CONCRETE CRACKS CAUSES AND TYPES BASIC INFORMATION

What causes concrete cracks? Several factors, when combined, can lead to restraint cracks in two-way reinforced-concrete slabs. Concrete slabs tend to shorten, and structurally stiff elements such as walls, elevator and stairwell cores, and columns can restrain the slab.

When the tensile stress exceeds the tensile strength of the concrete, a restraint crack occurs (ACI Committee 224, 1997). Depending on many factors, including the stiffness of the restraining elements and the length of the slab spans, multiple restraint shrinkage cracks may form.

The specific factors that cause shortening of concrete slabs include:
• Shrinkage of concrete
• Creep of concrete due to sustained loads (including precompression)
• Elastic shortening (prestressed slabs only)
• Fall in temperature


For a typical parking structure in Southern California with 70% ambient humidity and a moderate temperature variation of 40°F, the contributions of the above factors to slab shortening are as given in Figure 35.1 and Table 35.1. It is noteworthy that two thirds of slab shortening is typically due to concrete shrinkage.

Axial creep and elastic shortening, which are the only direct consequences of post-tensioning, contribute about one sixth of the total shortening.

To appreciate the magnitude of shortenings that are likely to occur in a post-tensioned slab, consider the example shown in Figure 35.2. For the 200 × 100-ft slab shown, the shortenings (if free to take place) are estimated to be 0.8 in. per 100 ft of slab length.

Obviously, this shortening cannot materialize in most cases, because the slabs are commonly tied to supporting structural elements. The interaction of the slab with its restraining structural elements is the crucial factor in the formation of cracks.

Referring to the breakdown of shortenings in Figure 35.2, only 18% of the calculated shortening is due to post-tensioning.

FIGURE 35.2 Reflected ceiling view of slabs: (a) post-tensioned slab; (b) reinforced concrete slab.

The balance is common to nonprestressed as well as post-tensioned slabs. This shows that little difference exists between post-tensioned and nonprestressed slabs as far as crack initiation is concerned; however, crack propagation is fundamentally different between the two types of slabs.


Prominent characteristics of cracks in unbonded post-tensioned slabs as compared to regular reinforced concrete are the following:

• Cracks are fewer in number; instead of a multitude of hairline cracks, fewer cracks form.
• Cracks are generally wider; they are spaced farther apart and generally extend deeper into the slab.

In regular reinforced concrete, the spacing between cracks is of the order of slab depth, whereas in post tensioned slabs it is more related to the span length and the overall dimensions of the slabs. In most cases, crack spacing is more than one quarter of the shorter slab span.

• Cracks are normally longer and continuous, and continuous cracks may extend over one span and beyond. In nonprestressed concrete, cracks are generally shorter in length.

• Cracks commonly do not coincide with locations of maximum moments. Restraining cracks do not necessarily develop at the bottom of midspan or the top of supports where the bending moments are maximum.

• Cracks occur at axially weak locations. Axially weak regions are typically found at construction joints, pour strips, cold joints, paths with reduced discontinuities in slab, and, finally, where precompression is reduced either due to termination of tendons or friction losses in tendons.

Figure 35.2 compares typical crack patterns on the soffit of an interior panel of a two-way slab construction. For the regular reinforced-concrete structure, the shrinkage cracks are shown coinciding with the locations of maximum tension.

Unbonded post-tensioned slabs generally exhibit poorer cracking performance as a result of lesser bonded reinforcement, which mobilizes the concrete in the immediate vicinity of a crack. Hence, a series of large slab segments separated by wide cracks rather than well-distributed small cracks is produced unless either the unbonded post-tensioning is accompanied by a sufficient nonprestressed reinforcement or inplane restraining actions are present that result in a similar improvement of the crack distribution.

Examples of common cracks in slabs, columns, and walls due to restrained movement are illustrated below. Due to the variety of member types and geometry and the array of crack initiation factors, it is imperative that each concrete member be reviewed individually and as part of the overall framing system during the design detailing process.

Concentrated load application and vulnerable member joint conditions may require a very localized review of concrete detailing. On the other hand, the overall framing layout may cause indirect load transfer due to geometry or member incompatibility, resulting in concrete cracking based on overall behavior of the framing system.

COMPRESSIVE STRENGTH OF HARDENED CONCRETE BASIC AND TUTORIALS

It is well recognized that silica fume can contribute significantly to the compressive strength development of concrete. This is because of the filler effect and the excellent pozzolanic properties of the material, which translate into a stronger transition zone at the paste–aggregate interface.

The extent to which silica fume contributes to the development of compressive strength depends on various factors, such as the percentage of silica fume, the water/cement + silica fume ratio, cementitious materials content, cement composition, type and dosage of superplasticizer, temperature, curing conditions, and age.

Superplasticizing admixtures play an important role in ensuring optimum strength development of silica-fume concrete. The water demand of silica-fume concrete is directly proportional to the amount of silica fume (used as a percentage replacement for Portland cement) if the slump of concrete is to be kept constant by increasing the water content rather than by using a superplasticizer.

In such instances, the increase in the strength of silica-fume concrete over that of control concrete is largely offset by the higher water demand, especially for high silica-fume content at early ages. In general, the use of superplasticizer is a prerequisite to achieving proper dispersion of the silica fume in concrete and fully utilizing the strength potential of the fume.

In fact, many important applications of silica fume in concrete depend strictly upon its utilization in conjunction with superplasticizing admixtures. Silica-fume concretes have compressive strength development patterns that are generally different from those of Portland cement concretes.

The strength development characteristics of these concretes are somewhat similar to those of fly-ash concrete, except that the results of the pozzolanic reactions of the former are evident at earlier ages. This is due to the fact that silica fume is a very fine material with a very high amorphous silica content.

The main contribution of silica fume to concrete strength development at normal temperatures takes place between the ages of about 3 and 28 days. The overall strength development patterns can vary according to concrete proportions and composition and are also affected by the curing conditions.

Carette and Malhotra (1992) reported investigations dealing with the short- and long-term strength development of silica-fume concrete under conditions of both continuous water curing and dry curing after an initial moist-curing period of 7 days. Their investigations covered superplasticized concretes incorporating 0 and 10% silica fume as a replacement by weight for Portland cement and water/cement + silica fume ratios ranging between 0.25 and 0.40.

As expected, the major contributions of silica fume to the strength took place prior to 28 days; the largest gains in strength of the silica-fume concrete over the control concrete were recorded at the ages of 28 and 91 days, although this gain progressively diminished with age. For concretes with water/cement + silica fume ratios of 0.30 and 0.40, the gain largely disappeared at later ages.

Under air-drying conditions, the strength development pattern was found to be significantly different from that of water-cured concretes up to the age of about 91 days; thereafter, however, air drying clearly had some adverse effect on the strength development of both types of concrete.

The effect was generally more severe for silica-fume concrete, where some reduction in strength was recorded between the ages of 91 days and 3.5 years, especially for concretes with water/cement + silica fume ratios of 0.30 and 0.40.

These trends of strength reduction have not yet been clearly explained, but they appear to stabilize at later ages and therefore are probably of little practical significance.

Curing temperatures have also been shown to affect significantly the strength development of silicafume concrete. This aspect has been examined in some detail by several investigators in Scandinavia. In general, these investigations have indicated that the pozzolanic reaction of silica fume is very sensitive to temperature, and elevated-temperature curing has a greater strength-accelerating effect on silica-fume concrete than on comparable Portland cement concrete.

The dosage of silica fume is obviously an important parameter influencing the compressive strength of silica fume concrete. For general construction, the optimum dosage generally varies between 7 and 10%; however, in specialized situations, up to 15% silica fume has been incorporated successfully in concrete.

STEEL REINFORCED CONCRETE STRUCTURES ASSESSMENT AND REPAIR OF CORROSION FREE EBOOK DOWNLOAD LINKE

STEEL REINFORCED CONCRETE STRUCTURES ASSESSMENT AND REPAIR OF CORROSION FREE EBOOK
Free E-Book Download Link Of Steel-Reinforced Concrete Structures: Assessment and Repair of Corrosion

A Practical Guide to Maintenance
Carrying a billion-dollar price tag, corrosion of reinforced concrete is the enemy of every country’s investment in real estate. The widespread and long-term use of reinforced concrete makes its correct and proper examination, maintenance, and repair paramount.

Steel-Reinforced Concrete Structures: Assessment and Repair of Corrosion explains the corrosion of reinforced concrete from a practical point of view, highlighting protective design and repair procedures.

The book begins with a discussion of the corrosion phenomena, the effect of concrete properties on corrosion, and the precautions available in the construction stage to mitigate corrosion. It covers the theoretical and practical methods in evaluating the concrete structures and new, practical methods to protect steel reinforcement.

The book also includes methods established in the last decade that provide new ways of protecting steel-reinforced bars and the traditional and advanced repairing methods. The author explains the importance of implementing an integrity management system to provide a comprehensive maintenance strategy and concludes with coverage of the traditional, time-tested, and advanced repair techniques.

A special feature is a chapter focusing on the advance maintenance plan philosophy and risk-based maintenance for reinforced concrete structures.

The author examines economic analysis procedures and the probability of structural failures to define structure risk assessment. He covers precautions and recommendations for protecting the reinforced concrete structures from corrosion based on codes and specifications. He uses case histories from all over the world to demonstrate the widespread application and range of advanced repair techniques and presents a practical guide to the maintenance of concrete structures. The book provides procedures for corrosion diagnosis and determining the appropriate methods for repair, as well as economic models for on-site decision making.

The Author
Mohamed A. El-Reedy's background in structural engineering. His main area of researches is reliability of concrete and steel structure. He has provided consulting to different engineering companies and oil and gas industries in Egypt and to international companies as the International Egyptian Oil Company (IEOC) and British Petroleum (BP). Moreover, he provides different concrete and steel structure design package for residential buildings, warehouses and telecommunication towers and electrical projects with WorleyParsons Egypt. He has participated in Liquified Natural Gas (LNG) and Natural Gas Liquid (NGL) projects with international engineering firms. Curently, Dr. El-Reedy is responsible for reliability, Inspection and maintenance strategy for onshore concrete structures and offshore steel structure platforms. He has performed these tasks for hundred structures in Gulf of Suez in the red sea.

Dr. El-Reedy has consulted with and trained executives at many organizations, including the Arabian American Oil Compnay (ARAMCO), bp, Apachi, Abu Dhabi Marine Operating Company (ADMA), the Abu Dhabi National Oil Company and King Saudi's Interior ministry, Qatar Telecom, EGPC , (SAPIC) , the Kuwait Petroleum Corporation, and Qatar petrochemical Company (QAPCO). He has taught technical courses about repair and maintenance for reinforced concrete structures and the advanced materials in concrete industry worldwide , especially in the Middle East, malaysia and singapore.

Dr. El-Reedy has written numerous publications and presented many papers at local and international conferences sponsored by the ASCE, CSCE, ACI, API and in technical committe for OMAE conference sponsor by ASME.He has published many research papers in international technical journals and has authored four books about total quality management, quality management and quality assurance, economic management for engineering projects, and repair and protection of reinforced concrete structures. He received his bachelor's degree from Cairo University in 1990, his master's degree in 1995, and his Ph.D from Cairo University in 2000.

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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.

WELDED-WIRE FABRIC (WWF) BASICS AND TUTORIALS

WELDED-WIRE FABRIC (WWF) BASIC INFORMATION
What Are Welded Wire Fabric?


Welded-wire fabric is an orthogonal grid made with two kinds of cold-drawn wire: plain or deformed. The wires can be spaced in each direction of the grid as desired, but for buildings, usually at 12 in maximum.

Sizes of wires available in each type, with standard and former designations, are shown in Table 9.6.

Welded-wire fabric usually is designated WWF on drawings. Sizes of WWF are designated by spacing followed by wire sizes; for example, WWF 6 12, W12/ W8, which indicates plain wires, size W12, spaced at 6 in, and size W8, spaced at 12 in. WWF 6 12, D-12/D-8 indicated deformed wires of the same nominal size and spacing.

All WWF can be designed for Grade 60 material. Wire and welded-wire fabric are produced to conform with the following ASTM standard specifications:

ASTM A82, Plain Wire

ASTM A496, Deformed Wire

ASTM A185, Plain Wire, WWF

ASTM A497, Deformed Wire, WWF

Epoxy-coated wire and welded wire fabric are covered by the ASTM specification A884/A884M. Applications of epoxy-coated wire and WWF include use as corrosion-protection systems in reinforced concrete structures and reinforcement in reinforced-earth construction, such as mechanically-stabilized embankments.

CONCRETE CORROSION INHIBITORS ADMIXTURE BASIC AND TUTORIALS

CONCRETE CORROSION INHIBITORS ADMIXTURE BASIC INFORMATION
What Are Corrosion Inhibitors Admixtures For Concrete?


Reinforcing steel in concrete usually is protected against corrosion by the high alkalinity of the concrete, which creates a passivating layer at the steel surface.

This layer is composed of ferric oxide, a stable compound. Within and at the surface of the ferric oxide, however, are ferrous-oxide compounds, which are more reactive.

When the ferrous-oxide compounds come into contact with aggressive substances, such as chloride ions, they react with oxygen to form solid, iron-oxide corrosion products.

These produce a fourfold increase in volume and create an expansion force greater than the concrete tensile strength. The result is deterioration of the concrete.

For corrosion to occur, chloride in the range of 1.0 to 1.5 lb /yd3 must be present. If there is a possibility that chlorides may be introduced from outside the concrete matrix, for example, by deicing salts, the concrete can be protected by lowering the water-cement ratio, or increasing the amount of cover over the reinforcing steel, or entraining air in the concrete, or adding a calcium-nitrate admixture, or adding an internal-barrier admixture, or cathodic protection, or a combination of these methods.

To inhibit corrosion, calcium-nitrate admixtures are added to the concrete at the time of batching. They do not create a physical barrier to chloride ion ingress. Rather, they modify the concrete chemistry near the steel surface.

The nitrite ions oxidize ferrous oxide present, converting it to ferric oxide. The nitrite is also absorbed at the steel surface and fortifies the ferric-oxide passivating layer.

For a calcium-nitrite admixture to be effective, the dosage should be adjusted in accordance with the exposure condition of the concrete to corrosive agents. The greater the exposure, the larger should be the dosage.

The correct dosage can only be determined on a project-by-project basis with data for the specific admixture proposed. Internal-barrier admixtures come in two groups. One comprises waterproofing
and dampproofing compounds.

The second consists of agents that create an organic film around the reinforcing steel, supplementing the passivating layer. This type of admixture is promoted for addition at a fixed rate regardless of expected
chloride exposure.

MODIFIED PORTLAND CEMENTS BASICS AND TUTORIALS

DIFFERENT TYPES OF MODIFIED PORTLAND CEMENTS BASIC
What Are The Different Types Of Modified Portland Cements?


Increasingly, modern concretes contain a blend of Portland cement and other cementitious materials. When other materials are added to Portland cement at the time at which the concrete is batched, they are referred to as mineral admixtures; however, there are also hydraulic cements, which are produced either by forming other compounds during the burning process or by adding other materials to the clinker and then intergrinding them.

The common types of such modified cements are described in the following sections.


Portland Pozzolan Cements
Portland pozzolan cements are blends of Portland cement and a pozzolanic material. The role of the pozzolan is to react slowly with the calcium hydroxide that is liberated during cement hydration.

This tends to reduce the heat of hydration and the early strength but can increase the ultimate strength of the material. These cements tend to be more resistant to sulfate attack and to the alkali–aggregate reaction.

Portland Blast-Furnace Slag Cements
Ground granulated blast-furnace slag (GGBFS), which is a byproduct of the iron and steel industry, is composed largely of lime, silica, and alumina and thus is a potentially cementitious material. To hydrate it, however, it must be activated by the addition of other compounds.

When the GGBFS is to be activated by lime, the lime is most easily supplied by the hydration of the Portland cement itself. Slags may be present in proportions ranging from 25 to 90%. They react slowly to form C–S–H, which is the same product that results from the hydration of the calcium silicates.

In general, because they react more slowly than Portland cement, slag cements have both lower heats of hydration and lower rates of strength gain.

On the other hand, they have an enhanced resistance to sulfate attack. When the GGBFS is to be activated with calcium sulfate (CaSO4), together with a small amount of lime or Portland cement, the material is known as supersulfated cement.

This cement is available mostly in Europe, where it is used for its lower heat of hydration and its resistance to sulfate attack.

Expansive Cements
Expansive cements were developed to try to offset the drying shrinkage that concrete undergoes. This is particularly important when the concrete is restrained against contraction or when it is to be cast against mature concrete in repair situations.

In both cases, severe cracking may occur as a result of the shrinkage. Expansive cements are based on the formation of large quantities of ettringite during the first few days of hydration; however, they are little used today, in large part because it is very difficult to control (or predict) the amount of expansion that will take place for a particular concrete formulation.

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.

CYCLONE RESISTANT BUILDING BASICS AND TUTORIALS

BUILDINGS THAT ARE CYCLONE RESISTANT
How To Make Cyclone Resistant Building?


A cyclone is a storm accompanied by high speed whistling and howling winds. It brings torrential rains. A cyclone storm develops over tropical ocean and blows at speed as high as 200–240 km/hour.

It is usually accompanied by lightning, thunder and continuous downpour of rain. Cyclones extend from 150 km to 1200 km in lateral directions with forced winds spiralling around a central low pressure area.

The central region of light winds and low pressure, known as the ‘eye’ of cyclone has an average diameter of 20 to 30 km. This central eye is surrounded by a ring of very strong winds extending up to 40 to 50 km beyond centre.

This region is called ‘wall cloud’. In this region strongest winds and torrential rains occur. Beyond this region winds spiralling extend outwards to large distances, which goes on reducing with the distance from the centre of the cyclone.

The following care should be taken in designing buildings in cyclone prone areas:

1. Foundations should be deeper

2. R.C.C. framed structures are to be preferred over load bearing structures

3. Sloping roofs should be avoided.

4. Cantilever projections should be avoided.

5. Roof and parapet wall should be properly anchored to the columns and walls.

6. Height of the buildings should be restricted.

7. Suitable wind load should be considered in the building design.

8. Openings in the wall should be less.

9. Structure should not rest on loose soil.

REINFORCED CEMENT CONCRETE (RCC) BASICS AND TUTORIALS

REINFORCED CEMENT CONCRETE (RCC) BASIC INFORMATION
What Is Reinforced Cement Concrete (RCC)?

Concrete is good in resisting compression but is very weak in resisting tension. Hence reinforcement is provided in the concrete wherever tensile stress is expected. The best reinforcement is steel, since tensile strength of steel is quite high and the bond between steel and concrete is good.

As the elastic modulus of steel is high, for the same extension the force resisted by steel is high compared to concrete. However in tensile zone, hair cracks in concrete are unavoidable. Reinforcements are usually in the form of mild steel or ribbed steel bars of 6 mm to 32 mm diameter.

A cage of reinforcements is prepared as per the design requirements, kept in a form work and then green concrete is poured. After the concrete hardens, the form work is removed.

The composite material of steel and concrete now called R.C.C. acts as a structural member and can resist tensile as well as compressive stresses very well.

Properties of R.C.C./Requirement of Good R.C.C.

1. It should be capable of resisting expected tensile, compressive, bending and shear forces.

2. It should not show excessive deflection and spoil serviceability requirement.

3. There should be proper cover to the reinforcement, so that the corrossion is prevented.

4. The hair cracks developed should be within the permissible limit.

5. It is a good fire resistant material.

6. When it is fresh, it can be moulded to any desired shape and size.

7. Durability is very good.

8. R.C.C. structure can be designed to take any load.

Uses of R.C.C.

It is a widely used building material. Some of its important uses are listed below:

1. R.C.C. is used as a structural element, the common structural elements in a building where

R.C.C. is used are:

(a) Footings (b) Columns

(c) Beams and lintels (d) Chejjas, roofs and slabs.

(e) Stairs.

2. R.C.C. is used for the construction of storage structures like

(a) Water tanks (b) Dams

(c) Bins (d) Silos and bunkers.

3. It is used for the construction of big structures like

(a) Bridges (b) Retaining walls

(c) Docks and harbours (d) Under water structures.

4. It is used for pre-casting

(a) Railway sleepers (b) Electric poles

5. R.C.C. is used for constructing tall structures like

(a) Multistorey buildings (b) Chimneys

(c) Towers.

6. It is used for paving

(a) Roads (b) Airports.

7. R.C.C. is used in building atomic plants to prevent danger of radiation. For this purpose R.C.C. walls built are 1.5 m to 2.0 m thick.