Acceptance Criteria for ½-in. Diameter Prestressing Strand in Pretensioned Applications




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Research Proposal


Acceptance Criteria for ½-in. Diameter Prestressing Strand in Pretensioned Applications


By


University of Nebraska-Lincoln


March 2011

Acceptance Criteria for ½-in. Diameter Prestressing Strand in Pretensioned Applications


Background


The performance of prestressed concrete members is dependent on the ability of strands to bond with concrete. Quantifying this performance parameter has been a focus of the prestressing industry for many years. As a result, five experimental techniques have become available to classify strand bond. They all determine bond quality of strand to the surrounding concrete as a function of the pullout strength.


The development of these methods began with the Moustafa Test (1974). It involves a simple setup where untensioned strands are pulled out of large concrete blocks. Within recent years, the Post-Tensioning Institute (PTI) investigated pulling untensioned strands out of cement mortar in a process referred to as the PTI Test (NCHRP 603, 2008). In a modification of this procedure, the North American Strand Producers (NASP) provided pooled funds to finance a study to modify the PTI Test. The study was primarily conducted by Professor B.W. Russell (Final Report NASP Round III Strand Bond Testing, 2001). One of the primary changes was to replace the cement mortar with sand cement mortar. Numerous tests were done to investigate repeatability of the NASP Bond Test (NCHRP 603, 2008).


The Moustafa test was able to find the worst performing strand in a select group. However, the method did not suggest any means of determining robust absolute values. The PTI Test performed slightly better, as it was able to determine the best and worst performing strands in the same select group. Both tests had a weak correlation to transfer length. In comparison, the NASP Bond Test accurately identified the best and worst strands in conjunction with the possibility that additional research could identify reliable robust absolute values. Correlation between the NASP Bond Test and transfer length was reasonable (NCHRP 603, 2008).


In light of these results, the NASP Bond Test was chosen for further analysis. In NCHRP Project 12-60, the NASP Bond Test was modified to evaluate the bond of 0.5 in. and 0.6 in. diameter strands in concrete, and equations were developed to predict strand bond for a given concrete strength as given below. Also, NASP Bond Test results were used to develop formulas to predict the bond strength of 0.5 in. and 0.6 in. diameter strands as a function of concrete strength.


For 0.5 in. strand diameter: P (kips) = 14.08 fc0.31 (ksi)

For 0.6 in. strand diameter: P (kips) = 7.98 fc0.56 (ksi)

Where: P = NASP pull out bond value

fc= 1-day concrete strength


The NASP bond test specimen consists of an 18 in. long, 5 in. diameter, and 1/8 in. thick steel pipe bolted to a 6 in. by 1/4 in. square steel plate. The plate is attached by 4 bolts and nuts to one end of the steel pipe in order to confine concrete during placement and provide a flat surface for loading. A hole is made in the plate for passing the strand. A picture of this setup can be viewed in Figure 1 and Figure 2.

c:\afshin\nasp test\picture nasp\201002a0\20022010160.jpg


Figure 1: NASP Bond Test Specimen at the University of Nebraska-Lincoln (Hatami, 2010)




Figure 2: NASP Bond Test Specimen Setup at the University of Nebraska-Lincoln (Hatami, 2010)


Each test specimen is prepared by casting sand-cement mortar in the steel pipe around a single prestressed strand. The sand-cement-water ratio is 2:1:0.45 and the cement used is Type III. The sand-cement mortar is proportioned to produce a strength value of 4,500 to 5,000 psi at 24 hrs using standard curing. Additionally, the sand-cement mortar is required to produce a flow in the range of 100% to 125% as measured by ASTM C 1437. After the mortar has been cast for 24 hrs, the strand is pulled out of the mortar at a displacement rate of 0.10 in./min. The pullout force is measured in relation to the movement of the free end of the strand to the hardened mortar. The NASP bond test records the pullout force that corresponds to 0.10 in. of free strand end slip. Each NASP bond test consists of six individual test specimens; the average value from the six specimens becomes the “NASP Bond Test Value.” Values corresponding to 0.01 in. strand slip at the free end are also recorded. The loading rate during testing is limited to 8,000 lb/min for 0.5 in. diameter strands.


NCHRP 603 developed a testing program to correlate transfer and development length to the NASP bond test. Specimens for this program are 12 in. deep, 6 in. wide, and 17 ft long rectangular beams. The beams have two levels of prestressing strand which can be seen in Figure 3. Flexural loading for the test was done for concrete strengths ranging from 4 ksi to 10 ksi, and development lengths approximately 80 percent and 100 percent of the AASHTO and ACI development length requirements.








Figure 3: NCHRP 603 Details for Transfer Length Testing (NCHRP 603, 2008)


A modified version of the NASP Bond Test has been proposed as an ASTM standard test protocol called the Standard Test Method for the Bond of Prestressing Strands. Acceptance is dependent on the ability of researchers to analyze, validate and/or justify modification of the test’s provisional acceptance threshold value of 10,500 lbs for ½ in. 270 ksi strand – a value developed in Report 603 of the National Cooperative Highway Research Program (NCHRP). As previously mentioned, this value refers to the pullout force that corresponds to 0.10 in. of free strand end-slip. A Due Diligence report commissioned by PCI concluded that a threshold value of 10,500 lbs is based on insufficient data provided by round robin testing, and that a more reasonable threshold value would be an average 12,000 lbs of multiple specimens with no single specimen experiencing a pullout force less than 10,500 lbs.


The last two methods available for classifying strand bond strength are performed using tensioned strands. The first of these methods is ASTM A981–97 (2007), Standard Test Method for Evaluating Bond Strength of 0.6 in. Diameter Prestressing Steel Strand, Grade 270, Uncoated, Used in Prestressed Ground Anchors. The second is “Simple Quality Assurance Test for Strand Bond” which was developed by Robert J. Peterman in 2009. The Peterman method is a more versatile alternative to the Standard Test Method for the Bond of Prestressing Strands because it is more relevant to flexural applications and it can be used for a wider range of strand diameters.


The Peterman Test is based on a single-strand flexural specimen cast using standard batching, placement, consolidation, curing, and detensioning methods. The ends of the strand are ground flush with the concrete at the ends of the beam in order to visually detect strand end-slip. The beam is then loaded to 85% of the calculated nominal moment capacity of the section. It is inspected for cracks and strand end-slip over a period of at least 24 hours in order to monitor for signs of increasing distress in the beam. Lastly, the beam is loaded to its full nominal capacity and held at that load for a minimum of 10 min. to determine if the beam passes or fails based on whether or not the beam collapses. Figure 4 is a detail of the test setup for 0.5 in. diameter strands as provided by Peterman in his report on the Simple Quality Assurance Test for Strand Bond.



Figure 4: A Simple Quality Assurance Test for Stand Bond (Peterman, 2009)


Passing or failing the test does not necessarily mean that other products made from the same constituents with multiple strands and various confinement conditions will necessarily perform in the same manner. However, it is a test that can provide reasonable confidence that the unique combination of mixture ingredients, strand source, batching, placement, and detensioning procedures results in members that can meet the current design assumptions for bond as measured by development length in flexural members (Peterman, 2009).

Peterman recommends that his test be conducted on a minimum of 2 beams (one at 28 day strength and the other upon reaching design strength). Strand should be tested twice a year for bond-critical applications as defined by the assumption that a 60% increase in ACI 318-08 transfer or development length would result in structural failure. Any new combination of standard batching, placement, consolidation, curing, and detensioning methods should also be tested before use in a prestressing application.


While it is recommended that strand manufacturers verify the pullout strength of their products on a regular basis, no set schedule has been established. Instead, manufacturers prefer to conduct easy and fast quality control tests based on surface characteristics. The following tests are proposed in NCHRP Report 621 as an alternative method to the Standard Test Method for the Bond of Prestressing Strands: loss on ignition, contact angle measurement, change of corrosion potential, and organic residue extraction.


Loss on ignition refers to the weight of compounds that can be volatilized or burned off the strand at high temperatures. These compounds are assumed to be organic residues accumulated during the manufacturing and storage process.


A contact angle test is performed on the strand as-is, after immersion in a lime dip, and after ignition. It is a measurement of surface tension based on the projected shadow of a small droplet of distilled water applied to the strand surface. The purpose of this test is to measure the “wetablity” of a strand upon casting with concrete. Organic residues which accumulate on concrete have been known to cause a chemical reaction with strands that acts as a wax-like water repellent and reduces bond strength (NCHRP 621, 2008). This process can be simulated by calcium hydroxide exposure (i.e. lime dip).


Organic compounds have also been suspected of making strand more susceptible to corrosion. This can be measured by submerging the strand in a Ca(OH)2 solution after it has passed through the ignition process (NCHRP 621, 2008). Corrosion potential is measured by the amount of ferrous ions in the solution versus the length of time the strand is submerged (6 hours).


The test for organic residue extraction is done according to ASTM C114 for organic materials in cement. The weight of residue extraction is determined by an analytical balance and the material is then defined by a Fourier transform infrared spectroscopical (FTIR) analysis. Organic residue is classified as being either warm water soluble or insoluble based on hydrocholoric acid and chloroform extraction. This information is useful in determining the best method for cleaning strands prior to pretensioning applications (NCHRP 621, 2008).


All but the last test can be conducted without specialized training or equipment. NCHRP 621 threshold values for acceptance of strand are given in Table 1. Research has been able to find direct correlations to the NASP Bond Test for the contact angle after a lime dip and the change in corrosion potential after 6 hours. Correlations to the NASP Bond Test are given in Table 2. It should be noted that an unfortunate downside to this test method is the frequency of testing (weekly) and the corresponding effect this has on the overall cost of strand manufacturing (NCHRP 621, 2008).


Table 1: Threshold values for NCHRP 621 Quality Control Testing based on Surface Characteristics of Strand




Table 2: Correlation of NCHRP 621 to NASP Bond Test




Similar to the Standard Test Method for the Bond of Prestressing Strands, an inadequate quantity of studies was conducted to provide threshold values for the acceptance of strand based on surface characteristics. A comparison of the revised NASP Bond Test and the method proposed in NCHRP Report 621 is needed to establish which threshold values are the most adequate for both the strand manufacturing industry and the precast/prestressed concrete industry (PCI). A threshold must be low enough to consistently produce material meeting that value, however high enough to provide a significant amount of confidence in the product. Confidence for prestressing members is defined as the certainty of which a member will meet ACI 318 specifications on transfer and development length.


Transfer and Development Length


Transfer length is defined as the distance along the length of a prestressed member for the fully effective prestress (jacking stress minus losses) to be transferred to the beam. There is zero stress at the end of the member/strand. From this point onward, the mechanisms of bond gradually increase the stresses in the strand until the strand is fully stressed. Figure 5, below, gives an ideal representation of the stress in the strand vs. length along the strand. One can see the gradual increase in force in the strand until it reaches a plateau. This transfer, or anchorage region, is present at the end of every pretensioned member, as well as at the beginning of the bonded region on every debonded or shielded strand.


The transfer length is not of utmost importance in the design of a prestressed member for flexure; however, it is often necessary for the design of shear at the critical section, as well as for the design of debonded strands to relieve the concrete of excessive stresses at prestress transfer to the concrete. It would be best to design with an overestimated or upper bound value for the transfer length. This would result in conservative designs, ensuring adequate safety in shear resistance.. Conversely, when checking concrete stresses, under estimating the transfer length would be a conservative design practice as there would be less moment from the girder weight at that section.


The development length is the distance along the length of a prestressed member for the strand stress corresponding to the full flexural design strength of the cross section. Similar to the transfer length, bond stresses must increase the stresses in the strand until the strand reaches the peak flexural strength. After the effective prestress is attained, exterior forces, such as tension in the strand caused by a moment on the member, which would require the stress in the strand to increase, can only be developed as the amount of bond is available. If there is not enough bond stress to reach the full design prestress in the member, a strand slip relative to the concrete occurs and a bond failure is likely to occur. Some researchers (Russell and Bums, 1993; Shahawy, 2001) acknowledged that premature bond failure can be caused by propagation of flexural/shear cracks though the transfer length. This statement could have significant effects on debonded strands as well as on thin web members susceptible to web shear cracking.





Figure 5 – ACI Idealized Steel Stress vs. Distance from End of Member (ACI 318-08, 2008)


ACI 318-08 Chapter 18 specifies the nominal resistance and effective prestess that can be achieved using prestressing strand:


ACI 318-08 18.6.1 – To determine effective stress in the prestressing steel, fps, allowance for the following sources of loss of prestress shall be considered:

  1. Prestressing steel seating at transfer;

  2. Elastic shortening of concrete;

  3. Creep of concrete;

  4. Shrinkage of concrete;

  5. Relaxation of prestressing steel stress;

  6. Friction loss due to intended or unintended curvature in post-tensioning tendons.


ACI 318-08 18.7.2 – As an alternative to a more accurate determination of fps based on strain compatibility the following approximate value of fps shall be permitted to be used if fse is not less than 0.5fpu

  1. For members with bonded tendons

fps = fpu {1 – γP / β1P fpu / fc + d / dp (ω – ω’)]} (ACI 318-08 18-3)


where ω is ρ fy / fc, ωis ρfy / fc, and γP is 0.55 for fpy / fpu not less than 0.80; 0.40 for fpy / fpu not less than 0.85; and 0.28 for fpy / fpu not less than 0.90.


If any compression reinforcement is taken into account when calculating fps by Eq. (ACI 318-08 18-3), the term


P fp / fc+ d / dp (ω – ω’)]


Shall be taken not less than 0.17 and d’ shall be no greater than 0.15dp.


ACI 318-08 Chapter 12 specifies the transfer and development length which can be achieved using prestressing strand:


ACI 318-08 12.9.1 – Except as provided in 12.9.1.1, seven-wire strand shall be bonded beyond the critical section, a distance not less than


ld = (fse / 3000) db + (fpsfse) db / 1000 (ACI 318-08 12-4)


The expressions in parentheses are used as constants without units.


ACI 318-08 12.9.1.1 – Embedment less than ld shall be permitted at a section of a member provided the design strand stress at that section does not exceed values obtained from the bilinear relationship defined by Eq. (ACI 318-08 12-4)

lt = (fse / 3000) db (ACI 318-08 Fig. R12.9)


A number of mechanisms have been identified as creating the concrete to steel bond. Adhesion, friction, strand expansion and contraction due to longitudinal stresses (Poisson's Effect), and mechanical interlock all in some way contribute to the bond stress. Each of these is briefly addressed below.


Adhesion is the bond between the concrete and the steel created when fresh concrete hardens. The bond due to adhesion is effective only until its failure. At this point it is gone, and as such it cannot be counted on. Any differential slip between the two materials effectively removes any effects from adhesion. In the transfer region, the effect of adhesion is zero, as the transfer length can be defined as a function of strand slippage (Guyon, 1960). Slip also occurs at the edges of cracks which pass across the strand as very high stresses in the steel are attained and strand diameter changes due to Poisson's effect.


Friction plays a significant role in the bond stress active during transfer and development length. Experiments by Janney (1954) with prestressed wire were able to isolate the effects of friction, as there were no deformations on the wire to enable mechanical resistance. Friction is only present when the two materials are forced against each other due to radial stresses. Radial stresses can be increased with the advent of concrete shrinkage, Poisson's effect, or mechanical interlock. These stresses can be reduced by any changes in strand diameter, which if large enough could remove friction entirely. Friction can also be increased through strand surface quality (Janney, 1954) and the wedging action from small particles that break from the surrounding concrete.


Hoyer's effect (or Poisson's Effect) was named after E. Hoyer, who in 1939 investigated the mechanisms of bond in pretensioned concrete and recognized the mechanism. As a material is loaded in one direction, the material elongates in that direction and therefore contracts in the others, as dictated by Poisson's ratio. In this case the strand is tensioned and released into hardened concrete, at the end of the member there is zero stress and the strand is at its normal diameter. As the strand gains stress it also contracts until the effective prestress is reached, at which point the diameter remains constant. The same effect takes place along the rest of the girder as additional strand tension is applied. This difference in diameter, specifically in the anchorage zone, creates a wedging action called Hoyer's Effect. Without Hoyer's effect the effects of friction are greatly reduced or eliminated. Janney (1954) and Hansen and Kaar (1959) noted this as the cause of a bond failure. The reduction of the strand's diameter in the transfer region would cause a successive collapse of anchorage such that a bond failure could occur. For this reason the anchorage zone is suggested to have ample reinforcement to protect the strands from catastrophic cracking.


The helical shape of the seven wire strand creates what is known as mechanical interlock, similar to the deformations on a reinforcing bar. When the concrete is cast around a strand, the strand cannot strictly pull out. It must either break the concrete which has filled the ridges and cracks between the wires or twist as can be seen in Figure 6.


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Figure 6 – Ridges Formed by Concrete When Cast Around 7- Wire Prestressing Strand at UNL


Mechanical interlocking is considered to be the largest contributor to flexural bond stresses (Russell and Bums, 1993), and is similar to the deformations on mild steel reinforcement. When cracking occurs, small slips are created which increases the effect of the mechanical interlock by causing the strand to react against the concrete in the strand's helical deformations.

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