Strength Comparison of Accelerated Cured and Normal Cured Fly-Ash Based Concrete

The most important property of concrete is its compressive strength, which is carried out after 28-days of proper curing of concrete. This test is affected by other factors like the condition of curing, water to cement ratio, method of transportation, handling of the concrete, extent of vibrations and quality of the ingredients of mix proportion. This research study is an attempt to develop a simple mathematical model, by using linear regression analysis to estimate the 28-day f c ’ (Compressive Strength) of concrete from the test results carried out at early age. This simple linear equation develops a relationship of 28.5 hours. These results show that most of the predicted values of compressive strength, calculated via equations, lie within permissible range difference for compressive strength achieved by experimental method, which is clear indication of credibility of the equations obtained for compressive strength at different age of concrete. The results show that compressive strength of concrete increases with the increase in content of FA (Fly Ash) upto 30% replacement, and the compressive strength of the concrete starts decreasing beyond 30% FA substitution. This argument is totally in line with all the literature carried out for this research.

to test moist-cured cylinders at 23°C after 28-days of casting. Sometimes even placement for different areas up to rate of 400 yd 3 /hour. large structures might get completed even before the 28-days strength. By depending on the test results in order to be sure that proper strength and quality of concrete is satisfied, there is a chance that noticeable quantity of inferior concrete might have been placed before the detection. After the test results, it might not be possible to substitute the inferior quality of concrete without damaging the acceptable quality of concrete in the structure. The cost increase due to this misshape is a consequence of this practice. The 7-days strength of cylinder is not the solution due to unreliable result for the 28-days strength prediction. Besides, even 7-days wait for the test results is a large amount of time to waste in the modern fast construction era. This fact is evident that the compressive strength of concrete is important in today's concrete construction industry [1][2]. The need to be able to predict the 28-days strength of concrete at early age is influenced by the following factors: (1) The rush and tight schedule of construction in the modern fast and machined world.
(2) Quality control issue is to be observed strictly via testing. (3) To be able to know that the quality of concrete for the said project is suitable even before 28 days testing.
(4) To have safety factor in terms of time in case of any misshape regarding strength.
To be able to apply the service loads on the structure prior to the said schedule.

MATERIALS AND METHODOLOGY
Materials used in this research are listed as follow:

Natural Coarse Aggregate (Margalla Crush)
Maximum aggregate size used is 19mm. The Fineness Modulus was calculated to be 7.32. Specific Gravity was calculated to be 2.5, and the water absorption of coarse aggregate was found to be 1.1%.

Fine Aggregate (Lawrencepur Sand)
Lawrencepur sand is known for its superior quality and higher strength. The fineness modulus of fine aggregate was calculated to be 2.55. the bulk specific gravity of fine aggregate was calculated to be 2.78. and the water absorption for fine aggregate was to found to be 1.2%.

Portable Water (Free from Impurities)
Ordinary tap water of PH from 6-8, colorless, odorless and free from impurities is used.

Ordinary Portland Cement (Maple Leaf Cement)
Maple Leaf cement is the best available cement in the market which gives the highest compressive strength. It is slightly expensive than the other brands of cement.

Fly Ash
The FA used in this research was of Class-F as per the specification of ASTM C-618. This class is produced from older and harder coal and it contains more iron.

Methodology
For casting of concrete specimens, 6 division of test

RESULTS AND DISCUSSION
The average of each day's compressive strength is mentioned in Table 2.

FIG. 3. COMPRESSIVE STRENGTH OF 28-DAYS TEST RESULT WITH INCREASE OF FLY ASH
The incorporation of FA in the concrete affects its compressive strength. Each variation of FA from results shown in Table 2 were compared with the control sample's test for each day. And the percentage variation is listed in

Linear Regression
Linear regression analysis is an attempt to develop a model which shows a relationship between two variables. In this research, the number of days is explanatory variable and 7, 14, 21 and 28-days compressive strength is dependent variables. A linear equation is for a straight line and is stated as Y = mX + c, where Y is the dependent variable and X is the explanatory variable, "m" denotes the gradient of the line and "c" is the y-intercept when the value of "x" is zero.

R-Squared
It is the measure of how close the regression equation is to the data observed. Statistically it is also known as coefficient of determination. This value varies between 0-100%. The closer this value to 100% the most accurate will be the results obtained by putting the data into it. It can simply be stated that higher value of R-squared means that model fits the data more accurately. Usually any value more than 80% is acceptable in a research. It means that there is an 80% chance/probability, that the model would give nearly 80% accurate value to the data.

Linear Regression Analysis
Regression analysis carried out for the data observed from the compressive strength values shown in Table 2 has been done via developing graphs (scatter plots   Fig. 4.

Linear Regression Equations
The mathematical equations obtained after carrying out regression analysis are listed in Table 4.

Difference between Actual and Predicted Values
The mathematical models obtained after carrying the *R 2 is the determination coefficient, which shows the compatibility of equation for predicting a certain value (i.e. in this research it is the compressive strength of concrete). In these equations "x" represents the day on which the compressive strength is to be calculated. Moreover, "y" is the compressive strength for "x" days.

CONCLUSIONS
This

RECOMMENDATIONS
Stated below are some of the recommendations to be carried out in further studies by the researchers. (2) Further study is required to establish a direct relationship of accelerated cured and normally cured FA based concrete.

ACKNOWLEDGEMENT
This research work has been carried out in the Department of Civil Engineering, The University of Lahore, Lahore, Pakistan.