CONCRETE FINISH PROBLEMS BASIC INFORMATION

What Are The Usual Problem In Concrete Finishing?

The skill required by carpenters to make and erect form work for concrete is seldom fully appreciated. The formwork must remain ‘true to line and level’ despite substantial loading from the wet concrete. Column and wall faces have to be strictly vertical, and beam soffits strictly level, or any departure will be easily visible by eye.

Formwork for concrete which is to remain exposed to view has to be planned and built as carefully as if it were a permanent feature of the building. Many methods have been tried to make the appearance of exposed concrete attractive: but any of them can be ruined by honeycombing, a bad construction joint, or by subsequent weathering revealing that one pour of concrete has not been identical with adjacent pours, or that the amount of vibration used in compacting one panel has been different from that used in others.

If concrete has to remain exposed to public view, then the resident engineer should endeavour to agree with the contractor what is the most suitable method for achieving the finish required if the specification or drawings do not give exact guidance on the matter. The problem is that if, through lack of detailed attention, a ‘mishap’ on the exposed surface is revealed when the formwork is struck, it is virtually impossible to rectify it.

Sometimes rendering the whole surface is the only acceptable remedy. Where concrete will not remain exposed to view, minor discrepancies can be accepted. ‘Fins’ of concrete caused by the mix leaking through butt joints in the formwork should be knocked off. Shallow honeycombing should be chiselled out, and a chase cut along any defective construction joint.

The cut-out area or chase should be washed, brushed with a thick cement grout, and then filled with a dryish mortar mix. This rectifying work should be done as soon as possible so the mortar mix has a better chance of bonding to the ‘green’ concrete.

Shrinkage cracking of concrete is a common experience. The shrinkage of concrete due to drying is of the order of 0.2–0.5mm/m for the first 28 days. Subsequently concrete may expand slightly when wet and shrink on drying.

The coefficient of temperature expansion or contraction is very much smaller, of the order of 0.007mm/m per degree centigrade of change. Rich concrete mixtures tend to shrink more than lean mixes. The use of large aggregate, such as 40 mm instead of 20 mm, helps to minimize shrinkage. To avoid cracking of concrete due to shrinkage, wall lengths of concrete should be limited to about 9 m if restrained at the base or ends.

Heavy foundations to a wall should not be allowed to stand and dry out for a long period before the wall is erected, because the wall concrete bonding to the base may be unable to shrink without cracking. Concrete is more elastic than is commonly appreciated, for example the unrestrained top of a 300 mm diameter reinforced concrete column 4m high can be made to oscillate through nearly 1 cm by push of the hand.

PRESTRESSED-CONCRETE BEAM DESIGN GUIDES AND TUTORIALS

Calculation Procedure:

1. Evaluate the results obtained with different forms of tendons The capacity of a given member is increased by using deflected rather than straight tendons, and the capacity is maximized by using parabolic tendons. (However, in the case of a pretensioned beam, an economy analysis must also take into account the expense incurred in deflecting the tendons.)

2. Evaluate the prestressing force For a given ratio of yj/ye the prestressing force that is required to maximize the capacity of a member is a function of the cross-sectional area and the allowable stresses. It is independent of the form of the trajectory.

3. Determine the effect of section moduli If the section moduli are in excess of the minimum required, the prestressing force is minimized by setting the critical values offbf and/, equal to their respective allowable values.

4. Determine the most economical short-span section For a short-span member, an I section is most economical because it yields the required section moduli with the minimum area. Moreover, since the required values of Sb and St differ, the area should be disposed unsymmetrically about middepth to secure these values.

5. Consider the calculated value of e Since an increase in span causes a greater increase in the theoretical eccentricity than in the depth, the calculated value of e is not attainable in a long-span member because the centroid of the tendons would fall beyond the confines of the section. For this reason, long-span members are generally constructed as T sections. The extensive flange area elevates the centroidal axis, thus making it possible to secure a reasonably large eccentricity.

6. Evaluate the effect of overload

A relatively small overload induces a disproportionately large increase in the tensile stress in the beam and thus introduces the danger of cracking. Moreover, owing to the presence of many variable quantities, there is not a set relationship between the beam capacity at allowable final stress and the capacity at incipient cracking. It is therefore imperative that every prestressed-concrete beam be subjected to an ultimate-strength analysis to ensure that the beam provides an adequate factor of safety.

PRE CAST TEES AND SLABS BASIC INFORMATION AND TUTORIALS

Precast slabs are available in hollow, cored, and solid varieties for use on floors, walls, and roofs. For short spans, various types of panel and channel slabs with reinforcing bars are available in both concrete and gypsum. Longer spans and heavy loads most commonly involve cored units with prestressed wire.

The solid panel and channel slabs are available in heavyweight and lightweight aggregates. The thicknesses and widths available vary considerably, but the maximum span is generally limited to about 10 feet.

Some slabs are available tongue-and-grooved and some with metal-edged tongue and- groove. These types of slabs use reinforcing bars or reinforcing mesh for added tension strengths.

These lightweight, easy-to-handle nail, drill, and saw pieces are easily installed on the job over the supporting members. A clip or other special fastener should be used in placing the slabs.

Cored units with prestressed wire are used on roof spans up to about 44 feet. Thicknesses available range from 4 to 16 inches with various widths available, 40 and 48 inches being the most common.

Each manufacturer must be contacted to determine the structural limitations of each product. The units generally have high fire resistance ratings and are available with an acoustical finish. Some types are available with exposed aggregate finishes for walls.

Specifications.

The type of material used and the manufacturer specified are the first items to be checked. The materials used to manufacture the plank, type and size of reinforcing, and required fire rating and finish must be checked. The estimator should also note who cuts the required holes in the planks and who caulks the joints, and the type of caulking.

CONCRETE FINISHING AND ESTIMATES BASIC INFORMATION

All exposed concrete surfaces require some type of finishing. Basically, finishing consists of the patch up work after the removal of forms and the dressing up of the surface by troweling, sandblasting, and other methods.

Patch-up work may include patching voids and stone pockets, removing fins, and patching chips. Except for some floor slabs (on grade), there is always a certain amount of this type of work on exposed surfaces.

It varies considerably from job to job and can be kept to a minimum with good quality concrete, with the use of forms that are tight and in good repair, and with careful workmanship, especially in stripping the forms.

This may be included with the form stripping costs, or it may be a separate item. As a separate item, it is much easier to get cost figures and keep a cost control on the particular item rather than “bury” it with stripping costs.

Small patches are usually made with a cement-sand grout mix of 1:2; be certain that the type of cement (even the brand name) is the same used in the pour because different cements are varying shades of gray. The labor hours required will depend on the type of surface, the number of blemishes, and the quality of the patch job required. Scaffolding will be required for work above 6 feet.

The finishes required on the concrete surfaces will vary throughout the project. The finishes are included in the specifications and finish schedules; sections and details should also be checked.

Finishes commonly required for floors include hand or machine troweled, carborundum rubbing (machine or hand), wood float, broom, floor hardeners, and sealers. Walls and ceilings may also be troweled, but they often receive decorative surfaces such as bush hammered, exposed aggregate, rubbed, sandblasted, ground, and lightly sanded.

Finishes such as troweled, ground, sanded, wood float, broom finishes, and bush hammered require no materials to get the desired finish, but require only labor and equipment. Exposed aggregate finishes may be of two types. In the first type, a retarder is used on the form liner, and then the retarder is sprayed off and the surface is cleaned.

This finish requires the purchase of a retarding agent, spray equipment to coat the liner, and a hose with water and brushes to clean the surface. (These must be added to materials costs.) The second method is to spray or trowel an exposed aggregate finish on the concrete; it may be a two- or three-coat process, and both materials and equipment are required.

For best results, it is recommended that only experienced technicians place this finish. Subcontractors should price this application by the square foot. Rubbed finishes, either with burlap and grout or with float, require both materials and labor hours plus a few hand tools. The burlap and grout rubbed finish requires less material and more labor than the float finish. A mixer may be required to mix the grout.

Sandblasting requires equipment, labor, and the grit to sandblast the surface. It may be a light, medium, or heavy sandblasting job, with best results usually occurring with green to partially cured concrete.

Bush hammering, a surface finish technique, is done to expose portions of the aggregates and concrete. It may be done by hand, with chisel and hammer, or with pneumatic hammers. The hammers are commonly used, but hand chiseling is not uncommon. Obviously, hand chiseling will raise the cost of finishing considerably.

Other surface finishes may also be encountered. For each finish, analyze thoroughly the operations involved,material and equipment required, and labor hours needed to do the work.

The finishing of concrete surfaces is estimated by the square foot, except bases, curbs, and sills, which are estimated by the linear foot. Since various finishes will be required throughout, keep the takeoff for each one separately.

Materials for most operations (except exposed aggregate or other coating) will cost only 10 to 20 percent of labor. The equipment required will depend on the type of finishing done. Trowels (hand and machine), floats, burlap, sandblasting equipment, sprayers, small mixers, scaffolding, and small hand tools must be included with the costs of their respective items of finishing.

Estimating Concrete Finishing.

Areas to be finished may be taken from other concrete calculations, either for the actual concrete required or for the square footage of forms required.

Roof and floor slabs, and slabs on grade, pavements, and sidewalk areas can most easily be taken from the actual concrete required. Be careful to separate each area requiring a different finish. Footing, column, walls, beam, and girder areas are most commonly found in the form calculations.

ESTIMATING CONCRETE WORKS FOR CIVIL ENGINEERING PROJECTS

Concrete is estimated by the cubic yard (cy) or by the cubic foot (cf) and then converted into cubic yards. Concrete quantities are measured in cubic yards as it is the pricing unit of the ready-mix companies, and most tables and charts available relate to the cubic yard.

Roof and floor slabs, slabs on grade, pavements, and sidewalks are most commonly measured and taken off in length, width, and thickness and converted to cubic feet and cubic yards (27 cf # 1 cy). Often, irregularly shaped projects are broken down into smaller areas for more accurate and convenient manipulation.

When estimating footings, columns, beams, and girders, their volume is determined by taking the linear footage of each item times its cross-sectional area. The cubic footage of the various items may then be tabulated and converted to cubic yards.

When estimating footings for buildings with irregular shapes and jogs, the estimator must be careful to include the corners only once. It is a good practice for the estimator to highlight on the plans which portions of the footings have been figured.

When taking measurements, keep in mind that the footings extend out from the foundation wall; therefore, the footing length is greater than the wall length.

In estimating quantities, the estimator makes no deductions for holes smaller than 2 sf or for the space that reinforcing bars or other miscellaneous accessories take up. Waste ranges from 5 percent for footings, columns, and beams to 8 percent for slabs.

The procedure that should be used to estimate the concrete on a project is as follows:

1. Review the specifications to determine the requirements for each area in which concrete is used separately (such as footings, floor slabs, and walkways) and list the following:

(a) Type of concrete

(b) Strength of concrete

(c) Color of concrete

(d) Any special curing or testing

2. Review the drawings to be certain that all concrete items shown on the drawings are covered in the specifications. If not, a call will have to be made to the architect-engineer so that an addendum can be issued.

3. List each of the concrete items required on the project.

4. Determine the quantities required from the working drawings. Footing sizes are checked on the wall sections and foundation plans. Watch for different size footings under different walls.

Concrete slab information will most commonly be found on wall sections, floor plans, and structural details. Exterior walks and driveways will most likely be identified on the plot (site) plan and in sections and details.

PRECAST AND PRESTRESSED DECK SLABS BASIC AND TUTORIALS

The deck is usually the first element in a bridge to deteriorate and to require funds for rehabilitation. In situations where traffic volumes are high, it is often necessary to rehabilitate or replace the deck in sections during off-peak periods.

Because of the time required for site-cast concrete to cure, a number of replacement strategies have been developed using prefabricated deck slabs (Issa et al., 1995a,b). Most of the systems involve a transverse segment (Figure 33.11) connected to the supporting beams with a rapid-curing polymer or hydraulic cement concrete.

FIGURE 33.11 Prestressed deck slabs. (From Sprinkel, M.M., Prefabricated Bridge Elements and Systems, NCHRP Synthesis 119, Transportation Research Board, Washington, D.C., 1985.)


Shear transfer between adjacent slabs is achieved through the use of grouted keyways, site-cast concrete, and post-tensioning. Composite action is achieved through the use of studs on steel beams that extend into voided areas in the slabs that are then filled with polymer or hydraulic cement concrete.

Precast deck slabs can behave in a full-composite manner when connected to steel stringers with studs and epoxy mortar and when keyways are grouted with epoxy mortar (Osegueda et al., 1989).

An earlier study identified some suitable connection details and concluded that the deck slabs are more economical than site cast concrete because of the structural efficiency provided by post-tensioning and prestressing and because of the reduced construction time (Berger, 1983).

Improved connection details for the use of panels on steel beams and prestressed concrete beams have been developed (Tadros and Baishya, 1998).

More recently, a special loop bar reinforcement detail has been developed to provide live load distribution across transverse and longitudinal joints (see FHWA, 2004). A new full-depth precast prestressed concrete bridge deck slab system has been developed that includes stemmed slabs, transverse grouted joints, longitudinal post-tensioning, and welded threaded and headless studs (Tadros and Baishya, 1998).

The deck slabs are thinner and lighter than a conventional deck and can be constructed faster. Prestressed deck slabs typically have been used on major bridge deck replacement projects (Figure 33.12) such as the Woodrow Wilson Bridge (Lutz and Scalia, 1984).

FIGURE 33.12 Prestressed post-tensioned deck slabs were installed at night to replace the deck of the Woodrow Wilson Bridge.


Also, most replacements have involved the use of transverse slabs. The decks on the George Washington Memorial Parkway were replaced using precast longitudinally post-tensioned transverse deck slabs (Jakovich and Alvarez, 2002).

A latex-modified concrete overlay was placed over the slabs. The truss spans of the deck on I-95 in Richmond, Virginia, were recently replaced with night lane closures using the full-depth transverse deck slabs (Figure 33.13).

FIGURE 33.13 Special loop bar connection detail for deck slabs. (From FHA, Prefabricated Bridge Elements and Systems in Japan and Europe, Summary Report, International Technology Exchange Programs, Federal Highway Administration, Washington, D.C., 2004)


The slabs were also used to replace the deck on Route 50 in Fairfax County, Virginia (Babaei et al., 2001). The Virginia Department of Transportation first used transverse precast deck slabs to replace a deck on Route 235 over Dogue Creek in Fairfax County in 1981 (Sprinkel, 1982).

Longitudinal slabs were successfully used to rehabilitate the Freemont Street Bridge (Smyers, 1984), and a new bridge was built in Thailand (Zeyher, 2003).

Longitudinal, partial-depth, or full-depth deck slabs that that are precast on one or more concrete or steel beams have also been used successfully (FHWA, 2004). The superstructure elements are set next to each other and are typically connected by transverse post-tensioning in the deck and diaphragms between the beams.

Keyways in the deck are grouted. The deck on I-95 in Richmond, Virginia, was recently replaced with night lane closures using the full-depth deck slabs on steel beam superstructure elements. When partial depth deck superstructure elements are set next to each other, reinforced site-cast concrete facilitates the connection of the elements.

FIBERS FOR CONCRETE MIXES BASIC AND TUTORIALS

As used in concrete, fibers are discontinuous, discrete units. They may be described by their aspect ratio, the ratio of length to equivalent diameter. Fibers find their greatest use in crack control of concrete flatwork, especially slabs on grade.

The most commonly used types of fibers in concrete are synthetics, which include polypropylene, nylon, polyester, and polyethylene materials. Specialty synthetics include aramid, carbon, and acrylic fibers. Glass fiber-reinforced concrete is made using E-glass and alkali-resistant (AR) glass fibers. Steel fibers are chopped high-tensile or stainless steel.

Fibers should be dispersed uniformly throughout a mix. Orientation of the fibers in concrete generally is random. Conventional reinforcement, in contrast, typically is oriented in one or two directions, generally in planes parallel to the surface.

Further, welded-wire fabric or reinforcing steel bars must be held in position as concrete is placed. Regardless of the type, fibers are effective in crack control because they provide omnidirectional reinforcement to the concrete matrix. With steel fibers, impact strength and toughness of concrete may be greatly improved and flexural and fatigue strengths enhanced.

Synthetic fibers are typically used to replace welded-wire fabric as secondary reinforcing for crack control in concrete flatwork. Depending on the fiber length, the fiber can limit the size and spread of plastic shrinkage cracks or both plastic and drying shrinkage cracks. Although synthetic fibers are not designed to provide structural properties, slabs tested in accordance with ASTM E72, ‘‘Standard Methods of Conducting Strength Tests of Panels for Building Construction,’’ showed that test slabs reinforced with synthetic fibers carried greater uniform loads than slabs containing welded wire fabric.

While much of the research for synthetic fibers has used reinforcement ratios greater than 2%, the common\ field practice is to use 0.1% (1.5 lb /yd3). This dosage provides more cross-sectional area than 10-gage welded wire fabric. The empirical results indicate that cracking is significantly reduced and is controlled. A further benefit of fibers is that after the initial cracking, the fibers tend to hold the concrete together.

Aramid, carbon, and acrylic fibers have been studied for structural applications, such as wrapping concrete columns to provide additional strength. Other possible uses are for corrosion-resistance structures. The higher costs of the specialty synthetics limit their use in general construction.

Glass-fiber-reinforced concrete (GFRC) is used to construct many types of building elements, including architectural wall panels, roofing tiles, and water tanks. The full potential of GFRC has not been attained because the E-glass fibers are alkali reactive and the AR-glass fibers are subject to embrittlement, possibly from infiltration of calcium-hydroxide particles.

Steel fibers can be used as a structural material and replace conventional reinforcing steel. The volume of steel fiber in a mix ranges from 0.5 to 2%. Much work has been done to develop rapid repair methods using thin panels of densely packed steel fibers and a cement paste squeegeed into the steel matrix.

American Concrete Institute Committee 544 states in ‘‘Guide for Specifying, Mixing, Placing, and Finishing Steel Fiber Reinforced Concrete,’’ ACI 544.3R, that, in structural members such as beams, columns, and floors not on grade, reinforcing steel should be provided to support the total tensile load. In other cases, fibers can be used to reduce section thickness or improve performance. See also ACI 344.1R and 344.2R.

PRACTICAL POINTS IN PRODUCING GOOD CONCRETES

Provided certain simple rules are followed good concrete can be achieved by methods varying from the ‘bucket and spade’ hand-labour method to use of the most sophisticated weigh-batching and mixing plant. The following shows the principal matters that should receive the resident engineer’s attention.

First, choose good aggregates. The best guide is to use well-known local aggregates that have been and are being used satisfactorily on other jobs elsewhere. A reputable supplier will be able to name many jobs where his aggregate has been used, and the resident engineer will not be over-cautious if he visits one or two of these where the concrete is exposed to view.

When the aggregates are being delivered on the job (not just the first few loads, but the loads when the supply has really got going), random loads as delivered should be examined. Handfuls of aggregate should be taken up and examined in detail, looking for small balls of clay, soft spongy stones, flaky stones, pieces of brick, soft shale, crumbly bits of sandstone, and whether clay or dirt is left on the hands after returning the handful.

If the engineer finds more than one or two pieces of weak stone, or more than a single small piece of clay from a few handfuls, he should request the contractor to bring this to the notice of the supplier. He need not reject the load out of hand, but it will do no harm to let the supplier know the aggregates are being watched.

If a load contains numerous weak stones or several pieces of clay, it should be rejected. Diagnosing whether an aggregate is likely to give rise to alkali-silica reaction (which can cause expansion and disruption of concrete in a few years in the presence of moisture) requires specialist knowledge.

The most practical approach for the engineer is to ask the supplier if his aggregate has been tested for this; if not, structures built some years previously with the aggregate should be checked for signs of cracking due to alkali-silica reaction. Guidance and precautions are set out in certain publications (References 1 and 2), but if it is proposed to use an aggregate not used before, the site staff should refer the problem to the engineer.

Second, choose tested cement. The same principle applies to cement as with the choice of aggregates; find the supplier of cement to other jobs and request a recent test certificate. Troubles can start when imported cement has to be used or cement from a variety of suppliers.

Overseas it is not unusual for a small contractor to buy his cement a few bags at a time from the local bazaar. Testing such cement on site before any concrete is placed in an important part of a structure is essential. BS 12 provides methods for testing the compressive strengths of 1:3 mortar cubes or 1:2:4 concrete cubes but, if this is difficult to arrange, the flexural test mentioned in Section 19.3 can be applied on site.

Third, ensure reasonably graded aggregates. In delivery and stockpiling of coarse aggregate there is a tendency for the mix to segregate, the larger material remaining on top. Care has to be taken to ensure that certain batches are not made up from all the coarsest material and others from most of the fines.

Crushed rock often has a considerable amount of dust in it, although this does not normally present a problem one does not want a batch made up mostly from dust and fines taken from the bottom of a stockpile.

Fourth, use washed aggregates. Unwashed aggregates suitable for concreting are rare: they are usually comprised of crushed clean homogeneous rock. Sometimes a river sand is supplied unwashed – it being assumed that the sand has already been ‘washed’ by the river.

This should not be accepted as a fact, since a river also carries silts and clays. Sea-bed or beach sands must be washed in fresh water to remove the salt from them.

Fifth, achieve the right workability. Mechanical mixers are seldom at fault with regard to mixing, and hand mixing can also be quite satisfactory; but it is the water content of a mix that requires the most vigilant attention. The site engineer should never let ‘slop’ be produced.

Although the slump test and the compacting factor test are useful in defining the degree of stiffness of a mix, in practice judging the water content of a mix ‘by eye’ is both necessary and possible.

The right sort of mix should look stiff as it comes out of the mixer or when turned over by hand on mixing boards. It should stand as a ‘heap’ and not as a ‘pool’ of concrete. When a shovel is thrust into such a pile, the shovelcut should remain open for some minutes.

Such a mix will look quite different after it is discharged and worked into some wet concrete already placed. As soon as it is worked with shovels or vibrated, it will settle and appear to flow into and become part of the previously placed concrete.


The same characteristic makes it possible to judge the water content by noticing what happens if the freshly mixed concrete is carried in a dumper hopper to the point of discharge. The ‘heap’ of stiff concrete discharged from the mixer to the dumper hopper will appear to change to a pool of concrete as the dumper bumps its way round the usual site roads.

When the dumper hopper is tipped, however, the concrete discharged should again appear stiff. But if, in transport, the concrete slops as a semi-fluid over the side of the dumper hopper, this shows too much water has been added.

A simple density test on freshly mixed concrete may assist in finding if the mix has too much water.

Sixth, ram the concrete well in place. Properly shovelled, rodded, or vibrated, the concrete should be seen to fill the corners of shuttering and to easily wrap around the reinforcing bars. When hand shovelling or rodding is adopted, it is scarcely possible to over-compact the concrete.

But when mechanical vibrators are used the vibration should not be so prolonged as to produce a watery mix on the surface. Vibrators of the poker immersion type should be kept moving slowly in and out of the concrete.

They should not be withdrawn quickly or they may leave an unfilled hole in the concrete; nor should they be left vibrating continuously in one location. Where vibrators are used, it is necessary for the contractor also to have available suitable hand rammers in case the vibrators break down in the middle of a pour.

Seventh, ensure the mix has sufficient cement in it. Normally contractors will use a little more cement than is theoretically necessary and this is helpful since batches of concrete vary.

But if a contractor becomes too keen on cutting the cement to the bare minimum, a number of the cube crushing tests may fail to reach the required strength, and much delay may be caused by conducting the investigations required to seek out the cause.

CONCRETE CHARACTERISTICS BASIC AND CIVIL ENGINEERING TUTORIALS

CONCRETE CHARACTERISTICS BASIC INFORMATION
What Are The Characteristics Of Concrete?


Characteristics of Concrete
1) Convenient for use: the new mixtures have good plasticity that can be cast into components and structures in various shapes and sizes.

2) Cheap: raw materials are abundant and available. More than 80% of them are sand and stone whose resources are rich, energy consumption is low, according with the economic principle.

3) High-strength and durable: the strength of ordinary concrete is 20 - 55MPa with good durability.

4) Easy to be adjusted: the concrete with different functions can be made just by changing the varieties and quantities of composing materials to meet various demands of projects; steel bar can be added to concrete to improve its strength, and this kind of concrete is a composite material (reinforced concrete) which can improve its low tensile and bending strength in order to meet the needs of various structural engineering.

5 ) Environment-friendly: concrete can make full use of industrial wastes, such as slag, fly ash and others to reduce environmental pollution. Its major shortcomings are high dead weight, low tensile strength, brittle and easy to crack.

FORMS OF SILICA FUMES BASIC AND TUTORIALS

FORMS OF SILICA FUMES BASIC INFORMATION
What Are The Forms Of Silica Fumes?


Silica fume is available commercially in several forms in both North America and Europe:

• As-produced silica fume is silica fume collected in dedusting systems known as bag houses. In this form, the material is very fine and has a bulk density of about 200 to 300 kg/m3, compared with 1500 kg/m3 for Portland cement (Malhotra et al., 1987).

As-produced silica fume is available in bags or in bulk. Because of its extreme fineness, this form poses handling problems; in spite of this, the material can be and has been transported and handled like Portland cement.

• Compacted silica fume has a bulk density ranging from 500 to 700 kg/m3 and is considerably easier to handle than as-produced silica fume.

To produce the compacted form, the as-produced silica fume is placed in a silo, and compressed air is blown in from the bottom of the silo. This causes the particles to tumble, and in doing so they agglomerate.

The heavier agglomerates fall to the bottom of the silo and are removed at intervals. The air compaction of the asproduced silica fume is designed so the agglomerates produced are rather weak and quickly break down during concrete mixing.

Mechanical means have also been used to produce compacted silica fume.

• Water-based silica fume slurry overcomes the handling and transporting problems associated with as-produced silica fume; the slurry contains about 40 to 60% solid particles. Typically, these slurries have a density of about 1300 kg/m3.

Some slurries may contain chemical admixtures such as superplasticizers, water reducers, and retarders. One such product (known as Force 10,000®) has been successfully marketed in North America.

EFFECT OF FLY ASH ON CONCRETE STRENGTH BASIC CIVIL ENGINEERING TUTORIALS

EFFECT OF FLY ASH ON CONCRETE STRENGTH BASIC INFORMATION
What Is The Effect Of Fly Ash On Concrete Strength?


The first difference among fly ashes is that some are cementitious even in the absence of Portland cement; these are the so-called ASTM Class C, or high-calcium, fly ashes, usually produced at power plants that burn subbituminous or lignitic coals.

In general, the rate of strength development in concretes tends to be only marginally affected by high-calcium fly ashes. Concrete incorporating high-calcium fly ashes can be made on an equal-weight or equal-volume replacement basis without any significant effect on strength at early ages.

Yuan and Cook (1983) examined the strength development of concretes with and without high-calcium fly ash (CaO = 30.3 wt%). The data from their research are shown in Figure 2.5 and Figure 2.6.

Using a simple replacement method of mixture proportioning (Table 2.6), they found the rate of strength development of fly-ash concrete to be comparable to that of the control concrete, with or without air entrainment.

Low-calcium fly ashes, the so-called ASTM Class F fly ashes, were the first to be examined for use in concrete. Most of what has been written on the behavior of fly-ash concrete examines concretes that use Class F ashes.

In addition, the ashes used in much of the early work came from older power plants and were coarse in particle size, contained unburned fuel, and were often relatively inactive pozzolans. Used in concrete and proportioned by simple replacement, these ashes showed exceptionally slow rates of strength development.

This led to the erroneous view that fly ash reduces strength at all ages. Gebler and Klieger (1986) evaluated the effect of ASTM Class F and Class C fly ashes from 10 different sources on the compressive strength development of concretes under different curing conditions, including effects of low temperature and moisture availability.

Their tests indicated that concrete containing fly ash had the potential to produce satisfactory compressive strength development. The influence of the class of fly ash on the long-term compressive strength of concrete was not significant.

In general, compressive strength development of concretes containing Class F fly ash was more susceptible to low curing temperature than concretes with Class C fly ash or the control concretes. Gebler and Klieger concluded that Class F fly-ash concretes required more initial moist curing for long-term, air-cured compressive strength development than did concretes containing Class C fly ashes or the control concretes.

GREEN CEMENT BASIC AND TUTORIALS

GREEN CEMENT BASIC INFORMATION
What Are Green Cement?


The concrete industry is the largest user of natural resources in the world and thus has a considerable environmental impact. Each ton of Portland cement requires about 1.5 tons of raw material for its production.

This industry is not only energy intensive but is also a major contributor of greenhouse gases, in the form of CO2. Each ton of Portland cement that is produced involves the release into the atmosphere of about one ton of CO2.

Indeed, according to Mehta (1999), the cement industry is responsible for about 7% of global CO2 emissions; thus, there is considerable interest now in developing cements that are more environmentally friendly. One such cement (CEMROC), based on blast-furnace slag, has recently been described by Gebauer et al. (2005).

This cement, produced by Holcim in Europe, is reported to show close to zero CO2 emission during its production (only about 100 pounds per ton of cement).

It is similar to the supersulfated cement described above and is particularly well suited for use in structures exposed to aggressive environments. Other cements of this general type will almost certainly be developed in the future.

Another (and simpler) approach is to use much greater proportions of fly ash in concrete. A great deal of development is being conducted on what is referred to as high-performance, high-volume fly ash concrete (Malhotra, 2002; Malhotra and Mehta, 2002).

Such concretes may be defined as:

• Containing at least 50% fly ash by mass of the cementing materials
• Having a Portland cement content of less than 200 kg/m3
• Having a water content of less than 130 kg/m3
• Having a water/cementing materials ratio of less than 0.35

These concretes reach their full strength potential rather more slowly than conventional concretes, but the end result is a low-permeability, durable concrete.

UNSATISFACTORY CONCRETE TEST RESULTS CAUSES BASIC AND TUTORIALS

UNSATISFACTORY CONCRETE TEST RESULTS CAUSES BASIC INFORMATION
What Are Some Causes Of Unsatisfactory Concrete Test Results


The two most common kinds of failure are:
• failure to get the required strength, the concrete being otherwise apparently good;
• structural failures, such as honeycombing, sandy patches, and cracking.

Failure to get the right strength in cubes taken from a concrete pour can sometimes have a very simple cause. Among such causes are the following:
• the cube was not compacted properly;
• it was left out all night in hard frost or dried out in hot sun;
• there was a mix-up of cubes and a 7-day old cube was tested on the assumption it was 28 days old;
• the cube was taken from the wrong mix.

Such simple errors are not unusual and must be guarded against because they cause much perplexity and waste of time trying to discover the cause of a bad test result.

The concrete must be fully compacted in the mould, which is kept under damp sacking until the next day when the mould can be removed and the cube marked for identity.

It is then best stored in water at ‘room temperature’ for curing until sent to the test laboratory. If poor cube test results appear on consecutive batches, an error in the cement content of batches may be suspected, or else the quality of the cement itself.

Honeycombing is most usually caused by inadequate vibration or rodding of the concrete adjacent to the face of formwork.

Sometimes too harsh a mix is used so there are insufficient fines to fill the trapped interstices between coarse aggregate and formwork, or the larger stones cause local arching.

Sand runs – patches of sandy concrete on a wall surface which can be scraped away with a knife – can be due to over-vibration near a leaking joint in the formwork which allows cement and water to pass out of the mix.

One simple, and not infrequent, cause of poor concrete is use of the wrong mix due to a ‘failure of communication’ with the batching plant operator or ready-mix supplier. An experienced concreting foreman should be able to detect a ‘wrong mix’ the moment it is discharged.

FIBERS FOR CONCRETE MIXES BASICS AND TUTORIALS

FIBERS FOR CONCRETE MIXES BASIC INFORMATION
What Are Concrete Mixes Fibers?


As used in concrete, fibers are discontinuous, discrete units. They may be described by their aspect ratio, the ratio of length to equivalent diameter. Fibers find their greatest use in crack control of concrete flatwork, especially slabs on grade.

The most commonly used types of fibers in concrete are synthetics, which include polypropylene, nylon, polyester, and polyethylene materials. Specialty synthetics include aramid, carbon, and acrylic fibers.

Glass-fiber-reinforced concrete is made using E-glass and alkali-resistant (AR) glass fibers. Steel fibers are chopped high-tensile or stainless steel.

Fibers should be dispersed uniformly throughout a mix. Orientation of the fibers in concrete generally is random. Conventional reinforcement, in contrast, typically is oriented in one or two directions, generally in planes parallel to the surface.

Further, welded-wire fabric or reinforcing steel bars must be held in position as concrete is placed. Regardless of the type, fibers are effective in crack control because they provide omnidirectional reinforcement to the concrete matrix.

With steel fibers, impact strength and toughness of concrete may be greatly improved and flexural and fatigue strengths enhanced.

Synthetic fibers are typically used to replace welded-wire fabric as secondary reinforcing for crack control in concrete flatwork. Depending on the fiber length, the fiber can limit the size and spread of plastic shrinkage cracks or both plastic and drying shrinkage cracks.

Although synthetic fibers are not designed to provide structural properties, slabs tested in accordance with ASTM E72, ‘‘Standard Methods of Conducting Strength Tests of Panels for Building Construction,’’ showed that test slabs reinforced with synthetic fibers carried greater uniform loads than slabs containing welded wire fabric.

While much of the research for synthetic fibers has used reinforcement ratios greater than 2%, the common field practice is to use 0.1% (1.5 lb /yd3). This dosage provides more cross-sectional area than 10-gage weldedwire fabric.

The empirical results indicate that cracking is significantly reduced and is controlled. A further benefit of fibers is that after the initial cracking, the fibers tend to hold the concrete together.

Aramid, carbon, and acrylic fibers have been studied for structural applications, such as wrapping concrete columns to provide additional strength. Other possible uses are for corrosion-resistance structures. The higher costs of the specialty synthetics limit their use in general construction.

Glass-fiber-reinforced concrete (GFRC) is used to construct many types of building elements, including architectural wall panels, roofing tiles, and water tanks. The full potential of GFRC has not been attained because the E-glass fibers are alkali reactive and the AR-glass fibers are subject to embrittlement, possibly from infiltration of calcium-hydroxide particles.

Steel fibers can be used as a structural material and replace conventional reinforcing steel. The volume of steel fiber in a mix ranges from 0.5 to 2%. Much work has been done to develop rapid repair methods using thin panels of densely packed steel fibers and a cement paste squeegeed into the steel matrix.

American Concrete Institute Committee 544 states in ‘‘Guide for Specifying, Mixing, Placing, and Finishing Steel Fiber Reinforced Concrete,’’ ACI 544.3R, that, in structural members such as beams, columns, and floors not on grade, reinforcing steel should be provided to support the total tensile load. In other cases, fibers can be used to reduce section thickness or improve performance. See also ACI 344.1R and 344.2R.

WATER REDUCING ADMIXTURES FOR CONCRETE BASIC AND TUTORIALS

WATER REDUCING ADMIXTURES FOR CONCRETE BASIC INFORMATION
What Are Water Reducing Concrete Admixtures?


Water-Reducing Admixtures
These decrease water requirements for a concrete mix by chemically reacting with early hydration products to form a monomolecular layer of admixture at the cementwater interface.

This layer isolates individual particles of cement and reduces the energy required to cause the mix to flow. Thus, the mix is ‘‘lubricated’’ and exposes more cement particles for hydration.

The Type A admixture allows the amount of mixing water to be reduced while maintaining the same mix slump. Or at a constant water-cement ratio, this admixture allows the cement content to be decreased without loss of strength.

If the amount of water is not reduced, slump of the mix will be increased and also strength will be increased because more of the cement surface area will be exposed for hydration. Similar effects occur for Type D and E admixtures. Typically, a reduction in mixing water of 5 to 10% can be expected.

Type F and G admixtures are used where there is a need for high-workability concrete. A concrete without an admixture typically has a slump of 2 to 3 in. After the admixture is added, the slump may be in the range of 8 to 10 in without segregation of mix components.

These admixtures are especially useful for mixes with a low water-cement ratio. Their 12 to 30% reduction in water allows a corresponding reduction in cementitious material.

The water-reducing admixtures are commonly manufactured from lignosulfonic acids and their salts, hydroxylated carboxylic acids and their salts, or polymers of derivatives of melamines or naphthalenes or sulfonated hydrocarbons. The combination of admixtures used in a concrete mix should be carefully evaluated and tested to ensure that the desired properties are achieved.

For example, depending on the dosage of admixture and chemistry of the cement, it is possible that a retarding admixture will accelerate the set. Note also that all normal-set admixtures will retard the set if the dosage is excessive.

Furthermore, because of differences in percentage of solids between products from different companies, there is not always a direct correspondence in dosage between admixtures of the same class. Therefore, it is
important to consider the chemical composition carefully when evaluating competing admixtures.

Superplasticizers are high-range water-reducing admixtures that meet the requirements of ASTM C494 Type F or G. They are often used to achieve highstrength concrete by use of a low water-cement ratio with good workability and low segregation.

They also may be used to produce concrete of specified strengths with less cement at constant water cement ratio. And they may be used to produce self-compacting, self-leveling flowing concretes, for such applications as longdistance pumping of concrete from mixer to formwork or placing concrete in forms congested with reinforcing steel.

For these concretes, the cement content or watercement ratio is not reduced, but the slump is increased substantially without causing segregation. For example, an initial slump of 3 to 4 in for an ordinary concrete mix may be increased to 7 to 8 in without addition of water and decrease in strength.

Superplasticizers may be classified as sulfonated melamine-formaldehyde condensates, sulfonated naphthaline-formaldehyde condensates, modified lignosulfonates, or synthetic polymers.

CONCRETE FLOORS AT GRADE BASICS AND TUTORIALS

CONCRETE FLOORS AT GRADE BASIC INFORMATION
What Are Concrete Floors At Grade?


Floors on ground should preferably not be constructed in low-lying areas that are wet from ground water or periodically flooded with surface water. The ground should slope away from the floor.

The level of the finished floor should be at least 6 in above grade. Further protection against ground moisture and possible flooding of the slab from heavy surface runoffs may be obtained with subsurface drains located at the elevation of the wall footings.

All organic material and topsoil of poor bearing value should be removed in preparation of the subgrade, which should have a uniform bearing value to prevent unequal settlement of the floor slab. Backfill should be tamped and compacted in layers not exceeding 6 in in depth.

Where the subgrade is well-drained, as where subsurface drains are used or are unnecessary, floor slabs of residences should be insulated either by placing a granular fill over the subgrade or by use of a lightweight-aggregate concrete slab covered with a wearing surface of gravel or stone concrete.

The granular fill, if used, should have a minimum thickness of 5 in and may consist of coarse slag, gravel, or crushed stone, preferably of 1-in minimum size. A layer of 3-, 4-, or 6-in-thick hollow masonry building units is preferred to gravel fill for insulation and provides a smooth, level bearing surface.

Moisture from the ground may be absorbed by the floor slab. Floor coverings, such as oil-base paints, linoleum, and asphalt tile, acting as a vapor barrier over the slab, may be damaged as a result.

If such floor coverings are used and where a complete barrier against the rise of moisture from the ground is desired, a twoply bituminous membrane or other waterproofing material should be placed beneath the slab and over the insulating concrete or granular fill (Fig. 3.8).

The top of the lightweight-aggregate concrete, if used, should be troweled or brushed to a smooth level surface for the membrane. The top of the granular fill should be covered with a grout coating, similarly finished. (The grout coat, 1⁄2 to 1 in thick, may consist of a 1:3 or a 1:4 mix by volume of portland cement and sand. Some 3⁄8- or 1⁄2-in maximum-sized coarse aggregate may be added to the grout if desired.)

After the top surface of the insulating concrete or grout coating has hardened and dried, it should be mopped with hot asphalt or coal-tar pitch and covered before cooling with a lapped layer of 15-lb bituminous saturated felt.

The first ply of felt then should be mopped with hot bitumen and a second ply of felt laid and mopped on its top surface. Care should be exercised not to puncture the membrane, which should preferably be covered with a coating of mortar, immediately after its completion. If properly laid and protected from damage, the membrane may be considered to be a waterproof barrier.

Where there is no possible danger of water reaching the underside of the floor, a single layer of 55-lb smooth-surface asphalt roll roofing or an equivalent waterproofing membrane may be used under the floor. Joints between the sheets should be lapped and sealed with bituminous mastic.

Great care should be taken to prevent puncturing of the roofing layer during concreting operations. When so installed, asphalt roll roofing provides a low-cost and adequate barrier against the movement of excessive amounts of moisture by capillarity and in the form of vapor.

In areas with year-round warm climates, insulation can be omitted. (‘‘A Guide to the Use of Waterproofing, Dampproofing, Protective and Decorative Barrier Systems for Concrete,’’ ACI 515.1R, American Concrete Institute.)

CEMENT HYDRATION AND CONCRETE CURING BASICS AND TUTORIALS

CEMENT HYDRATION AND CONCRETE CURING BASIC INFORMATION
Cement Hydration And Concrete Curing Information

Concrete curing is not simply a matter of the concrete hardening as it dries out. In fact, it is just the opposite. Portland cement is a hydraulic material. That is, it requires water for curing and can, in fact, fully cure to a hardened state even if it is completely submerged in water.


Portland cement is anhydrous—it contains no water or moisture at all. The moment it comes in contact with water, a chemical reaction takes place in which new compounds are formed. This reaction is called cement hydration.

The rate of hydration varies with the composition of the cement, the fineness of the cement particles, the amount of water present, the air temperature, and the presence of admixtures. If the mixing water dries out too rapidly before the cement has fully hydrated, the curing process will stop and the concrete will not harden to its intended strength.

Curing will resume if more water is introduced, but at a slower rate. Hydration occurs more rapidly at higher air temperatures. Cement hydration itself generates heat, too. This heat of hydration can be helpful during cold-weather construction, and potentially harmful during hot-weather construction.

The chemical reaction between water and cement first forms a paste which must completely coat each aggregate particle during mixing. After a time, the paste begins to stiffen or set, and after a few hours has lost is plasticity entirely.

The rate of this setting, however, is not the same as the rate of hardening. A Type-III high-early-strength cement may set in about the same time as a Type-I general-purpose cement, but the Type III hardens and develops compressive strength more rapidly after it has set.

Concrete normally cures to its full design strength in 28 days. Curing is slower in cold weather, and at temperatures below 40°F, the concrete can be easily and permanently damaged if it is not properly protected.

Concrete must be kept moist for several days after it is placed to allow the portland cement in the mix to cure and harden properly. Concrete that is not kept moist reaches only about 50% of its design strength. Figure 2-19 shows the differences in concrete strength for various periods of moist curing.

If it is kept moist for at least three days, it will reach about 80% of its design strength, and for seven days, 100% of its design strength. If the concrete is kept moist for the full 28- day curing period, it will reach more than 125% of its design strength.

CONCRETE MIXING TECHNIQUES TIPS TRICKS BASICS AND TUTORIALS

MIXING OF CONCRETE TUTORIALS
Links On Concrete Mixing Tips and Tutorials

Concrete is one of the most used materials in civil and structural construction. May it be a high rise building, or a bridge that carries a heavy load, even in simple household projects, concrete is used.

An important aspect on the design of such is the desire to have a good concrete mix. Below are links to articles that teaches basics and advance concrete mixing techniques.

How To Mix Concrete
Concrete is a mixture of cement, sand, gravel and water. Variations in the ratio of the ingredients produces a mix suitable for different jobs but we will be instructing you on a general puepose mix suitable for most garden tasks and shallow foundations not house foundations. Mix 1 part cement, 2 parts sand and 4 parts of gravel. What you use to measure the parts depends on how much concrete you need normally buckets are the best measure, one part being a level bucket full. Read more... 

The Perfect Concrete Mix Design
Is there such a thing as a Perfect Mix Design? If you are looking for one mix design recipe that will  work perfectly for every job and every application,  the answer is definitely "NO". However, there is a  right mix design for every job and every  application, and with the right knowledge and good  communication between a concrete contractor and  his supplier, you can find The Perfect Mix Design  for every one of your jobs. Read more...

High Strength Concrete Mix Design
The type of concrete generally with compressive strength of 6000psi (40MPa) or greater is called as HSC. We need high strength concrete in our modern infrastructures in order to put the concrete into service at much earlier stage, To build up high rise building by reducing column sizes and increasing available space or in case of long span bridges Now what is mix design? Although the American Concrete Institute (ACI) doesn't use the term, preferring mix proportioning. Read more...

Concrete Mix Evaluator
Concrete mix design is no longer just a recipe of  proportions of cement, sand, stone and water. Today the  concrete supplier is held responsible for the performance  of the in-place concrete. Curling, cracking, dusting, color variation, moisture  transmission, and strength, are all concerns for the quality of the in-place concrete.  Much  of what happens is outside the responsibility or control of the concrete producer.  Concrete of good quality can become undesirable in the hands of an inexperienced  finisher, but poor concrete, that of inappropriate proportions, will doom the final product  regardless of the experience of the concrete finisher. Read more...

How to Properly Mix Concrete
How to hand mix concrete so it delivers maximum strength and durability. Mixing isn't complicated and when done well, the concrete should last a lifetime. Mixing bags of concrete isn't complicated. You add some water, stir it up and pour it out. But to get the most strength from the concrete, you have to recognize when it has just the right amount of water mixed in. Too little water and the particles in the mix won't stick together. Too much water weakens the concrete. In this article, we'll show you what the perfect mix looks like. We'll also show you a mixing technique that will ensure thoroughly mixed concrete with a minimum of effort. Read more...

AGGREGATE MOISTURE CONTENT BASICS AND TUTORIALS

MOISTURE CONTENT OF AGGREGATES USED IN CONSTRUCTION BASIC INFORMATION
What Is The Value Of Moisture Content Of Aggregates?


Aggregates can hold water in two ways: absorbed within the aggregate porosity or held on the particle surface as a moisture film. Thus, depending on the relative humidity, recent weather conditions, and location within the aggregate stockpile, aggregate particles can have a variable moisture content.

For the purposes of mix proportioning, however, it is necessary to know how much water the aggregate will absorb from the mix water or how much extra water the aggregate might contribute. Figure 1.10 illustrates four different moisture states:

• Oven-dry (OD)—All moisture is removed by heating the aggregates in an oven at 105°C to constant weight.
• Air-dry (AD)—No surface moisture is present, but the pores may be partially full.
• Saturated surface dry (SSD)—All pores are full, but the surface is completely dry.
• Wet—All pores are full, and a water film is on the surface.


Of these four states, only two (OD and SSD) correspond to well-defined moisture conditions; either one can be used as a reference point for calculating the moisture contents. In the following discussion, the SSD state will be used. Now, to determine how much water the aggregate may add to or take from the mixing water, three further quantities must be defined:

• The absorption capacity (AC) represents the maximum amount of water the aggregates can absorb. From Figure 1.10, this is the difference between the SSD and OD states, expressed as a percentage of the OD weight:


AC = Wssd - Wod/ Wod x 100%


where W represents weight. It should be noted that, for most common aggregates, the absorption capacities are of the order of 0.5 to 2.0%. Absorption capacities greater than 2% are often an indication that the aggregates may have potential durability problems.

• The effective absorption (EA) refers to the amount of water required for the aggregate to go from
the AD to the SSD state:

EA = Wssd - Wad/ Wssd x 100%

To calculate the weight of the water absorbed (Wabs) by the aggregate in the concrete mix:

Wabs = (EA)Wagg

• The surface moisture (SM) represents water in excess of the SSD state, held on the aggregate surface:

SM = Wwet - Wssd/ Wssd x 100%

Thus, the extra water added to the concrete from the wet aggregates will be:

Wadd = (SM) Wagg

PROPERTIES OF HARDENED CONCRETE BASICS AND TUTORIALS

HARDENED CONCRETE BASIC PROPERTIES
What Are The Properties of Hardened Concrete?

Fully cured, hardened concrete must be strong enough to withstand the structural and service loads which will be applied to it and must be durable enough to withstand the environmental exposure for which it is intended. When concrete is made with high-quality materials and is properly proportioned, mixed, handled, placed, and finished, it is one of the strongest and most durable of building materials.

When we refer to concrete strength, we are generally talking about compressive strength which is measured in pounds per square inch (psi). Concrete is strong in compression but relatively weak in tension and bending.

It takes a great deal of force to crush concrete, but very little force to pull it apart or cause bending cracks (Figure 2-3). Compressive strength is determined primarily by the amount of cement used but is also affected by the ratio of water to cement, as well as proper mixing, placing, and curing.

Tensile strength usually ranges from 7 or 8% of compressive strength in high-strength mixes to 11 or 12% in low-strength mixes. Both tensile strength and flexural bending strength can be increased by adding steel or fiber reinforcement.

Structural engineers establish required compressive strengths for various building elements based on an analysis of the loads which will be applied and the soil conditions at the project site. Actual compressive strength is verified by testing samples in a laboratory using standardized equipment and procedures.

On commercial projects, numerous samples are tested throughout construction to verify that the concrete being put into place actually has the specified strength. Laboratory testing is not often required in residential work, except perhaps on large, high-end projects or on projects with difficult sites where special foundation designs make concrete strength critical.

For most residential projects, required concrete strength will be in the range of 2,500 to 4,000 psi, depending on the intended use (Figure 2-4). A concrete that is stronger than necessary for its intended use is not economical, and one that is not strong enough can be dangerous.

The primary factors affecting concrete compressive strength are the cement content, the ratio of water to cement, and the adequacy and extent of hydration and curing, all of which are discussed later in this chapter.

Durability might be defined as the ability to maintain satisfactory performance over an extended service life. Satisfactory performance is related to intended use. Concrete that will be walked or driven on must be abrasion resistant so that it doesn’t wear away.

Concrete that will be exposed on the outside of a building must be weather resistant so that it doesn’t deteriorate from repeated freezing and thawing. Concrete in which steel reinforcement is embedded must resist excessive moisture absorption in order to protect the metal from corrosion.

Natural wear and weathering will cause some change in the appearance of concrete over time, but in general, durability also includes the maintenance of aesthetic as well as functional characteristics. Just as concrete mix designs can be adjusted to produce a variety of strengths, appropriate concrete ingredients, mix proportions, and finishes can and should be adjusted on the basis of required durability.