APPLICATION OF POST-TENSION TECHNOLOGY IN MODERN CONSTRUCTION
Modern developers and designers in their work strive to provide a high level of structural flexibility of structures to meet the ever-changing demands of customers. Using the post-tension system, large spans of buildings are constructed with a lower structural height of the beam, which allows to increase the area free from columns. As a result, the internal layout does not depend on the placement of the column grid. Prevention of deflection, cracks and the use of jointless floors free designers from the need to comply with numerous restrictions on the construction of reinforced concrete structures.
With the help of a post-tension system without coupling prestressed reinforcement with concrete, our organization erected many buildings. The system involves the use of ropes, which are individually tensioned and firmly fixed using special elements.
The participants of the commercial construction segment benefit from the use of monostrand after tension , since the thinning of the floors reduces the weight construction. In addition, there is a reduction in the duration of construction work due to the possibility of earlier dismantling of the formwork.
Facilitation of the building structure due to prestressing reinforcement in the shell, as well as the organization of uninterrupted power supply and architectural lighting are the main advantages in the construction of multi-storey parking lots using post-tension compared to the use of prefabricated structures. Tensile reinforcing ropes for soil slabs help to extend their service life, as they prevent cracking and increase resistance to the influence of aggressive environments: swelling soil, underwater and sewage.
After-tension floor slabs increase the earthquake resistance of the structure. The advantage of the after-stress system in the construction of silos, nuclear reactors and fuel storage tanks is a constant level of compressive force, which eliminates cracks and prevents leaks.
In our monostrand post-stress system, we use ropes with a diameter of 15, 2 mm 0, 6 ”, coated with a special anti-corrosion coating and enclosed in an extruded plastic sheath. Coating and plastic sheath serve as double anti-corrosion protection and prevent the rope from adhering to concrete.
The plastic shell is made of polyethylene and has a thickness of 1.5-2.0 mm. To provide corrosion protection in an aggressive environment, special adapters are used that connect the plastic shell with anchors. Each anchor has a protective cap. Characteristic features of the monostrand system are the presence of factory anticorrosive grease, low friction losses, etc. Light and flexible elements of the system are mounted quickly and simply, which makes it possible to talk about its economy.
1.1. General information
Over the years, the post-tension system has played an important role in the construction of bridges and storage tanks. The reason for success lies in decisive technical and economic advantages.
The following are the most important benefits of monostrand post-tension:
• Compared to traditional technology, when using this technology, there is a saving in the concrete and reinforcement used, which makes the structure more flexible;
• high resistance to cracking and good corrosion protection of steel;
• operational reliability under excessive load, since after the termination of the load the cracks converge and disappear;
• high degree of strength due to variation in the amplitude of the tension under the influence of a variable load.
In addition to the previously mentioned general characteristics of the after-voltage system, the advantages of post-tension floor slabs over reinforced concrete slabs can be listed:
• Cost-effectiveness of structures through the use of high-strength steel, which differs in its performance characteristics from traditionally used steel;
• large spans and flexibility of the structure - this leads to a decrease in the weight of the structure, which favorably affects the columns and the foundation, as well as to a decrease in the overall height of the structure (Fig. 2);
• resistance to the formation of deflections under the influence of a constant load;
• high shear strength.
Fig. 2 Dependence of plate thickness on span length
This technology is the most preferable for the construction of flat slabs with sizes from 6 to 10 meters. The design design for such overlap involves the use of a monostrand grid (0.6 "- 0.62") with a cross-sectional area of 140-150 mm2. Strands are placed along the spatial lattice for a long-term floor, anchors - along the perimeter and fixed in the center. The half-term load value is calculated by adding half the value of the temporary load to the dead weight of the structure Under these conditions, the slab is subjected only to axial forces without bending moment and shear.
If the variable load is below 30% of the total load, the calculation is statistically determined, since the bending moment is created only when +/- half of the variable load is applied.
Typically, the compressive stress in floor slabs with spans of 6-10 m is less than 2.5 N / mm2.
Advantages of using this technology:
• Lack of automatic calculations
• Lack of bending moment of vertical structures (supporting columns and walls), arising under the influence of a long load, and as a result, the consumption of the bending stability margin to counter horizontal loads
• Smooth floor surface
• Water tightness of the surface of the floors due to the absence of cracks
• Reduction of construction time due to the possibility of earlier form stripping (tension is possible 2-3 days after concreting, upon reaching concrete strength Fck> = 150 kg / cm2)
• Possibility of making holes in the structure itself and, in the presence of special conditions, in supporting columns
• When constructing this type of structure, the safety coefficient is approximately 3, and during the construction of conventional structures, the value of this coefficient approaches 1.75. The use of low-skilled labor leads to minor construction flaws, but generally does not affect the design
itself. Highly qualified specialists are required only for rope tensioning operations (1 or 2 people).
1.2 Post-tension with or without subsequent traction of prestressing reinforcement with concrete
1.2.1 Post-tension followed by traction of prestressing reinforcement with concrete
As you know, the method of post-tensioning consists in placing a steel strand in a round corrugated channel former (or channel) and in its subsequent tension. After injection of the channel with cement mortar, the rope adheres to concrete. Since in thin-walled floors of a building there is a strong decrease in the eccentricity of prestressed steel - especially at the intersection points - flat channels are most popular (see Fig. 4). As a rule, 4 strand ropes with a nominal cross-sectional diameter of 15 mm are placed in such channels.
1.2.2. Post-tension without subsequent adhesion of prestressed reinforcement to concrete
In the early stages of the emergence of the post-tension system in Europe, the method was used without coupling prestressed reinforcement with concrete. Over time, this method has been used less and less. And only after many years, structures began to be erected again through post-tensioning with clutch.
In the USA, the first application of this method had its own peculiarities: a steel rope was wrapped in grass and wrapped in wrapping paper in order to facilitate its longitudinal movement during tension.
Over the past few years, the method of manufacturing a plastic shell has become widely used. It consists in the fact that the strand is coated with anticorrosive grease at the manufacturing plant or company that provides services for tensioning reinforcement. Then the strand is placed in an extruded plastic pipe made of polyethylene or polypropylene, 1,5/2,0 mm thick. A plastic pipe is the first degree of corrosion protection, and lubrication is the second. Such strands are called monostrends. (Fig. 3) The nominal cross-sectional diameter of the used rope is 15,2/15,7 mm
Fig. 3. The structure of the rope with anti-corrosion grease, placed in a plastic shell.
1.2.3 Preference for one of two types of overtension.
The issue of preference for one of the two types of overtension has been and remains relevant to this day. We will not consider it in detail, but only give the most compelling arguments for and against.
Arguments for a post-tension system:
• The maximum eccentricity of the rope due to its minimum diameter (is of particular importance for thin-walled ceilings) (see Fig. 4);
• corrosion protection is applied to reinforcing strands at the manufacturing plant;
• simplicity and speed of strand placement;
• minimal loss of stress forces due to friction;
• lack of need for pouring with cement mortar;
• more economical method.
Fig. 4 Comparison of eccentricity can be made with different types of strands.
Arguments for a Post-tension clutch system:
• Greater ultimate bending moment;
• in case of local destruction of the strand (as a result of fire, explosion, earthquake), a limited section is damaged.
2. DETAILED PROJECT CHARACTERISTICS
2.1 Placement of the strands
The thickness of post-tensioned floors is less than the thickness of traditional reinforced concrete floors. This is primarily due to the effect of load distribution (balancing), as shown in Fig. 5-6. Along the spans, the deflecting force caused by the deflection of the rope acts on the concrete as opposed to the load.
Fig. 5 Transverse components and forces resulting from overtension
In places where the bending of the reinforcement changes direction, for example, on supports, the deflecting force of the reinforcement acts downward, directing the stress forces to the vertical elements. Thus, the system shown in Fig. 7, can be compared with a grating stretched between supports, the voltage of which is transmitted to concrete. The amount of necessary reinforcement is taken so that it can guarantee a given percentage of stability and withstand the weight of the floor. This percentage depends on the ratio of total load and long-term load, and, as a rule, varies between 70 and 130%. For floors of residential or office buildings with a temporary load of 3 to 4 kN / m2 and a long load of 1 kN / m2,
70-90% of its own weight is taken into account, for floors with a greater long-term load - 100% of its own weight. In addition to improving the characteristics of the slabs due to resistance to formation, one should remember the effect on the behavior of the slab, which is created by axial compression stress during post-stress. Provided that there are no significant restrictions, the compression stress neutralizes a part of the stress that occurs when a part of the loads is unbalanced by the forces induced by the reinforcement. Typically, in post-stressed ceilings, the compression stress indicator varies from 1.0 to 2.5 N / mm2.
Fig. 6 Principle of load balancing
Let's look at the ratio of height to span. With a small load of up to 3.5 kN / m2 and provided that the shear stress index at pressure is not critical, the thickness of the after-stress floor can be 1/40 of the length of the largest span (for internal panels), while in reinforced concrete floors the thickness is 1/30 span. If the columns have overhead slabs, then the height to span ratio can be increased to 1/45 for post-voltage ceilings and 1/35 for reinforced concrete floors. With increasing load, the height to span ratio decreases.
To balance the increased random load, you can’t just increase the amount of post-tension , but you need to design larger thickness ceilings taking into account the sag.
The use of post-tensioned concrete, in particular concrete with reinforcement without adhesion, requires consideration of some design principles that differ from standard ones. The problem that most often occurs in the design of buildings is the location of the joints of floors, walls and between walls and floors. Unfortunately, it is impossible to provide general explanations on this issue, since there are a number of aspects of the pros and cons. There are basically two main aspects to consider:
• Ultimate Strength (Safety)
• Horizontal displacement (limiting working condition)
2.2.1. Impact on ultimate load capacity
If only the loss of the bearing capacity of the structure is taken into account, it is recommended to completely avoid joints, because each of them represents interrupts the integrity of the structure and reduces its strength. In the case of prestressed ceilings with reinforcement without adhesion, the monolithic structure contributes to the action of bending, which leads to an increase in ultimate load.
Fig. 8 Influence of the action of the joints type on the bearing capacity
2.2.2. Serviceability limit
In buildings without joints under the influence of horizontal displacements, cracks in the supporting structure can occur. Offsets can be caused by the following factors:
• Concrete shrinkage
• Elastic shortening due to tension
• Creep of concrete
In the concrete structure, the following average values of reductions and elongations take place:
Concrete shrinkage DLcs = -0.25 mm / m
Thermal expansion DLct = -0.25 mm / m to +0.15 mm / m
Elastic shortening DLcel = -0.05 mm / m
(for medium centric
overvoltage 1.5 N / mm2 and
El = 30kN / mm2
Creep of concrete DLcc = -0.15 mm / m
These indicators should be appropriate to local environmental conditions.
After deciding on the location of the joints, the full displacements of the floors, walls and supports, their dynamics and the deformation of the foundation must be taken into account.
Horizontal offsets can be partially reduced or prevented by appropriate measures (e.g. clearances).
Shrinkage: Concrete is subject to shrinkage. The degree of shrinkage depends on the following factors: the proportion of water and cement, the size of the cross section and the percentage of humidity in the environment. Shortening due to shrinkage can be reduced by half with the help of temporary joints.
Thermal expansion: Regarding the effect of temperature, the difference in temperature between the individual structural components and the different coefficients of thermal expansion of the materials is important. In structures of a closed type, not subject to atmospheric phenomena, floors and walls are subject to low temperature fluctuations. Exterior walls and exposed ceilings are subject to large temperature fluctuations. Particular attention should be paid to the connections between floors and elements made of other materials.
3. PRODUCTION OF CONSTRUCTION WORKS
3.1. General information
The process of building post-tensioned floors is very similar to the construction of ordinary reinforced concrete floors. The difference lies in the placement of the reinforcement, its tension and the time to complete the work. Rebar placement work is divided into three stages. First, reinforcement is placed around the perimeter of the future structure, then channel formers are laid and firmly fixed. The next step is to place the top layer of conventional reinforcement. Strand tension and, in the case of using reinforcement with a clutch, injection are additional work, if we compare this process with the construction of a conventional reinforced concrete floor. Given that tensioning work can only be carried out by qualified specialists who have the right to carry out these works, the construction company workers can continue their main work without stopping.
An important distinguishing feature of this technology is the speed of dismantling the formwork. Depending on the quality of the concrete and the ambient temperature, the minimum period between pouring concrete and dismantling the formwork is 48/72 hours. As soon as concrete gains the necessary strength, full or partial tension of the floor begins, after which it is possible to disassemble and reassemble the formwork for the construction of the next section of the floor. This separation is dictated by the geometry of the structure itself, dimensions, layout, sequence of work, the use of formwork, etc.
The weight of the newly poured slab should be distributed by formwork to the underlying slabs. Given that this weight is less than the weight of already constructed floors from ordinary concrete, the cost of supporting structures is usually lower.