Cement Hydration and Concrete Curing

Cement Hydration and Concrete Curing Concrete curing is not simply a matter of the concrete hardening as it dries out. In fact, it is just the opposite. Portland cement is a hydraulic material. That is, it requires water for curing and can, in fact, fully cure to a hardened state even if it is completely submerged in water. Portland cement is anhydrous—it contains no water or moisture at all. The moment it comes in contact with water, a chemical reaction takes place in which new compounds are formed. This reaction is called cement hydration. The rate of hydration varies with the composition of the cement, the fineness of the cement particles, the amount of water present, the air temperature, and the presence of admixtures. If the mixing water dries out too rapidly before the cement has fully hydrated, the curing process will stop and the concrete will not harden to its intended strength. Curing will resume if more water is introduced, but at a slower rate. Hydration occurs more rapidly at higher air temperatures Cement hydration itself generates heat, too. This heat of hydration can be helpful during cold-weather construction, and potentially harmful during hot-weather construction. The chemical reaction between water and cement first forms a paste which must completely coat each aggregate particle during mixing. After a time, the paste begins to stiffen or set, and after a few hours has lost is plasticity entirely. The rate of this setting, however, is not the same as the rate of hardening. A Type-III high-early-strength cement may set in about the same time as a Type-I general-purpose cement, but the Type III hardens and develops compressive strength more rapidly after it has set.

Concrete normally cures to its full design strength in 28 days. Curing is slower in cold weather, and at temperatures below 40°F, the concrete can be easily and permanently damaged if it is not properly protected. Concrete must be kept moist for several days after it is placed to allow the portland cement in the mix to cure and harden properly. Concrete that is not kept moist reaches only about 50% of itsdesign strength. The Figure shows the differences in concrete strengthfor various periods of moist curing. If it is kept moist for at least three days, it will reach about 80% of its design strength, and for seven days, 100% of its design strength. If the concrete is kept moist for the full 28- day curing period, it will reach more than 125% of its design strength.

LEED FOR RESIDENTIAL BUILDINGS

LEED is a rating system developed by the United States Green Building Council. The rating system outlines important sustainable design features that residential buildings should incorporate, and provides a scoring system to rank success. The rating system is available on the USGBC website, (www.usgbc.org/leed/ratingsystems). The LEED rating system for residential buildings provides a guideline process for architects and builders to produce a more sustainable product.

The LEED for homes checklist is extensive. It starts with innovation in the design process, and then proceeds through site issues, water issues, energy and atmosphere issues, material issues, indoor environment issues, and finally awareness for the owner and the public. A detailed checklist is available on the USGBC website at new.usgbc.org/leed/rating-systems. The following is a summary of the main sections of this rating system.

The innovation and design process section has a total of 11 points and includes integrated project planning, durability management, and innovative or regional design opportunities. This section reinforces the importance of including all the design professionals early in the design process so innovation can happen. The location and linkages section has a total of 12 points and includes site selection, preferred locations, infrastructure, community resources, and access to open spaces. These criteria are echoes of New Urbanism concepts of walkable communities linked by mass transit.

The sustainable sites section has a total of 22 points and includes site stewardship, landscaping, local heat island effects, surface water, nontoxic pest control, and compact development. This section is focused on responsible landscaping with plants that can survive in the location without extensive watering and also awards points to dense development that uses up less land per house.

The water efficiency section has a total of 15 points and includes water reuse, irrigation systems, and indoor water use. Here the focus is minimizing water use through efficient fixtures, efficient irrigation, and possible gray water reuse.

The energy and atmosphere section has a total of 38 points and includes sections on insulation, air infiltration, windows, the heating and cooling equipment and distribution, the water heater, lighting, appliances, renewable energy, and refrigerant management. This section has the highest point total because reducing energy use is very important. A review of the list above indicates that reducing energy use requires extensive attention to detail across many areas of the home design.

The material and resources section has a total of 16 points and includes material efficient framing, environmentally preferable products, and waste management. This section addresses the material choices to be incorporated in the home and the environmental efficiency of the construction process.

The indoor environmental quality section has a total of 21 points and includes combustion venting, moisture control, outdoor air ventilation, local exhaust, distribu tion of heating and cooling, air filtration, contaminant control, radon protection, and garage pollutant protection. Indoor environmental quality is an increasingly important issue as homes are designed to be more air tight to reduce energy use. This section addresses this problem with an extensive list of requirements and a significant number of points to be gained.

The final section is awareness and education, which has a total of 3 points and includes education of the home owner and or tenant, and education of the building manager. The final section only has 3 points but it is very important that the users of a high performance home or multifamily building understand what they have and how to appropriately use the features provided.

This list is extensive and all of it is good practice. Even if it is only used as a guideline to good practice with no intention of applying for the formal rating from the USGBC, the architect, builder and eventual homeowner, tenant, building manager, and or building owner will be well served.