Questions and Answers on practical civil engineering

1. What is the difference between epoxy grout, cement grout and cement mortar?
Epoxy grout consists of epoxy resin, epoxy hardener and sand/aggregates. In fact, there are
various types of resin used in construction industry like epoxy, polyester, polyurethane etc.
Though epoxy grout appears to imply the presence of cement material by its name, it does
not contain any cement at all. On the other hand, epoxy hardener serves to initiate the
hardening process of epoxy grout. It is commonly used for repairing hairline cracks and
cavities in concrete structures and can be adopted as primer or bonding agent.
Cement grout is formed by mixing cement powder with water in which the ratio of cement
of water is more or less similar to that of concrete. Setting and hardening are the important
processes which affect the performance of cement grout. Moreover, the presence of
excessive voids would also affect the strength, stiffness and permeability of grout. It is
versatile in application of filling voids and gaps in structures.
Cement mortar is normally a mixture of cement, water and sand. They are used as bedding
for concrete kerbs in roadwork.


2. In soil compaction test, if a test result exceeds 100%, should engineers accept the
result?
Soil compaction is the process of increasing the soil density by reducing the volume of air
within the soil mass.
Soil compaction depends mainly on the degree of compaction and the amount of water
present for lubrication. Normally 2.5kg rammers and 4.5kg rammers are available for
compaction in laboratories and the maximum dry densities produced by these rammers
cover the range of dry density obtained by in-situ compaction plant.
Regarding the second factor of water content, it affects the compaction in the following
ways. In low water content, the soils are difficult to be compacted. When water content is
increased gradually, water will lubricate the soils and this facilitates the compaction
operation. However, at high water content, as an increasing proportion of soils is occupied
by water, the dry density decreases with an increase in water content.
For soil compaction tests, the dry density obtained from compaction carried out in-situ by
vibrating roller/vibrating plate is compared with the maximum dry density conducted in
laboratories using 2.5kg rammer of compaction with similar soils. In essence, the in-situ
compaction is compared with the compacting effort of using 2.5kg (or 4.5kg) rammer in
laboratories. In case the compaction test results indicate values exceeding 100%, it only
means that the in-situ compaction is more than that being carried out in laboratories which
is treated as the basic criterion for satisfactory degree of soil compaction. Therefore, the
soil results are acceptable in case compaction test results are over 100%. However,
excessive compaction poses a risk of fracturing granular soils resulting in the reduction of
soil strength parameters.