Role of FM of sand

The grading and maximum size of aggregates is important parameters in any concrete mix. They affect relative proportions in mix, workability, economy, porosity and shrinkage of concrete etc. Experience has shown that very fine sands or very coarse sands are objectionable – the former is uneconomical, the latter gives harsh unworkable mixes. Thus the object in this paper is to find the best fineness modulus of sand to get the optimum grading of combined aggregate (all-in-aggregate), which is most suitable, and for economy. In general, the grading of aggregates, which do not have a deficiency or excess of any size of aggregate and give a smooth grading curve, produce the most suitable concrete mix. Further a cohesive mix is also desired for the pumped concrete produced by RMC Plant. In the present investigations, effect of the grading of river sand particles has been investigated for a good Concrete mix. Sand has been sorted in three categories i.e. Fine, Medium, and Coarse. These were mixed with coarse aggregate in different proportions so as to keep the combined Fineness Modulus (all-inaggregate) more or less the same. Various proportions of such aggregate are mixed in preparing M 30 grade of Concrete mix. Effect is studied on concrete workability, cube strength, flexural strength and permeability. The results indicate that with the change in fineness of sand, workability gets affected. The details of findings and its effect on compressive and flexural strength and permeability, influencing durability are reported in this paper.

Introduction

Fineness Modulus is a term used as an index to the fineness or coarseness of aggregate. This is the summation of cumulative percentage of materials retained on the standard sieves divided by 100. It is well–known that aggregate plays an important role in achieving the desired properties of concrete. Though, aggregate constitute 80 to 90% of the total volume of concrete, yet very littleattention is given in controlling the grading and surface texture of aggregate to optimize the properties of concrete. Improper blend of aggregate influences the cement and water demand for a given concrete mix and affects workability, compactibility, and cohesion characteristics of pumpable concrete mix. It also influences the compressive strength, flexural strength and other properties like permeability & durability of concrete.

Review of Provisions in Different Specifications

IS 383: "Specifications for Coarse and Fine Aggregates from Natural Sources for Concrete." This publication deals with specifications for Coarse and Fine aggregates from natural sources for Concrete. These specifications do not specify any limit for fineness modulus to be used in concrete. It divides the sand in four zones i.e. from Zone I to Zone IV. Zone I–Sand being very coarse and Zone 4 sand is very fine. It is generally recommended by code to use sands of zones I to Zone III for Structural concrete works.

AASTHO Designation: M6-93- "Standard Specification for Fine Aggregate For Portland Cement Concrete"- It indicates that the fineness modulus of sand will not be less than 2.3 and nor more than 3.1. Further, fine aggregate failing to meet the fineness modulus requirement as above may be accepted, provided concrete made with similar fine aggregate from the same source has an acceptable performance record in similar concrete construction; or in absence of a demonstrable service record, provided, it is demonstrated that concrete of the class specified, made with the fine aggregate under consideration, will have relevant properties at least equal to those of concrete made with the same ingredients, with the exception that a reference fine aggregate be used which is selected from a source having an acceptable performance record in similar concrete construction.

ASTM Designation: C33-93- standard specification for concrete aggregates"–The fine aggregate shall have not more than 45% passing any sieve and retained on the next consecutive sieve and its fineness modulus will not be less than 2.3 and not more than 3.1. Rest is the same as for AASTHO M6-93.

U.S.B.R: The code has specified that the fineness modulus of sand shall not be less than 2.50 and not more 3.0.

Experimental Investigation

In the present investigations, the effect of Fineness Modulus of sand has been investigated. The Fine aggregate (Sand) taken is Yamuna river sand and coarse aggregate taken is Dolomite limestone in crushed form. It has been sorted in several categories starting the Fineness Modulus (FM) of sand from 2.0 to 3.0. These were mixed in different proportions to get a consistent combined FM. The combined FM is determined like All-in-aggregate FM. In the present study we have selected M30 Grade of concrete mix. To find out the effect of fineness modulus (FM) of sand on concrete, sand of different FM from 2.0 to 3.0 is chosen. Two sizes of coarse aggregate particles: i.e. 20 & 10 mm, which are generally used in standard concrete mix, were chosen for the investigations.

Concrete Mix Selected:

Concrete Grade : M30

Water-Cement Ratio : 0.45

Cement: OPC 53 grade (350 Kg)

Aggregate to Cement Ratio : 5.52

Admixture: Super plasticizer (as required)

Fine Aggregate : Yamuna River Sand (Average 777Kg)

Coarse Aggregate: Dolomite;

Crushed Stone (Average 1155Kg)

When we choose very fine sand (i.e. FM 2.0), and very coarse sand (i.e. of FM 3.0), and if the proportion of sand is fixed in the mix then due to poor all-inaggregate grading, the mix may become very harsh or not give correct results. So in present study proportions of coarse aggregate and fine aggregate, are slightly adjusted in the mixes to keep allin- aggregate grading within envelope of desired all-inaggregate grading given in IS: 383. The FM of combined mix is kept in range of 4.94 to 4.97 as seen from Table 2.

In this study water-cement ratio (W/C) of mix is kept constant for all the trial mixes with sand of different fineness modulus. Workability of mix is also fixed in range of 45 to 55 mm slump. Since mix with so different fineness modulus of sand, will result in different water demands, so watercement ratio is kept constant and to adjust workability slight adjustments in admixture dosages has been made. Various proportions of such ingredients are mixed in laboratory mixer of 0.1 capacity for preparing M30 grade of Concrete mix. Cubes (150 x 150 x 150 mm size), cylinders (150 Ö x 150 mm height) and beams (150 x 150 x 700 mm length) are cast. Effect of varying FM of sand is studied on concrete density, workability, compressive strength, flexural strength and permeability.

Observations & Discussion of Results

A. Workability of Concrete Mix: The workability of concrete mix was measured with the help of 300 mm standard size slump cone. A little amount of admixture dose was added to concrete mix. Each time concrete mix was examined for the behavior in slump, segregation and Bleeding etc. The slump observed was about 50 mm in all cases. No segregation or bleeding was observed in the mix.

-shows the type of slump observed. The results indicate that with the increase in fineness modulus of sand, water demand in the mix got affected consequently workability gets affected. Since water-cement ratio is kept constant, so to keep workability in the same range of 50 mm, admixture dosages were varied. The admixture dosages reduced considerably as fineness of sand increases.

· Admixture Dosage reduced from 1.0 percent to 0.2 percent as sand fineness modulus increases from 2.0 to 3.0.

· For Every 5% increase in FM of sand, admixture dosage reduced by 0.1%.

Effect of Fineness of Sand on Density of Concrete

After measuring the slump, several 150 mm cubes were filled. These were cured in water tank for 28 days. After curing, each cube was weighed using electronic balance and density of concrete was calculated. The variation of density with FM of sand for different cases. it is evident that there is slight increase in density i.e. 0.80 to 1.20 percent, when fineness modulus increases from 2.0 to 3.0.

Cubes of 150 mm were tested for compressive strength at 7 and 28 days. This compressive strength is given in table 3 for varying FM of sand. The figure indicates that:

· As fineness modulus of sand changes from 2.0 to 2.5 there is an increase in compressive strength from 43.07 to 49.00 MPa. i.e. strength increases by 14%. On the other hand by increasing Fineness Modulus from 2.5 to 3, Compressive strength increases from 49.00 to 56.83 MPa resulting in 16% increase in strength.

· For Every 0.1 increase in FM of sand from 2.0 to 3.0, 28 days Compressive Strength increases by 2.5 to 3.0%.

· 7 days compressive strength also increases in the similar proportion.

· There is faster increase in strength towards coarser side of sand.

Effect of Fineness of Sand on Compressive Strength of Concrete

Effect of Fineness of Sand on Flexural Strength of Concrete

Flexural strength is calculated from 28 days testing of beam of size 150x150x700 mm by using following formula.

Flex. Strength = P x 1000 x L / [b x d x d], for a > 200 mm but less than 200 mm

= P x (3000 x a) / (b x d x d), for a > 170 mm but less than 200 mm

= Result is discarded when a > 170 mm

Where,

b = width of sample beam (150 mm).

d = depth of sample at the point of failure (1500 mm).

a = distance between the line of fracture and the nearest support (recorded for each sample after test).

P = failure Load.

L = total support length of specimen (600 mm).

The variation of flexural strength with respect to different parameters is also given  This figure indicates the following:

· As fineness modulusincreases from 2.0 to 2.5 there is an increase in 28 days flexural strength from 3.82 to 4.25 MPa i.e. strength increases by 11.25%. On the other hand by increasing Fineness Modulus from 2.5 to 3, the strength increases from 4.25 to 4.81 MPa resulting in 13.1% increase in strength.

· For Every 0.1 increase in FM of sand from 2.0 to 3.0, Flexural Strength increases by 2.1 to .5%

· The increase in strength is more towards coarser side of sand.

Effect of Fineness of Sand on Permeability of Concrete

Permeability of concrete is determined by using cylinder specimen having 150 mm diameter and 160 mm height. They were applied water pressure of 7 Kg/ cm2 for 96 hours in the Permeability Apparatus

Immediately after 96 hours cylinders were split under line load test. The depth of penetration of water in cylinder was measured as well as volume of water lost is recorded.

The results are interpreted as:

1. Average depth of water penetration in cylinder is measured

2. Coefficient of permeability is calculated as volume of water lost divided by volume of concrete penetrated with water i.e.

Permeability coefficient = vol. of water lost / (Area of cylinder x Average depth of concrete having effect of penetration of water).

The Permeability Coefficient of concrete Vs FM of sand is plotted in figures 6. It is

seen from figure that permeability coefficient is more or less constant with respect to fineness of sand. Thus FM of sand has very little impact on Permeability Coefficient of Concrete and the value remains more or less constant.

Failure Pattern of Beams & Cubes

1. It is generally seen that the failure occurs at the interface of aggregate and mortar.

2. In flaky aggregate, some voids are observed at the interface of concrete and mortar. Elongated aggregate pieces are broken.

3. Mortar matrix isgenerally crushed.

Cost Benefit Ratio

Cost of Concrete mix per is calculated on the basis of unit cost of each ingredient material in the mix. The following market rates have been taken for Cement, Sand, Coarse aggregate, Admixture and a nominal cost for water. No labor cost has been added in the calculation as it will remain constant.

Cement: Rs. 4.25 per Kg

Sand* : Rs. 0.30 to Rs. 0.32 per Kg (depending upon Fineness of Sand)

Coarse Aggregate: Rs. 0.75 per Kg

Admixture: Rs. 40.00 per Kg

Water: Rs. 0.10 per Kg.

* Rate of Sand for FM 2.0 to 2.3 is Rs. 0.30 per Kg, for FM 2.4 to 2.7 is Rs. 0.31 per Kg, and for F. M. 2.8 to 3.0 Rs. 0.32 per Kg. Cost of concrete is calculated by taking above rates and quantities given in table On the basis of cost calculated for concrete and the corresponding 28 day compressive strength, the cost benefit ratio is calculated as follows.

a. Cost of concrete is calculated in terms of quantity of material used & market rates as given above.

b. Cost Benefit Ratio is calculated as:

C/B ratio = Total cost of Concrete/28 days Compressive Strength

A curve has been plotted between FM of sand Vs C/B Ratio as shown in Figure 6. From this graph, it is seen that C/B ratio reduces considerably as the FM of sand increases. From FM varying from 2.0 to 3.0 the C/B ratio reduces by 71%. Thus, it is advisable to use coarser sand in Concrete.

Conclusion

Fineness Modulus of Sand affects Compressive and flexural strength of Concrete. Sand, with higher FM, results in higher strength of concrete. It is evident by cost benefit ratio that overall concrete mix is becoming economical if we use sand with higher FM. The results indicate that with the increase in FM, workability gets affected considerably. The cement demand also gets modified. Some of the observations are given below:

· Fineness Modulus has larger impact on 28 days Compressive & Flexural Strength.

· Fineness Modulus has very little impact on Permeability of Concrete. Permeability coefficientis changed by about 2% for FM from 2.0 to 3.0.

· Fineness Modulus also affects the density of concrete. It increases by about 2.3% as the FM increases from 2.0 to 3.0. Optimum value of density and other parameters are obtained when FM is 2.8.

· The optimum value of strength can be taken when workability of concrete is also good. It is obtained when Fineness Modulus is about 2.7.

· The net cost of Concrete reduces when FM of sand increases. It reduces by about 6.5% for an increase of FM from 2.0 to 3.0.

· As the fineness modulus of Sand increases, the Cost/Benefit Ratio reduce by a very large factor. This is 29% when FM changes from 2.0 to 3.0. That means we can get large advantage by using concrete having Coarse Sand.

· A well adjusted grading (all-inaggregate) of concrete mix is also suitable for pumped concrete produced through RMC Plant. This is achieved by using a sand having FM of around 2.5.