1. How do water reducers work?
Water reducers (WR) have detergent like properties. They are called “surface active agents” and carry a negative charge (-). In cement pastes, there is tendency for cement particles to form large flocs (agglomerates). The water reducer will allow cement particle not to collect together, more cement surface area is available for reaction with water.
Modified lignosulphonates, naphthalene, melamine and polycarboxilates based high range water reducer (HRWR) are like WR surface active agents and perform in similar manner, but more powerful: more cement dispersion, more water reduction, higher strength achievable.
2. Effects of WR and HRWR on fresh concrete
2.1 Water reduction
WR and HRWP can be used to decrease water, decrease water per cement ratio (W/C). It is resulted in increasing strength with same workability. If W/C is fixed, the workability is increased. Another affects from water reduction is to reduce cement content while workability keep unchanged.
2.2 Time of setting
As per ASTM C494, set retarding admixture shall retard initial setting at least 1 hour later than control and retard final setting not more than 3.5 hours later than control. Set accelerating admixture shall accelerate initial setting at least 1 hour (maximum 3.5 hours) earlier than control and retard accelerate setting not more than 1.0 hours later than control.
2.3 Air entrainment
Air entraining properties of admixtures are generally controlled by formulation modifications unless specified in technical data sheet. WR and HRWR are not designed to act as air entrainers. If the project calls for air entrained concrete (typically with an air content of 4 to 6%) a special admixture shall be used.
Concrete using admixture shall show less segregation when vibrated and shall have better flowability. By reducing water content, coarse and fine aggregate content are increased and easier placing and good finishing can be obtained.
Generally use of WP or HRWR admixture has beneficial effect in terms of reducing the bleeding segregation of fresh concrete. But, when admixtures are used at high dosage for producing high slump concrete or flowing concrete, trials are still required to check actually bleeding.
2.6 Rate of slump loss
WR or HRWR with set retarding effect will reduce slump loss. But many factors can still affect the slump loss like dosage of admixture, cement characteristics, concrete temperature, outside temperature, initial slump… Trials are always required to fine tune mix design.
2.7 Heat of cement hydration
WR and HRWR can have only an indirect beneficial effect on heat generated by cement hydration. When the use of admixture allows water reduction and cement reduction (keep same W/C ratio than control mix). Thus, lower heat is generated than in control mix.
Use of WP or HRWP admixtures gives general better concrete finishing quality. But for concrete with highly reduce water content; finishing can become more difficult due to decrease of bleeding. Trials are required.
2.9 Shrinkage and early cracking
In concrete with highly reduced water content, the decrease of bleeding can lead to possible risk of plastic shrinkage if surface is not cured properly (by fogging or use of curing compound). This is related to the fact that plastic shrinkage cracks result from a loss of water by evaporation higher than the rate of bleeding.
Overall, WP and HRWR admixtures still have generally a beneficial effect in term of fresh or wet concrete cracking reduction. This is due to the fact that cement paste shrinks but the aggregates build a rigid formwork.