Advanced concrete technology1 fresh concrete

Advanced concrete technology1 fresh concrete Advanced concrete technology1 fresh concrete Advanced concrete technology1 fresh concrete Advanced concrete technology1 fresh concrete Advanced concrete technology1 fresh concrete Advanced concrete technology1 fresh concrete Advanced concrete technology1 fresh concrete Advanced concrete technology1 fresh concrete Advanced concrete technology1 fresh concrete Advanced concrete technology1 fresh concrete Advanced concrete technology1 fresh concrete

Fresh concrete

P.L D o m o n e

Fresh concrete is a transient material with continuously changing properties It is, however, essential that these are such that the concrete can be handled, transported, placed, compacted and finished to form a homogenous, usually void-free, solid mass that realizes the full- potential hardened properties A wide range of techniques and systems are available for these processes, and the concrete technologist, producer and user must ensure that the concrete is suitable for those proposed or favoured

Fresh concrete technology has advanced at a pace similar to many other aspects of concrete technology over the past three decades, and indeed many of these advances have been inter-dependent For example, the availability of superplasticizers has enabled workable concrete to be produced at lower water/binder ratios thus increasing the in-situ strength

In this chapter, we will start by considering the property known as workability*,

including its definition and common methods of measurement We will point out the limitations of these, and show how this leads to the need for a more fundamental scientific description of the behaviour of fresh cement pastes and concrete We will then describe how this has been achieved by applying the principles of rheology, and explain the development and use of test methods which give a more complete understanding of the behaviour We will then discuss the effect on the rheological properties of a range of constituent materials, including admixtures and cement replacement materials, and how

a knowledge of these properties can be used to advantage The factors that influence the loss of workability before setting are then briefly considered

*The alternative term 'consistence' is often used, particularly in specifications and standards

We will not discuss the specific properties required for particular handling or placing techniques such as pumping, slipform construction, underwater concreting etc These are covered in various chapters in Volume 3 of this series, but hopefully the more general description given in this chapter will be of value when reading these We will, however, describe the principles of ensuring that the concrete is correctly placed and compacted to give a uniform, homogenous result Finally, we will discuss the behaviour of the concrete after placing but before setting, with particular reference to segregation and bleed

1.2.1 Terminology and definitions

Problems of terminology and definition are immediately encountered in any discussion

of the fresh properties of concrete Every experienced concrete technologist, producer and handler has an understanding of the nature and properties of the material, and can choose from a wide variety of terms and expressions to describe it; examples include harsh, cohesive, lean, stiff, rich, etc Unfortunately, all these terms, and many others, are both subjective and qualitative, and even those that purport to be quantitative, e.g slump, give a very limited and sometimes misleading picture, as we will see This is not to say that such terms and values should not be used, but that they must be used with caution, particularly when trying to describe or specify the properties unambiguously

A satisfactory definition of workability is by no means straightforward Over 50 years ago, Glanville, et al (1947), after an extensive study of fresh concrete properties, defined

workability as 'the amount of work needed to produce full compaction', thereby relating

it to the placing rather than the handling process A more recent ACI definition has encompassed other operations; it is 'that property of freshly mixed concrete or mortar which determines the ease and homogeneity with which it can be mixed, placed, consolidated and finished' (ACI, 1990) This makes no attempt to define how the workability can be measured or specified A similar criticism applies to the ASTM definition of 'that property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity' (ASTM, 1993)

Such definitions are clearly inadequate for the description, specification and quality control of fresh concrete, and many attempts have been to provide a more satisfactory definition which includes quantitative measurements These are sometimes more restrictive, for example the ACI (1990) definition of consistency as 'the relative mobility or ability

of freshly mixed concrete to flow', which is measured by the slump test This difficulty illustrates that no single test or measurement can properly describe all of the required properties of the fresh concrete

(Tattersall 1991) has proposed a division of the terminology relating to workability into three classes:

Class 1:

Class 2:

Qualitative, to be used in a general descriptive way without any attempt to

quantify, e.g workability, flowability, compactability, stability, pumpability

Quantitative empirical, to be used as a simple quantitative statement of behaviour

in a particular set of circumstances, e.g slump, flow table spread

Fresh concrete 1/5

Class 3: Quantitative fundamental, to be used strictly in accordance with the definitions

in BS 5168: Glossary of rheological terms, e.g viscosity, mobility, fluidity, yield value 1

Such a division is helpful in that it clearly exposes the limitations of many of the terms, and it will be useful to keep this in mind when reading this chapter

1.2.2 Measurement of workability by quantitative

As long ago as 1947, twenty-nine single-point tests were described as the more important

of those developed up to that time (Glanville et al., 1947) A recent compendium of tests has included sixteen single-point tests, and therefore at least this number are likely to be

in current use (RILEM, 2002) Few, if any, of the tests described are suitable for the complete range of workabilities used in practice Indeed, many have been developed in the past two decades in response to the use of increasingly higher workability concrete, including, most recently, self-compacting concrete

Four tests have a current British Standard: slump, compacting factor, Vebe and flow table (or more simply, flow), and will now be discussed together with the slump flow test,

an adaptation of the slump test for self-compacting concrete, and the degree of compactability test, which has replaced the compacting factor test in the recent European Standards The tests are shown and described in Figures 1.1-1.6 Table 1.1 gives the principles on which they operate, and some comments on their use

The slump test (Figure 1.1), which is simple, quick and cheap, is almost universally used for nearly all types of medium and high workability concrete As well as the drawbacks listed in Table 1.1, there are also some differences in practice with its use in different countries, particularly with respect to the British and American standards

First, the British and European Standards specify that the slump should be measured

to the highest point of the concrete, whereas the American standard specifies measurement

to the displaced original centre of the top surface of the concrete (as shown in Figure 1.1) Clearly, the same test on the same concrete can give different values depending on where

it is performed

Second, the British standard only recognizes values from a true slump as valid, and does not allow recording of values from either shear or collapsed slump (Figure 1.1); the American standard includes a similar restriction for shear slump, but allows measurements

of a collapsed slump, and values of 250 mm and above are often reported The recent European standard states that the test is sensitive to changes in consistency corresponding

to slumps between 10 and 200 mm, and the test is not considered suitable beyond these extremes

The flow table (Figure 1.4) test was introduced initially to German standards when superplasticizers and high workability flowing concrete (i.e collapsed slump) started to

1 A list of the relevant standards can be found at the end of the chapter

Table 1.1 Common single-point workability tests

Slump (Figure 1.1) measures a flow property of

concrete under self-weight after standard compaction

Vebe

(Figure 1.3)

measures the amount of work (time at constant vibration) for full compaction

as in the slump test, measures

a flow property of concrete under self-weight, but after self-weight compaction measures the effect of a standard amount of work (dropping the concrete from the edge of the container) on compaction

• suitable for medium and high workability concrete

• sensitive to small changes in water content

• very simple, suitable for site use

• heavily operator dependent

• suitable for low, medium and high workability mixes

• fairly simple, but requires scales

• less operator dependent than slump

• suitable for very low and low workability mixes

• greater relation to concrete placing conditions than slump

• more complex than other methods, requires standard vibrating equipment

• sometimes difficult to define end point

• suitable for high and very high workability mixes

• gives some indication of tendency of mix to segregate

• fairly simple, but, like slump, operator dependent

• developed for self-compacting concrete

• very simple, suitable for site use

• operator dependent, but less so than slump

• an alternative to the compacting factor test

• simple, suitable for site use

• likely to be operator dependent

1 The cone is filled with

concrete in three equal

layers, and each layer is

compacted with twenty-five

tamps of the tamping rod

3 The slump is measured using the upturned cone and slump rod as a guide

Types of slump

Figure 1.1 The slump test (BS 1881 Part 102: 1983; BS EN 12350-2: 2000; ASTM C 143-90a)

Upper hopper

Lower hopper

300 x 150 mm cylinder

/ /

,,,'-,

%° Oo°° ~

App 1 metre

1 Concrete is loaded into the upper hopper

2 The trap door is opened, and the concrete falls into the lower hopper

3 The trap door is opened, and the concrete falls into the cyinder

4 The concrete is struck off level with the top of the cylinder

5 The cylinder + concrete is weighed, to give the partially compacted weight of concrete

6 The cylinder is filled with fully compacted concrete

7 The cylinder + concrete is weighed, to give the fully compacted weight of concrete

Compacting factor = weight of partially compacted concrete

weight of fully compacted concrete

F i g u r e 1.2 The compacting factor test (BS 1881 Part 103: 1993)

Clear perspex disc JJ _

3 Vibration at a standard rate is applied with a vibrating table

Vebe degrees is the time (in seconds) to complete covering of the underside

of the disc with concrete

Figure 1.3 The Vebe test (BS 1881 Part 104: 1983, BS EN 12350-3: 2000)

2 The free edge of the board is lifted against the stop and dropped 15 times

Flow = final diameter of the concrete (mean of two measurements at right angles) Figure 1.4 The flow table test (BS 1881 Part 105: 1984, BS EN 12350-5: 2000)

/ I /

I

1 A slump cone (see Figure 1.1) is filled without compaction

2 The cone is lifted and the slump flow is final diameter of spread (mean of two diameters at right angles)

3 The time to reach a spread of 500 mm is sometimes also measured

4 The baseboard must be smooth, clean and level Figure 1.5 The slump flow test

become popular in the 1970s However, this test was criticized (Dimond and Bloomer, 1977) even before its first inclusion in British Standards in 1983, for several reasons, including:

• The test is operator sensitive, potentially more so than the slump test;

• When the spread exceeds 510 mm, the recommended minimum for flowing concrete (Cement and Concrete Association, 1978), the concrete thickness is about the same as

a 20 mm aggregate particle, and the test cannot therefore be a satisfactory measure of the bulk concrete properties;

• There is a high degree of correlation between the initial spread before jolting and the final spread after jolting, and thus no extra information is gained by the jolting

Dimensions in mm

1 The container is filled with concrete, using a trowel, from all four edges in turn

2 Excess concrete is struck off with a straight edge

3 The concrete is compacted by vibration

4 The height s is measured at the mid-point of each side, and the mean of the four

readings calculated

Degree of compactability = h/(h- s)

(to two decimal places)

Figure 1.6 The degree of compactability test (BS EN 12350-4: 2000)

The relationships between slump and flow table results from three sources are shown

in Figure 1.7; two of these indicate an S-shaped relationship showing the increased sensitivity of the flow table test at higher slumps, but the third is linear between slumps

of 100 and 250 mm However, the scatter is sufficiently wide to encompass both forms of the relationship

The slump flow test (Figure 1.5) could be considered as an alternative to the flow table test, and, as already mentioned, is widely used for testing high-fluidity self-compacting

concrete (it has been standardized for this purpose in Japan) The only extra complication over the slump test is that the result is more sensitive to the surface condition of the board

on which the test is performed The relationship between slump amd slump flow from three test programmes is shown in Figure 1.8 Not surprisingly this shows that, at slumps above about 200 mm, the latter is much more sensitive to changes in the concrete fluidity The best-fit relationships diverge at higher slumps, which may reflect differences in practice, e.g in the measurement of slump as discussed above

Slump flow (mm) 8OO

600

400

200

Domone (1998) Khayat et al (1996) /

Slump (ram) Figure 1.8 The relationship between slump and slump flow measurements

Since the tests listed in Table 1.1 are based on several different principles, and measure different properties, it is not surprising that only a very wide degree of correlation is obtained between them, with considerable scatter This is illustrated by the data plotted

in Figure 1.9, from a single but comprehensive test series These broad general relationships are reflected in the consistence classes given in the European standard for concrete specification, EN 206: 2000, which are listed in Table 1.2 The standard states that the classes are not directly related, but they are consistent with the relationships shown in Figures 1.7 and 1.9

Table 1.2 Consistence classes according to BS EN 206-1" 2000

Figure 1.9 Typical spread of results from single-point workability tests (data from Ellis, 1977)

Table 1.3 Slump, Vebe and compacting factor results from four mixes (data from Ellis, 1977)

by compacting factor: Mix B ~ Mix A ~ Mix C ~ Mix D

These different rankings are clearly unsatisfactory - not only do the tests have limitations, but they can also be misleading

For a greater understanding of the behaviour in general, and an explanation of the anomalies that can arise from single point testing in particular, we need to turn to the science of rheology, and to consider the developments in the application of this to fresh concrete that have taken place over the past thirty years or so

1.2.3 Rheology of liquids and solid suspensions

Rheology is the science of the deformation and flow of matter, and hence it is concerned with the relationships between stress, strain, rate of strain and time We are concerned with flow and movement, and so we are interested in the relationship between stress and rate of strain

Fluids flow by the action of shear stress causing a sliding movement between successive adjacent layers, as illustrated for laminar (non-turbulent) flow in Figure 1.10 The relationship between shear stress (~:) and rate of shear strain ('~) is called the flow curve, and can take

a variety of forms, as shown in Figure 1.11 The simplest form is a straight line passing

applied shear stress (x) = force/area = P/A

shear strain (y) = shear displacement/base length = x/y

rate of change of shear strain = dy/dt =

Figure 1.10 Shear flow in a fluid under the action of a shear force

through the origin This is called Newtonian behaviour, and is a characteristic of most simple liquids, such as water, white spirit, petrol, lubricating oil, etc., and of many true solutions, e.g sugar in water The equation of the line is

1 : = r l - ? and the single constant 11 (called the coefficient of viscosity) is sufficient to fully describe the flow behaviour

The other forms of flow curves in Figure 1.11 all intercept the shear stress axis at some positive, non-zero value, i.e flow will only commence when the shear stress exceeds this threshold value, which is often called the yield stress This is a characteristic of solid suspensions, i.e solid particles in a liquid phase, of which cement paste, mortar and concrete are good examples A wide range of equations have been proposed to model the various shapes of flow curves found in practice, but for our purposes it is sufficient to consider a general equation of the form:

Rate of shear strain (~,)

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annular gap between the inner and outer cylinders is typically of the order of few millimetres, and this is suitable for testing cement paste, in which the maximum particle size is about

100 pm

For low shear rates and/or viscous liquids, the flow is uniform and laminar and can readily be analysed; the resulting linear equation relating the measured torque (T) to the rotational speed (N) for a Bingham fluid is:

_ m I n

and hence Xy and p can be calculated from the measured flow curve of T versus N, and the instrument geometry (Note: this is known as the Reiner-Rivlin equation; a full analysis can be found in Tattersall and Banfill, 1983.) There has been considerable debate over the requirements for and methods of avoiding particle sedimentation and slippage on the cylinder surfaces Although these have not been fully resolved, a consensus of typical behaviour can be identified

First, taking the simplest mixture of cement and water, varying the water/cement ratio produces a fan-shaped family of flow curves such as that in Figure 1.13, which shows that both the yield stress and plastic viscosity reduce with increasing water content Figure 1.14 shows some values for Portland cement mixes, from which it can be seen that the magnitude of the changes of both the yield stress and plastic are similar, i.e adding or subtracting water produces similar proportional changes in both properties

Shear stress (x) ~ , ~

~ e a s i n g

Wat~er/cement

Rate of shear strain (~,)

Yield stress (Pa)

Fresh concrete 1/15

The behaviour is, however, somewhat different when the fluidity of the paste is increased with a superplasticizer Typical data for the addition of a naphthalene formaldehyde superplasticizer to pastes with three different water/cement ratios are shown in Figure 1.15 With increasing admixture dosage, the proportional reduction in the plastic viscosity

is much less than that in the yield stress

The data of Figures 1.14 and 1.15 can be combined into a single diagram of yield stress versus plastic viscosity, as in Figure 1.16 From the individual data points lines of equal water/cement ratios and superplasticizer dosages can be drawn The latter are much steeper than the former and indeed, are near vertical over much of the range Clearly the mechanisms of fluidity increase by water and superplasticizer must be different- both make the flow easier to initiate, i.e they reduce the yield stress, but superplasticizers maintain the viscosity Such diagrams are extremely useful in showing these interactive effects, and we will use them later to describe the more complicated behaviour of concrete

Yield stress (Pa)

0.1

0.01

wlc 0.3

0.4 "

Sp dosage (% solids by wt cement) Sp dosage (% solids by wt cement)

Figure 1.15 Typical effect of superplasticizer on Bingham constants for cement paste (Domone and

Plastic viscosity (Pa.s)

superplasticizer dosage (constructed from the data in Figures 1.14 and 1.15)