Mortar Workability Questions

Mortar Workability Competition Questions

Q: We are writing to you because we have a doubt about the Mortar Workability Competition technical rules. According to our Engineering Materials class, we learned that all liquid admixtures incorporated in the mixture should be considered as part of the water. According to the competition rule, the water/cementitious materials + mineral fillers ratio should be less than 0.5. In that rule, is water just pure water or should any other liquid admixture be considered as part of the “water”?

A: What your instructor told you is entirely true: the water incorporated in any admixture should be included in the calculation of w/cm. However, for this competition, the committee has decided not to do this, and calculate w/cm (or w/b) purely based on the water in the mixture, not on the water content in the admixtures. The main reasoning behind this is to avoid the complexity for the judges to see if a team complies with the rules. The number of potential chemical admixtures is so high that the judging team will not be able to verify all information on time. So briefly, for this competition, the w/cm does not include the water contained in chemical admixtures.

Q: We have a question about the requirements of the aggregate requirements to meet ASTM C33. Through several iterations, we have defined the sand grading that meets at the same time the three parameters required by the competition rules (percentages of each sieve, fineness modulus and “No more than 45% passing one sieve and retained on the next”). As a result of this analysis, we found that the grading is very unique, and there are only a few possibilities or combinations that meet at the same time the three parameters. Using one of the few possibilities meeting these requirements will definitely affect the rheological performance of our mortar because the sand won't be very fine (won't have a low fineness modulus), and we wanted to improve its workability by decreasing the fineness modulus. However, we are having a lot of problems meeting the requirement: “No more than 45% passing one sieve and retained on the next,” because even when we meet the requirements of percentage limits for each sieve and the rank of the fineness modulus, it is very difficult to meet that clause. In this regard, how strict will be the evaluation of the judges be? Because we think that some teams won’t meet these requirements and we don’t want to be on disadvantage with those other teams.

As an additional query, we would like to use as a filler some powdered glass within our mortar, and we don't see any restrictions regarding to this on the competition rules, so we want to be sure that it's allowed.

A: The rules on the sand grain size distribution will be enforced and the sand used should comply with the rules. I understand your concern that the requirements for the sand will affect the rheology of your mortar. However, allowing an exception to your team may give you an advantage to teams who do follow the rules. Grain size distribution requirements of the sand will initially be screened in the reports submitted by the teams. Any team who has reported a grain size distribution of the sand which is close to the limits is more likely to undergo a verification by the judges on site or in a nearby testing laboratory.

Concerning the powdered glass: You can use this material for sure. As it is a material which is not listed in the table for cost calculation, please provide as much information as possible on this material. A material data sheet is the strict minimum which should be provided, but any additional information is welcome and will make the task for the judges easier. For the cost calculation, please include the market price for this material, but the judges have the right to adjust the cost of the material to market values in the United States and to recalculate the cost of your mixture.

Q: We have some doubts about the competition and its materials, i thank you for your reply regarding our problem with fine aggregate grading, but i just need to be sure that we understood the same, because i think the problem we had was caused by a misinterpretation of the ASTM C33 Specification, especifically with the "No more than 45% passing one sieve and retained on the next" clause. Our interpretation, in the simplest way, is that the percentage retained on one individual sieve must not be greater than 45%. Is that correct? I really hope so because we had a bad time interpretating that statement. If that is not correct, could you please explain the correct interpretation to me? We had another options too, another possible interpretation is that the sum of the percentages retained on two consecutive sieves must not be greater than 45%, but we can't really decide which one is correct.

A: Your interpretation is correct: the maximum percentage retained on an individual sieve cannot be larger than 45%. This means, for example, if you have e.g. 30% cumulative passing on the No. 30 sieve, you should have a maximum of 75% cumulative passing on the No. 16 sieve.

Q: a. Cement type: Portland Cement production is not a common thing in our country, however our team used; on the National Competition, a similar cement. This cement is the combination of Clincker Portland type and limestone (proportions of 6% to 20% by mass, according to Data sheet). Our question is if we can use this type of cement or do we have to find pure Portland cement?

b. Sand condition: the day of the competition, does the sand have to be saturated surface dry (SSD) or dry?

c. Transportation of materials: is there a possibility to get a letter from ACI International with the purpose of easing the process on the airport with the materials, especially the chemical admixtures?

A: a. The rules do not allow for the use of a blended cement, so in principle, you should use a pure Portland cement. However, if no pure Portland cement is at all available, the blended cement can be used. However, a detailed description of the different constituent elements in the cement should be provided and the blend of cement and other materials should still be compliant with the rules (e.g.: a blended cement with 50% Portland and 50% slag cannot be used as it exceeds the maximum SCM replacement rate). Furthermore, if no exact details on the composition of the cement are included in the report (e.g. no exact percentage of limestone, but rather a range), the judges will use the worst case scenario principles for compliance checks and cost calculations.

b. In the report, the mix design should be expressed in SSD condition. For the competition though, we realize it could be difficult to keep the material in SSD condition, especially considering traveling. The judges will accept materials which are between dry and SSD conditions. Materials wetter than SSD are not allowed.

c. The invitation letter that is created for the team will note the purpose of your visit to the United States ie. This should be sufficient. We have held this competition in the past and have had no problems reported.

Q: a. In Costa Rica the cements produced and sold by the industry contain additions which are already incorporated as part of the cement. In our country Portland pure cement is not produce and for this reason the use it is difficult for the competition.

b. We want to know whether we can use cements sold in Costa Rica that are already added instead of using Portland cement and additions separately.

A: a. For the specification for the cement: There is ASTM C595, but this only allows for a maximum of 10% ground limestone, and it is assumed that the cement in Costa Rica may have higher percentages of limestone. Provide in your report a substantial amount of documentation so the judges can evaluate the cement, and mention to which standard this cement complies to in Costa Rica. But keep in mind that the limestone in the cement can be maximum 30%, and that any combination of the incorporated limestone and other added SCMs or mineral fillers cannot be larger than 30%.

b. For the specification for the cement: There is ASTM C595, but this only allows for a maximum of 10% ground limestone, and it is assumed that the cement in Costa Rica may have higher percentages of limestone. Provide in your report a substantial amount of documentation so the judges can evaluate the cement, and mention to which standard this cement complies to in Costa Rica. But keep in mind that the limestone in the cement can be maximum 30%, and that any combination of the incorporated limestone and other added SCMs or mineral fillers cannot be larger than 30%.

Q: Materials proportions of the mixture are to be reported to produce 1 cubic yard of mortar, but the admixtures used on the mixture are going to be considered as part of that 1 cubic yard volume? We had this doubt when calculating the costs in the Official Mix and Cost Worksheet; that worksheet includes two tables, the first one has a column intended to report the volume of each material and the sum of all these volumes has to equal 27 cubic feet or 1 cubic yard of mortar. But on that table there is no space to include the volume of the admixtures used. So, are the admixtures (with their density) going to be considered as part of the volume, or as part of the reaction water, or they will not be considered as part of that 1 cubic yard volume?

A: To calculate the total volume of the mixture, the volume of chemical admixtures can be neglected, as density determination of chemical admixtures can sometimes by tricky. Furthermore, the amount of water in the admixtures is not taken into account for the w/cm, but this just for practical reasons. Normally, this should be done. For cost calculations though, we need the amount of the chemical admixtures added. Cost calculations will be verified by the judges.

Q: All the team members are going to be able to participate on the mixing of the mortar at the competition? Or just two of them can participate on the mixing process?

A: Two members will be allowed within the competition zone to prepare the mixture and pass it to the judges. Other team members are allowed to be present and advise the two members who are mixing, but they need to stay outside the perimeter set up for the competition. The reason the number of students is limited is to avoid overcrowding in the competition area.

Q: How is the Official Mix and Cost Worksheet going to be filled and calculated? It has a column where it requires the specific gravity of all materials used, but in the end, that value is not even used. And the material volume is actually calculated with the density of that material in pounds/cubic feet; lastly, table requires you to convert the pounds of the materials used to tonnes; it is confusing because it passes from one unit system to another and some values (like the specific gravity ones) are not used. Also, we have found that there are some units that have different values according to the country or the unit system (e.g. there are short tonnes, metric tonnes and long tonnes, and there are UK gallons and US gallons, or short cwt and long cwt and UK ounce and US ounce) so which ones have to be used? The US' ones?

A: Students are supposed to use specific gravity to calculate the volume (specific gravity and density are related). We did not have the equation embedded in the excel file, but we assume students can do the conversions. All units should be US units (no UK units, metric units, …).

Q: In terms of complying with competition rules, do we need to factor chemical admixtures into our w/c ratio?

A: The amount of water in the chemical admixture does not need to be taken into account in the w/cm in this competition, this to keep judging easy and efficient. It should be done in real case scenarios though.

Q: We want to know if we can put color to the Mortar Mixture, unless it disqualifies us from the competition. Also, we want to know what you mean in sections 4.d.ii, 4.d.iii, and 4.d.iv of the rules, we get confused with the time of the flowability test, it says three (3) minutes is the maximum amount of time for the mixture to flow through the mold, but then it says a time larger than thirty (30) seconds will receive a score of zero (0). Can you please clarify this?

A: Technically speaking, it’s not forbidden to add color to your mixture. However, we leave it up to the judges to qualify the added material as a powder (counts towards the 30% replacement of the cement), or a chemical admixture. Information on the coloring agent needs to be provided, including cost, data information, etc., to ensure it is included in the cost calculation. However, the competition team discourages you to use color, as the cleaning of mixer bowls, paddles and testing equipment will become a lot more labor intensive.

Concerning the flow test: Sections 4.d.ii says that the judges will perform the flow test within 3 minutes after you delivered the container with mortar.

Section 4.d.iii says that the judges can wait up to 3 minutes after filling the mold to determine the filling percentage, if the mold is not completely filled.

Section 4.d.iv states that: if you fill the mold, you get 60/100, regardless of the time. Filling it in less than 30 s delivers additional points (max 40). Filling it in more than 30 s will just give you the 60% for the filling, zero for the time.

Q: We also had a question with regard to the funnel — once the team member removed their finger from the bottom of the funnel, will the tip of the funnel then be placed in the opening of the mold, or should a gap be left between the funnel and the mold?

A: The tip of the funnel will be placed on the opening of the mold. No gap will be left.

Q: Section 2.e mentions "The mixture must contain 60% (by mass of the whole mixture) as a minimum aggregate."

Our concern is whether the sand that is mentioned should be taken as 60% of dry sand or applies for 60% of the sand in saturated surface-dry condition (SSD).

A: All sand calculations are based on SSD conditions in the report (and for compliance).

Q: Section 2d of the Rules says, "Chemical admixtures meeting ASTM C494 OR C1017 may be used."

But our air-entraining admixtures meet ASTM C260; and all the air-entraining admixtures that our team could find meet ASTM C260 and not ASTM C474. Therefore, can we use our attached admixture meeting ASTM C260? Also, what kind of concrete/mortar/cement mixer is going to be used at the competition?

A: This is OK. Feel free to use the air-entraining admixture.

Q: The cost sheet you have provided us has the prices of materials—do we use that price to get the total cost price?

A: Yes. However, if other materials were used which are not listed, please provide us sufficient information so that we can make an adequate estimate of the price on the U.S. market.

Q: The rules ask us to compute the cost of our mixture in the official spreadsheet. This spread sheet is a PDF online. We have modeled an Excel file to reflect the PDF, but wanted to check—are we missing an existing competition cost Excel file?

A: The cost sheet has been updated to Excel format. You need to enable Edit Mode.

Q: Will the mixing bowl be damped for each team prior to mixing?

A: Each team is allowed to damp the bowl and paddle prior to mixing. The choice is up to the team.

Q: Is lightweight aggregate allowed to be used? If so, is ground slate acceptable? Is silica sand allowed?

A: Lightweight material and silica sand are allowed, as long as all criteria for the mix design are respected (sand = minimum 60% total mass, grain size distribution should comply with ASTM C33).

Q: We have a doubt about the materials that are allowed in the competition; can we use bentonite as an admixture?

A: The use of bentonite is not excluded, but it should not be regarded as a chemical admixture. It needs to be classified as a mineral filler and will thus count toward the maximum total replacement of cement and toward the total mass of the sample.

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