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The effect of mixing time on the compressive strength of masonry
mortars
THE EFFECT OF
MIXING
TIME ON THE COFIFRESSIVESTRENGTH OF MASONRY MORTARS
by
J.1, Davison
The lack of quality conrrol measures during t h e mixing of
masonry mortars an construction sites is e v i d e n t to observers of f i e l d
practice. Sand and c e m e n t i t i o u s materials a r e often measured with a shovel, water by the bucket o r w i t h a h o s e , and o n l y on rare occasions
is the mixing time checked.
Observations indicate that t h e length o f mixing time usually reflects the demand f o r mortar at t h e working site rather t h a n concern
f o r
its quality. Fox example, first t h i n g i n t h e morning o r j u s t a f t e rlunch, when the masons a r e s t a r t i n g work and a l l demanding mortar at the same time, the mixing machine is dumped a l m o s t as soon as the last
materials are added. A t the other extreme,
in
mid-morning ormid-afternoon,
when a l l the morrar boards a r e f u l l b u t t h e supply in the mixer box needs replenishing, t h e machine may b e allowed to sun 20 to 30 minutes beforebeing dumped.
T h e s e observations r a i s e questions about t h e e f f e c t of various mixing times on mortar p~operties, and a study was undertaken t o provide
some answers. Water requirement, flow, r e t e n t i v i t y , a i r content and compressive strength were i n c l u d e d i n t h e i n v e s t i g a t i o n . The latter is one of t h e m a j o ~ concerns of the masonry contractor and t h i s paper
reports t h e effect of mixing times on i t .
THE P R O G M
The s t u d y included t h r e e mortar types, M, S and N, listed in the C S A ~ and A S T M ~ Mortar S p e c i f i c a t i o n s . Five material combinations were used for each mortar t y p e . F o s Types M and
S,
one P o r t l a n d cementwas combined w i t h two hydrated limes and t h r e e masonry cements, w h i l e
the Type N mortars included t w o P o r t l a n d cement-hydrated l i m e combinations and three masonry cements. Mixing t i m e varied f r o m I to 30 minutes.
Compressive s t r e n g t h t e s t s were conducted on 2-in. cubes after 28-day c u r i n g p e r i o d s .
MATERIALS
A 1 1 cemcntitious materials were available locally and were known to conform to t h e i r r e s p e c t i v e CSA Standards.
Masonry cements Nos. 1 and 2 are known to be the interground cement clinker-limestone varieties, while No. 3 is a b l e n d e d P o r t l a n d
cement-hydrated lime. A l l contain air-entraining a g e n t s and a l l had
mixing instructions on the hag
-
Nos. 1 and 2 recommended 1 p a r t masonrycement and 3 p a r t s s h a r p , c l e a n sand; No. 3 required I bag to 18 No. 2
s h o v e l s f u l of sand and machine mixing for a maximum o f 5 minutes. The
instructions on a l l t h r e e bags were designed t o produce Type N mortars. The aggregate used was 50 per c e n t Standard and 50 per cent Graded Ottawa sand,
MIXING
The materials were proportioned by weight, transposed from
a
volume b a s i s u s i n g t h e fallowing u n i t weights: Portland cement,87 l J 2 l b ; masonry cements Nos, 1 and 2, 70 l b ; masonry cement No, 3 ,
55 Ib; and h y d r a t e d lime, 50
Ib.
The amount of water added was t h a t required ta produce a flow value of 120 5 2 per cent. The different mortar mixes, the weights of materials and q u a n t i t i e s of water per batcha r e l i s t e d in Table 1 .
The mortars were mixed in a Hobart mixer, using t h e procedures outlined in CSA Standard ~ 8 except ~ , r h a t the " w a i t periodM before t h e f i n a l mixing was increased fram 1 1 / 2 to 3 minutes. For the purpose of t h i s s t u d y mixing time is d e f i n e d as the l e n g t h of t i m e t h e mortar was mixed after the final ''wait period"'.
During the study it was found t h a t the " c o n t r o l f r f l a w value decreased with i n c r e a s i n g mixing time until the mortar was no longer workable. A t t h a t p o i n t s u f f i c i e n t water w a s added to restore t h e
i n i t i a l flow value. Following addition of the extra water the mortar w a s mixed for 1 minute at medium speed.
The practice of adding water was considered to be similar to retempering
an
the construction s i t e where t h e mixer tender, or t h e mason, adds water u n t i l t h e m o r t a r regains satisfactory w o r k a b i l i t y . CURING AND TESTINGAll t h e mortar cubes were moulded, cured and t e s t e d f o r
compressive s t r e n g t h by procedures d e s c r i b e d in CSA Standard A179, A i r
c a n t e n t determinations, using the d e n s i t y method, were carried out on separate batches o f mortar, Results are shown graphically
in
Figures 1,2 and 3. DISCUSSION
Type M Mortars (Table 2 )
Portland Cement-Lime, Compressive s t r e n g t h values were h i g h e s t
a f t e r 3 to 5 minutes mixing and t h e n dropped steadily. The d e c r e a s e is attributed to the extra water that was required to retemper t h e mortar.
A t 30 minutcs t h c "e>rtrnl' water was - I [ ) p o r c c n t o f t h e original w;ltcr
c o n t e n t . Mcanwhi l e t h e a i r content decrcascd.
Portland Cement - Masonry Ccmcnt. The mortar c o n t a i n i n g masonry cement No. 1 had t h e least drop in compressive s t r e n g t h - a f t e r 30 minutes' m i x i n g timc o f any of the
five
Type M mortars. I t sair
content dropped 3 per cent a f t e r 30 m i n u t e s ' m i x x i g while t h e water q u a n t i t y w a s 22 per cent above the original value. Compressive strengthf o r mortar containing masonry cement No. 2 exceeded the required 2500 psi at I minute, b u t dropped steadily with increasing mixing time until at 30 minutes it was o n l y 42 per cent of t h e o r i g i n a l value. Compressive
s t r e n g t h for t h e mortar c o n t a i n i n g masonry cement No. 3 , i n i t i a l l y below
t h e 2500 p s i requirement, dropped steadily with increasing mixing t i m e
and at 30 minutes was 53 per cent o f t h e c o n t ~ o l value. A t 30 minutes 11 t o 15 p e r cent "extra" water had been added t o t h e s e l a s t t w o m o r t a ~ s
and a i ~ c o n t e n t s were 40 to 45 per cant above t h e l e v e l for the c o n t r o l m o r t a r s .
Type S Mortars (Table 3)
Port la n d Cement- Lime. Compressive s t r e n g t h s were maximum a f t e r 3 m i n u t e s 9 i x i n ~
-
and then decreased until values were 20 to2 3 per cent below control values after 30 minutes' mixing, at which t i m e 30 to 4 0 p e r c e n t extra water had been added and air c o n t e n t s dropped
eo
25 t o 45 p e r c e n t of the c o n t r o l values.Portland Cement-Masonry Cement. The mortar containing masonry
cement No. 1 had a h i g h e r compressive strength a f t e r 30 minutesq mixing than it d i d initially. Twenty-one per cent more water had been added
while the a i r c o n t e n t was 10 peT cent below t h e i n i t i a l value. Compressive s t r e n g t h s f o r the mortars c o n t a i n i n g masonry cements Nos. 2 and 3 dropped below t h e 1300 p s i requirement a f t e r 2 minutesmixing and at 30 minutes s t r e n g t h s were only 52 and 27 per c e n t o f control values respectively. The decrease is attributed to m increase of 14 and 24 per cent i n water content, and an increase of 40 to 52 per cent in a i r c o n t e n t ,
Type N Mortars (Table 4')
Portland Cement-Lime. Compressive s t ~ e n g t h values were maximum at 3 and 5 m i n u t c s and t h e n dropped steadily until a f t e r 30 minutes' mixing they wcrc 61 and 80 pcr c e n t of t h e control values, with water c o n t e n t s
4 2 and 34 per c e n t above, and a i r contents reduced t o 30 and 4 0 per cent o f t h c values f o r , c o n t r o l m o r t a r s .
Masonry Cement. Maximum compressive s t r e n g t h values occurred
a f t e r I and 3 minutes' mixing. As mixing t i m e was increased compressive
s t r e n g t h values dropped - a t 5 minutes values for mortars containing masonry cements Nos. 2 and 3 were below t h e 750 p s i requirement and at
15 minutes mortar c o n t a i n i n g masonry cement No. 1 was a l s o below. S t r e n g t h values levelled o f f after 15 minutes, b u t at 30 minutes they were only 4 9 , 29 and 25 p e r cent of the c o n t r o l values. A t 30 minutes increased water requirements were 3 6 , 27 and 7 per cent above t h e i n i t i a l
amounts and air c o n t e n t s had increased 7 , 54 and 82 p e r cent, respectively,
f o r t h e t h r e c mortars.
There was a c o n s i s t e n t p a t t e r n
of
dropping compressive s t r e n g t h s with increasing mixing times for Types M, S and N mortars. Reductions in s t r e n g t h f o r cement-lime mortars are a t t r i b u t e d to t h e a d d i t i o n a l waterr e q u i r e d t o maintain satisfactory workability, while
in
mortars c o n t a i n i n g masonry cement, reductions a r e a t t r i b u t e d to a combination of increasing water and air contents.Strength values f o r t h e cement-lime mortars a f t e r 30 minutes"
m i x i n g , d e s p i t e substantial decreases due to the prolonged mixing p e r i o d , remain well above the minimum requirements i n mortar specifications.
Types M and S mortars c o n t a i n i n g masonry cement No. 1 are not seriously affected by longer mixing, b u t s t r e n g t h f o r Type N m o r t a ~ s falls below the 750 p s i requirement a f t e r 15 rninutestmixing.
Values f o r t h e mortars containing masonry cements Nos. 2 and 3
drop w e l l below the specification requirements a f t e r a small increase in mixing time. Prolonged mixing of these mortars i s accompanied by an
increase in air c o n t e n t , and t h i s combined with t h e extra water q u a n t i t i e s , r e s u l t s i n the lower strength v a l u e s .
Requirements far mixing time in mortar s p e c i f i c a t i o n s have been i n c o n s i s t e n t , probably r e f l e c t i n g a lack o f knowledge of its significance. The c u r r e n t ASTM C270 r e q u i r e s a minimum of 3 minutes'mixing and psior to
1975 CSA A179 required a maximum of 10 minutes' mixing. The 1975 revision o f the CSA Specification c o n t a i n s both t h e maximum and the
minimum requirements. The results of t h i s s t u d y emphasize the importance
of t h e maximum requirement, and,
in
fact, i n d i c a t e t h a t some mortarss h o u l d n o t b e mixed l o n g e r than 5 minutes. RECOMMENDATIONS
Mixing time requirements, based on s t u d i e s of t h e e f f e c t of mixing time on mortar p r o p e r t i e s , should be contained in mortar
s p e c i f i c a t i o n s .
Mixing time could be regulated on construction s i t e s by
p l a c i n g an automatic t i m e r on t h e mixing machine, one t h a t would stop the mixer a t t h e correct time, or a timer with an alarm bell to a l e r t the mixer t e n d e r when the d e s i r e d time has been reached.
ACKNOWLEDGEMENTS
Thc author wishes to express his a p p r e c i a t i o n to T. R i t c h i e for h i s h e l p f i l l comments durlng p r e p a r a t i o n of t h i s paper-
REFERENCES
I . CSA Standard A179-1975 - Mortar and Grout for Unit Masonry.
2. AS'I1M C270-73 - Standard S p e c i f i c a t i o n f o r Mortar f o r Lhit Masmry. 3. CSA Standard A8-1970
-
Masonry Cement.TABLE I
MIXING
PROPORTIONS
Mortar ~ a t e s i a i Proportions Weight per Batch [gm) Wat ex
Type
(Parts by Volume)- -
P.C. M . C - H . L .-
-
Sand Volume (ml]M . C . = Masonry Cement
S = Sand
P C . = P o r t l a n d Cement
Mixing Mortar T h e Mix (minl lP.C.:lM.C.#1:6S 1 Water Volume [ml 1 225 I n i t i a l Flow
( % I
120.8 Air Content ( % 3 15.6 16.0 16.5 16.5 14.5 1 4 -0 12.5Compx ess ive Strength (psi) 3644 3704 3642 3575 3352 3582 3169 Retempered Na II Yes II Yes 120 - 1 Yes 121.0 1q 120.5 No 120 - 9 11 125.5 I I 119.6 Yes 119. T I t 9 2 0 . 3 11 119.7 11 120.7
TABLE 3 TYPE 5 MORTARS Mixing Mortar T i m e Mix (rnin) ~P.c.:~M.C.#~:~S 1 Water V o l u m e I m l ) 215 I n i t i a l Flow
(%I
120.5 A i r Content(%I
16. U 16-6 17.0 15.8 15-7 15-5 14 - 4 Compressive S t r e n g t h (psi1 2834 2702 2642 2725 2732 2 709 2936 Reternper ed No n Yes 11 Yes 11 Y e s 119.9 No 120.9 11 118.8 Y e s 120.3 II 120-8 11 120.7 11 1213.8 II 120.2 PJo 120-7 1 4 120.3 Y e s 120.3 I F 120.4 I 1 120.9 I# 120-4TABLE 4 TYPE N MORTARS
Mixing Water
Mortar Time Volume
Mix
Iminl
(mil?M.C.#1:3S 1 2 06 Initial Flow Re tempered I%) No 120.2 A i r C o n t e n t ($1 19.6 21-0 2 1 . 6 21-8 21.5 25 -2 21.0 Compressive Strength ( p s i ) 1330 1215 I313 1040 1085 697 676 Y e s 119-7 II 121.0 11 119 - 9 I n 120-3 Y e s 119.9 Yes 120 - 6 If 119.5 Yes IT I 1 It 11 Yes I 1
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M I X ' I N G TIME, M I N F I G U R E 1 T Y P EM
M O R T A R S-
C O N S T A N T F L O WN o . 1 N o . 2 N o . 3 N o . 1 N o . 2 0 2 6 10 1 4 1 8 2 2 2 6 3 0 M I X I N G T I M E , M I N FIGURE 2 T Y P E 5 M O R T A R S - C O N S T A N T F L O W
C S A L I M I T
