Effect of Aggregate Properties on Water Requirement
 

Introduction

The water requirement of a concrete mix is the amount of water needed to produce one cubic metre of concrete, of the desired workability.

The water requirement of a concrete mix is important as it has a direct effect on the economy of the mix (high water requirement = high binder content = high cost), and on the drying shrinkage of the concrete (high water content = high drying shrinkage).

The water requirement of any specific mix is primarily a function of the physical properties of the sand, and is modified to a lesser degree by coarse aggregate properties, binder properties and the use of admixtures.

This tip discusses the effects of particle size, shape and surface texture on the workability and water requirement of concrete. Reference is made to SABS 1083:1994 and to the Cement & Concrete Institute’s Commentary on that standard.

Fine Aggregates in Concrete


Particle Size, Grading and Dust Content

As a general rule coarse sands have lower water requirements than fine sands, but this is not always true.

Well-graded sands tend to have lower water requirements than single-sized sands and increasing dust contents tend to increase the water requirement of sands. However, there are exceptions. For example, pit sands from the Klipheuwel, Malmesbury and Saldanha areas tend to be of average fineness, modulus and contain excessive amounts of dust, but they have very low water requirements - among the lowest in the country.

Many attempts have been made to try and correlate water requirement with particle size distribution and aggregate surface area (logic suggests that a sand with a low surface area would have a low water requirement as there would be less surface to wet), none being particularly successful. Some mix design methods use a modified aggregate surface area method to determine a first estimate of the water requirement of the concrete aggregate properties.

Particle Shape

Fine aggregates in concrete have a particle shape that has significant influence on the workability and water requirement of concrete.

It is intuitive that sands with well-rounded particles will be less thirsty and make more workable concrete than sands with flaky, elongated particles. This is one reason (dust content and surface texture being the others) why crusher sands tend to have higher water requirements than natural sands.

Particle Surface Texture

In general, sands with a rough surface texture will have a higher water requirement than sands with smooth particle surfaces.

Again this is intuitive. On the other hand, sands with a slightly rough surface texture give slightly higher concrete strengths because of improved bond. In some cases (for example crushed dolerite from De Aar) there may be a significant strength increase.

Water Absorption

All aggregates absorb water to a greater or lesser degree. The higher the water absorption the higher the water requirement will be, but the water absorbed into the aggregate will not affect the effective water: binder ratio or the strength. It will however lead to rapid slump loss if absorption is excessive, say >1% by mass.

In general it is preferable to avoid concrete aggregate properties with water absorptions of more than 1 to 1,5% by mass.

Coarse Aggregates

The same factors affect water requirement as is the case with sands, but the effect is smaller, more predictable and easier to quantify:

Grading

Concrete made with well-graded stone generally has a lower water requirement than that made with single-sized stone. Other factors being equal the difference is of the order of 10 to 15 litres per cubic metre.

The workability of concrete made with single-sized stone is, however, more sensitive to water content than the workability of concrete made with continuously graded stone.

Particle Size

The larger the coarse aggregate, the less water is required. Using 19-mm stone as a reference, changing to 26,5-mm stone will require 5 litres less, and to 37,5-mm 15 litres less.

Changing to 13,2-mm stone will require about 15 litres more water.

Particle Shape and Surface Texture

The particle shape and surface texture of crushed coarse concrete aggregate properties affects the water requirement in the same manner as sands, but to a lesser degree. Angular, smooth particles have lower water requirements, but produce concrete with slightly lower flexural strengths.

A useful measure of particle shape is the voids content of the aggregate. This is calculated as follows:

Voids (%) = 100(1- Consolidated Bulk Density/1000*Relative Density)

The average voids content of South African sands is 35% with a range from 28 to 44%. For stone, the respective figures are 46% average, ranging from 34 to 50%.

In general, concrete aggregate properties with low void contents have low water requirements.

Example of the Effect of Water Requirement on Concrete Materials Costs

It is common practice when tendering for contracts to phone around for prices for sand and stone, and then to price the concrete accordingly, using the cheapest available sand and stone.

Unfortunately, the blend of the cheapest available materials does not necessarily produce the most cost-effective combination once the water requirements of the aggregates are taken into account.

Take a mix design for 25 MPa concrete where there is a choice of two sands, sand “A” at R50/m3 and sand “B” at R75/m3. Their water requirements are 210 and 185 litres/m3 respectively. Cement cost taken at R650/T, stone at R90/T.

The mixes and costs are tabulated below: (mix proportions by mass are in kg based on dry aggregates. The sand volumes are bulked volumes assuming 25% bulking).

Material

Sand “A”

Sand “B”

Cost mix “A”

Cost mix “B”

Water

Cement

Stone

Sand

210

315

1150

725 (0,60m3)

185

280

1150

785 (0,65m3)

-

204.75

103.50

30.00

-

182.00

103.50

48.75

Total

   

338.25

334.25

In this example, concrete made with the cheap sand actually costs R4 more per cubic metre. This is not always the case but it is essential that concrete mixes are priced on the basis of the total material’s cost per cubic metre, not on the basis of the cheapest available materials.