aggregate particles move like ball bear-ings, lubricated by the surroundingcement paste and by other plasticizingingredients, such as hydrated lime orentrained air. Some indications of work-ability are:

� The mortar spreads easily with thetrowel.

� The mortar supports the weight ofthe masonry units.

� The mortar adheres to masonry sur-faces (is sticky).

� The mortare x t r u d e sreadily fromt h e j o i n twhen them a s o napplies pres-sure to theunit.

In thefield, themason mea-sures workabilityby the responseof the mortar tothe trowel. In thelaboratory,we mea-sure work-ability withs tandard-ized testso f w a t e rr e t e n t i v i t y,flow, consis-tency, plastici-ty, cohesion(ability to sticktogether), and adhe-

sion (ability to stick to other materials).Factors that affect mortar workabilityinclude air content, lime content, sizesand shapes of sand particles, and amountof water.

Water-retentivity is the ability of themortar to resist rapid loss of mixing waterto air and to absorptive masonry units.If mortar does not have good water-reten-tivity, it stiffens quickly, making it very difficult to obtain water-resistant mortar

B Y R O C H E L L E C . J A F F E

Before specifying mortarfor a project or using it inthe field, it is important tounderstand why the per-formance of mortar isvital to the successful per-

formance of masonry and how the ingre-dients in mortar affect its performance.Mortar is the material that binds togethermasonry units and binds joint reinforce-ment and connectors to the masonry units.Mortar also is used as a spacer betweenmasonry units and as a means of levelingand plumbing the units.

More importantly, mortar plays a cru-cial role in the resistance of masonry towater penetration. In a typical claymasonry wall (or in a water-repellent-treated concrete masonry wall), waterdoes not significantly permeate themasonry units or the mortar itself.However, water may penetrate the inter-face between mortar and masonry unit ifthe bond between the two materials isdeficient.

Finally, mortar affects the appearanceof the masonry assembly. Both the colorof the mortar and the method of finishing(tooling) the joints affect the visualimpression that masonry makes.

Mortar propertiesThree important properties of mortar

are workability, bond, and compressivestrength.

Workability. Workability is perhapsthe most important property of plastic(fresh and not yet hardened) mortar.Workability is the ease with which themortar moves under the trowel. In freshmortar, workability is achieved when the

Understanding

MortarIt is critical to appreciate

mortar’s properties and how the ingredients inmortar affect performance

on largely because it is easily measured.Compressive strength is increased withmore cement in the mix but is decreasedwith higher ratios of water to cementitiousmaterials and higher ratios of sand tocementitious materials.

Mortar’s compressive strength has sig-nificantly less influence on the compres-sive strength of the masonry assemblythan does the compressive strength of the

masonry units. For example,the compressive

strength of Type Smortar is 140%greater than thatof Type N mortar,but Type S mortarincreases the

strength of themasonry assembly

by only about 20%.However, when the

compressive strength ofthe masonry units isincreased 50%, the com-pressive strength of themasonry assemblyincreases by about 40%.*

Mortar materialsMortar is a combination

of water, aggregate, andcementitious materials.Each ingredient serves animportant purpose in themortar mix.

Water. Water facilitatesmixing of the aggregate and

cementitious materials. Sufficientwater is essential for hydration, thechemical process that gives mortar itsstrength. Additional water, above andbeyond the amount needed to hydratethe cement, is also needed for workabil-ity, for absorption by the masonry units,and to account for evaporation. Potablewater should be used in mortar becausewater that is safe to drink generally doesnot have contaminants that may adverse-ly affect mortar properties.

Aggregate. Aggregate is the granularmaterial, usually sand, that is used in themortar mix to reduce the required propor-tion of cementitious materials and to resistshrinkage of the cement. For workability

joints in the masonry assembly. Bond. Bond is an important property

of hardened mortar. Two facets of bondcritical to a masonry assembly’s perfor-mance are extent-of-bond and bondstrength (known to structural engineers asflexural tensile strength).

Extent-of-bond is a measure of the actu-al contact area at the interface of the mor-tar and masonry unit. Goodextent-of-bond

existswhen

sile stress required to break the bondbetween mortar and masonry unit (that is,to create a crack at the interface). Factorsthat affect bond strength include: mor-tar composition, especially cement con-tent and air content; masonry unit prop-erties such as surface texture, initial rateof absorption, and moisture content;quality of workmanship; and conditions

of curing.

the mortar-to-unit con-tact is complete and intimate. Good extent-of- bond prevents water penetration throughthe masonry assembly and is achievedwhen the mortar is workable and water-retentive, the masonry units have a mediuminitial rate of absorption (IRA), and theworkmanship is good, with completelyfilled mortar joints. Extent-of-bond can bemeasured directly by a microscopic exami-nation of the cross section per ASTM C1324 (Ref. 1) or indirectly by a water pene-tration test in accordance with ASTM E 514(Ref. 2).

Bond strength is a measure of the ten-

Using bond-wrench equipment in ac-cordance with ASTM C 1072 (Ref. 3)or using the procedures of ASTM C 952(Ref. 4) the laboratory can measurebond strength.

Compressive strength. By reading mostproject specifications and seeing theemphasis placed on this property by archi-tects and engineers, one would think thatcompressive strength is the single mostimportant mortar property. In fact, it is notas important to the performance of themasonry assembly as workability andbond. Architects and engineers single outcompressive strength as a selection criteri-

*Based on tabulated values of compressive strengthof masonry in the Masonry Standard Joint Committee’sSpecification for Masonry Structures (Ref. 12).

and strength, each particle of aggregatemust be coated with a matrix ( combina-tion) of cementitious material and water.If sand particles of uniform size (large orsmall) are used in the mortar, the total vol-ume of voids between sand particles isgreater and more of the cementitiousmatrix is required in the mix than if sandparticles of varying sizes are used.

Well-graded sand, containing particlesof varying sizes, is desirable because therequired proportion of matrix to aggre-gate is decreased and the total volume ofwater used in the mix is decreased. Lowerproportions of cement and water result inreduced shrinkage of the mortar. Lessshrinkage means a lesser tendency for themortar to crack.

Particle sizes and gradations ofmasonry sand are specified by ASTM C144 (Ref. 5). Sand not complying withthese grading requirements can still meetthe requirements of that standard, how-ever, provided that it can be used to pro-duce a mortar that complies with theproperty specification of ASTM C 270(Ref. 6).

Cementitious materials. Cementitiousmaterials have adhesive and cohesiveproperties both when in a plastic state andwhen hardened. Mortar includes one ofthree categories of cementitious materi-als: portland cement (sometimes com-bined with other hydraulic cements) andlime, masonry cement, or mortar cement.

Portland cement, which is governedby ASTM C 150 (Ref. 7), is a hydrauliccement (it can harden even when underwater) that is produced by pulverizingclinker. Portland cement is used in mor-tar to increase compressive strength,bond strength, and durability. However, amortar containing portland cement as theonly cementitious material lacks plastici-ty, has low water-retentivity, and is harsh(less workable).

Lime is used in conjunction with port-land cement in a mortar mix. The form oflime may be hydrated lime, governed byASTM C 207 (Ref. 8), or may be quick-lime mixed with water per ASTM C 5(Ref. 9). The advantages of combininglime with portland cement in mortar

include increased workability and waterretentivity, and the ability to deformslowly in the hardened state, therebyaccommodating some structural move-ment.

Masonry cement is a proprietaryprepackaged blend of portland cement orblended hydraulic cement with plasticiz-ing materials (such as hydrated lime orpulverized limestone) and other ingredi-ents. The standard that governs masonrycement, ASTM C 91 (Ref. 10), does notplace limitations on what materials maybe used to manufacture a masonrycement. The standard does, however,state physical requirements for thecement, such as fineness, compressivestrength, air content, and water retention.

Masonry cements include an air-entraining additive that gives the mixexcellent workability. Because of thisentrained air, however, masonry cementmortars may have lower bond strengthsthan non-air-entrained portland cement/lime mortars. According to most build-ing codes, allowable flexural tensilestress (bond strength) values for unrein-forced masonry with masonry cementmortar or air-entrained portlandcement/lime mortar are 40% to 50% lessthan when the masonry uses non-air-entrained portland cement/lime mortar.

Although there is research evidence tosupport the building code reduction inallowable flexural tensile strength ofmasonry with masonry cement mortar,the evidence is not clear relative to theextent of bond. While some researchersreport reduced extent of bond andincreased water penetration in masonryconstructed with masonry cement mortar,other researchers report no difference inthe water resistance of masonry withmasonry cement mortar compared to thatof masonry with non-air-entrained port-land cement/lime mortar.

Mortar cement is similar to masonrycement in that it is also a proprietaryprepackaged blend of materials intendedto be mixed with sand and water to pro-duce mortar. However, unlike masonrycement, the governing standard for mor-tar cement, ASTM C 1329 (Ref. 11),includes a minimum bond-strengthrequirement in addition to requirementsfor fineness, time of setting, autoclaveexpansion, compressive strength, aircontent, and water retention. The intentof the standard’s bond-strength require-ment is to produce a mortar that is equiv-alent in bond strength to the same type ofportland cement/lime mortar.

Like masonry cement, mortar cementincludes an air-entraining additive to

Mortar should always be of lower compressive strength than the masonry units

that make up the masonry assembly.

A mortar withgood work-ability extrudesreadily from thejoint when themason appliespressure to theunit.

the default method given by ASTM C270. A property specification, on theother hand, dictates minimum or maxi-mum values for certain physical proper-ties of a laboratory-prepared mortar mix.The physical properties addressed byASTM C 270 are minimum compressivestrength, minimum water retention, andmaximum air content.

Other properties, such as flexural ten-sile (bond) strength, may be specified bythe project architect/engineer if consid-ered important to the successful perfor-mance of the masonry but are not includ-ed in ASTM C 270. Property require-ments in addition to those included in theASTM standard are seldom specified.

Mortar mixes are designated as TypeM, S, N, or O. These are listed in orderof highest to lowest compressive strengthand also in order of lowest to highestworkability. Because no single mortartype is ideal, mortar type should be spec-ified based on the best mix for the projectand not simply based on high compres-sive strength. Mortar should always be oflower compressive strength than themasonry units that make up the masonryassembly.

Within each mortar type, any of thethree categories of cementitious materials(portland cement/lime, masonry cement,or mortar cement) may be used unlessrestricted by the project documents. Formortar specified by proportion, the rela-tive volumes of the ingredients are givenin Table 1 for portland cement/lime mor-tar, and Table 2 for masonry cement mor-tar and mortar cement mortar.

The mortar mix need not adhere to thematerial proportions exactly as shownbecause the standard for mortar gives arange for these proportions. For example,Type N portland cement/lime mortar isshown (in Table 1) to consist of 1 partportland cement, l part lime, and 6 partssand. In fact, for each part of portlandcement, between 0.5 part and 1.25 parts oflime can be used and between 3.38 and6.75 parts of sand can be used (2.25 to 3times the combined volume of cementand lime).

Table 3 gives the minimum and maxi-mum properties for mortars specified bythe property method. These propertiesare evaluated on a laboratory-preparedmix of mortar. The laboratory mortarmix differs from the field mortar mix inthat the amount of water that is added to

give the mix workability. However, theASTM standard for mortar cement lim-its entrained air to a lower volume per-centage than is permitted for masonrycement. The ASTM standard for mortarcement was first published in 1996.Many design professionals are unfamil-iar with this mortar, and it is not oftenspecified.

Mortar mixesMortar is specified by one of two

methods: by proportion or by property. Itis inappropriate to combine requirementsfrom the two methods of specifying mor-tar. A proportion specification dictatesthe relative quantities of each ingredientto be included in the field-prepared mor-tar mix. The proportion specification is

TABLE 1: PORTLAND CEMENT / LIME MORTAR BY PROPORTION SPECIFICATION

PORTLANDMORTAR TYPE CEMENT LIME SAND

M 1 1/4 3 3/4

S 1 1/2 4 1/2

N 1 1 6

O 1 2 9

TABLE 2: MASONRY CEMENT AND MORTAR CEMENT BY PROPORTION SPECIFICATION

MASONRY CEMENT PORTLANDTYPE OR MORTAR CEMENT OR SAND

MORTAR TYPE CEMENT TYPE BLENDED CEMENT

M 1 3

or

M 1 1 6

S 1 3

or

S 1 1/2 4 1/2

N 1 3

O 1 3

the laboratory mix is limited and is basedon a standardized measurement of flow.

In the field, the mortar standard doesnot limit the amount of water that amason can add to the mix. This is a sig-nificant difference from concrete con-struction. Water in mortar is not limitedin the field because some of the mixingwater will be absorbed by the masonryunits and some will be lost to evapora-tion. The mason is able to judge the cor-rect amount of water to add to the mixbased on the type of masonry unit andambient conditions.

Water content in mortar is self-regulat-ing: If too much water is added, themasonry units will float on the mortar,and the mason will not be able to laythem; if too little water is added, the mor-tar mix will be unworkable, the masonwill not be able to adequately spread themortar, and the mortar will not stick to theunits. In recognition of the necessaryinput by the mason in the field, ASTM C270 states that mortar is to be mixed “…with the maximum amount of water toproduce a workable consistency.”

An understanding of how water,aggregate, and cementitious materialsaffect the properties and field perfor-mance of mortar helps assure the designand construction of water-resistant andattractive masonry buildings.

Rochelle C. Jaffe is a principal archi-tect/structural engineer with ConstructionTechnology Laboratories Inc. (CTL) inSkokie, Ill. She specializes in the investi-gation, evaluation, and rehabilitation ofexisting, deteriorated, and damaged

structures and hasover 17 years ofexperience in thisfield. A licensedarchitect and alicensed structuralengineer, Jaffe hasbeen an activemember of TheMasonry Societyand the Masonry

Standards Joint Committee and holds aMaster of Architecture degree (structuresmajor) from the University of Illinois atChicago.

REFERENCES

1. ASTM C 1324, “Standard Test Method forExamination and Analysis of HardenedMasonry Mortar.”

2. ASTM E 514, “Standard Test Method for WaterPenetration and Leakage Through Masonry.”

3. ASTM C 1072, “Standard Method forMeasurement of Masonry Flexural BondStrength.”

4. ASTM C 952, “Standard Test Method for BondStrength of Mortar to Masonry Units.”

5. ASTM C 144, “Standard Specification forAggregate for Masonry Mortar.”

6. ASTM C 270, “Standard Specification forMortar for Unit Masonry.”

7. ASTM C 150, “Standard Specification forPortland Cement.”

8. ASTM C 207, “Standard Specification forHydrated Lime for Masonry Purposes.”

9. ASTM C 5, “Standard Specification forQuicklime for Structural Purposes.”

10. ASTM C 91, “Standard Specification forMasonry Cement.”

11. ASTM C 1329, “Standard Specification forMortar Cement.”

12. Specification for Masonry Structures(ACI 530.1-99/ASCE 5-99/TMS 602-99),Masonry Standards Joint Committee.

TABLE 3: MORTAR BY PROPERTY SPECIFICATION†

MORTAR MIN. 28-DAY MIN. MAX. AIR CONTENT %

TYPE COMPRESSIVE WATER Masonry Mortar CementSTRENGTH, PSI RETENTION % Cement or PC/L

M 2500 75 18 12

S 1800 75 18 12

N 750 75 20* 14**

O 350 75 20* 14**

† Based on ASTM C 270*Maximum air content when structural reinforcement is incorporated into masonry cement mortar shall be 18%.

**Maximum air content when structural reinforcement is incorporated into mortar cement mortar or portland cement/lime mortar shall be 12%.

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