Bio-friendly Slag Cement Benefit to the Drainage Systems for the City of Houston
Written by Juan Valdez, University of Houston
Professor: Arash Rahmatian
Houston is a high rainfall city and it’s called bayou city. During the months of June to November, Houston can experience an average of about 49 inches of rainfall with over 89 inches of rainfall being the most to fall in one year. In a city so prone to heavy rain with a poor drainage system, it is no wonder that we have had devastating flood events with the most recent occurring during Hurricane Harvey in 2017. The precipitation intensity in some area reached to more than a foot per hour.
Figure 1. A common parking lot near Downtown Houston during Hurricane Harvey.
Being a Houstonian that has observed this problematic situation year after year, I feel that finding a suitable solution is necessary to ensure this city and its citizens remain safe from preventable disasters. Having said this, the problem is the city is not prepared adequately to deal with the rainfall intensity. With this concern in mind, when I learned of Slag Cement and its various properties , it was only natural that I felt the desire to explore this versatile material and how it can help to remedy our situation. The idea is to design a concrete with Slag Cement to promote water flow at a higher rate of speed.
As a relatively new introduction to the construction industry, Slag Cement has already proven itself to be a massive contender when it comes to cementitious materials . One of the greatest aspects of Slag Cement is being a recycled material. As a byproduct of iron manufacturing, the production of Slag Cement does not contribute to carbon emissions in and of itself. However, the benefits of Slag Cement far exceed that of simply being a recycled material.
Several studies have confirmed that Slag Cement can improve water flow, strength  and fineness of surface . This is an especially interesting and appropriate area of evaluation given the recent drainage system difficulties Houston has faced following recent hurricanes or even simply heavy rain fall.
This research will focus on evaluating the observed water percolation of 4” x 8” concrete cylinders containing Slag Cement to determine if the properties of Slag Cement make it a superior option for improving drainage systems. In other words, I believe the addition of Slag Cement can decrease the coefficient of friction between water and concrete thus promoting water flow. Samples from the ACI Pervious Concrete competition were considered for this evaluation 
The scope of the competition focused on a cementitious efficiency mix design that allowed the displacement of 9” of water through the cylinder using a falling-head Permeameter.
Figure 1.1 Permeameter apparatus, measures the rate of water displaced through the pervious concrete cylinder.
During our mix design several ASCM were also utilized and tested, it was found that Slag cement had the most favorable results. The mix design consisted of 3/4” aggregates, a 0.38 water per cement ratio with 20% air voids computed to promote water percolation. All the samples created were cured for 28 days before testing was conducted. It was noticed during the mixing that the paste containing Slag cement was much more viscous compared to our baseline mixture. In addition, further observed the cured Slag Cement samples had a much smoother and less porous cylindrical perimeter finishing compared to the Portland cylinder.
Figure 1.2 Cylinder permeability rates for displacement of 9 inches of water by 4-inch diameter.
After several trials in the mix design, final mix design with highest strength, smooth surface utilized in the tests.
Given the dosage provided on SCA website , the mix was proceeded with concrete paving dose ranging from 20%-50% max.
For more study regarding to permeability  and Slag ratio in the mix design the following results in Figure 1.3 obtained with a few modifications; however, it is observed that slag is the most sensitive variable in these results and more slag percentage in the mix design increase the permeability.
Figure 1.3. Cylinder permeability rate with increased Slag Cement
This finding is notable because it demonstrates very obviously that Slag cement significantly reduces the porosity of any cement mixture. Like the first observation, this reinforces the assumption that Slag cement is a highly bonding material reducing voids created in conventional Portland cement (see figure 1.4).
Figure 1.4. Portland versus Portland & Slag.
Ideally, drainage systems seek to move water quickly from the city and streets to bayous, pump stations, reservoirs and other water retention areas, consequently the water speed in the channels, culverts and concrete pipes can influence directly in the water ponding and delay in drainage time. This issue substantially tried to be eliminated by bigger size drainages in the code but the backup flow with low water velocity due to high friction of pipe surface doesn’t let the water drain in time and severe flood can be expected with this severe issue. In addition this smooth surface improves sediment movement and less amount of sediment can be accumulated is drainage system which enhances the drainage system age of service. This can save cost of maintenance, construction of sewer and drainage system in addition to the flood and ponding damages.
Taking this information, it’s expected that the rate of flow is increased by reducing the friction coefficient. To further explore the characteristics of Slag, an experimental test that compares a Slag-free cement design to a Slag cement design is conducted. The objective is to create two simulated model of a parking lots, one using a base mix made only of Portland, and the second using a mix that includes Slag cement. Both slabs of cement will be tested for their respective water retention rates and water flow rates.
The slabs will be placed on different angle slopes and a constant amount of water will be discharged over the slab. The amount of water collected at the bottom of the slope will be measured and the amount absorbed by the cement slab will be determined. The water flow rate will focus on how quickly the water moves across the slabs. This will be done by measuring how much time it takes for the entire amount of water poured over the slabs to exit the bottom of the slab. In another test sediment movement in flow and surface impact on accumulation of sediment is tested.
In conclusion, I believe Slag Cement can make a huge difference in improving any city’s sewer and drainage system as well as communities around the world. Ideally, placing a greater emphasis on including Slag Cement in cement mixtures will offset the heavy necessity placed on Portland. Furthermore, by reducing the demand of Portland, we would be able to reduce the production of Portland and ultimately reduce carbon emissions from cement production altogether. During this entire process, I had the privilege of coming across many articles and research discussing the various facets of Slag cement with its numerous uses and benefits. I hope that many will begin to also see the usefulness of this material and find more ways to incorporate it into the concrete industry and begin to build a better future.
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