By Adam D. Neuwald
Two of the most commonly specified requirements for concrete used in the manufactured concrete products industry are the design compressive strength (f’ c) and the maximum water-to-cement ratio (w/c). These two values are inversely related, which means that as the water-to-cement ratio increases, the compressive strength decreases. Not only does the w/c ratio have a strong influence on compressive strength, but it also affects the permeability and ultimately the durability of the concrete. Both of these properties become extremely important when the precast product will be subjected to a corrosive environment or freeze-thaw conditions, or when it is required to provide a watertight structure.
Concrete is designed to withstand a certain maximum load per area before failing, known as compressive strength A number of factors influence the concrete’s ability to withstand the force from an applied load, such as the size, type, quantity and gradation of aggregates, the type and quantity of cement and/or supplementary cementitious materials, the amount of mix water, the age or maturity of the concrete, and the production practices used in placing, consolidating and curing the concrete. Small changes in any of these variables can have a profound effect on the concrete’s compressive strength, permeability and durability. To account for such variables, mixes are designed to meet an average or required compressive strength (f’ cr), which is greater than the design strength. Procedures for determining the average or required compressive strength are addressed in chapter 5 of ACI 318 and are covered in the May/June 2004 MC magazine article titled “Standard Deviation” (available at www.precast.org).
Once general requirements such as the required compressive strength, air content and slump have been established, initial mix designs may be developed following the guidelines in ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete.”
Water-to-cement ratio
The maximum water-to-cement ratio may be established by the customer or authority having jurisdiction based on anticipated exposure conditions. The target w/c ratio can also be selected from available data on the actual materials that will be used. If no such data is available the w/c ratio can be selected from table 6.3.4(a) of ACI 211.1 based on the required compressive strength. The lower of the two w/c ratios should be used for the mix design.
The water-to-cement ratio is the weight of water provided in a mix divided by the weight of cementitious materials. The total weight of water includes all batch water and free water from the surface of aggregates. If the amount of water is provided in gallons, it can easily be converted to pounds by multiplying the total gallons by 8.34 pounds per gallon. Cementitious materials include portland cement, blended cements and supplementary cementitious materials such as fly ash, silica fume and slag. Because of this, the water-to-cement ratio may be referred to as the water-to-cementitious materials ratio (w/cm). When calculating the w/c ratio, the total weight of all cementitious materials is used in the denominator.
1 gallon of water = 8.34 pounds of water
Table 6.3.3 of ACI 211.1 may be used to select the required amount of batch water based on the desired slump and maximum aggregate size. The amount of cement and/or cementitious materials is then determined by dividing the selected water weight by the w/c ratio. As the amount of batch water is increased to achieve greater workability, so is the amount of cement in order to maintain the required water-to-cement ratio. The workability of a concrete mix is provided by the paste, which fills the voids between aggregates. The paste acts as a lubricant that reduces internal friction between aggregates while increasing workability. As the aggregate decreases in size, the amount of paste must increase to account for an increase in aggregate surface area.
For both economical reasons and concerns with durability, it is often desirable to use the largest size aggregate possible to minimize the amount of paste in the system. Water-reducing chemical admixtures are often incorporated into a mix to achieve the required fresh properties for placing and consolidating concrete, ensuring that both a lower w/c ratio and a paste content can be maintained. Water alone should never be used to improve the workability of fresh concrete. Using water to assist in finishing operations or working bleed water back into the top surface of the concrete should also be avoided as these practices will increase the water-to-cement ratio of the top layer of concrete, which will lead to future durability problems.
Hydration is the result of a chemical reaction that occurs between the cement and water. Initially the cement grains are dispersed throughout the system and are separated by water (Fig. 1 at right). During this stage of hydration, which typically occurs in the first 15 minutes, a rapid exothermic chemical reaction takes place, which produces a considerable amount of heat. Following this initial reaction, the hydration process enters a dormant period of any where from two to four hours. This dormant period allows for the transportation and placement of the concrete.
Rather than adding additional water to increase the concrete’s workability, water-reducing admixtures can improve the dispersion of the cement particles to increase the workability.
Following the dormant period, the cement will continue to hydrate, producing reaction products that will begin to fill the voids between the cement particles (Fig. 2 at right). The formation of reaction products ultimately creates the binding material between aggregates. A basic mix will typically reach its initial set after about four hours of hydration. At this time, concrete is no longer workable and will typically have a compressive strength of about 500 psi. The cement will continue to hydrate, producing additional reaction products that will fill the voids provided by the initial mix water. As long as there is available room for the reaction products to form and water present to further hydration, the hydration reaction will persist and the concrete will continue to gain strength. However, once the available water has been exhausted or the voids have been filled, the hydration of the cement will cease and the strength gain of the concrete will plateau.
In theory, 100 percent hydration of cement can be achieved when enough water has been provided to react with the available cement and enough space has been provided by the initial mix water for the hydration products to form. Although 100 percent cement hydration does not actually occur, we will proceed as if it does. Roughly 1.1 unit volumes of water are required to completely hydrate 1 unit volume of cement, meaning that 1 cubic foot of cement will produce 2.1 cubic feet of hydration product formed from the available cement and water. This translates to a w/c ratio of 0.36. However, in order to achieve complete hydration, all the pores within the system must be completely filled with water throughout the hydration reaction. If a w/c ratio of 0.36 were used, the pores would not remain full during the entire reaction; thus to achieve 100 percent hydration, a w/c ratio of 0.42 is required.
Some concretes are produced with w/c ratios lower than 0.2 and as high as 0.7, although these ratios are not recommended for quality concrete. Concretes with higher water-to-cement ratios ultimately contain more water than is required for complete hydration of the available cement. This additional water creates additional voids known as capillary pores. As the w/c ratio increases, so does the capillary porosity, and it has a strong influence on the strength and permeability of the concrete as illustrated in the following graphs. A concrete with a high porosity will not provide a watertight structure and will likely deteriorate at an accelerated rate when exposed to severe freeze-thaw conditions or a corrosive environment.
Because of this effect, both the American Concrete Institute (ACI) and the National Precast Concrete Association have established maximum w/c ratio limits for various applications. The “NPCA Quality Control Manual for Precast Products” sets a maximum w/c ratio of 0.45 for concrete exposed to freezing and thawing and a maximum limit of 0.40 for concrete that will be exposed to deicer salts, brackish water or seawater. A maximum w/c ratio of 0.48 is set for watertight products containing fresh water. In order to produce concrete with a lower water-to-cement ratio, chemical admixtures can reduce the required amount of mixing water and still obtain the desired fresh properties to facilitate the placement and consolidation of the concrete.
One of the key parameters in producing high-strength concrete is the use of a low w/c ratio. As explained earlier, this means that not all of the cement will hydrate because of the lack of available space within the system for hydration products to form and because of the lack of free water available to hydrate all of the cement (Fig. 3 at right). This is why moisture curing of high-strength concrete is extremely important.
Aggregate moisture corrections
Aggregates are not completely solid but rather contain a certain level of porosity. Pores may be located in the center of the aggregate, while others may actually connect to the surface of the aggregate. When calculating the bulk specific gravity of an aggregate, take both the volume of the aggregate and all its pores into consideration. These pores will likely contain a certain level of moisture that will affect the performance of the concrete if appropriate corrections are not made to account for the actual moisture content of the aggregates. There are four different moisture conditions for aggregates, two of which may be achieved in a laboratory, while the other two occur naturally on a daily basis in aggregate stockpiles.
Oven-dry (OD): This is achieved under laboratory conditions when the aggregate is heated to 220 F (105 C) for an extended period. Under this condition, all moisture is removed from the aggregate’s pores.
Air-dry (AD): The surface of the aggregate is dry and the internal pores may be partially filled with water. This condition may occur on a hot summer day or in an arid region. The aggregates will likely absorb water from the mix, which may affect the workability of the concrete unless proper adjustments are made to the aggregate and water batch weights.
Saturated surface-dry (SSD): This is achieved under laboratory conditions when all the pores are completely filled with water but no free water remains on the surface of the aggregate. Aggregates in this condition will not contribute free water nor absorb water from the mix.
Damp or Wet: All the pores are completely filled with water and the surface of the aggregate contains free water. Aggregates in a stockpile will typically be in this condition, meaning additional water will be added to the mix unless proper adjustments are made to the aggregate and water batch weights.
Aggregate mixture proportions are developed using either the oven-dry or saturated surface-dry condition. It is important to know this information when adjusting mix designs to account for actual aggregate moisture contents. Mix designs are typically developed using the oven-dry condition, but some may be developed using the saturated surface-dry condition. According to Ken Hover of Cornell University, one advantage of designing a mix based on SSD conditions is that the total weight of the batched materials will be the same before and after aggregate moisture corrections. Corrections to aggregate batch weights can be made using a correction factor, while the batch water weight is easily calculated by subtracting the weight of the cement and adjusted aggregates from the original design weight of all materials.
The actual aggregate moisture content and the absorption value of the aggregates must be known in order to accurately adjust the batch weights. The aggregate supplier should be able to provide you with the absorption value for each aggregate; otherwise they may be calculated following the procedures in ASTM C127 for coarse aggregates and ASTM C128 fine aggregates.
A = Absorption
W SSD = Weight saturated surface-dry aggregate
W OD= Weight oven-dry aggregate
The moisture content for each aggregate must also be calculated. Aggregate moisture contents will vary throughout a stockpile, with wetter aggregates located near the bottom of the pile. It is extremely important to calculate the aggregate moisture content at least once a day and perhaps more frequently when producing self-consolidating concrete (SCC), which is more sensitive to changes in aggregate moisture contents. Some batching systems are equipped with probes that read the moisture content of aggregates while being discharged from the hopper. These systems are typically tied directly into the batch computer and will automatically adjust the batch weights for correct proportions and w/c ratio. For batching systems without moisture meters or probes, the aggregate moisture content must be determined manually.
ASTM C566, “Standard Test Method for Total Evaporable Moisture Content of Aggregate by Drying,” should be followed when determining the aggregate moisture content. Take a representative sample from the aggregate stockpile, avoiding the first few inches as this material is probably dry and not representative of the entire lot. Take the samples in accordance with the procedures established in ASTM D75, “Standard Practice for Sampling Aggregates,” except for the sample size.
Weigh the collected sample and record it prior to drying. Use a hot plate, microwave oven or some other means of drying. Note that very rapid heating may cause some particles to explode, resulting in the loss of particles, which may render your calculations inaccurate. The sample is considered dry when further heating would cause less than 0.1 percent additional loss in mass. Allow the sample to cool to avoid damaging the scale. Weigh the sample to the nearest 0.1 percent. Calculate the total aggregate moisture content (MC) using the follow equation:
MC = Moisture Content
W initial = Weight of the sample prior to drying
W OD = Weight of the sample after drying
By using the moisture content and absorption of the aggregates, you can adjust the batch weights to account for the actual moisture condition. If the moisture content is higher than the aggregate’s absorption value, the aggregates will contribute free water to the mix. If the moisture content is below the absorption value, the aggregates will absorb a portion of the mix water.
For mix designs based on raw materials in an oven-dry condition, make the following adjustments.
Calculate the adjusted coarse aggregate (CA BW) and fine aggregate (FA BW) batch weights using the following equation for each material:
AGG BW = Weight of adjusted aggregate to be batched (calculate for CA BW and FA BW)
AGG DW = Mix design weight of aggregate (CA DW and FA DW)
MC = Moisture content as a percentage (MC CA and MC FA)
Calculate the adjusted water batch weight (W BW) using the following equation:
W BW= Weight of water to be batched after adjustment
W DW= Mix design weight of water
CA DW = Mix design weight of coarse aggregate
MC CA = Moisture content of coarse aggregate as a percentage
A CA = Absorption of coarse aggregate as a percentage
FA DW = Mix design weight of fine aggregate
MC FA = Moisture content of fine aggregate as a percentage
A FA = Absorption of fine aggregate as a percentage
For mix designs based on raw materials in a saturated surface-dry condition, make the following adjustments.
Calculate coarse aggregate (CA BW) and fine aggregate (FA BW) batch weights by multiplying each aggregate design weight (AGG DW) by its respective correction factor (CF) using the following equation:
CF = Correction factor must be calculated for each aggregate (CF CA and CF FA)
MC = Moisture content of aggregate as a percentage (MC CA and MC FA)
A = Absorption of aggregate as a percentage (A CA and A FA)
AGG BW = Weight of adjusted aggregate to be batched (calculate for CA BW and FA BW)
AGG DW = Mix design weight of aggregate (CA DW and FA DW)
CF = Correction factor must be calculated for each aggregate (CF CA and CF FA)
Determine the amount of batch water by subtracting the sum of the corrected batch weights (cement, CA BW and FA BW) from the sum of all the initial design weights, including the water. This concept is illustrated below.
W BW= Weight of water to be batched after adjustment
C DW = C BW ; The weight of the cement does not change form the initial design
CA DW = Mix design weight of coarse aggregate
FA DW = Mix design weight of fine aggregate
CA BW = Adjusted batch weight of coarse aggregate
FA BW = Adjusted batch weight of fine aggregate
The following examples show to adjust how mix design weights to account for aggregates of varying moisture contents.
Example 1: Adjusting Mix Designs Based on Oven-Dry Conditions
The following information is provided for the initial mix design:
- Cement = 650 lbs
- Coarse Aggregate (OD) = 1,836 lbs
- Absorption = 0.5%
- Moisture Content = 2.0%
- Fine Aggregate (OD) = 1,243 lbs
- Absorption = 0.7%
- Moisture Content = 5.20%
- Water = 315 lbs
Calculate the adjusted aggregate batch weights
Calculate the adjusted water batch weight:
The new batch weights are as follows:
- Cement = 650 lbs
- Coarse Aggregate = 1,873 lbs
- Fine Aggregate = 1,308 lbs
- Water = 231 lbs
Example 2: Adjusting Mix Designs Based on Saturated Surface-Dry Conditions
The following information is provided for the initial mix design:
- Cement = 650 lbs
- Coarse Aggregate (SSD) = 1,610 lbs
- Absorption = 0.5%
- Moisture Content = 1.8%
- Fine Aggregate (SSD) = 1,245 lbs
- Absorption = 0.7%
- Moisture Content = 4.8%
- Water = 310 lbs
Total weight of materials = 
Calculate the adjusted aggregate batch weights
Calculate the adjusted water batch weight
The new batch weights are as follows:
- Cement = 650 lbs
- Coarse Aggregate = 1,631 lbs
- Fine Aggregate = 1,296 lbs
- Water = 237 lbs
What would happen if the design batch weights in the above examples were used without making corrections to account for the actual aggregate moisture contents? The w/c ratio in the first example would have changed from roughly 0.48 to 0.61, and the w/c ratio in the second example would have changed from roughly 0.48 to 0.59. This would mean that the 28-day compressive strength of each mix would likely be reduced by 1,000 psi, not to mention that the w/c ratios may no longer comply with the limits established by the authority having jurisdiction.
Whether you are using your own batch plant with automated moisture probes or purchasing ready-mixed concrete, it is extremely important that all individuals involved with the batching, mixing and casting of concrete understand the importance of maintaining the specified water-to-cement ratio. All additional water added to a mix should be measured and accounted for by adjusting mix proportions to ensure the maximum water-to-cement ratio is not exceeded. Having tight control on the w/c ratio will remove one of the many variables that influence the strength and durability of finished products.
this is nice.:)..
Nice
This was very helpful. Thank you.
Interesting comment regarding increased porosity degrading the freeze thaw resistance.
nice job and thanks this is very helpfull
What is the standard for achieving SSD aggregates?
Achieving SSD in aggregates can be done using methods ASTM C127 Standard Test Method for Relative Density (Specific Gravity) and Absorption of Coarse Aggregate and C128 Standard Test Method for Relative Density (Specific Gravity) and Absorption of Fine Aggregate.
Usually, SSD weights are given in the aggregate mill certificates through the Absorption Capacity. Take the AC and multiply it by the oven dry weight, and you get your SSD weight. I would happy to go over this with you in detail if need be.
How do you calculate for the moisture correction for hot aggregates which is lying open yard at Batching plant site. Aggregates will drink more water because of hot. Then how do you calculate that water content other than considering water absorption and moisture content of aggregates.
Thanks for the comment Janagarajan.P. I consulted with the technical department here at NPCA, and they advised that determining water adjustments for aggregates is usually only based on moisture content and absorption capacity. The aggregate can only absorb as much water as its porosity will allow, regardless of temperature. If the aggregate is so hot that it evaporates mix water that comes in contact, then looking at ways to lower that temperature may be considered. Some precast concrete companies will sprinkle aggregate with water. Others will erect means of shading to keep temperatures down.
Thanku sir.Very useful information.
Very helpful for civil engineers, thanks
what is the normal slump of girders concrete to achieve a good strength for this water cement ratio o.40?
i m waiting for your good response;;;;;;;;;;;;;;;;;;
AME NESPAK PAKISTAN
Thank you for the comment ejaznespak. Evan Gurley, one of our technical engineers, has given the following response:
“Concrete must always be made with a workability, consistency and plasticity suitable for job conditions. The slump test is generally used to measure concrete consistency. Consistency is the ability of freshly made concrete to flow. When used with different batches of the same mix design, a change in slump indicates a change in consistency and in the characteristics of the materials, mixture proportions, water content, mixing, time of test or the testing itself.
The reality is that if we measure the slump, the only thing we really know at this point is the slump. The slump of a concrete mix is influenced by everything. Changes in any of the following can change the slump of the concrete:
– Content, proportions, chemistry, fineness particle size distribution, moisture content and temperature of cementitious materials
– Content, proportions, size, texture, grading, cleanliness and moisture contents of the aggregates
– Dosage, type, combination, interaction, sequence of addition of chemical admixtures
– Air content
– Batching, mixing and delivery/placing methods
– Temperature of the concrete
– Sampling, slump-testing technique and the condition of the test equipment
– Amount of free water in the concrete
– Time since batching at the time of testing
Slump is usually indicated in the job specifications as a range or as a maximum value not to be exceeded. If slump is not specified, ACI 211.1 Table 6.3.1 has established recommended slumps based on the various types of construction.”
How to calculate the volumetric batch of 1:2:4, and how to apply corrections in moisture of aggregates and bulking in sand.
Thank you for your comment Rod. Kayla Hanson, one of technical engineers, has provided the following response.
“Volumetric Batching: First, the 1:2:4 ratio is the ratio of cement to fine aggregate to coarse aggregate, by material volume. There are five parameters we need to identify prior to calculating the mix proportions.
1. Required strength
2. Minimum cementitious materials content or maximum water-to-cementitious materials ratio
3. Nominal maximum aggregate size
4. Air content
5. Desired slump
Next, ACI 318 outlines in detail numerous guidelines and requirements that dictate certain factors. Additionally, you’ll need to refer to ACI 211.1, ACI 211.2, ACI 211.3 or ACI 237R-07 for proportioning, depending on what type of mix you’re developing – normal, heavyweight or mass concrete; structural lightweight concrete; no-slump concrete; or self-consolidating concrete. If you have further questions after reviewing the ACI codes, NPCA’s technical services is happy to answer them for you.
Moisture Adjustments: To determine moisture correction factors, we first need to know the moisture content of both the coarse aggregate and fine aggregate. We also need to know the certain inherent aggregate properties.
For example, if the nominal maximum coarse aggregate size is 1 inch, and the fine aggregate has a fineness modulus of 2.40, ACI 211.1 will tell us the recommended bulk volume of coarse aggregate is 0.71. If the coarse aggregate has a unit weight of 100 lb/ft3, this will require 1,917 pounds of coarse aggregate per cubic yard of concrete (0.71 * 100 lb/ft3 * 27 ft3/yd3 = 1,917 lb/yd3). If the coarse aggregate has a moisture content of 1.5%, we will instead need to batch about 1,946 pounds of coarse aggregate per cubic yard of concrete (1,917 lb/yd3 * 1.015 = 1,946 lb/yd3).
Similar calculations also apply to fine aggregate moisture adjustments. Adjusting any aspect of the batch will alter the characteristics of both plastic and hardened concrete, so be sure to account for any and all adjustments that are made, and also be aware of how those adjustments will affect other raw material quantities and the fresh and hardened concrete properties.”
Thanks guys for your helpful information . You guys are on the ball with everything
Base on all the information that is been shade Is there anyone on your team have ever achieve 4000 PSI in 24 hours ? If yes can someone explain to me how is it been done
What are the key factor in doing this design ?
Please help
I am not a concrete professional, but for that type of performance, I think you will need to use calcium aluminate cement instead of portland cement.
Thank you for the informative article.
🙂
Do you know what the standard or typical values for absorption of coarse aggregates that are to be used in concrete mix design are?
Thank you for the comment Isah. Aggregate absorption values and specific gravities can vary widely depending on the geology and geography of the aggregate site. This can even vary within close regional pit locations. I suggest to contact your local aggregate supplier to obtain absorption values, which would be valid for your mix design.
What if the materials are not in OD or SSD condition? Which adjustment do you make?
Thank you for the comment RHC. Eric Carleton, vice president of technical services, provided the following response:
“The answer to your question is actual aggregate conditions are rarely oven dry or surface-saturated dry. Those are laboratory established ideal conditions used to develop the mix design. Prior to production, all aggregates need to be tested no less than daily (and possibly more depending on local conditions) to determine the actual moisture content of the stored aggregate. The mix water can then be adjusted to match the original mix design water content developed for that particular aggregate when in OD and SSD condition. The arithmetic adjustments used, as described in the article, would depend on which way you want to develop the original mix design.”
Is it required to reduce the selection of water content in calculation of mixed design when fine aggregate has a surface moisture content of 2%?if yes, then how?
Thank you for the comment Ussu. To answer your question, when aggregates are wet beyond an SSD state, the excess surface water on the aggregates contributes to the mix water. The amount of water batched into the mixer should be adjusted to account for the aggregate moisture content (and the additional water added to the mix by the wet aggregates) so that the total amount of water in the mix abides by the mix design qualifications.
Why would two plants batching concrete using the same materials use different amount of water, say 120litres and 146litres? More so when before the two plants were using the same amount of water? (Please note that one of the plant is automated and the other is not when it comes to making moisture corrections?).
Thank you for your comment Richard. Eric Carleton, director of codes and standards, provided the following response:
“Greetings Richard, you pose an interesting question. As you may know, water serves two purposes in a concrete mix design. First, it starts the hydration process with the cementitious materials. The minimum water-to-cementitious materials ratio needed is between 0.19 and 0.26, depending on the materials, grind fineness, etc. Second, water in excess amounts of the needed hydration minimum fill the pores and provide lubricity or workability to the mix. This characteristic is measured via a slump test. More water creates greater slump and workability. However, there is diminishing returns because adding water beyond the optimized hydration amount can lead to reduced compressive strength, possible aggregate segregation and reduced durability. Many standards limit the maximum w/cm ratio for quality concrete.Fortunately, modern chemical admixtures can increase concrete workability as well without the need for excessive water.
Which now brings us back to your initial question, why would two plants using identical raw materials have mix designs with different water quantities? When you say “one plant is automated and the other is not when it comes to making moisture corrections,” I assume the automation only applies to the mix and batch operation and not that one plant manufactures products via an “automated” process. In that case, “automated” would mean one plant employs a dry-cast automated production method and the other doesn’t, which would explain everything.
However, assuming both plants use wet cast or SCC mixes, the answer could be a number of reasons:
1. If the raw materials used are the same, each plant’s mix design may be slightly different. It could be one plant is using more cement requiring greater amounts of water to maintain a similar slump or slump flow range needed for casting. Note, the added cement may not be intentional, so checking weigh scales or ensuring material gates open and close properly, may also solve the problem.
2. The use of automated moisture controls when operating properly can provide added consistency to batching. It is important moisture sensors are calibrated with the aggregates being used. The NPCA Quality Control Manual for Precast concrete Plants, Section 5.2.2, “Moisture Content,” requires the calibration to occur weekly for conventional, dry-cast and SCC mixes. This is typically accomplished by either ASTM C70 or C566 and compares it to automated readings to ensure they are within an acceptable range. If the moisture sensors are not reading properly that could lead to varied water content. A slump test would stress this problem.
3. Are both plants maintaining identical slump or slump flow range for the produced products? And are they actively testing fresh mixes? If not, that could also be the reason for the water difference. Additionally, if one plant is using a water reducing or plasticizer admixture in different proportions, that would account for different batching weights of water.
4. Are both aggregates stored similarly? If a sand or course aggregate is stored outside for one plant, it may dry out faster than a plant that keeps its aggregate stored inside. This can have a greater water demand.
In a perfect world theoretically both mixes of “identical materials” should have identical water demand. However, concrete batching is not a perfect science and there are minor variables to always be considered. Therefore, the answer to your question boils down to either: mix design differences, equipment error or human error.”
Is there a formula or estimate for the impact on cement requirements to maintain required strength when additional fine aggregate is added. With a known surface area for the additional sand can I calculate the additional cement required to avoid strength loss?
Nice job:):D
I am trying to determine the range or spec. of the W/C ratios on 2 machines that we run in a dry cast processing plant. What we call our Columbia is the 50 machine running 307 lbs. cement with 8.7 gallons of water, my calculation comes to 0.263 W/C ratio.
On the Teksam machine we are running 413 lbs. cement with 12.2 gallons of water with my calculation of 0.246 W/C ratio. Remember this is a dry cast plant. Is there a specific range that we should be in as far as WC as I see it vary from week to week? Most of the info I get is pertained to wet cast.
Thank you in advance,
Jorge
Hi Jorge,
Thank you for submitting your question! We passed it along to our technical services engineers, and Kayla Hanson has this response for you:
If we assume one gallon of water weighs 8.36 pounds, aggregate at SSD and no admixtures, we get the following values for situation #1, where you’re using 307 lb of cement and 8.7 gallons of water: It appears that you may have transposed the decimal numbers in your inquiry.
8.7 gal x 8.36 lb/gal = 72.732 lb water
72.732 lb water / 307 lb cement = 0.2369
We get the following values for situation #2, where you’re using 413 lb of cement and 12.2 gallons of water:
12.2 gal x 8.36 lb/gal = 101.992 lb water
101.992 lb water / 412 lb cement = 0.2476
The reason you don’t get exactly these water-cement ratios for every batch of dry cast concrete you produce could be due to a variety of reasons. Is your water meter calibration up to date? Is it functioning within appropriate limits? Do you use supplementary cementitious materials in your mix designs? Do you monitor both the fine aggregate and coarse aggregate moisture contents accurately and consistently with probes or by moisture burns (ASTM C70 or ASTM C566)? Are your aggregates clean? Do you adjust your mix water depending on how wet/dry your aggregates are? Do you use chemical admixtures in your mix designs? The water in the admixtures may actually count toward the total amount of mix water in your concrete. This isn’t an exhaustive list of the possible causes, but these could be good places to check first.
As far as an appropriate w/c ratio range is concerned, that will depend on what the design calls for and how much variance can occur in the w/c while still producing the same strengths, same durability, same performance, etc. ACI 211.3, “Guide for Selecting Proportions for No-Slump Concrete,” will provide valuable information and guidelines for you.
are both examples based on trial mixes done in the lab ? cause they have me confused?
Thank you for your comment Leslie. I forward your question to our Technical Services engineers. The following response is from Kayla Hanson.
The examples use sample values that one might see either in a plant or in a lab setting. Example 1 shows how to adjust for aggregate moisture if a mix design is proportioned based on oven dry aggregate, and Example 2 shows how to adjust for aggregate moisture if a mix design is proportioned based on saturated surface dry aggregate. Further information on concrete mix design, adjustments, and examples can be found in ACI 211, “Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete.”
It’s important to remember that aggregates have pores that can hold water. Oven dry (OD) describes the state where the aggregate surface is completely dry and the pores/internal structure of the aggregate are also completely dry. Air dry (AD) describes the state where the surface of the aggregate is dry and the internal pores are partially filled with water. Saturated surface dry (SSD) describes the state where the surface of the aggregate is dry, but the internal pores are completely full of water. Wet describes the state where all the internal pores are fully saturated plus the surface of the aggregate is fully saturated.
If aggregates are dryer than SSD, they can absorb mix water because their pores are not full, and the aggregate tries to get to a state of equilibrium. If aggregates are more wet than SSD, they can contribute water to the mix because there is excess water (no matter how little) on the aggregate surface. Because of this, aggregate moisture contents can affect the water-to-cementitious materials ratio of a mix. However, aggregates are never going to be in a perfect SSD state, and rarely will the entire stockpile of aggregate be in an OD state. Therefore, we need to determine the absorption capacity of each aggregate type. Before making the first batch of the day, we measure the aggregate’s actual moisture content at that time and compare it to its absorption capacity. The values will indicate what state the aggregate is in, and from that we can either batch additional water into the mixer or hold back some of the water to ensure we will have the appropriate w/cm ratio.
What are the main differences between IS method and the ACI method of concrete mix design?
Which is more affordable one?
Thank you for your comment Paven. Since we primarily represent North American precasters, we are unfamiliar with Indian Standard codes. I’d suggest contacting the Bureau of Indian Standards to get a more accurate response to your questions. If you are needing information about precast concrete mix design, we have many helpful resources on our website. Search “mix design” in the main search bar for case studies, technical documents and more. If you have anymore questions, please let me know.
Our design mix is suppose to be for 80mm slump, the w/c ratio is 0.35. The slump we got was 40mm. how are we going to adjust the slump to 80mm without altering the w/c ratio? Is the batch adjustment for wet aggregates only? how about if the aggregates are dry. Are we going to use batch adjustment even the aggregates are dry?
Thank you for your comment Jun. I forwarded your question to our Technical Services engineers. The following response is from Kayla Hanson, P.E..
If the aggregates are dry, they will absorb the mix water and reduce your slump and water-cement ratio. Prior to batching, you should check your aggregate moisture content and compare it to how the mix design was proportioned. If your mix design assumes you’ll be using aggregates in a saturated surface dry state, but the aggregates you actually use are dryer than SSD, they will absorb mix water. Dry aggregates can also absorb any liquid admixtures in the mix, which can greatly reduce their effectiveness. Similarly, if your aggregates are wet, they will add water to your mix, resulting in greater slump and greater w/c. ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete” provides further information.
we are using sand, 20mm and 10mm aggregates. do we still have to get the moisture content of 10mm?
Thank you for your comment Jun. I forwarded your question to our Technical Services engineers. The following response is from Kayla Hanson.
Yes. The absorption capacity and moisture content of each aggregate needs to be determined (sand, 20 millimeter and 10 millimeter). Each ingredient in the mix, including each aggregate type and size, can affect the mix and its fresh and hardened properties and behavior.
I would like to know just what aggregate particle shape, round versus fractured face play in the water demand in a given mix design?
Thank you for your comment Ken. I forwarded your inquiry to our technical services engineers. The following response is from Eric Carleton, P.E., director of codes and standards.
Many concrete design documents acknowledge that rounded course aggregates have a reduced water demand than crushed angular. This statement has been repeatedly verified through physical concrete batching, but the answer to “why” lies in the surface area of the two shapes. Rounded aggregate of similar gradation sizes has less exposed surface area than jagged faced aggregate. More surface area requires more paste to cover the area. Also, on a microscopic level, angular aggregate can have slightly increased void space between them then rounded aggregate. This leads to increasing the requirements of fine aggregate or concrete paste to fill this void and increases water demand.
I have this bookmarked on my computer, absolute gold.
Thank You! This was very helpful!