Video from my YouTube channel

Subscribe to My You tube channel

AIR-ENTRAINING ADMIXTURES

By -
Freezing environment posing a serious challenge for concrete structures. freezing and thawing cycle significantly reducing the durability of concrete. during winter and when the temperature goes down sharply. the water freeze inside void or capillary cavities of concrete. the volume of water increase to 9% after freezing. this will cause tensile stress in concrete and it will lead to cracks and disintegration of concrete sometimes.
The introducing of air-entrained admixture to concrete will significantly improve concrete durability in harsh environments. the air-entrained admixture generates millions of tiny bubbles that relieve the pressure resulted from water expansion after freezing. freezing water inside capillaries will expand and will be pushed to air entrainment pores under pressure. this process will prevent the formation of harmful pressure.







the air-entrained created by adding liquid admixtures manufactured for this purpose to the concrete mix. the required air-entrained volume will vary according to the severity of environmental conditions and aggregate size. for example

            Aggregate size                            sever expouser        moderate expouser
    
                            9.50mm(3/8in)                                   7.50%                       6.00%
                             25mm (1 in)                                       6.00%                      4.50%
                                         
the increasing of air-entrained volume decreasing the strength of concrete. a decrease of 5% in concrete strength expected for 1% of air-entrained. therefore, the volume of air-entrained should be controlled and monitored. another consideration is the spacing. spacing is defined as the maximum distance needed by water to reach the void reservoir to release the harmful pressure. according to ASTM C457 spacing factor should not be greater than 0.2 mm(0.008in).








The air-entrained bubbles increasing concrete cohesiveness and reducing concrete bleeding and segregation. another merit of using air-entrained admixture is enhancing concrete workability. the presence of millions of air-entrained voids will ease the relative movement of the aggregate particle. an approximate increase of slump from 20 mm to 50 mm for air-entrained concrete VS similar concrete mix with no air-entrained voids.
It might be logical to expect an increase in concrete permeability because the air-entrained concrete is more porous than plain concrete but it is not necessary for air-entrained levels required to satisfactory frost protection. maintaining concrete permeability related to two reasons. the first reason: using air-entrained admixture will increase concrete workability, therefore, reducing W/C ratio and reducing of capillary pores. second reason: the air voids acting as impermeable open pores blocking capillary structure, therefore, ingress and fill these pores with water occurred at a very slow rate under normal conditions. therefore it doesn't participate in capillary action of transferring liquid water in concrete. 


Comments

Post a Comment

Videos from My Youtube channel

Popular posts from this blog

Pile cap

By -
Image
Pile cap used to transfer the loads from superstructure to the piling. The pile cap is thick concrete mat rests on piles. It is part of the foundation and used to distribute the loads over the piles. Piles used when the soil bearing is not enough to carry loads of the structure. Figure 1  construction management: concrete construction bridge construction:How to become a bridge engineer Pile cap construction procedure Excavation around piles: After 7 days of the casting of the pile the concrete will gain approximately, 70% of it is compressive strength. The soil around piles will be removed to the required levels and dimension that allows construction of the pile cap. A step shall be made if the excavation is more than 1.5 m to prevent soil collapse. Figure 2 Pile head cutting: Concrete will be removed up to cut off level. If concrete under the cut off level is unsound. It shall be removed, and the pile will be repaired...
Read more

Binding wire for steel reinforcement

By -
Image
  Binding wires are used to tie steel bars together. These wires are playing a significant role in maintaining the reinforcement stability and rigidity. Wires are used to tie the steel bars at intersections points. By tying the steel bars together, we ensure that the steel bars will not move from their locations during construction or during the time of concreting. In slabs, binding wires are used to tie longitudinal and transverse bars together. In columns, it is used to tie vertical bars with stirrups.   Figure 1 There are various types of binding wires such as  Black annealed baling wire, Stainless Steel Binding Wire, and PVC coated binding wires.  Black annealed baling wire used to tie black steel. The popular size for black wire ranges from 16 to 22 gauges. Stainless steel binding wires are used to bind stainless steel reinforcement. Stainless steel reinforcement is used in a harsh environment where the black steel gets rusted quickly. Therefore, stainless ...
Read more

Bridge Diaphragm

By -
Image
The main function of diaphragms is to provide a stiffening effect to deck slab in case bridge webs are not situated directly on top of bearings. Therefore, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be directly transferred to the bearings.On the other hand, diaphragms also help to improve the load-sharing characteristics of bridges. Diaphragms also contribute to the provision of torsional restraint to the bridge deck. Diaphragm will be used also to resist stresses that occurs during the post-tensioning process. Post-tensioning hardware are casted inside the diaphragm. Figure 1 construction management: concrete construction bridge construction:How to become a bridge engineer Types of diaphragm: Diaphragm at the span end: this type of diaphragms will be located between two bridge spans or at the bridge abutments. Expansion joints will be used to connect bridge spans. This ...
Read more

Bar Bending schedule (BBS)

By -
Image
 The bar  bending schedule is a table that contains the details of reinforcement for a specific structural member such as footing, column, slab, and others. The bar bending schedule will contain the bars' dimensions to be used, the shape of the bars, the size of bars, bar mark, number of bars, and the weight of bars. BBS will be used to determine the weight of reinforcement required for a specific structural member.  there are various advantages of bar bending schedule such as: estimating the quantity of reinforcement for each structural member. So we can estimate the amount of reinforcement required for a construction project.  BBS will help in reducing the losses of steel by procuring the precise quantity of different rebar diameter.  BBS will reduce the wastage of reinforcement by providing the engineers with the exact dimension and shape of rebars. Knowing the exact dimension and shape will reduce the time of cutting and bending rebars.  BBS will reduce...
Read more

stress-strain diagram for aluminium alloy and rubber

By -
Image
figure no:1 showing the stress-strain diagram for aluminum alloy. we can notice from the stress-stress diagram that aluminum has a considerable ductility. despite the absence of explicit yield point as in structural steel . the initial portion of the stress-strain curve is linear with a recognizable proportional limit. the proportional limit for aluminum alloy ranges from 70 to 410 Mpa (10 to 60 Ksi). aluminum alloy undergoes large strain before failure. ultimate stress ranges from 140 to 550 Mpa (20 to 80 Ksi). construction management: concrete construction bridge construction:How to become a bridge engineer Figure-1 Yield stress for aluminum alloy can be determined by drawing offset line parrel to the linear portion of the stress-strain curve. the straight line offset by a standard strain such as 0.002. the intersection point with the stress-strain curve is the point of yield stress. figure no:2 illustrates the concept of offset method. yield stress acquire...
Read more

Non-Working Pile Testing and Static Compression Load Test

By -
Image
Purpose of Non-Working Piles Non-working piles are constructed at the beginning of the project to evaluate soil bearing capacity and determine the maximum settlement. These piles are loaded up to twice the working load. Based on the test results, the pile length may be adjusted. If the measured settlement exceeds the allowable limit, the pile length will be increased accordingly. construction management: concrete construction bridge construction:How to become a bridge engineer Static Compression Load Test The static compression load test provides insight into how the pile responds to loading. Settlement is measured at different load increments to determine the appropriate pile design length. The maximum applied load is typically twice the working load (2 × working load) or as specified by local standards. The following steps outline the testing procedure: A. Steel Cage Preparation The steel reinforcement cage is prepared in accordance with the approved shop drawings. ...
Read more

Prestressing stands debonding

By -
Image
  Prestressing bridge concrete beams contributes to resisting the tensile stresses caused by the traffic loads and the beam's weight. Stressing concrete has various benefits, such as increasing concrete members' capacity, reducing the concrete member section, and increasing the span lengths for structures such as bridges and buildings. However,  The stressing of bridge beam ends can result in unwanted cracks. Therefore AASHTO calling for debonding a specific portion of strands at the beam end to minimize the stresses and reduce cracks. Figure 1 Debonding is achieved by wrapping the strands at the end of the beam with plastic. Wrapping the strand with plastic wrapping will prevent the strands from bonding with concrete, thus avoid transferring unwanted stress to the beam end. As a result, the unwanted cracks will be prevented by reducing the stress levels at the end of the beam.  
Read more

Repairing of concrete bulging

By -
Image
Improper designing and installation of formwork for a concrete member can result in concrete Bulging. During the placing and compacting of concrete, forms may deflect largely in the weak areas. The deflection of forms will cause concrete to expand at this location, and this known as concrete Bulging. Concrete Bulging will spoil the appearance of a concrete member. Therefore we should always ensure that the forms are appropriately installed, and it is sufficiently supported to resist the concrete during placing and compacting. Bulging should be repaired in case we couldn't avoid it. The solution of concrete Bulging, offset, or surface irregularities are surface grinding. However, excessive surface grinding can weaken the concrete.  Figure 1 If the bulging or surface irregularities are small, grinding of concrete surface will be sufficient to remove surface irregularities. If the Bulging of concrete is significant, then the concrete should be chipped, and the surface...
Read more

Determinate and indeterminate structure

By -
Image
structure can be called determinate if the equilibrium equation provides enough and sufficient condition for equilibrium. if all forces for a structure can be determined using equilibrium equations only, this structure is determinate, but if the unknown forces are more and cant be determined using equilibrium equations then this structure is indeterminate.in general we can determine if a structure is statically determinate or indeterminate by drawing free body diagram for a structure or part of it and comparing a number of unknown forces and moment component with number of available equation of equilibrium. for coplanar structures, we have  three equilibrium equations, if n is the total number of parts and r is the number of unknown forces and moment component then: r=3*n determinate r>3*n indeterminate  construction management: concrete construction bridge construction:How to become a bridge engineer statically indeterminate structure cant ...
Read more

Bundled Bars

By -
Image
sometimes the required reinforcement bar will not fit for a given section of beam or girder, in this case, we resort to bundling a group of parallel bars, up to four bars can be bundled together,  ACI 7.6.6.2 stating that bundled bars should be enclosed within stirrups or ties, bundled bars should be tied, wired and fastened together to ensure remaining in position whether vertical or horizontal, ACI 7.6.6.3 stating that bars have diameter greater than 36 mm(#11) shall not be bundled in beams, the limitation of bundled bars size is for beam or girders in building, the small size of building members make it unpractical to bundle bars with size larger than 36mm(#11), in bridges the size of members are bigger and therefore it is allowed to bundle up to two bars with size greater than 36mm (#11). figure 1 showing bundle bar arrangements. figure 1 construction management: concrete construction bridge construction:How to become a bridge engineer when two or more ...
Read more