Role of water/cement ratio on strength development of cement mortar S.B. Singh n , Pankaj Munjal, Nikesh Thammishetti Civil Engineering Department, BITS Pilani, 333031, India article info Article history: Received 28 March 2015 Received in revised form 3 September 2015 Accepted 3 September 2015 Available online 8 September 2015 Keywords: Abrams’ law Cement mortar Compressive strength Split tensile strength Water/cement ratio (w/c) abstract The effect of water/cement (w/c) ratio on the mechanical properties such as compressive strength and split tensile strength of cement mortar cylinders and cubes was investigated experimentally for 28 days curing period as per IS standard. Based upon the experimental results, empirical equations have been developed to predict the strength of cement mortar mixes with various w/c ratios. It is observed that Abrams’ law is applicable for the cement mortar also. The cement mortar contains varying proportions of portland pozzolana cement (PPC) and river sand such as 1:3, 1:4, 1:5, 1:6, 1:7, 1:8 with different w/c ratios. An empirical equation has been developed between split tensile strength and compressive strength of cement mortar. Results show that compressive strength and split tensile strength of cement mortar decreased with an increase in the w/c ratio. It is observed that minimum w/c ratio required to make the cement mortar workable is 0.5. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction A well established fact in the cement industry speaks that an excessive water content leads to reduction in strength of cement mortar, but insufficient water content incurs a poor workability. Hence, a method for determining the optimum water content and influence of w/c ratio on cement mortar is obviously desirable. Quality control unit involves tight control of w/c ratio for concrete materials. But in the case of cement mortar, more water is deemed requisite by the mason to make the mixture workable enough for his comfort zone. Therefore, it is appropriate to see the influence of w/c ratio for mortar strength. Haach et al. [1] investigated the influence of aggregate grading and w/c ratio on the workability and compressive strength of mortar. Authors [1] observed that increase in w/c ratio has reduced the value of mechanical properties and increased the workability. In another study by Schulze [2], the influence of w/c ratio and cement content on the properties of polymer-modified mortar has been of acute interest. Kim et al. [3] observed that for increase in w/c ratio of cement mortar from 0.45 to 0.60, porosity went up to 150% and compressive strength has reduced to 75.6%. Zhou et al. [4] observed that dynamic compressive strength of cement mortar increased with decrease in water content. The dynamic compres- sive strength of saturated specimen was 23% lower than that of totally dry specimen. Ji-Kai and Li-Mei [5] observed that fracture behavior of low w/c ratio mortar is more brittle than that of mortar with high w/c ratio. Zivica [6] studied the effect of low w/c on the pore structure and compressive strength of the cement paste. Fi- neness modulus of sand also influences the w/c ratio of the mortar. Lim et al. [7] have stated that finer sand grading specimen requires a higher w/c ratio to achieve an equivalent workability. It has also been observed by authors [7] that mortar with coarse sand has higher compressive strength than those of the finer sand when the w/c ratio is lower. Study has also shown that influence of sand grading affects the properties of mortar [8–11]. By experimental investigations, Authors [8] observed 55–60% reduction in tensile bond strength as fineness modulus of sand changes from 3.21 to 1.72. As the surface area of sand increases, more paste is needed to cover the surface to attain certain viscosity [10–11]. In case of concrete, It has been shown that compressive strength varies inversely with the w/c ratio through the Abram’s generalization law. The Abram’s law developed for strength of concrete is given below [12]. K K Strength 1 1 2 w c = () where K 1 and K 2 are constants, w is mass of water and c is mass of cement. This formula is valid over the range of w/c ratio of 0.3– 1.20. Abrams’ law is well known for relation of strength and w/c ratio of the concrete. Yeh [13] has confirmed that application of the Abrams’ law is valid to any duration between 3 and 365 days of concrete age. Rao [14] developed the empirical model expressions to predict the compressive strength and split tensile strength of Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jobe Journal of Building Engineering http://dx.doi.org/10.1016/j.jobe.2015.09.003 2352-7102/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail addresses: [email protected], [email protected](S.B. Singh), [email protected](P. Munjal), [email protected](N. Thammishetti). Journal of Building Engineering 4 (2015) 94–100
Transcript
Journal of Building Engineering 4 (2015) 94–100
Contents lists available at ScienceDirect
Journal of Building Engineering
http://d2352-71
n CorrE-m
sbsinghpankaj7nikesht
journal homepage: www.elsevier.com/locate/jobe
Role of water/cement ratio on strength development of cement mortar
S.B. Singh n, Pankaj Munjal, Nikesh ThammishettiCivil Engineering Department, BITS Pilani, 333031, India
a r t i c l e i n f o
Article history:Received 28 March 2015Received in revised form3 September 2015Accepted 3 September 2015Available online 8 September 2015
Keywords:Abrams’ lawCement mortarCompressive strengthSplit tensile strengthWater/cement ratio (w/c)
x.doi.org/10.1016/j.jobe.2015.09.00302/& 2015 Elsevier Ltd. All rights reserved.
The effect of water/cement (w/c) ratio on the mechanical properties such as compressive strength andsplit tensile strength of cement mortar cylinders and cubes was investigated experimentally for 28 dayscuring period as per IS standard. Based upon the experimental results, empirical equations have beendeveloped to predict the strength of cement mortar mixes with various w/c ratios. It is observed thatAbrams’ law is applicable for the cement mortar also. The cement mortar contains varying proportions ofportland pozzolana cement (PPC) and river sand such as 1:3, 1:4, 1:5, 1:6, 1:7, 1:8 with different w/cratios. An empirical equation has been developed between split tensile strength and compressivestrength of cement mortar. Results show that compressive strength and split tensile strength of cementmortar decreased with an increase in the w/c ratio. It is observed that minimum w/c ratio required tomake the cement mortar workable is 0.5.
& 2015 Elsevier Ltd. All rights reserved.
1. Introduction
A well established fact in the cement industry speaks that anexcessive water content leads to reduction in strength of cementmortar, but insufficient water content incurs a poor workability.Hence, a method for determining the optimum water content andinfluence of w/c ratio on cement mortar is obviously desirable.Quality control unit involves tight control of w/c ratio for concretematerials. But in the case of cement mortar, more water is deemedrequisite by the mason to make the mixture workable enough forhis comfort zone. Therefore, it is appropriate to see the influenceof w/c ratio for mortar strength.
Haach et al. [1] investigated the influence of aggregate gradingand w/c ratio on the workability and compressive strength ofmortar. Authors [1] observed that increase in w/c ratio has reducedthe value of mechanical properties and increased the workability.In another study by Schulze [2], the influence of w/c ratio andcement content on the properties of polymer-modified mortar hasbeen of acute interest. Kim et al. [3] observed that for increase inw/c ratio of cement mortar from 0.45 to 0.60, porosity went up to150% and compressive strength has reduced to 75.6%. Zhou et al.[4] observed that dynamic compressive strength of cement mortarincreased with decrease in water content. The dynamic compres-sive strength of saturated specimen was 23% lower than that of
totally dry specimen. Ji-Kai and Li-Mei [5] observed that fracturebehavior of loww/c ratio mortar is more brittle than that of mortarwith high w/c ratio. Zivica [6] studied the effect of low w/c on thepore structure and compressive strength of the cement paste. Fi-neness modulus of sand also influences the w/c ratio of the mortar.Lim et al. [7] have stated that finer sand grading specimen requiresa higher w/c ratio to achieve an equivalent workability. It has alsobeen observed by authors [7] that mortar with coarse sand hashigher compressive strength than those of the finer sand when thew/c ratio is lower. Study has also shown that influence of sandgrading affects the properties of mortar [8–11]. By experimentalinvestigations, Authors [8] observed 55–60% reduction in tensilebond strength as fineness modulus of sand changes from 3.21 to1.72. As the surface area of sand increases, more paste is needed tocover the surface to attain certain viscosity [10–11].
In case of concrete, It has been shown that compressivestrength varies inversely with the w/c ratio through the Abram’sgeneralization law. The Abram’s law developed for strength ofconcrete is given below [12].
K
KStrength
1
1
2
wc
=( )
where K1 and K2 are constants, w is mass of water and c is mass ofcement. This formula is valid over the range of w/c ratio of 0.3–1.20. Abrams’ law is well known for relation of strength and w/cratio of the concrete. Yeh [13] has confirmed that application ofthe Abrams’ law is valid to any duration between 3 and 365 days ofconcrete age. Rao [14] developed the empirical model expressionsto predict the compressive strength and split tensile strength of
Table 1Physical properties of portland pozzolana cement(PPC).
PPC properties Test results
Blaine Fineness (m2/kg) 375Specific Gravity 3.15Normal Consistency (%) 30.8Initial and Final setting time (min) 28 and 550% Flyash addition 30Soundness: Le-chat expansion (mm) 1.00
Table 2Chemical properties of portland pozzolana cement (PPC).
S.B. Singh et al. / Journal of Building Engineering 4 (2015) 94–100 95
mortar using w/c ratio based upon Abram’s law and observed thatit is applicable to mortars with w/c ratio greater than 0.40. Gen-erally, It has been observed that mechanical properties of the ce-ment mortar are primarily affected by the w/c ratio, cement/sandratio, type of cementitious material, and properties of aggregate.
The objective of this paper is to determine the influence of w/cratio on the cement mortar’s mechanical properties such ascompressive strength and split tensile strength and examine thevalidity of the Abram’s law for cement mortar. Moreover, empiricalequations are developed to predict the strength of cement mortarfor different proportions of w/c ratios.
Fig. 1. Particle size distrib
2. Experimental details
2.1. Material and mix design
An adequate number of cement mortar specimens with variousw/c ratios have been prepared to study its effect on the strength ofmortar. Portland pozzolana cement (PPC) as a binder and localriver sand for the fine aggregate were used to prepare the mortarspecimens. Portland pozzolana cement is ordinary portland ce-ment blended or interground with pozzolanic materials such as flyash, calcined clay, rice husk ash, etc. Tables 1 and 2 depict thephysical and chemical properties of cement. The fine aggregatespassing through 4.75 mm sieve has been used and its particle sizedistribution is given in Fig. 1. The material properties of fine ag-gregate is given in Table 3. In this work, five mixes of cement: sandproportions (1:3, 1:4, 1:5, 1:6 and 1:7) with different w/c ratiovarying from 0.5 to 1.2 were prepared by weight batching. Thecement mortar mix were prepared in the Hobart mixer for 2–3 min of mixing. After mixing the mortar, six cylinders of size76.2 mm�152.4 mm and three cubes of size(70.7�70.7�70.7 mm3) were cast. A thin layer of release agentwas spread on the interiors of the moulds using a clean brush andthen a paste of cement mortar was poured into the mould. Excessmortar was struck off with a metallic trowel across the top of themould. The mould was then placed on the vibrating table andvibrated for 2 min at a speed of 12,0007400 per minute toachieve full compaction. Specimens were left in the mould insidethe moist room (temperature 2772 °C and relative humidity 65%75) for a period of 24 h. The specimens were removed from themould and placed inside the curing tank at temperature of2772 °C for 28 days.
ution curve of sand.
Table 4Compressive strengths of cube and cylinder specimens from experimental andanalytical predicted.
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2.2. Experimental methods and test procedure
2.2.1. Compression strengthThe compressive strength of cement mortar is considered to be
one of the most important aspects of masonry structures. Threecylindrical and three cube specimens were tested for each mix inUniversal Testing Machine as per IS 2250:1981 [15] after 28 days.Compressive strength (sC) was measured by placing the specimensin the contact of bearing surface of the Universal Testing Machine(UTM) and the load was applied at the rate of 2–5 N/mm2 perminute until failure occurs. The compressive strength was calcu-lated by dividing the maximum load applied to the specimenduring the test by cross sectional area.
2.2.2. Split tensile strengthTensile strength is one of the important properties of cement
mortar. In the case of masonry, influence of bond–strength is whatmakes the study of tensile strength more significant. The tensilestrength of cement mortar was measured as per the IS 5816-1999[16] and BS 1881-Part 117 [17] standards. IS 5816 [16] indicatesthat maximum tensile stress can be calculated using Eq. (2). In thisequation P is load applied to the specimen; l, d are length anddiameter of the specimen, respectively and fct is split tensilestrength.
fPdl
22ct π
= ( )
3. Determination of optimum w/c ratio
It is necessary to evaluate optimum w/c ratio to fully exploit themechanical properties of cement mortar. Thanh [18] developed anapproach to calculate water quantity of cement mortar based upon
two purposes: firstly amount of water required to hydrate thecement and second to lubricate the sand particle, based on itsspecific surface. This can be represented by Eq. (3)
m m w m w 3w a a c c= + ( )
where mw is optimum mass of water, ma and mc are mass of ag-gregate and cement, respectively and wa and wc are fractions. Inthis paper, Eq. (3) is used to calculate the optimum water contentrequired for cement mortar. In this equation, optimum mass ofwater required for given mass of cement (mc) and sand (ma) iscalculated using the fractional values of wa and wb. The values ofthe fraction wa are around 0.08–0.11 based upon the specific sur-face of sand (3.2–8.2 m2/kg) while the value of fraction wc is takenas 0.21. According to the above formula, optimum water contentrequired for cement mortar (1:3, 1:4, 1:5, 1:6, 1:7) for givingproperties in Tables 1–3 are 0.54, 0.57, 0.61, 0.68 and 0.72 of w/cratios, respectively. From the experimental results, compressiveand split tensile strengths are maximum at w/c ratio of approxi-mately 0.6 for cement mortar of 1:3, 1:4, 1:5 and 0.7 for 1:6, 1:7.
4. Experimental results and analysis
4.1. Compressive strength
The results of compressive strength of cube and cylindricalspecimens with coefficient of variations (% COV) are presented inTable 4. The ratio of compressive strengths of 76.2 mm diametercylinder to compressive strength of 70.7 mm cube varied from0.82 to 0.9. It is observed that the strength increases initially withaddition of water because of proper hydration of cement pastewith increasing water content. It may be noted that initially thewater content in the mortar was not sufficient for proper hydra-tion process resulting into low strength. However, subsequent
Table 6Split tensile strengths of cylinder specimens from experimental and analyticalpredicted.
Fig. 7. Split tensile strength of cylindrical specimen.
Fig. 8. Failure pattern of cylindrical specimen during split tensile strength.
S.B. Singh et al. / Journal of Building Engineering 4 (2015) 94–10098
water addition leads to the reduction in strength as expected.Various failure patterns of specimens subjected to compressivestrength test are shown in Figs. 2–4. Due to the high w/c ratiospecimens have undergone lateral bulging during the compressivestrength test as shown in Fig. 2. It has been observed that verticalaxial crack developed in the most of the specimens (Fig. 3). In caseof cubes under compression test, initial cracks were developed attop and propagated to bottom with increase in load and then thecracks are widened at failure along the edge of the cube shown inFig. 4.
Abram’s Law [14] is valid for high strength mortar when w/c
ratio is greater than 0.40. Results show that compressive strengthof cement mortar with a varying w/c ratio follow the Abram’s Law.Rao [14] has developed a general formula relating strength to w/cratio of the mortar proportions of 1:2, 1:2.5, 1:3. In this paper, amore general expression for all the five mortar mixes i.e. 1:3, 1:4,1:5, 1:6 and 1:7 are evaluated. The experimental results of com-pressive strength of cube specimens with w/c ratio of the all fivemortars at the age of 28 days are shown in Fig. 5.
Generalized correlation has been derived as shown in Fig. 6 topredict the 28-day compressive strength of cement mortar for allthe five mix groups as a function of w/c ratio using regressionanalysis. In general, relationship between the 28 days compressivestrength and w/c ratio of cube specimens of cement mortar forvarious cement sand ratio is given by Eq. (4).
n w c/ 4nc 1
2σ = ( ) ( )
where parameters n1 and n2 are given in Table 5.Based on the above equation, design strength of mortar for any
practical purpose could be calculated. The difference between theresults from above equation and experimental is less than 15% asshown in Table 4.
Bolomey’s Eq. (5) has been used for relating the cement/waterratio to compressive strengths of concrete [19]. A, B are the con-stants depending on the material properties. In the present paper,parameters A and B are evaluated in the Table 5 for all the fivemortar mixes (1:3, 1:4, 1:5, 1:6 and 1:7) of cube specimens.
fA
w cB
/ 5ct =( )
+( )
4.2. Split tensile strength
The results of split tensile strength of cement mortar is pre-sented in Table 6 with coefficient of variations (% COV). Failuremodes of various split tensile strength specimens are shown inFigs. 7 and 8. In case of cylinders subjected to split tensile strength,the cylinder got splitted into two pieces as shown in Fig. 8. The 28-day Split tensile strength data is presented in Fig. 9. It is shownthat with increase in water content, the split tensile strength de-creases, as expected. Rao [14] developed the equations to predictthe split tensile strength of mortar with w/c ratio in all threemortar mixes (1:2, 1:2.5, 1:3). In this paper, an empirical expres-sion (Eq. (6)) has been derived (Fig. 10) to predict the split tensilestrength of cylinder specimens of cement mortar for various ce-ment sand ratio at an age of 28 days.
f n w c/ 6n
ct 34= ( ) ( )
where parameters n3 and n4 are presented in Table 5 for all the five
Fig. 9. Relationship between experimental split tensile strength of cement mortar and water cement ratio.
Fig. 10. Relationship between predicted split tensile strength of cement mortar and water cement ratio.
Fig. 11. Relationship between split tensile strength and compressive strength of cement mortar.
S.B. Singh et al. / Journal of Building Engineering 4 (2015) 94–100 99
S.B. Singh et al. / Journal of Building Engineering 4 (2015) 94–100100
mortar mixes.Similar to the compressive strengths of mortar, an expression
(Eq. (7)) has been developed for split tensile strength of cylinderspecimens also, where parameters C and D are presented in Ta-ble 5 for all the five mortar mixes.
fC
w cD
/ 7ct =( )
+( )
From the above equations (Eqs. (6) and (7)) expressing re-lationships between split tensile strength of mortar and w/c ratio,it would be possible to estimate the design strength of mortarrequired for any practical purpose. The difference between ex-perimental and predicted results (Eq. (6)) is less than 14% as pre-sented in Table 6.
Fig. 11 shows the nonlinear relation between the 28 days splittensile strength and compressive strength of mortar. The splittensile strength of any mortar at the age of 28 days can be esti-mated using Eq. (8) as a function of compressive strength ofmortar. The regression coefficient is 0.97 for Eq. (8).
f 0.21 8ct c0.66σ= ( ) ( )
where fct and sc are in MPa.
5. Conclusions
The compressive strength of cement mortar at the age of 28days has decreased with an increase in cement-to-sand propor-tions. The influence of w/c ratio and cement-to-sand proportionson compressive strength and split tensile strength of mortar arepresented through empirical equations. An expression for opti-mum water content required for making workable mortar hasbeen performed. The decrease in cement content requires morewater for making mortar workable. Abrams’ law is inferred to beapplicable in the case of cement mortar too with different para-meters. Empirical equations have been developed to predict thecompressive as well as split tensile strength of cement mortar fordifferent cement-to-sand ratio in terms of w/c. These results willbe helpful in design of cement mortar mix for masonry structures.
Acknowledgment
The authors would like to acknowledge the support from the
Department of Science and Technology (DST), New Delhi (Grant#SR/S3/MERC/051/2012) and Aditya Birla Group.
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