1. CONCRETE MIX DESIGN PRACTICE OF ETHIOPIA AND ITS APPLICATION ON CONSTRUCTION SITES Presented by: Adiamseged Eyassu WORKSHOP ON CONCRETE CONSTRUCTION PRACTICES IN ETHIOPIA AT THE LAUNCHING CEREMONY OF ACI – ETHIOPIA CHAPTER

2. February 4, 20192 Outline of the Presentation 1.Introduction 2.Mix Design in Ethiopian Context 3.Problems in Application of Mix Design on Sites 4.Conclusion and Recommendation

3. February 4, 20193 Introduction  Concrete like other engineering materials needs to be designed for properties like strength, durability, workability and cohesion.  Concrete mix design is the science of deciding relative proportions of ingredients of concrete, to achieve the desired properties in the most economical way.  Mix Design methods are mostly based on empirical relations, charts, graphs, and tables developed as outcomes of extensive experiments and investigations of locally available materials.

4. February 4, 20194 Advantages of Mix Design Mix design has the following advantages: Quality concrete meaning:  Better strength,  Better imperviousness and durability and  Dense and homogeneous concrete Economy  Economy in cement consumption - It is possible to save cement content with the help of concrete mix design.  Best use of available materials - Mix design can give an economical solution based on the available materials if they meet the basic requirements.

5. February 4, 20195 Factors Influencing the Choice of Mix Proportions It is essential that the concrete mix is prepared as economically as possible by using the least possible amount of cement content per unit volume of concrete, with due regard to the strength and durability requirements. The properties most usually specified are:  The workability of the fresh concrete  The compressive strength at a specified age usually 28 days  The durability, by means of specifying the minimum cement content and/or the maximum free-water/cement ratio and, in some cases, requiring the use of selected types of materials

6. February 4, 20196 Factors Influencing the Choice of Mix Proportions Selection of concrete proportion should be based on test data with the materials actually to be used. The minimum laboratory test data required are:  Sieve analysis of fine and course aggregates  Unit weight of coarse aggregates  Bulk specific gravities and absorption of aggregates  Mixing water requirements of concrete developed from experience with available aggregates  Relationship between strength and water-cement ratio or ratio of water-to-cement plus other cementations materials  Specific gravity of Portland cement and other cementious materials, if used

7. February 4, 20197 Flow Chart for Mix Design Aggregate Shape & Texture Required Workability Method of Compaction Size of section and spacing of reinforcement Max size of Aggregate Proportion of each size Fraction Aggregate/ Cement Ratio MIX PROPORTIONS Grading of Aggregates Minimum Strength Concrete Mix Structural Requirements Target Mean Strength Durability Age at which Strength is required Water/Cement ratio Type of Cement

8. February 4, 20198 Mix Design in Ethiopian Context  EBCS 2, Chapter 8: Materials and Workmanship provides mix proportions for grades C5 to C30 as shown in table below. Concrete Grade Nominal max. size of Aggregate (mm) 40201410 WorkabilityMediumHighMediumHighMediumHigh Mediu m High Limits of slump that may be expected (mm) 30 to 6060 to 12020 to 5050 to 10010 to 3030 to 6010 to 25 25 to 50 C5 Total Aggregate (kg) Fine Aggregate (%) Vol. of finished concrete (m 3 ) 640 30-45 0.312 550 30-45 0.275 540 35-50 0.277 480 35-50 0.252 ---- C15 Total Aggregate (kg) Fine Aggregate (%) Vol. of finished concrete (m 3 ) 370 30-45 0.200 330 30-45 0.183 320 35-50 0.178 280 35-50 0.160 ---- C20 Total Aggregate (kg) Fine Aggregate (%) Vol. of finished concrete (m 3 ) 305 30-35 0.165 270 30-40 0.165 280 30-40 0.156 250 35-45 0.143 255 35-45 0.146 220 40-50 0.130 240 40-50 0.137 200 45-55 0.121 C25 Total Aggregate (kg) Fine Aggregate (%) Vol. of finished concrete (m 3 ) 265 30-35 0.147 240 30-40 0.137 240 30-40 0.137 215 35-45 0.127 220 35-45 0.130 195 40-50 0.118 210 40-50 0.124 210 45-55 0.110 C30 Total Aggregate (kg) Fine Aggregate (%) Vol. of finished concrete (m 3 ) 235 30-35 0.134 215 30-40 0.127 210 30-40 0.124 190 35-45 0.115 195 35-45 0.115 170 40-50 0.106 180 40-50 0.109 150 45-55 0.097 Standard Mixes for Ordinary Structural Concrete per 50kg Bag of Cement

9. February 4, 20199 Mix Design in Ethiopian Context  Minimum cement content with maximum water/cement ratio per m 3 of concrete for different exposure is also provided.  However there is no standardized concrete mix design method provided.  It is clearly stated in this chapter that: The chapter is neither intended as, nor extensive for, a contract document The standard mixes specified in the table are rich in cement and are intended for use where the cost of trail mixes or of acceptance cube testing is not justified The cement used shall be PPC or OPC only

10. February 4, 201910 Mix Design in Ethiopian Context  Based on the guidance of the code mix proportion of 1:2:3 with required cement content is being specified in contract documents.  “Reinforced concrete quality C - 25, 360kg of cement/m 3, filled into prepared surface and vibrated around rod reinforcement, mix ratio should be 1:2:3”  Standard box sizes for the mix proportion - 40x50x16, 40x50x18 and 40x50x20.  This proportion was used for small and big projects and it is still being used in most projects.  As per the recommendation of EBCS the cement required for C25 is 360kg per m 3.  From mix design data to produce C25 concrete cement content in the range of 285- 320kg using OPC and 375-420kg using PPC is required depending on the aggregates used.  Therefore as stated in the code the code recommendation should only be used for small jobs where the cost of trail mixes or of acceptance cube testing is not economical.

11. February 4, 201911 Problems in application of Mix Design on Sites Common problems encountered on site are the following:  Even though mix design is done, in most of the projects the result is filed and will not be used up to the end of the project.  Cement types specified during the mix design are changed without changing the mix design.  All materials should be accepted only after testing and only if they meet the acceptance criteria. Since there is no site laboratory in most projects once tests are carried out in the laboratory for mix design the materials are only tested when the source is changed.

12. February 4, 201912 Problems in application of Mix Design on Sites Common problems encountered on site continued …:  Every mix design is specific to materials used for mix design. Hence for any minor variation in material properties of sand and coarse aggregate received at site, corrections in mix proportions may be necessary at the site. Correction for surface moisture Correction for Fineness Modulus of sand Correction for size and shape of coarse aggregate Correction for workability of concrete Correction for Bulk density in case of Volume batching  Concrete mix design assumes that aggregates used in concrete are in saturated surface dry condition and hence should be wetted before concreting to bring them in SSD condition.

13. February 4, 201913 Problems in application of Mix Design on Sites Common problems encountered on site continued …:  The slump given in mix design should never be exceeded and strictly controlled on site. Slump test are not done in most projects.  Use of measured cans for adding water in the mixer is not a common practice. However water is usually added with the judgment of the mixer operator.  Mix design is based on the strength of cement tested at laboratory that is usually fresh. If age of cement is more than 1 month or if it is not stored properly, the strength of cement will be affected. In such cases, corrections in mix design may be required to account for reduced strength.  Standard deviation of samples of cube results should be regularly checked. Higher standard deviation means poor control at site on concrete operation.

14. February 4, 201914 Problems in application of Mix Design on Sites Common problems encountered on site continued …:  Cement consumption should be checked every day and proper record of cement actually consumed versus the design consumption should be maintained in a concreting work data sheet.  The mix design should be regularly revalidated from the laboratory for any changes in material properties.  The box sizes used for proportioning sometimes have different sizes. In addition the boxes are not always equally filled.  Mix design is usually done only at the start of the project. The same mix design is used up to the end of the project. Sometimes even when the source of materials are changed the same mix design is used.

15. February 4, 201915 Conclusion and Recommendation The above problems are mostly attributed to the following reasons:  Lack of awareness – Most projects are run by foreman’s who doesn’t have the know-how of mix design  Negligence both by consultants and contractors  Interference from Client – when consultant want to do tests client doesn’t want to waste time and order contractors to continue without mix design or other tests  There is no standardized concrete mix design method

16. February 4, 201916 Conclusion and Recommendation The following are recommended for proper use of mix design in the country:  A lot of trainings and workshops should be organized to change the trend in mix design and its application on site.  Aggregate production in the country should be standardized and properties of all materials should be studied  A standardized concrete mix design method for the country should be developed  There should be a directive from the government regarding quality control in construction sites in general

17. February 4, 201917 THANK YOU!