What comprises concrete?
Concrete is a mixture of Portland cement, sand, gravel, water, and other chemical admixtures.
What affects the concrete’s strength?
The cement content and water are the two factors that impact the strength of the concrete. These components of the mixture are also commonly known as the water/cement ratio. The water/cement ratio has the most significant effect on the concrete’s strength, but it is also affected by correct sand and gravel grading.
What causes concrete to crack?
Concrete cracks over time and cracks because of a variety of reasons. Concrete contracts and gets affected in the curing, drying, and hardening stages that can result in cracks. If the bond between the cement mixture and the aggregates is not strong enough, the concrete will not survive stress induced by different factors and cause the concrete to crack. One of the most effective ways to prevent cracking is placing control joints every 2 ½ to 3 times the depth of the concrete slab. It will help prevent the appearance of cracks. Control joints sawed into the concrete should take care of the cracking. Improper sub-grade also leads to cracking.
How long can the concrete mixture stay in the bucket truck before it can no longer be used?
The concrete is considered “no good” two hours after it was batched, considering that the temperature is between 60 F and 80 F. The concrete may take a little longer to harden in lower temperatures. You should dispose of the balance of the truck if the temperature reaches 90 F or higher. It is crucial to monitor the concrete temperature to prevent accelerated hardening.
How much water should I add to the mixture?
More water cannot be added to the mixture because the water/cement ratio affects the overall strength of the concrete. If you add water to the mixture and compromise the water/cement ratio, the strength of the concrete will be affected. If you go beyond the allowable water/cement ratio, it will greatly affect the strength of the concrete. The concrete strength can decrease by 200 psi. To put it simply, you shouldn’t add more water when not necessary because it can do more harm than good.
Is it possible to just pour the concrete rather than placing it?
Concrete is designed with workability, which means it can be transported to the site by using a smooth rounded ramp and still keep its composition without it segregating. If you add extra water to the mixture to make it looser, it can result in segregation. And of course, there is an effect on the strength of the concrete. It will also affect the setting time and increase the chances of cracking. You should add a chemical admixture to the mix to make it more pourable, without causing adverse effects on its strength. You should be careful with the amount of admixture you will add.
What are the types of concrete?
Concrete mixtures are poured for specifically designed strengths and with specific components. For residential concrete, we should use concrete that is designed for 3000 psi. The maximum stone should be ¾” and the slump is 5″. For commercial concrete, we usually use a designed strength of 4000 psi or even higher and a maximum stone of ¾” and a slump of 5″. If you go beyond the allowable water/cement ratio, it will greatly affect the strength of the concrete. The concrete strength can decrease by 200 psi. You can always check with your contractor if you have questions about your application.
What tests do you run on fresh and hardened concrete?
A slump is used to measure the workability of fresh concrete. We also test the concrete’s air content, unit weight, as well as its temperature. We examine the compressive and flexural strength on hardened concrete.
Is a high slump better than a slow slump? Vice versa?
This depends mainly on the design of the concrete mix and its application. A moderate slump of 3″ to “is ideal for normal concrete. We typically use a high slump when the concrete must be placed somewhere with a high concentration of reinforcing steel. Low slump, on the other hand, can be used if the concrete is placed in large open areas. The mixture of concrete should be specifically designed for these particular applications. You should always check with your contractor when you need clarifications about your application.
Can we do something to make the concrete more workable if the workability is too low?
First, what do you exactly mean by too low? The design for commercial and industrial concrete applications is usually a maximum of 3 inches slump. The maximum slump for residential concrete, on the other hand, is designed at 5 inches. You should immediately contact the concrete plant if the concrete delivered to the site is so hard that it cannot be poured from the load. If you add extra water to the concrete mixture, above the desired design value, it may cause issues when the concrete begins to harden.
Can there still be something to do if the stump is too high?
Unfortunately, not much. Send the truck back to the plant.
Why is temperature essential?
Temperature is essential to concrete in significant ways. Concrete gets its strength from a complex chemical reaction that involves the hydration process of the cement mixture. The process of hydration is heavily dependent on temperature. If the temperature is too low, below 32 degrees F, the mixture will have a hard time hydrating. The water in the cement mixture can turn to ice and affect the setting and development of the designed strength. In this case, it may not achieve the desirable designed strength. If the temperature is too high, somewhere above 90 degrees F, the cement mixture will cause the mix to hydrate faster than average. The cement mixture may set and harden even before it is appropriately placed. Between 50 F and 85 F is the recommended temperature for proper concrete placement. It may require special preparation if the placement of concrete goes beyond these recommended ranges. You can ask your contractor for suggestions and clarifications.