The importance of applying the water – cement relation in the criteria for mixture designs
Concrete is a material that is fabricated directly at the site or plant with at least 4 components: water, cement, fine aggregate and thick aggregate; however, the need to add a fifth element has been demonstrated: the additive. These compounds of such diverse nature, used in different proportions and circumstances, make concrete a heterogeneous material with properties which can almost adapt to the whim of the maker.
The Water – Cement Relation concept must be understood as the relation between the water mass used in the concrete or mortar mix and the cement mass.
When concrete is made, the use of stone materials generates a water demand caused by the effects of the structure (Absorption) and state (Humidity) of the materials, which must be corrected in the total water of the mix.
We are attaching in this article a chart that links the water – cement relation with the compressive strength of 28-day-old concrete cured in standard form. The data has been obtained from numerous experiences with aggregates from different regions in the country and with different cements from the Selvalegre family
Some external variables to take into consideration to achieve our goals:
Aggregates must comply with the rules of granulometry, form and size of the particles, abrasion resistance, absence of lightweight, elongated, flat particles or covered with strange substances, with a total lack of organic material and rough surfaces.
The water used must be drinking water or, at least, must not contain suspended particles, organic material or chemical substances that oppose the reaction in the cement.
The cement must be from known sources that guarantees its suitability.
In all practicality, it is difficult to have all these variables under control; therefore, the mixture design must be in the hands of qualified professionals who can validate the implications for each one of these variables and who can mitigate impacts.
In spite of complying with these conditions, the variability of the materials is such that the mixture designs should be made in a laboratory and require verification at the site.
Lesser resistances of 21 and 25 MPa can be obtained with a small safety margin without the aid of additives; however, we recommend their use.
Resistances of 28, 30, 35 and 40 MPa require the use of a high range plasticizer and, as are intended for the construction of large-scale works, the mixture designs must be made in specialized labs, following the ACI 211.1, ACI 318 S guidelines and the Ecuadorian Construction Regulations.
To obtain 45 and 50 MPa compression resistance, the use of superfluidifying additives and even hyperfluidifying additives, or a mix, is required and must comply with the requirements previously indicated.
- The storage warehouse should be roofed and ventilated to prevent the accumulation of moisture.
- Cement bags should be stored in dry places, separating the bags from the floor and the wall.
- Place the bags over pallets or planks separated from the floor.
- Place bags 20 cm (7.87 in) away from walls.
- If there is a risk of the bags getting wet due to rain or condensation, it is better to cover the cement with resistant plastic sheets.
- Stow at most up to two pallets with 8 bags, one on top of the other. A pile of several pallets may compact the cement.
- Prolongued storage, even in optimal conditions, may produce lumps in the cement (which do not alter the product’s performance).
- Ensure a suitable rotation of the stowed cement. Store the bags so that you use the older ones first.
- Do not throw the bags from above or drag them across the floor.
- An input/output control sheet should be used to avoid lagging bags.
- Use the product preferably within 30 days after the date of preparation.