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Use of Lightweight Concrete and Drying Times

Another typical phone call. An owner wants to know how long it will take for the lightweight concrete in the elevated slabs of his new building to dry before he can place the floor covering. The slabs were placed 4 months ago, but tests still show a moisture-vapor-emission rate of 8 pounds per 1000 square feet in 24 hours. The floor-covering manufacturer requires the concrete to be at 3 lbs/1000 sf/24 hrs. Delaying floorcovering installation will delay building occupancy. The owner has never had this problem in other buildings he has constructed. Why won’t this concrete dry? Concrete is concrete, right?

Well, unfortunately it’s not. Many owners and contractors have told us they’ve experienced project delays while waiting for lightweight concrete to dry. Though we couldn’t find any data regarding the drying time of lightweight concrete, field experience tells us that lightweight concrete takes longer to dry than normal-weight concrete. To help fill this information gap, CONCRETE CONSTRUCTION devised a testing program to find out how long it takes lightweight concrete to dry.

The test program

Three normal-weight concretes with water-cement ratios of 0.31, 0.37, and 0.40 were delivered to a testing lab in 1-cubic-yard loads. A lightweight concrete mix with a water cement ratio of 0.40 was also delivered to the testing lab. The proprietary mixes were supplied by the ready-mix division of CAMAS Colorado, Denver. The normal-weight concrete moisture-vapor-emission test results were reported in THE CONCRETE PRODUCER (Ref. 1). Here we compare the lightweight concrete moisture-vapor-emission test results with those for normal-weight concrete having the same water-cement ratio. The producer tightly controlled water content and water-cement ratio by closely monitoring aggregate moisture and water left in the drum. The fresh and hardened properties of the lightweight concrete were as follows:

■ 3-inch slump

■ 4.5% air content

■ 124-pound-per-cubic-foot unit weight

■ 48° F temperature

■ 6850-psi 28-day compressive strength

Workers placed and vibrated the concrete in 3-foot-square test slabs 2, 4, 6, and 8 inches thick. After striking off the surface with a 2x4 and floating it by hand, they covered the slabs with plastic sheeting for 3 days. Calciumchloride moisture-emission testing began after the sheeting was removed.

Moisture-emission tests were conducted in accordance with the test manufacturer’s instructions. Technicians ran one test on each slab at the 3-day age but thereafter ran two tests on each test slab and reported an average test value.

The calcium-chloride test kits were left in place for 72 hours on slabs stored inside the test lab at a 70±3° F and a relative humidity of 28±5%. This is the normal indoor environment during the winter in the Rocky Mountain region, where the tests were conducted, and represents the environment found in many buildings without relative-humidity controls.

Drying in months instead of weeks

The table shows moisture-vapor emission rates for the test slabs after drying for up to 183 days. Drying times are measured from the end of the 3-day curing period to the end of the 72-hour moisture-emission test. Thus the 3-day measurement was taken 6 days after concrete placement. The test data support the following conclusions.

Lightweight concrete dries slower. Regardless of the test slab thickness, the lightweight concrete took about 6 months to dry to a moisture-vapor emission rate of 3 lbs/1000 sf/24 hrs. Normal-weight concrete of the same water-cement ratio took only 6 weeks of drying in laboratory air to reach the same level (Ref. 1). From previous work (Ref. 2), we know that laboratory drying represents the fastest drying time. So field conditions that include wet-dry cycles will increase the actual time for the slab to reach the specified moisture-vapor-emission limits.

Thickness effects

As with normal-weight concrete, the moisture-vapor-emission rates were unaffected by slab thickness.

Don’t blame the contractor

Lightweight concrete offers many advantages. However, any owner using lightweight concrete that’s to be covered by a moisture-sensitive floor covering or any architect/engineer specifying this combination should consider the slower drying time as an important part of the construction schedule. Once lightweight concrete is specified, the contractor can’t change its drying characteristics. If owners want the benefits of lightweight concrete and a fast-track schedule, they may need to consider applying a polymer coating or sheet product to reduce moisture emissions.

References

1. Bruce A. Suprenant and Ward R. Malisch, “Quick-Dry Concrete: A New Market for Ready-Mix Producers,” THE CONCRETE PRODUCER, May 1998.

2. Bruce A. Suprenant and Ward R. Malisch, “Are Your Slabs Dry Enough for Floor Coverings?” CONCRETE CONSTRUCTION, August 1998.

By Bruce A. Suprenant and Ward Malisch

Floor and roof systems in reinforced concrete structures

9:35 PM Civil Engineering , Concrete , RCC Structural Designing , Structural Analyses , structures

Overview

Reinforced concrete structural systems can be formed into virtually any geometry to meet any requirement. Regardless of the geometry, standardized ﬂoor and roof systems are available that provide cost-effective solutions in typical situations. The most common types are classiﬁed as one-way systems and two-way systems. Examined later are the structural members that make up these types of systems.

It is common for one type of ﬂoor or roof system to be speciﬁed on one entire level of building; this is primarily done for cost savings. However, there may be cases that warrant a change in framing system. The feasibility of using more than one type of ﬂoor or roof system at any given level needs to be investigated carefully.

One-Way Systems

A one-way reinforced concrete ﬂoor or roof system consists of members that have the main ﬂexural reinforcement running in one direction. In other words, reactions from supported loads are transferred primarily in one direction. Because they are primarily subjected to the effects from bending (and the accompanying shear), members in one-way systems are commonly referred to as ﬂexural members.

FIGURE 1 One-way slab system.

Members in a one-way system are usually horizontal but can be provided at a slope if needed. Sloped members are commonly used at the roof level to accommodate drainage requirements.

Illustrated in Fig. 1 is a one-way slab system. The load that is supported by the slabs is transferred to the beams that span perpendicular to the slabs. The beams, in turn, transfer the loads to the girders, and the girders transfer the loads to the columns.

Individual spread footings may carry the column loads to the soil below. It is evident that load transfer between the members of this system occurs in one direction.

FIGURE 2 Standard one-way joist system.

Main ﬂexural reinforcement for the one-way slabs is placed in the direction parallel to load transfer, which is the short direction. Similarly, the main ﬂexural reinforcement for the beams and girders is placed parallel to the length of these members. Concrete for the slabs, beams, and girders is cast at the same time after the forms have been set and the reinforcement has been placed in the formwork. This concrete is also integrated with columns. In addition, reinforcing bars are extended into adjoining members. Like all cast-in-place systems, this clearly illustrates the monolithic nature of reinforced concrete structural members.

A standard one-way joist system is depicted in Fig. 2. The one-way slab transfers the load to the joists, which transfer the loads to the column-line beams (or, girders). This system utilizes standard forms where the clear spacing between the ribs is 30 in. or less. Because of its relatively heavy weight and associated costs, this system is not used as often as it was in the past.

FIGURE 3 Wide module joist system.

Similar to the standard one-way joist system is the wide-module joist system shown in Fig. 3. The clear spacing of the ribs is typically 53 or 66 in., which, according to the Code, technically makes these members beams instead of joists. Load transfer follows the same path as that of the standard joist system.

Reinforced concrete stairs are needed as a means of egress in buildings regardless of the number of elevators that are provided. Many different types of stairs are available, and the type of stair utilized generally depends on architectural requirements. Stair systems are typically designed as one-way systems.

Two-Way Systems

As the name suggests, two-way ﬂoor and roof systems transfer the supported loads in two directions. Flexural reinforcement must be provided in both directions.


FIGURE 4 Two-way beam supported slab system.

A two-way beam supported slab system is illustrated in Fig. 4. The slab transfers the load in two orthogonal directions to the column-line beams, which, in turn, transfer the loads to the columns. Like a standard one-way joist system, this system is not utilized as often as it once was because of cost.

A ﬂat plate system is shown in Fig. 5. This popular system, which is frequently used in residential buildings, consists of a slab supported by columns. The formwork that is required is the simplest of all ﬂoor and roof systems. Because the underside of the slab is ﬂat, it is commonly used as the ceiling of the space below; this results in signiﬁcant cost savings.

FIGURE 5 Flat plate system.

Similar to the ﬂat plate system is the ﬂat slab system (Fig. 6). Drop panels are provided around the columns to increase moment and shear capacity of the slab. They also help to decrease slab deﬂection. Column capitals or brackets are sometimes provided at the top of columns.

The two-way system depicted in Fig. 7 is referred to as a two-way joist system or a wafﬂe slab system. This system consists of rows of concrete joists at right angles to each other, which are formed by standard metal domes. Solid concrete heads are provided at the columns for shear strength. Such systems provide a viable solution in cases where heavy loads need to be supported on long spans.

FIGURE 6 Flat slab system.

FIGURE 7 Two way joist system.

Antifreeze Admixtures for Concrete during Cold Weather Concreting

7:10 PM Civil Engineering , Concrete , Construction Management , Environmental Design & Construction , Special Concrete

Freezing reduces strength of concrete by 20 to 40 % when fresh concrete is subjected to freezing. Antifreeze admixtures of concrete, its properties and uses in cold weather concreting are discussed.

The resistance of the fresh concrete against the freeze and thaw cycle is given by the durability factor which is also lowered by 40 to 60%. There is 70% decrease in the bond between the reinforcement and the concrete that is normally cured.

Cold Weather Concreting

Hence it is very essential during the concreting in cold weather conditions to ensure that the concrete will not undergo freezing in its plastic state.

There are two methods for carrying out cold weather concreting:

Provision of normal ambient temperatures for the concrete. This can be done through the heating of the concrete ingredients or bley providing heating enclosures.
The addition of chemical admixtures.

Conventional Chemical Admixtures in Cold Weather Concrete

Conventional concrete used calcium chloride as accelerating admixtures to offset the retarding effects of slow hydration of concrete in low temperatures. This admixture is not effective below the freezing temperatures.

This is found to be a drawback in the conventional form of admixtures. Hence, for arctic weather conditions, special admixtures are necessary. One such is antifreeze admixtures.
Antifreeze Admixtures for Concrete

The antifreeze admixtures affect the physical condition of the mix water used in the concreting. These can depress the freezing point of the water to a large extent and can be used in temperatures lesser than -30 degrees Celsius. This can enable the extension of the period of the construction activity.

Chemical Composition and Action of Antifreeze Admixtures

There are two groups of antifreeze admixtures that provide the characteristics of antifreeze and the accelerated setting and hardening properties.

They are:

1. First Group

This includes chemicals, weak electrolytes, sodium nitrite, sodium chloride and non-electrolytic organic compounds which lower the freezing point of the water used in the concrete. But these group acts as weak accelerators to promote the setting and hardening.

2. Second Group

These include binary as well as ternary admixtures which contains potash and additives based on calcium chloride, sodium nitrite, calcium chloride with sodium nitrite, calcium nitrite -nitrate-urea and other chemicals.

These have effective antifreeze properties and accelerating property to promote the setting and hardening. These are used in larger dosages compared to that of conventional admixtures.

One such example is the use of 8% of sodium nitrite to keep the liquid at a temperature of -15-degree Celsius.

These admixtures function by lowering the liquid phase freezing point and by accelerating the cement hydration at the freezing temperatures.

Based on the dosage in the mixture, the non-chloride admixture enables the mix (concrete or the mortar) to be placed at sub-freezing temperatures. This hence reduces the need of protective measures required during the cold weathering works.

The method improves the quality of the concrete and as it facilitates early setting, early stripping of formworks can also be carried out. This helps in the reuse of the form within a small duration and hence speed up the construction.

The table-1 shows the significant difference is strength gain at 3, 7 and 28 days for plain concrete and antifreeze admixture used concrete.

Table.1: Concrete Compressive strength with and without antifreeze admixture

(As per Ratinov and Rosenburg)

Property	Plain Concrete	Freeze-protection Admixture
Set time (-4 degree Celsius)	–	–
Compressive strength (MPa)
-4 degree Celsius (3 days)	3.4	9.24
-10 degree Celsius (7 days)	8.3	39.3
-10 degree Celsius (28 days)	18.1	49.9

It is possible for the incorporation of other admixtures that contains superplasticizers to be incorporated with the antifreeze admixtures. The main advantage of such combination is that in totality there will be a reduction of water.

The water reduction will reduce the freezable free water content in the mix. This freezable water content is the one that serves as the heat sink for the heat liberated by the initial hydration reactions. This will hence reduce the number of antifreeze admixtures.

Selection of Antifreeze admixtures

The factors based on which the selection of antifreeze admixtures is carried out are:

The type of structure
The operating Conditions
Protecting methods employed in winter concreting
Cement brand and aggregate types

A laboratory test must be carried out with the operating materials and the dosage of antifreeze admixtures that are intended to be used in the field.

The incorporation of other admixtures like retarders, superplasticizers with antifreeze admixtures is not restricted in cold weather concreting. The dosage of all the admixture that are used must be established experimentally.

Application and Advantages of Antifreeze admixtures

The antifreeze admixtures are technologically simple and beneficial for cold weather concreting. The admixture helps in improving the cohesiveness, cold joint minimizations, sand streaking, and plasticity. These are estimated to provide large cost saving than other methods of steam curing or concrete enclosures.

The combination of antifreeze agents with water reducing agents or air-entraining agents will help in increasing the resistance of concrete towards the frost action and corrosion.

Overview: Shotcrete and its Types

5:07 PM Civil Engineering , Concrete

The gunite alias shotcrete or sprayed concrete is formed by mixing cement and sand, in the ratio of 1:3. It refers to mortar or small-aggregate concrete that is employed through a wet or dry process. It is conveyed through a hose and pneumatically projected at high velocity onto a surface under a pressure of about 20 to 30 N/cm².

Guniting is mostly recommended for repairing concrete work that has been defective because of substandard work or other reasons. It is applied for delivering a watertight layer. Gunite is sprayed pneumatically onto the surfaces at a high velocity surface under.

Shotcreting in progress

Gunite is very effective for tunnels, underground structures, slope stabilization, structural repairs, and pools. It is normally reinforced with traditional steel rods, steel mesh, or fibers. When the gunite is used it starts a instantaneous method of compression and settling.

Shotcrete is normally a comprehensive term for both the wet-mix and dry-mix versions. In pool construction, however, shotcrete stands for wet mix and gunite to dry mix. In this respect, these terms are not exchangeable.

Shotcrete is arranged and consolidated simultaneously, because of the force with the nozzle. It is sprayed into different types of shapes or surfaces along with vertical or overhead areas.

Categories of Shotcrete : Dry Shotcrete and Wet Shotcrete

The shotcrete may be used in dry or wet condition. In dry condition, the components are set in a hopper and driven pneumatically via a hose to the nozzle. The inclusion of water at the nozzle is restricted that is mixed up as soon as the material touches the surface. With dry shotcrete or guniting it is possible to modify the water content that should be provided into the mix immediately. It facilitates superior placement process devoid of the inclusion of accelerators. Dry application is suitable when the process entails repeated stops throughout the process of application.

Prepared concrete or the ready mixed concrete is utilized in wet-mix shotcrete. Application of compressed air is prepared at the nozzle that moves the wet mixture over the receiving surface. Less rebound and less dust with regards to dry application of shotcrete has transformed the wet-mix application as the most common process. The benefit of the wet-mix process is to arrange greater volumes in less time.

Benefits of Guniting

Setting and compression takes place at the same time.
Shotcrete will follow surfaces better than that of regular concrete.
Shotcrete is also applied with steel fibers to be utilized as a substitution of welded wire mesh. It allows greater flexural strength, ductility and toughness.
It is inexpensive with reference to conventional concrete.
It offers lowered shrinkage and permeability.

Use of gunite

Gunites are mostly found in slope stabilization, tunneling, retention walls, water tanks and pools, artificial ponds, ditches and channels as well as structural reinforcement and mining applications.

Spreadsheet: Bar Bending Schedule of a Box Culvert

2:38 PM Civil Engineering , Concrete , Estimation , Spreadsheets , Steel

Definition of Bar bending

It is the method of bending reinforcing steel into shapes which are important for reinforced concrete construction.

Definition of Bar bending schedule (BBS)

Bar bending schedule alias schedule of bars refers to a list of reinforcement bars, a specified RCC work item that is shown in a tabular form for a smooth view. This table sums up all the necessary particulars of bars ranging from diameter, shape of bending, length of each bent and straight portions, angles of bending, total length of each bar, and number of each type of bar. This information can be used for making an estimate of quantities.

Download: Bar Bending Schedule of a Box Culvert

It includes all the details essential for fabrication of steel like bar mark, bar type and size, number of units, length of a bar, shape code, distance between stirrups (column, plinth, beam) etc.

While generating bar schedules, it is important to take proper care about length. In case of bending, bar length will be raised at the bending positions.

Benefits of the Bar Schedule:

When bar bending schedule is applied along with reinforcement detailed drawing, it makes the quality of construction superior.

Once bar bending schedule is prepared, cutting and bending of reinforcement is performed at factory and shipped to job site. This improves quick implementation at site and minimizes construction time and cost as fewer workers are needed for bar bending. Bar bending also circumvents the wastage of steel reinforcement (5 to 10%) and thus project cost is saved significantly.

It offers the perfect estimation of reinforcement steel requirement for all the structural members which are applied to workout complete reinforcement requirement for whole project.

Bar bending schedule offers the steel quantity requirement in a better way and thus delivers an option to make optimal use of the design in case of cost overflow.

The process becomes simple for site engineers to validate and approve the bar bending and cutting length throughout inspection prior to positioning of concrete with the support of bar bending schedule and thus facilitates in superior quality control.

It becomes easier to handle the reinforcement stock necessary for identified time duration.

It will facilitate the fabrication of R/F with structure.

Download

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Dutch inaugurate the 3D Printed Reinforced Concrete Bridge Designed by Technical University of Eindhoven

11:54 PM 3D printing , Bridges , Civil Engineering , Concrete , Construction Management , structures

3D printing has emerged in the recent past as a smart and quick alternative to many fields of production. Its applications in the field of construction are no exception. It is however relatively easier to handle 3D printing of small or medium sized products, ensuring stability, strength and durability in case of a structure for the built environment pose serious challenges.

The cycle bridge is part of the Noord-Om project which is a new section on the ring road around the village of Gemert in Netherlands. Although the team claims it to be World's 1st 3D printed bridge, there was already a 3D printed pedestrian bridge built this year in Madrid, Spain which in our opinion is the rightful deserver of the title '1st 3D printed bridge'. Another project was to be initiated by a company named MX3D to print a steel bridge with huge robotic arms but the project appears to be still.

Dutch inaugurate the 3D Printed Reinforced Concrete Bridge Designed by Technical University of Eindhoven

Inauguration of the 3D printed bridge in Netherlands (Image courtesy: BAM Infra)

This Dutch bridge is a practical bridge for cyclists which was printed using a custom built cement printer at the Technical Universty of Eindhoven. Individual layers each having a thickness of 1cm are printed by injecting liquid mortar into the printhead through the storage. Steel reinforcing wire is fed down before a layer dries and the process goes on. The printer builds up sections layer by layer which are approximately 1m high.

(Images courtesy: BAM Infra)

3D-printed concrete bridge from Royal BAM Group nv

Six of the printed sections were transported to the site to be glued together to form a bridge that is 8 m long, 3.5 m wide and is 0.9 m thick. During the testing phase, the bridge was found to support an imposed load of 5 Tons which is far greater than that of the cyclists. The expected life of the bridge is 30 years. The bridge was built by the company BAM Infra and the team claims to have gained valuable experience from this versatile project which will allow the members to print much larger sections for much bigger structures.

3D printing in construction has endless possibilities and benefits. A very important aspect is the economical use of cement and other materials. Another aspect is not requiring a formwork or minimum if at all needed which also saves costs and material wastage.

What is a Frost Wall? Types and Uses of Frost Walls

6:00 PM Civil Engineering , Concrete

Frost wall or frost protected wall construction is to prevent soil beneath the building from freezing for protection of foundations in freezing temperature climates. Types of frost walls, their requirements and uses are discussed.
Frosting is a serious issue for the building structures during the colder climates. These undesirable effects are more pronounced and observed on the building foundation.

Any damage to the building foundation will affect the whole stability of the structure. In regions where the frosting is a persistent issue, the most common remedy is to construct deep foundation that will be lying over a footing level much below the frost line.

Hence the construction of buildings and structural elements in extreme climates is a very challenging procedure. The temperature characteristics of the building materials used for the construction is a sole factor that influence the construction.

The concrete placing and mixing in colder climate results in the contraction of the mix. These contractions of concrete will result in internal stresses.

If these issues of contraction and the internal stresses are not taken into consideration seriously there will be extreme issues of internal strain. The internal strain accumulation is a great danger for the structural integrity and the serviceability of the structure.

Need for Frost Protection Wall

Frost protection wall is constructed with an intention to prevent the soil around the building from freezing under high freezing temperatures. A form of heat conversion is used to transfer from the building to the soil beneath so that the soil does not freeze.

Construction of Frost Wall Foundation in Freezing Temperature_engineersdaily.com

Fig 1: Construction of Frost Wall Foundation in Freezing Temperature

As we know from the basics of soil mechanics, the soil matrix consists of voids that is filled with water and air. In dry soil, these soil voids will be filled with air. In the case of saturated soils, the voids will be filled with water that will get converted into ice under freezing temperature. The volume of water in a void will increase when the water is converted into ice.

The soils underneath the foundation is mostly filled with water. If the construction is on a colder region, these waters will get converted into ice. Any fall in temperature will melt the ice into water. Hence a procedure of freezing and thawing is experienced. This will result in an upward showing of the structure due to expansion and contraction.

Penetration of water in the interior of building as the walls get wet_engineersdaily.com

Fig.2: Penetration of water in the interior of building as the walls get wet

This frost heaving phenomenon will increase with the conversion of water in soil into ice. These frozen ices in the soil is called an ice lens. These ice lens will push the nearby soil mixture extensively. Any structure that is lying over such expanded soil will shove the structure in the upward direction.

Hence the only way to prevent such issues is to bring a means that will stop the freezing of the soil. The frost wall is such a unique technique implemented widely for this purpose.

What is Frost Wall?

The frost wall can be defined as an insulated wall that are constructed around the periphery of the foundation. These are constructed deep and beneath the frost line. As the frost wall are placed beneath the soil, the foundation won’t be subjected to upward pressure from the frost heave process.

The term frost wall is also used to mention walls that are constructed above the ground in the interior of the building structure. This will hence act as an insulation to maintain warmth for the building interior. These frost walls also collect heat from the structure and prevent the soil surrounding the building from freezing and related issues.

Types of Frost Wall

Based on the requirements of the load, temperature and the building features there are different types of frost wall that can be constructed. One such classification is:

Load Bearing Frost Walls
Non-Load Bearing Frost Walls

1. Load Bearing Frost Wall

This construction of frost wall will place the responsibility of foundation over the frost wall. The frost wall will itself act as a foundation wall by constructing it beneath deep the soil.

This will be clearly constructed beneath the frost line of the area. These type of frost walls are constructed in extreme weather conditions (freezing temperatures).

2. Non–Load Bearing Frost Walls

As the name implies, these frost walls are constructed just as an insulating wall. This is constructed in homes that are not insulated. These insulated non-load bearing walls will be constructed inside the building.

Non – load bearing frost walls helps in preventing the escape of heat through the foundation. The interior frost wall constructed must not be in contact with the exterior wall. Special care should be taken while constructing the same.

A gap is maintained between both the walls. It is also recommended to have a barrier to prevent moisture else the moisture will get converted into ice within the wall structure.

Requirements for Frost Wall Construction

The construction of the frost wall provides better performance if all the structural elements that accompany this construction too are of required properties.

Some of the basic features related to its requirements are mentioned below:

The basement wall constructed beneath the wall must be patched to prevent any open gaps. Mostly these basement walls are constructed with the help of cinder blocks. The gaps can be filled with the help of brick fillers.
If the basement walls are constructed with concrete, it is necessary to clear any cracks present in it with the help of a paint sealer. Special paints are available in the market that will help in preventing the penetration of moisture into the basement.
All structural elements must be built with a primary intention to prevent moisture penetration.

Application of Frost Wall

Below mentioned are the working and the construction of the frost wall for the preventing the shallow foundation from freezing and for non-heated buildings.

Frost Wall for protection of Shallow Foundations

The frost wall constructed with an intention to protect the shallow foundation is non-bearing frost walls. This type is used where the frost wall construction as a deep foundation is not at all feasible for the area or it does not bring any sort of economy.

The frost wall here is constructed by leaving a specified gap as per the constructor recommendation with the foundation. This is arranged such a way that the soil does not lose the heat from it.

These types of frost wall construction are constructed surrounding the foundation so that the heat radiated from the building is warmed up efficiently.

Insulation horizontally throughout the foundation_engineersdaily.com

Fig.3. Insulation horizontally throughout the foundation

A rigid foam of insulation is constructed on the exterior of the foundation vertically and on the basement of the foundation horizontally. The construction of these insulation makes the heat formed within the interior of the building to move down the soil and prevent them from freezing.

Horizontal and Vertical Insulation for the frost walls_engineersdaily.com

Fig.4: Horizontal and Vertical Insulation for the frost walls

Frost Wall for non-heated buildings

The frost wall explained in above case provides warmth for the building only if the building constructed is a heated building. This type of frost wall won’t work for an unheated building type.

An alternative for such issue is to design a horizontal layer placed under the foundation of the entire building. This horizontal layer has to extend throughout the building area outward also. There is no form of vertical insulation provided.

The insulation provided are laid over the layer of gravel. Hence, the warmth will be entrapped within the soil and prevent the soil from freezing.

Cathodic Protection of Reinforcement in Reinforced Concrete Structures

3:10 PM Civil Engineering , Concrete

CATHODIC PROTECTION OF REINFORCEMENT

1. Introduction

If an investigation shows that the principal reason for the corrosion of reinforcement is the presence of chlorides in the concrete, then normal repair methods will not be durable, resulting in the need for a continuous programme of repair. This leads to the question of what can be done to ensure a satisfactory and durable repair. The main method to deal with this problem is the use of cathodic protection.

2. General principles of cathodic protection

Cathodic protection has been used for many years for the protection of steel and iron structures such as jetties, storage tanks, and underground pipelines.

Cathodic Protection of Reinforcement in Reinforced Concrete Structures

Cathodic protection of reinforcement (Image courtesy: Conrehab)

In the early 1960s highway engineers in the USA and Canada started trials on the use of cathodic protection for the reinforcement in bridge decks which had been severely damaged by chloride attack from deicing salts. It is now used extensively in both USA and Canada for this purpose, mainly the impressed current system. In the UK cathodic protection is being increasingly specified to combat chloride corrosion of rebars, mainly in highway bridges.

Corrosion of steel is an electro-chemical process and exposed steel in a moist environment will corrode due to differences in electrical potential on the surface of the metal itself. These areas form anodes and cathodes, and this allows an electric current to flow from anode to cathode; the metal suffers corrosion at the anodic areas. The objective in cathodic protection is to ensure that the steel to be protected forms the cathode.

There are two practical ways of introducing cathodic protection to steel:

1. by connecting the steel to a metal which is ‘less noble’ in the electrochemical series (this forms the anode); this is known as sacrificial anode protection;

2. by the application of an external electric current of sufficient intensity to ‘swamp’ the corrosion current; this is the impressed current technique.

A. Sacrificial anodes

The following list shows part of a basic series in which the metals high on the list become anodic to those lower down and hence provide protection. This can be described as sacrificial anode protection and depends on on metal being designed to corrode and so prevent another metal connected to it from corroding.

• sodium

• magnesium

• zinc

• aluminium

• mild steel

• cast iron

• stainless steel (cromium-based)

When two dissimilar metals are in contact in an electrolyte, a current is produced as a function of the electrochemical series of metals. For example, if steel is in electrical contact with zinc in an electrolyte, a current will flow from the zinc to the steel because zinc is anodic to steel and the zinc will corrode but the steel will not. This is the principle of the use of sacrificial anodes.

B. Impressed current

Protection can be provided by the introduction of an impressed current (dc). The structure to be protected is connected to the negative supply (the cathode) and the positive to an introduced anode which is specially selected to have semi-inert or non-corrodable properties. The range of suitable anodes is limited and is designed to last very much longer than the ‘sacrificial’ anodes referred to in the previous paragraph, and a useful life of 20 years or more can be expected. The materials available are graphite, platinized titanium, high-silicon iron, tantalum or niobium; lead/silver alloys are often used in marine installations.

The relevant British Code of Practice is BS 7361: Cathodic Protection; the USA Code is ASTM B.843. In considering cathodic protection to an existing structure the following matters need careful attention:

1. The type of protection which will be the most suitable in the given circumstances, i.e. sacrificial anodes or impressed current.

2. The use of cathodic protection to reinforcement encased in concrete introduces special problems. The pore water in the concrete acts as the electrolyte of the corrosion cells.

3. The impressed current must be adequate to suppress the corrosion current. Concrete has a high resistance to the flow of electric current and this has to be overcome.

4. It has been claimed that cathodic protection can cause hydrogen embrittlement but I have not seen a reliable report of such a case.

5. If cathodic protection by means of impressed current is used in a building, care must be taken to ensure that corrosion of unprotected ferrous metals in the building does not occur.

The use of a cathodic protection system for a reinforced concrete structure requires special knowledge and experience in concrete technology in addition to that required for ‘normal’ cp systems used for steel structures. For the system to be successful in a reinforced concrete structure a number of factors should be taken into consideration, the principal two being:

1. the possibility of initiating alkali-silica reaction;

2. the existence of significant discontinuities in the reinforcing steel.

Repair of Concrete Structures by Steel Plates & Fiber Reinforced Plastics (FRPs)

2:00 PM Civil Engineering , Concrete , structures

1. REPAIRS USING EPOXY-BONDED STEEL PLATES

Repair of Concrete Structures by Steel Plates & Fiber Reinforced Plastics (FRPs)

Repair using epoxy bonded steel plates
(Image courtesy: http://www.sh-horse.com)

1.1 Introduction

It appears that the use of steel plates bonded to the tension face of reinforced concrete beams was developed in France and South Africa in the 1960s. In the UK the first recorded use of resin-bonded steel plates to strengthen an existing building was in about 1966; for the strengthening of road bridges the first use was in 1975 for the Quinton Interchange on the M5.

The use of steel plates bonded to the sides of beams to increase shear resistance would appear to be a possibility, but I have not found any record of such use. However, it has been used to stiffen reinforced concrete floor slabs by bonding the steel plates to the top surface of the slabs, and to strengthen the connections between beams and columns.

1.2 Information on the technique

This technique depends on composite action between the steel plates and the concrete, and therefore requires maximum bond between steel epoxy resin-concrete. This in turn requires very careful preparation of the contact surfaces. The steel must also be adequately protected against corrosion on the exposed surfaces. When the work is properly carried out and the bond strength has fully developed, the strength of the epoxy joint should exceed that of the concrete which will fail in horizontal shear.

The strength of epoxy resins deteriorates rather rapidly at temperatures in excess of about 65°C and this constitutes a fire hazard when used in building structures, but is of little significance in bridges.

A considerable amount of work has been carried out on the development of this technique at the Transport and Road Research Laboratory at Crowthorne, and at Sheffield University, and the universities of Dundee and Warwick.

The technique is discussed in detail in the Report by the Standing Committee on Structural Safety, set up by the Institution of Civil Engineers and the Institution of Structural Engineers. The Report recommends caution in the use of this method and emphasizes that it should not be used to strengthen structures in which the concrete is poor quality, or the reinforcement has suffered chloride attack, or the concrete has been damaged by alkali-silica reaction, unless these basic defects are first corrected.

2. THE USE OF FIBER-REINFORCED PLASTICS

According to Research Focus, No. 21, April 1995, the use of fiber reinforced plastic (FRP) as a substitute for steel has been investigated in Switzerland, Germany and the USA, and at Oxford Brookes University under an EPSRC (Engineering and Physical Sciences Research Council) grant.

2.1 Carbon fibre composites

An alternative to this method of strengthening reinforced concrete beams by epoxy-bonded steel plates by using carbon-fibre reinforced composites was reported by Dave Parker. It is stated in the article that to provide the additional strength required, the epoxybonded steel plates would have had to be 7mm thick, but these were replaced by a 1mm thickness of carbon fibre composite (a Sika Carbodur System). The work was part of a major refurbishment.

Crack Injection in Reinforced Concrete Structures

12:42 PM Civil Engineering , Concrete , structures

1. Introduction

Deep cracks in massive concrete members and cracks which pass right through a structural member can often be repaired satisfactorily by crack injection using a selected polymer such as an epoxy resin.

When properly carried out, crack injection will significantly improve the structural strength of the member. If corrosion or rebars and spalling of concrete has already occurred, then a satisfactory method of repair may be to remove defective concrete down to the rebars, clean off the rebars, remove all grit and dust etc., inject the crack with a suitable resin and then fill in the cut-out section of concrete with an SBR modified mortar. In appropriate cases, this repair method can be combined with crack injection, e.g. to avoid cutting out on both sides of the member.

2. Essential features of crack injection

The essential crack injection consists of injecting a suitably formulated resin into the cracks. This should bond the concrete together across the crack and should form an effective seal against ingress of water or other liquids, and reduce the ingress of carbon dioxide.

Correct formulation of the resin is of vital importance; present-day polymer resins provide considerable scope for variations in the formulation so as to obtain optimum characteristics for each particular job.

The resins in general use are epoxy, polyurethane and polyester, in that order. Desirable qualities for the formulated resin include:

a. low viscosity, (to facilitate penetration into the crack);

b. formation of good bond to damp concrete;

c. suitability for injection in a wide temperature range;

d. low-curing shrinkage;

e. toughness (low modulus of elasticity combined with high yield point);

f. curing time to suit injection conditions;

g. resistance to aggressive chemicals may be required;

h. durability under service conditions.

3. The injection process

This work is highly specialized and should only be entrusted to firms with a proven record of success. Unsuccessful crack injection by one firm is likely to result in another (more experienced firm) being unable to rectify the situation; I have come across this unfortunate state of affairs on more than one occasion.

The crack injection process is carried out in the following phases:

a. preparation of the cracks;

b. location of injection points and surface sealing;

c. injection of resin;

d. removal of injection nipples (if used) and plugging the holes

e. removal of sealing strip and application of any surface treatment which may be required.

4. Preparation of the cracks prior to injection

This should consist of the removal of any dirt or loose weak material on the surface, followed by cleaning out of the crack if this is considered necessary. It is seldom that cracks less than 0.5 mm wide require cleaning unless they have been fouled by the use to which the structure has been put. Compressed air can be used for this cleaning work; solvents may be required in special cases.

5. Location of injection points and surface sealing

The distance apart of the injection points will depend largely on the depth and width of the crack. The object is to have as few injection points as possible consistent with maximum resin penetration with low operating pressure. They are either holes drilled on the line of the crack, or nipples screwed into the concrete; the use of nipples is usually reserved for high pressure work (See Figure 1.).

Crack Injection in Reinforced Concrete Structures

Figure 1 Diagram showing alternative sequences of injection points in concrete wall.

6. Injection of the resin

As stated above, crack injection work should only be entrusted to specialist firms with a good ‘track record’ and preferably to firms which formulate their own resins. The correct formulation of the resin to suit the particular requirements of each job is of primary importance. If the formulator and the applicator are the same firm, any question of divided responsibility is avoided. For larger jobs, some adjustments in formulation may be required during the execution of the work to meet unforeseen conditions and this requires close co-operation between the staff on site and the laboratory.

Firms specializing in crack injection develop their own techniques and equipment. Some firms favor simple means of injection by gravity feed or pressure guns, the resin being premixed in batches. Others use sophisticated equipment for continuous feed of freshly mixed resin and hardener through separate feed pipes, bringing the two materials together at a specially designed nozzle. One firm supplements pressure-feed by the use of a vacuum mat to assist penetration.

Figure 2 Diagram of crack injection and coating of thermal contraction crack in concrete wall.

In appropriate cases, the degree of penetration can be checked by coring or by a UPV survey or impulse radar. The aim of all injection processes is to obtain uniform penetration of the resin and complete filling of the cracks. It has been found in practice that a deliberate fluctuation in the injection pressure can be more effective than an increase in continuous pressure. The volume of resin used in filling cracks is very small.

Where cracks are inclined or vertical, it is usual to commence injection at the lowest injection point and work upwards. For horizontal cracks there is no fixed order of work; the injection can start at one end and work along the crack to the other end, or start in the middle and work first left and then right, or alternately left and right.

7. Final work following injection

It is usual to remove the injection nipples (if they are used) and seal the holes when the resin has set. Crack injection, except where the cracks are very wide on the surface, is likely to be much less conspicuous than cutting out and repairing with mortar. Nevertheless, the injected crack will be visible. Where appearance is important, surface grinding and some ‘cosmetic’ treatment will help to mask the crack.

Figure 2 shows a repair using crack injection.

Following articles might also be of interest to you:

☞ How to Prevent Plastic Shrinkage Cracking of Concrete?
☞ Cracks in Concrete Floors
☞ How to Repair Cracks in Concrete Floors?
☞ Cracks in Concrete Construction

☞ Investigations for Structural Defects of Reinforced Concrete Structures

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