Bench Project

BRAE 433 Concrete Bench Project Labs 2, 4, and 7

Alex Jensen, Matt Hart, Nadya Alexander, Daniel deGraaf, Luke Schaner, Adam Scheuber BRAE 433 11-16-09

Introduction: The Bench crew was assigned with the task to create concrete benches for the Swanton Pacific Ranch. Several different designs have been tried over the years with varying success. Problems with the designs ranged from rough edges to bubbles forming in the concrete on the top of the bench. This year’s group decided to try to minimize the difficulties by analyzing what the groups from previous years had done. Team Sittin' Pretty, from last years bench team, had plans for a quality bench which was made last year. The team decided to build an identical form based on last years design. BRAE 433 stamps would be fabricated to put lettering into the finished product. To add style to the benches after they were poured and cured, acid etching would be used to give the benches a finish.

Procedure: Design: The team decided to use and modify last years form design. After examining the plans and the execution of the previous bench it was determined that there were some alterations that could be made that would improve the design. With two identical forms, it would be possible to pour two identical benches at the same time. The team used drawings from the previous group to create the pieces needed for the bench. Pieces were cut out using the CNC plasma in shop 5 and were assembled using a welder. All pieces Figure 1 Cutting out the needed to be placed with precision in order to reduce the bench form pieces on the amount of leakage of concrete when the benches were to be CNC plasma poured. The two main pieces of the form were held together with bolts. Modification of the existing design included reducing the amount of steel on top of the bench which trapped air bubbles in last years bench. To strengthen the bench and to prevent cracking #5 rebar was bent to fit the form and tied together to make a frame that fit into the form as the benches were filled with concrete. There were three pieces of rebar along with two adobe blocks on each end to hold up the rebar so that no steel would be exposed on the finished product. They were held together with Figure 2 Modified edge ties and five pieces of rebar cut to fit between the forms contour to reduce the perpendicular to the three main rebar pieces. These gave the possibility that air bubbles rebar skeleton enough strength to hold itself together while the would become trapped at the surface concrete was poured.

To finish the bench when they were poured two sets of stamps that read BRAE 433 were cut out of 10 gauge steel. The letters were cut with the CNC plasma and a piece of flat stock was welded to the top so they could be pressed into the concrete as it Figure 3 Fiiting the 4 BRAE 43 concrete dried and then removed to leave Figure reinforcing steel to the finishing stamp fabrication imprints of the letters in the top forms of the bench. Mold Preparation: On the day of the pour, the bench form pieces were gathered. Each face of the form which contacted the concrete was sprayed with WD-40. The coat of WD-40 kept the concrete from attaching to the steel mold as the concrete set. The reinforcing steel was set in place after the forms were bolted together and sprayed with lubricants. The rebar was kept separate to ensure that no WD-40 came into contact with its surface. A strong bond is desired between the reinforcing steel and concrete to ensure that shock and tension loads are Figure 5 Applying Lubricant (WD40) to the form transferred to the steel efficiently. Table 1 Batch Ingredient Weights Concrete Mix: Once the concrete bench forms were determined during lab 3 assembled, the concrete was prepared. The concrete was mixed with a 0.40 water to cement ratio in order Lab Three 0.40 Recommended Proportions to maximize the compressive strength of the benches. Water 28 lb Cement 70 lb The initial concrete weights (Table 1) were adjusted Sand 88.9 lb to the proportions determined in Lab 3. The adjusted Coarse Aggregate 138.4 lb Moisture 2 % proportions have the same weights of sand and coarse % Actual Sand 90.7 lb 26.2 lb aggregate as the initial proportions. Because the Actual Water Weight 325.3 lb mixture was very dry initially, water and cement were Total Total Volume 2.2 ft^3 added in 0.4:1.0 until the mixture was workable. The final proportions have the same 0.40 ratio, but have an additional 20.5 lb of water and 50 lb of cement. The final proportions are shown in Table 2; the total volume of the final mix was estimated to be 3.1 ft3. The estimated volume for two benches is 7.6 ft3, so 2.5 batches, or 7.75 ft3, of Table 2 Final Concrete Mix for one bench

Final 0.40 Proportions Water 28 lb Cement 120 lb Sand 88.9 lb Coarse Aggregate 138.4 lb % Moisture 2 % Actual Sand 90.8 lb Actual Water 46.7 lb Total Weight 395.9 lb Total Volume 3.1 ft^3

Table 3 Total amount of material mixed for two Benches

Lab Four Concrete Weights Batch 1 Batch 2 Batch 3 (Half) Sand (lb) 95 100 45 Coarse Aggregate (lb) 138.5 139 69 Water (lb) 47 48 24 Cement (lb) 120 120 60 Water:Cement Ratio 0.39 0.40 0.40 Approximate Total Volume (ft^3) 3.1 3.1 1.55

concrete were mixed. The proportions of each batch are shown in Table 3; the average water to cement ratio was 0.40. Mold Filling and Finishing: The well mixed concrete was shoveled into the bench molds carefully so that the reinforcement skeleton was not dislodged and the concrete filled all

spaces adequately. To ensure proper filling in tight spaces, a rod compacted the concrete as shown in the image below. This let air escape to reduce and eliminate air pockets in the mold. Tapping of the form with a hammer also aided in releasing air pockets within the concrete. Once the mold was adequately filled, the surface was worked to bring Figure 7 Filling the Form smooth cream to the top of the with concrete surface. Exposed edges of the mold were pressed to reduce a profound and painful edge. Note, the finishing process occurred Figure 8 (left) : finishing after excess moisture disappeared in the bench (this took hours the surface, (right): removing trapped air after the last shovel of concrete was added to the mold). within the concrete Acid Etching: The final step in the concrete bench construction was to stain the concrete using a process known as acid etching. This is a chemical reaction in which the stain is applied to the cured concrete. The acid penetrates into the concrete and allows the coloring to be absorbed deep into the bench, providing a variety of colors and a surface finish similar to marble. The process of acid etching is remarkably simple. The bench surface is first cleaned with water to remove any surface dust or contaminates. The acid is then applied either with a sprayer, brush, or in our case a rag. The acid is poured onto the surface slowly and wiped around so as not to have any puddles of the acid sitting since that will cause areas of serious discoloration. The top surface of the bench was relatively easy to stain since the acid was easier to manipulate and spread. The vertical sides of the bench were trickier since the acid just wanted to run down the sides so that it didn’t have time to penetrate. Several different techniques were used to try to apply the acid to the sides with no single method working Figure 9 Concrete Bench after better than the rest. The simplest was to pour the acid onto the acid etching rag and then apply it to the bench, this resulted in a little waste but overall was very effective. The final appearance of the bench was a reddish golden color with the desired marble look. The clean up was simple and the final step was to rinse off any acid residue and apply a concrete sealer to protect the stain and give a satin surface finish to protect the color. The process was simple, effective and cheap ($80 for acid and sealer) and was more than enough to stain and seal two benches. Conclusion: The modified design resulted in less air bubbles being trapped under the form on the top. The acid etching provided a very provided a good finish after the concrete has cured. Some recommendations for next year would be to bevel the edges of bench through form modification. Another recommendation might be to experiment with different colored stains.