We know many impurities in water used in concrete mix produces different affect on concrete properties. Deleterious substances may also be included in concrete through aggregate (both fine and coarse aggregate). Here we will learn about effect of sugar in fresh concrete.
Though the title of this post is accidental application of sugar in concrete but in some important projects sugar is used intentionally for specific purposes. Sugar has retarding influence on concrete. Now consider about accidental inclusion; sugar mainly included due to use of sugar bags to transport aggregates to laboratory or when fresh concrete is transported by molasses bags.
This effect was discovered when investigation was conducted for very low strength of lab sample with proper mix design, careful sampling and curing procedures. The effects depend on quantity of sugar used/included and confusing results were found in previous tests.
It was found that careful application of sugar in small quantity resulted satisfactory retarding effect. The dose and retarding effects are as follows
Dosage: 0.05% of cement (measured by mass)
Retarding effect: about four hours
The probable cause of retardation is preventing action of sugar to form C-S-H. But actual rate of retardation depends on chemical composition of cementitious materials. Like other retarder, for this reason, performance of sugar as retarder should be ensured by different trail mixes (with actual cement that will be used in this job).
Sugar, thus, can be used in many untoward situation like when mixer or in some cases agitators become out of order and concrete cannot be placed due to site condition. The important fact is that it is an inexpensive retarder. Molasses was applied in construction of channel tunnel when residual concrete was required to be retarded due to difficulty in washing out underground.
Channel tunnel was constructed between Folkestone, Kent, England and Coquelles, Pas-de-Calais, France. The tunnel was opened on 6 May 1994. Construction began in 1988 and our reference was in 1990. A dose of (0.2-1) % of cement (by mass) will actually prevent total setting of cement.
Now we have to know about final strength of concrete on this application. When this retarder is applied in controlled manner, obviously, early strength becomes very low but after 7 days, the strength is increased by several percent relative to mix that are not retarded. The probable cause is hydrated cement gel becomes denser due to delayed setting.