Civil Engineering Materials Department of Civil, Structural and Environmental Engineering Trinity College Dublin Dr. Roger P. West (TCD) and Mr. Peter Flynn (ARUP)

Section A: Concrete A1 Basic Materials: A2 Fresh Concrete Properties: A3 Hardened Concrete Properties: A4 Concrete Mix Design: A5 Reinforced Concrete: A6 Pre-stressed Concrete:

Section A.2 Fresh Concrete Properties The first 48 hours are very important for the performance of the concrete structure. It controls the long-term behaviour, influence strength, Young’s Modulus, creep and durability.

Section A.2 Fresh Concrete Properties 1. Workability 2. Addition of Water 3. Voids 4. Hydration 5. Segregation 6. Settlement

Section A.2 Fresh Concrete Properties 7. Shrinkage 8. Compaction 9. Honeycombing 10. Grout Loss 11. Surface Finishes

Section A.2 Fresh Concrete Properties 1. Workability “the ability of concrete to flow in a mold or formwork, perhaps through congested reinforcement, the ability to be compacted to a minimum volume, perhaps the ability to perform satisfactorily in some transporting operation or forming process, and maybe other requirements as well”

Section A.2 Fresh Concrete Properties 1. Workability The amount of internal work required to produce complete compaction, or the energy required to overcome the internal friction between particles. But concrete also needs to be “cohesive” in order to resist “segregation” and “bleeding”. Consistence is the ease with which concrete flows Consistence which is a combination of workability and cohesion is a measured using the Slump Test.

Section A.2 Fresh Concrete Properties 1. Workability Water Content - higher water content leads to higher workability Aggregate Type Aggregate Shape Grading - uniform grading leads to better workability Aggregate – Cement Ratio Admixtures - accelerators / retarders Fineness of cement - finer cement, faster loss of workability Time - delay of cement hydration Temperature - higher temperatures lead to loss of workability

Section A.2 Fresh Concrete Properties 1. Workability

Section A.2 Fresh Concrete Properties 1. Workability

Section A.2 Fresh Concrete Properties 1. Workability

Section A.2 Fresh Concrete Properties 1. Workability A check on the day to day or hour to hour variation in materials or constituents Increase in slump may indicate increased moisture states of aggregates or added water Changes in slump may indicate changes in aggregate gradation Indicates the relative amount of coarse to fine aggregates

Section A.2 Fresh Concrete Properties 1. Workability Compacting factor Best test to measure the amount of work needed to achieve full compaction Higher the value, the more workable the concrete - Up to a point!

Section A.2 Fresh Concrete Properties 1. Workability

Section A.2 Fresh Concrete Properties 2. Bleeding (or “water gain”) Water in the mix tends to rise to the surface of fresh concrete Inability of solid constituents to hold all the water together Top layer becomes too wet, resulting in a porous and permeable layer Concrete should never be finished until the bleed water has evaporated

Section A.2 Fresh Concrete Properties 2. Bleeding (or “water gain”) Causes of Bleeding Too much water (high water-cement ratio) Less fines in the mix (cement, sand) Poor grading of aggregates Overworking of concrete

Section A.2 Fresh Concrete Properties 2. Bleeding (or “water gain”) Water is lighter Leaves pockets of low density Water may be reabsorbed No bleeding at all also may be a problem

Section A.2 Fresh Concrete Properties 3. Air Voids Entrapped air Capillary air Entrained air

Section A.2 Fresh Concrete Properties 3. Air Voids

Section A.2 Fresh Concrete Properties 3. Air Voids There should be sufficient air voids within the concrete mix to ensure sufficient freeze thaw resistance

Section A.2 Fresh Concrete Properties 3. Air Voids Removal of air from concrete by agitating

Section A.2 Fresh Concrete Properties 4. Hydration

Section A.2 Fresh Concrete Properties 4. Hydration

Section A.2 Fresh Concrete Properties 4. Hydration

Section A.2 Fresh Concrete Properties 4. Hydration

Section A.2 Fresh Concrete Properties 4. Hydration Curing is the maintenance of satisfactory moisture content and temperature in concrete for a period of time immediately following placing and finishing so as to develop the desired properties of concrete CONCRETE MUST BE PROPERLY CURED TO DEVELOP OPTIMUM PROPERTIES

Section A.1 Basic Materials 4. Hydration

Section A.2 Fresh Concrete Properties 4. Hydration

Section A.2 Fresh Concrete Properties 4. Hydration

Section A.2 Fresh Concrete Properties 4. Hydration

Section A.2 Fresh Concrete Properties 4. Hydration

Section A.2 Fresh Concrete Properties 5. Segregation Separation of constituents of the concrete mixture caused by excessive handling or vibration, or improper gradation of aggregates. Results in a non-uniform distribution Coarser particles separate out Cement and water mix separates out Causes: Differences in particle sizes Improper grading

Section A.2 Fresh Concrete Properties 5. Segregation Laitance: Coarse aggregates separate towards the bottom and cement paste forms a scum on the top To reduce segregation: Increase air content Reduce water-cement ratio Increase rate of hydration

Section A.2 Fresh Concrete Properties 6. Settlement

Section A.2 Fresh Concrete Properties 7. Shrinkage (Plastic Cracking) Cracking that occurs when the concrete is still plastic Cracks are usually parallel to each other Moisture loss from the surface is the cause If bleed water replaces the water lost, no cracking results Evaporation increased by: High surface temperatures High wind speed Low humidity

Section A.2 Fresh Concrete Properties 7. Shrinkage

Section A.2 Fresh Concrete Properties 7. Shrinkage Minimized by: Moist aggregates Cover concrete Dampening sub-grade Erection of windbreaks & sunshades Add plastic fibres

Section A.2 Fresh Concrete Properties 8. Compaction Immediately upon placing, the concrete must be compacted by means of hand tools or vibrators. The compaction by high frequency power driven needle vibrators (frequency : 8000-15000 rpm, and amplitude : 0.50 - 0.80 mm) has the advantage that they allow the use of stiff mixes for the same strength. The spacing of needle vibrator and its duration depend upon several factors. It requires an experienced operator to make the necessary judgement at the actual site. The spacing of needle vibrator depends upon its radius of influence in concrete. It may be used for about 30 sec to 60 sec at each location. Such compaction ensures more impermeable and dense concrete. Under compaction as well as over compaction are bad for concrete.

Section A.2 Fresh Concrete Properties 9. Honeycombing Honeycombing occurs when voids are left in concrete due to failure of the mortar to effectively fill the spaces among the coarse aggregate particles. It occurs due to lesser quantity of fine sand leading to a harsh concrete mix. 

Section A.2 Fresh Concrete Properties 10. Grout Loss

Section A.2 Fresh Concrete Properties 11. Surface Finishes

Section A.2 Fresh Concrete Properties 11. Surface Finishes