How much should the concrete strength of prestressed concrete reach before tensioning

The strength grade of prestressed concrete should not be lower than C30, and when carbon steel wire, steel strand, and heat-treated steel bars are used as prestressed tendons, the concrete strength grade should not be lower than C40.

Prestressed concrete is to make up for the phenomenon of premature cracks in concrete, before the use (loading) of the component, give a pre-pressure to the concrete in advance, that is, in the tensile area of the concrete, the steel bar is stretched by the method of artificial afterburner, and the retraction force of the steel bar is used to make the concrete tensile area under pressure in advance. This stored pre-pressure, when the member bears the tensile force generated by the external load, first offset the pre-pressure in the concrete in the tension zone, and then with the increase of the load, the concrete is tensile only, which limits the elongation of the concrete, delays or does not make the crack appear, this is called prestressed concrete.

Some requirements for concrete in prestressed concrete:

The strength should be high, and it should be adapted to the high-strength steel bar, so as to ensure that the prestressed steel bar can play a full role, and can effectively reduce the cross-sectional size of the component and reduce the self-weight.

The shrinkage and creep should be small to reduce the loss of prestress.

Fast hardening and early strengthening, enabling the application of prestress as soon as possible, speeding up the construction progress and improving the utilization rate of the equipment.

The tension of prestressed concrete requires the strength value of concrete, which is generally determined according to the design; If the design is not clear, the concrete should reach 100% of the design strength before it can be tensioned.

The strength value should meet the design requirements, if the design is not clear, it should reach 100% of the concrete strength grade of the component.

It should not be lower than the design concrete strength.

75% of the grade value, prestressed reinforcement can be carried out.

Tension (application of prestress).

GB50666-2011 Code for construction of concrete structure engineering.

According to the requirements of the code, the concrete strength of the prestressed steel bar shall not be less than 75% of the design strength when the prestressed steel bar is tensioned!

When the concrete strength grade reaches 75%, the prestressed steel bar can be tensioned.

Hello, now I am here to answer the above questions for you. What are the prestressed tensile specifications, prestressed tensile specifications, I believe that many friends still don't know, now let's take a look at the pre-response. <>

The tensile control stress of the tendon must comply with the design regulations.

When the stress control method is used to tension the prestressed tendon, the elongation value should be checked.

3. The difference between the actual elongation value and the theoretical elongation value should meet the design requirements; When the design is not specified, the difference between the actual elongation value and the theoretical elongation value should be controlled within 6.

4. Otherwise, the tensioning should be suspended, and the tensioning can only be continued after the reason is found out and measures are taken.

When the prestress tension is made, it should be adjusted to the initial stress The initial stress should be the tension control stress 10 l5, and the elongation value should be measured from the initial stress.

The anchorage of the prestressed tendon should be carried out in a stable state of tension control stress, and the inward shrinkage of the tensile end of the tensile tendon in the anchoring stage shall not be greater than the design or specification.

What is the strength level of concrete soil that can be used for prestressed steel tensioning? (1) The strength of concrete shall meet the design requirements, and when the design is not required, it shall not be less than 75% of the design value of the strength. (2) Curved prestressed tendons or linear prestressed tendons with a length greater than or equal to 25m should be tensioned at both ends.

Linear tendons with a length of less than 25 m can be tensioned at one end. (3) Adopt symmetrical tensioning in batches and phases. It is advisable to start in the middle, then up, down or on both sides.

4) The number of broken wires, slippery wires and broken tendons in the tensioning process shall not exceed the regulations. The steel bar is not allowed to break, the total number of broken steel wires and steel strands shall not exceed 1%, and the broken wires of each bundle of steel wires and steel strands shall not exceed 1 wire (root).

The tensioning of the prestressed concrete pre-tensioning method, the strength of the concrete is as follows: the compressive strength of 75% of the concrete is obtained by the test, and the cube specimen with a side length of 150mm is used as the standard size specimen of the compressive strength of the concrete below the latest standard C60 strength. In accordance with the "Standard for Test Methods for Mechanical Properties of Ordinary Concrete" GB T50081-2002, the cube with a side length of 150mm is made under the standard curing conditions (temperature 20 2, relative humidity above 95%).

Curing to 28 days of age, the ultimate compressive strength measured by the standard wheel wax test method is called the standard cubic compressive strength of concrete, which is expressed as FCU. According to the GB50010-2010 "Code for Design of Concrete Structures", in the overall distribution of the ultimate compressive strength of the cube, the compressive strength of the Lina-selling body specimen with a strength guarantee rate of 95% is called the standard value of the compressive strength of the concrete cube (calculated as MPa), which is represented by FCU and K.

The compressive strength with a 95% guarantee rate measured according to the standard experimental method is used as the concrete strength grade. According to GB50010-2010 "Code for Design of Concrete Structures", ordinary concrete is divided into 14 grades, namely: C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75, C80.

For pre-tensioned prestressed structures, it is required that concrete pouring should be carried out in the time after prestress tensioning.

Dear, hello, I am glad to answer for you for the pre-tensioning prestressed structure, it is required that the concrete should be poured within 24 hours after the prestressing tension is completed, and the concrete should be poured immediately after the completion of all the tensioning work. When the concrete has not been poured for more than 24 hours, the prestressed tendons must be checked again; When there is an error in the brightness value of the wide force, it should be re-tensioned.

The matters that should be paid attention to when tensioning prestressed concrete are as follows:

1. Before tensioning, the strength data of the concrete test block should be checked to confirm that the concrete strength meets the requirements of tensioning, and then the tensioning construction can be carried out. Before tensioning, check whether the formwork has sinking, whether there are cracks and other quality problems and concrete defects in the components (beams, etc.). If the problem is serious, it should be studied and dealt with, and tensioning should not be carried out rashly.

Check and calibrate the tensioning equipment and anchors. Develop construction safety measures.

2. Attention should be paid to safety during construction. When tensioning, it is forbidden to stand at both ends of the steel bar. When striking the cone plug or wedge of the anchor, the force should not be too strong, so as not to break the prestressed tendon and hurt people, but it should be anchored reliably.

When the temperature is lower than 2, the danger of brittleness of the tendon should be considered. In order to check whether the internal force of each steel wire is consistent after tensioning, the internal force of the steel wire can be measured by using a dynamometer.

3. Prepare tension records** and record personnel. Pay attention to the situation in tensioning, if you find slippage or breakage, stop tensioning in time and check it. The specification stipulates that for post-tensioning components, it is strictly forbidden to exceed 3% of the total number of pre-tensioned steel in the same section of the structure.

And only one bundle of steel wire is allowed.

The design and specification requirements of prestressed tensioning and grouting are as follows:

1. The main control of tensile stress, the actual elongation value of the method of checking, accurate control of each grading tension, the measurement of the corresponding elongation and record and calculation, the difference between the actual elongation value and the theoretical value should be controlled within 6%, otherwise the tension should be suspended, to find out the cause and take the adjustment. The tensioning order is n1, n2, and n3.

2. The principle of symmetrical tension is adopted to prevent the beam from overstressing, the component from torsion and side bending, and the frame from remaining in place. Processing and installation of steel strands, steel strands are cut by grinding wheel cutters. The cutting length of the strand is the length of the hole plus the working length at both ends.

The working length at both ends is 2000px, and the design length should be reviewed before blanking.

Answer]: B

This question examines the construction of prestressed concrete engineering. When the tensile prestressed steel is wide and void, the strength of the concrete of the component should not be less than 75% of the design compressive strength of the concrete.

Answer]: A This Tong Brigade question examines the prefabricated construction of the bridge superstructure. When the compressive strength of the concrete of the beam body reaches more than 1 3 of the design strength and the elastic modulus is not less than 50% of the design value, the initial tension of part of the prestress can be carried out, but the tensile stress should not exceed the number of the prestressed steel bundle and the tensile stress of the initial tension of the design tension control stress of 1 3 should conform to the provisions of the fire meter.

The final tension of the prestressed steel bundle of the box girder should be carried out after the compressive strength of the concrete reaches 80% of the design strength and the elastic modulus is not less than 80% of the design value.