Preparation of Concrete

Production of concrete involves mixing cement with water, aggregates and other additives in order to improve their quality in terms of durability, strength, workability and other criteria. There are several mixing procedures involving the preparation of concrete mixture. The most common one is the Normal Mixing Process (NMP) which is according to ASTM C192.

1. Normal Mixing Process (NMP)

Procedure:

1. Add coarse aggregates with half of the water measured for mixing, into the mixer, and mix it for 30 seconds.

2. After that, add all cementitious material, together with fine aggregates and a slow addition of water, and mix them for 3 minutes.

3. After that, leave the mixture for 3 minutes.4. Finally, continue mixing for another 2 minutes.

This procedure is as according to ASTM C192 recommendation, hence it is recommended to use the procedure for normal concrete production.

2. Geopolymer Concrete (GPC)

Generally, GPC is a mix proportion consists of an alkali activated fly-ash based Geopolymer Concrete

Basic GPC reaction mechanism involves 3 stages:

1. Dissolution of silicates and aluminium from the source material such as fly ash.2. Hydrolysis or gelation3. Condensation which forms a 3D network of silico-aluminates

Source of silicates and alumina will be obtained from fly ash, which contains both of them, which will help in enhancing some properties of concrete such as workability and reducing heat of hydration. There are two classification of fly ash (FA) available, which is Class C and Class F. Class C is not favored because of its high content of calcium oxide which could reduce the setting time, and also durability issues. Thus, Class F flying ash was chosen, mainly because its alkaline activated properties.

There are 3 proposed mixing procedures for GPC.

1.

Procedure:

a) Add all aggregates and flying ash together into the mixer, and mix it for 2 minutes.b) Add alkaline liquid with plasticizer and water and mix it for another 3 minutes.

2.

Procedure:

a) Add fly ash with hydroxide into the mixture and mix it for 3 minutes.b) After that, add silicates and continue mixing for another 2 minutes.c) Add aggregates into the mixture and continue mixing for 3 minutes.d) Last, add water and plasticizer and mix all the mixture for 2 minutes.

3.

Procedure:a) Saturated and surface dry (SSD) aggregates were dry mixed with fly ash for 2 min.b) The silicates solution was then added to the dry mix and mixed for 3 min. c) Hydroxide solution was then added and mixed for 2 more minutes. d) Additional water along with any admixtures was then added and mixed for an

additional 2 min period.

3. Two-Stage Mixing Approach

Procedure:

a) Coarse aggregates and cementitious materials were mixed for 1 minb) Half of the water was added and then mixed for 1 minc) Fine aggregate and remaining water is added with chemical admixtures and then

mixed for 2 min.

Among 3 of the suggested procedure, the 3rd procedure is chosen as the best recommended procedure because it does not need any pre-preparation, and is very similar to the process generally used for OPC concrete.

5. High Strength Concrete with Nano particles

Procedure:

a) Putting fine materials (cement – silica fume) in the mixer and mixing them together for one minute.

b) Stirring a half of the superplasticizer required dosage for the mixture to half of the required amount of water and then adding them to the required amount of nano particles with high speed rate mixing for two minutes.

c) Adding the mixed nano particles in step (b) to the dry mix in step (a) and mixing for two minutes.

d) Stirring the rest half dosage of the superplasticizer to the rest half of the amount of water rest and then adding to the pan mixer gradually and mixing for other four minutes until getting homogeneous paste.

e) Adding the fine aggregate gradually, followed by the coarse aggregate. Mixing continued for other 5 min until getting homogeneous concrete paste.

6. Ultra High performance Concrete (UHPC)

Procedure:

a) All powders and sand fractions are added into the mixer for dry mixing (30 s at low speed).

b) Then, around 75% of water is added into the mixer. After mixing for 90 s (low speed), the mixer is stopped for 30 s.

c) Afterwards, the remaining water and SP are added, and the mixture is mixed at low speed for 180 s.

d) Finally, the mixture is mixed at high speed for 120 s.

The mixing is always executed under laboratory conditions with dried and tempered aggregates and powder materials. The room temperature while mixing and testing is constant at around 21 oC.