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close this bookJournal of the Network of African Countries on Local Building Materials and Technologies - Volume 1, Number 4 (HABITAT, 1991, 48 p.)
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View the documentHousing in Africa, problems, prospects and strategies*
View the documentKenya: Towards the development of a national code of practice for structural masonry - the Kenyan approach*
View the documentNigeria: Research and development in the promotion of standards and specifications for stabilized soil blocks*
View the documentEthiopia: Light-weight concrete made with Ethiopian pumice*
View the documentMauritius: Use of calcarenite blocks in housing construction in Rodrigues*
View the documentGhana: Optimum firing temperature for some clay bricks*
View the documentEthiopia: Construction of mud houses - an alternative to the traditional methods of house construction*
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Ethiopia: Light-weight concrete made with Ethiopian pumice*

* By Mikyas Abayneh, Faculty of Technology, Addis Ababa University, Ethiopia.

This paper was presented to the Seminar on Local Materials for Housing, Third International Seminar of the African Network of Scientific and Technological Institutions (ANSTI), Civil Engineering Subnetwork held at the University of Mauritius, Reduit, March 1990. ANSTI is a UNESCO-sponsored network.

Abstract

At present, a considerable amount of building activity is taking place all over East Africa. The demand for light materials to reduce the weight of structures and the expense on foundations thereof is increasingly being felt. Many parts of East Africa are covered with volcanic materials and light-weight rocks such as pumice and scoria which are found in abundance, especially in the Rift Valley region. A study has been carried out on how to produce concrete using light-weight Ethiopian pumice. The unit weights of concrete varied around 1500 kg/cu m. The flexural strengths were high as compared with their compressive strengths. It was found that compressive and flexural strengths and the modulus of elasticity increased with decrease in water to cement ratio.

Introduction

In the last few years, a marked increase in building activity has taken place all over East Africa. As a result, East African engineers and builders, like their counterparts in any part of the world, are faced with the problem of building efficiently and economically making effective use of indigenous raw materials.

At present, reinforced concrete is rapidly replacing timber as a material for structural purposes. In fact, it is becoming the most commonly used material in construction from small dwelling houses to large modern buildings. Moreover, the demand for light masonry derived from the desire to reduce the weight of multi-storey structures and the expenses of the foundations thereof, has compelled producers of building materials to find better means of producing building elements such as load or non-load bearing blocks. In their endeavours to produce light-weight concrete, producers are trying to replace part of the normal stone aggregates by natural light-weight aggregates. The light-weight materials include pumice and scoria of which there are quite large reserves, especially in the Rift Valley region.

Over the last few years, considerable activity has taken place both in the exploitation of new deposits and techniques of using natural light-weight aggregate for producing concrete. However, the development of these techniques is not based on scientific studies and results, but purely on practical experience, resulting from field usage. Hence, the use of these abundant natural resources has been rather limited.

Natural light-weight aggregates have good potential to be increasingly used in the future, in which case the knowledge of their properties and of the concrete made with them will certainly be essential. This paper presents part of the results of investigations conducted with the objective of studying the properties of samples of pumice aggregates, finding out the most suitable mix design, and studying the properties of the concrete made with them.

Materials, preparation of test specimens, and test results

The pumice aggregates used in the investigation had a white-grey colour a fairly soft surface of mostly granular texture, and tube like interconnected pores. The maximum fraction size of aggregate that could be obtained at the quarry was less than one inch, hence crushing was not found necessary but, instead, the particles were used as quarried.

It was decided to use maximum sizes of 3/4 inch, and accordingly, the grading curves were compared with the requirements of ASTM C 330. The grading curves were out of the standard ranges, therefore, the supplies from the quarry were screened and separated into a number of sizes which were kept in separate containers. For each preparation of the concrete mix, the different aggregate sizes were recombined to obtain the specific gradings required. As the materials were deficient in the passing sieve No. 4, it was decided to use natural sand in the concrete mixes. Trial and final mixes were made under various climatic conditions. However, the results reported hereunder are those at 20°C (see table 1). It was realized that the specific gravity and absorption of particles could not be reliably determined by the simple ASTM standard procedure. Therefore, the specific-gravity-factor method (ACI 613A-59) of mix design was used instead of the conventional absolute volume proportioning. All trial mixes were made following a standardized procedure and the resulting proportions of materials were used for the final mixes. Test results are given in table 2.

Table 1. Quantities of materials

Mix No.

Aggregates (percentage volume)

Water/
cement ratio (weight)

Quantities/cu m of fresh concrete (kg)


f.a*

c.a**


Cement

Water

f.a*

c.a**

P-1

39.5

60.5

1.71

199

342

743

280

P-2

39.5

60.5

1.63

199

324

805

283

P-3

39.5

60.5

1.45

200

291

860

298

P-4

37

63

1.14

297

340

707

278

P-5

37

63

1.09

305

333

740

291

P-6

37

63

0.97

300

291

758

298

P-7

34.5

65.5

0.86

396

339

625

278

P-8

34.5

65.5

0.81

404

329

656

288

P-9

34.5

65.5

0.73

402

292

697

300

* f.a. represents fine aggregate
** c.a represents coarse aggregate

Evaluation of lest results and conclusions

The conventional methods of mix design, generally accepted in East Africa, call for the determination of exact values of specific gravity and absorption capacity of the aggregate used. The determination of the specific gravity of the pumice aggregate is very difficult, if not impossible. As found out in the study, in addition to the problem of determining the exact specific gravity, the absorption characteristic of the aggregate is also very complex. The ACI specific-gravity factor of mix design by trial method was found useful in the sense that neither measurement of specific gravity nor any knowledge of the absorption property of the aggregates was necessary. The use of the method resulted in fairly good yield of concrete and accurate cement factor.

Table 2. Properties of concrete

Mix No.

Consistency

Slump (mm)

Air content (percentage)

Measured unit weight (kg/m3)

Compressive strength 28 days (Mpa)

Flexural strength (Mpa)

Modulus of elasticity (GPa)

P-1

Flowing

120

3.3

1603

6.3

1.74

7.1

P-2

Plastic

40

3.5

1655

6.9

1.85

8.4

P-3

Stiff

0

3.2

1623

6.5

2.22

9.2

P-4

Flowing

140

3.5

1623

10.2

2.43

9.5

P-5

Plastic

50

2.5

1635

12.7

2.65

10.8

P-6

Stiff

0

3.7

1637

12.6

2.16

11.4

P-7

Flowing

140

2.6

1632

11.0

2.69

9.9

P-8

Plastic

70

2.8

1623

12.8

3.07

11.6

P-9

Stiff

0

2.7

1628

12.0

3.12

13.5

The workability of all final mixes was fairly good. This was obtained at the expense of a higher percentage of natural sand and a consequent sharp increase in unit weight of the concrete.

As expected, slump measurements resulted in lower values than are normally accepted for ordinary concrete. Because of the tendency of the pumice to separate from the matrix, the lowest slump corresponding to required consistency had to be employed. Compaction by hand of the plastic and flowing pumice concretes was found to be sufficiently good for reducing segregation.

To obtain an indication as to which variable of the mix influenced most the strength properties of the resulting concrete, two separate statistical regression analyses of the test data were made. In the first regression analysis of the test, the independent variable was the water/cement ratio, while the dependent variable was the (150 x 150 x 150 mm) cube crushing strength at the twenty-eighth day. In the second regression analysis, the corresponding variables were the cement content and the cube crushing strength at the same age. Significant coefficients of regression were obtained leading to the conclusion that the compressive strength of pumice concrete may be correlated either to its cement content or to the water/cement ratio.

The flexural strengths under third point loading were generally high compared with the cube crushing strengths. In practically all cases the flexural strength represented over 20 per cent of the corresponding compressive strength. The ratio of the flexural strength to the compressive strength was higher with the weaker concretes. The secant modules of elasticity, determined at 25 per cent of the ultimate load, increased with an increase in compressive strength.

In summary, the following may be concluded:

(a) Pumice aggregates, when stored in bulk, tend to segregate. To avoid such segregation, it is recommended to screen separately and keep different stockpiles of the various sizes at a building site;

(b) Mix design of concrete using pumice is satisfactorily done with the specific-gravity-factor method. The concrete mixes require a relatively high percentage of sand for proper workability;

(c) Because of the tendency for the aggregate to float to the surface, leading to segregation of the mix, pumice concrete of flowing or plastic consistency might be suitable for precast work where compaction could be better controlled. If used in cast in-situ work, the degree of compaction must be consistently controlled;

(d) The air-dry unit weight of pumice concrete with natural sand and 200 to 400 kg/cu m cement could vary between 1500 to 1750 kg/cu m;

(e) The strength of pumice concrete is related to its water/cement ratio;

(f) An increase in cement content increases the strength of pumice concrete. But cement content is dependent on the consistency of the concrete;

(g) Pumice concrete has high flexural strength compared with its compressive strength;

(h) The flexural strength and the modulus of elasticity increase with an increase in cement content and decrease in water cement ratio, however, the relationship is dependent on the consistency of the concrete.