Engineering and Construction

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During construction, excavations must be kept free of standing water.
Such water may come from precipitation or it may come from ground-water seepage originating from any of a number of sources, such as surface water percolating through the soil, underground streams, perched water moving over impervious soil strata, or adjacent permanently sat-urated soil areas where the excavationextends below the water table.

Some shallow excavations in relatively dry soil conditions may remain free of standing water without any intervention. But most excavations require some form of dewatering, or extraction of water from the excavation or surrounding soil. The most common method of dewatering is to remove water by pumping as it accumulates in pits, called  sumps, created at low points in the excavation. Where the volume of ground-water flowing into the excavation is great, or with certain types of soils, particularly sands and silt, that may be softened by constant seepage, it may be necessary to keep ground-water from entering the excavation at all. This can be done either by pumping water from the surround-ing soil to depress the water table below the level of the bottom of the excavation or by erecting a watertight barrier, such as a slurry wall, around the excavation (Figure 2.26).

Figure 2.26 Two methods of keeping an excavation dry, viewed in cross section. The water sucked from well points depresses the water table in the immediate vicinity to a level below the bottom of the excavation. Watertight barrier walls work only if their bottom edges are inserted into an impermeable stratum that prevents water from working its way under the walls.

 Well points are commonly used to depress the water table. These are vertical sections of pipe with screened openings at the bottom that keep out soil particles while al-lowing water to enter. Closely spaced well points are driven into the soil around the entire perimeter of the excavation. These are connected to horizontal header pipes leading to pumps that continually draw water from the system and discharge it away from the building site. Once pumping has drawn down the water table in the area of the excavation, work can continue “in the dry” (Figures 2.27 and 2.28). For excavations deeper than the 20 feet (6 m) or
so that cannot be drained by a suction pump stationed at ground level, two rings of well points may be required, the inner ring being driven to a deeper level than the outer ring, or a single ring of deep wells with submersible force pumps may have to be installed.

Figure 2.27 Well point dewatering. Closely spaced vertical well points connect to the larger-diameter header pipe.

Figure 2.28 An excavation is kept dry despite the close proximity of a large body of water. Two dewatering pumps are visible in the foreground.  A pair of header pipes and numerous well points can be seen surrounding the excavation.

In some cases, well points may not be practical: they may have insufficient capacity to ensure that an ex-
cavation remains dry; restrictions on the disposal of groundwater may limit their use; reliability due to power outages may be a concern; or lowering of the water table may have serious adverse effects on neighboring buildings by causing consolidation and settling of soil under their foundations or by exposing untreated wood foundation piles, previously protected by total immersion in water, to decay. In these cases, a watertight barrier wall may be used as an alternative (Figure 2.26). Slurry walls and soil mixed walls (pages 40–45) can make excellent watertight barriers. Sheet piling can also work, but it tends to leak at the joints.  Soil freezing is also possible. In this method, an array of vertical pipes similar to well points is used to continuously circulate coolant at temperatures low enough to freeze the soil around an excavation area, resulting in a temporary but reliable barrier to groundwater. Watertight barriers must resist the hy-drostatic pressure of the surrounding water, which increases with depth, so for deeper excavations, a system of bracing or tiebacks is required. A watertight barrier also works only if it reaches into a stratum of impermeable soil such as clay.
Otherwise, water can flow beneath the barrier and rise up into the excavation.

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One of the principal advantages of steel decking is that it acts as a working  platform. The loads that it supports arise mainly during the concreting operation.

Lightweight concrete (LWC), therefore, has advantages in terms of its reduced self-weight in comparison to normal-weight concrete (NWC). In the UK the main form of LWC comprises Lytag and sand with a dry density range of 1800–1900kg/m3.The compressive strength of Lytag LWC is similar to NWC, although the elastic modulus is lower.

Concrete is usually placed by pump,mainly for reasons of speed, but also because it is difficult to ‘skip’ the concrete with the decking in place above. Indeed, it is usual practice to deck-out a number of floors ahead of the concreting  operation. The decking is attached to its supports by shot-fired pins or self-tapping screws and later by welded shear-connectors, welding or bolting.

Unlike traditional reinforced concrete, there is no need to restrict bay sizes during construction because the decking serves to distribute early age and shrinkage strains, thereby eliminating the formation of wide cracks.Typical pumped pours are 500–1000m2. LWC also has a higher tensile strain capacity than NWC, reducing the tendency for cracking.The mesh reinforcement is of a nominal size to control cracking at supports, and to act as ‘fire reinforcement’. A142 or A193 are the common sizes specified.

The concrete grade (cube strength in N/mm2) is normally specified as C30 to C40. Pumped concrete also contains additives to aid lubrication. For  this reason the slump test is not a good measure of workability and so the flow-meter method is often used. The concrete is usually tamped level by fixing the tamping rails to the support beams. As these beams deflect, the slab level adopts that  of the support beams. In propped construction, the slab level deflects further on removal of the props.

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Plastic shrinkage cracking is defined as the cracking of the surface of concrete that develops in freshly mixed concrete soon after it has been placed and while it is still being finished. Because the concrete is still plastic, this type of shrinkage cracking is termed plastic shrinkage to distinguish it from drying shrinkage described in the previous sections. Plastic shrinkage cracking is most often associated with climate conditions, such as high temperatures, low humidity, and high winds that result in rapid evaporation of moisture from the concrete surface. Especially in the desert areas of the southwestern United States, the water may be quickly drawn from the concrete resulting in plastic shrinkage.

Plastic shrinkage cracks develop because of the tensile stresses generated in the concrete due to the evaporating water. The concrete cracks often form in distinct patterns of short and irregular cracks. The lengths are generally from a few inches to a few feet in length with spacing similar to crack lengths. For a discussion of the measures to prevent plastic shrinkage cracking, see Design and Control of Concrete Mixtures (PCA, 1994).

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One option to control the location of shrinkage cracking is to use contraction joints. A contraction joint, also known as a control joint, is usually formed as a continuous straight slot in the top of the concrete slab. The slot forms a plane of weakness in which the shrinkage crack will develop. A common method of constructing the slot is to saw-cut the concrete after it is strong enough to resist tearing or other damage by the saw blade. Normally the concrete slab-on-grade foundation is saw-cut 4 to 12 h after the concrete begins to harden. It is often recommended that the concrete slab-on-grade be saw-cut to a depth equal to 1/4 the thickness of the slab. If the slab is not saw-cut to an appropriate depth, then the drying shrinkage crack will deviate from the saw-cut, such as shown in Fig. 12.13.

There are many other methods for installing contraction joints in concrete slab-on-grade foundations. For example, during the finishing of the top surface of the concrete, grooves can be formed in the concrete. Another possible option is to install plastic strips in the top of the slab before the concrete has set, such as shown in Fig. 12.14.



FIGURE 12.14 View of a plastic strip that was used to form a contraction joint. The plastic
strip is sometimes referred to as a zip-strip. The gaps that have opened-up on both sides of the
plastic strip were caused by drying shrinkage.





The goals of contraction joints are to force the shrinkage cracking to develop at specific areas and to minimize the width of the shrinkage cracks. By limiting the width of the shrinkage crack, vertical loads are transmitted across the contraction joint by aggregate interlock between the opposite faces of the crack. In order to increase load transfer across contraction joints, steel reinforcement or steel dowels can be placed across the joint.

Design and Control of Concrete Mixtures (PCA, 1994) provides two tables that can be useful for the design of contraction joints, the first (Table 9-1) provides dowel sizes and spacing at contraction joints and the second (Table 9-2) provides maximum spacing of contraction joints as a function of floor slab thickness, slump, and maximum-size aggregate. As this document indicates, contraction joints should not be random,  rather they should consist of a standard square pattern with the spacing of contraction joints as recommended by the text. The joint spacing should be decreased if the concrete is suspected of having high shrinkage characteristics and the resulting panels should be approximately square.

Contraction joints are commonly used for buildings such as warehouses that have large concrete slab-on-grade foundations. It should be recognized that if there is no steel reinforcement across the contraction joints, they become planes of weakness in the foundation. For example, Fig. 12.15 shows about 1 in. (25 mm) opening of a contraction joint due to slope movement that pulled apart the foundation at the contraction joints. Contraction joints placed at the living areas of slab-on-grade foundations can also result in damage to brittle floor coverings placed over the contraction joints. For example, Fig. 12.16 shows the opening of a contraction joint (lower arrow) that has resulted in the cracking of shower tiles (upper arrow) that span the contraction joint.

As previously mentioned, concrete having a low volume-to-surface ratio will produce more drying shrinkage.

This is important because excessive shrinkage is often observed for slab-on-grade foundations that are too thin.

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There can be many different reasons for the development of excessive concrete shrinkage. For example, excessive shrinkage cracking can develop in concrete if the mix was inadequately prepared with a water-cementitious materials ratio that is too high (i.e., too much water in the mix). Another possibility is that water may be added to the concrete at the job site in order to make the concrete easier to place and consolidate, in which case the extra water will cause excessive drying shrinkage. Other possibilities are that the concrete slab-on-grade is too thin (i.e., low volume-to-surface ratio) or it is not adequately cured. Figures 12.11 and 12.12 show two examples of excessive concrete shrinkage cracks in a slab-on-grade foundation.

There are three basic methods that can be used to limit or prevent the development of shrinkage
cracks in concrete slab-on-grade foundations, as follows (PCA, 1994):

1. Post-tensioned slabs. A concrete slab-on-grade foundation can be built with post-tensioning cables, which basically consist of steel strands in ducts that are tensioned after the concrete hardens. By tensioning the cables, the concrete is subjected to a compressive stress that will counteract the development of tensile stress due to concrete shrinkage. If the compressive stress is high enough, an essentially crack-free foundation can be created. Especially for a long slab-on-grade foundation, special effort may be needed to reduce subgrade friction.

2. Steel reinforcement. It has been stated that steel reinforcement equal to 0.5 percent of the cross-sectional area of the slab-on-grade foundation will be sufficient to essentially eliminate visible shrinkage cracking.

3. Expansive cements. Expansive cement is defined as  hydraulic cement that expands slightly during the early hardening period after setting. Concrete that contains expansive cement can be used to offset the amount of anticipated drying shrinkage. However, steel reinforcement is still needed in order to produce compressive stresses during and after the expansion period of the cement.

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Concrete expands slightly with a gain in moisture and contracts with a loss in moisture. This contraction of the concrete with a loss of moisture has been termed drying shrinkage. The United States Department of the Interior (1947) performed drying shrinkage tests on small concrete beams (4 × 4 × 40 in.) and they measured the drying shrinkage for a period of 38 months. The small concrete beams were initially moist-cured for 14 days at 70°F and then stored for 38 months in air at the same temperature and 50 percent relative humidity. The results of the study showed that 34 percent of the drying shrinkage occurred within the first month and 90 percent of the drying shrinkage occurred within 11 months. The remaining 10 percent of the shrinkage occurred from 11 months to the end of the test (38 months). Hence the results of this study on small concrete specimens showed that most drying shrinkage occurs within the first year after placement, with the amount of shrinkage decreasing with time.

It has also been observed that concrete near the surface dries and shrinks faster than the inner concrete core (Hanson, 1968). Other studies have shown that the volume-to-surface ratio is very important in the amount of drying shrinkage (Hansen and Mattock, 1966). Concrete having a low volume-to-surface ratio will shrink much more than the same concrete having a high volume-to-surface ratio. In addition, it has been observed that drying shrinkage continues longer for larger masses of concrete.

For a constant volume-to-surface ratio of concrete, the factors governing the amount of drying
shrinkage are as follows (PCA, 1994):

1. Water per unit volume of concrete. The amount of water per unit volume of concrete is by far the most important factor affecting the amount of drying shrinkage. The more water in the concrete mix, the greater the amount of drying shrinkage. One study at M.I.T. showed that for each 1 percent increase in mixing water, concrete drying shrinkage was increased by about 2 percent (Carlson, 1938). Another study showed that doubling the water in the concrete mix from 200 to 400 lb/yd3 caused a five times increase in the amount of drying shrinkage.

2. Increased water requirement. Any alteration of the mix design that increases the water requirement of the cement paste will lead to an increase in drying shrinkage. For example, additional mix water may be needed to increase the slump or to compensate for high temperatures of the freshly mixed concrete. Likewise, more water per unit volume will be required for those concrete mixes that have a high proportion of fine aggregates.

3. Coarse aggregates. The coarse aggregates physically restrain the drying shrinkage of the cement paste. Hence dense and relatively incompressible coarse aggregates will be more difficult to compress and will reduce the amount of drying shrinkage as compared to soft and deformable aggregates. Examples of dense and relatively incompressible aggregates that reduce the amount of drying shrinkage include aggregates composed of quartz, granite, and feldspar (ACI, 1980).

4. Curing. Curing of concrete is accomplished by maintaining a satisfactory moisture content and temperature in the concrete for a specified period of time immediately following the placement and finishing of the concrete. In essence, the objective of curing is to prevent the loss of moisture from the concrete. Curing can improve many desirable concrete properties, such as strength, dura- bility, watertightness, abrasion resistance, and resistance to freezing and thawing. In addition, because the loss of water from concrete causes it to shrink, the process of curing will reduce the amount of drying shrinkage.

5. Admixtures. Some admixtures require an increase in the water content of the concrete mix and hence they will cause an increase in drying shrinkage. In addition, studies have shown that accelerators, such as calcium chloride, can substantially increase the drying shrinkage of concrete.

6. Cement. The type of cement, cement fineness and composition, and the cement contents may have some effect on the amount of drying shrinkage, but the effect is usually small.

The basic reason for the development of shrinkage cracks in concrete slab-on-grade foundations is that drying shrinkage produces tensile stresses that are in excess of the concrete’s tensile strength.

Development of Shrinkage Cracks in Concrete Slab-on-grade Foundations.   There can be many different reasons for the development of excessive concrete shrinkage. For example, excessive shrinkage cracking can develop in concrete if the mix was inadequately prepared with a water-cementitious material...

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The soil or groundwater samples should be obtained after the site has been graded and the location of the proposed building is known. For the planned construction of shallow foundations, near surface soil samples should be obtained for the sulfate testing. For the planned construction of deep foundations that consist of concrete piles or piers, soil and groundwater samples for sulfate testing should be taken at various depths that encompass the entire length of the concrete foundation elements.

The geotechnical engineer should select representative soil samples for testing. If the site contains clean sand or gravel, then these types of soil often have low sulfate contents because of their low capillary rise and high permeability that enables any sulfate to be washed from the soil. But clays can have a higher sulfate content because of their high capillary rise that enables them to draw up sulfate bearing groundwater and their low permeability that prevents the sulfate from being washed from the soil.

Sulfate concentrations in the soil and groundwater can vary throughout the year. For example, the highest concentration of sulfates in the soil and groundwater will tend to occur at the end of the dry season or after a long dry spell. Likewise, the lowest concentration of sulfates will occur at the end of the rainy season, when the sulfate has been diluted or partially flushed out of the soil. The geotechnical engineer should recognize that sampling of soil or groundwater at the end of a heavy rainfall season will most likely be unrepresentative of the most severe condition.

Soil that contains gypsum should always be tested for sulfate content. This is because soluble sulfate (SO4) is released as gypsum (Ca SO4⋅ 2H2O) weathers. Gypsum is an evaporate and can rapidly weather upon exposure to air and water.

There are many environments that could lead to the chemical attack of concrete. For example, if the site had been previously used as a farm, there may be fertilizers or animal wastes in the soil that will have a detrimental effect on concrete. A corrosive environment, such as drainage of acid mine water, will also lead to the deterioration of concrete. Another corrosive environment is seawater, which has both moderate soluble sulfate content and a high salt content that can attack concrete through the process of chemical reactions involving sulfate attack and deterioration of concrete due to the physical growth of salt crystals.

As indicated in Table 18.2, of the types of Portland cements, the most resistant cement is type V, in which the tricalcium aluminate content must be less than 5 percent. Depending on the percentage of soluble sulfate (SO4) in the soil or groundwater, a certain cement type is required as indicated in Table 18.2. In addition, minimum water-cementitious ratios and minimum unconfined compressive strengths f'C are required depending on the amount of sulfate in the soil or groundwater (see Table 18.2).

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The sulfate resistance of concrete depends on many different factors. In general, the degree of sulfate attack of concrete will depend on the type of cement used, quality of the concrete, soluble sulfate concentration that is in contact with the concrete, and the surface preparation of the concrete
(Mather, 1968).

1. Type of cement. There is a correlation between the sulfate resistance of cement and its tricalcium aluminate content. As previously discussed, it is the chemical reaction of hydrated calcium aluminate and gypsum that forms ettringite. Therefore, limiting the tricalcium aluminate content of cement reduces the potential for the formation of ettringite. It has been stated that the tricalci- um aluminate content of the cement is the greatest single factor that influences the resistance of concrete to sulfate attack, where in general, the lower the tricalcium aluminate content, the greater the sulfate resistance (Bellport, 1968).

2. Quality of concrete. In general, the more impermeable the concrete, the more difficult for the waterborne sulfate to penetrate the concrete surface. To have a low permeability, the concrete must be dense, have high cement content, and a low water-cementitious materials ratio. Using a low water-cementitious materials ratio decreases the permeability of mature concrete (PCA, 1994). A low water-cementitious materials ratio is a requirement of ACI (1990) for concrete subjected to soluble sulfate in the soil or groundwater. For example, the maximum water-cementitious materials ratio must be equal to or less than 0.50 for concrete exposed to moderate sulfate exposure and 0.45 for concrete exposed to severe or very severe sulfate exposure (see Table 18.2). There are many other conditions that can affect the quality of the concrete. For example, a lack of proper consolidation of the concrete can result in excessive voids.

Another condition is the corrosion of reinforcement, which may crack the concrete and increase its permeability. Cracking of concrete may also occur when structural members are subjected to bending stresses. For example, the tensile stress due to a bending moment in a footing may cause the development of microcracks that increases the permeability of the concrete.

3. Concentration of sulfates. As previously mentioned, it is often the geotechnical engineer who obtains the soil or water specimens that will be tested for soluble sulfate concentration. In some cases after construction is complete, the sulfate may become concentrated on crack faces. For example, water evaporating through cracks in concrete flatwork will deposit the sulfate on the crack faces. This concentration of sulfate may cause accelerated deterioration of the concrete.

4. Surface preparation of concrete. An important factor in concrete resistance is the surface preparation, such as the amount of curing of the concrete. Curing results in a stronger and more impermeable concrete, which is better able to resist the effects of salt intrusion (PCA, 1994).

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There has been considerable research, testing, and chemical analysis of sulfate attack. Two different mechanisms of sulfate attack have been discovered: chemical reactions and the physical growth of crystals.

Chemical Reactions. The chemical reactions involving sulfate attack of concrete are complex and studies have discovered two main chemical reactions (Lea, 1971; Mehta, 1976). The first is a chemical reaction of sulfate and calcium hydroxide (which was generated during the hydration of the cement) to form calcium sulfate, commonly known as gypsum. The second is a chemical reaction of gypsum and hydrated calcium aluminate to form calcium sulfoaluminate, commonly called ettringite (ACI, 1990).

As with many chemical reactions, the final product of ettringite causes an increase in volume of the concrete. Hurst (1968) indicates that the chemical reactions produce a compound of twice the volume of the original tricalcium aluminate compound. Concrete has a low tensile strength and thus the increase in volume fractures the concrete, allowing for more sulfates to penetrate the concrete, resulting in accelerated deterioration.

Physical Growth of Crystals. It has been shown that there can be crystallization of the sulfate salts in the pores of the concrete. The growth of crystals exerts expansive forces within the concrete, causing flaking and spalling of the outer concrete surface. Besides sulfate, the concrete, if porous enough, can be disintegrated by the expansive force exerted by the crystallization of almost any salt in its pores (Tuthill, 1966; Reading, 1975).

Damage due to crystallization of salt is commonly observed in areas where water is migrating through the concrete and then evaporating at the concrete surface. Examples include the surfaces of concrete dams, basement and retaining walls that lack proper waterproofing, and concrete structures that are partially immersed in salt-bearing water (such as seawater) or soils. Figures 12.3 and 12.4 show two examples of concrete patio deterioration due to the crystallization of salts.

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Introduction
Sulfate attack of concrete is defined as a chemical and/or physical reaction between sulfates (usually in the soil or groundwater) and concrete or mortar, primarily with hydrated calcium aluminate in the cement-paste matrix, often causing deterioration (ACI, 1990). Sulfate attack of concrete occurs throughout the world, especially in arid areas, such as the southwestern United States. In arid regions, the salts are drawn up into the concrete and then deposited on the concrete surface as the groundwater evaporates. Sulfate attack of concrete can cause a physical loss of concrete and unusu- al cracking and discoloration of the concrete. Sulfate attack can also cause scaling, pitting, and etching of the concrete surface (Detwiler et al., 2000). Figures 12.1 and 12.2 show sulfate attack of a concrete patio.

Typically the geotechnical engineer obtains the representative soil or groundwater samples to be tested for sulfate content. The geotechnical engineer can analyze the soil samples or groundwater inhouse, but a more common procedure is to send the samples to a chemical laboratory for testing.

There are different methods to determine the soluble sulfate content in soil or groundwater. One method is to precipitate out and then weigh the sulfate compounds. A faster and easier method is to add barium chloride to the solution and then compare the turbidity (relative cloudiness) of barium sulfate with known concentration standards.

The geotechnical engineer is often required to provide soluble sulfate specifications and recommendations for concrete foundations. In addition to concrete foundation deterioration due to sulfate attack, there could also be deterioration of concrete or the steel reinforcement due to chloride attack and acid attack. According to AASHTO (1996), laboratory testing of soil and groundwater samples for sulfates, chloride, and pH should be sufficient to assess deterioration potential. When chemical wastes are suspected, a more thorough chemical analysis of soil and groundwater samples would be required.