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Overview: Problems with Foundations

3:03 PM Civil Engineering , Construction Materials

Introduction

Foundations must be designed to support the loads from a structure, taking proper account of ground conditions. The design of foundations and the calculations involved will not be considered in detail, but only the diagnosis of foundation defects which may be observed in the course of investigations into defects in buildings.

Image courtesy: Sinai construction

Normal foundations comprise concrete strips on which the walls are erected, together with prepared foundations to support any solid floors. It is well recognized that strip foundations must have sufficient width to provide an adequate bearing area to support the imposed load on the ground support concerned; the depth of the foundation strip is important in terms of the ability of the foundations to resist local fluctuations in the bearing capacity of the ground without developing fractures, and the total depth of the foundations is important if it is necessary to excavate sufficiently deeply to provide bearings on better compacted soil or perhaps even rock at a greater depth. The importance of careful preparation of foundations for solid floors is not so widely recognized, as floor loads are much less and the requirements much less severe, but problems commonly arise if the floor slab support is poorly compacted, or support varies on a sloping site between excavated ground at one end and inadequately compacted fill at the other. If the load bearing capacity of the ground is poor, construction on a substantial reinforced concrete raft may provide the best means for spreading the foundation load, and various piling systems may be preferred to excavating deep foundation trenches.

Settlement and subsidence

The differences between settlement and subsidence failures are not widely recognized, although the different causes are obviously important in relation to remedial works. The causes may also be critical should damage result in a dispute for negligence or breach of contract in design or construction.

Settlement is due to normal compaction of the supporting ground as the building loads are imposed on the foundations. Some settlement always occurs during construction, but there is usually some further settlement or creep following completion. Settlement is normal and must be anticipated in design, so that damage due to settlement is an indication of inadequate design in relation to ground conditions or failure to observe the design during construction. General settlement usually occurs on loose ground such as sand or shingle or on readily compressed ground with a high organic and moisture content, such as peat. It is only troublesome because the external walls, the internal walls and solid floors impose different loads and therefore settle to a different extent, the most obvious damage being doming or an increase in height of the floors, although actually it is the walls that are settling around the floors, or settlement of the heavily loaded peripheral walls in relation to the internal partition walls. In one extensive housing development on sandy soil in the south of England, extensive damage was caused by the preferential settlement of the partition walls, and this was found to be due to inadequate foundations. Inadequately compacted landfill is also a serious problem, particularly if it contains biodegradable materials, such as wood, which will cause further settlement over a prolonged period. Certainly the worst settlement problems are associated with sites in which the load bearing capacity of the ground varies so that part of the foundations settle, sometimes causing massive structural fractures. In extreme cases, diagonal fractures may be identified as resulting from the settlement of a distinct part of the structure; in other cases fractures, generally through openings which represent the weakest parts of walls, will be wider at the tops of the walls than at the bottoms, indicating that local settlement has occurred to one side of the fracture, such damage being common on inadequately compacted fill, on land with patches of fill, or on sloping sites where walls are on excavated ground at one end but fill or inadequately compacted ground at the other. Once settlement has been fully relieved a damaged building may become structurally stable and no remedial works are necessary, other than the repair of the settlement fractures, unless structural weakening has occurred which requires the fractures to be re-bonded to restore structural integrity, as when vertical fractures occur between gables and adjacent bracing walls. However, if foundations are seriously inadequate and settlement is continuing, it may be necessary to provide additional support by inserting deeper foundations or perhaps supporting piles.

A special situation arises when buildings are constructed on shrinkable clay soils. Normally clay soils will be moist and in their expanded condition, but abnormal drying may result in shrinkage. Such problems are not usual in the British Isles, but serious damage occurred in this way through the exceptionally dry weather in 1976.

In addition, many of the houses constructed on shrinkable clay during the exceptionally dry weather have since suffered damage due to expansion of the clay on subsequent wetting. Buildings themselves and associated patios, paths, drives and roads all reduce rain penetration into the ground and can result in clay shrinkage damage following construction. For these reasons deeper foundations must always be used on clay soils to provide support below the clay or at a depth within the clay which will not be affected by such moisture content changes.

Subsidence follows from some unexpected event following construction. Ground water percolation may result in removal of material and loss of support. Water percolation can occur in this way through natural groundwater movement, but usually some event prompts subsidence, such as the diversion of a stream, the overflow of a drainage system, or even a fracture in the rainwater or sewage drains associated with the building itself, although one of the most common causes of subsidence is the fracture of a water main due to frost or traffic damage. Landslip is also another example of subsidence, usually because apparently stable ground has been fluidised through the accumulation of an exceptional amount of rainwater, sometimes causing severe damage to houses and gardens constructed on sloping sites.

Tree root damage

Tree root damage results most obviously from the penetration of tree roots into masonry and beneath foundations, and rupturing due to progressive root growth. Such damage can usually be readily identified by excavation and does not justify special comment, except in relation to safe separation between trees and buildings.

A more serious problem is the presence of trees in conjunction with shrinkable clay. Deciduous trees will remove water from the clay during the summer and the clay will shrink, but the tree will have no demand for water during the winter so that the clay moisture content will increase and it will expand. This seasonal movement can only be avoided by ensuring that buildings are constructed a sufficient distance from established trees, or new trees are planted a sufficient distance from a building. If these requirements cannot be satisfied deeper foundations are necessary to penetrate below the clay, or sufficiently deeply in the clay for moisture content fluctuations to be minimal. If a new building is constructed in the summer on a site from which trees have been recently removed, clay soil may have an unusually low moisture content; subsequent wetting may cause expansion and damage to the new building, unless the foundations are sufficiently deep to avoid the effect as previously explained.

Safe separations between trees and buildings depend upon the tree species and are summarized in Table 1.

Table 1 Recommended minimum distances between trees and buildings

Tree species	Minimum separation on shrinkable clay soil	Normal maximum tree heIght (H) (m)
Oak	1H	16-23
Poplar	1H	24
Lime	(1/2)H	16-24
Ash	(1/2)H	23
Plane	(1/2)H	25-30
Willow	1H	15
Elm	(1/2)H	20-25
Hawthorn	(1/2)H	10
Maple/sycamore	(1/2)H	17-24
Cherry/plum	1H	8
Beech	(1/2)H	20
Birch	(1/2)H	12-14
White beam/rowan	1H	8-12
Cypress	(1/2)H	18-25

Further reading:
☞Book: Settlement Calculation on High Rise Buildings by Xiangfu Chen
☞Foundation Engineering Handbook by Robert W.Day
☞Causes of Failures of Foundations and Preventive Measures

Biological Damage to Masonry Structures

Although most masonry surfaces will be colonised by various organisms, damage is usually restricted to porous stone and is usually associated with atmospheric pollution. The main pollutant today is sulphur dioxide which dissolves in rainwater to produce sulphurous acid and reacts with the calcium carbonate of limestones to form calcium sulphite. However, sulphite is never found on limestones, but only sulphate which is produced by much more aggressive sulphuric acid. Various chemical explanations have been given for this oxidation from sulphite to sulphate, such as ultra-violet radiation, but it is a fact that sulphate deposits are always associated with the presence of sulphating bacteria, particularly Thiobacillus species. In areas where coal fires are still used extensively and in some industrial areas, nitrous oxide pollution also occurs which should give nitrous acid and nitrites on limestones, but only nitrates are actually found, indicating oxidation to the more aggressive nitric acid. In these circumstances nitrating bacteria, particularly Nitrobacter species, are always found.

Urban areas appear to be cleaner since the introduction of the Clean Air Act, but this is only because particulate emissions have been reduced from industrial chimneys and there has been a progressive decline in most areas in the use of coal for domestic heating. Unfortunately sulphur dioxide pollution has become steadily worse, partly through the increasing use of less expensive heavy oil for heating large commercial and industrial premises as this fuel has a relatively high sulphur content, but also partly through a feature of the Clean Air Act which limits only emission concentrations rather than amounts; if an operator is emitting excessive sulphur dioxide concentrations these can be easily reduced by injecting air into the flue, but the total emissions of sulphur dioxide remain unchanged.

Image courtesy: Historic environment Scotland Slime producing algae may result
in slippery surfaces.

Sulphate formed from sulphur dioxide in this way is a source of crystallisation damage, but damage is not confined to limestones. On mortar the sulphate in urban areas may be sufficient to react with the tricalcium aluminate in ordinary Portland cement to cause the expansion and cohesion failure usually known as sulphate attack. However, deterioration problems attributable to bacteria are not confined to urban areas. In rural areas ammonia generated by bacteria from urine in stables and byres can be absorbed on stone walls or asbestos-cement roofs where it is converted by Nitrosomonas species to nitrites and then by Nitrobacter species to nitrates, frequently causing spalling damage.

Bacteria are not the only organisms which colonise damp masonry surfaces. If the surfaces are warm with sufficient light, algae will develop in the water film on the surface, typically producing a bright green coloration, although sometimes dark green, brown and pink colorations occur. Algae often colonise a surface within one or two hours of rainfall, but the algal coloration disappears just as rapidly as the surface dries. Many of the algae are killed by drying but sufficient remain to redevelop and multiply when dampness returns. The humus accumulating on the masonry surface from dead algae and other sources eventually allows mosses, liverworts, grasses and even trees to develop, their root systems often causing serious damage. Organic deposits on the surface also encourage fungi to develop, such as Cladosporium, Phoma, Alternaria and Aureobasidium species; some species are associated particularly with the high nitrogen levels that develop on masonry contaminated by bird droppings.

Serious masonry deterioration is sometimes associated with growth of lichenised fungi or lichens, symbionts of algae growing within fungi, usually Ascomycetes. The fungal hyphae penetrate deeply into stone, exploring fractures but also generating organic acids such as oxalic acid. Oxalates are formed in carbonaceous stones which are usually deposited in or near the thallus or surface growth; eventually these accumulations of phosphates can kill the thallus, leaving a lichen ‘fossil’ of calcium oxalate on the surface of the stone which is sometimes mistaken for lichen growth; repeated applications of biocide sometimes fail to control lichen growth because the growth is, in fact, a dead calcium oxalate fossil formed in this way. If the calcium oxalate is deposited just below the surface, densification can occur which is similar in texture to the calcium sulphate densification that can occur on limestones in urban atmospheres, causing similar spalling damage to the surface of the stone, particularly if it is also microporous and subject to frost or salt crystallisation damage. Where lichens grow on roofs, the oxalic and other lichen acids can cause severe damage to lead, copper, zinc and aluminium roof coverings and gutters; these acids can even cause etching on glass and apparently resistant stones, such as granites.

There are basically three types of lichen, classified according to the shape of the thallus. In the crustose lichens the thallus forms a flat crust on the surface of the stone, the diameter of the thallus giving an accurate indication of the age of the growth; the diameter of the largest growths in millimetres will indicate approximately the years since the stone was installed, a useful feature for identifying original and replacement stones in old masonry. The crustose lichens cause densification of the stone surface on limestones and sandstones, the stone within the centre of the thallus often spalling away to leave bare stone which is then rapidly colonised by the growth. Sensitive species cannot develop in polluted atmospheres, but resistant species become very active in the absence of competition, particularly on limestone, cast stone and concrete surfaces on which acid pollutants are neutralised; Lecanora and Candelariella species are particularly common in these circumstances. Crustose lichens vary greatly in size from minute growths within pores to enormous plates 300mm (12") or more across.

Foliose lichens have thalli like leaves or scales projecting as a group from a point of attachment to the stone. Fruticose lichens also originate from a point in this way but their thalli are branched. Foliose and fruticose lichens are not so common on buildings, except in exposed and relatively unpolluted areas on western coasts, conditions that actually encourage the development of many different lichens. Particular species tend to be associated with particular conditions. Lecanora and Candelariella have previously been mentioned as species which tolerate pollution, particularly when growing on acid neutralising substrates such as limestone, carbonaceous sandstone, asbestos-cement tiles, render and concrete. Calaplaca species are also commonly found on limestones in reasonably unpolluted conditions, whilst Tecidia and Rhizocarpa species are more often found on sandstones.

It will be appreciated from these comments that identification of lichen growths can often indicate both the nature of the substrate on which it is developing and the pollution to which it is subject. Very heavy lichen growth on limestone headstones in a cemetery was found to be causing continuous and rapid stone erosion and spalling damage, each sequence of spalling removing the lichen thallus layer with a thin layer of attached stone, thus exposing a fresh stone surface with lettering still engraved in it but with the detail becoming blurred. Identification of the lichen suggest that it was a species which particularly favoured surfaces with a high nitrogen content and which was usually associated with contamination through bird droppings, although none were present on the headstones and all surfaces were virtually identically affected. The explanation for the abnormal growth was pollution through dust discharges from a neighbouring fertiliser factory.

This abnormally heavy lichen growth was associated with Portland limestone headstones, but adjacent memorials constructed in French Euville limestone with a rather different texture developed instead a heavy coating of black slime fungus. Slime fungi are strange organisms which form a heavy gelatinous coating over the stone in which algae are trapped, giving the coating a colour characteristic of the algae involved. Slime fungi can develop externally or internally on building materials. Green, brown or red slime fungi commonly develop on masonry surfaces in churches in humid areas where the periodic heating results in excessive condensation; this is a common problem in churches in Cornwall.

Overview: Penetration of Dampness in Buildings

12:24 PM Civil Engineering , Construction Materials

Water penetration through walls is generally avoided through precautions in design and construction. Overhanging eaves considerably reduce rain penetration into wall surfaces, except during windy weather, and even where flush eaves are employed, gutters and downpipes are provided to ensure that walls are not exposed to roof water discharge. However, these precautions should not be necessary as walls should be capable of resisting rain penetration in any case. In solid walls very thick construction is used so that absorbed penetrating rainwater can be accommodated without becoming apparent at the internal surface, the accumulated water later dispersing by evaporation. In cavity walls complete penetration and saturation of the external skin is expected, but precautions are taken to prevent absorption of this water into the internal skin. In both cases defects can occur and internal dampness problems are often encountered.

Image: http://www.taliesin-conservation.com

In solid walls penetrating dampness may result from insufficient absorptive capacity, penetration occurring because the wall is too thin, either throughout the building or at local thin points, such as window reveals. Alternatively, penetration results from excessive permeability. With carbonaceous stones, that is limestones or sandstones with a carbonate cementing matrix, atmospheric acids will cause slow erosion and a progressive increase in permeability. In many cases dampness follows the pattern of the masonry, developing in either the mortar or the stone, whichever is more permeable. This effect can be seen most clearly if the internal surface is finished in limewash, the custom in many old churches in some parts of the British Isles. Construction in impermeable granite presents a particularly interesting example. The impermeability of the stone would seem to be a good protection against penetrating dampness, but the same amount of rainwater will be incident on an impermeable granite wall as on any other wall; all the water flowing down the wall will be absorbed into the porous mortar which, with its limited absorptive capacity, will quickly become saturated throughout the thickness of the wall, although the mortar might have perfectly adequate capacity if used in combination with a stone of average porosity which could absorb some of the water. In fact, many granite walls are not constructed in this way, but comprise two separate skins with a rubble core. Penetrating water can drain through the core, emerging at the interior at some distance from the original source, making it difficult to positively identify a defect such as an area of faulty pointing.

Penetrating dampness resulting from inadequate absorptive capacity tends to result in uniform internal dampness, except where the upper parts of walls are drier as they are protected from rainfall by overhanging eaves. However, rainwater absorbed into a wall will tend, at least in durable macroporous materials, to drain towards the base. If a damp-proof course is provided it is likely that this water will accumulate on top, giving an appearance very similar to rising dampness, although often the wall is dry underneath the damp-proof course. Whilst this accumulating dampness may appear similar to rising dampness, treatment is obviously entirely different and it is thus essential to ensure that dampness of this type is correctly diagnosed. Similarly, if dampness is concentrated at the top of a wall it must be suspected that there is a roof or adjacent gutter defect, and heavy flow from a defective gutter or downpipe may cause a vertical band of dampness.

Parapet and valley gutters, as well as flat roofs, frequently drain into hoppers which lead to downpipes. In some large buildings the hoppers may be fitted with spouts at a higher level through which water can discharge if the hopper should become blocked. Dampness in many buildings can be attributed to the failure to keep hoppers free from accumulating leaves, moss, lichen and pieces of stone, so that the entire roof discharge eventually occurs through the overflow spout, or simply overflows down the wall if spouts are absent.

Some years ago extensive renovation works at a Scottish castle were followed by the development of dampness at the junctions between walls and ceilings, immediately below every parapet, a problem that had never occurred previously. Investigation showed that the parapet copings had a slight fall towards the roof, but their edges did not project beyond the parapet walls so that water was discharging down the roof side of the parapet, being absorbed and percolating downwards to the accommodation beneath. The problem had not occurred previously because the roof asphalt had been continued up the inner faces of the parapets and across the copings, a detail that had been introduced by the well-known architect Lorimer when the castle was repaired and extensively reconstructed in about 1908 following a fire. This dampproof membrane feature was considered unacceptable by the architect preparing the scheme for the recent renovations and it was removed, but unfortunately the architect failed to appreciate the essential damp-proofing function of the asphalt covering. The dampness was cured by reconstructing the parapets with damp-proof courses coupled with flashings at the top of the asphalt upturns.

In theory the use of cavity walls should completely prevent penetrating dampness, but unfortunately practice is not as perfect as theory! Dampness at the top of walls can arise through roof, parapet and gutter defects, as for solid walls, but even direct penetration can occur, spreading across the cavity through mortar droppings or slovens which accumulated on wall ties during construction, causing small patches of dampness internally. Sometimes this problem develops only when cavity fill insulation is introduced, the original ventilation of the cavity being sufficient to prevent significant water penetration across the ties but the cavity fill destroying this ventilation and causing the dampness to become apparent as patches on the interior surfaces of the walls. Cavities should preferably extend to well below internal damp-proof course level or, alternatively, if the damp-proof courses are continuous, they should be stepped so that the level in the external skin is lower than in the internal skin. Occasionally the situation is reversed, either through carelessness or ignorance, and penetrating dampness accumulating on the damp proof course at the base of the external skin may flow into the internal skin; in one example this defect occurred in all the houses throughout a large building development.

In another house sulphates in the bricks reacted with the cement in the mortar of the external skin, initially causing expansion and distortion. This prompted some reconstruction of the brickwork in some areas, but where the original brickwork had been retained the mortar eventually became very friable; sulphate attack in brickwork will be discussed in a separate article. One day there was a severe thunderstorm, the thunder shocks causing the friable mortar to run from the external skin into the cavities, accumulating on the trays over openings and providing bridges through which the heavy rainfall penetrated from the external into the internal skin, damaging the interior decorations!

It has become normal practice in recent years to restrict cavity ventilation to improve thermal insulation. In fact, cavity ventilation has a very important function in evacuating humid air; if vents are omitted dampness penetrating through the external skin will increase the humidity of the cavity air. If the wall cavity is continuous with the roof space, and the roof space is similarly unventilated, condensation will eventually occur if the roof covering is impermeable, water accumulating in the supporting boarding of a wood roof deck, perhaps resulting in the development of fungal decay within the boarding and the supporting joists, and in severe cases causing dampness staining through condensation dripping onto the ceilings beneath. Cavity fill insulation reduces heat loss by restricting the convection circulation that transfers heat across the cavity from the inner to the outer skin, but fill materials must be chosen with care. Non-wettable pelletised materials, such as expanded polystyrene, can combine excellent thermal insulation with freedom from disadvantages, but some mineral fibre materials can conduct moisture across cavities; the mineral fibre should be treated with a water repellent but there have been many instances of cavity fill without water repellency, either because of a fault in manufacture or because the water repellent has been destroyed by biodeterioration. Even foams formed in-situ can have considerable disadvantages; wettable injected foams can conduct moisture across the cavity, and if they are incorrectly formulated they can actually collapse to release moisture and cause dampness. There are also two forms of dampness development which are common to all forms of cavity fill insulation. If mortar slovens were permitted to accumulate on cavity ties during construction, they will tend to conduct moisture across the cavity from the outer skin to the inner skin, but in a normal ventilated cavity the evaporation from these sloven bridges is usually sufficient to prevent any dampness becoming apparent within the accommodation. However, all forms of cavity fill restrict ventilation and increase the likelihood of dampness developing in isolated patches through sloven bridges. In addition, the vibration caused by drilling into the outer skin in preparation for cavity filling often loosens considerable debris from the inner face of the outer leaf which then accumulates at the bottom of the cavity, perhaps bridging trays over openings and damp-proof courses, causing dampness to develop in the accommodation; this problem is particularly severe in seaside properties in which sea spray absorbed into the outer leaf can cause crystallisation damage to the inner surface which can then be loosened by the drilling vibration.

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Sulfate and Sulfide Analyses of Concrete

7:40 PM Civil Engineering , Concrete , Construction Materials

A sulfate analysis may be required to determine if sulfate has entered the concrete from the environment, if sulfate compounds (e.g., calcium sulfate hemihydrate-plaster) have been added to the concrete as an admixture, to determine cement content, or to determine reasons for unusual
setting times.

Sulfate and Sulfide Analyses for Concrete

Image: http://www.understanding-cement.com

Because most cements and aggregates do not contain sulfide sulfur, analyses for total sulfur are often an accurate measure of sulfate sulfur. Instrumental methods for sulfur, such as x-ray fluorescence or evolutiontitration, are thus generally acceptable. However, if the result appears too high for the estimated cement contribution, sulfate analyses should be performed. The difference between total sulfur and acid-soluble sulfate determinations, each calculated as SO₃, is usually a measure of sulfide sulfur. Sulfide sulfur minerals may cause popouts or rust discoloration if they are close to the surface of a concrete member.

Sulfate Procedure

Digest 5 grams of concrete, pulverized to pass a No. 30 screen, in 20 ml of 1:4 hydrochloric acid solution. Filter through a coarse-textured paper inside or over a fine-textured paper into a 400 ml beaker. Determine sulfate by the procedures of ASTM C114 or as follows: place a small piece of filter paper in the beaker and hold down with a glass rod. Bring the solution to boiling and add drop-wise, through a pipet, 20 ml of barium chloride solution (10 g per 100 ml water). Boil until the white precipitate settles, then digest below boiling in the glass-covered beaker.

Filter through a retentive filter paper, wash the paper 10 times with hot distilled water, place in a tared porcelain crucible, ignite over a flame without inflaming, then in a muffle furnace at 820°C. Weigh. Calculate SO₃ as 0.343 × weight of precipitate.

Sulfide Procedures

Sulfide may be determined by evolution of hydrogen sulfide using the procedure of ASTM C114. Alternatively, total sulfur may be determined by evolution/titration (e.g., LECO furnace) and sulfate sulfur by Sulfate Procedure. The difference between these values, both calculated as SO3, multiplied by 0.4, is sulfide sulfur.

'Q' Carbon Surpassed Diamond as the Hardest Material on the Planet - Extreme Discovery

2:12 PM Civil Engineering , Construction Materials , General , Mechanical Engineering , News

It is a well-known fact that diamond is the hardest material on the planet. However, diamond lost its status of the hardest substance when scientists at the North Carolina State University discovered a new, harder-than-diamond form of carbon.

The research team found a new phase of carbon, called the Q-carbon.

'Q' Carbon Surpassed Diamond as the Hardest Material on the Planet - Extreme Discovery

(Image modified from: Jagdish Narayana & Anagh Bhaumik)

The new phase of carbon is quite different from graphite and diamond. Q-Carbon shows some unique and quite unexpected properties like ferromagnetic nature. Another unusual feature of this phase of carbon is that it begins to glow in the presence of energy. See more images from various experiments below:

Q carbon engineersdaily.com

(Image Source: Jagdish Narayana & Anagh Bhaumik)

The research has been published in the Journal of Applied Physics. The Q-phase of carbon was created by focusing a laser beam on amorphous carbon for 200-nanoseconds. The temperature of the amorphous carbon sample rose quickly and was cooled down by the process of quenching to create Q-Carbon.

This recent discovery will be a massive breakthrough in the domain of structures and materials. The new hardest material on the planet might be used for the fabrication of prosthetic structures, improvement of the equipment used for deep drilling, as well as developing new, brighter screens for the smartphones and televisions.

Overview - Aggregates for Concrete

10:39 AM Civil Engineering , Construction Materials

The importance of using the right type and quality of aggregates cannot be overemphasized. The fine and coarse aggregates generally occupy 60% to 75% of the concrete volume (70% to 85% by mass) and strongly influence the concrete’s freshly mixed and hardened properties, mixture proportions, and economy. Fine aggregates (Fig. 1) generally consist of natural sand or crushed stone with most particles smaller than 5 mm (0.2 in.).

Fig. 1. Closeup of fine aggregate (sand).
Image courtesy: University of Memphis

Coarse aggregates (Fig. 2) consist of one or a combination of gravels or crushed stone with particles predominantly larger than 5 mm (0.2 in.) and generally between 9.5 mm and 37.5 mm (3⁄8 in. and 1 1⁄2 in.). Some natural aggregate deposits, called pit-run gravel, consist of gravel and sand that can be readily used in concrete after minimal processing. Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed.

Fig. 2. Coarse aggregate. Rounded gravel (left) and
crushed stone (right). Image courtesy: University of Memphis

Crushed stone is produced by crushing quarry rock, boulders, cobbles, or large-size gravel. Crushed air-cooled blast-furnace slag is also used as fine or coarse aggregate. The aggregates are usually washed and graded at the pit or plant. Some variation in the type, quality, cleanliness, grading, moisture content, and other properties is expected. Close to half of the coarse aggregates used in Portland cement concrete in North America are gravels; most of the remainder are crushed stones.

Naturally occurring concrete aggregates are a mixture of rocks and minerals (see Table 1). A mineral is a naturally occurring solid substance with an orderly internal structure and a chemical composition that ranges within narrow limits. Rocks, which are classified as igneous, sedimentary, or metamorphic, depending on origin, are generally composed of several minerals. For example, granite
contains quartz, feldspar, mica, and a few other minerals; most limestones consist of calcite, dolomite, and minor amounts of quartz, feldspar, and clay. Weathering and erosion of rocks produce particles of stone, gravel, sand, silt, and clay.

Table 1: Rock and Mineral Constituents in
Aggregates

Recycled concrete, or crushed waste concrete, is a feasible source of aggregates and an economic reality, especially where good aggregates are scarce. Conventional stone crushing equipment can be used, and new equipment has been developed to reduce noise and dust

Aggregates must conform to certain standards for optimum engineering use: they must be clean, hard, strong, durable particles free of absorbed chemicals, coatings of clay, and other fine materials in amounts that could affect hydration and bond of the cement paste. Aggregate particles that are friable or capable of being split are undesirable. Aggregates containing any appreciable amounts of shale or other shaly rocks, soft and porous materials, should be avoided; certain types of chert should be especially avoided since they have low resistance to weathering and can cause surface defects such as pop outs.

Identification of the constituents of an aggregate cannot alone provide a basis for predicting the behavior of aggregates in service. Visual inspection will often disclose weaknesses in coarse aggregates. Service records are invaluable in evaluating aggregates. In the absence of a performance record, the aggregates should be tested before they are used in concrete. The most commonly used aggregates—sand, gravel, crushed stone, and air-cooled blast-furnace slag—produce freshly mixed normal-weight concrete with a density (unit weight) of 2200 to 2400 kg/m³ (140 to 150 lb/ft³).

Aggregates of expanded shale, clay, slate, and slag (Fig. 3) are used to produce structural lightweight concrete with a freshly mixed density ranging from about 1350 to 1850 kg/m³ (90 to 120lb/ft³).

Fig. 3. Lightweight aggregate. Expanded clay (left) and
expanded shale (right).
Image courtesy: University of Memphis

Other lightweight materials such as pumice, scoria, perlite, vermiculite, and diatomite are used to produce insulating lightweight concretes ranging in density from about 250 to 1450 kg/m³ (15 to 90 lb/ft3). Heavyweight materials such as barite, limonite, magnetite, ilmenite, hematite, iron, and steel punchings or shot are used to produce heavyweight concrete and radiation-shielding concrete (ASTM C 637 and C 638). Only normal-weightaggregates are discussed in this article

Normal-weight aggregates should meet the requirements of ASTM C 33 or AASHTO M 6/M 80. These specifications limit the permissible amounts of deleterious substances and provide requirements for aggregate characteristics. Compliance is determined by using one or more of the several standard tests cited in the following sections and tables. However, the fact that aggregates
satisfy ASTM C 33 or AASHTO M 6/M 80 requirements does not necessarily assure defect-free concrete.

For adequate consolidation of concrete, the desirable amount of air, water, cement, and fine aggregate (that is, the mortar fraction) should be about 50% to 65% by absolute volume (45% to 60% by mass). Rounded aggregate, such as gravel, requires slightly lower values, while crushed aggregate requires slightly higher values. Fine aggregate content is usually 35% to 45% by mass or volume of the total aggregate content.

By Kieu Hai Dang

Classification of Soil Shear Tests According to Drainage Conditions

6:00 AM Civil Engineering , Construction Materials , Soil Mechanics

The shear strength parameters in the case of saturated soils depend very much upon the drainage conditions and therefore in the laboratory shear test, the drainage condition expected in the field for a particular problem should be simulated. Based on drainage condition the shear tests are classified as following 3 types.

Unconsolidated Undrained Test (UU test)
Consolidated Undrained Test (CU test)
Consolidated Drained Test (CD test)

1. Unconsolidated Undrained Test (UU)

Drainage is not permitted throughout the test. In the case of direct shear test drainage is not permitted during the application of both normal stress and shear stress. In the case of triaxial compression test drainage is not permitted during the application of both cell pressure and deviator stress. Since the test is conducted fast allowing no time for either consolidation of sample initially or dissipation of pore pressure in later stage, the test is also called quick test.

2. Consolidated Undrained Test (CU)

In this type of shear test the soil specimen is allowed to consolidate fully under initially applied stress and then sheared quickly without allowing dissipation of pore pressure. In the case of direct shear test the specimen is allowed to consolidate fully under applied normal stress and then sheared at high rate of strain to prevent dissipation of pore pressure during shearing. In the case of triaxial compression test the specimen is allowed to consolidate fully under the applied cell pressure and then the pore water outlet is closed and the specimen is subjected to increasing deviator stress at higher rate of strain.

3. Consolidated Drained Test (CD)

In this type of shear test drainage is allowed throughout the test. The specimen is allowed to consolidate fully under the applied initial stress and then sheared at low rate of strain giving sufficient time for the pore water to drain out at all stages. The test may continue for several hours to several days.

Causes of Cracks in Concrete

6:00 AM Civil Engineering , Concrete , Construction Materials

Cracks in concrete are caused due to following factors.

One of the main causes of cracks in concrete is the cooling and contraction due to setting of concrete. Volume change and stresses due to shrinkage are independent of any external load or stress applied.
Cracks may develop in a smaller section attached to a large section due to differential expansion and contraction. Therefore a joint should be provided at the change of section. There is more possibility of cracking of fixed members than those which are free to expand and contract as simply supported beams.
Repeated expansion and contraction or alternate wetting and drying which may result in gradual disintegration of poor concrete.
Rapid drying due to hot weather and high speed winds or absorption of water from the concrete by wooden forms also a cause of cracking in concrete. Therefore the form work on which fresh concrete is placed must be damped, or it should be waterproof so that it does not absorb water from fresh concrete.
Loose form work can also lead to cracks in concrete. so form work should be of adequate strength to bear the pressure of the wet concrete without swelling, spreading or any movement.
Concentration of tensile reinforcement at square openings or re-entrant angles (as in corners of door and window openings) causes cracks. This can be avoided by suitably placing reinforcements having adequate covering. Sufficient thickness of concrete should be given at the points where bars are bent up and anchored.
Minute cracks on the tension side of a reinforced concrete member are unavoidable due to poor tensile strength of concrete as compared to steel and which must crack when steel reinforcement taken its load. Those cracks, however, should be fine enough for moisture penetration to prevent corrosion of the reinforcement.
Hair cracks are the result of unequal shrinkage of the surface concrete and the mass behind it. Delayed finishing and final floating of concrete can avoid these cracks up t o a certain limit.
Surface cracks are also caused by surface dressing with a mortar having too rich in cement. Too much water, insufficient curing, or from over trowelling. One method of avoiding such hair cracks is to remove the surface skin of the concrete by brushing it with a stiff brush soon after setting.
Contraction of concrete is more harmful than expansion as it sets up tensile stresses in the structure, particularly those with a large surface area and thus form cracks. Such contracting cracks may be prevented by inserting reinforcement near the surface. Closely spaced reinforcement of small diameter and near the surface is more effective than large diameter bars further apart from the surface.

Precautions During Concrete Placement

5:00 AM Civil Engineering , Concrete , Construction Materials

The concrete, as it comes out of the mixer or as it is ready for use on the platform, is to be transported and placed on the formwork.

Following precautions should be taken during placing of concrete.

The formwork or the surface which is to receive the fresh concrete should be properly cleaned, prepared and well watered.
It is desirable to deposit concrete as near as practicable to its final position.
The large quantities of concrete should not be deposited at a time. Otherwise the concrete will start to flow along the formwork and consequently the resulting concrete will not have uniform composition.
The concrete should be dropped vertically from a reasonable height. For vertical laying of concrete, care should be taken to use stiff mix. Otherwise the bleeding of concrete through cracks in forms will take place. The term bleeding is used to mean the diffusion or running of concrete through formwork.
The concrete should be deposited in horizontal layers of about 150 mm height. For mass concrete, the layers may be of 400 mm to 500 mm height. The accumulation of excess water in upper layers is known as the laitance and it should be prevented by using shallow layers with stiff mix or by putting dry batches of concrete to absorb the excess water.
As far as possible, the concrete should be placed in single thickness. In case of deep sections, the concrete should be placed in successive horizontal layers and proper care should be taken to develop enough bond between successive layers.
The concrete should be thoroughly worked around the reinforcement and tapped in such a way that no honeycombed surface appears on removal of the formwork. The term honeycomb is used to mean comb or mass of waxy cells formed by bees in which they store their honey. Hence, if this precaution is not taken, the concrete surface so formed would have a honeycomb like surface.
The concrete should be placed on the formwork as soon as possible. But in no case, it should be placed after 30 minutes of its preparation.
During placing, it should be seen that all edges and corners of concrete surface remain unbroken, sharp and straight in line.
The placing of concrete should be carries out uninterrupted between predetermined construction joints.

6 Common Types of Rollers Used for Compaction Work

5:00 AM Civil Engineering , Construction Materials , Soil Mechanics

Depending upon the project requirement and soil to be compacted, different types of rollers are used for compaction work. The various types of rollers which are used for compaction are:

Cylindrical Rollers
Sheepsfoot Rollers
Pneumatic tyred Rollers
Smooth wheeled Rollers
Vibratory Rollers
Grid Rollers

The following types of road rollers are generally used.

1. Cylindrical Roller

This is a light roller of iron, concrete or stone; drawn by hand or bullocks. The size varies, but it is generally about 1 metre in dia. and about 1.5 metre long.
This ground pressure generated by this type of roller is about 7 kg/cm².

2. Sheepsfoot Roller

Sheepsfoot Roller

As the name indicates, this type of roller consists of a drum having many round or rectangular shaped protrusions or “feet” on it. These rollers are also called tamping rollers.

Various types are available having different diameters and widths of drum and different lengths and shapes of feet. The most common type is the one having two drums 1.22 meters wide and 1.06 either as taper-foot or club-foot rollers according to the shape of the feet.
Area of each protrusion can vary from 30 to 80 cm².
The coverage area is about 8 to 12%.
The thickness of compacting layer is kept about 5 cm more than the length of each foot.
This type of roller mostly used for compaction of cohesive soils such as heavy clays and silty clays. Not effective with sandy soils.
The weight of the drum can be increased by filling the drum with water or damp sand.
The factors that governs the amount of compaction of soil are as follow:
- Gross weight of the roller
- Area of each feet
- No of feet or lugs in contact with ground
- Total no of feet per drum
- Maximum pressure is exerted on soil when a foot is vertical.
The soil is supposed to be consolidated when the impression by the projecting teeth is not more than 12 mm deep or when the surface has been rolled 16 to 20 times.
10 to 20 passes are generally required to give complete coverage.
The density of the consolidated soil should be about 1.48 kg/cm³. The top layer has to be finished with a smooth wheel roller.
Pressure on the feet may be increased by filling the drum with wet sand or some other material, which may be 4 to 7kg/cm² for light rollers and upto 25 to 70 kg/cm² for giant rollers.

3. Pneumatic Tyred Rollers

6 Common Types of Rollers Used for Compaction Work

Pneumatic tyred roller

This type of roller consists of a heavily loaded wagon with several rows of four to six closely spaced tyres. This is also called rubber tyred roller.
It provided uniform pressure throughout the width.
2 factors governing the amount of compaction are as follow
- Tyre pressure
- Area of contact
Tyre pressure may be upto about 7 kg/cm²
The coverage area is about 80%.
The gross weight of the roller is about 6 to 10 tonnes which can be increased to 25 tonnes by ballasting with steel section or other means.
The maximum density can be achieved by 8 passes of the roller. The optimum speed of roller is between 6 to 24 km/h.
Used for compacting cold laid bituminous pavements, soft base course materials or layers of loose soil. These rollers are also suitable for compacting closely graded sands, and fine-grained cohesive soils at moisture content approaching their plastic limits, though the compaction is not as high as that with the smooth wheel roller.
They are particularly efficient when used to finish off the embankment compacted by sheepsfoot roller or on loose sandy soils.

4. Smooth Wheeled Roller

Tandem Roller

This type of roller consists of a large steel drum in front and one or two wheels or drum on the rear end.
Depending upon the number of wheels on the rear, it can be of following two types:

Tandem rollers (having one wheel at rear and one wheel in front)
Three wheeled rollers (having two wheel at rear and one in front)

The weight of tandem roller varies from 2 to 8 tonnes and that of two wheeled roller varies from 8 to 10 tonnes.

Three wheeled roller
It ground coverage provided by smooth wheeled roller is 100%.
The weight of the roller can be increased by filling the inside space of the drum with water or wet sand. This is called ballasting.
The ground pressure exerted by tandem rollers is about 10 to 17 kg/cm².
Performance of smooth wheel roller depend upon it load per cm width and diameter of the roll.
The speed and number of passes of a smooth wheeled roller depends on the type of soil to be compacted and project requirements. The optimum working speed has found to be 3 to 6 km/h and about 8 passes are adequate for compacting 20 cm layer.
Smooth wheel rollers are most suitable for consolidating stone soling, gravel, sand, hard core, ballast and surface dressings. Not suitable for consolidating embankments and soft sub-grades, but are better suited than any other plant for compacting silty and sandy soils and with fewer passes. When the moisture content is a little more than optimum it will compact more easily.
The two types (i.e. steam and diesel) are very much alike, the difference being mainly in power unit. Adjustable weight devices are available which can be fitted to the wheels so that the rolling pressure can be varied to suit different consolidation requirements. When engaged on heavy work, the sliding weights must always be at the rear of the roller. The sliding weight must never be moved when the roller is on a gradient.
The steam road roller can stand heavier wear and tear and is much simpler to work than the diesel roller but it takes over an hour to start up and cannot be temporarily shut off, while the diesel type can be started up and shut down in a few minutes and does not consume fuel when standing temporarily idle on a job. Steam road rollers are now getting outdated. Diesel rollers are cheaper in running cost.
Some rollers are made with its prime movers or engine as a separate unit which is a tractor, and is mounted on the roller, and which has its own advantages.
Scrappers are provided on all the wheels in adjustable positions covering the full width of the roll, with water sprinkling arrangement, for scraping of the mud and keeping the wheels clean during rolling.
The maximum grade a road roller can climb is 1 in 5.

5. Vibratory Rollers

Vibratory Roller

This type of roller is fitted with one or two smooth surfaced steel wheels 0.9 m to 1.5 m in diameter and 1.2 m to 1.8 m wide.
Self propelled vibratory rollers are now available weighing from 4 to 6 tonnes.
Vibrations are generated by the rotation of an eccentric shaft inside.
A vibratory roller is used for compacting granular base courses. It is sometimes used for asphaltic concrete work.

6. Grid Rollers

Grid Roller

These rollers have a cylindrical heavy steel surface consisting of a network of steel bars forming a grid with squire holes and may be ballasted with concrete blocks.
They are generally towed units and can operate at speeds between 5 and 24 km/h.
Typical weights vary between 5 tonnes net and 15 tonnes ballasted.
Grid rollers provide high contact pressure but little kneading action and are suitable for compacting most coarse grained soils.

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