The present invention relates to cement mixtures which prevent or reduce the formation of micro-cracks and, more particularly, to an improved cement mixture of a conventional cement and a cement that is micro-encapsulated in time-release polymer coatings.
Concrete made with conventional cements has a tendency to crack. Great care must be taken in the mixture formulation, joint design, and steel reinforcement design to minimize the chances of detrimental cracks developing. It will be appreciated that joints and steel reinforcement add substantial cost to the concrete application. Although additives such as chemical and mineral admixtures and plastic and metal fibers have often been used in concrete to minimize cracking, these can add considerably to the cost of the concrete.
Patents of interest in this field include the following U.S. Pat. Nos. 5,639,298 (Miller et al.); 4,069,063 (Ball); 4,419,136 (Rice); and 5,728,209 (Bury et al). Briefly considering these patents, the Miller et al patent discloses a composition for controlling the setting time of cements, the composition including a sulphate component modified by the inclusion of magnesium sulphate in an amount of from about 0.5 to about 12% by weight. The Ball patent discloses a composition for controlling the setting time of cements wherein carbon dioxide is added to the cement mixture in an amount from about 0.1% to about 6.0% by weight. The carbon controls the setting time and stabilizes the hydrated products. The Rice and Bury et al patents relate to additives used with cements.
According to the invention, there is provided an improved cement mixture for preventing or reducing the formation of micro-cracks in a concrete structure, the cement mixture comprising a conventional portland cement and a second cement which is micro-encapsulated in a time-release polymer coating so the second cement does not hydrate for at least a few, i.e., about three, hours after the bulk of the conventional cement is hydrated so as to provide autogenous healing of small cracks in the concrete structure.
The second cement, i.e., the cement which is micro-encapsulated in the time-release polymer coating, is preferably portland cement (like the first cement) or expansive hydraulic cement. The amount of cement which is micro-encapsulated in the time-release polymer coating in the mixture is preferably between 10% to 50% of the total mixture. In a more preferred embodiment, the mixture comprises 10% micro-encapsulated cement and 90% conventional portland cement.
Other features and advantages of the invention will be set forth in, or apparent from, the following detailed description of the preferred embodiments of the invention.