The Cathedral of Our Lady of the. Angels. Los Angeles, California

Transcription

1 The Cathedral of Our Lady of the Angels. Los Angeles, California Rafael Moneo

2 Concrete Mix Design Goal: Design a mixture of optimum strength th at minimum cement and acceptable workability Cement $/ton (8/1/11) Aggregates 6-8 $/ton (6/1/11) Minimize cement conte ent Use lowest w/c ratio allowable Use largest size aggrega ate with good gradation Do not use too much fine aggregate Use admixtures fly ash, superplasticizers, etc.

3 How do I design a concrete mix? Produce a trial batch using the ACI 211 Design Procedure: 1. Water and air entrainment determines slump 2. w/c determines strength (durability requirements also) 3. Compute cement from 1 & Use as much coarse aggregate as you can (compute from empirical tables). 5. Fine aggregate fills the rest of the space (compute by subtracting other ingredient ts from total estimated weight) 1. Adjust weights of aggregate g e and water added based on moisture contents and absorption capacity of the aggregates g

4 Trial Batch hing Steps Use Table 1 to determine th e amount of water per cubic yard req d to obtain the desired slump Use Table 2 to determine the w/c ratio req d to obtain the desired strength Calculate the amount of cement Use Table 3 to determine th e amount of coarse aggregate Calculate the amount of fine aggregate using the estimated weight of concrete from Table 4 Fine Weight = Total Weight Coarse Weight Cement Weight Water Weight Adjust the amounts of coars e and fine aggregates and amount of mix water based on level of moisture in aggregate

5 Mix De esign I Concrete for an interior column in a three-story building The desired properties are: Slump = day compressive strength, th f c = 4000 psi No air-entrainment required Coarse aggregate: Nominal maximum size = 1 Dry-rodded weight = 105 lb/ft 3 Absorption capacity (AC) = 1.0 0% Moisture content (MC) = 0.5% Fine aggregate: Fineness modulus (FM) = 2.70 Absorption capacity (AC) = 0.7% Moisture content (MC) = 1.5%

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7 Table 1

8 Table 2

9 Table 3

10 Table 4

11 Level of Moistu re in Aggregate Oven-dry (OD). All moisture is removed. All pores connected to the surface are empty and the aggregate is fully absorbent. Air dry (AD). All surface moisture is removed, but internal pores connected to the surface are partially filled with water. The aggregate is somewhat absorbent. Saturated surface dry (SSD). All pores connected to the surface are filled with water, but the su rface is dry. The aggregate is neither absorbent nor does it contribute water to the concrete mix. Wet. All internal pores connec cted to the surface are filled with water and there is excess moisture on the surface. The aggregate contributes water to the concrete mix.

12 Adjustments for A ggregate Moisture Aggregate weights. Aggregate quantities are calculated based on oven dry (OD) unit weights, but aggregate is typically batched based on actual weight. Therefore, any moisture in the aggregate will increase its weight and stockpiled aggregates almost always contain some moisture. Without correction, the batched ag ggregate volumes will be incorrect. Amount of mixing i water. If the bat tched aggregate is anything but saturated surface dry (SSD) it will absorb water (if oven dry (OD) or air dry (AD)) or give up water (if wet) to the cement paste. This causes a net change in the amount of water available in the mix and the amount of water added to the mix must be adjusted accordingly.

13 Adjusting for aggregate moisture The actual quantities of aggregate to be weighed out during batching of a concrete mix must allow for the specific amount of moisture in the aggregate. Generally, the aggregates will be moist and their dry weights should be increased by the amount of water they contain, both absorbed and surface. The mixing water added to the batch must be reduced by an amount equal to the free moisture contributed by the aggregate (i.e., moisture content of the mix minus absorption capacity of aggregate, or the what is reported as the surface moisture). Remember that water absorbed into the aggregate does not become part of the available mixing water. If the MC is less than the AC, more mix water needs to be provided as some of the mix water will be absorbed by the aggregate. If the MC is more than the AC, then surface water is included in the mix and less mix water needs to be added.

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15 On-Going Research Area

16 Mix De esign II Concrete for an exterior slab of a parking deck The desired properties are: Slump = day compressive strength, th f c = 3500 psi Air-entrainment required Coarse aggregate: Nominal maximum size = 3/4 Dry-rodded weight = 100 lb/ft 3 Absorption capacity (AC) = 1.5 5% Moisture content (MC) = 1.8% Fine aggregate: Fineness modulus (FM) = 2.60 Absorption capacity (AC) = 1.0% Moisture content (MC) = 2.0%