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Post-Tensioned Concrete Masonry Wall Design

August 2018

INTRODUCTION

The 1999 Building Code Requirements for Masonry Structures, ACI 530/ASCE 5/TMS 402 (ref. 1), was the first masonry code in the United States to include general design provisions for prestressed masonry. Prestressing masonry is a process whereby internal compressive stresses are introduced to counteract tensile stresses resulting from applied loads. Compressive stresses are developed within the masonry by tensioning a steel tendon, which is anchored to the top and bottom of the masonry element (see Figure 1). Post-tensioning is the primary method of prestressing, where the tendons are stressed after the masonry has been placed. This TEK focuses on the design of concrete masonry walls constructed with vertical post-tensioned tendons.

Advantages

Prestressing has the potential to increase the flexural strength, shear strength and stiffness of a masonry element. In addition to increasing the strength of an element, prestressing forces can also close or minimize the formation of some cracks. Further, while research (refs. 14, 15) indicates that ductility and energy dissipation capacity are enhanced with prestressing, Building Code Requirements for Masonry Structures (ref. 1) conservatively does not take such performance into account.

Post-tensioned masonry can be an economical alternative to conventionally reinforced masonry. One major advantage of prestressing is that it allows a wall to be reinforced without the need for grout. Also, the number of prestressing tendons may be less than the number of reinforcing bars required for the same flexural strength.

Post-tensioning masonry is primarily applicable to walls, although it can also be used for beams, piers, and columns. Vertical post-tensioning is most effective for increasing the structural capacity of elements subjected to relatively low axial loads. Structural applications include loadbearing, nonloadbearing and shear walls of tall warehouses and gymnasiums, and commercial buildings, as well as retaining walls and sound barrier walls. Post-tensioning is also an option for strengthening existing walls.

Figure 1—Schematic of Typical Post-Tensioned Wall

MATERIALS

Post-tensioned wall construction uses standard materials: units, mortar, grout, and perhaps steel reinforcement. In addition, post-tensioning requires tendons, which are steel wires, bars or strands with a higher tensile strength than conventional reinforcement. Manufacturers of prestressing tendons must supply stress relaxation characteristics for their material if it is to be used as a prestressing tendon. Specifications for those materials used specifically for post-tensioning are given in Table 1. Other material specifications are covered in references 9 through 12. Construction is covered in Post-Tensioned Concrete Masonry Wall Construction, TEK 03-14 (ref. 3).

CORROSION PROTECTION

As with conventionally reinforced masonry structures, Building Code Requirements for Masonry Structures (ref. 1) mandates that prestressing tendons for post-tensioned masonry structures be protected against corrosion. As a minimum, the prestressing tendons, anchors, couplers and end fittings in exterior walls exposed to earth or weather must be protected. All other walls exposed to a mean relative humidity exceeding 75% must also employ some method of corrosion abatement. Unbonded tendons can be protected with galvanizing, epoxy coating, sheathing or other alternative method that provides an equivalent level of protection. Bonded tendons are protected from corrosion by the corrugated duct and prestressing grout in which they are encased.

DESIGN LOADS

As for other masonry structures, minimum required design loads are included in Minimum Design Loads for Buildings and Other Structures, ASCE 7 (ref. 5), or the governing building codes. If prestressing forces are intended to resist lateral loads from earthquake, a factor of 0.9 should be applied to the strength level prestress forces (0.6 for allowable stress design) as is done with gravity loads.

STRUCTURAL DESIGN

The design of post-tensioned masonry is based on allowable stress design procedures, except for laterally restrained tendons which use a strength design philosophy. Building Code Requirements for Masonry Structures (ref. 1) prescribes allowable stresses for unreinforced masonry in compression, tension and shear, which must be checked against the stresses resulting from applied loads.

The flexural strength of post-tensioned walls is governed by either the flexural tensile stress of the masonry (the flexural stress minus the post-tensioning and dead load stress), the masonry compressive stress, the tensile stress within the tendon, the shear capacity of the masonry or the buckling capacity of the wall.

Masonry stresses must be checked at the time of peak loading (independently accounting for both short-term and long-term losses), at the transfer of post-tensioning forces, and during the jacking operation when bearing stresses may be exceeded. Immediately after transfer of the post-tensioning forces, the stresses in the steel are the largest because long-term losses have not occurred. Further, because the masonry has had little time to cure, the stresses in the masonry will be closer to their capacity. Once long-term losses have transpired, the stresses in both the masonry and the steel are reduced. The result is a coincidental reduction in the effective capacity due to the prestressing force and an increase in the stresses the fully cured masonry can resist from external loads.

Effective Prestress

Over time, the level of prestressing force decreases due to creep and shrinkage of the masonry, relaxation of the prestressing tendons and potential decreases in the ambient temperature. These prestressing losses are in addition to seating and elastic shortening losses witnessed during the prestressing operation. In addition, the prestressing force of bonded tendons will decrease along the length of the tendon due to frictional losses. Since the effective prestressing force varies over time, the controlling stresses should be checked at several stages and loading conditions over the life of the structure.

The total prestress loss in concrete masonry can be assumed to be approximately 35%. At the time of transfer of the prestressing force, typical losses include: 1% seating loss + 1% elastic shortening = 2%. Additional losses at service loads and moment strength include:

relaxation	3%
temperature	10%
creep	8%
CMU shrinkage	7%
contingency	5%
total	33%

Prestress losses need to be estimated accurately for a safe and economical structural design. Underestimating losses will result in having less available strength than assumed. Overestimating losses may result in overstressing the wall in compression.

Effective Width

In theory, a post-tensioning force functions similarly to a concentrated load applied to the top of a wall. Concentrated loads are distributed over an effective width as discussed in the commentary on Building Code Requirements for Masonry Structures (ref. 1). A general rule-of-thumb is to use six times the wall thickness as the effective width.

Elastic shortening during post-tensioning can reduce the stress in adjacent tendons that have already been stressed. Spacing the tendons further apart than the effective width theoretically does not reduce the compressive stress in the effective width due to the post-tensioning of subsequent tendons. The applied loads must also be consolidated into the effective width so the masonry stresses can be determined. These stresses must be checked in the design stage to avoid overstressing the masonry.

Flexure

Tensile and compressive stresses resulting from bending moments applied to a section are determined in accordance with conventional elastic beam theory. This results in a triangular stress distribution for the masonry in both tension and compression. Maximum bending stress at the extreme fibers are determined by dividing the applied moment by the section modulus based on the minimum net section.

Net Flexural Tensile Stress

Sufficient post-tensioning force needs to be provided so the net flexural tensile stress is less than the allowable values. Flexural cracking should not occur if post-tensioning forces are kept within acceptable bounds. Flexural cracking due to sustained post-tensioning forces is believed to be more severe than cracking due to transient loading. Flexural cracks due to eccentric post-tensioning forces will remain open throughout the life of the wall, and may create problems related to water penetration, freeze-thaw or corrosion. For this reason, Building Code Requirements for Masonry Structures (ref. 1) requires that the net flexural tensile stress be limited to zero at transfer of the post-tensioning force and for service loadings with gravity loads only.

Axial Compression

Compressive stresses are determined by dividing the sum of the post-tensioning and gravity forces by the net area of the section. They must be less than the code prescribed (ref. 1) allowable values of axial compressive stress.

Walls must also be checked for buckling due to gravity loads and post-tensioning forces from unrestrained tendons. Laterally restrained tendons can not cause buckling; therefore only gravity compressive forces need to be checked for buckling in walls using laterally restrained tendons. Restraining the tendons also ensures that the tendons do not move laterally in the wall when the masonry deflects. The maximum compressive force that can be applied to the wall based upon ¼ buckling is P_e, per equation 2-11 of Building Code Requirements for Masonry Structures (ref. 1).

Combined Axial and Flexural Compressive Stress

Axial compressive stresses due to post-tensioning and gravity forces combine with flexural compressive stresses at the extreme fiber to result in maximum compressive stress. Conversely, the axial compressive stresses combine with the flexural tensile stresses to reduce the absolute extreme fiber stresses. To ensure the combination of these stresses does not exceed code prescribed allowable stresses, a unity equation is checked to verify compliance. Employing this unity equation, the sum of the ratios of applied-to-allowable axial and flexural stresses must be less than one. Unless standards (ref. 5) limit its use, an additional one-third increase in allowable stresses is permitted for wind and earthquake loadings, as is customary with unreinforced and reinforced masonry. Further, for the stress condition immediately after transfer of the post-tensioning force, a 20% increase in allowable axial and bending stresses is permitted by Building Code Requirements for Masonry Structures (ref. 1).

Shear

As with all stresses, shear stresses are resisted by the net area of masonry, and the wall is sized such that the maximum shear stress is less than the allowable stress. In addition, the compressive stress due to post-tensioning can be relied on to increase allowable shear stresses in some circumstances.

Post-Tensioning Tendons

The stress in the tendons is limited (ref. 1) such that:

the stress due to the jacking force does not exceed 0.94f_py, 0.80f_pu, nor that recommended by the manufacturer of the tendons or anchorages,
the stress immediately after transfer does not exceed 0.82f_py nor 0.74f_pu, and
the stress in the tendons at anchorages and couplers does not exceed 0.78f_py nor 0.70f_pu.

DETERMINATION OF POST-TENSIONING FORCES

Case (a) after prestress losses and at peak loading:

Assuming that the moment, M, due to wind or earthquake loadings is large relative to the eccentric load moment, the critical location will be at the mid-height of the wall for simply-supported walls, and the following equations apply (bracketed numbers are the applicable Building Code Requirements for Masonry Structures (ref. 1) equation or section numbers):

The 1.33 factor in Equation [2-10] represents the one- third increase in allowable stress permitted for wind and earthquake loadings. If the moment, M, is a result of soil pressures (as is the case for retaining walls), the 1.33 factor in Equation [2-10] must be replaced by 1.00.

Note that if the tendons are laterally restrained, P_pf should not be included in Equation [2-11].

(under the load combination of prestressing force and dead load only)

Additional strength design requirements for laterally restrained tendons:

Equation 4-3 above applies to members with uniform width, concentric reinforcement and prestressing tendons and concentric axial load. The nominal moment strength for other conditions should be determined based on static moment equilibrium equations.

Case (b) at transfer of post-tensioning:

Assuming that vertical live loads are not present during post-tensioning, the following equations apply. The worst case is at the top of the wall where post-tensioning forces are applied.

For cantilevered walls, these equations must be modified to the base of the wall.

If the eccentricity of the live load, P_l, is small, neglecting the live load in Equation [2-10] may also govern.

Case (c) bearing stresses at jacking:

Bearing stresses at the prestressing anchorage should be checked at the time of jacking. The maximum allowable bearing stress at jacking is 0.50f’_mi per Building Code Requirements for Masonry Structures (ref. 1) section 4.9.4.2.

DESIGN EXAMPLE

Design a simply-supported exterior wall 12 ft (3.7 m) high for a wind load of 15 psf (0.72 kPa). The wall is constructed of concrete masonry units complying with ASTM C 90 (ref. 6). The units are laid in a full bed of Type S Portland cement lime mortar complying with ASTM C 270 (ref. 7). The specified compressive strength of the masonry (f’_m) is 1,500 psi (10.3 MPa). The wall will be post-tensioned with ⁷/₁₆ in. (11 mm) diameter laterally restrained tendons when the wall achieves a compressive strength of 1,250 psi (8.6 MPa). Axial load and prestress are concentric.

Given:
8 in. (203 mm) CMU
t_f = 1.25 in. (32 mm)
f’_m = 1,500 psi (10.3 MPa)
f’_mi = 1,250 psi (8.6 MPa)
F_bt = 25 psi (0.17 MPa) (Type S Portland cement/lime mortar)
f_py = 100 ksi (690 MPa) (bars)
f_pu = 122 ksi (840 MPa)
A_ps = 0.14 in² (92 mm²)
E_s = 29 x 10⁶ psi (200 GPa)
E_m = 900 f’_m = 1.35 x 10⁶ psi (9,300 MPa)
n = E_s/E_m = 21.5
d = 7.625/2 in. = 3.81 in. (97 mm) (tendons placed in the center of the wall)
unit weight of CMU wall = 39 psf (190 kg/m²) (ref. 13)

Loads: M = wh²/8 = (15)(12)²/8 = 270 ft-lb (366 N-m)
P_d at mid-height = (39)(12)/2 = 234 lb/foot of wall (3,410 N/m) (P_l = 0)

Maximum tendon stresses:
Determine governing stresses based on code limits (ref. 1):

At jacking:	0.94 f_py = 94.0 ksi (648 MPa)
	0.80 f_pu = 97.6 ksi (673 MPa)
At transfer:	0.82 f_py = 82.0 ksi (565 MPa)
	0.74 f_pu = 90.3 ksi (623 MPa)
At service loads:	0.78 f_py = 78.0 ksi (538 MPa) ⇒ governs
	0.70 f_pu = 85.4 ksi (589 MPa)

Because the tendon’s specified tensile strength is less than 150 ksi (1,034 MPa), f_ps = f_se (per ref. 1 section 4.5.3.3.4).

Prestress losses: Assume 35% total loss (as described in the Effective Prestress section above).

Tendon forces:
Determine the maximum tendon force, based on the governing tendon stress determined above for each case of jacking, transfer and service. At transfer, include 2% prestress losses. At service, include the full 35% losses.
Tendon capacity at jacking = 0.94 f_pyA_ps = 13.3 kips (59 kN)
Tendon capacity at transfer = 0.82 f_pyA_ps A x 0.98 = 11.4 kips (51 kN) (including transfer losses)
Tendon capacity at service = 0.78 f_pyA_ps A x 0.65 = 7.2 kips (32 kN) (including total losses)

Try tendons at 48 in. (1,219 mm) on center (note that this tendon spacing also corresponds to the maximum effective prestressing width of six times the wall thickness).

Determine prestressing force, based on tendon capacity determined above:
at transfer: P_pi = 11.4 kips/4 ft = 2,850 lb/ft (41.6 kN/m)
at service: P_pf = 7.2 kips/4 ft = 1,800 lb/ft (26.3 kN/m)

Wall section properties: (ref. 8)
8 in. (203 mm) CMU with full mortar bedding:
A_n = 41.5 in.²/ft (87,900 mm²/m)
I = 334 in.⁴/ft (456 x 10⁶ mm⁴/m)
S = 87.6 in.³/ft (4.71 x 10⁶ mm³/m)
r = 2.84 in. (72.1 mm)

At service loads:
At service, the following are checked: combined axial compression and flexure using the unity equation (equation 2-10); net tension in the wall; stability by ensuring the compressive load does not exceed one-fourth of the buckling load, P_e, and shear and moment strength.

Check combined axial compression and flexure:

Check tension for load combination of prestress force and dead load only (per ref. 1 section 4.5.1.3):

Check stability:
Because the tendons are laterally restrained, the prestressing force, P_pf, is not considered in the determination of axial load ( per ref. 1 section 4.5.3.2), and the wall is not subject to live load in this case, so equation 2-11 reduces to:

Check moment strength:
Building Code Requirements for Masonry Structures section 4.5.3.3 includes the following criteria for moment strength of walls with laterally restrained tendons:

In addition, the compression zone must fall within the masonry, so a < t_f.

where 1.3 and 1.2 are load factors for wind and dead loads, respectively.

At transfer:
Check combined axial compression and flexure using the unity equation (equation 2-10) and net tension in the wall.

Check tension for load combination of prestress force and dead load only (per ref. 1 section 4.5.1.3):

Therefore, use ⁷/₁₆ in. (11 mm) diameter tendons at 48 in. (1,219 mm) o.c. Note that although wall design is seldom governed by out-of-plane shear, the shear capacity should also be checked.

NOTATIONS

A_n net cross-sectional area of masonry section, in.² (mm²)
A_ps threaded area of post-tensioning tendon, in.² (mm²)
A_s cross-sectional area of mild reinforcement, in.² (mm²)
a depth of an equivalent compression zone at nominal strength, in. (mm)
b width of section, in. (mm)
d distance from extreme compression fiber to centroid of prestressing tendon, in. (mm)
E_s modulus of elasticity of prestressing steel, psi (MPa)
E_m modulus of elasticity of masonry, psi (MPa)
e_d eccentricity of dead load, in. (mm)
e_l eccentricity of live load, in. (mm)
e_p eccentricity of post-tensioning load, in. (mm)
F_a allowable masonry axial compressive stress, psi (MPa)
F_ai allowable masonry axial compressive stress at transfer, psi (MPa)
F_b allowable masonry flexural compressive stress, psi (MPa)
F_bi allowable masonry flexural compressive stress at transfer, psi (MPa)
F_bt allowable flexural tensile strength of masonry, psi (MPa)
f_a axial stress after prestress loss, psi (MPa)
f_ai axial stress at transfer, psi (MPa)
f_b flexural stress after prestress loss, psi (MPa)
f_bi flexural stress at transfer, psi (MPa)
f’_m specified compressive strength of masonry, psi (MPa)
f’_mi specified compressive strength of masonry at time of transfer of prestress, psi (MPa)
f_ps stress in prestressing tendon at nominal strength, psi (MPa)
f_pu specified tensile strength of prestressing tendon, ksi (MPa)
f_py specified yield strength of prestressing tendon, ksi (MPa)
f_se effective stress in prestressing tendon after all pre-stress losses have occurred, psi (MPa)
f_y specified yield strength of steel for reinforcement and anchors, psi (MPa)
h masonry wall height, in. (mm)
I moment of inertia of net wall section of extreme fiber tension or compression, in.⁴/ft (mm⁴/m)
M moment due to lateral loads, ft-lb (N⋅m)
M_n nominal moment strength, ft-lb (N⋅m)
M_u factored moment due to lateral loads, ft-lb (N⋅m)
n modular ratio of prestressing steel and masonry (E_s/E_m)
P_d axial dead load, lb/ft (kN/m)
P_du factored axial dead load, lb/ft (kN/m)
P_e Euler buckling load, lb/ft (kN/m)
P_l axial live load, lb/ft (kN/m)
P_lu factored axial live load, lb/ft (kN/m)
P_pi prestress force at transfer, lb/ft (kN/m)
P_pf prestress force including losses, lb/ft (kN/m)
r radius of gyration for net wall section, in. (mm)
S section modulus of net cross-sectional area of the wall, in.³ /ft (mm³/m)
t_f face shell thickness of concrete masonry, in. (mm)
w applied wind pressure, psf (kPa)
¤ strength reduction factor = 0.8

REFERENCES

Building Code Requirements for Masonry Structures, ACI 530-02/ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee, 2002.
Building Code Requirements for Structural Concrete, ACI 318-99. Detroit, MI: American Concrete Institute, Revised 1999.
Construction of Post-Tensioned Concrete Masonry Walls, TEK 03-14. Concrete Masonry & Hardscapes Association, 2002.
International Building Code. International Code Council, 2000.
Minimum Design Loads for Buildings and Other Structures, ASCE 7-98, American Society of Civil Engineers, 1998.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90-01a. American Society for Testing and Materials, 2001.
Standard Specification for Mortar for Unit Masonry, ASTM C 270-01. American Society for Testing and Materials, 2001.
Weights and Section Properties of Concrete Masonry Assemblies, CMU-TEC-002-23, Concrete Masonry & Hardscapes Association, 2023.
Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, CMU-TEC-001-23, Concrete Masonry & Hardscapes Association, 2023.
Mortars for Concrete Masonry, TEK 09-01A. Concrete Masonry & Hardscapes Association, 2001.
Grout for Concrete Masonry, TEK 09-04. Concrete Masonry & Hardscapes Association, 2005.
Steel for Concrete Masonry Reinforcement, TEK 12-04D. Concrete Masonry & Hardscapes Association, 1998.
Weights and Section Properties of Concrete Masonry Assemblies, CMU-TEC-002-23, Concrete Masonry & Hardscapes Association, 2023.
Schultz, A.E., and M.J. Scolforo, An Overview of Prestressed Masonry, TMS Journal, Vol. 10, No. 1, August 1991, pp. 6-21.
Schultz, A.E., and M.J. Scolforo, Engineering Design Provisions for Prestressed Masonry, Part 1: Masonry Stresses, Part 2: Steel Stresses and Other Considerations, TMS Journal, Vol. 10, No. 2, February 1992, pp. 29-64.
Standard Specification for Steel Strand, Uncoated Seven-Wire for Prestressed Concrete, ASTM A 416-99. American Society for Testing and Materials, 1999.
Standard Specification for Uncoated Stress-Relieved Steel Wire for Prestressed Concrete, ASTM A 421-98a. American Society for Testing and Materials, 1998.
Standard Specification for Uncoated High-Strength Steel Bar for Prestressed Concrete, ASTM A 722-98. American Society for Testing and Materials, 1998.
Standard Specification for Compressible-Washer-Type Direct Tension Indicators for Use with Structural Fasteners, ASTM F 959-01a. American Society for Testing and Materials, 2001.

Evaluating Fire-Exposed Concrete Masonry Walls After a Fire

August 2018

INTRODUCTION

Fire safety requires that a wall not only halt the spread of fire from one area to another, but also retain its structural integrity throughout the fire and fire-fighting operations. If occupants, firefighters and building contents are to be fully protected, the structure must not collapse, add fuel to the fire nor emit toxic gases during the fire.

Concrete masonry fire walls provide maximum safety during and after severe fire exposure. Because concrete masonry is a noncombustible structural material which neither adds fuel to a fire nor emits toxic gases, it is widely used to provide compartmentation—containing a fire until it can be brought under control by fire fighters. In addition, even after severe fires, concrete masonry walls can typically be repaired by simply patching cracks and tuckpointing mortar joints, rather than requiring demolition and replacement. Experience with building fires has shown that the most damage to concrete masonry walls during a fire often occurs due to lost support rather than as a direct result of fire on the masonry.

This TEK provides general information on assessment methods and repair techniques and discusses what can be expected after concrete masonry walls have been subjected to fire.

EVALUATING FIRE-EXPOSED WALLS

Preliminary Inspection

After a fire occurs, a preliminary inspection should be conducted as soon as possible to assess: the condition of the structure, the type and severity of problems observed in the affected area(s), the feasibility of rehabilitation and the need for conducting a detailed investigation. After collecting data on the building structure and the fire event, the preliminary investigation should take place as soon as safe entry into the building can be arranged.

The first step in the preliminary investigation is a visual inspection of structural members in the fire-affected areas. Indications of cracking, spalling, deflections, distortions, misalignment of elements and/or exposure of steel reinforcement should be documented. Measurements of deflections, deformations and geometry can be taken of any suspect members for comparison to unexposed members in the same structure. These observations should be recorded, documenting the type of damage and its severity for each affected member. This summary helps identify damaged members in need of more detailed investigation, as well as the extent and nature of any necessary repairs.

As an adjunct to visually assessing the structural members in fire affected areas, the building contents in these areas should be observed. The melting points of various materials (see Table 1) indicate the temperature ranges that have occurred in localized areas, providing an estimate of the maximum temperatures achieved during the fire. These estimated maximum temperatures help establish the severity of the fire relative to the Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E 119 (ref. 2) fire test, or to another recognized baseline. If the maximum temperatures during the fire are similar to those of the E 119 test, the potential damage to the concrete masonry is somewhat predictable, based on the history of E 119 testing on concrete masonry.

The ASTM E 119 fire test time/temperature protocol is shown in Figure 1.

There is a large body of data on concrete masonry walls tested according to the ASTM E 119 protocol. This test method evaluates walls subjected to the standard test fire. Performance criteria include: resistance to temperature rise on the unexposed side of the wall; resistance to the passage of hot gases or flames through the wall; structural stability during the test; and resistance of the masonry to deterioration under direct exposure to a fire hose stream immediately following the fire test. Research has shown that the fire resistance ratings of concrete masonry walls are invariably determined by the temperature rise on the cold (unexposed) side of the wall.

Field Testing Procedures

Part of the preliminary inspection is determining the need for further testing and evaluation. Nondestructive field tests, such as the use of an impact rebound hammer, are typically not used with concrete masonry, as the hollow cells interfere with obtaining meaningful results in many cases. In addition, extensive field testing is not always prudent, as removal and replacement of the fire-damaged element can sometimes be more economical than extensive testing. A solid understanding of both structural engineering and the effects of fire on building materials is invaluable to this decision-making process. When necessary, destructive test methods can be used to assess the strength of the in-situ concrete masonry (see reference 3). However, nonuniform fire damage on opposite sides of the wall and corresponding strength differences can lead to unreliable results. In most cases, strength testing is unnecessary.

ASSESSING THE CONCRETE MASONRY

In general, structural concrete masonry without excessive deformations, deflections, misalignments or large extensive cracks can typically be repaired rather than replaced. When these types of conditions are present, it indicates that the member’s load-carrying capacity may be impaired, which would require removal and replacement of the suspect members.

Fire distress such as soot and smoke deposits, pitting of aggregates, hairline cracks, shallow spalling and other surface damage generally require only cosmetic repairs. The following sections provide some more detailed guidance on assessing various concrete masonry characteristics after a fire.

Cracks

Cracks wider than about 1/16 in. (1.6 mm) should be further investigated to determine the potential structural impact. When the reinforcing steel in conventionally-reinforced masonry has not been exposed, the load-carrying capacity of the wall can typically be assumed to be relatively unaffected (see also Exposed Reinforcement, below).

Mortar Joint Damage

Mortar joints often appear to be more affected by fire exposure than the adjacent surface of the masonry units. When concrete masonry walls are subjected to a fire hose immediately after fire exposure in ASTM E 119 testing, mortar in the dehydrated state is sometimes flushed out, typically to a depth of about 1/4 in. (6.7 mm). In actual fires, mortar joints subjected to the most severe fire exposures can become softened or chalky, although this damage is typically not deeper than about 3/4 in. (19 mm). However, this loss of mortar does not affect the load-carrying ability of the concrete masonry wall (ref. 4), so can most often be adequately repaired by tuckpointing.

Exposed Reinforcement

Reinforcement exposed during or after a fire must be evaluated for quenching, buckling and/or loss of prestress. The investigator must consider that any exposed steel may have been quenched during fire fighting operations. This rapid cooling causes a loss of ductility in the steel that can reduce the load-carrying capacity of the member. A visual inspection of any exposed structural reinforcement can help asses the potential structural damage. This assessment must be tied to the element under consideration: either a conventionally-reinforced wall or prestressed wall, as follows. In a conventionally-reinforced wall, local buckling of exposed reinforcing bars usually indicates that the steel has been directly exposed to fire. When steel is exposed to temperatures of 1,100 o F (593 o C) or higher, the bars lose about half of their yield strength and buckling occurs. If the bars are exposed but not buckled or otherwise deformed, spalling may have occurred after the fire exposure. In general, flexural reinforcement that is not visibly deformed is unlikely to have suffered significant permanent damage. Similarly, if the spalling does not expose the reinforcement, i.e. the cover protection remains intact, the wall strength is unlikely to be compromised.

In prestressed concrete masonry walls, on the other hand, significant loss of prestress can occur without any visible distress to exposed tendons. Therefore, for prestressed masonry, any exposed prestressing tendons should indicate the need for a more in-depth structural evaluation. Tendon buckling is rarely observed, as the tendon typically remains in tension, even with significant loss of prestress.

EFFECT OF FIRE EXPOSURE ON WALL STRENGTH—EXPERIMENTAL RESULTS

One effect of fire exposure, as determined by testing (ref. 4), was reduced wall compressive strength due to the gradual dehydration of the cement and, depending on the aggregate type, to the expansion and changes in the physical properties of the aggregate used in the concrete masonry units. Reductions in compressive strength for 8-in. (203-mm) units exposed to 3 to 3 1/2 hours of fire varied widely, resulting in maximum reductions of 50 percent for some types of concrete masonry units. Lightweight aggregates, manufactured by expanding certain minerals in a kiln, are stable under fire exposure, so minimize loss of strength. During testing, limestone aggregate concrete masonry units also showed substantial stability and minimized loss of strength after fire exposure (ref. 4). For the wide range of masonry units tested, the wall strength after fire exposure remained directly proportional to the concrete masonry unit compressive strength before fire exposure.

A number of 8-in. (203-mm) walls underwent 2 1/2 to 3 1/2 hours of fire exposure, were cooled, then subjected to another 2 1/2 hour fire before being tested for compressive strength. These results showed that these walls were able to carry the same, or slightly higher, loads as similar walls exposed once for three to four hours, as well as serving as an effective fire barrier during the second fire.

PREPAIRING FIRE-EXPOSED CONCRETE MASONRY

For fire-exposed concrete masonry free from large cracks or deflections, repairs should be minimal. Crack repair and mortar joint tuckpointing procedures and recommendations are covered in detail in Maintenance of Concrete Masonry Walls, TEK 08-01A (ref. 5). Recommended cleaning procedures are covered in Cleaning Concrete Masonry, TEK 08-04A (ref. 6).

SUMMARY

In conventionally-reinforced concrete masonry, if reinforcing steel is not exposed, there is little likelihood of structural damage.
Lintels and beams free from excessive deflections are unlikely to be structurally impaired.
Softening of the top surface of mortar results in little loss of load carrying capacity and can be easily repaired by tuckpointing.
Walls subjected to fire one time without structural damage can be expected to perform just as well in a second fire.
Field tests are typically not conducted to assess fire damaged concrete masonry walls. Post-fire investigation typically consists only of visual inspection.
If no severe distortion, cracking or displacement of concrete masonry walls is present, complete reinstatement of the wall can usually be accomplished by patching cracks and tuckpointing mortar joints.

REFERENCES

Assessing the Condition and Repair Alternatives of Fire-Exposed Concrete and Masonry Members. National Codes and Standards Council of the Concrete and Masonry Industries, August, 1994.
Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E 119-05. ASTM International, 2005.
Evaluating Existing Concrete Masonry Construction, TEK 18-09A. Concrete Masonry & Hardscapes Association, 2003.
Menzel, Carl A. Tests of the Fire Resistance and Strength of Walls of Concrete Masonry Units. Portland Cement Association, January, 1934.
Maintenance of Concrete Masonry Walls, TEK 08-01A. Concrete Masonry & Hardscapes Association, 2004.
Cleaning Concrete Masonry, TEK 08-04A. Concrete Masonry & Hardscapes Association, 2005.

Post-Tensioned Concrete Masonry Wall Construction

August 2018

INTRODUCTION

Prestressing is the general term used when a structural element is compressed prior to being subjected to building loads. This initial state of compression offsets tensile stresses from applied loads. Post-tensioning is a specific method of prestressing where tendons are stressed after the wall has been placed. The other type of prestressing, called pretensioning, involves tensioning the tendon prior to construction of the masonry. Because virtually all prestressed masonry built to date has been post-tensioned, the two terms are often used interchangeably as they apply to this form of masonry design and construction.

Post-tensioned concrete masonry walls have been built for schools, retail, manufacturing, highway sound barriers, warehouses and other types of structures. In addition, posttensioning has been used to strengthen and repair existing masonry walls. This TEK addresses new concrete masonry walls laid in running bond and built with unbonded vertical posttensioning tendons. Post-Tensioned Concrete Masonry Wall Design, TEK 14-20A (ref. 1) addresses the structural design of vertically post-tensioned walls.

POST-TENSIONING

In post-tensioned construction, hollow concrete masonry units are laid conventionally and prestressing tendons are either placed in the concrete masonry cells or in the cavity between multiple wythes. Current design codes (ref. 3) typically address post-tensioning of masonry walls laid in running bond. The cells or cavity containing the tendons may or may not be grouted. Grouting helps increase cross-sectional area for shear and compressive resistance, but increases construction cost and time.

Prestressing tendons are either installed during wall construction, or access ports are left in the walls so the tendons can be slipped in after the walls are completed. In either case, the tendons are tensioned only after the walls have cured for approximately three to seven days.

MATERIALS

Construction of a post-tensioned wall proceeds similarly to that of conventional masonry. The materials are the same, with the addition of hardware to develop the posttensioning forces, steel prestressing tendons which can be wires, bars or strands, and sometimes prestressing grout.

Concrete Masonry Units

Open-ended (Aand H-shaped) concrete masonry units (Figure 1) are particularly suited to post-tensioned masonry, as these units can be placed around the tendons without having to lift the units over the tendons. While these two-core units are commonly used, proprietary units are also being developed that are specifically intended for use with tendons.

The net area strength of concrete masonry units must be at least 1,900 psi (13.1 MPa) per Standard Specification for Loadbearing Concrete Masonry Units (ref. 2). However, stronger units are often specified for post-tensioned walls to utilize the higher compressive strength.

Mortar and Grout

Type S mortar is commonly used for conventional loadbearing masonry, and Type S is a good choice for posttensioned masonry as well. Higher early strength mortars can accommodate earlier stressing.

Because mortar must be placed on concrete masonry webs adjacent to grouted cores to contain the fluid grout, full mortar bedding is sometimes specified when grout is used. Mortar bedding is a design issue as well, as the section properties of a wall with face shell mortar bedding are different from those of a fully bedded wall.

Because this TEK addresses unbonded tendons only, the grout discussed here is conventional grout (ASTM C 476, ref. 6), not prestressing grout. Prestressing grout is only used with bonded tendons. Encasing tendons in conventional grout restrains the tendons, but they are still considered unbonded.

Tendons

In the United States, tendons are usually high-strength bars joined by couplers, although Building Code Requirements for Masonry Structures (ref. 3) also allows steel strands or wires to be used. Couplers allow the use of shorter bars which minimizes the height of lifting. To date, there are no code provisions for tendons which are not steel.

Important features of the tendons are their size, strength, and relaxation characteristics. Most tendons currently available in the United States are bars between 7/16 and 1 in. (11 and 25 mm) in diameter, with strengths between 60,000 and 100,000 psi (413 and 690 MPa), depending on the supplier. Steel strand tendons are generally 270,000 psi (1,860 MPa). Tendons are usually placed in hollow cells of masonry units with little or no grouting, except for certain shear walls (these must be identified on the design drawings). In addition, the open-ended units shown in Figure 1 must be grouted to meet minimum web requirements in ASTM C 90 (ref. 2).

Tendon Corrosion Protection

Tendons must be protected from moisture deterioration, and the design documents should indicate the type of protection required. Tendons in walls with a likelihood of high moisture levels (single wythe exterior walls in areas of high humidity and interior walls around swimming pools, locker rooms, etc.) must have corrosion protection in addition to that provided by the masonry cover, such as hot-dipped galvanizing (ref. 3). In practice, most prestressing tendons are supplied with a hotdipped galvanized coating. It is considered good practice to use additional corrosion protection, such as flexible epoxy-type coatings, for tendons in moist environments.

Grouting

While the need for grouting is minimized compared to conventionally reinforced walls, grout is still needed for mild reinforcement, anchorages for the tendons, such as in bond beams, and tendon restraints.

Anchorages

Each tendon is anchored at the foundation and extends to the top of the wall. Building Code Requirements for Masonry Structures (ref. 3) requires that tendons be anchored by mechanical embedments or bearing devices or by bond development in concrete. Tendons can not be anchored by bond development into the masonry. The foundation anchorage is embedded in the wall or footing while the top anchorage utilizes a special block, a precast concrete spreader beam or a grouted bond beam.

Unless the design documents call out specific bottom anchors, the contractor must select the anchor appropriate to the conditions. The cast-in-place bottom anchor (Figure 2a) is preferred for shear walls and for fire walls. While they are the best anchors for capacity, cast-in-place anchors are the most difficult to align. Cast-in-place anchors are often set by the foundation contractor, not the mason. Thus, quality control is a concern with these anchors.

The mason controls bottom anchor placement when either adhesive anchors are installed in the foundation (Figure 2c), or when an anchor is used which does not rely on the foundation for support (Figure 2b). If the anchor in Figure 2b is used, foundation dowels are grouted into the wall to lock it in place. In some instances, tendons can also begin at an upper floor and not at the foundation. In this case, the foundationless anchor is used with a bond beam, similar to Figure 2b.

The mechanical post-installed anchors can be used for nearly all applications, while the adhesive type should not be used for fire walls.

CONSTRUCTION

Key steps of post-tensioning concrete masonry walls include: selecting and setting the bottom anchorages; installing the tendons; selecting and setting the top anchorages; and a tensioning the tendons.

Bottom Anchors

Bottom anchors are most critical to the proper construction of post-tensioned walls. Alignment is essential to ensure that the tendons are placed exactly as intended.

Tendons

Tendons are usually placed concentric with the wall. However, they may be placed off-center to counteract bending moments due to eccentric vertical forces or lateral forces from a single direction. However, tendons should not be placed such that tensile stresses develop in the wall due to the combination of prestressing force and dead load.

Laterally-unrestrained tendons are free to move within the cell or cavity and are the simplest to construct. Laterally restrained tendons are not free to move within a cell or cavity. Restraint is accomplished by grouting the full height of the tendon or by providing intermittent restraints—either grout plugs or mechanical restraints—at the quarter points of the wall height.

Placing tendons is much like that of mild reinforcement. They may be installed after the masonry is constructed provided the design allows laterally-unrestrained tendons. If laterally restrained tendons are required, the tendon placement should proceed simultaneously with the masonry to allow the restraints to be installed unless the cells will be grouted.

Tendon positioners (see Figure 3) are useful to maintain the tendon location within the wall during construction of the masonry. Positioners may also function as restraints if their capacity is determined by testing.

In all details, the tendons must be able to slip freely. If grout encases the tendon either totally or at restraints or bond beams, a bond breaker such as poly tape should be used to allow the tendon to slip.

Tendons can also be either bonded or unbonded. Bonded tendons are encapsulated by prestressing grout in a corrugated duct which is bonded to the surrounding masonry by grout. Both the prestressing grout inside the duct and the grout around the duct must be cured before the tendons are stressed. Thus, bonded tendons are also laterally-restrained. All other tendons are unbonded. However, unbonded tendons may be either laterally-restrained or unrestrained. Walls with laterally unrestrained and unbonded tendons do not require grouting and are generally the most economical to construct. However, the wall performance will not be as good as with laterally restrained tendons. The designer must specify which system will be used.

For some conditions, primarily seismic, grouted conventional reinforcement is used in addition to post-tensioning tendons to provide minimum requirements of bonded reinforcement. However, post-tensioned walls are most economical when the grouting is minimized or eliminated totally in comparison to a conventionally reinforced wall. The higher cost of the post-tensioning materials is more than offset by the savings of placing fewer tendons compared to reinforcing bars and eliminating most of the grouting.

Top Anchors

The top anchor must be placed on solid masonry, a grouted bond beam or a precast concrete unit. The anchor should not be supported by mortar.

Figure 4 shows a means for supporting the top of a wall when the top anchor is placed on a bond beam in a lower course. This detail can also be used for interior partitions.

Tensioning

At the time the tendons are stressed, the masonry is considered to have its initial strength (f ‘mi). The project specification should include either the minimum f ‘mi and minimum specified compressive strength of masonry ( f ‘m), or the amount of curing required before stressing can occur.

The sequence of tensioning, whether it is accomplished by fully stressing each tendon sequentially or by stressing the tendons in stages, is a function of the design specifications. Prestressed masonry design, and therefore the structural integrity of these walls, relies on an accurate measure of the prestress in the tendons. To ensure the required level of accuracy, Specification for Masonry Structures (ref. 4) requires that the following two methods be used to evaluate the tendon prestressing force:

1. measure the tendon elongation and compare it with required elongation based on average load-elongation curves for the prestressing tendons, and either:

2a. use a calibrated dynamometer to measure the jacking force on a calibrated gage, or

2b. for prestressing tendons using bars of less than 150 ksi (1,034 MPa) tensile strength, use load-indicating washers complying with Standard Specification for Compressible-Washer-Type Direct Tension Indicators for Use with Structural Fasteners, ASTM F 959 (ref. 5). If the two values determined by methods 1 and 2 are not within 7 percent of each other, the cause of the difference must be corrected.

QUALITY ASSURANCE

Post-tensioned walls must be constructed in conformance with masonry standards applicable to conventionally reinforced masonry. In addition to these, Specification for Masonry Structures (ref. 4) requires the following for posttensioned masonry:

In the out-of-plane direction, the tolerance for the tendon placement shall be + 1/4 in. (6 mm) for masonry beams, columns, walls, and pilasters with cross-sectional dimensions less than 8 in. (203 mm). For cross-sectional dimensions greater than 8 in. (203 mm), the tolerance increases to + 3/8 in. (10 mm).
In the in-plane direction, the tolerance for tendon placement is +1in. (25 mm).
If tolerances exceed these amounts, the Architect/Engineer should evaluate the effect on the structure.

REFERENCES

Post-Tensioned Concrete Masonry Wall Design, TEK 14-
20A. Concrete Masonry & Hardscapes Association, 2002.
Standard Specification for Loadbearing Concrete Masonry
Units, ASTM C 90-01a. ASTM International, 2001.
Building Code Requirements for Masonry Structures, ACI
530-02/ASCE 5-02/TMS 402-02. Reported by the Masonry
Standards Joint Committee, 2002.
Specification for Masonry Structures, ACI 530.1-02/ASCE
6-02/TMS 602-02. Reported by the Masonry Standards
Joint Committee, 2002.
Standard Specification for Compressible-Washer-
Type Direct Tension Indicators for Use with Structural
Fasteners, ASTM F 959-01a. ASTM International, 2001.
Standard Specification for Grout for Masonry, ASTM C
476-01. ASTM International, 2001.