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We have only considered elements with normal strain in the previous article, those that elongate or shorten, but remain rectangular. Now, we consider shear strain, which quantifies deformation in which element edges no longer form right angles.

1. Shearing corresponds to internal surfaces that slide parallel to themselves and elements that distort. Take a rectangular block of rubber, attach metal plates to its top and bottom, and etch a grid of square elements on its side face.

Move the upper plate relative to the lower plate in the direction parallel to the dimension a, maintaining the plates parallel. The etched grid distorts as shown here.

Not all elements deform the same, but elements located away from the left and right edges distort identically. Their lower and upper faces do not lengthen. The side faces are no taller, but they have tilted.

2. Shear strain is quantified by the angle change at an element corner when it is deformed. Here is the key feature of shear strain: element edges, which were originally perpendicular, make angles that are greater than and less than 90° or π/2 radians when deformed. Let be the change from the initial right angle.

The greater is β, the greater is the distortion or shear of the element. The shear strain,γ, is defined in terms of the angular distortion β.

γ=tanβ

If β in radians is small (β<<1), then tanβ≈β, so γ=β. Also, when β is small, the change in length of the vertical elements is negligible.

3. Shear strain can be related to the shear displacement divided by the thickness of the sheared layer. Using trigonometry, we can find the angle β in terms of the relative shear displacement. Let u be the displacement (motion) of a plate in the shear direction. β is related to the relative shear displacement, Δushear, (i.e., utop - ubottom) and the height, h, perpendicular to the shearing.

4. Besides two oppositely acting shear forces, there must be additional loads to balance moments. While one can readily picture the shear strain, it is more difficult to picture the forces on the plates that shear the block. Here are several possibilities (only the second and third are in equilibrium).

5. Bodies that shear must have internal forces (shear stresses) acting on both horizontal and vertical planes. Assume the forces to be as in the third case above. Consider the internal forces at a surface parallel to F0. There must be a horizontal shear force V equal to F0. We call this a shear force because it acts parallel to, rather than normal to, the face on which it acts.

Now cut across the block vertically. There must be a vertical shear force, although its magnitude is unclear because it depends on how the moment is balanced. Shear stress, τ, is defined as the shear force per unit area. The shear stress is rarely uniform, but the average value can be found in terms of the shear force, V, and the area, A, on which it acts.

Flexure (bending) is associated with lateral deformation of a member under a transversely applied load. Consider a reinforced concrete beam subjected to the uniform load shown in Figure 1a. In cast-in-place concrete construction, beams act as a monolithic unit with the supporting columns. However, for design purposes the beam can be modeled as a simply supported member, as shown in Figure 1b. A bending moment diagram for this beam is shown in Figure 1c. The top portion of the beam is subjected to compression whereas the bottom is under tension. Concrete has a limited ability to carry tension and cracks once its tensile strength has been reached in the region of maximum bending moments (in this case at the beam midspan). To increase the bending resistance of a cracked beam, steel reinforcement (often called tension reinforcement or tension steel) is placed inside the beam near the bottom to resist the tensile stresses.

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Positive bending causes tension at the bottom and compression at the top of a flexural member. The positive bending moment is shown on the bending moment diagram above the longitudinal axis of the member according to the North American convention.

Types of Flexural Members

An isometric view of a concrete floor structure subjected to gravity load is shown in Figure 2. The structure consists of a slab supported by beams, which are in turn supported by columns. In this case, beams are provided in one direction only. Consequently, the slab transfers the applied load in the direction perpendicular to the beams. This type of slab is called a one-way slab. One-way slabs are flexural members that behave essentially like wide beams. In cast-in-place concrete construction, beams are usually cast monolithically with the slab. The portion of the slab cast monolithically with a beam contributes to the beam moment resistance; this additional capacity is taken into consideration in the beam design. The term T-beam is used in this case because the slab and the beam form a T section for positive bending. In design practice, beams cast monolithically with the slab may be designed either as T-beams or as rectangular beams for positive bending. If the slab contribution is ignored, the beam is considered to act as a rectangular beam. The analysis of rectangular beams is simpler than that of T-beams. and it is a good starting point for understanding the flexural resistance of concrete members.

Reinforced concrete beams and slabs can be classified into simple and continuous structures. Continuous structures are statically indeterminate. Simple (or simply supported) structures span across two supports, as shown in Figure 3a, whereas continuous structures span across three (or more) supports (see Figure 3c). When subjected to a gravity load such as the uniform load in Figure 3a, only positive bending moments develop in simple structures (see Figure 3b), while both positive and negative bending moments develop in continuous structures subjected to the same load, as shown in Figure 3d. The deformed shape of the continuous beam subjected to a uniform load is shown in Figure 3e. The points where the curvature of the deflected shape changes from the sagging to the hogging shape are called inflection points (or points of contraflexure) and are denoted by IP on the diagram in Figure 3e. The same points correspond to the locations of zero moments on the bending moment diagram in Figure 3d.

In general, reinforced concrete members are characterized by regular cross-sectional dimensions. Members with constant cross-sectional dimensions within one span are called prismatic members (see Figure 4a). In some cases, the cross-sectional properties of a beam within a span are varied. Beams and slabs with variable cross-sectional dimensions along their length are called nonprismatic members. Haunched beams are nonprismatic members commonly found in design practice. These beams are characterized by larger cross-sectional dimensions in the support regions, which could also taper toward the midspan (see Figure 4b ). Haunched beams are sometimes considered to be a more effective design solution for longer-span continuous structures.

The new trend of using high-strength concrete in construction has caused a need for the use of 4 x 8 in. cylinders for assurance testing. A controlling factor that affects the size of specimen that can be tested in a compression machine is the strength of the concrete on evaluation. Some testing machines are not able to produce the force needed to break high-strength 6 x 12 in. concrete cylinders. If 4 x 8 in. cylinders are to be used in quality assurance testing, the relationship between fc4 and fc6 needs to be understood in order to ensure that concrete with sufficient strength is provided. If the average compression machine operates safely, rarely exceeding 80% of its capacity, and has a capacity of 250,000 lbs, the machine can test a 6 x 12 in. cylinder with a compressive strength of approximately 7,000 psi. The same machine can test a 4 x 8 in. cylinder of approximately 16,000 psi.

A 4 x 8 in. cylinder weighs about 9 lb compared to a 6 x 12 in. cylinder, which weighs about 30 lb. This might suggest that because 4 x 8 in. cylinders are lighter and can easily be handled, collection of quality control and assurance specimens would be easier for contractors and inspectors. The advantages of using smaller specimens are: 1) easier handling; b) less required storage space; c) less capacity required of testing machines.

This research project was born from the need to determine a correlation between the strength of the standard size 6 x 12 in. cylindrical specimen and the strength of a 4 x 8 in. cylindrical specimen made from the same batch of concrete. The objectives of this study are to review the factors that may affect the compressive strength, those that may affect the strength obtained by 4 x 8 in. and 6 x 12 in. cylinders, and the variability associated with these tests. An extensive laboratory testing program was developed to evaluate the desired goals of the project. A total of 359 4 x 8 in. and 357 6 x 12 in. cylinders were tested.

The factors that were studied to evaluate the effect of cylinder size on concrete compressive strength were aggregate size, technician, compressive strength, and age of specimen at testing. It was determined that compressive strength was the only factor significant in affecting the ratio of 4 x 8 in. cylinder strength to 6 x 12 in. cylinder strength. Compressive strength was also the only factor significant in affecting the within-test variability of each batch of concrete. It is recommended that 4 x 8 in. cylinders may be implemented for quality assurance testing if the design strength of concrete is greater than 5,000 psi and the capacity of the testing machine will not allow the testing of 6 x 12 in. cylinders based on the design strength. However, if 4 x 8 in. cylinders are used, a correlation between the 4 x 8 in. and 6 x 12 in. cylinders should be determined using a capable machine for the project.

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A step-by-step procedure for modeling and analysis of frame structure using ETABS is explained through a simple example. Subsequently an example of seismic analysis of regular frame structure and irregular frame structure are solved manually and through ETABS.

A plan of five storey reinforced concrete (RC) frame structure is considered for modeling and analysis using ETABS.
Beam sizes 300×450 mm

Storey Height 3.2 m

Columns sizes 300×450 mm

Live Load 3 kN/m²
Slab thickness 120 mm

Floor Finish Load 1 kN/m²
Concrete grade M25

Steel Fe415

In this thesis, a special reinforced concrete shear wall building was designed per ASCE 7-05, and then the performance was investigated using the four analysis procedures outlined in ASCE 41-06. The proposed building was planned as a 6-story office building in San Francisco, CA. The structural system consisted of a two-way flat plate and reinforced concrete columns for gravity loads and slender structural walls for seismic loads. The mathematical building models utilized recommendations from ASCE 41-06 and first-principle mechanics. Moment-curvature analysis and fiber cross-section elements were used in developing the computer models for the nonlinear procedures.

The results for the analysis procedures showed that the building met the Basic Safety Objective as defined in ASCE 41-06. The performance levels for the nonlinear procedures showed better building performance than for the linear procedures. This paper addresses previously found data for similar studies which used steel special moment frames, special concentric braced frames, and buckling restrained braced frames for their primary lateral systems. The results showcase expected seismic performance levels for a commercial office building designed in a high seismicity region with varying structural systems and when using different analysis procedures.

This manual demonstrates step by step procedures involved in modeling, analysis and design of typical reinforced concrete row houses in the famous structural engineering software CSI ETABS. Contents of the manual are as mentioned below,

1. Start Model with Template

2. Define Material Properties
3. Define and Assign Beams Sections
4. Define and Assign Column Sections
5. Define and Assign Slab Section Properties
6. Add Stairs to Model
7. Assign Load on Beam~ and Slab Panels
8. Define Load Cases and Load Combination
9. Replicate Model
10. Specify Meshing Option for Shell Element (Slab Panels)
11. Draw Dummy Walls and Assign Wind Load
12. Assign Spring Support
13. Run Analysis
14. View Analysis Results
15. Start RC Design

This manual has been developed by ACECOMS Asian Institute of Technology(AIT), Thailand. It can be downloaded from link below. Professional engineers, students and other professionals are encouraged to share similar helpful manuals/tutorials for common good.

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Multiple elements are used to transmit and resist external loads within a building. These elements define the mechanism of load transfer in a building known as the load path. The load path extends from the roof through each structural element to the foundation. An understanding of the critical importance of a complete load path is essential for everyone involved in building design and construction.

The load path can be identified by considering the elements in the building that contribute to resisting the load and by observing how they transmit the load to the next clement. Depending on the type of load to be transferred, there are two basic load paths:

• gravity load path 
• lateral load path

Both the gravity and lateral load paths utilize a combination of horizontal and vertical structural components, as explained below.

1. Gravity Load Path

Gravity load is the vertical load acting on a building structure, including dead load and live load due to occupancy or snow. Gravity load on the floor and roof slabs is transferred to the columns or walls, down to the foundations, and then to the supporting soil beneath. Figure 1 shows an isometric view of a concrete structure and a gravity load path.

The vertical gravity load acts on a slab (1), which transfers the load to the beams (2), which in turn transfer the load to the columns (3) and then down to the foundations (4). The gravity load path depends on the type of floor slab, that is, whether a slab is a one way or a two-way system. In the one-way system in Figure 2a, the effect of external loads is transferred primarily in one direction, shown with an arrow. The slab-beam and-girder floor is an example of a one-way system. The gravity load acting on this system is transferred from the slab (1) to the beams (2) and then to the girders (3). Finally, the girders transfer the load to the columns (4).

The load path in a two-way system is not as clearly defined. The slab transfers gravity load in two perpendicular directions; however, the amount carried in each direction depends on the ratio of span lengths in the two directions, the type of end supports, and other factors. For example, in the slab with beams system shown in Figure 2b, the load is transferred from the slab (l) to the beams aligned in the two directions (2) and then to the columns (3).

2. Lateral Load Path

The lateral load path is the way lateral loads (mainly due to wind and earthquakes) are transferred through a building. The primary elements of a lateral load path are as follows:,

• vertical components: shear walls and frames;
• horizontal components: roof, floors, and foundations.

Figure :3 Lateral Load Path

Figure 3 shows a reinforced concrete structure and the elements constituting the lateral load path: roof and floor systems (I) transfer the load to the walls (2), which in turn transfer the load to the foundations (3). Roof and floor systems (also called diaphragms) take horizontal forces from the storeys at or above their level and transfer them to walls or frames in the storey immediately below.

Shear walls and frames are the primary lateral-load resisting elements; however, these members also carry gravity loads. Shear walls receive lateral forces from diaphragms and transmit them to the foundations. Foundations form the final link in the load path by collecting the lateral forces from all storeys and transmitting them to the ground.

Tributary Area

The tributary area is related to the load path, and is used to determine the loads that beams, girders, columns, and walls carry. The reader is expected to be familiar with the concept of tributary area from other design courses, as it also applies to design of timber and steel structures; however, a brief overview is presented in this section. The tributary area for a beam or a girder supporting a portion of the floor is the area enclosing the member and bounded by the lines located approximately halfway between the lines of support (columns or walls), as shown in Figure 4. For example, a tributary area for the reinforced concrete beam AB that is a part of the one-way floor system is shown hatched in Figure 4a. A typical column has a tributary area bounded by the lines located halfway from the line of support in both directions (shown hatched in Figure 4b). In the case of uniformly loaded floors, tributary areas are approximately bounded by the lines of zero shear, that is, the lines corresponding to zero shear forces in the slabs, beams, or girders supported by the element for which the tributary area is determined. Zero-shear locations are generally determined by the analysis. For buildings with a fairly regular column spacing, the zero-shear locations may be approximated to be halfway between the lines of support.

Figure :4 Tributary area for reinforced concrete members: a) beams; b) columns.

It is very important for novice designers to thoroughly understand the design process, including the steps involved and the time allocated to and spent on performing each step.
Major steps in the design process are

This step involves the identification of project constraints, including cost, building shape, and architectural form, and functional constraints, including column spacing, materials, and serviceability limits. Conceptual design is the most important part of the entire design process. At this stage, the structural engineer has the pivotal role of developing a practical structural concept that strikes the proper balance between the external constraints and the project objectives. To develop a good structural concept, the designer needs to have not only a sound background in reinforced concrete design but also a strong appreciation for the architectural aspects of the project, constructability issues, and the owner's overall design objectives. The final goal is to develop a structural concept that is simple to build, aesthetically pleasing, functionally effective, and affordable to the owner. Once all the design issues have been identified, the designer should be able to make a schematic drawing of the structural system and decide on the general arrangement of structural elements. Further on, the structural designer can estimate gravity and lateral loads and develop trial sizes of key structural members.
 

This process may require a few iterations before the optimal solution is found. Next, the preliminary concrete outlines to be used for both architectural and structural drawings are developed. Finally, a preliminary construction budget is determined. Depending on the complexity of the project, this phase could take I 0% to 20% of the total time on the project.
 

Detailed (final) Design

This step involves the detailed analysis, evaluation, and sizing of members and more refined calculation of gravity and lateral loads. At this stage, the designer needs to ensure the safety and serviceability of the structure by carefully following the requirements of pertinent building codes. However, the designer also needs to keep in mind that several external factors may have an adverse effect on the performance and safety of reinforced concrete structures. Whenever possible, the designer should take advantage of available opportunities to increase the structural capacity, that is, to provide a reserve capacity in the structure. In general, reserve capacity may be required to account for construction errors in the field, errors in load estimates, load increases due to design modifications by the owner or architect, change of building use leading to load increase, variations in soil capacities, etc.

The best design solutions involve good judgment based on experience and knowledge, consideration of the economy and construction issues, repetition, simplicity in rebar placements, reduction of potential field errors, etc. A detailed design takes approximately 20% of the total time spent on the project. 

Development of Contract Documents

The main focus in this step is to transfer the concept and details from the mind of the designer to those involved in the construction. The contract documents include drawings and specifications. Drawings are a graphical representation of the design, whereas specifications are written descriptions of materials and construction procedures. A well-designed building will not perform in a satisfactory manner when poorly constructed. Therefore, preparation of clear and correct contract documents is an essential part of the design and may take up to 50% of the total time spent on the project.

Coordination

The designer needs to keep in mind that structural design is only one component of the overall building design. Other disciplines include architectural, electrical, mechanical, geotechnical, and civil engineering. The structural designer needs to coordinate the design with other disciplines. This stage might be rather time-consuming and may take up to 10% of the total time spent on the project.
 

Services During Construction

Structural designers are routinely involved in the review of shop drawings, concrete mix designs, and laboratory testing reports. The designer must make regular visits to the construction site to observe the construction, answer questions, and clarify contract documents. The main objective of these visits is to verify that the work is progressing in the manner intended by the design. The involvement of a designer at this stage varies with design and construction complexity; however, it may take about 10% to 20% of the total time spent on the project.

Concrete is one of the most versatile construction materials, offering potentially unlimited opportunities for developing diverse forms of construction. Concrete is what is known as a universal material, as its ingredients. namely cement, sand, aggregates, and water, are available all over the globe. Furthermore, concrete structures can be built with all different levels of technology, ranging from the simplest hand tools to computerized equipment. Concrete also has the excellent characteristics of fire resistance and durability and requires substantially less maintenance than other materials. Its mechanical properties can be enhanced by the use of steel reinforcement. Reinforced concrete construction makes use of the high compressive strength of concrete and the high tensile strength of steel reinforcement.

The structural (or framing) system is the skeleton of a building, and it supports the rest of the structure. Structural systems characteristic of reinforced concrete buildings are (see Figure 1)

• moment-resisting frames
• bearing-wall systems
• frame/shear-wall hybrid systems

Moment Resisting Frame

A moment-resisting frame (or moment frame) consists of columns and beams that act as a three dimensional (3-D) space frame system, as shown in Figure la. Both gravity and lateral forces are resisted by bending in beams and columns, while strong rigid joints between columns and beams have a special role in providing stability in moment frames. The frames are often infilled with masonry partitions (usually hollow concrete blocks or hollow clay tiles); such a system is called a concrete frame with masonry infills. The moment-frame system has been used often for office buildings in the world. The predominant use is in the five to ten storey range. Intermediate moment resisting frames (IMRF) and special moment resisting frames (SMRF) are other popular variants of moment resisting frames.

Bearing Wall System

A bearing-wall system consists of reinforced concrete bearing walls located along exterior wall lines and at interior locations as required (see Figure 1b). These bearing walls are also used to resist lateral forces, in which case they are called shear walls. Shear walls are designed to resist lateral forces from floor structures and transmit them to the ground. Ideally, these shear walls are continuous structures, extending from the foundation to the roof of the building. This system is usually characterized by a rectangular plan, with a centrally located elevator and stair core and uniformly distributed walls. Many residential and office buildings in Canadian cities utilize the bearing-wall system.

Frame/Shear Wall Hybrid System

A frame/shear-wall hybrid system utilizes a complete 3-D space frame to support gravity loads and shear walls to resist lateral loads (see Figure 1c). The main lateral load resisting system consists of reinforced concrete shear walls forming the elevator core (central core formed by the elevators and stairs in the building), and additional walls located elsewhere in the building as required. The role of the concrete frame is to transfer gravity loads only, so it is often called the gravity frame. The columns typically support concrete flat slab structures or two-way slabs with beams. The interaction of the frame and shear walls is essential for limiting lateral deformations due to wind and earthquake loads. These buildings are generally characterized by a symmetrical plan of square, circular, or hexagonal shape with a centrally located elevator core. This system is commonly found in modern office and residential high-rise buildings throughout the world.

Structural Components of a Reinforced Concrete Building

Concrete is artificial stone made from two main components: cement paste and aggregates.

Aggregates usually consist of natural sand and gravel or crushed stone. The paste hardens as a result of the chemical reaction between cement and water and glues the aggregates into a rock-like mass. Reinforced concrete structures utilize the best qualities of concrete and steel - concrete's high compressive strength and steel's high tensile strength. The main idea behind reinforced concrete is to provide steel reinforcement at locations where tensile stresses exist that the concrete cannot resist. Due to its strength, only a relatively small amount of steel is needed to reinforce concrete. Steel's ability to resist tension is around 10 times greater than concrete's ability to resist compression. It is very important to note that reinforcement in concrete structures is effective only if it is appropriately used, strategically placed, and in proper quantity.

Prestressed concrete is a special type of reinforced concrete in which internal compression stresses are introduced to reduce potential tensile stresses in the concrete resulting from external loads. High-strength steel tendons are embedded within the concrete and subjected to a tensile stress imposed by special equipment (jacks). The two main methods of prestressed concrete construction are

These structural components can be classified into horizontal components (Floors, roofs, and beams) and vertical components (columns and walls). According to another classification, the part of the building above ground is called the superstmcture, while the part below ground (including foundations, basemen!, and other underground structures) is called the substructure. The role of each structural component is briefly explained below.

The floor and roof systems are the main horizontal structural components in a building. They carry gravity loads and transfer them to the vertical components (columns and/or walls), and also act as horizontal diaphragms by transferring the lateral load to the vertical components of a structure. The most common floor and roof systems are listed below (see Figure 2):
 

• Slab-beam-and-girder: The slabs are supported by beams, which are in turn supported by girders (see Figure 2a). A girder is a large beam that carries loads from the beams framing into it. Beams around the outside edges of the floor are called spandrel beams.
 

• Slab band: This is a uniform slab with a thickened slab portion along the column lines parallel to the longer spans (see Figure 2b).

• Flat slab: This is a system without beams, where a slab is supported by round or square columns (see Figure 2c). In this system, the design may also require a flared cone shaped cap on the top of the column, called the capital, and a thickened slab above it, called the drop panel.

• Flat plate: This is similar to the flat slab, except that there are no drop panels or capitals, as shown in Figure 2d. Columns are typically of circular or square shape.

• Slab with beams: The beams frame into columns and support floor or roof slabs, as illustrated in Figure 2e. They provide moment interaction with the columns (this interaction is essential for the frame to resist lateral loads).

• Joist floor (pan joist): This system consists of a series of closely spaced joists (similar to small beams), spanning in one or two directions, topped by a reinforced concrete slab cast integrally with the joists, and beams spanning between the columns perpendicular to the joists (see Figure 2f).

• Waffle slab: This is a two-way reinforced concrete joist floor. Waffles are hollow spaces between the joists.

Slab on grade is a very common form of slab construction that is placed directly on the ground. It is also called "floor on ground." It is possible to confuse this term with the term "floor system." The basic difference is that a slab on grade is supported by the earth beneath it, whereas a floor system is supported only by columns at a few distinct locations.

Beams transmit the loads from the floors to the vertical supports (columns). Beams are usually cast monolithically with the slab and are subjected to bending and shear. 

Columns are vertical components that support a structural floor system. Columns are usually subjected to combined axial load and bending.

Walls provide the vertical enclosure for a building. Bearing walls carry gravity loads only, whereas shear walls have a major role in carrying lateral loads due to wind and earthquakes. Concrete walls built in the basements of buildings are subjected to lateral soil pressure in addition to gravity loads - such walls are called basement walls. 

Foundations transmit the weight of the superstructure to the supporting soil. There are several types of foundations. Spread footings transfer the load from the columns to the soil. Walls are supported by strip footings. Other types of foundations include combined footings, which support more than one column; piles which may be driven into dense soil strata beneath; and raft foundations, where several columns rest upon a raft or a mat distributing the column or wall loads over a uniform soil bearing area.

Specifically, the development lengths and splice lengths for straight bars in tension as well as compression are determined. Also, the development length for standard hook bars is determined. The provisions for development and splice lengths are included for high seismic risk applications per ACI 318-05, Chapter 21. There is also a worksheet which contains reinforcing bar data tables. This version is based on the ACI 318-05 Code.

1. This program follows the procedures and guidelines of the ACI 318-05 Building Code, Chapters 12 and 21.
2. The "Calc Development" worksheet, for a given reinforcing bar size, determines the straight development and  splice lengths for a "top" bar and an "other" bar in tension, the straight development and splice lengths for the  bar in compression, and the tension development length of the bar as a standard hook, all at one time.  A complete table of reinforcing bar development and splice lengths is also created for #3 through #18 bars.
3. The "Rebar Data" worksheet contains tables of reinforcing bar data which include various bar properties,  reinforcing bar areas based on spacing, tension development and splice lengths for straight bars, tension  development lengths for 90 degree standard hooks, tension lap splice criteria, compression development  and splice lengths for straight bars, maximum spacing for column ties, and various plain welded wire fabric  properties.
4. This program contains numerous “comment boxes” which contain a wide variety of information including  explanations of input or output items, equations used, data tables, etc. (Note: presence of a “comment box”  is denoted by a “red triangle” in the upper right-hand corner of a cell. Merely move the mouse pointer to the  desired cell to view the contents of that particular "comment box".)

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Bonding fresh, plastic concrete to old, hardened concrete increases the strength of the composite material. Fresh patches, concrete adjacent to construction joints, and overlays all benefit from bonding to the hardened concrete substrate. Bond is not, however, guaranteed. It must be ensured through proper surface preparation, material choice and use, and curing. Ignoring one of these components does not mean a one-third decrease in bond; it may result in the total loss of bond.
 

Surface Preparation
 

All damaged, loosened, or unbonded portions of existing concrete should be removed by chipping hammers or other mechanical methods. Prepare the exposed concrete by wet sandblasting, water blasting, or shot blasting. Then clean it and allow it to dry thoroughly. This removes any laitance, soft mortar, dirt, wood chips, form oil, or other foreign materials that may interfre with proper bonding of the new concrete.
 

Pressurized water and air is commonly used for surface preparation. Be sure that water used in cleaning is itself clean and also that no contaminants are present in the compressed air. ACI 503 (Ref. 2) recommends that all equipment supplying compressed air be equipped with efficient oil and water traps to prevent surface contamination from the compressed air supply.
 

Acid etching was once considered another way to prepare a surface, but experience shows that this method is not as dependable as mechanical methods (ACI 503R). Also, some cleaning acids contain chlorides that can start rebar corrosion. Acid etching is not recommended unless no other means of cleaning is possible.

Patches

Patches are easier to make and more successful if they are made as soon as practical, preferably when the concrete is still green. Successful patches can be made, however, at any time. The edges of the defective area should be chipped or cut straight and at right angles to the surface, or slightly undercut to provide a key at the edge of the patch. Most contractors saw cut around the defective material. This helps define the scope of work for the laborers and provides a right angle surface cut.

Whatever the method, no feather edges should be permitted (Figure1). When chipping around reinforcement leave at least a l-inch space around each exposed bar. Always leave rebar partially embedded.
 

Construction Joints

Workers can prepare the surface of construction joints during the first concrete placement. For horizontal construction joints, the top surface of fresh concrete can be roughened while still plastic. Green concrete, 12 to 24 hours old, can be easily cut or wire brushed to create a roughened surface.
 

Sometimes workers sprinkle or mix retarders into the top layer of concrete. Delaying the set allows the surface to be roughened up to 48 hours after the pour. For vertical construction joints placed against bulkheads, the concrete surface is generally too smooth to permit proper bonding. Stiff- wire brushing may be sufficient if the concrete is less than 3 days old. Otherwise bushhammering or sand or waterblasting may be needed. Follow this by washing to remove all the dust and loose particles.

Overlays

For bonded two-course floors, the surface of the partially set base course is usually brushed with a coarse wire broom to remove laitance and score the surface. Then it should be wet cured for 3 days. Don’t use curing compounds as they can interfere with bonding. If the slab is being repaired with a bonded overlay, then techniques developed for the pavement industry are typically appropriate. Bonding of two-course floors is difficult.
 

Without close attention to detail, the bond won’t be successful. Slabs on grade and pavements now can be cleaned fast with high production, self-propelled cold milling equipment and improved blasting techniques. The type of coarse aggregate in the existing pavement usually dictates the least costly way to prepare a surface. Most agencies specify the surface cleaning method and minimum depth of surface removal. The US Army Corps of Engineers requires removal of at least 1/4 inch from the surface by scarification followed by high-pressure water flushing and air blowing. The Portland Cement Association (PCA) recommends that the surface be scarified to remove unsound concrete and cleaned by sandblasting or other means.
 

Bonding Medium

The most practical and economical bonding agents are sand-cement and water-cement grouts. Epoxy resin grouts specially formulated for each application also are on the market. ACI 503 details the use and specification of epoxies for bonding fresh to hardened concrete. The bonding sand-cement grouts usually consist of 1 part cement, 1 part sand, and enough water (about 1/2 part) to form a creamy consistency.

The sand should pass the No. 30 sieve. Proportion water- cement bonding grouts at the rate of 1 bag of cement to 6 to 7 gallons of water. Some project specifications permit a water-cement grout with a water cement ratio of 0.62. This allows the grout to be sprayed on the surface to a depth of about 1⁄16 inch.
 

Epoxy resins and their hardeners or curing agents are co-reactants in a chemical reaction that allows the material to harden. The proportioning of the resin and hardener is extremely important; they must be mixed thoroughly to produce a homogenous mixture. This ensures a complete reaction. Epoxy resins can be formulated for different temperatures and for dry or damp surfaces.

The ability to be used on a damp surface is sometimes an advantage. Regardless of the bonding medium, a minimum bond strength is required. Based on laboratory and field tests, Felt (Ref. 4) concludes that bond strengths greater than 400 psi may be achieved, but that strengths of 200 psi or less may be adequate. The bond strength of 200 psi is generally used as a guide in designing bonding media.
 

Bonding Procedure

After preparing the surface, the contractor need only decide if the concrete should be dry or damp before brooming or brushing the bonding medium into place. Most agencies recommend a damp surface free of water, especially in hot, windy weather. Protect the bonding medium from drying above and below. Hot, windy weather dries the bonding medium from above. From below, porous aggregates or concrete can absorb enough water to prevent complete hydration. This produces a weak bond interface or the porous surface can absorb enough epoxy to starve the glue line.
 

Apply the grout immediately before placing the new concrete. Place only as much grout as can be covered with fresh concrete before the grout dries. The amount of grout varies with weather, equipment, and crew. After applying the bonding medium, place the concrete as usual.
 

Curing

Start curing as soon as possible after placing the fresh concrete. Use wet burlap, wet sand, plastic sheets, curing paper, tarpaulins, curing compounds, or a combination. Moisture and temperature both affect the curing of bonded concrete. Differential shrinkage, thermal movements, or moisture gradients can cause enough stress to break the bond during the curing period. This is especially important when the new concrete has different properties (modulus of elasticity, coefficient of thermal expansion, shrinkage strains) than the underlying concrete.

References

1. Hutchinson, R. L., “Resurfacing with Portland Cement Concrete,” NCHRP 99, Transportation Research Board, December 1982.

2. “Use of Epoxy Compound with Concrete,” ACI 503R-80, Manual of Concrete Practice, American Concrete Institute, P.O. Box 19150, Detroit, Michigan 48219, 1986.
 

3. “Standard Specification for Bonding Plastic Concrete to Hardened Concrete with a Multi-Component Epoxy Adhesive,” ACI 503.2-79, Manual of Concrete Practice, American Concrete Institute, 1986.

4. Felt, E. J., “Resurfacing and Patching Concrete Pavement with Bonded Concrete,” Highway Research Board Proceedings, Vol. 35, Highway Research Board, National Research Council, Washington, DC, pages 444-69, 1956.
 

5. Kosmatka, S. and W. Panarese, “Design and Control of Concrete Mixtures,” 14th Edition, Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.