Quality control in preparing masonry mortar

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BUILDING

-3ACTICE

I

A

NOTE

QUALITY COHntoL IN PREPARING MA!5ONRY

MORTAR

Division of Building Research, National Research C-cil of Canada

QUALITY CONTROL IN PREPARING MASONRY MORTAR

by

J.I.

Masonry mortar is one of the few remaining building materials manufactured on the construction sits. In sharp contrast to quality control measures associated with the production of factory-made building components or the more comparable ready-mix concrete, the mixing operation of masonry mortar has been entrusted to unskilled

labourers who have l i t t l e knowledge of the materials they are using, or of the mortar they produce.

The introduction of the load-bearing masonry w a l l which, i n effect

,

control. Custcmmrily the designer takes great care in specifying and selecting quality materials and the correct mortar combination. He may require t e s t s on the "construction" mortar to reassure himself that strength values are adequate. &rt the quality of the mortar rests with the mixer operator, who, in most cases, has limited knowledge of the material and works with minimum supemision.

Recognition of t h e importance o f quality control i s reflected in

a number of provisions in CSA Specification A179 'mortar and Grout

for Unit Masonry.'' Diligent adherence to these inexpensive measures would improve the masonry tfimagell in the eyes

of

Storage of Materials

Cementitims materials and aggregates arriving on-site should be stored so as to prevent deterioration OT intrusion of foreign

material. Tt is especially important that cementitious materials should be protected from water, They should be p i l e d on pallets to prevent water wicking up from the ground, and covered with tarpaulins or some other waterproof sheeting to protect them from rain or snow.

The aggregate (sand] used in mortars is also sensitive to moisture: bulking occurs under the influence of moisture; m a x i m bulking occurs when the moisture content is between zero arrd ten per cent. To ensure uniform moisture content. in the stock-pile, aggregate should be stored an a raised platform and covered for protection against the elements.

Batching

Batching by weight, similar to the method used in ready-mix concrete plants, is the ideal way, but there are very few masonry jobs that are large enough to justify the c o s t of t h e required equipment.

Volume measurement thus remains the practical solut5on.

Cementitims materials should be added by the bag - a bag generally contains one cubic foot of material. Sand is usually batched with a shovel, a practice that is as widely condemned as it is used. The use of a properly gauged hopper is recommended. If no suitable hopper is available, a 1 cu. ft. gauge box can be hilt on s t t e . With a little ingenuixy the gauge box can be positioned to empty'directly into the mixer. As an alternative this box can be used to check the shovel measurement- In this case the operator should determine the number

of shwelfuls required to fill the gaugk box twice a day.

The sand quantity should also be adjusted f o r bulking. Bulking will vary with the gradation o f the sand and the moisture content of the pile

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of

moisture content of damp sands on construction sites usually varies

between 4 and 8%, t h e range in which maximum bulking occurs. If the stock-pile is not protected frm the weather the moisture content w i l l vary from day to day. Loose damp sand can vary in weight from

76 to 105 lb. per cubic foot. If no compensation is made for variations i n bulking the resulting mortars will contain different

h o u n t s o f sand and have variable compressive strengths.

There is a simple t e s t for bulking in CSA Specification A179, based on the fact that the volume of saturated sand

is

it is for dry sand. A 2-pound jar, or similar container, is filled two-thirds full w i t h

the

be

water so that it is entirely saturated. I t is then rodded to remove any air, and the "new1* depth is measured- It will be

less than before; the difference between the new and original levels will be the itmount of bulking. It can be expressed as a

percentage of dry sand which can be used to correct the batch

quantity. Bulking should be checked after each new shipment of sand is received, and after any severe change in the weather.

In-plant dry batching has recently been introduced in some areas. The cementitious materials and aggregate are weighed, m5xed and

delivered an-site in a sealed truck. The operator adds w a t e r and mixes the mortar. This sophistfcated approach should result in very

Consistent batdhiag procedures for cementitious materials and sand will, in addition to ensuring more I m i f m properties 5n mortars, minimize colour variations in m o r t a r joints that are often

enbarrassing to masonry contractors. These variations are caused by a change in the cemmtztiaus materials or sand, or by variations in batch- procedures.

The w a t e r retpirement for a mortar batch will vary with a number of factors, including fineness, mo5sture content and quantfty of aggregate, and ambient ueather conditions. It will therefore be much easier to deteraine a m e a s u r a b l e w a t e r requirement after proper

batching of the sand has been established, When this has been established for a particular m o r t a r batch, it should be used consistently until the _{batch proportions are changed. The final} water adjustment will be made by the m a s o n at the working face. He uses w a t e r to temper the moTZar until t h e required workability is obtained.

Mortar materials mst be thoroughly m i x e d to obtain good workability and the maximum potential of the properties of the

plastic mortar. Mortar should be mixed Zn a suitable mechanicdl mixer. If the size of the job does not warrant use of a mechanical mixer, hand-mixing may be p e m i t t e d w i t h the consent of the

purchaser.

About three-quarters of the water requirement should be put into the machine, then half the sand, followed by all the

cementitious materials, After these materials have been mixed briefly the remainder of the sand and then the ~ e s t of the water should be added,

Mixing time is anoTher neglected aspect of mortar preparation, This is well illustrated by conflicting requi-ts that, until recently, appeared in ASTM and CSA mortar specifications. The

American specification ~equired a slinimusl of 3 minutes; the Canadian

specification required a maximum of I0 minutes. It has been noticed in field observations that l i t t l e attention is paid to specification requirements and t h a t mixing time is usually determined by the demand

at the working face, rather than by any concern about mortar

properties. Far example, f i ~ s t thing in the morning or just after lunch, when all the masons are demanding mortar, t h e machine is

dumped almost as s o m as the last material is added. Streaks of lime, sand, or cement that are often present in the mortar indicate

inadequate mixing. A t The other extreme, in mid-morning or mid-

afternoon, when the mortar boards are relatively well supplied but the reserve in the mixer box is exhausted, it is not lmcomnmn to f i n d the machine running for 20 to 30 minutes as the operator replenishes the

A recent 1 aborat o r y study' indicates that the water requirement and t h e air content levels increase as mixing time is increased, which in t u r n results in lauer compressive strength values.

This

particularly critical for proprietary mortar materials (masonry cements) which contain air-entxainfng or other additives; it is less critical for cement-lime mortars w i t h minimal air contents.

Mixing time should be adequate to ensure complete mixing of all ingredients (this is covered by the amrent requirement in CSA A179:

a minimum 3 minutes and a maximum 113 minutes]. Mixing time can easily be cmtro3led by placing an automatic timer on the machine, one that could either stop the machine at the required time, or ring

a bell to alert the operator.

Special provisions f o r mixing mortars

in

contained in "Recommended Practices and Guide Specifications for Cold Weather Masonry Construction" published by the International Masonry Industry All-Weather Council-

Mortars containing cement that are not used within 2 hours of mixing when the ambient temperature is above, XO'F or within 3 hours when the temperature is below 80°F, should be discarded. Mortars t h a t have stiffened by evaporation of moisture, within these time limits, can be retempered to satisfactory workability by adding water.

The extra water will result i n some reduction i n compressive strength,

but t h i s will be more than compensated for by a better extent of bond as a result of the improved workability.

Proarietarv Mortars

Proprietary mortar materials [ e . g . , masonry cemenz) containing air-entraining or other additives are becoming increasingly popular. Their formulations, which are not published, have been developed by competent, technical persons and are designed to meet specification requirements. In most instances, instructions regarding aggregate quantities and/or mixing time are printed on the bag containing these materials. Adherence to these instructions will ensure maximum

performance from the mortar. If the designer or the mixer operator alters the instructims the mortar properties should be checked for compliance with the specification.

Other Tests

CSA A179 contains several other test procedures to assess the quality of mortar used on s i t e . One is a procedure for molding cubes of job-mixed mortar for compression t e s t s . U s e o f t h i s method will correct injustices resulting from incorrect past practice. In assessing compressive strength values it is important to remember t h a t they w i l l be less than those for laboratory-prepared mortar

because construction mortar contains more water than t h a t prepared in the laboratory.

The CSA specification a l s o contains a test procedure t o

determine the r a t i o of aggregate to cernentitious material i n fresh mortar. In the test a mortar sample i s washed on a No. 100 sieve. The cementitious materials pass through t h e s i e v e and the aggregate

is retained. The results can be used to check the r a t i o of aggregate to cementitious material against the specification requirement.

The Specification does not contain a test method for determining

a i r content levels. Entrained a i r i s an essential ingredient in

masonry cements, as it provides the workability contributed by lime in cement-lime mortars. But entrained air is also a s i g n i f i c a n t f a c t o ~ in determining t h e ultimate compressive s t r e n g t h - as a i r

content l e v e l s increase, compressive strength decreases.

There a r e several simple tests for air content t h a t can b e used by the mixer operator or an inspector, as quality control checks on

construction mortar. One method involves a modified pressure p o t , the familiar test used to determine a i r content levels i n concrete. The second involves the Chace air meter, a small g l a s s tube with a

graduated s t e m . In the t e s t the air bubbles are released from a

known quantity of mortar by agitating the tube containing the mortar sample and some isopropyl alcohol. As the air i s released, the l e v e l o f the tube contents lowers, and indicates the volume of entrained

a i r . Although not as accurate as a laboratory test, the method is reliable in indicating variations in air contents that sometimes

occur because of, f o r example, abnormal aggregate grading or

contaminants in aggregates or cementitious materials. A i r content

values, like compressive strength values, w i l l be lower in

construction mortars than in laboratory m o r t a r s . The d i f f e r e n c e , in the order of 4%, reflects the difference in mixing between the

laboratory Hobart mixer and the mechanical mixers used on construction sites.

Quality control in the preparation of masonry mortar h a s assumed a new importance because of ( 1 ) the comparison of the conventional masonry process with sophisticated, quality-controlled, prefabricated building components, and 12) the use of m o r t a ~ as an engineering

material i n load-bearing w a l l construction. Quality control during mortar preparation can be improved by adherence to recornendations in CSA A179 concerning on-site storage of materials, batching, and mixing procedures. The Specification contains a r e s t method that will a s s i s t

the mixer operator in determining sand bulking, and methods t h a t the

i n s p e c t o r can use to prepare specimens EOT compression tests and to determine the r a t i o between aggregate and cementitious material in

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in

safety factors in design codes. Bib 1 iography

Hendry, A.W., The Effect of Site Factors in Masonry Performance. Proceedings, F i r s t Canadian Masonry Symposium, University of Calgary, Calgary, Alberta. June 1976. p . 182-198.

Mortar for Brick Masonry Selection and Contmls. Brick Institute

of h e ~ i c a , McLean, Virginia, Technical Note

an

Davison, J , I , , The Effect of fixing Time on the Compressive Strength o f Masonry Mortars. National Research Council of Cmada,

Divisim of Ehilding Research, Ottawa, Ontario, Building Research Mote No. 114, July 1976.

Mortar and Gmut for Unit Masonry. CSA Standard A179-1975.

Mortar for Unit Masonry. ASTM Specification C270-73.

Concrete Masonry Handbook, Portland Cement Association, Skokie, Illinois, 1976.