1. Introduction
The purpose of this lab is to introduce the students to the process, procedures and to determine important physical properties of mixing fresh concrete. With an understanding of concrete properties and the means of which to calculate these properties the students can demonstrate their knowledge in a laboratory environment. The students are expected to achieve this task by designing and mixing a batch of concrete based on specific requirements provided in the ASTM C231. The procedure requirements include strength, slump, air entrainment, volume, aggregate type and size. Students are split into groups and each group is assigned specific parameters to base their designs on. These designs and parameters are used to complete the concrete mixing procedures.
The specific parameters for Thursdays Lab Group # 1 were a compressive strength of 8000 psi (55 MPA), limestone aggregate with a maximum size of ¾”. The mixture required moderate air entraining, a supplementary cementitious material called fly ash, and a target slump of 3”- 4”. This design should produce a mixture that is relatively high in strength, durability and workability. The assumption was made that the aggregate properties calculated we accurately described.
A sample of this mixture design was created in the lab using the equipment provided. The mixture was then subjected to testing using the slump cone test. The ASTM C231 requires the sample to be tested using an air pot to determine the air content. However, the air pot failed to preform and we were provided with the air content value to aid in continued calculations. The concrete was then placed into three 3” diameter, 4” tall cylinder molds and placed in the curing room. Complete procedures for this lab can be located in Appendix A.
2. Results
Characteristics of this mix design can be found in Table 1. Results of this group can be found in table 1 line W1. Sample calculations can be found in Appendix B. Batch proportions and for a cubic yard (cubic meter) and adjusted batch proportions are presented in tables 1 and 2 respectively. The concrete mix design specifications were stated in assignment # 2 and subsequent calculations can be found as an attached image in Appendix B.

Table 1: Characteristics of mix design (Orton Spring 2013). Group # | Water/cement ratio | Cement (lb/yd3)(kg/m3) | Water (lb/yd3) | Expected Unit Weight (lb/yd3)(kg/m3) | Measured Unit Weight (lb/yd3)(kg/m3) | Air content of fresh concrete (%) | Slump inches(mm) | | Theoretical | Actual | | Theoretical | Actual | | | | | T1 | 0.55 | 0.48 | 637.2(378) | 351 | 307.5 | 3810.8(2261) | 4039.2(2396) | 3.5 | 3(76.2) | T2 | 0.36 | 948(562.4) | 346 | 3888(2307) | 4018(2384) | 3.5 | 2.3(58.4) | T3 | 0.4 | 0.43 | 870.3(516) | 378.4 | 3852(2285) | 3882.6(2304) | 3 | 5.5(139.7) | W1 | 0.34 | 672.8(399) | 305 | 3780.1(2242) | 3969.3(2355) | Airpot was broken | 3.5(88.9) | W2 | 0.32 | 0.34 | 954(566) | 304.1 | 345.6 | 3836(2276) | 3877.2(2300) | | 2(50.8) | W3 | 0.36 | 954(566) | 346.6 | 3885.8(2305) | 4051.7(2404) | | 2(50.8) |

Table 2: Batch proportions for cubic yard (cubic meter). | Cement | Air Entrainment | Water | Coarse Aggregate | Fine Aggregate | Fly Ash | Cubic Yard | 672.8 lbs | 49.7 mL | 305.0 lbs | 1575.8 lbs | 1002.2 lbs | 224.3 lbs | Cubic Meter | 514.7 lbs | 38.1 mL | 233.3 lbs | 1205.5 lbs | 766.7 lbs | 171.6 lbs |

Table 3: Adjusted batch proportions. | Cement | Air Entrainment | Water | Coarse Aggregate | Fine Aggregate | Fly Ash | Batch Volume | 12.5 lbs | 1.2 mL | 6.4 lbs | 29.3 lbs | 18.6 lbs | 4.2 lbs |

3. Discussion
The measured slump average for this lab was found to be 3.05”. Our group achieved the desired slump range of 3-4”, with a measured slump of 3.5”. On average the workability of the mixtures seemed low when compared overall. What our group expected is the slump to be directly related to workability, where the higher a concrete’s slump, the greater its workability. It appeared that groups with fibrous material in the design seemed to have less workability then those designed with fly ash. The fly ash appeared to increase workability, acting as a lubricant, without lowering its slump. Because of this, a concrete with admixtures and low slump might be more workable than one with no admixtures and a very high slump. This is a way in which slump might not be a good indication of workability.
Based on the observable characteristics of each Wednesday lab group, those that incorporated with both a higher slump and workability improving admixtures were more workable than those without any admixtures. Our group (W1) had the highest slump with the design incorporating a 25% fly ash into the mix. Whereas, the other groups W2 had no add mixtures in the design and W3 who used poly fibers had the same slump amounts regardless of the additive. These two groups appeared to have low workability which corresponded with low slump measurements. This is most likely due to procedural errors, such as incorrect proportions and placing time.
Other admixtures which might affect the workability of the concrete include silica fume and steel or poly fibers. The silica fume would require more water, as it is a cementations material. If this is accounted for in the design stage it wouldn’t affect the workability. Adding fibers to a concrete mixture would most likely reduce workability. They would act as a large, uniformly shaped aggregate, adding to the inter-particle friction of the concrete mix.
An air entraining agent is another type of admixture which can be considered in concrete mix design. This type if admixture affects the workability, slump, and other properties of post mix concrete. Air entertainers improve workability by creating tiny air bubbles. These air bubbles then reduce the friction between the various parts of the concrete, thus increasing workability. This increase in workability comes at a cost, after the concrete has set, air bubbles create tiny cavities. These cavities increase the amount of voids present and decrease the overall strength of the concrete.
Based on our design mixture, the theoretical unit weight for this batch was 140.0lbft3. After mixing and measuring the batch that was created, the actual unit weight was 147.01lbft3, showing a difference of 7.01lblb3. Possible reasons for this discrepancy could be due to incorrect measurements, scale error or procedural errors. Although the actual unit weight was found to be slightly higher, it is still well within reasonable tolerances for typical values usually around 150lbft3. Again, the most likely cause of this was the resolution on the scales in the lab. Most of the scales had a resolution of 0.1 lbs, but some would only read to the nearest 0.5 lbs. When considered that four quantities had to be weighed, this could have easily resulted in a higher density.
The conclusion that can be drawn concerning the relationship between water content and slump is shown in Figure 1. The water content of the concrete is directly related to the slump of the concrete. The data obtained in the lab shows that the higher the water content the larger the slump will be. However, when admixtures are added, the water content has less of an impact on the slump of the concrete. It is important to take both water content and slump into account when determining a mix design, because it can affect the workability of the concrete.

Figure 1: Slump vs. W/C ratio (Orton Spring 2013).

4. Conclusion
The unit weights, slump, air content, amount of cement and water of each concrete were presented in this paper, as well as relationships between various aspects of the concrete mix and workability. The unit weight of each concrete mix was appropriate. The properties fall within 98% of the average unit weight of concrete (150 lb/ft3). The highest observed unit weight being 150.06 lbft3 and the lowest being 143.8lbft3. The group’s experimental unit weight was comparable to the design unit weight, only having a difference of +7.01lbft3. This increase can be attributed to decisions made during the mixing process. It can be quantified that those concrete mixes with a greater water content or workable improving admixtures were more workable. It was observed that admixtures, which attempt to increase the strength of the concrete, would tend to increase the friction within the concrete, decreasing workability. Air entraining admixtures would have the opposite effect, decreasing strength, but improving workability.