EXECUTIVE SUMMARY
The main objectives of the proposed project are to develop scientific data to demonstrate the effective use of fly ash and bottom ash from burning of Illinois coal in deep foundations, and to develop suitable composites containing coal combustion residues that could be used to construct deep foundations. To achieve the intended objectives, the execution of the project was divided into three distinct phases. Phase I of the investigation consisted of laboratory tests on several different mixtures using Portland cement, natural coarse and fine aggregates, and Illinois pulverized coal combustion (PCC) fly ash and bottom ash to identify suitable mixes that have potential of performing satisfactorily in actual field loading conditions. The purpose of Phase II of the investigation was to determine the effect of the actual environmental conditions on the strength, stiffness, and durability characteristics of the selected PCC matrices. Phase III of the investigation consisted of installation of full-size piles (12-inch diameter and 25 feet long) in field and testing the piles for compressive, uplift, and lateral loads.
Laboratory Testing on Concrete Composites (Phase
I)
Several Portland cement based composites having different matrix constituents and proportions were prepared and tested in compression, split tension, and flexure at various curing ages. Table 1 shows the mix designations with percent of different matrix constituents used to prepare the composites.
Figure 1 shows the compressive strength of concrete composites plotted with respect to the percent of Portland cement replaced with Illinois PCC fly ash. Figure 1(a) is for the concretes in which 100 percent of fine aggregate was replaced with Illinois PCC dry bottom ash and Figure 1(b) is for the concretes in which 50 percent of fine aggregate is replaced with Illinois PCC dry bottom ash.
Based on the results obtained after 60 days of curing, two mixture proportions were selected for installation of Phase II and Phase III piles. These mix proportions are (1) 100 percent replacement of natural aggregate with Illinois PCC dry bottom ash and 10 percent replacement of Portland cement with Illinois PCC fly ash, and (2) 50 percent replacement of natural aggregate with Illinois PCC dry bottom ash and 20 percent replacement of Portland cement with Illinois PCC fly ash.
Although, Figures 1(a) and 1(b) show that long-term strength of concrete composites (180 days curing age) with 0 percent fly ash have higher strength than those of concretes with any amount of fly ash, it was concluded that the difference between the strengths of concretes selected and those with 0 percent fly ash is not significant for the purposes of construction of piles.
Strength Comparison Between Laboratory, Field, and Core Samples (Phase
II)
Figure 2 (a) shows compression strength comparison between two types of samples (1) the samples prepared and cured in laboratory (referred to as lab samples) and (2) the samples prepared in field during pile installation but cured in the laboratory (referred to as field samples). Figure 2 (b) shows compression strength comparison between three type of samples (1) lab samples (2) field samples, and (3) core samples which where cored from the Phase II piles after the piles cured under actual environmental conditions for 90 days.
From the results presented on Figures 2(a) and (b), it was observed that compressive strength of field and core samples is generally less than that of lab samples. This may be due primarily to slightly higher water-to-cementitious materials ratio of the field mixtures (0.51) compared to that of the lab mixtures (0.47). It was concluded that in general, the strength difference is not significant for all practical purposes.
Installation and Testing of Phase III piles
A total of 15 piles were installed and 12 piles were tested to determine compression, uplift, and lateral load capacities of the piles. The piles installed were 12 inch in diameter and 25 feet deep.
Figure 3 shows the lateral load-deflection response of piles constructed using the concrete composites selected and conventional concrete. Similar response in axial pull-out test for all three types of piles is shown in Figure 4. Both Figures 3 and 4 show that the behavior of piles constructed from concrete composites having varying amounts of Illinois PCC fly ash and dry bottom ash is similar to that of the piles constructed using conventional concrete. Figure 3 shows that lateral load load-deflection behavior of piles constructed from composite "MA" is significantly better than that of piles constructed from conventional concrete. However, better resistance shown by this pile may be because of slightly bigger pile head of this pile compared to other piles.
Table 1: Mixture Constituents
Mixture
Designation |
Binders
(%) |
Fine
Aggregates
(%) |
Remarks |
||
Portland
Cement |
PCC Fly
Ash |
PCC Dry Bottom
Ash |
Natural
Sand |
||
TH |
100 |
0 |
100 |
0 |
|
MA |
90 |
10 |
100 |
0 |
|
MB |
80 |
20 |
100 |
0 |
|
MC |
70 |
30 |
100 |
0 |
|
MD |
90 |
10 |
50 |
50 |
|
ME |
80 |
20 |
50 |
50 |
|
MF |
70 |
30 |
50 |
50 |
|
MG |
100 |
0 |
50 |
50 |
|
CM |
100 |
0 |
0 |
100 |
Control
mix |
Figure 1. Influence of fly ash on compressive strength of concrete
composites
(a) (b)
Figure 2. Compression Strength Comparison
of Laboratory, Field, and Core Samples
Figure 3. Comparison of Lateral Load Response of Piles Constructed from Concrete Composites (MA and ME) and Conventional Concrete (CM)
Figure 4. Comparison of Axial Pull-out Response of Piles Constructed from Concrete Composites (MA and ME) and Conventional Concrete (CM)