefficients. The former correlation is complementary to an existing similar correlation for the exponential size diminution coefficients, while the latter is consistent with earlier theoretical correlations based on very limited data. The data supporting the correlations extend over virtually the whole range of stream lengths, solids concentrations, and bed sediment particle sizes found on Earth. This universality strongly suggests that there are underlying mechanisms common to all kinds of mobile bed subaerial aqueous flows. However, the scatter of the data for the correlations is significant and is mostly attributable to variations in hydraulic conditions and sediment properties rather than measurement errors. Some of the conditions and properties have been identified, but others remain obscure. Copyright (C) 1999 John Wiley & Sons, Ltd.

Performance of Permeability-Reducing Admixtures in Marine Concrete Structures

The use of permeability-reducing admixtures is a potential preventative of the chloride-induced corrosion of steel reinforcement, which is the main cause of the deterioration of concrete structures exposed to coastal environments. This paper presents an experimental investigation into the effectiveness of two typical commercially available permeability-reducing admixtures: one characterized by crystallization activity and the other by hydrophobic and pore-blocking effects. Concrete specimens were exposed to simulated coastal environments, and chloride concentration profiles at 28-, 365-, and 730-day exposures were determined by X-ray fluorescence spectrometry. The results suggested that the incorporation of the admixture, characterized by hydrophobic and pore-blocking effects, appeared to considerably enhance the concrete durability with respect to chloride-induced corrosion. The inclusion of the admixture characterized by crystallization activity however, seemed to have almost no detectable effect. This implies the necessity of exercising a degree of caution during specification. Copyright

Cracking of Plastic Concrete

Abstract The ACI recommendations for the prevention of the cracking of plastic concrete by limiting the evaporation rate from exposed surfaces are reviewed, and are shown to overestimate the evaporation from the surface of desiccated concrete. Pore moisture effects in desiccated concrete and the stresses arising from these effects are described. The stresses due to desiccation of early age concrete and its strength are compared, and crack initiation and propagation associated with these stresses are discussed. It is shown that ambient conditions conducive to cracking in early age concrete are encountered everywhere, and that, under favourable conditions, shallow cracks in desiccated concrete can propagate almost instantaneously to considerable depths.

Generalized Calibration of a Nuclear Moisture/Density Depth Gauge

The application of nuclear moisture/density depth gauges to soil deposits has generally relied on calibration by direct correlation with measured values of moisture content and density. In very soft variable deposits such as coal tailings, direct calibration is not possible. A generalized calibration can be developed from first principles based on the elemental composition of the soil being tested and the application of existing theories. Density and moisture content calibrations are considered separately for two different access tubes, and reasonably reliable calibrations are obtained. In applying these calibrations, an iterative procedure is required since the moisture content has a direct bearing on the elemental composition of the moist soil. The paper gives an insight into the operation and important features of a nuclear gauge.

Suctions, Fracture Energy and Plastic Cracking of Cement Mortar and Concrete

Plastic cracking of cement mortar and concrete is primarily attributable to desiccation by evaporation from unprotected surfaces. This causes high suctions (negative pressures) to develop in the pore water adjacent to these surfaces. Dissolved salts in the pore water can also contribute significantly to suctions. Quantitative expressions are available for all of the components of the total suction. The development of suctions over time is illustrated by the results of desiccation tests conducted on cement mortars, supplemented by data from the literature. It is shown that ambient conditions conducive to plastic cracking can arise almost anywhere, but that the extremely high suctions that develop in mature cement mortar and concrete do not imply that compression failures should occur A high value of fracture energy is derived from data from the desiccation tests that implies that plastic cracking is characterized by a significant zone of plastic straining or microcracking.

Crack Depths in Desiccating Plastic Concrete

The depths of cracks in desiccating plastic concrete are estimated by considering the effects of the suction (negative pore pressure) associated with desiccation and applying five failure models derived from fracture, theories combined with theories drawn from geotechnical engineering under the assumption that plastic concrete is a frictional particulate material. The estimated crack depths vary with the depth of desiccation, the suction profile, and a small number of material parameters that depend on the model adopted and are comparatively easy to estimate accurately. Four of the models predict excessively large crack depths. The fifth, however, predicts shallower crack depths that increase with the age of the concrete and are consistent with those of analogous desiccation cracks in coal mine tailings. It thus offers a relatively robust method of estimating the depth of desiccation cracks. Confirmation of this with data for plastic concrete is clearly desirable but not possible at present.