Journal of Civil Engineering Research 2015, 5(3): 67-73 DOI: 10.5923/j.jce.20150503.03

Evaluation of the Concrete Works Produced Locally in River Nile State

Mohamed Y. Mustafa1, Fathelrahman M. Adam2,*

1Civil Engineering Department, Nile Valley University, Atbara, Sudan 2Civil Engineering Department, Jazan University, Jazan, KSA, on leave from Nile Valley University, Sudan

Abstract This paper aim to evaluate the concrete produced locally in River Nile State (RNS) where the output results lead to improve the work of casting the concrete in-situ in order to acquire the design values and raise the level of quality control. A data concern the concrete casting in-situ in the RNS was collected by visiting a 24 sites included information about the materials used in concrete such as cement, sand and gravel also information about the procedure followed for quality control and the tests done for the concrete and materials and its results. Many samples of materials on sites were collected and a lab test for each had been done. The study also include visiting of 20 sites constructed through past 10 years and the available data had been collected. This paper adopted a criterion to classify the sites included by this study according to the quality control. Statistical analysis had been done for the information collected and for the results obtained from lab test. The study concluded that for more than 57% of the sites studied it is found that the quality control followed for construction is fair and poor. Many reasons of that were summarized and the necessary recommendations were given.

Keywords Concrete Work, Quality Control, Mix Design, Concrete Materials Tests

1. Introduction Sudan is developing country and its infrastructures are

under construction such as bridges, roads, water dams, etc., so the construction industry continues in expand in order to meet the increase in population and the consequent increase in the life requirements. In the near future is expected to increase these facilities as well as the steps taken to achieve peace and political stability that will attract foreign investment to contribute in this area within Sudan.

Concrete is a composite material composed of coarse granular embedded in a hard matrix of material (cement) that fills the space between the aggregate particles and water glues them together. The mixture when placed in forms and. allowed to cure becomes hard like stone. The hardening is caused by chemical action between water and the cement and it continues for a long time, and consequently the concrete grows stronger with age. The strength, durability and other characteristics of concrete depend upon the properties of its ingredients, on the proportions of mix, the method of compaction and other controls during placing, compaction and curing. In Sudan most of construction companies do not pay attention to adopt the correct methods for production the concrete, as

* Corresponding author: fat470@yahoo.com (Fathelrahman M. Adam) Published online at http://journal.sapub.org/jce Copyright © 2015 Scientific & Academic Publishing. All Rights Reserved

they in many cases employs unskilled labor, resulting in a disparity in the quality of concrete produced, and leads to the appearance of defects in structures and sometimes turns disintegration in concrete which affects the performance and durability and may lead to unsafe use.

The building in the RNS are categorized to three types, these are building construct using reinforced concrete frame, building using steel frame and building built directly on load bearing walls. Most of buildings are built using reinforced concrete frame and this is because the materials are available.

The selection of the relative proportions of cement, water, and aggregate is called mix design. The requirements of mix designs are workability, strength, durability and economy [1]. The workability describes the ease or difficulty with which the concrete is handled, transported and placed between the forms with minimum loss of homogeneity. The main factor affecting workability is the water cement in the mix also the maximum size of aggregates its grading, texture and shape. Good concrete can be made by using different types of aggregates and the quality of concrete chiefly depends upon the quality of aggregate because these are the aggregates that cover at least 3/4th of the total volume of concrete Also the aggregate greatly influence the durability and structural performance of concrete. Cement is by far the most important constituent is about 10 per cent of the volume of the concrete mix, it is the active portion of the binding medium and the only scientifically controlled ingredient of

68 Mohamed Y. Mustafa et al.: Evaluation of the Concrete Works Produced Locally in River Nile State

concrete. The cement commonly used is Portland cement and in common situations, cements with a greater fineness level will tend to hydrate rapidly and result high early age strength, due to this fact, use of finer cements in the mix. design of high early strength concrete is common. Moreover, finer cement causes a stronger reaction with alkali reactive aggregate, and makes the cement, though not necessarily concrete, display a higher shrinkage and a greater proneness to cracking [2]. Generally, cement requires about 0.3 of its weight of water for hydration. Hence the minimum water-cement ratio required is 0.35. But the concrete containing water in this proportion will be very harsh and difficult to place. Additional water is required to lubricate the mix, which makes the concrete workable. This additional water must be kept to the minimum, since too much water reduces the strength of concrete. The water-cement ratio is influenced by the grade of concrete, nature and type of aggregates, the workability and durability. If too much water is added to concrete, the excess water along with cement comes to the surface by capillary action and this cement-water mixture forms a scum or thin layer of chalky material known as laitance. This laitance prevents bond formation between the successive layers of concrete and forms a plane of weakness. The excess water may also leak through the joints of the formwork and make the concrete honeycombed. As a rule, the smaller the percentage of water, the stronger is the concrete subject to the condition that the required workability is allowed for [3]. The impurities in water may obstruct the setting of cement, may badly influence the strength of the concrete or produce discoloration of its surface, and may also cause rusting of the reinforcement. The compressive strength of concrete is one of the most important mechanical properties. In most structural applications, concrete is employed primarily to resist the compressive forces. In those cases where other stresses (for e.g. tensile) are of primary importance, the compressive strength is still frequently used as a measure of the resistance because this strength is the most convenient to measure. For the same reason, the compressive strength is generally used as a measure of the overall quality of the concrete, when strength itself may be relatively unimportant [4]. The strength of a hardened concrete largely depends upon the (i) water-cement ratio, (ii) the quality and characteristics of cement, (iii) the degree of compaction obtained in the concrete, (iv) curing and (v) the age of the concrete. The strength increases, as the concrete becomes older [4].

The strength of concrete is influence by the curing which is the process led to keep the concrete moist and warm enough so that hydration of cement can continue. If curing is neglected in the early period of hydration, the quality of concrete will experience a sort of irreparable loss. Concrete, while hydrating, releases high heat of hydration. This heat is harmful from the point of view of volume stability. If the heat generated is removed by some means, the adverse effect due to the generation of heat can be reduced. This can

be done by a thorough water curing which it is a method of curing and another methods of curing is the membrane curing where the concrete covered with membrane which will effectively seal off the evaporation of water from it. Different methods of curing are fully detailed in Reference [5].

Compaction of concrete is the process adopted for expelling the entrapped air from the concrete. In the process of mixing, transporting and placing of concrete air is likely to get entrapped in the concrete. The lower the workability, higher is the amount of air entrapped. In other words, stiff concrete mix has high percentage of entrapped air and, therefore, would need higher compacting efforts than high workable mixes. If this air is not removed fully, the concrete loses strength considerably [5]. More details about methods of adopting compaction in References [2, 5].

The successful placement of concrete is dependent upon careful mixing which can vary from hand to machine mixing. The successful mixture is achieved by proper batching of all materials. Quality assurance, suitable arrangement of materials and equipment, and correct weighing of the materials are the essential steps that must be completed before any mixing takes place [6].

The transferring of concrete from the mixing plant to the construction site must be done in careful in order to prevent segregation and to not reduce the workability of the mix. This transportation process must be well thought out and organized efficiently. As a general rule of thumb, thirty to sixty minutes of transportation are acceptable on small jobs. At a central or portable ready-mix plant, concrete should be discharged from a truck mixer or agitator truck within two hours. If non-agitating transporting equipment is used, this time is reduced to one hour. All delays must be avoided in order prevent honeycombing or cold joints [6].

The placing of concrete can be done either by using pumps, jacks or by using metal or plastic containers. If the placing of concrete can be done directly from a truck or concrete pump, the concrete must be place vertically into the face of concrete already in place and never allow the concrete to fall more than 1 to 1.5 meters. Concrete is placed in its final position before the cement reaches its initial set and concrete is compacted in its final position within 30 minutes of leaving the mixer and once compacted it should not be disturbed. In all cases the concrete is deposited as nearly as practicable directly in its final position and should not be re-handled or caused to flow in a manner which may cause segregation, loss of materials, displacement of reinforcement, shuttering or embedded inserts or impair its strength. For locations where direct placement is not possible and in narrow forms suitable drop and Elephant Trunks to confine the movement of concrete is provided. Special care is taken where concrete is dropped from a height especially if reinforcement is in the way particularly in columns and thin walls [3].

This paper study and evaluate the cast in-situ concrete either ready mix or blended manually in RNS. The study depends on design a questionnaire to collect data on the

Journal of Civil Engineering Research 2015, 5(3): 67-73 69

materials used in the concrete industry, as well as methods of mixing and pouring it on site in addition to do field visits to sites to collect the necessary data that help on study.

2. Concrete Materials Available in RNS The Ordinary Portland Cement is richly available in RNS

because of most manufacturers of cement factories exist in RNS. The aggregate also available, both crushed and uncrushed (natural). The natural aggregate sometimes exist in poor-graded and need blending between two types or more to be comply with the standards specification for aggregates [7]. Also That there are some harmful substances that are likely presence in the aggregate but as general the aggregate in RNS has no alkaline materials in chemical composition [7].

3. Evaluation of the Concrete Works This paper adopt two methods for evaluation of concrete

work in RNS, one method is by measuring the quality control of concrete directly and the other is by using statistical analysis by calculating the standard deviation and coefficient of variation. The evaluation has been done by way of classifying the quality control of concrete work for the sites visited to four classes

1- Class A, mean the quality control is Excellent. 2- Class B, mean the quality control is Good. 3- Class C, mean the quality control is Fair. 4- Class D, mean the quality control is Poor.

3.1. Evaluation According to the Requirements of Quality Control

A forms has been prepared on the bases of quality control requirements which bases on three items as reference to evaluate, these are:

1. The materials specifications contain cement and aggregate according to the following details:

a. For cement: the results of test done before using and the suitability of it for use according to ASTM requirements.

b. For aggregate (fine and course): their classification, grading and strength all these are according to ASTM requirements.

2. The preparing phase and executing phase for the concrete production. The preparing phase include the mix design and the method followed according to BS Code. The executing phase include the method of mixing, the method of placing and transporting of concrete and compaction as specified and recommended by technical books and researchers [1, 2, 4-11] with check for workability by applying slump test according to ASTM requirements.

3. The procedure and type of curing done according to the technical and recommended procedure and compressive strength of the cubes sampled from concrete according to ASTM requirements.

The evaluation was organized by given percentages for the three items above with comparing the data collected and the results of tests obtained with the requirements of each item as detailed above. The forms used for evaluation are shown in Tables 1, 2 and 3.

Table 1. Form to Evaluate Concrete Constituents

Relative Wt. of Component Concrete Constituents (20%)

Item cement

Aggregates

Fine Course

Zone Crushing Value Grading

Test of each Batch Testing Per Brand Not Tested 1 2 3 4 Acceptable Not Acceptable Well Poor

Relative Wt. of Item

Table 2. Form to Evaluate Workmanship

Relative Wt. of

Component Workmanship (50%)

Item

Control of Mix Proportions (30%) Mixing and placing (20%)

Mix Design

Type of Batching Type of Vibration Slump Time Between*

Mixing & Placing(Minutes)

Type of Vibration

Relative Wt. of Item

Yes No By Wt.

By Volume Batching Plant or Ready Mix

Mech. Manual Meas-ured Not

Meas-ured <45 45-60 >60 Mech. Manual

Excellent Good Fair Poor

* The Time Limits Indicated are used for Normal Concrete without any additives., If concrete Admixtures are used, then these Limits are Amended Accordingly.

70 Mohamed Y. Mustafa et al.: Evaluation of the Concrete Works Produced Locally in River Nile State

Table 3. Form to Evaluate Curing and Sampling

Relative Wt. of Component Curing (20%) Samples Taken and Tested (10%)

Item

Curing Contractor's Samples

Repaired Type & Method Period (Days)

continuously Wet

Spraying with Water

(Intermittent) ≥ 7 6-4 3-2 < 2 Always Not

Always None

Relative Wt. of Item

The percentage for the three items and their branches are detailed in Table 4.

Table 4. Site Evaluation Form

Site Details: Percentage

Item No. Description Details According to the Requirements

According to the Site Evaluation

1 Material specification cement 8%

aggregate 12%

2 Preparing phase Mix design procedure 30%

Executing phase Mixing, placing, compaction and slump test 20%

3 Curing Type and time 20%

Compressive Strength Samples of Cubes of concrete 10%

Total 100%

The sites are classified according to the total of percentage gained for the quality control (QC) according to the limitations shown in Table 5.

Table 5. Evaluation Classes with Reference to the Percentage of QC

Percentage % Limitation Class

100 – 90 A

89 – 70 B

69 – 50 C

49 – 0 D

3.2. Evaluation According to the Statistical Analysis

This method is based on measuring the standard deviation (σ) as reliable method of measuring used for evaluating the quality control statistically and also calculating the coefficient of variation (ν).

The standard deviation can be measure from the formula:

𝜎𝜎 = �(𝜒𝜒𝑖𝑖−𝜒𝜒�)2

𝑛𝑛 (1)

Where: n is the number of concrete cubes χi is the compressive strength of concrete cubes after

28-days where i the counter from 1 to n. �̅�𝜒 is the mean of the compressive strengths for concrete

cubes and can be calculated from the formula:

�̅�𝜒 = ∑ 𝜒𝜒𝑖𝑖𝑛𝑛

𝑛𝑛𝑖𝑖 (2)

The coefficient of variation can be calculated from the following formula:

𝜈𝜈 = 𝜎𝜎𝜒𝜒�

x 100 (3)

To set the classes according to values of standard deviation (STD) and coefficient of variation (CV), Table 6 explained the criterion.

Table 6. Evaluation Classes with Reference to the values of STD and CV

Standard Deviation (N/mm2) Coefficient of Variation (%) Class

≤ 3.4 ≤ 12 A

3.5 – 4.0 13 – 15 B

4.1 – 5.5 16 – 18 C

> 5.5 > 18 D

The above criterion is near to criterion adopted by ACI-214-77 and Walker [11].

4. Collecting and Analysis of Data The data are collected from 24 sites under construction

and from 20 sites constructed within 10 years later. For the 24 sites the ways of collecting data are restricted

on visiting the sites, take samples of materials like cement,

Journal of Civil Engineering Research 2015, 5(3): 67-73 71

aggregate (coarse and fine) and fresh concrete, general seen inspection, tools and types of doing mixing, placing of concrete, compaction and curing, meeting contract engineer and supervision engineer and query them about labors and their skills, tools used in construction, whether and climate of mixing and placing of concrete and any admixture used for concrete. All these data are collected and tabulated in Tables and analyzed according to the criterion mentioned before. And also all the necessary tests for the materials cement, aggregate and fresh concrete in addition to compressive strength for cubes in 28-daye had been done and the results were recorded.

For the constructed 20 sites, all the necessary data for analysis in addition to the results of test are collected from

existence files concern these sites from the authorized and confidential institution.

For the data collected for all sites and for the results obtained from the laboratory tests an analysis had been done by the two methods described before by using evaluation form shown in Table 1 and Equations 2 and 3.

5. Results and Discussion According to the results of evaluation all sites were

classified according to the criterion described before and shown in Tables 2 and 3. The classification results were summarized as shown in Table 7 and more clarify in Figures 1, 2 and 3.

Table 7. Classification Results

Class 24 sites Percentages 20-sites Percentage All Sites Percentages

A (Excellent) 5 21% 7 35% 12 27%

B (Good) 4 17% 3 15% 7 16%

C (Fair) 11 46% 3 15% 14 32%

D (Poor) 4 16% 7 35% 11 25%

Total 24 100% 20 100% 44 100%

Figure 1. Classification of 24-Sites

Figure 2. Classification of 20-Sites

72 Mohamed Y. Mustafa et al.: Evaluation of the Concrete Works Produced Locally in River Nile State

Figure 3. Classification of All 44-Sites

The results shown in Table 7 and Figures 1, 2 an 3 are the classification of sites as done according to criterion adopt in this paper in order to evaluate the quality control of the production of concrete in sites studied. According to the results obtained we see only 12% of 24 sites under construction gained excellent QC and 17% have good QC and the remainder have fair and poor, also for 20-sites only 19% of sites are excellent and good and the remainder are under good, this results reflect that there is a problem in the methods and types used for QC and this maybe refer to use acceptable materials but not high quality, use unskilled labor, the types and tools used for mixing, compacting, placing and curing are not typical to recommended and specified tools and types and also may refer to week supervision.

As general the 20 existing sites have best QC than 24-sites under construction, where 20-sites have 50% of sites of QC above B-class where for 24-sites have only 38% sites above B-class. This lead to sign a recommending to the contractors for 24-sites to be care for the reasons lead to poor QC in order to raise it.

6. Conclusions A 24-sites under construction and 20-sites constructed

within 10 years past were studied in order to evaluate the quality control of the concrete work in the River Nile State. The evaluation have been done according to the data collected that concern the quality control like material data (cement, aggregate and fresh concrete) and technical data about the types and methods used to execute the concrete work from the mixing, handling and placing the concrete and all functions accompanied to verify quality control like compaction and curing and the necessary field and laboratory tests. The evaluation had been done by adopting total percentage for each site according to results of tests and according to the technical method followed to verify the quality control. A criterion has been used for classifying the sites to classes according to the evaluation percentage obtained for each site. A four classes were adopt that are A

for excellent QC, B for good QC, C for fair QC and D for poor QC. A statistical analysis had been done based on standard deviation and coefficient of variation and these also used to classify the sites to the same classes. From the results obtained we concluded that about 57% of the total sites studied have QC under class B. This mean the QC is weak and more attention must be think about in order to verify as less a good QC by seeking about the reasons of declining the QC and see how to avoid them and make attention to the unskilled labor and using tools and types that precise the work with anticipated to use in future the ready mix concrete.

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Journal of Civil Engineering Research 2015, 5(3): 67-73 73

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