Fresh Concrete Lecture Notes PDF

Lecture 1 (2024-2023) Objectives of Concrete Production: 1. Achieving the requirements of strength and durability over time. 2. Meeting the requirements for handling, transporting, and compacting the concrete (a certain degree of workability). Workability is: The amount of effective work r...

Lecture 1 (2024-2023) Objectives of Concrete Production: 1. Achieving the requirements of strength and durability over time. 2. Meeting the requirements for handling, transporting, and compacting the concrete (a certain degree of workability). Workability is: The amount of effective work required to mix, transport, pour, and completely compact (consolidate) the concrete without its particles separating (to overcome the friction between the mix components, mix components and mixer walls and blades, mix components and transport means, mix components and form sides + reinforcing bars). Effect of good compaction on Concrete Strength  Fresh Concrete How do we determine the required workability of concrete for a specific project??? This is determined based on the following: 1. Type of labor. 2. Manufacturing location (on-site or pre-mixed). 3. Equipment that will be used for mixing, transporting, and pouring. 4. Concrete dimensions. 5. Reinforcement density.  Fresh Concrete Different Degrees of Workability Dry Concrete Mix (Roadway Slabs) Plastic Concrete Mix (Conventional Construction) Flowing (Wet) Concrete Mix (Pumped Concrete)  Fresh Concrete Tests to Measure Workability: A workability test is expected to measure one or more of the following variables: 1. The ability and ease of compacting the concrete. 2. The ease of concrete flow within the formwork and around the reinforcement. 3. The stability and cohesion of the mix components during handling.  Fresh Concrete Tests to Measure Workability: Slump Test Apparatus:  Fresh Concrete Measured Variable: Consistency Slump Levels Dry Mixes: 0-30 mm Plastic Mixes: 50-100 mm Wet (flowing) Mixes: 100-150 Collapsing Mixes : >200 (SCC? 230+)  Fresh Concrete Advantages of the Test: Simple, easy, does not require high skill, and is inexpensive. Used to monitor the consistency of concrete production. Disadvantages of the Test: May give the same slump for completely different mixes in terms of component ratios. For the same mix, the slump value may vary depending on the person conducting the test (up to 30 mm). Does not distinguish between mixes with low workability (all may show no slump) or between collapsing mixes.  Fresh Concrete Tests to Measure Workability: Compaction Test Apparatus: Measured Variable: Ability of Concrete to be compacted under its own weight Levels of Compaction Index: 0.9 – 0.8 : Dry Mixes 0.95 – 0.9 : Plastic Mixes 0.95 < : Flowing Mixes  Fresh Concrete Advantages of the Test: Objectively measures the ability of the concrete to compact. Disadvantages of the Test: Difficult to use on‐site. Some dry mixes stick to the walls of the cone.  Fresh Concrete Tests to Measure Workability: Reshaping (VB) Test Apparatus Measured Variable: Time (=Energy) to reshape the cone into a cylinder  Fresh Concrete Advantages of the Test: The only device suitable for very dry mixes. Disadvantages of the Test: Not suitable for flowing (wet) mixes (with a VB time less than 5 seconds).  Fresh Concrete Tests to Measure Workability: Flow Table Test Apparatus Measured variable: The amount of flow of a standard cone of fresh concrete under the effect of a standard shaking process.  Fresh Concrete Advantages of the Test: Best Suited for Flowing (wet) and collapsing mixes Disadvantages of the Test: Not suitable for dry mixes. Fresh Concrete Tests to Measure Workability: Comparing Tests  Fresh Concrete Factors Affecting Workability Mix Environment Components Temperature Water Content Aggregates Time Cement Additives  Fresh Concrete Mix Components Water Content AND NOT W/C RATIO Example: Two concrete mixes, the first contains 400 kg of cement with a water/cement ratio (w/c) = 0.5, and the second contains 300 kg of cement with a w/c = 0.6. Which one has higher workability? Answer: The one with higher water content (mix 1)  Fresh Concrete Mix Components Aggregates Shape Texture Grading Max Nominal Agg. Size Fineness of fine Agg. Absorption and Moisture Content  Fresh Concrete Mix Components Cement Cement Fineness Heat of Hydration The effect is the rate of loosing the workability, not the actual initial workability, following how quick the cement will react and consume the water. Fresh Concrete Mix Components Admixtures Water reducers (plasticizers) Set accelerators?  Fresh Concrete Environment Temperature and Time  Fresh Concrete Concrete setting: It results from the setting of the cement paste. What is cement paste setting? What causes it to occur? How is the setting of cement paste measured?  Fresh Concrete What is false setting? It occurs if: The temperature rises during grinding to 100 deg. Celsius. Gypsum crystals (CaSO₄∙2H₂O) lose 1.5 molecules of water. When cement is mixed with water, gypsum crystals regain the lost water and harden immediately. It can be overcome by remixing.  Fresh Concrete What is flash setting? It occurs if: The gypsum content in the cement is insufficient (2.5 m free fall. Too wet or too dry mixes without appropriate consolidation High percentage of Max Nominal agg. Size > 25mm Big difference between specific gravities of coarse and fine agg. Avoid the aforementioned !  Fresh Concrete Issues to avoid in the Fresh Concrete Phase How to Avoid Bleeding? Reduce water content as much as possible. Use finer cement. Use mineral pozzolanic admixture (SF, FA,…). Use air entraining admixtures. Use well graded aggregates.