Retard-Bonded Steel Strand

Strands for Prestressed Concrete

First, the origin of prestressing technology

Origin: In 1866, the United States first used prestressing for concrete structures;

Unsuccessful: low values of prestress are quickly lost after concrete creep and shrinkage;
Theoretical breakthrough: In 1928, French engineer Freixinai applied prestress to concrete structures again;
Success: Adopt high-strength steel and high-strength concrete, and enter the substantive stage;
After the Second World War, there was a shortage of steel, and a large number of prestressed concrete structures replaced steel structures to repair war-damaged structures. The prestressed concrete technology has been vigorously developed. Establishment of the Association: The International Prestressed Concrete Association (FIP), established in 1950, is committed to promoting the development of prestressed concrete technology.
Domestic start:
In the 1950s, it was initially used for prestressed steel chord concrete sleepers, and it developed the fastest in railway and bridge engineering;
In the late 1970s, almost all bridges built in my country were prestressed concrete structures.
In recent years, prestressed concrete technology has been promoted nationwide and is widely used in railways, bridges, and civil fields

II. Classification of prestressing technology

1. Different classification according to prestressing method

①The first method:
On the pedestal or steel form, the steel bars are first tensioned and fixed with temporary fixtures, and then the concrete is poured. After the concrete reaches a certain strength, the prestressed tendons are loosened to make the concrete generate prestress. Suitable for prefabricated elements.
②Post-tensioning method:
The components or structures are made first, and then the concrete is stretched after reaching a certain strength, so that the concrete generates pre-stress. Applies to cast-in-place components.

2. According to the different bonding methods:

①No bonding:
Refers to the prestressed concrete in which the prestressed steel strand is free to expand and deform and does not bond with the surrounding concrete. The full length of the unbonded prestressed steel strand is coated with grease and protected by a plastic tube.
②With bonding:
It refers to the prestressed concrete in which the prestressed steel strand is completely bonded and wrapped by the surrounding concrete or cement paste. Both pre-tensioned prestressed concrete and post-tensioned prestressed concrete with pre-set channels pierced and grouted belong to this category.
③Slow bonding:
It means that the prestressed tendons can be stretched and deformed freely during the construction stage, and do not bond with the surrounding retarding adhesive, but the prestressing tendons are bonded with the surrounding concrete through the cured retarding adhesive within a predetermined period after the construction is completed. The prestressed tendons and the surrounding concrete form one body and work together to achieve a sticky effect.

3. Classification by prestressing degree

①Fully prestressed structure:
Prestressed concrete that does not allow tensile stress at the tensile edge under full service load. Applicable to the requirement that the concrete is not open
cracked structure. Partially prestressed structure: Prestressed concrete that allows certain tensile stress or cracks to occur at the tensile edge under full service load.
②Partially prestressed structure: 
Prestressed concrete that allows certain tensile stress or cracks to occur at the tensile edge under full service load.

Key ※:

    In the 1980s, Japanese scholars first proposed that "if we can develop a prestressed technology that is as easy to construct as the unbonded prestressed concrete technology, but also has the good bonding performance and structural performance of the bonded prestressed concrete technology, it will be a great success." It will greatly promote the development of prestressed structures". In Japan in the 1980s, from the perspective of convenient construction and reasonable force transmission mechanism, a new type of prestressed concrete technology was developed on the basis of bonded prestressed and unbonded prestressed concrete structures. Joint prestressed concrete technology. The technology inherits the advantages of simple and easy construction of unbonded prestressed structures; it also has the force transmission mechanism of bonded prestressed concrete structures, and has excellent seismic performance.

    Retard-bonded prestressing steel strand is composed of high-strength prestressed steel strand, sheath with regular transverse and longitudinal ribs, and densely filled between the sheath and prestressed steel strand. Consists of a slow-bonding adhesive that cures gradually over a set period of time. Before the tensioning construction is completed, the slow-bonding adhesive has fluidity, and the prestressed steel strands can slide freely in the slow-bonding adhesive layer, and the construction is simple and easy like "unbonded prestressing"; After the construction is completed, the slow-bonding adhesive gradually solidifies, bonding the prestressed steel strand and the sheath, and at the same time, the slow-bonding adhesive and the sheath are tightly "engaged" through the transverse ribs and longitudinal ribs formed together, thereby making the slow-bonding The prestressed steel strand creates a strong, long-lasting bond with the concrete.

Application fields, technological processes and international standards of prestressed steel strands

   I. Product application areas:
    Highways, railway bridges, airports, long-span building beams, stadiums, mine support, LNG natural gas liquid storage tanks, wind power generation foundations, photovoltaic power generation brackets, offshore wind power platforms, foundation support, mountain support and other fields.

Second, the production process:   
    The prestressed steel strand is a stranded steel cable composed of 2, 3, 7 or more high-strength cold-drawn smooth steel wires, and is subjected to stress relief treatment (ie stabilization treatment). The production process is shown in Figure 2 below. Generally, high carbon steel 82B wire rod is used as the raw material. After pickling surface treatment, it is cold drawn into steel wire. Stabilized.

The key links in the production process of prestressed steel strands are described as follows:

Figure 1: Process flow for the production of prestressed strands

1. Pickling

    The raw material used in production is high-carbon steel 82B wire rod, and the surface is relatively clean, but in order to ensure the phosphating effect of the subsequent sequence, further acid cleaning treatment is required. Unbundle and loosen the raw wire rods and then immerse them in the pickling tank. The pickling solution is dilute hydrochloric acid between 10% and 15%. Soak and pickle them at room temperature for about 30 minutes. After pickling, lift the wire rods to the pickling tank. Hanging on the top of the bracket, and slowly shaking in a small range, so that the wire rod will bring out the acid liquid and flow into the tank. The residence time is as long as there is no acid dripping. The liquid is further removed. The acid solution is reused, and new acid is regularly replenished and replaced according to the consumption situation. When the waste acid is replaced, the emission concentration is about 5%, and it is sent to the tannery for leather tanning without being discharged. After pickling and water washing, the wire rod enters the phosphating process, and the acid waste water produced by water washing enters the sewage treatment station of the factory for treatment, and part of it is used for green water reuse, and part of it is discharged up to the standard.

2. Phosphating

    After pickling and water washing, the wire rod enters the phosphating tank for phosphating treatment. The low-temperature rapid phosphating process is adopted, and there is no need for heating and heating during the phosphating process. Phosphating is a process of chemically and electrochemically reacting to form a phosphate chemical conversion film. The formed phosphate conversion film is called a phosphating film. The main purpose of phosphating is to protect the base metal and improve the corrosion resistance of the base.

    The main components of the phosphating solution are aqueous solutions of phosphoric acid and dihydrogen phosphate. The phosphating solution will not be discharged after repeated use. After phosphating, there is no need to wash it with water. The operation process is the same as the acid control process after washing. After the phosphating solution is nearly dried, it enters the saponification process.

3. Saponification

    The operation process of the saponification process is the same as that of the phosphating process. The composition of the saponification liquid is an aqueous solution of sodium soap, and its purpose is to increase the lubricity of the surface of the wire rod and prepare for the subsequent wire drawing section. The saponification liquid is not discharged out, and it does not need to be washed with water after saponification. After the saponification liquid is controlled to dry, it enters the wire drawing section.

4. Wire drawing and plying

    The advanced high-speed straight-forward wire drawing machine is selected, and the cold drawing process is adopted. The wire drawing process is divided into nine stages of drawing. The customer needs to twist several finished yarns into shape, and then straighten the strands through the tensioning wheel.

5. Stabilization

    The stranded wire is heated by induction to improve its physical properties and enhance its strength and toughness.

Three, product quality control:

    Since its establishment, the company has been adhering to the corporate policy of "scientific management, technology first, quality first, service first". The company passed the Noah Group ISO9001:2015 quality management system certification, ISO14001:2015 environmental management system certification, and ISO45001:2018 occupational health and safety management certification in December 2021. The quality control work has run through the preparation of product production process, production and manufacturing In the whole process of , inspection, transportation and after-sales service, all kinds of products have obtained corresponding qualification certification. The products implement the Chinese National Standard GB/T5224-2003 (see Table 2 for details), American Standard ASTM A 416 (see Table 3 for details), British Standard BS 5896 (see Table 4 for details) and Japanese Standard JIS 3536 (see Table 5 for details) . The company's products have all passed the inspection of the National Construction Steel Quality Supervision and Inspection Center.

Table 2: Chinese National Standard GB/T5224-2014

Structure	Nominal Diameter (mm)	Tolerance (mm)	Cross-sectional area (mm)	Every 1000m Theoretical weight (Kg/1000m)	Nominal Strength (Mpa)	Yield strength is not small (%)	Elongation not less than (%)	1000h relaxation rate % is not greater than the initial load is 70% of the maximum load
1*2	10	0.25	39.3	308			3.5	2.5
	12	-0.1	56.5	444
1*3	10.8	0.2	58.9	462	1470	1320	3.5	2.5
	12.9	-0.1	84.8	666	1570	1410
1*7(standard)	9.50	0.3	54.8	430	1670	1500	3.5	2.5
	11.1	-0.15	74.2	582	1720	1550
	12.7	0.4	98.7	775	1860	1670
	15.2	-0.2	140	1101	1860	1760
1*7(Draft type）	12.7	0.4	112	890			3.5	2.5
	15.2	-0.2	165	1295

Table3: American Standard ASTM A 416

level	Nominal diameter(mm)	Tolerance(mm)	Cross-sectional area(mm²)	Theoretical weight per 1000m (kg)	Breaking load (KN)	At 1% elongation Minimum Load (KN)	Elongation (%)	Slack value 1000hrs
								Apply 70% force	Apply 80% force
250	9.53	±0.40	51.61	405	89.0	80.1	3.5	2.5	3.5
	11.11		69.68	548	120.1	108.1
	12.7		92.9	730	160.1	144.1
	15.24		139.35	1094	240.2	216.2
270	9.53	0.65	54.84	432	102.3	92.1	3.5	2.5	3.5
	11.11	-0.15	74.19	582	137.9	124.1
	12.7		98.71	775	183.7	165.3
	15.24		140	1102	260.7	234.6

Table4: British Standard BS 5896

Model	Nominal Diameter (mm)	Tolerance (mm)	Cross-sectional area (mm簡)	Theoretical weight per 1000m (kg)	tensile Strength (Mpa)	Specified characteristic value 0.1% yield Load (KN)	Load (KN) at 1% elongation	Load (KN) at 1% elongation	Slack value 1000h
									Apply 60% force	Apply 70% force	Apply 80% force
Standard	9.3	0.3	52	408	1770	92	78	81	1	3.5	4.5
7	11	-0.15	71	557	1770	125	106	110
Silk	12.5	0.3	93	730	1770	164	139	144
	15.2	-0.15	139	1090	1670	232	197	204
		0.4
		-0.2
		0.4
		-0.2
Super	9.6	0.8	55	432	1860	102	87	90	1	3.5	4.5
7	11.3	-0.15	75	590	1860	139	118	122
Silk	12.9	0.3	100	785	1860	186	158	163
	15.7	-0.15	150	1180	1770	265	225	233
		0.4
		-0.2
		0.4
		-0.2
Die-Draft	12.7	0.4	112	890	1860	209	178	184	1	3.5	4.5
7	15.2	-0.2	165	1295	1820	300	255	255
Silk	18	0.4	223	1750	1700	380	323	323
		-0.2
		0.4
		-0.2

Table5: Japanese Standard JIS 3536

Model	Nominal Diameter (mm)	Tolerance (mm)	Cross-sectional area (mm²)	Every 1000 m Theoretical weight	Breaking Load (KN)	Load at 0.2% permanent elongation (KN)	Elongation (%)	Slack value 1000h (%)
SWPR7A	9.3	0.4	51.61	405	88.8	75.5	3.5	3
	10.8	-0.2	69.68	546	120	102
	12.4		92.9	729	160	136
	15.2		138.7	1101	240	204
SWPR78	9.2	0.4	54.84	432	102	86.8	3.5	3
	11.1	-0.2	74.19	580	138	118
	12.7		98.71	774	183	156
	15.2		138.7	1101	261	222