THE EFFECT OF MIXING SEQUENCE ON THE PROPERTIES OF CONCRETE

Abstract

Concrete is the most demanded material only second to water as a substance, with this, it can only be imagined that huge expense and resources have been put into it. In consideration with the aforementioned fact on concretes‟ demands coming in tune with the advancements of the 21^st century production researches, cost free methods are being optimized to boost production efficiency. This research entitled “The effect of mixing sequence on the properties of concrete” is about the latter statement, as it deals directly with finding the most suitable mixing sequence to optimize production efficiency without any added resources. The research focused on the fresh and hardened properties of concrete. British Research Establishment (BRE) Method of mix design was used. A total of 720 cubes were cast for tests at the following ages of 7, 14, 28 and 56 days. Twenty four (24) mixing sequences were assessed. Among the mixing sequences, two standardized mixing sequences from the ASTM and ACI were chosen. It was found that samples mixed using the standard mixing sequences of the ASTM and ACI passed all the standard requirements for all properties of concrete assessed. However the samples produced using ACI mixing sequence had better hardened concrete properties when compared with the samples of concrete made using ASTM mixing sequence. All the concrete batch samples from the various mixing sequences passed the standard conditions for compacting factor and plastic density. Samples mixed using mixing sequences (22, 19 and 17) did not pass the standard condition for slump. Samples mixed using mixing sequences (14 and 17) did not pass the standard condition for air content. Samples from mixing sequence 24 had optimum compressive strengths of 23.40N/mm², 25.52 N/mm², 30.84 N/mm² and 35.56 N/mm² for 7, 14, 28 and 56 days of testing respectively and passed all the standard conditions for all properties tested. It surpassed samples from the standardized mixing sequences in grade by at least 4.7% for every test age. During concrete production, with all processes, materials and proportioning considered, two sets of concrete samples from two different mixing sequences can differ in average from 0.91% to 41.85% under water absorption while two sets of concrete samples from two different mixing sequences can differ in average from 0% to 100% under abrasion resistance. From the properties of concrete tested, the various mixing sequences used have the most positive effects on plastic density followed by air content then slump and finally compressive strength. Due to the differences in the properties of concrete samples under the influence of different mixing sequence, it is recommended that good mixing sequences, when used should be consistent under a particular job. Mixing sequence 24 had optimum compressive strengths and can be used to make the best concrete. Uniformity should be checked firstly for fresh concrete properties when determining which or what mixing sequence to use.

Concrete is the most useable building materials in today‟s construction industry. Its ability to be cast into infinite desirable shapes and fashion makes it applicable for most building purposes. Its relatively long life and low maintenance adds to its popularity. It is such that it does not rot, rust, decay, and resistant to wind, water, rodent and insects. It does not combust making it fire resistant and has the ability to withstand high temperatures (Assist, 2009). There are many factors that affects the properties of concrete. Mixing sequence as a factor amongst others affects the properties of concrete (Fitzpatrick and Serkin, 1949)

Mixing sequence is the order of introduction of constituents into the mixing process during concrete production. Mixing sequence is as old as concrete manufacture itself. It is an essential, integral and inevitable part of concrete production. Concrete has been formed in rigid molds since its invention in antiquity. (Malinowski and Garfinkel, 1991).

A stepping stone in the history of concrete was the invention of Portland cement by Joseph Aspdin, (Francis et al., 1977). The way to go about how to mix this cement or other cementitious materials to get an improved and efficient concrete of high quality became essential and sustainable. This only requires at first research which is vital to every development.

Fitzpatrick and Serkin (1949) were the first persons recorded to investigate mix sequence of constituents. Their work found that, mixing sequence has a significant effect upon the properties of concrete which includes workablity, strength, density, surface finish and absorption. These properties determine concrete quality.

The quality of concrete is determined by how suitable a concrete is in terms of its practicability to properly designed properties by influences of production techniques and by the homogeneity of the material after mixing and placement. The methodology to determine the quality of the concrete mixed is often referred to as the measurement of the efficiency of the mixer. The efficiency parameters of a mixer are affected by the mixing sequence, the type of mixer, and the mixing energy (power and duration) as pointed out by Ferraris (2001). Ferraris (2001) suggested that there should be methodology to check the quality of concrete produced, because only one attempt of standardization was found. National Concrete Pavement Technology Centre (NCPTC, 2007) shows that concrete mixing is a complex process in which different factors influence the quality of produced concrete during production. These factors include loading sequence, mixing time, mixer type, and the time and rate of adding chemical admixtures.

The sequence of charging a mixer is a great factor of whether the constituents would mix properly or not, American Concrete Institute (ACI Committee 304, 2000). There are no general rules in mixing sequence of concrete (Neville and Brooks, 2010). The sequence of introduction of constituents into a mixer varies from plant to plant, (ACI Committee 304, 2000).

The American Society for Testing and Materials (ASTM, 1994a) specifies in its sequence that some water and aggregates should be mixed first, then cement should be added, and finally, the remaining portion of the water is added with not more than one-fourth of the total mixing time elapsed. Any liquid admixture should be added together with water. (Mass, 1989) strongly suggested experimentally that, mixing cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete. ACI Committee 304 (2000) specifies something different to (Mass 1989). The ACI Committee 304 (2000) says that coarse aggregates should be poured first, followed by the fine aggregates. Then add in sequence the required water and cement. ASTM C 305, (1998c) specifies a different order of sequence from above sequences. This sequence calls for first mixing water, cement and fine aggregate altogether until it produces a uniform mortar, then the coarse aggregate is added.

Then again, the mixing sequence developed by Tam et al., (2005) called the ‘two stage mixing approach’ is aimed at improving the quality of concrete. It indicates that all aggregate should be mixed first, then half of water after 60 seconds, cement after another 60 seconds and the other half of the required water after 30 seconds and then wait for 120 seconds for the concrete constituents to be properly mixed. Tam et al., (2005) investigated in comparism to what they considered the normal mixing method, which is pouring fine aggregate, cement, coarse aggregate and water and mixing respectively.

Soga and Takagi, (1986) reported that the addition rate of the mixing water and the rotational speed of the mixing drum control fresh concrete characteristics. In particular, the bleeding rate of fresh concrete decreases as the rate of adding water decreases. The bleeding rate of fresh concrete decreases as the rotational speed of the mixer is increased. Similarly, Tamimi (1994) and Mitsutaka and Yasuro (1982) studied the effects of mixing water but in two separate stages. In the first stage, aggregate and a weight of water equal to 25 percent of the weight of cement were mixed for 30 seconds. Next, cement was added and mixed for 60 seconds. And finally, the remaining water was added and mixed for 90 seconds. Concrete produced by this technique was labeled “sand enveloped with cement concrete” (SEC). The goal was to investigate the effectiveness of this mixing sequence in reducing the bleeding rate of fresh concrete and increasing the compressive strength at various stages of curing. Tamimi (1994), Mitsutaka and Yasuro (1982) concluded that this method did in fact reduce the bleeding rate of fresh concrete and improved the compressive strength over conventionally-mixed concrete. Tamimi (1994) took the research further to prove that adding water using this formula leads to a greater gel-to-space ratio in the interfacial transition zone (ITZ), creating a more intimate bond, lower porosity, and increased micro-hardness in the ITZ.

Gaynor (1996) studied the influence of concrete truck mixers on concrete properties. He concluded that non-uniformity in truck-mixed concrete is caused by agglomerations of concrete materials inside the mixer. This included head packs and cement balls. To remedy the non-uniformity problems, Gaynor suggested that one-fourth of the mixing water be added as the last ingredient and that the mixer rotate at 20–22 rpm. Traditional concrete mixing practice is today regulated by a specific mixing time required to achieve specified performance of the fresh and hardened concrete. This mixing time is based on a long experience of developed correlations between the mixing process and the mixers performance, and is generally detailed in specifications, National Concrete Pavement Technology Centre (NCPTC, 2007). ASTM (1998a) has shown that insufficient mixing time can lead to lower compressive strength and inhomogeneous concrete. Excessive mixing time can cause aggregate breakdown and decreased air content. Beitzel (1981) studied the influence of mixing time on the quality of concrete, where quality was defined as the uniform distribution of water, cement and fine aggregate. Results showed that the optimum mixing time is different for different concrete properties, and that there should be upper and lower limits on the mixing time. Beitzel (1981) developed a qualitative empirical representation of the optimum relationship between mix separation and uniformity.

Cable and McDaniel (1998) also investigated the effects of mixing time on a variety of concrete characteristics, including workability, the air content of cured concrete, and segregation caused by truck mixing. Cable and McDaniel (1998) concluded that a minimum mixing time of 60 seconds is effective for all mixer types; this is in order to achieve an acceptable level of concrete performance from the final product. And this time should only be reduced if measures are taken to ensure all aggregate particles are completely coated upon discharge from the mixer.

American Petroleum Institute (API) Specification 10A, (2002) states the requirements for a mixer which include mixing speed and time. Shetty (2005) says mixing more than 2 minutes would not very significantly increase strength. Assist (2009) says 1.5 to 3 minutes is sufficient to obtain a good mixture. Mixing more than 3 minutes will not improve the quality of the mixture. NCPTC (2007) indicates that the continued mixing of cement pastes can delay setting time and that the setting time could be delayed; although hydration does not stop, this slows bonding. Generally speaking, mixing sequence needs to be studied in conformity with the parameters that bind it (such as mixing time, loading intervals e.t.c). The time and speed of rotation (mixing energy) is known to improve the quality of concrete (homogeneity) up to a certain level at which it becomes inconceivable after optimum time. The speed of rotation of a mixer affects the optimum time within closed limit for different mix proportion and water-cement ratios. Added variation in quality can be deduced from mixing sequence. The method of loading which primarily includes mixing sequence is the most influential variant in the process of concrete production and has the greatest effect on batch uniformity (Irtishad et al, 2002). Reasonably fixed time is needed to characterize the quality of different mixing sequence in relation to one another. All the other entities (optimum time, mixer efficiency, mixing energy e.t.c) could be measured and calibrated for optimum production, but only the mixing sequence cannot and needs to be determined. Lastly, Different sequences may need adjustments in the mixing time (kosmatka et al., 2003).

Factors that affect mixing sequence in concrete production are basically, the constituent materials used, mixing time, type of mixer, speed of mixing (Debbie, 2017). Constituent materials affect mixing sequence because their properties determine how mixing sequence affects quality firstly which is empirical. The number of materials being introduced adds to the order of sequencing. The number of times a particular material is introduced into the mixing process affects the order of introduction of materials into the mixing process which exclusively is referred to as mixing sequence, hence the quality of concrete. Batch size in relation to size of aggregate also affects mixing sequence invariably (Debbie, 2017). This is because the physical properties of constituent materials is a general factor in determining how these materials are most preferably introduced into a mixing process in relations to batch size, mixer type and mixing speed. Time, mixer type and mixing speed evidently affect mixing sequence (Debbie, 2017). The choice of mixing sequence is vital to concrete production and is primarily determined to a broader aspect by researches, relevant standards, specifications and experience. This cannot be overemphasized as mixing sequence is key to concrete production and mix uniformity which is used to assess concrete quality (Ferraris, 2001).

1.2 Statement of the Research Problem

With the advancements of the 21^st century production researches, methods are being optimized to boost concrete production efficiency. It is true that concrete production is cunningly a complex process (NCPTC, 2007). Concrete structures are a great deal more than sand, gravel, cement and water blended and left to harden into useful shaped lumps. Considerable care, study and knowledge are needed to manufacture good quality concrete (Controls, 2014). Knowing good and proper sequence from poor ones prevents the manufacture of poor quality concrete moreover, provides the knowledge in the manufacture of good quality concrete. Properly manufactured concrete is inherently an environmentally friendly material as it can be demonstrated readily with a life-cycle analysis. The challenge is derived mainly from the fact that Portland cement is not environmentally friendly, (Micheal et al, 2002). Someone could therefore reduce this problem by simply using as much concrete with as little cement as possible, (Meyer, 2005). This can be achieved through improved mechanical properties by harnessing the strength in concrete. One efficient way is by knowing the best mix sequence to use. This adds to the environmental friendliness for  given quantity of cement in concrete by increased strength and durability, prolonging life circles of infrastructures or by reduced cement needed in production as per requirements.

Concrete is in fact, the most demanded material only second to water as a substance. Much effort is being used to provide target in the cost of concrete. This is especially as a result of its huge demand in comparative to its accumulated cost and overall price impact. The right sequence in which cement, aggregate and water are mixed could add to optimize concrete strength, mixer performance and production efficiency. This reduces the manufacturing cost of the most useable building material basically by reducing the amount of cement in concrete of a specified concrete grade and durability requirements needed for a particular job. This is met with needed quality, more so, without any added effort in production. The observation made could amount to much difference as the overall demand for concrete is huge. So to say, the above situations determine the case and criteria of this study and needs to be addressed proficiently.

1.2 Justification of the Study

The construction industry has gone far and great lengths to harness strength in concrete. Strength that could be got should be tapped especially if it causes no extra human, material, time or effort. Knowing the best sequence of mixing to be used adds to optimize strength and durability of concrete during concrete production processes.

Strength of concrete is considered the most important property of concrete as it denotes and relates the general quality of concrete and its overall internal structural arrangement. However, durability is nevertheless very important as from the onset of Portland cement concrete hitherto, structures are more and more being built in and/or to a toxic environment injurious to concrete which require enough durability to resist, withstand and sustain these likely injuries. So to say, durability is critical as it is beneficial because durable concretes keep the useful life of structures by ensuring its internal mass and surfaces are not penetrated, degraded, worn or leached. These properties, strength and durability are of viable importance and so, immense consideration is needed to improve these properties in concrete effectively because they, on a major scale affect essential construction items like cost, sustainability, material and a related other useful items. These items are necessary and play a major role in determining key related activities in the construction industry.

Strength and durability, a primary determinant in the assessment of the most useable building material cannot be overstated. The improvement of these concrete properties; strength and durability can be achieved by knowing and using the proper mixing sequence. This does not result in the addition of any added effort of sort in concrete production. These rewarding benefits provided a platform for the need of study in this research work.

1.3   Aim and Objectives

1.4.1    Aim

The aim of this research is to investigate the effect of mixing sequence on the properties of concrete with a view to establishing an optimized mixing sequence in the properties of concrete

1.4.2    Objectives

1. To determine the properties of the concrete ingredients such as specific gravity, density.
2. To assess the fresh properties of concrete produced such as workability, air content, plastic density, with different mixing sequences.

• To assess the hardened properties of concrete produced such as strength and durability with different mixing sequences.

1. To establish the effect of mixing sequence on the properties of concrete such as workability, air content, plastic density, strength and durability.

1.5 Scope and limitations

1.5.1    Scope

The research covered the manufacture of concrete using hand fed concrete mixer fed up to a reasonable capacity of 35 to 45 percent of gross drum volume, (ASTM, 1978) . Batches were mixed for 2 minutes in accordance with Indian Standard (IS 456, 2000), with 20 seconds charging intervals. Same batch size was used for each individual mixing sequence. Tests on specimens such as abrasion resistance test, water absorption test, air content test, plastic density test, workability and compressive strength test were carried out, on the fresh and hardened samples in accordance with the relevant standards.

Limitations

Fresh concrete tests like segregation and bleeding were not carried out because the mixes do not possess significant properties being assessed by such tests. Non-destructive test method is more accurate to assess uniformity using compressive strength test as all samples can be tested at once and reused and interference of hydration on uniformity can be monitored from subsequent ages across same batch samples but the equipment to be used was not available.