DEVELOPMENT OF READY-MIX CONCRETE TECHNOLOGY G. Shakhmenko Riga Technical University, Riga, Latvia 1. Introduction

At present ready-mix concrete production is

prevailing in concrete industry. The use of mixes with special reological properties (such as pumping, self-compacting, etc.) is now increasing. As a result it gives an impetus to develop concrete mix design methods. The proposed method facilitates determining of grading for different types of concrete mixes more precisely.

2. Restructurization of ready-mix concrete industry

During last 10 year considerable changes in concrete industry in Latvia have taken place. Construction of multi-storied residential and industrial panel building was stopped and large precast concrete factories were closed. However, last years building construction is developing much more efficiently, than other fields of industry. At present mostly monolite concrete constructions are used in buildings construction. This factor influences fast development of ready-mix concrete industry. During last 7 years ready mix concrete production in Latvia has increased about 2.5 times. The following main tendencies are evident. • Decreasing of role of little ready-mix concrete

plants where old technology and equipment are used.

• Active development of new large concrete ready-mix plants equipped with modern technology and computer aided control. It enables increasing of production quality. It therefore allows to introduce ISO 9000 quality system.

• Use of concretes with higher compressive strength. It makes use of mechanical properties of material

more effectively. For example, in 1995 average ordered concrete class was B15 - B20, but in 2000 – B25 - B30.

• Active use of new special types of concretes: !"pumping concrete (in 2000 it share was aproks. 50%

from the total produced concrete amount); !"high performance concrete for bridges,

hydrotechnical structures etc. and high strength concrete [1];

!"high performance light concrete with density 1700 - 2000 kg/m3 [2];

!"self-compacting concrete (SCC), that allows to make durable constructions when minimum skilled workers are employed. This type of concrete was first developed in 1988 and at present is being used more widely [3].

3. Optimum models of aggregate mix compositions

Active development of ready-mix concrete industry requires effective and flexible methods for concrete mix design. Special attention to aggregate composition must be paid because the aggregate forms main skeleton of material and determines reological properties of concrete mix. Specially prepared aggregates as well as natural ones are used. The task is to find the appropriate aggregates and its proportions for given concrete mix. This task consists of two parts. On one hand, aggregate mix with maximum dense packing and minimum void content must be obtained. On the other hand, the best properties of concrete mix complying with the given technology must be taken into account: pumpability, compactability without segregation and bleeding, etc..

Different aggregate mix design methods are used [4].

Former method of "course aggregate packing" was applied, but nowadays it becomes obsolete because it has a restricted

field of application (it allows using only one type course aggregate). Packing models method allows to obtain dense packing, but necessary workability of concrete mix is not guaranteed in some cases. The most steady results may be obtained by means of "ideal" grading method.

At present aggregate mix optimum proportions mostly are calculated taking into account "ideal" grading restricting areas. The most known of them are DIN 1045 curves, but these have some drawbacks. On one hand, these curves are given for discrete aggregate maximum size – 8, 16, 32 and 64 mm. In practice aggregates with intermediate finess are mostly used (especially natural aggregates). On the other hand, universal requirements for aggregate grading, used for special concrete types, have not been worked out. The task is to find usable formula for different cases.

The large part of "ideal" grading curves is based on "Fuller's" curve [5]. In common case this curve can be determined by the following equation:

YTi = T (Xi - Xo)0.5 here YTi - is “ideal” (theoretical) passing, %; Xi - is sieve hole size, mm Xo =0.063 - is aggregate minimum size, mm

(beginning point of an “ideal” curve);

T - is the coefficient depends on aggre- gate minimum and maximum size.

This equation without corrections can't be used in

practice. Comparing Fuller's curve to aggregate "ideal" grading restricting areas in accordance with DIN 1045 it is evident that large part of Fuller's curve is situated inside optimum grading (Fig 1), but contains an increased amount of fine particles (below 0.125 and 0.25 mm). Corresponding correction coefficients for "ideal" passing YTi are proposed:

K0.125 = 0.4; K0.25 = 0.9 It must be stressed, The Fuller's curve provides

quite dense packing of aggregates. It successfully may be applied for stiff concrete mixtures if shock or vibro pressing compacting methods are used as well as for mix with medium workability (slump class S1 in accordance with EN 206 [6]). Flowable and pumping concrete mixes require increasing mortar part. SCC

concrete mixes also require just more mortar part and in addition special fillers must be used. Some authors [7] propose to determine SCC concrete mix composition by specifying gravel content in total absolute volume and sand content in mortar volume. This approach gave good results for well known, fractionated aggregates and can not be applied for non-standard aggregates.

To determine required "ideal" curves for different types of concretes it is suggested to use the modified Fuller's curves equations:

YTi = (100/(Xmax - Xo)n) (Xi - Xo)n

here n - is degree of an “ideal” curve equation; Xmax - is aggregate maximum size, mm

(finish point of an “ideal” curve).

Parameter n can be changed depending on type and application of concrete mix, parameter Xmax determines maximum size of course aggregate. In addition, corrections for sieves 0.063, 0.125 and 0.25 mm must be taken into account. Determined parameters for different types of concrete mixes are shown in Table 1.

Table 1. "Ideal" curve parameters

Corrections for "ideal

curve YTi in sieves with

hole dimension, mm:

Field of application

of concrete mix

Degree n of an "ideal"

curve 0.063 0.125 0.25 Earth-wet mix and

slump class S1 0.50 ∗ 0 ∗ 0.4 ∗ 0.7

Pumpable concrete, slump class > S2

0.40 - 0.48 ∗ 0 ∗ 0.4 ∗ 0.7

Self-compacting concrete (SCC)

0.38 (128 / Xmax)

^ 0.3 * 2

∗ 0.7 ∗ 0.9

4. Modified Fuller's curve application for aggregate mix design

Obtained mathematical model of "ideal" grading curve has been applied for aggregate mix calculation for different types of concrete. The practical examples of calculation are shown in Fig 1, 2 and 3. Natural sand and gravel (with big amount of fine particles) are used as aggregates. Limestone powder is used as filler for self-compacting concrete. Calculations are carried out by means of computerized

numerical method [4]. Results of calculations (Tab 2) show, that the same materials in different proportions can be used for different types of concrete mix (stiff concrete mix, pumping concrete and SCC).

Fig 1. Grading curves for stiff concrete mix

Fig 2. Grading curves for pumpable concrete mix

Fig 3. Grading curves for self-compacting concrete mix

Table 2. Aggregate mix proportions for given aggregates

Type of concrete

Gravel Sand Talsi Limestone powder

Stiff 0.66 0.33 0.01 Pumpable 0.61 0.38 0.01

SCC 0.53 0.41 0.06

This "ideal" curve model has been successfully applied

in producing some laboratory mixes as well as positive results are obtained in ready-mix concrete plant. It must be emphasized, that calculation of cement paste content is the next part after aggregate mix design. Workability of concrete mix depends on the total mix composition, and therefore such parameters as paste content, mortar content and total fine particle content (cement + aggregate <0.125mm) also must be controlled.

5. Conclusions

Ready-mix concrete production takes a new quality

level that requires a new approach for concrete mix design. Parameters for modified Fuller' curve are proposed and

enable defining optimum aggregate grading for ordinary, pumpable and self-compacting concrete mixes.

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The same aggregates may be used in different proportions to obtain concrete mixes with different applications. Computer-aided program allows to modify different "ideal" grading curves and find optimum aggregate mix for given application. References 1. S. Helland. Bulletin High Strength Concrete. State of the

Art Report // FIP/CEB Bulletin d'Information No. 197, p 5-29.

2. Technical and Economical Mixture Optimization of High Strength Lightweight Aggregate Concrete // EuroLightCon Document BE96-3942/R9, March 2000.

3. P. Billberg. Self-compacting concrete for Civil Engineering Structures – the Swedish Experience // Swedish Cement and Concrete research Institute, August, 1999.

4. G. Shakhmenko. Optimal aggregate mix design // Proceedings of the 6th International Conference "Modern Building Materials, Structures and Techniques" held on May 19-22, 1999, Vilnius, Lithuania, Vol. 1. Vilnius: Technika, 1999, p. 86-91.

5. Ю. Биршс. Разработка режимов формования железобетонных изделий по ударной технологии. - Диссертация на соискание ученой степени кандидата технических наук. Москва, 1987.

6. EN 206. Concrete - part 1: Specification, performance, production and conformity.

7. H. Okamura. Self-compacting concrete // Structural Concrete, Volume 1, Number 1, March 2000.

Įteikta 2001 02 15 ……………………………………………………………………... Genadij SHAKHMENKO. MSc., Chief of laboratory, PhD student. Dept. of Building Materials. Riga Technical University (RTU), Azenes st. 16, LV-1048, Riga, Latvia. E-mail: gena@bf.rtu.lv

Graduate of Riga Technical University (1991, civil engineer). MSc (1995) at RTU. Author and co-author of 4 research papers. Research interest: investigations in concrete mix design and optimization.