Fly ash is both environmentally friendly and less expensive than cement. The disposal of fly ash and wood ash can be another problem at the end of use as well. However, both can be a good alternative to replace cement. Fly ash and granulated blast-furnace slag are among materials which develop great cementitious properties when mixed with water. High dosage levels of fly ash, about 40% to 60%, can be used in structural applications, producing concrete with good mechanical properties and durability [1
]. Besides concrete, fly ash is used up to almost 50% in other construction materials such as blocks and bricks. Fly ash itself is being used in different engineering applications such as structural fill, soil stabilization procedures, as an alternative filler material for aggregates, highways and roads sub-bases, and grout filling [2
Recently, renewable fuel such as wood-chips, sawdust, hard chips, and wood bark are used in power generation plants, which results in a rise in timber waste such as wood ash. Around 70% of wood ash is dumped in the ground during the common land-filling process [3
], and it is an expensive process. In modern era there should be better solutions for wood waste rather than disposal. Recently, compressive strength tests have been performed by partially replacing the cement with wood ash in the production of concrete. The test outcome shows acceptable structural grade concrete. This research helps in managing wood ash. Thus, wood ash can be used as a construction material with cement to decrease the cost of production of cement.
The use of waste wood ash in concrete (5, 10, 15, 20, 25, and 30%) by weight influences significantly the strength of concrete which decreases with the increase in wood ash content and it increases with time because of pozzolanic actions [6
]. Depending on the type of wood ash or wood species, the chemical compositions change. Thus, it is the main factor to be considered while producing wood ash concrete. Some other factors such as combustion technology and combustion temperature can be used for its quality and quantity performance [4
Recycled aggregates are particles of stones attached with old cement mortar produced by crushing demolished concrete waste, however recycled concrete aggregate (RCA) helps not only sustainability but also has a practical economic consideration in the construction industry. Recent studies show that pozzolanic materials such as fly ash, blast furnace slag, wood ash and silica fume contain silica in a reactive form which can be a partial replacement for cement and are also environment-friendly as well as economical [7
]. About 50% of fly ash is currently in use in various construction materials, such as concrete, fly ash bricks, etc. It explains the fact that about 7% of the world’s total CO2
comes mainly from manufacturing plants which produce cement [2
Many studies have been focusing on alternative concrete strength as shown in Table 1
. Each study used different types of ashes from different sources to replace cement and analyzed compressive strength at different water/cement ratios. As shown in the table, a higher water cement ratio reduces the compressive strength of the concrete. This was proved by Teixeira’s study [8
] on the valorization of wood/fly ash on concrete. According to his study, the mechanical strength of concrete is dependent on the percentage of cement replacement and the curing time. In general, wood/fly ash addition decreases the compressive strength. In addition, a loss on ignition (LOI) study [9
] shows that the concretes made with high-LOI (8%) fly ash can be successfully produced for structural concrete. The study presented the experimental results of properties of concrete incorporating fly ash with LOIs of 5% and 8%.
The objectives of the study are to introduce a home-produced concrete for small projects such as sidewalks, backyards, or fixing existing concrete economically. Bringing concrete from a concrete plant or local stores is not always affordable, not only it is expensive, but it takes time as well. Hiring local contractors to fix very small areas around homes is not always efficient. The materials being used in this study are available locally, especially wood ash. Wood is burned at homes in most parts of Connecticut, so wood ash can be used as an alternative to replace cement which will help with the reduction of cement in home-produced concretes.
The study focuses on methods to analyze the compressive strength and conduct economic analysis of alternative concrete with sustainable materials using different mix designs. The concrete mix designs will have different percentages of fly ash, wood ash and recycled concrete aggregates. Other admixtures will not be added. The material to be investigated is a conventional mixture in which the Portland cement will be replaced by Class C Fly ash, wood ash and regular natural aggregates will be replaced by recycled concrete aggregates. In order to achieve the objectives, four trials of fly ash concrete mix having 30%, 50%, 75%, and 100% fly ash, were mixed. Four trials of wood ash concrete mix with 18%, 25%, 33%, and 50% wood ash and four mix designs were prepared for concrete with recycled concrete aggregates with 30%, 50%, 80%, and 100%, respectively. All three types of concrete were tested for 7 and 28 days. The target strength of the concrete was 2500 psi as a minimum strength.
4. Results and Discussion
4.1. Compressive Strength
The target strength was 2500 psi (17 MPa) as a minimum strength to be used for small home projects. 2500 psi concrete can be useful for driveways, walkways, sidewalk, or crack repair. The ACI 318 Standard indicates a minimum specified compressive strength of 2500 psi for structural concrete. Simply stated, no structural concrete can be specified with a strength less than 2500 psi. As a reference, concrete compressive strength can vary from 2500 psi for residential concrete to 5000 psi (35 MPa) and higher in commercial structures. Some applications use higher strength, greater than 10,000 psi (70 MPa) [21
Concrete compression tests were conducted based on the standard ASTM C39/C39M-18 [24
]. An INSTRON compressive strength machine (3380 series) with a capacity up to 100 kN (22,500 lbf) was used for testing the compressive strength of the cylinders. Figure 2
summarizes the compressive strength between regular concrete and concrete containing different proportions of fly ash, recycled aggregate and wood ash for 7 and 28 days, respectively. The standard deviation of concrete compressive strength measured in testing varied from 1 to 448 psi. The average of standard deviation for 28-day testing was 121.8 psi.
As shown in Figure 2
, the strength of concrete with fly ash is dependent on the percentage of cement replacement and the curing time. With 30% replacement, the 26% concrete strength dropped down compared to the strength of 0% fly ash replacement in 28-day testing. The concrete strength became meaningless by replacing fly ash over 50%. The strength of 30% fly ash concrete is just 50 psi lower than the target strength of 2500 psi in 28-day testing. The 7-day strength was approximately 50–60% of the 28-day concrete strength for 30% and 50%.
With 50% replacement by wood ash, the strength became lower than half of 0% replacement in 28-day testing. When a target strength is considered, the wood ash replacement percentage must be approximately 30% or less. This is very similar to fly ash concrete. The concrete strength becomes meaningless when replacing wood ash over 33% in 28-day testing. The 7-day strength was approximately 75–77% of the 28-day concrete strength for 18% and 25%.
Recycled aggregate concrete shows a much lower decrease in strength compared to other two alternative concretes. When 100% recycled aggregate is replaced, the 28-day strength decreased only 10% while the 7-day strength decreased 25%. The concrete strength with 100% was still almost 3000 psi for 28-day.
The strengths of the (alternative) concrete depend on the target strength of the proposed (design) concrete. For example, Kalra [2
] shows the average strength of 25% wood ash concrete with a water–cement ratio (w/c) of 0.45 is 4018 psi while the average strength of 20% wood ash concrete with a w/c of 0.49 is 6817 psi. In the case of Smith’s fly ash concrete [27
], the fly ash concrete strength with a w/c of 0.42 was measured to be about 5200 psi. A study with 30% coal ash concrete [28
] shows 3200 psi with a w/c of 0.58. A higher w/c ratio (0.75) was used in this study to reduce the cost of cement and the proposed strength was 2500 psi (17 MPa).
Ramadevi and Chitra [29
] found in their study that there was no big difference in strength between concrete with natural aggregates and concrete with 30% recycled concrete aggregates. Safiuddin et al. [30
] stated in his research paper that the hardened properties of recycled concrete aggregates significantly depend on its source, type, content, gradation, and physical properties. As a general principle, up to 30% (on a weight basis) of natural aggregates may be replaced by recycled concrete aggregates without significantly affecting the hardened properties of concrete.
4.2. Cost Analysis
Cost of materials is the major concern in the construction industry, and construction materials can have negative impacts on the environment frequently. By using recycled concrete aggregate as an alternative for natural aggregate—fly ash and wood ash as alternatives for cement in the construction industry—the cost of materials can be reduced to produce concrete product. The following information was analyzed during this study.
As per mix designs during this research study, the ratios and prices of coarse and fine aggregates and water were assumed to be constant. The only factor affecting the cost of concrete was the use of cement, wood ash, fly ash and recycled aggregate. A bag of 94-lb Portland cement Type I can be purchased at a local store at a price of 13.8 cents per pound [31
]. The cost of fly ash is 1.75 cents per pound for fly ash while wood ash can be purchased for 1.2 cents per pound [32
] although the wood ash can be obtained for free from each house’s fireplace. The official recycled concrete aggregate cost is not available but a local aggregate suppliers, State Crushing, Inc. [33
], lists it for sale and the cost varies from USD 16 to USD 18 per cubic yard since it depends on the purchase amount. If a ½ cubic yard of recycled aggregate is purchased, it costs USD 13 when cheapest. The price varies very frequently. However, the recycled concrete aggregates could be obtained for free from many places. The one used in this study was obtained from Yale University Science Building crane tower’s footing without any cost and they were crushed manually. The price of coarse and fine aggregates has been obtained from a Connecticut-based gravel and sand supplier, Lombardi Gravel and Excavation [34
], and their cost range is from USD 16 to USD 19 per ton. Water per 1000 gallons in Connecticut is about USD 9 per thousand gallons for residential class [35
There are many other significant factors in cost analysis such as transportation, tax, delivery fee, and labor cost. However, there is high uncertainty that could be considered in the analysis since those factors are depend on the size of project, location of project, seasonal variation, market variation and so on. Thus, the cost analysis for this study is focused on materials.
The cost analysis was performed for regular concrete and other alternative concretes in Table 9
, Table 10
and Table 11
. The cost comparison was made for 1 cubic yard of concrete with regular concrete, 30 and 50% fly ash concrete, 25% wood ash concrete, and 100% recycled aggregate concrete. Based on the achieved costs per cubic yard, the fly ash concrete with 30% fly ash was 15.2% cheaper than regular concrete although the strength was 50 psi lower than the target strength of 2500 psi. Fly ash concrete with 50% fly ash can save 26.4%, however, the strength did satisfy the minimum of 2500 psi. Regular concrete was 14.9% more expensive than the wood ash concrete with 25% wood ash. When wood ash was provided for free, there was an additional 2% saving as shown in Table 10
The use of recycled concrete aggregates could replace natural aggregates up to 100% but there was no significant saving (5.6%), however, when the recycled aggregate was considered to be free then it reduced 19% of cost compared to regular concrete.
If Ready-mix concrete is ordered for 1 cubic yard concrete project, it costs much more. Although it depends on project location, the order price was about USD 175 to pour ready-mix concrete for a short load of less than 10 cubic yards [36
]. In addition, Quikrete (premixed concrete) would be an option for a home project. The typical strength of Quikrete is 3000 to 5000 psi so it fully satisfies any kind of home project. A 60-lb bag of 4 ksi Quikrete is USD 3.35 in a local recycling plant [37
] and it costs about USD 180 for a 1 cubic yard concrete project (Table 11
). This is much higher than other concretes discussed above. Table 12
shows the normalized cost for each concrete type. As shown in the table, Quikrete and Ready-mix concrete were twice as expensive as regular concrete while all other alternative concretes were cheaper than the regular concrete. However, transportation and labor costs are not considered in the analysis and this may change the results significantly.
4.3. Regional Factor
The concrete material for aggregates varies depending on suppliers’ locations. The U.S. Geological Survey (USGA) mineral year book [38
] lists the average value and quantity of construction sand and gravel sold or used by producers in the U.S. by state. The value shown in the report typically reflects wholesalers for large consumers and is not appropriate to be used for this study directly since this study focuses on home produced concrete for a small consumer. Thus, the regional cost analysis was conducted by normalizing the value based on the assumption that the value is correlated linearly to the local retailer prices of construction materials in Connecticut. The most recent report of the USGA reflects the data from 2017 and Figure 3
shows the averaged the normalization factor compared to Connecticut.
The lowest normalized aggregate cost reported by the report is South Dakota in Midwest and the value is determined to be 0.49 times the price in Connecticut, while Hawaii is 2.1 times more expensive than Connecticut. The normalized Midwest aggregate price for Connecticut ranges from 0.49 (South Dakota) to 0.9 (Ohio). Other than the Midwest and Southwest, the values are very similar to Connecticut. Using the Midwest normalization factor of 0.71 for aggregates, the cost for fly ash decreases to USD 56.28 (additional USD 9 saving) from Table 10
. Water price is of minimal impact and was not considered in the regional factor calculation. Cement price is lower in a typical store, such as Lowes or Home Depot and the price is similar nationwide. The regional factors for fly ash, wood ash, and recycled aggregates were not considered because of limited information and sources for other regions.
4.4. Carbon Emission Factors (Global Warming Potentials)
A tremendous amount of concrete is being used in the construction industry which makes the cost of construction so high. The vast amount of construction demolition waste from old concrete structures can be reused as alternative concrete by the construction industry. It is motivated not only by environmental protection, but also by the conservation of natural aggregate resources, the storage of waste disposal land, and the increasing cost of waste treatment before disposal. In China, it was estimated approximately 15.5 million tons of construction waste annually [39
Carbon dioxide has been considered the main threat which causes global warming. Many possible attempts have been made to reduce the emissions of CO2 by replacing the vast usage of cement with other alternative cementitious materials. Among all those materials, fly ash and wood ash are an effort to reduce the cement content in concrete. Moreover, the use of ash in concrete industry reduces the scarcity of land-filling and waste management.
Thus, environmental effects of alternative concrete were also evaluated and part of the Life Cycle Assessment (LCA) methodology in terms of CO2
emission factor (or Global Warming Potential, GWP) was performed. Greenhouse Gas (GHG) emission factors such as CO2
equivalents for concrete material production have been developed by the Portland Cement Association (PCA), US Census Bureau, the U.S. Geological Survey (USGS), and many individual researchers and universities [27
]. There is a notable range among each study and the emission factors used for this study are appropriately derived from selected studies. The emitted CO2
equivalents for the materials are then converted to the corresponding common fuel quantity such as gasoline (gal) and followed by multiplying by the 2020 market cost for regular gasoline in the U.S. According to The Environmental Protection Agency (EPA) data—Greenshouse Gases Equivalencies [47
], the common conversion factor is 8887 g of CO2
emissions per gallon of gasoline consumed. This indicates that the heat content of the fuel per gallon can be multiplied by the kg CO2
per heat content of the fuel to obtain the number of grams of CO2
emitted per gallon of gasoline combusted. Table 13
displays the emission factors with the associated direct cost saving calculation. The averaged regular gasoline price in Connecticut was USD 2.196 [48
] as of August 9, 2020.
As shown in Table 13
, the total CO2eq
for 30% fly ash concrete was 299.9 lb, while the regular concrete emits 419.9 lb for 1 cubic yard of concrete. This converts to equivalent gasoline and an equivalent indirect cost of USD 13.47 that could be saved by using 30% fly ash concrete. The lowest gasoline price as of August 2020 is USD 1.82 in Mississippi while the highest one is USD 3.24 in Hawaii. When this price range is considered, the equivalent saving could be from USD 11.17 to USD 19.88 for fly ash. The figures used in the study data do not include transportation associated costs for purchasing purposes since there are many unknowns such as distance from house to store, store to manufacturing factory, vehicle type and the gas efficiency of a vehicle, and so on.
The recycled aggregate had 25 times less carbon GWP than new aggregates, however, the aggregates have 99.7% less GWP than cement. This shows that reducing the cement content of the concrete is the most significant measure to be sustainable and environmentally friendly in the concrete industry. It is also very interesting that fly ash had almost 17 times higher GWP than wood ash, yet the concrete produced using 30% fly ash had lower total GWP since less cement was required using fly ash to produce a similar strength. In addition, recycled aggregate concrete saves only about 1% CO2 emission compared to regular concrete while fly ash saves more than 28.5% and wood ash saves almost 24.5%.