Civil Engineering

Advantages and Disadvantages of Reinforced Concrete

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Reinforced concrete, as a structural material, is widely used in many types of structures. It is competitive with steel if economically designed and executed.

The advantages of reinforced concrete can be summarized as follows:

It has a relatively high compressive strength;
It has better resistance to fire than steel;
It has a long service life with low maintenance cost;
In some types of structures, such as dams, piers, and footings, it is the most economical structural material;
It can be cast to take the shape required, making it widely used in precast structural components. It yields rigid members with minimum apparent deflection.

The disadvantages of reinforced concrete can be summarized as follows:

It has a low tensile strength of about one-tenth of its compressive strength;
It needs mixing, casting, and curing, all of which affect the final strength of concrete;
The cost of the forms used to cast of concrete placed in the forms;
It has a low compressive strength as compared to steel (the ratio is about 1:10, depending on materials), which leads to large sections in columns of multistory buildings;
Cracks develop in concrete due to shrinkage and the application of live loads.

Design Philosophy

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For the designer, the primary boundary conditions are stability and allowable settlement, both as a function of time. This chapter therefore approaches the design process from these two aspects; the intention is not to trivialize the other aspects, but rather to simplify the subject. In the present chapter, ‘safety’ is not a specific concept, as it is in Chapter 3; the word ‘safety’ is here used in its generally accepted meaning.

An earth structure requires additional space because a difference in level as compared with ground level has to be spanned – above (in the case of filling) or below (in the case of excavation) – mostly by means of a slanting surface or slope. The extra space occupied is partly determined by the properties of the subsoil; the slope angles which can be achieved depend on the strength and deformation properties of the foundation, and, in the case of filling, of the filling material. In addition, aspects such as maintenance and management may affect the design. We have in mind factors such as the possibility of mowing by machine, the need for a maintenance platform, etc.

The designer’s task is to devise acceptable solutions for safely transferring the load to the subsoil. The load usually consists of the earth structure’s own weight. A load may also result from the function of the structure itself, such as for example traffic on roads and depots at industrial sites. An excavation is the equivalent of a negative load. In addition, in a country such as the Netherlands, the load exerted by the groundwater flow must not be underestimated. This load occurs is areas where there is permanent polder pumping, or in areas affected by temporary high sea and river water levels or by temporary pumping. These examples show that the concept of load must therefore be understood at its widest.

Every load on soil in principle upsets the prevailing pattern of the in situ stress. The science of mechanics teaches us that a change in the stresses leads to deformation. If the soil is not capable of absorbing the increased stresses, very large deformations may occur, possibly leading to collapse.

In addition, it is the importance of a particular earth structure which usually dictates the safety margin in the design. The design technique applied, in combination with the nature and scope of the soil investigation, is the key factor with respect to safety.

The primary focus in building a structure on subsoil of low bearing capacity and high compressibility is the control of stability and deformation. There are two important questions in this connection: is the structure stable in all circumstances and are the deformations in the structure allowable?

For any given load, the stability or equilibrium of an earth structure is determined by the shear strength of the soil. The shear strength, in its turn, depends on the magnitude of the strength parameters, such as cohesion and the angle of internal friction and the effective stress level. The methods which have been developed to make it possible to build safe earth structures are intended to:

· Restrict the shear stress; for example by gradually bridging the height differences by means of a gradual slope, which may or may not be combined with a berm or carefully raising the stress level by phased filling;

· Increase the shear strength in situ; for example by soil replacement or soil improvement.

In principle, it is always possible to construct a safe earth structure on highly compressible subsoil using traditional filling materials such as

Geotechnical Mechanisms

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Good design means not only determining the methods to be employed, but also planning well ahead which geotechnical mechanisms may be involved and which methods of calculation are needed to evaluate stability and deformation.

Some major geotechnical mechanisms are:

§ Shearing;

§ Uplift;

§ Squeezing;

§ Settlement;

§ Horizontal deformation;

§ Negative skin friction.

It should be borne in mind that the following explanatory notes on geotechnical mechanisms make no claim to be complete; there are other mechanisms and phenomena which have not been mentioned, but can occur in specific circumstance, and have a crucial effect on the geotechnical behaviour of the earth structure.

Shearing

A loss of equilibrium can occur in soil of low bearing capacity beneath the edges of fillings and excavations. In such cases, the shear resistance in the soil is insufficient to prevent the continuous movement of a particular piece of ground along a sliding surface. This mechanism is called shearing.

The origin of the shearing movement is usually the self weight of the soil.

In the case of steep slopes, because in the latter case the shear resistance of the soil can usually be mobilized over a greater length.

Sliding surface along which shearing occurs can in principle be of any kind. In practice they often seem to be curved, with the result that many methods of calculation are based on circular sliding surface; other sliding surfaces have however also been noted, in particular as a consequence of the layering of soils.

The risk of loss of stability due to shearing can be lessened by consolidation of the subsoil and also by making the slope more gradual, raising supporting berms or using a lightweight material for the slope. Stability can also be improved by lowering the water table.

Uplift

Whenever an impermeable clay or peat layer of low bearing capacity overlies a sand layer with relatively high water pressure, an unstable situation can arise in the clay or peat layer due to differences in water pressure. The water pressure in the impermeable layer may be lower than that in the sand layer after draining of the clay or peat layer or pumping out of a building excavation or trench; the difference in water pressure may also be due to the fact that the head of water in the sand layer is determined by a relatively high external water level. In such cases, the water pressure on the underside of the poorly permeable layer may cause the layer to swell or burst if its own weight is too low.

A phenomenon of this kind can be observed at floor level in excavations, for instance, or in a polder behind a dyke retaining a temporarily high external water level.

Squeezing

When a layer of low bearing capacity situated above a firm sand layer is subjected to a load from a fill whose horizontal dimensions are restricted, for instance in the case of a dyke or artificial mound, it is possible that this intermediate low bearing capacity layer may be forced horizontally outwards while the underlying layer is only slightly deformed and the top layer merely undergoes settlement. This phenomenon is known as the toothpaste effect or squeezing.

Settlement

The materials commonly found in the surface layers of the Northen and Western Netherlands, such as clay and peat, can be considerably compressed by filling, resulting in settlement of the top of the earth structure. The compression process in these water saturated and relatively incompressible materials usually goes through three phases.

Initial settlement, which occurs immediately and/or during the application of the load, e.g. the fill, and is relatively minor.
Consolidation settlement, which occurs gradually and in parallel with pore water dissipation.

Excess water pressures are set up by the application of the load, but because of the relatively low permeability of the material and the relatively long drainage route they are released only gradually. By far the greater part of the total anticipated settlement will be consolidation settlement.

Secondary settlement, also known as creep.

This settlement progresses more slowly with time; eventually, the rate of settlement tends asymptotically towards zero.

Horizontal deformation

At the edges of fills and excavations horizontal stresses build up in the highly compressible layers, leading to horizontal deformation. Since in layers of this kind it is only possible to mobilise a low level of passive earth pressure, these deformations can be relatively marked; they can often be observed up to ten metres in front the toe of the slope.

The result of horizontal soil deformations can be horizontal loads on any rigid foundation elements such as basement walls or foundation piles; loads of this kind, which are often unforeseen, can cause serious damage to structures.

Negative skin friction

As a result of the compression of the peat and/or clay layers, vertical shearing forces may affect foundation elements such as basement wall or foundation piles. The skin friction of filling materials is usually greater than that of the highly compressible layers. The greater the thickness of the fill, the greater the total vertical force on the foundation elements. This force is called negative skin friction; this mechanism can considerably reduce the allowable bearing capacity of a foundation. Moreover, negative skin friction can also increase the normal stress in foundation piles; in addition, extra deformation of foundation elements can trigger off settlement and differential settlement in the construction affected.

Other mechanisms

A considerable change in the shear stress level, for instance resulting from dynamic loading, may weaken loosely packed saturated sand and silt formations. Weakening of this kind is also known as liquefaction; it manifests itself as a sudden loss of stability of a mass of soil which may be of quite large extent; the ultimate result is predominantly extremely gradual slopes.

In certain circumstances, other mechanisms may also play a part. These include phenomena such as micro-instability, swelling, piping and erosion.

One possibly less obvious example is the effect of plants and animals on the stability of the structure, such as the influence of plant and tree roots or the possible consequences of holes dug by burrowing animals.

All of the phenomena listed under this heading, as well as other possible mechanisms are however excluded from consideration in the present volume.

Earth Structures

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Earth structures can be divided into two types:

§ Earth structures which impose a load on the subsoil, such as dams, road and rail beds, general land fill, banks and depots for soil and waste;

§ Earth structures which remove a load from the subsoil, such as water courses, river widenings, harbours, excavations for roads or railway lines, building excavations and trenches.

Below a number of comments on each of these structures is given.

General

When a load is applied to highly compressible layers of low bearing capacity by fill material, the water pressures increase because in the first instance the load is supported by the pore water. The effective stresses present in these layers do not undergo any significant changes during this period, so that the shear resistance also remains unaltered. As the excess water drains away, the effective stresses increase and the ground consolidates. However, it takers a long time for the drainage process to be completed because of the layer thickness which is often great, and the low permeability of the layers to be drained. The filling time, i.e. the time required to apply the load, is therefore much shorter in practice than the time needed for the water to drain away and for the soil to consolidate. During filling operations, therefore, shear stresses may occur at the edges of the fill which cannot be absorbed by the shear resistance at that point. As soon as the filling operations cease, however, the excess water pressures no longer increase; from that time on the soil consolidates. In practice filling operations are usually phased, i.e. the load is applied in stages and there is a waiting time between the stages.

Where a load is removed by excavation, the opposite effect occurs. The effective stresses in the subsoil are reduced, making the soil less firm. In the first instance, the only result is a reduction in the water pressure, while the effective stress and/or the shearing resistance remain unaltered at all levels. The result is that the slopes and excavation bases are initially stable. With time, however, the pore pressures rise with the result that the shear resistance in the layers in question diminishes. Contrary to filling, excavations undergo adverse effects if there are waiting times.

If the quality of the subsoil is such that filling would result in unacceptably high compression or unacceptably long waiting times between the phases, measures must be taken in the form of special foundation techniques.

These techniques are aimed at the following:

o Improvement of the subsoil, so that its composition is less sensitive to the effects of compression and failure.

This kind of improvement can be achieved by replacing the soil down to a certain level with soil of a better quality as regards its resistance to compression or failure. In practice this often means excavating a layer of peat and replacing it with sand. Installation of stone columns has a similar effect.

o Reducing the load on the subsoil so that the compression is less marked, and the resulting shear stresses are reduced.

The load can be reduced by introducing lightweight filling materials, such as for instance Flugsand, aerated concrete of PS rigid foam.

o Rapid drainage of the subsoil so that the waiting times can be shortened. Rapid drainage of the excess pore pressure water can be achieved by installing horizontal and/or vertical drainage systems. Vertical drainage is usually applied in such cases. In exceptional cases, suction is applied to the system, which results in acceleration of dissipation of excess water pressures.

Dams

Dykes, dams and quays are primarily water retaining structures. Earth structures of this kind have therefore been constructed from time immemorial using low permeability fill material such as clay. Nowadays, however, a low permeability facing layer of clay is usually applied to a structure consisting of a stable and incompressible material such as sand. The clay used must fulfil certain requirements as regard workability, shrinkability and permeability. Small quays and retaining dams are still however often constructed entirely of clay or clay-type material.

A distinction must be made between dams which have to cope with a constant difference in the water level, such as polder dykes and canal banks, and dams exposed to varying water levels, such as sea and river dykes. In the case of the latter, the loads and load combinations are of particular importance in the design.

The geometry and choice of materials for dyke construction are influenced to a great extent by the maximum outside water level, wave height, wave load and changes to the water table resulting from the load exerted by the dyke structure, etc. in the case of dykes of this kind, it is above all necessary to take measures over a wide area, e.g. covering slopes with a facing to prevent attack by waves on the outer slope. As a rule, materials such as stone, either tipped or placed in position, concrete elements or asphalt are used.

Extra attention must be paid to the level of the water table: situations where the water table intersects the inner slope, with the result that water could escape from the slope, must be avoided at all costs. This can be achieved by making use for example of a more gradual slope or by altering the water table with a ditch or by introducing a filter.

Most dykes, dams and quays are protected against further surface erosion by a layer of grass over a layer of topsoil; this layer too must meet special requirements.

In many cases, dams in the Netherlands are also required to carry traffic. In that case, even greater demands are placed on the structure.

Road and railway beds

Earth structures for road and railway beds railway beds are intended first and foremost to transmit the traffic loads to the subsoil without causing unacceptable deformations and in particular differential settlements; in any case, traveling comfort imposes stringent differential settlement requirements, both longitudinal and transversal.

The surface layers of structures of this kind, i.e. the road, pavement and the railway track must not only exhibit a minimal level of deformation but also possess a high bearing capacity. It is thanks to these qualities that traffic loads are absorbed at quite a shallow depth.

Connections between earth structures and bridge structures demand particular care. Bridge structures almost always exhibit different settlement behaviour from earth structures on account of the different type of foundations.

In recent years, widening of existing roads and railways has been the principal problem. Deformation in a transverse direction to the existing track must be kept to a minimum.

The slopes of a road or railway embankment are covered with topsoil, with grass or other plants sown on it; this facing is mainly intended to prevent erosion.

Banks

Banks are often raised to prevent the leakage of liquids and to reduce noise; their shape resembles that of dykes.

Banks are often erected around storage tanks for liquids; in the event of a disaster they prevent the liquid from escaping. Because of the nature of the liquids, banks of this kind are generally covered with an impermeable facing on the tank side, which must also cover the area between the toe of the bank and the tank. It is essential to take into account at the design stage the consequences of any contact between the liquid from the tank and the material from which the bank and the ground surface adjacent to the tank are constructed.

In horticultural areas, banks are sometimes used to create retention areas for rainwater.

Noise barriers are mostly positioned alongside roads and have no other purpose than to reduce the noise level in the adjacent residential area. In addition to the acoustic requirements, the design of the barrier is greatly influenced by the available space, the available material and aesthetic considerations. Dimensions, angle of slope and covering can therefore differ in each individual situation.

Dumps

Dumps for primary or secondary materials may be temporary or permanent. Primary material include naturally deposited materials such as for instance san and clay. Secondary materials are industrial residues, such as fly ash and blast furnace slag, as well as refuse, which may or may not be recycled, such as refuse incineration plant slag and material from crushing plants.

Both categories, primary and secondary, may be contaminated to a greater or lesser extent, so that sometimes an environmental impact report is necessary, and extra care has to be taken with the structure, in the form of a variety of protective measures. In such a case, extensive drainage arrangements are often necessary, as well as sealing systems above and below ground level in the form of impermeable sheeting, sand-bentonite layers or special screens.

Generally speaking, the slopes of these dumps have no other function than fulfil stability requirements, although measures to prevent surface erosion may also be necessary.

If the dump reaches a great height because of lack of space, for instance, and/or the stability of the subsoil is threatened, soil improvement is applied in the form of stabilization or soil replacement after partial excavation.

Excavations for waterways, ports, etc.

Excavations for ditches, drainage ditches, canals and ports are often carried out under water. The design must therefore not only take into account the nature of the subsoil but also, in particular, the available dredging equipment.

At water level, a bank revetment or berm is often built to counter the eroding effect of waves. Washing way or erosion of the lower levels is usually prevented by using gradual slopes and/or other design techniques.

Road and railways cuttings

Sections of roads and railways in built-up areas, and also in areas of particular natural or historic interest, are being laid more often in cuttings. So far as possible a cutting of this kind is carried out using the natural slope of the soil.

If the groundwater level in the areas in question is high, it will have to be lowered permanently, and a polder system is the result. Lowering the groundwater level can however have adverse environmental effects, such as subsidence of buildings, damage to crops and complications for surface water management. In order to restrict these effects, percolation screens or bentonite walls are installed.

Excavations

For buildings and engineering structures whose foundations have to be located relatively far below ground level, dry excavations of considerable size are required. The soil layers beneath the floor of such excavations will experience a certain reduction in ground pressure, which will however be pertly or completely reversed after the construction has been built and/or the excavation has been reviled. In compressible soils it is especially important not only to prevent adverse effects on the foundation of the building, resulting from deformation of the subsoil, but also damage to the surrounding area.

The stability of the slopes and the excavation must be guaranteed for a specified period of the construction time; a pumping system is usually installed to prevent water escaping from the slope and/or seeping up through the floor of the excavation due to local excess water pressures. The pumping system should lower the original water table to such an extent that it lies below the excavation depth. Sometimes pumping has to be carried out at several levels: open pumping or pumping from well points or any combination of the two.

Draining can however also lower the water level outside the excavation, resulting in settlements which may cause damage to the foundations of building and other structures.

Deformation of slopes and the excavation floor should also be kept to a minimum; in particular, deformations around the edge of the building pit may result in damage to the structure, such as for instance skewing of piles driven inside the excavated area.

Trenches

Trenches are usually dug for the laying of cables and/or pipe lines and are comparable to the excavations described above. Trenches are however much smaller in cross-section and as a rule remain open for a much shorter period. If personnel have to work in the trench, the stability of the trench wall must be taken into account. The longer the trench remains open and the greater the risk of reduction of the shear resistance, the greater the risk of loss of stability.

If drainage is carried out, just as in the case of excavations, it is necessary to take account of settlement in the vicinity of the trench.

Horizontal and vertical deformations near to the trench can also cause damage to adjacent constructions.

Low Bearing Capacity and Highly Compressible Subsoil

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The highly compressible subsoil which is the subject of this book occurs in large parts of the Western and Northern Netherlands. It consists of recently sedimented clay and peat layers, mainly deposited by rivers or the sea.

In this region, the layers extend to depths of 10-20 m below ground level; the ground-water level is high and in many cases comes almost up to ground level. Due to the manner in which they were formed, these layers have been found to contain a relatively large percentage of pores. Because of the weight of the layers lying above them, this porosity has gradually decreased over the years, but the layers in question are still much less dense than deeper and older deposits. The reason for this is the structure and composition of such deposits and the fact that the high groundwater level also results in a reduction in weight. When subjected to a load, by a few metres thickness of sand fill, for example, the porosity of these layers is further reduced, resulting in a noticeable compression of the layers and/or a considerable settlement of the ground level.

On the other hand, the removal of a load from these layers, for instance by excavating or raising the groundwater level, causes them to swell, though always to a lesser degree than the compression resulting from the load.

The large settlements resulting from normal stresses therefore means that this material is classified as ‘highly compressible’. It is however known that the consolidation behaviour of soil is closely related to the ultimate failure behaviour which occurs as a result of shear stresses. Highly compressible soil is generally also of low bearing capacity and is commonly referred to as ‘soft’, which means that it has poor resistance to deformation and has low bearing capacity.

Yet another characteristic of layers of this kind is their relatively low permeability. After a load has been applied, therefore, excess pore water pressure decreases only gradually; after removal of the load, there is a reduction in water pressure, which dissipates much more quickly. As the excess water pressure decrease with the dissipation of the water, the density of the material rises, which means that its resistance to deformation and failure also increases; the material becomes firmer, less soft. Removing the load gives the opposite result: the soil becomes softer. The process of drainage and consolidation under the effect of loading is known as the primary consolidation process.

Even after the excess pore water pressures have disappeared, in the layers in question deformation continues as a function of time, without the load being increased as a result of the ‘creep affect’. In the case of the types of soil under consideration, this secondary compression is certainly not negligible; in the case of sand, however, it can be disregarded.

The behaviour of soil layers of low bearing capacity and high compressibility as a basis for the building of earth structures is so important for the design and construction of these structures that it fully justifies the issuing of a manual such as the present publication.

Structural Concrete

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The design of different structures is achieved by performing, in general, two main steps: (1) determining the different forces acting on the structure using proper methods of structural analysis, and (2) proportioning all structural members economically, considering the safety, stability, serviceability, and functionality of the structure. Structural concrete is one of the materials commonly used to design all types of buildings. Its two component materials, concrete and steel, work together to form structural members that can resist many types of loadings. The key to its performance lies in strengths that are complementary: Concrete resists compression and steel reinforcement resists tension forces.

The term structural concrete indicates all types of concrete used in structural applications. Structural concrete may be plain, reinforced, prestressed, or partially prestressed concrete; in addition, concrete is used in composite design. Composite design is used for any structural member, such as beams or columns, when the member contains a combination of concrete and steel shapes.