J. C. Cripps

Aggregate-cement paste interface: Part I. Influence of aggregate geochemistry

The influence of aggregate geochemical properties on the development of the microstructure and bond strength at the interfacial transition zone (ITZ) between aggregate and the hydrating cement paste was studied. The mineral phases and microstructure at the ITZs of three commonly used concrete aggregates (quartzite, basalt, and limestone) with contrasting chemical properties were analysed using scanning electron microscopy with an energy dispersive X-ray analyser and X-ray diffraction. It was observed that differences in microstructure and mineral phases exist at ITZ between the three different aggregates studied. The limestone in particular produced a porous ITZ as a result of chemical interaction.

Clay materials used in construction

Concluding the trilogy on geological materials in construction by specially convened Geological Society working parties, this authoritative volume reviews many uses of clays, ranginf from simple fills to manufactured products. Comprehensive and international coverage is achieved by an expert team, including geologists, engineers and architects, who met over six years to produce the book. Packed with information prepared for a wide readership, this unique handbook is also copiously illustrated. The volume is dedicated to the memory of Professor Sir Alec Skempton. Various definitions of ‘clay’ are explored. Clay mineralogy is described, plust the geological formation of clay deposits and their fundamental materials properties. World and British clay deposits are reviewed. New compositional data are provided for clay informations throughout the British stratigraphic column. Investigate techniques and interpretation are considered, ranging from site exploration to laboratory asessment of composition and engineering performance. Major civil engineering applications are addressed, including earthworks, earthmoving and specialized roles utilizing clays. Traditional earthen building is included and shown to dominate construction in places. Clay-based construction materials are detailed, including bricks, ceramics and cements. The volume also includes a comprehensive glossary.

The engineering properties of mudrocks

Summary The ‘rock’ and ‘soil-like’ properties of British mudrocks are shown to be influenced by: (a) their lithology; (b) their geological history of loading (especially during exhumation); (c) the type and method of testing; and (d) the degree of weathering. In particular, unloading and weathering leads ultimately to a normally-consolidated clay of much the same undrained shear strength, irrespective of age and origin of the parent material. For this reason, the engineering properties of the unweathered mudrocks are illustrated separately in terms of classification indices, undrained and effective shear strengths and deformability in relation to geological age.

Aggregate-cement paste interface. II : Influence of aggregate physical properties

This paper describes part of a large project on aggregate-cement interactions and interface bonding mechanisms in concrete. This part of the study investigates the influence of aggregate physical properties on the nature of aggregate-cement paste interfacial bonding with the aim of establishing the bonding mechanisms as controlled by aggregate physical properties. A newly developed experimental technique to characterise quantitatively the aggregate surface texture using a surface profilometer is also presented. Three commonly used concrete aggregate rocks (basalt, limestone, and quartzite) were investigated. Significant differences in the measured bond strength and modes of failure of rock-hardened cement paste composite specimens under uniaxial tension were observed between the different rock types. Based on the results presented, it is apparent that for a given cement paste, the interfacial bond strength cannot be predicted from aggregate surface roughness alone.

A review of selected engineering geological characteristics of English Chalk

Abstract Chalk is a variable material, the properties of which are dependent upon its composition, textural features and diagenetic history. With the exception of certain horizons in the Lower Chalk that contain appreciable amounts of clayey material, the English Chalk is a remarkably pure micritic carbonate rock that generally can be divided into coarse and fine fractions. The latter comprises 70–80% of chalk. Cementation took place more or less contemporaneously with deposition so that the sediment was able to support relatively high overburden pressures. Hence, high values of porosity were retained. Chalk varies appreciably in density and hardness. The harder chalks are the result of diagenetic processes and bioturbation that brought about densification. In soft chalks the grains are only bound together at the points of contact by thin films of calcite. The latest classification of chalk is based on an assessment of intact dry density, discontinuity aperture and discontinuity spacing. Chalk tends to vary from moderately weak to moderately strong and its strength is significantly reduced on saturation. Under triaxial loading conditions diagonal shear failure tends to occur at lower confining pressures but at higher confining pressures barrel-shaped failure occurs indicating plastic deformation and textural disaggregation. Similarly, at low loading, chalk exhibits low volume compressibility but much more significant consolidation occurs if the yield stress is exceeded. Chalk undergoes dissolution and so solution features are found throughout its outcrop. Mineworkings in the Chalk extend back into the distant past, the most ancient being those excavated in the Neolithic Age for flint. Several types of workings exist. Collapse of old mineworkings, most of which are unrecorded, is difficult to predict. The potential for subsidence, caused by the collapse of both mineworkings and dissolution features, affects development and its occurrence can lead to the abandonment of property or, worse, the loss of lives.

Assessing the durability of mudrocks using the modified jar slake index test

As exposure to weathering processes or other changes in moisture content can be the cause of troublesome breakdown of many mudrocks, it is essential to have reliable means of predicting this behaviour. Although the ISRM slake durability test is the standard test used to distinguish between durable and non-durable mudrocks, the test does not always identify problematic materials. The dynamic nature of the test and the use of an arbitrary 2 mm drum mesh size are particular short falls. The test is too aggressive to characterize properly the behaviour of low durability mudrocks and slaking of samples to a fragment size greater than 2 mm is not measured. This paper reviews the various tests that have been used for predicting slaking potential. Comparative durability testing of 49 selected UK mudrocks ranging in age from Cambrian slates to Carboniferous Coal Measures is described. An evaluation of the performance of these materials in the one- and three-cycle ISRM slake durability test confirms the greater value of the latter. The modified jar slake test was found to be a suitably simple and accurate means of determining the durability of these mudrocks. Comparisons with the results of natural slaking trials confirm that the modified jar slake test presents a more detailed evaluation of the slaking potential of samples. In practice it is anticipated that the described test procedure will be used to provide data that would augment sample evaluation based on the ISRM slake durability test.

Aggregate-cement chemical interactions

This paper highlights the role played by aggregate with respect to the chemical interactions that take place within the interfacial transition zone between aggregates and cement paste matrix. Four commonly used aggregates with different chemical properties, basalt, limestone, silica sand, and quartzite, were investigated. It was observed that ions are both absorbed and released by the aggregates in all the aggregate-cement solution systems, with basalt being the most active in this respect. The findings reveal that aggregates are more chemically active than has been supposed and that a wide range of chemical interactions between the aggregates and cement should be anticipated.

Some engineering implications of chemical weathering of pyritic shale

Summary Previous research at the site of a landslip at Mam Tor, Derbyshire, has linked the oxidation of pyrite in Edale Shale to the generation of acid groundwater. Chemical analysis of surface seepages suggests that reactions occur between this acid water and sedimentary minerals, including clays. The objective of the present research is to relate changes caused by chemical weathering to modifications of the engineering properties of those materials. It has been found that in closed system acid leaching experiments, some constituents of shale react very rapidly. In order to quantify these changes in terms of the residual shear strength of shale, a Bromhead Ring Shear apparatus has been modified so that the composition of pore solutions may be altered during shearing. These tests indicate that the residual shear strength of the shale is sensitive to modification of pore solution composition. The changes are rapid, reversible and apparently chemically analogous to those at Mam Tor. Since the bulk properties of weathered shale depend both on the type of solid material and the pore solutions present, the weathering of Edale Shale is discussed in terms of solid and solution phase modifications.

PYRITE OXIDATION IN LOWER LIAS CLAY AT CONCRETE HIGHWAY STRUCTURES AFFECTED BY THAUMASITE, GLOUCESTERSHIRE, UK

The process of pyrite oxidation with the resulting formation of sulfates in disturbed pyrite-rich mudstone is recognised in the UK as a potential engineering hazard. This paper describes and quantifies the extent of pyrite oxidation in Lower Lias Clay in Gloucestershire using a large data set collected in 1998 and 1999 during the investigation of twenty-eight, approximately 30-year old highway structures affected by the thaumasite form of sulfate attack. Most of the structures have concrete spread footings founded on undisturbed and relatively unweathered Lower Lias Clay, and are surrounded by backfill of reworked Lower Lias Clay. Chemical, X-ray diffraction and X-ray fluorescence analyses are used to determine the composition of the soils. Differences are identified in the sulfides, sulfates and total sulfur concentrations between unweathered and weathered Lower Lias Clay and the backfill. The work is supported by pyrite oxidation trials in the laboratory on clay samples stored in bags and sealed tubes. The results indicate that pyrite oxidation, sulfate formation and leaching have occurred as the Lower Lias Clay weathers in situ and also during and following disturbance and reworking for highway construction. The amount and rate of pyrite oxidation are estimated, and comparison is made with the concentration of sulfate in the surrounding groundwater, and the extent of thaumasite sulfate attack measured at adjacent structures.

Sulfur species in geological materials––sources and quantification

Abstract Groundwaters containing dissolved sulfates and sulfuric acid can lead to chemical attack on construction materials such as steel and also concrete and cement stabilized soils forming complex hydrated calcium sulfo-aluminate species such as ettringite and thaumasite. Many factors contribute to aggressive conditions that need to be adequately determined prior to construction. The identification of acid environments is generally tested for, although in some circumstances corrosion may also take place under apparently neutral or alkali conditions. Sulfur reducing bacteria can give rise to such conditions. They thrive under acidic and aerobic conditions within the range of ambient temperatures found in the UK and where there is an availability of reduced sulfur and carbon compounds. As present methods of assessment for aggressive conditions tend to concentrate on sulfate and pH determinations, the problems posed by reduced sulfide species are not fully addressed. This paper describes research by the TRL Limited and the University of Sheffield for the Highways Agency to develop better test methods for sulfur compounds in structural backfills which may cause problems to buried steel and concrete structures.