modern driveway design ideas
hi, eric here with thirty by forty designworkshop, today we're going to be discussing board formed concrete which we're using ontwo large site retaining walls on our squid cove project here in maine. so, we've just cast a small mock-up and inthe process i've learned quite a bit and i'm eager to share that with you. now, we're all likely familiar with pouredin place concrete. it's one of the humblest of building materialsand it's unique in that it begins as a highly plastic slurry and after it cures it becomesits polar opposite - extremely hard and durable. and, aside from its inherent utility in foundationsand its weather-resistive properties, we can
exploit this transformation from slurry tosculpture in our architecture. typically, concrete is poured into large,panelized forms made of plywood or aluminum. this results - to varying degrees - in a relativelysmooth appearance. yet even smooth forms will telegraph knotsand surface imperfections to the finished face of concrete walls once the forms arestripped. now, board forming is actually an old techniquethat preceded the larger panelized systems commonly used today and it's a method thataccentuates concrete's plasticity. in lieu of plywood or metal panels, boardscomprise the container - or form - into which the concrete mix is poured.
this results in a finished wall imparted withthe reverse impression of the face of these boards, including all the imperfections, theknots, and the gaps. it's this texture that we're after when wespecify board formed concrete. it gives life to wall surfaces, subtly highlightingthe process of their making and for our project it also links it back to the other wood textureswe're using and the surrounding wooded site. so concrete is composed of four elements:water, cement, aggregate and sand. by altering the mix between these ingredientswe can control how the concrete will perform over its lifetime and in the specific environmentwhere we're using it. for exterior walls we want a dense concreteand a high quality paste.
it's the paste that predominates the colorof a concrete wall. so, we're using a higher strength concretewhich means more cement paste. and to that we're adding a super-plasticizertechnically known as a "high range water reducer." this is an admixture that affects the cementparticles, which are normally attracted to each other and they tend to clump togetherin the presence of water. a super-plasticizer binds to the cement particlesand in so doing, leaves space between them large enough for water molecules to enter. now what this does is it allows the waterto hydrate the cement more efficiently. hydration is the chemical reaction that causesconcrete to "cure" or harden.
superplasticizer allows us to use less water- which makes for stronger concrete - and still maintains its workability which is typicallya problem when we use less water in the mix. it also ensures that the concrete will readilyflow into all the imperfections in the boards that make up our forms. the concrete mix is arguably the most importantpart of our recipe. we want lots of texture so wood type and facefigure is important. for our project, we settled on a local spruce- which was readily available, it was inexpensive and it was pretty easy to work with. most board forms use softwoods like spruce,pine, fir, or larch.
now, we did experiment with finishes to accentuatethe surface texture. we tried pressure washing, wire brushing,sandblasting with walnut shells and finally with coal slag. for our test panel we also included regularmill-finished, un-planed boards. and this is probably a good time to discussthe necessity for sample panels, they're just an absolute must and you only get one chanceto get the real thing cast properly and looking the way you intended. so nate, our general contractor, built oursas a 2-foot by 2-foot by 8-inch thick panel which was enough to test out the board texturesand a few different edge finishing techniques.
next, you'll have to choose board orientationand width. now, these are fundamentally aesthetic decisions:whether it's vertical, horizontal, fixed or random width, broad or narrow but they canbe used to underscore a particular design vision. we chose a horizontal board which helps withthe site wall to gesture to the horizon which is helping us to reach out into the site. and we chose a fixed width board of 6-incheswhich is a comfortable, human-scaled proportion. and 6-inches also allowed our coursing toeasily correlate with other elements of the architecture like the deck, our lighting andthe interior and exterior steps which we've
designed which also relate back to the steppingsite. vertical boards will tend to make elementsfeel taller, while narrower board widths - regardless of orientation - will accentuate directionalityeven more, making vertical orientations seem taller and horizontal ones seem longer. the more boards though the more expensivethe forming labor becomes so there's a delicate balance to maintain there. to keep the concrete forms held together underthe immense weight and outward pressure of wet concrete the two sides of the form wallsmust be tied together. now, typically these form ties are made ofmild steel and the tips are broken off flush
with the wall face after the wall is cast. in most cases this is fine, but mild steelwill rust and because our board formed walls are acting as aesthetic elements in our project- site walls near a large deck and a gathering space we want them to look as good as possible. complicating this decision was the fact thatwe opted to use standard plywood forms for the outer shell of our forms. this was for ease of setup and bracing andallowed us to line the forms with our wood boards. what this also meant was that we have to useties that worked with our concrete contractor's
standard forms. so, we priced out stainless steel ties, butthe added cost was significant and, as it turned out, was more than our client wantedto spend. so, we're planning to use the standard mild-steelties and then we'll just come back and patch the holes after they've been broken off. now, fiberglass rod ties are another optionif you're not using a standard form. they aren't expensive, they don't rust, andthey're actually quite unobtrusive, but we had a hard time sourcing them locally andagain, they didn't work with our subcontractor's forms so it really wasn't a viable optionfor us.
now is also the time to think about the cornerdetails. our retaining walls are doubling as a seatingbench and a planter so the corners are becoming really important. hard edges on concrete don't perform wellover time as they tend to break and they sort of look terrible. so, our contractor nate mocked-up a few differentoptions for us to consider and we settled on a small chamfer, which from a distance,creates a shadow line that actually looks a lot like a hard-edged corner. we tried a true corner and a quarter-roundtooled edge as well but neither of those looked
quite as nice as the small chamfer. after casting the sample panel and discussingit with our client, we all agreed that the unaltered boards provided enough subtle texturefor our needs. and, this helped by reducing the extra laborcost of blasting every one of the boards. there's reinforcing: all concrete should havereinforcing to prevent cracking and with that you'll also need control joints per aci requirements. concrete will crack and control joints essentially"control" where that cracking occurs keeping it contained rather than letting it spider-webacross a beautifully formed wall face. especially long walls will require them andyou'll have to plan for these in the forms
as well. now, there's also things like planters, whichwe're using. you'll have to form those and you'll haveto reinforce them separately. there are bond-outs for integrated equipmentor devices, things like exterior lights, speakers, recesses, utility connections, drainage ifyou're using a planter that's especially important. and finally, you might choose to embellishthe wall with any number of embedments - dates, patterns, emblems, things like that. that all needs to find its way into the formwork. now for the concrete placement.
adding the superplasticizer will aid in theworkability of the concrete as it will tend to flow into voids readily. however, this doesn't eliminate the need tovibrate the concrete as it's placed. but, you have to be careful not to over-vibrateor over-consolidate or hit the rebar when vibrating as that will tend to bring the aggregateto the surface and negate all the hard work of the previous steps. vibrating keeps large voids or honeycombsfrom forming in the wall which are not only unsightly, but they can impact the longevityof a wall allowing water to pool and freeze and crack it over time.
also think about how you want to finish thetop surface. for us, we opted for a hand-troweled, smoothfinish. because of the variables involved it's hardfor me to say precisely what this will add to your project over a standard concrete wall. it will depend on height, thickness, job scope,details, board size, ties; in short, all the things we just discussed. however, the more information you can provideyour concrete sub and the more experience they have in placing board-formed concretethe better your final price will be. so you should come in prepared to discussthe requirements and expect to work through
the details together as you work to quantifythe added costs. so, to recap, the costs above and beyond astandard poured-in-place wall are: boards and any prefinishing that you choose to doto them sandblasting or whatever, extra labor to install the boarding, any special formties, stainless, fiberglass, superplasticizer, a higher strength concrete, any embedmentsor specialty bond-outs and potentially more stripping time. now, for our project's budget all of thiswas pretty manageable and the trade-off - we think - is priceless. i'll be sure to update the video once theactual site walls are poured and complete
and you can leave any questions you have inthe comments.